MICROSCOPIC ICE FRICTION OF POLYMERIC SUBSTRATES

Transcription

1 MICROSCOPIC ICE FRICTION OF POLYMERIC SUBSTRATES by CHRISTOS STAMBOULIDES B.A.Sc. (Ptychion), Aristotle University of Thessaloniki, Greece, 2002 M.A.Sc., The University of British Columbia, 2005 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Chemical and Biological Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) July 2010 Christos Stamboulides, 2010

2 ABSTRACT Interest in snow and ice friction comes from the need to understand and control phenomena of practical importance such as glacier and avalanche movement, traction of automobile tires, snow and ice sports. The need to minimize friction on ice and snow in competitive winter sports is the main motivation behind the present work. A novel tribometer was designed and utilized in conjunction with a conventional rheometer for measuring and understanding the mechanisms of ice friction over polymeric surfaces. Experiments were performed to measure friction between ultra-high molecular weight polyethylene, polytetrafluoroethylene and poly(methyl methacrylate), and ice as a function of sliding velocity, temperature, surface roughness and hydrophobicity. Various techniques were utilized to modify the properties and characteristics of the polymeric surfaces. Light microscopy and scanning electron microscopy as well as surface profilometry were utilized to perform surface analysis and characterize the surface. A goniometer set-up was used for the measurement of the water contact angle measurements and X-ray photoelectron spectroscopy for conducting the elemental analysis. Overall it was found that the magnitude of the sliding velocity and temperature play important roles in ice friction. The more hydrophobic polymers exhibit a lower coefficient of friction. Liquid fluorinated additives as well as a plasma enhanced chemical vapour deposition in a fluorinated gas can improve the hydrophobicity of a polymer and decrease its coefficient of friction over ice. These two concepts can directly be applied in snow winter sports and more specifically in ski and snowboard bases production and preparation where greater speeds, shorter times and therefore less friction are in high demand. ii

3 TABLE OF CONTENTS ABSTRACT... ii TABLE OF CONTENTS... iii LIST OF TABLES... vii LIST OF FIGURES... viii NOMENCLATURE... xiii ACKNOWLEDGEMENTS... xv DEDICATION...xvi 1 INTRODUCTION LITERATURE REVIEW History of Friction Ice/Snow Friction Phenomena Friction Components and Regimes Parameters Influencing Ice/Snow Friction Sliding Base and its Properties Snow and Ice Properties Area of Contact between Sliding Base and Ice/Snow Sliding Velocity and Load Ambient Weather Conditions Waxes Sliding Base Preparation Techniques Base Stone-Grinding Treatment Wax Application Polymer Surface Modification Polymer Additives...27 iii

4 2.6.2 Plasma Application SCOPE AND THESIS OBJECTIVES Introduction Thesis Objectives Thesis Organization MATERIALS AND METHODS Introduction Materials Experimental Equipment Concentric Parallel-Plate Rheometer/Tribometer Field Emission Scanning Electron Microscope (FESEM) Contact Angle Measurements Plasma Enhanced Chemical Vapour Deposition (PECVD) X-Ray Photoelectron Spectroscopy (XPS) Optical Surface Profiler (WYKO) Methodology Sample Preparation Plasma Fluorination of Polymer Substrates Contact Angle Measurements Friction Measurements SLIDING FRICTION OF UHMWPE ON ICE Introduction Background Effect of Velocity Effect of Temperature Effect of Surface Roughness Surface Patterns: Concentric and Cross-Sectional Summary EFFECT OF LIQUID ADDITIVES ON UHMWPE PROPERTIES iv

5 6.1 Introduction Hydrophobicity Effect of Additive Concentration on the C.o.F Effect of Velocity and Temperature on the C.o.F Field Testing Summary PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION (PECVD) TREATMENT OF UHMWPE Introduction Background Sample Preparation SEM, Roughness Analysis, and Hydrophobicity XPS Analysis Effect of PECVD Treatment on the C.o.F Effect of PECVD on Commercial Ski Bases Summary FRICTIONAL PROPERTIES OF DIFFERENT POLYMERIC SURFACES ON ICE Introduction Hydrophobicity Effect of Sliding Velocity on the C.o.F Effect of Temperature on the C.o.F Summary CONCLUSIONS AND CONTRIBUTIONS TO KNOWLEDGE Conclusions Contributions to Knowledge and Practical Implications Recommendations for Future Work BIBLIOGRAPHY v

6 APPENDIX A ADDITIONAL GRAPHS vi

7 LIST OF TABLES Table 4.1 Properties of UHMWPE...33 Table 4.2 Properties of PMMA...33 Table 4.3 Properties of PTFE...34 Table 4.4 Properties of liquid PFPAE...35 Table 4.5 XPS specifications...43 Table 4.6 Profiler specifications...43 Table 4.7 PECVD specifications...45 Table 5.1 Critical sliding velocities for the onset of lubricated friction...55 Table 5.2 Estimated time for the onset of lubricated friction in s...57 Table 5.3 The effect of preparation method, on the average roughness and the static contact angle of UHMWPE substrates...62 Table 6.1 Static contact angle of different surfaces...72 Table 7.1 Effect of various surface preparation methods on the average roughness of UHMWPE substrates and ski running bases...92 Table 7.2 Effect of various surface preparation methods on the static contact angle of UHMWPE substrates...92 Table 7.3 XPS analysis of pure UHMWPE substrates before and after PECVD in a CF 4 environment treatment...93 Table 7.4 XPS analysis of the surface of a ski base sample before and after PECVD in a CF 4 environment treatment as well as after sliding on ice for about an hour...94 Table 8.1 Static contact angle of different polymeric surfaces vii

8 LIST OF FIGURES Figure 2.1 Schematic of a ski/snowboard sliding base and ice/snow system...7 Figure 2.2 Effect of melt water thickness on the coefficient of friction and the characteristic ice/snow friction regimes...12 Figure 2.3 The molecular structure of an ultra-high molecular weight (UHMWPE) chain segment...14 Figure 2.4 Example of a hydrophobic (low wettability) and a hydrophilic (high wettability) surface...16 Figure 4.1 Chemical structure of perfluoropolyalkylether...35 Figure 4.2 Schematic of the Paar-Physica MCR 501 rheometer and the environmental chamber...36 Figure 4.3 Schematic of the concentric parallel plate rheometer/tribometer geometry...37 Figure 4.4 Specially designed ice dish holder...37 Figure 4.5 Schematic of the ring...38 Figure 4.6 Image of the ice dish and the rotating ring...39 Figure 4.7 Measurement of the static contact angle θ...40 Figure 4.8 Surface plasma fluorination...42 Figure 4.9 Drill press in cold-room...44 Figure 4.10 Compression-molding cycle for the preparation of the polymeric substrates...45 Figure 4.11 Static water contact angle analysis of a hydrophilic (a) and a hydrophobic surface (b)...46 Figure 4.12 a) Disk holder with an attached polymeric ring, b) the shaft with the disk holder...47 Figure 4.13 A typical transient friction experiment indicating the torque versus time graph. Note the constant normal load during the test, essential for these measurements...48 Figure 5.1 The effect of sliding velocity on the ice coefficient of friction...51 viii

9 Figure 5.2 The effect of sliding velocity on the c.o.f. for UHMWPE substrates sliding on ice at different temperatures...53 Figure 5.3 The effect of temperature on the ice c.o.f. of UHMWPE substrates sliding on ice at different sliding velocities...59 Figure 5.4 Topography (a) and contact angle (b) images of an UHMWPE substrate prepared with 400 grit sandpaper...61 Figure 5.5 Topography (a) and contact angle (b) images of an UHMWPE substrate prepared with 600 grit sandpaper...61 Figure 5.6 Topography (a) and contact angle (b) images of an UHMWPE substrate prepared with 1200 grit sandpaper...62 Figure 5.7 The effect of roughness on the c.o.f. for different sliding velocities at T = -1.5, -4, and -7 C...64 Figure 5.8 The effect of temperature on the ice c.o.f. for substrates of different roughnesses sliding on ice at , 0.079, 0.79, and 1.96 m/s...65 Figure 5.9 The effect of sliding velocity on the c.o.f. for substrates of different roughnesses at -1.5, -4 and 7 C...66 Figure 5.10 Surface patterns created by different polishing techniques: a) concentric grooves and b) cross-sectional grooves...67 Figure 5.11 Microscopic images of surface patterns created by different polishing techniques: a) concentric grooves and b) cross-sectional grooves.68 Figure 5.12 The effect of sliding velocity on the ice c.o.f. of two (A and B) commercial ski base samples having a cross-sectional and concentric surface patters sliding on ice at -1.5 C...69 Figure 6.1 The effect of lubricant concentration on the UHMWPE static contact angle...73 Figure 6.2 Static contact angle image and water droplet analysis for a substrate of pure UHMWPE...74 Figure 6.3 Static contact angle image and water droplet analysis for a substrate of UHMWPE + 5% PFPAE...74 Figure 6.4 Static contact angle image and water droplet analysis for a substrate of UHMWPE + 7.5% PFPAE...74 Figure 6.5 The effect of liquid additive (PFPAE) concentration on the c.o.f. of UHMWPE sliding on ice at 0.79 m/s and temperature of -1.5 C...75 ix

10 Figure 6.6 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at 1.96 m/s and temperature of -1.5 C...76 Figure 6.7 The effect of liquid additive (PFPAE) concentration on the c.o.f. of UHMWPE sliding on ice at 0.79 m/s and temperature of -4 C...76 Figure 6.8 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at 1.96 m/s and temperature of -7 C...77 Figure 6.9 The effect of temperature on the ice c.o.f. of UHMWPE and UHMWPE + 2.5% PFPAE substrates sliding on ice at m/s...78 Figure 6.10 The effect of temperature on the ice c.o.f. of UHMWPE and UHMWPE + 2.5% PFPAE substrates sliding on ice at 0.79 m/s...79 Figure 6.11 The effect of temperature on the ice c.o.f. of UHMWPE and UHMWPE + 2.5% PFPAE substrates sliding on ice at 1.96 m/s...79 Figure 6.12 The effect of sliding velocity on the ice c.o.f. of UHMWPE + 2.5% PFPAE substrates sliding on ice at different temperatures...80 Figure 6.13 The effect of temperature on the ice c.o.f. of UHMWPE + 2.5% PFPAE substrates sliding on ice at different sliding velocities...81 Figure 7.1 PECVD chamber...86 Figure 7.2 Illustration of the surface treatment under CF 4 plasma application...86 Figure 7.3 SEM image of a UHMWPE substrate before PECVD treatment. Magnification: x 2.5K...89 Figure 7.4 SEM image of a UHMWPE substrate before PECVD treatment. Magnification: x 10K...90 Figure 7.5 SEM image of a UHMWPE substrate after PECVD treatment. Magnification: x 2.5K...90 Figure 7.6 SEM image of a UHMWPE substrate after PECVD treatment. Magnification: x 10K...91 Figure 7.7 Topography (a) and contact angle (b) images of a UHMWPE substrate prepared with 1200 grit sandpaper...91 Figure 7.8 Topography (a) and contact angle (b) images of a CF 4 plasma treated UHMWPE substrate prepared with 1200 grit sandpaper...92 Figure 7.9 The effect of sliding velocity on the ice c.o.f. of a UHMWPE substrate sliding on ice before and after CF 4 treatment at -1.5 C...96 x

11 Figure 7.10 The effect of sliding velocity on the ice c.o.f. of a UHMWPE substrate sliding on ice before and after CF 4 treatment at -4 C...96 Figure 7.11 The effect of sliding velocity on the ice c.o.f. of a UHMWPE substrate sliding on ice before and after CF 4 treatment at -7 C...97 Figure 7.12 The effect of sliding velocity on the ice c.o.f. of commercial ski bases sliding on ice before and after CF 4 treatment at -1.5 C...99 Figure 7.13 The effect of sliding velocity on the ice c.o.f. of commercial ski bases sliding on ice before and after CF 4 treatment at -4 C...99 Figure 8.1 Static contact angle image and water droplet analysis for a substrate of pure UHMWPE Figure 8.2 Static contact angle image and water droplet analysis for a substrate of pure PMMA Figure 8.3 Static contact angle image and water droplet analysis for a substrate of pure PTFE Figure 8.4 The effect of sliding velocity on the ice c.o.f. of PTFE substrates sliding on ice at different temperatures Figure 8.5 The effect of sliding velocity on the ice c.o.f. of PMMA substrates sliding on ice at different temperatures Figure 8.6 The effect of temperature on the ice c.o.f. of PMMA substrates sliding on ice at different sliding velocities Figure 8.7 The effect of temperature on the ice c.o.f. of PTFE substrates sliding on ice at different sliding velocities Figure 8.8 The effect of temperature on the ice c.o.f. of the three polymeric surfaces under study at 0.79 m/s Figure 8.9 The effect of temperature on the ice c.o.f. of the three polymeric surfaces under study at 1.96 m/s Figure 9.1 The effect of temperature on the c.o.f. at high sliding velocities Figure 9.2 The effect of temperature on the c.o.f. at low sliding velocities Figure A.1 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at m/s and temperature of -1.5 C127 Figure A.2 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at m/s and temperature of -1.5 C.127 xi

12 Figure A.3 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at m/s and temperature of -4 C..128 Figure A.4 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at m/s and temperature of -4 C Figure A.5 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at m/s and temperature of -7 C..129 Figure A.6 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at m/s and temperature of -7 C xii

13 NOMENCLATURE A c A w C p c.o.f D F N f s F T F T, c F T, d F T, w L M Q R t c dry contact area during sliding wet contact area during sliding ice heat capacity coefficient of friction melt-water film thickness normal force area fraction of the solid-liquid interface tangential friction force resistance due to snow compression dry frictional force frictional force due to melt-water ski length torque heat flux at the interface roughness factor time necessary for the ice surface to melt Greek symbols γ sv, γ sl and γ lv z η θ interfacial tension at the solid-vapour, solid-liquid and liquid-vapour interfaces respectively height of the snow compaction absolute viscosity of water film water contact angle xiii

14 θ Y and θ W κ Young s intrinsic contact angle and Wenzel s apparent contact angle respectively ice thermal conductivity µ coefficient of friction μ d μ dry, μ wet, μ cap, μ plough and μ dirt ρ σ c σ n τ υ dry coefficient of friction coefficient of friction due to solid deformation, melt-water film, capillary drag effects, snow ploughing and dirt and snow accumulation respectively ice density compressive strength of snow normal stress at the interface shear strength of the softer material at the junction sliding base velocity xiv

15 ACKNOWLEDGEMENTS I wish to express my sincere gratitude to my thesis supervisor Prof. Savvas G. Hatzikiriakos for his skillful guidance and constructive criticism throughout this work. He has helped me become a better professional in various ways. I would also like to thank my co-supervisor Prof. Peter Englezos for his valuable suggestions, points and comments throughout this work. They have provided me with plenty of academic freedom in order to reach my research goals. Their continuous encouragement and support has been a source of motivation during the course of this work. Thanks to Own The Podium (OTP) 2010 for the financial support. I would also like to thank the technical staff, coaches and athletes from Alpine, Biathlon, Cross Country and Snowboard Canada for their technical support, feedback, and exchange of ideas as well as for supplying testing samples of commercial ski bases. My colleagues and ex-colleagues from Rheolab at UBC have all helped me in their own special ways. I wish to thank Anne-Marie Kietzig for the endless discussions on snow and ice friction. I would also like to thank Dr. Edward Budi Muliawan, Babak Derakhshandeh and Mahmoud Ansari for their friendship and support as well as the constant exchange of ideas in tribology and rheology. I want to thank all my close friends in Vancouver and Greece for their continuous friendship and support. Michali my thoughts and prayers are with you for this very important fight you have ahead of you. Last but not the least and from the depths of my heart I want to express my love to my parents Aristoteli and Athina Stamboulides. I owe them everything I am. Their unconditional love and support has always been and will always be a source of strength and a true inspiration to me. xv

16 DEDICATION To my parents, Aristoteli and Athina xvi

17 1 INTRODUCTION Friction is the resistive force acting between body surfaces in contact that tend to slide past each other. It is represented by the coefficient of friction, μ. It is usually distinguished as static and kinetic (or sliding) friction. Static friction is the force required to set one of the surfaces in motion when they initially are both at rest; kinetic is the force that tends to slow the motion between the surfaces when they are sliding over each other. Kinetic friction is one of the oldest problems in physics and definitely one of the most important from a practical point of view. It is accounted for a plethora of every day activities in industry, aviation, medicine, and engineering operations and sports. Specifically in sports, kinetic friction is mainly present in motor (car and bike racing) and winter sports. The performance in winter sports, which include skiing, snowboarding and ice sports, is highly dependent on the acting friction between the ski/board running surface (sliding base) and snow, and the ice blade runner and ice. The ice/snow friction is studied in this work. Sports are an indispensable part of our daily lives and the world economy. There is a large industry behind modern sports that promotes research and development for technological advancements. One of those areas is in the field of controlling and minimizing friction. Sliding over snow is necessarily more complicated than sliding over ice because of the large variety of possible snow conditions. Hence, understanding snow friction is difficult. As Kuzmin (2006) states: this sliding process is extremely complicated and it is very hard to carry out research in this area. Friction between the sliding base and snow is influenced by many parameters including the snow and weather conditions, the ski/snowboard sliding base, base preparation and the skier (Bowden and Hughes, 1939; Bowden, 1952; Glenne, 1987; Colbeck, 1992; Buhl et al., 2001). Specifically, snow characterization parameters include temperature, grain size, density, hardness and snow liquidwater content; weather condition parameters include the air temperature and 1

18 humidity; ski/snowboard sliding base parameters are the material of construction, possible additives used therein, surface hydrophobicity and topography, stiffness distribution and base preparation. In addition to the above parameters there is the sliding velocity of the base as well as the normal force applied. The stone-grinding technique followed by wax application is the most accepted used method for the preparation of the sliding base. Stone-grinding is used for scraping the base surface from impurities, polishing and creating different structures (patterns), i.e., some specially designed roughness. This method results in macroscopically structured surfaces. Waxing cleans the ski base surface, increases the hydrophobicity, modifies the hardness of the sliding surface and essentially provides better sliding properties. Typical waxes are paraffinic, fluoropolymers or perfluoropolymers and are available in the form of blocks, liquids or powders. It is known from various scientific areas that different physical and chemical methods exist for modifying the surface of polymers. Surface modification includes either the alteration of the main polymer composition or the application of different techniques to alter the topography of the surface and/or deposit different films on the surface. These methods are explored in this research work and the most suitable are implemented to modify sliding bases and reduce their ice coefficient of friction. The main focus is on finding ways of minimizing friction between skis and snowboards sliding on snow. Due to the fact that snow surfaces exhibit many complexities, the experimental work is conducted on ice surfaces instead of snow. Colbeck (1992) pointed out that a snow surface that has seen repeatedly passed by a slider resembles an ice surface. In this study, an extensive experimental work to measure the ice coefficient of friction is performed. The outcome of this work will be relevant to alpine and nordic ski as well as to snowboard where the improvement margin is believed to be limited due to the last decade s advancements in base materials and ski preparation technology 2

19 (Coyne, 2006; MacLean, 2006; Perks, 2006). It has been calculated that a 5% decrease of the coefficient of friction would lead into, approximately, 1% less time in alpine skiing (Green, 2006). Such improvements are significant, considering that the total times in alpine ski races are typically minutes and the podium times differ by a few tenths of a second. This is based on information from Olympic Games and World Cup official results which are available at the following websites, 3

20 2 LITERATURE REVIEW In this chapter a historical account of friction is firstly presented. A description of the fundamental friction phenomena and mechanisms, and parameters that influence snow and ice friction follows. Preparation techniques for snow and ski/snowboard running bases are also discussed in this chapter. 2.1 History of Friction Tribology is the science and technology of the mechanisms of friction, lubrication, and wear of interacting surfaces that are in relative motion. The term tribology is derived from the Greek word τριβή, meaning friction. The significance of friction and resistance to motion has been recognized throughout the ages, but a full appreciation of the significance of tribology in a technological society is a recent phenomenon (Dowson, 1998). The importance of lubricants for friction reduction was initially appreciated at the old Sumerian and Egyptian civilizations. The use of a lubricant, possibly grease, vegetable or animal oil or water, for friction reduction, was recorded as early as 2400B.C. (Dowson, 1998). A chariot from about 1400B.C. was found in the tomb of Yuaa and Thuiu (Egypt), along with traces of the original lubricant on the axle. Scientists suggested that it had been mutton or beef tallow, either of which would have proved suitable for axle lubrication in such a warm country. The traces of mortar between the layers of stones of the Pyramids, suggest that the builders used a squeeze-film mechanism of lubrication to slide the blocks properly into place (Lloyd, 1954). Aristotle ( B.C.) recognized the force of friction and further observed that it was lowest for round objects (Dowson, 1998). Leonardo da Vinci ( ) studied friction systematically and realized how important friction was for the workings of machines. He focused on all kinds of friction and drew a distinction between sliding and rolling friction. Da Vinci stated the two basic laws of friction 4

21 200 years before Newton ( ) even defined what force is. He simply stated that: a) the areas in contact have no effect on friction and b) if the load of an object is doubled, its friction will also be doubled. Da Vinci made the observation that different materials move with different ease and surmised that this was a result of the roughness of the material in question; thus, smoother materials will have smaller friction. Galileo ( ) concluded that objects retain their velocity unless a force - often friction - acts upon them (Dowson, 1998). The first published account of the empirical laws of friction was made by Amonton ( ) in Amonton formulated the two basic laws of friction that had been stated by Leonardo Da Vinci, and at the same time added that friction was predominately a result of the work done to lift one surface over the roughness of the other, or from the deforming or the wearing of the other surface. According to Dowson (1998), Amonton stated that the force of friction, F T, for two objects in sliding contact is proportional to the normal load of the object being moved. This is the first law of friction. In addition, he stated that the force of friction is independent of the apparent area of contact. This is the second law of friction. Euler ( ) was the first who distinguished between static and kinetic friction. He studied theoretically the mechanism of the sliding motion of a solid body block on an inclined plane and concluded that static friction must be always larger than kinetic. His work is also important because it developed a clear analytical approach to friction and introduced the coefficient of friction symbol μ (Dowson, 1998). Coulomb ( ) stated that for most materials and ordinary speeds, the force of friction is independent of the speed with which an object is traveling. Moreover, Coulomb stated that strength due to friction is proportional to compressive force, although for large bodies friction does not follow exactly this law. Thus, Amonton s first law of friction became "Amonton- Coulomb Law": 5

22 F = μ [2-1] T F N where F T is the tangential frictional force, F N is the normal (compressive) force and μ is the coefficient of friction. Bowden and Tabor (1950) gave a physical interpretation for the laws of friction. They determined that the true area of contact is a very small percentage of the apparent contact area and that it is formed by the asperities on the surface. As the normal force increases, more asperities come into contact and the average area of each asperity contact grows. The frictional force is shown to be dependent on the true contact area. 2.2 Ice/Snow Friction Phenomena Interest in snow friction arises mainly because of the interest in recreational skiing. Increased interest in competitive skiing over the past few decades has generated a lot of research (Colbeck, 1992; Nachbauer et al., 1996; Moldestad, 1999; Nikki et al., 2005; Fauve et al., 2005; Baurle et al., 2006a; Kuzmin, 2006). The focus of this study is the ice/snow coefficient of friction between a ski/snowboard sliding base and ice/snow. Consider the system ski/snowboard sliding base and snow as shown in Figure 2.1. This system consists of the moving sliding base and the snow that lies beneath it. There are several parameters affecting the frictional behaviour of the sliding base on ice/snow, namely the material of construction of the base, the ice/snow properties, the contact area between the base and ice/snow, the surface treatment e.g. wax treatments, additives to snow, sliding speed, normal load, and ambient conditions. It is important to note that the sliding process may induce a phase change of the water at the point of the sliding base and ice/snow contact which gives rise to melt water. Melting is induced by frictional heating and/or pressure melting. The latter however is minimal. According to Bowden and Hughes (1939) the thin layer 6

23 of water (melt-water film) which exists between the sliding base and the ice/snow is produced mainly by frictional heating. This layer of water provides a lubrication effect which reduces the friction between the bases and ice/snow. The friction between the base and ice/snow in the presence of the melt-water is known as wet friction. In the absence of the melt-water film the system shown in Figure 2.1 would involve what is known as dry friction. The process of dry friction plays very little role in the gliding enhancement of a ski/snowboard. It is more important though in the process that increases the ability of a sliding base to accumulate snow (Lind and Sanders, 2004). Figure 2.1 Schematic of a ski/snowboard sliding base and ice/snow system Glenne (1987) and Colbeck (1992) further established the critical lubricating role of the melt-water on the ice/snow coefficient of friction. During ski/snowboard gliding, the energy corresponding to the sliding motion is dissipated into irreversible compression and shearing of snow. This energy is converted into heat near the ski/snow surface. The amount of heat generated causes the local melting and discontinuous thin film creation. In addition to melt-water, the presence of two other types of water should also be considered as relevant to the system of interest. These are the ambient interstitial snow moisture (liquid-water content) and the percolation of snow liquid-water to the sliding interface due to snow compaction (Glenne, 1987). 7

24 The accumulation of more water at the base interface increases shearing and results in capillary suction that drastically increases the resistance (Colbeck, 1988). Additionally, there are some ice grains that do not come into contact with the sliding base when a ski/snowboard is sliding over snow. The existing spaces between those ice grains may serve as pores in the snow surface through which capillary columns (capillary bridges) of melt-water connect to the sliding base and produce a drag effect (capillary drag) on the ski (Lind and Sanders, 2006). An additional resistance to a slider s motion when the snow is compressed (compaction) and/or pushed aside (ploughing) may contribute to the total resistance to sliding over snow. Both these processes dissipate energy and slow forward progress. Another phenomenon that may add more resistance to gliding is snow and/or dirt accumulation under the sliding base. Often, solid-to-solid contacts at the surface of the sliding base causes electrostatic charge that attracts dirt. Finally, it should be noted that at low temperatures any source of heat should reduce friction since it would help compensate for heat loss by conduction into the old snow and/or ski, whereas at high temperatures heat production should be avoided as much as possible since too much melt-water is being produced. 2.3 Friction Components and Regimes With a thin film boundary layer between sliding base and ice/snow, a laminar resistance frictional force, F T, w, may be expressed as (Glenne, 1987): υaw FT, w = η [2-2] d where η is the absolute viscosity of the water film, υ is the sliding base velocity, A w is the wet contact area and d is the thickness of the film. This equation describes what is known as wet friction. The equation predicts linearly increasing resistance with increasing velocity and wet contact area, as well as decreasing 8

25 resistance with increasing water film thickness. This behaviour seems reasonable for pure wet-snow sliding resistance. The linear increase in friction force with increasing velocity is a characteristic of the hydrodynamic lubrication (Kavehpour and McKinley, 2004). Classically, dry friction (adhesion) considers shearing and ploughing at the junction of crystal asperities. With negligible ploughing forces, the dry frictional force F T,d may be written as (Glenne, 1987): F F = Aτ = τ = μ F N T, d c d N [2-3] σ c Where τ is the shear strength of the softer material at the junction (i.e.,, snow, ice or wax), σ c is the compressive strength of the snow and μ d the dry coefficient of friction, which is equal to τ / σ c. At low temperatures (usually lower than -10 C) snow crystals are sharp, hard and abrasive. To optimize sliding speed at low temperatures, competitive skiers and ski technicians use smooth and hard running surfaces. When snow is gradually compressed, it will yield until the snow stress equals the normal stress. At this point the dry contact area is given by (Glenne, 1987): A = / σ [2-4] c F N c Where σ c is the compressive strength of the snow and F N is the normal load. It is accepted that the kinetic snow friction is the sum of wet friction (melt-water film lubrication), dry friction, capillary drag effects and resistance caused by displacing and compressing the snow and dirt accumulation (Glenne, 1987; Colbeck, 1992; Buhl et al., 2001). The splitting of the total friction into individual components is related to the formation of the melt-water film between the sliding base and the snow surface. The thickness of the water film mainly determines the distribution of the different friction components. Furthermore, it is believed to be the crucial parameter influencing the kinetic friction. Since this water film is 9

26 formed primarily by frictional heating of the contact area, there is a number of influential parameters which can be divided into two categories: sliding base and ice/snow (Buhl et al., 2001). Sliding base parameters include: roughness, hardness, wettability and thermal conductivity. Ice/snow parameters include temperature, density, liquid-water content, grain size and shape (only for snow), thermal conductivity, roughness and hardness. In addition, the sliding velocity and the load also contribute to the system shown in Figure 2.1 (Buhl et al., 2001). Colbeck (1992) considered the friction phenomena mentioned above and expressed the total snow coefficient of friction (similar approximation can be applied for ice) as follows: μ = μ + μ + μ + μ + μ [2-5] wet dry cap dirt plough where µ is the coefficient of friction (c.o.f.) and the subscripts wet, dry, cap, dirt and plough represent the frictional contributions due to the melt-water film, solid deformation, capillary drag effects, dirt and snow accumulation, and ploughing respectively. Equation 2.5, would be valid if these phenomena took place independently. Colbeck (1992) assumed that wet and dry friction are parallel processes. He expressed the total coefficient of friction with respect to the meltwater thickness based on the following equation: μ wet μ dry μ = μ cap + [2-6] μ + μ wet dry In summary, based on the interaction that takes place between the interface and the melt-water the following regimes shown in Figure 2.2 can be defined. These regimes are similar for ice friction: Dry and Boundary Friction: dry friction indicates the type of friction when two surfaces are sliding over each other in the absence of any type of lubrication between them. The contacting surfaces usually exhibit geometrical characteristics in the millimeter, micro and nano levels. It is possible to have either well-designed patterns or anomalous surfaces that exhibit asperities, 10

27 waves, peaks and valleys, cones or ripples. In order for the two surfaces in contact to slide over each other, it is necessary to break the adhesive bonds that are created between the two surfaces. These bonds are created either physically or chemically and when the two surfaces in contact are moved relative to one another, the adhesive bonds are sheared. Dry friction is defined as the work necessary to break or shear the adhesive bonds between the two surfaces. It depends on the applied load and the hardness of the surfaces but is independent of the sliding speed (Bowden and Tabor, 2001; Bhushan, 2002). The main characteristic of the boundary friction (or boundary lubrication) regime is the existence of a very thin lubricating layer between the two surfaces in contact. It has been shown that even at very low temperatures it is possible to have a very thin (a few nanometers) liquid-like film (Petrenko, 1997) or even in the molecular level (Makkonen, 1994; Bhushan, 2002). The thickness of this film is less than the average height of the surface asperities and therefore it does not overlay them. Bluhm et al., (2000) reported a lubricated film thickness of less than 8 nm which did not contribute to the lubrication of the interface in their study. As the sliding velocity increases though, frictional heating increase which may cause the thin lubricating layer thickness to increase. The greater the thickness of the layer the less the contact between the surface asperities. This is known as the boundary lubrication regime. Figure 2.2 illustrates the transition from dry to boundary friction. In literature though, boundary and dry friction refer to the same mechanism (Evans et al., 1976; Glenne, 1987; Colbeck, 1992). Mixed Friction (or Mixed Lubrication): The thickness of the lubricating film is still less than the characteristic roughness of the surfaces but it keeps increasing which decreases the solid-to-solid contact and improves the surface lubrication. At the same time a wetting lubricant induces the build-up of capillary water bridges between the asperities. This is the mixed friction regime. In this regime the load of the slider is partly supported by the surface asperities and partly by the lubricating layer. The resistance from solid-to-solid contact decreases but resistance increases due to capillary drag. The 11

28 contributions of these two resisting mechanisms lead to a minimum value of the frictional force which is the characteristic of this regime (Bowden, 1953; Evans et al., 1976; Colbeck, 1992; Bhushan, 2002). Hydrodynamic: In this regime the thickness of the melt-water film is greater than the average height of the surface asperities. It therefore overlays the surface, it becomes capable of carrying the applied load and the phenomenon that dominates is the capillary drag and viscous shearing. There is no more solid-to-solid contact and the breakage and shearing of the adhesive bonds between the two surfaces in contact does not contribute to the resisting force. Coefficient of friction Dry to Boundary Friction Mixed friction / Mixed Lubrication Hydrodynamic friction / Capillary drag Melt water thickness Figure 2.2 Effect of melt water thickness on the coefficient of friction and the characteristic ice/snow friction regimes 2.4 Parameters Influencing Ice/Snow Friction Below, the role of the parameters influencing ice/snow friction is discussed in more detail. Most of the parameters discussed influence both ice and snow friction in the same way, and only when the effects are different, they will be pointed out. In order to elucidate what influences friction between ice/snow and a 12

29 sliding base (refer to schematic in Figure 2.1) and how to reduce it depends on: a) the type of sliding base and its properties; b) the ice/snow and its properties; c) the sliding speed and applied load; and d) the ambient weather conditions. The above parameters are not necessarily independent. For example, ambient temperature and humidity affect ice and snow properties. It is important to note that it is very difficult to alter and essentially optimize these parameters for optimal gliding at all given conditions. As Lehtovaara (1989) states: One particular ski can operate optimally only in a small range of track and air conditions Sliding Base and its Properties Material of construction: The earliest known skis bear little resemblance to the ones used today. Over the decades, the development of ski equipment has been phenomenal. The evolution of the ski base in chronological order has been as follows: spruce hickory hickory with synthetic edges Acrylonitrile Butadiene Styrene (ABS) High Density Polyethylene (HDPE) Ultra-High Molecular Weight Polyethylene (UHMWPE). It is obvious that the development of the sliding base was clearly shifting towards an increase in durability and hydrophobicity (Kuzmin, 2006). Ultra-high molecular weight polyethylene (UHMWPE) (Figure 2.3), is used as the main element of ski soles, because it has a relatively low friction coefficient, which remains the fundamental factor for ski performance and it is very resistant to abrasion (Ducret et al., 2005). There are two types of polyethylene base materials attached to the bottoms of skis and snowboards: sintered and extruded. Sintered base materials are fused polyethylene resin sheets produced by compression molding under prescribed temperature and pressure conditions, while extruded base materials are formed from molten, homogenized polyethylene resin forced through a die. Sintered base materials are more expensive to produce and are found on the bottom surfaces of most skis where high-performance characteristics are required. 13

30 -- CH 2 (CH 2 CH 2 ) n CH 2 -- Figure 2.3 The molecular structure of an ultra-high molecular weight (UHMWPE) chain segment Contact angle Hydrophobicity: The snow coefficient of friction strongly depends on the adhesion between the sliding ski base and ice/snow. At a macroscopic scale, the strength of the interaction between a solid and a liquid is most obviously characterized through the wetting behaviour. Weak interactions result in non-wetting behaviour, with large contact angles for a drop of liquid water resting on the solid substrate. This characterization is purely thermodynamic, and has, in principle, no direct influence on the nature of fluid flow past the interface (Cottin-Bizonne et al., 2003). A measure of adhesion is the contact angle, θ. In general, the surfaces that exhibit a high contact angle give a lower friction. The water contact angle is governed by the forces acting on the three phase contact line of the drop in the plane of the solid (Figure 2.4), which is where the solid/liquid, liquid/gas and solid/gas interfaces meet (Blossey, 2003). The surface tension forces act at this line, and their balance results the Young s equation (see Figure 2.4): γ sv = γ + γ cosθ [2-7] sl lv Y where γ ij denotes interfacial tension of the interface ij and s, l and v designate the solid, liquid and vapour phases respectively. Equation 2.7 does not apply to rough surfaces. Wenzel (1936) and Cassie and Baxter (1944) established that wettability is determined by roughness as well as surface energy. Wenzel (1936) proposed a model which states that the apparent contact angle θ W of the drop on the rough surface is related to Young s intrinsic angle θ Y on the smooth surface by: cos θw = r cosθ Y [2-8] 14

31 where r is the roughness factor, defined as the ratio of the total or actual area of a rough surface to the geometric area (the surface measured in the plane of the interface). Since r is always greater than one, the surface roughness enhances both the hydrophilicity of hydrophilic surfaces and the hydrophobicity of hydrophobic ones. In other words Wenzel s equation implies that wettability is improved by roughness for a hydrophilic surface (θ W < θ Y for θ Y < 90 ), but becomes worse for a hydrophobic one (θ W > θ Y for θ Y > 90 ). Cassie and Baxter (1944) proposed an equation describing the contact angle on a surface composed of a solid and air, assuming the water contact angle for air to be 180 : cos θ f cos θ + f 1 [2-9] C = S W S where f S is the area fraction of the solid/liquid interface. Regardless of the approach, the contact angle is always greater or equal on a rough surface, therefore creating different structures on running surfaces is an effective way to increase hydrophobicity. Wetting occurs when the contact angle, θ Y, is less than 90, whereas if the surface is hydrophobic, the value of the contact angle is greater than 90 as seen in Figure 2.4. The contact angle depends on several factors, such as roughness, the manner of surface preparation, and surface cleanliness (Burton and Bhushan, 2005). As already mentioned, one way to increase the hydrophobic properties of the surface is to increase its surface roughness (Bhushan, 2002). Water slides more readily on hydrophobic surfaces. In cases of excess lubrication, capillary forces would be higher on the less hydrophobic ski base. In view of this, it can be concluded that a hydrophobic surface would be advantageous in all snow (weather) conditions (Colbeck, 1992). According to Bowden (1953), at temperatures near 0 C, friction on snow of various surfaces is highest for the surfaces that wet easily and lowest for the surfaces which have a high contact angle. If the snow is saturated with water to 15

32 form slush, the friction with all surfaces will be appreciably higher than on dry snow at 0 C and the correlation between low friction and high contact angle is even more marked. The smaller the contact angle the more readily is the surface wetted by water. The decrease in contact angle becomes greater when the surfaces are sheared, which produces more liquid water. Y Y Figure 2.4 Example of a hydrophobic (low wettability) and a hydrophilic (high wettability) surface Chemical modification of surfaces can typically lead to water contact angles of up to 120 by using fluoropolymer coatings or silane layers (Shafrin and Zisman, 1964) but not more. To reach the extreme values of the contact angles near 180, a second ingredient has to come into play and that is surface structure (Bhushan, 2002; Blossey, 2003). Roughness Base patterns: At first it seems reasonable that the smoother the sliding base is, the lower the coefficient of friction would be. However, this is not always true. As mentioned above, surface friction may be reduced by surface roughness (Cottin-Bizonne et al., 2003; Burton and Bhushan, 2006). Traditionally, ski bases are roughened with small grooves under wet snow and high speed conditions. This way, it is believed that the capillary attachments generated at the interface between ice/snow and sliding base are reduced by channelling of the free water (Nachbauer, 1996). Roughening the surface reduces the drag at higher temperatures where melt-water film is plentiful (Colbeck, 1996). This is due to reduction of the contact area between the surface and the film, and to the presence of a liquid water film between the shearing surfaces. At low temperatures where melt-water lubrication is essential, a smooth 16

33 sliding surface would be desirable to allow water slippage, whereas at high temperatures, where water attachments increase capillary drag effects, a rougher sliding base would be useful to disrupt the water attachments (Colbeck, 1992). With increased roughness the number of asperities in contact is reduced, therefore the real area of contact is reduced which leads to decreased adhesion (Bhushan, 2002). Experimental studies on the base patterns by Colbeck (1994) and Kuzmin (2006) have shown that when the grooves are longitudinally with the ski, the film of meltwater tends to be thinner. When the grooves are oriented transversely across the ski the water film thickness increases as the fluid pressure increases on the upstream sides of the asperities. Thus, a transverse structure should be beneficial at low temperatures, whereas a longitudinal structure should be better at high temperatures. Thermal conductivity Effect of solar radiation: Temperature measurements on the surface of skis have consistently shown that ski bases warm up during motion. Higher weights and faster speeds produce more frictional heating and the base temperature rises accordingly. Low thermal conductivity skis do not conduct energy easily and thus a higher temperature increase is expected at the sliding base surface which essentially decreases the coefficient of friction. Skis respond directly to the presence or absence of solar radiation. Heat production calculations suggest that solar radiation absorption at a ski base can contribute significantly to the production of the melt-water film on which skis glide (Colbeck and Perovich, 2004). Since sliding base colour affects the energy balance at the base, colour should affect ski friction too. Black bases run at higher temperatures than white bases and this effect should be measurable during all daylight hours, even during overcast conditions, since diffuse solar radiation still penetrates the snow surface. Furthermore, black ski bases can offer advantages, or disadvantages, over white bases, depending on the specific conditions. At low temperatures, where heat production should be maximized to 17

34 overcome conductive heat losses, black bases run at higher temperatures apparently because of solar radiation absorption. On the other hand, white bases absorb much less solar energy and should offer advantages at high temperatures where too much melt-water is present and capillary drag increases snow friction (Colbeck and Perovich, 2004). Fauve and co-workers (2005) showed that the higher net radiation leads to shorter run times for old snow but longer run times for new snow. Quantitative models for the frictional heating theory were developed by Evans et al., (1976) and Oksanen and Keinonen (1982) and verified experimentally by Akkok et al., (1987). Frictional heat is conducted into both the sliding base and the snow, thereby raising the snow surface to its melting point. A highly conductive sliding base displays higher friction than one that is well insulated; indicating that heat conduction plays a major role in ice friction (Baurle et al., 2006a). Additives: There is little known about the additives used into the ultra-high molecular weight polyethylene sliding bases. Graphite is used as an additive to produce black ski bases. Ceramic additives are also being used in some sliding bases (Salomon skis). It is believed that the reason behind this is to alter the degree of crystallinity, therefore the wax absorption on the sliding bases (Perks, 2006). Degree of crystallinity: It is believed that degree of crystallinity affects the wax absorption (Coyne, 2006; McLean, 2006). This can be studied by considering ultra-high molecular weight polyethylene bases having various crystallinities Snow and Ice Properties The ice/snow temperature, liquid-water content, density, hardness and grain size are very important parameters influencing snow friction. It is very difficult to determine the effect of each parameter independently due to the unstable mass and energy balances at the ice/snow and running base interfaces (Colbeck, 18

35 1992). Moreover, the movement of the sliding base over the snow affects the temperature and the liquid-water content. Temperature: It is crucial to determine the influence of ice/snow temperature on friction in order to be able to optimize the ice/snow and slider surface system for reduced friction. This enables one to determine the importance of the individual friction components in different temperature ranges, which is the key for optimizing the ski parameters in order to reach the best gliding performance. According to Buhl et al., (2001), who performed sliding experiments of polyethylene on snow between -25 C and 0 C, the coefficient of friction reaches a minimum of 0.02 when the snow temperature is around -3 C. Further temperature increase (to 0 C) resulted into an increase of the c.o.f. Baurle et al., (2006b) performed experiments with sliding polyethylene on ice. They reported that at low temperatures, below -2 C, an increase in temperature leads to a decrease of the c.o.f. On the other hand, above -2 C, a temperature increase leads to a higher c.o.f. This is due to the increased amount of melt water that in turn leads to larger real contact areas. Liquid-water content: The liquid water content in the snow is probably the most critical parameter for the wax selection (Coyne, 2006; McLean, 2006; Perks, 2006). Snow is generally classified as dry and wet. In nature, snow contains ambient interstitial liquid-water which along with possible melt-water from frictional heating or local high pressures at the sliding interface modifies the dry sliding process, particularly above -5 C (Glenne, 1987). Ski friction increases significantly as liquid-water content increases (Nachbauer et al., 1996). This might be caused by an increased number of capillary bridges between wet snow and the sliding base. Snow density and hardness: Snow density correlates well with snow hardness except for very wet snow that has high density but very low resistance. Heavy ski traffic greatly reforms the natural ski field, producing what is generally known as hard-pack, a layer with a higher density. High density snow is considered when 19

36 it is more than 300 kg/m 3. Hämäläinen and Spring (1985) found the kinetic c.o.f., at velocities 1 m/s and 4 m/s to decrease with increasing snow hardness, whereas the static c.o.f. increased. The same observation was made by Fauve et al., (2005). They reported that for dry snow, run times decrease, essentially the coefficient of friction decreases, for increased snow hardness. When both snow hardness and density decrease, the ploughing effect increases. At the same time the contact area between the ski/snowboard sliding base and the snow increases, causing higher friction. Nachbauer et al., (1996) performed experiments on ski slopes to evaluate these effects and observed that decreasing snow hardness on dry and wet snow resulted into a significant friction increase. Friction also increased with decreasing snow density, where hardness is expressed by a penetration distance, which is inversely related to hardness. Grain size Type of snow: Snow surfaces are polished by repeating ski/snowboard sliding. Experiments performed by Colbeck (1992) show that these polished snow grains appear to be melt-water caps formed on snow surfaces. They can be generated by rubbing a flat surface over them and indicate melting and refreezing. These highly polished surfaces are known to have lower friction than unpolished surfaces and this is because ice exhibits a lower coefficient of friction than snow. Viscosity of the snow shear layer was found to be dependent on snow type (grain characteristics) but independent of temperature; the average kinematic viscosity was found roughly 5x10-5 m 2 /s for snow of density 350 kg/m 3 (Casassa et al., 1991). According to Fauve et al., (2005) larger grain size results into faster run times, essentially lower c.o.f. Furthermore, the type of snow (new or old) seems to be more important than grain size. To summarize, it is difficult to quantify the effect of all parameters independently because the friction, the produced heat, the temperature, the liquid-water content and the normal load are all interdependent. At a low temperature, the friction increases because the produced heat is insufficient for a complete lubrication of 20

37 the tribological interface. On the other hand, the frictional heating increases for a higher c.o.f., which increases the amount of heat in the contact area and enables to melt more snow. This delicate equilibrium between the components of the friction and heat production has not been to date described quantitatively (Buhl et al., 2001) Area of Contact between Sliding Base and Ice/Snow Very little is known about the actual contact area between ice/snow and sliding surface, yet this parameter is very important to the effects of the melt-water film and dry lubrication mechanisms (Perla and Glenne, 1981). The dimensions, topography, and nature of the contact between the ice and the sliding surface are the main uncertainties in ice friction. The actual contact area is a function of the asperity interactions on both the slider and ice surface as well as of the amount of melt-water between these two surfaces (Penny et al., 2007). Evidence of surface conditions during sliding is difficult to gather, and it is especially difficult to estimate the actual contact area during sliding (Ludema, 1984). The real area of contact is in fact very small and it varies with the load, but for flat steel surfaces it may be less than 1/1000 of the apparent area. It is little influenced by the shape and degree of roughness of the surfaces; it depends mainly on the load which is applied to it, and is in fact directly proportional to the load. This means that, even with lightly loaded surfaces, the local pressure at these small points of contact is very high (Bowden, 1952). Real contact occurs over isolated regions and the sliding resistance is due to the interaction between the solid surface and ice over these small areas of contact. The snow particles themselves are small, rounded and irregular grains of ice so that the interaction in the regions of contact with snow is similar to that observed on an ice surface. The real contact area between a slider and snow has been determined by observing the snow surface through a microscope after a sliding experiment and found to be around 4% (Kuroiwa, 1977). 21

38 The spatial distribution of the contact pressure between ski base and snow is very heterogeneous and depends essentially on the rheological and physicochemical state of snow, wax, polymer and morphology of ski bases. It is important to know the abrasive action of snow granularities in order to master the evolution of ski bases for better sliding performance (Ducret et al., 2005). The minimum area of contact is determined by the load and by the plastic flow of the surface. If the ski surface is the harder (at the temperature under consideration) it will be determined primarily by the plastic deformation of the ice. This is dependent upon the time of loading. On snow the small ice grains can pack and conform more to the shape of the ski, so that the real area of contact may be appreciably greater. At low temperatures we may expect that the snow grains will scrape the wax off the surface (Bowden, 1953). When the real area of contact increases, there are more asperity interactions between the two surfaces and adhesion and friction increase Sliding Velocity and Load Bowden and Leben (1939) performed friction tests on sliding surfaces on snow and concluded that when the surfaces were set in motion the c.o.f. was not greatly influenced by the speed of sliding until the speed became low. At low speeds the c.o.f. increases. An analysis of the sliding friction has shown that, even at high speeds, the kinetic coefficient of friction was not constant but was fluctuating rapidly. It was clear that the surfaces were not sliding continuously, but were moving by a process of stick and slip, and the general behaviour was very similar for metal surfaces. If the sliding speed is very low (a few cm/sec) the kinetic friction is very similar to the static friction. If the sliding speed is increased, however, there is a great drop in friction (Bowden, 1953). This decrease is due to water film formed locally in the regions of contact by the frictional heating of the rubbing surfaces. The friction is higher for a good thermal conductor because of the increased difficulty of forming the water film. Spring (1988) found that the c.o.f. increases with increasing 22

39 speed. With an increase in velocity, more snow granular bonds are destroyed, so that friction increases. Baurle et al., (2006b) reported the same observation for experiments performed close to the melting point. Thus, for the same snow type at constant temperature and constant normal stress, friction is strongly dependent on velocity (Casassa et al., 1991). Oksanen and Keinonen (1982) have shown that the kinetic coefficient of friction decreases with increasing speed at cold snow temperature (-15 C), although it increases with increasing speed at warm snow temperature (-1 C). This implies that increasing friction melting may be beneficial to sliding at low snow temperatures (lower than -5 C) but not at high ones (higher than -5 C). According to Bowden and Hughes (1939) friction is independent of the load for small and moderate loads. Load influences the coefficient of friction only in the low temperature region. The correlation between the temperature and the c.o.f. remains the same for any load. The influence of the weight of the skier contributes only for a snow temperature below -6 C. In this temperature region the heavier skier is faster. In the temperature region where only lubricated friction contributes to the total friction, the load has little influence on the tribological system. The thickness of the melt-water film is sufficient for a complete lubrication of the contact area, and the dry friction is eliminated. In the high temperature region, between -3 C and 0 C, too much water is produced, which results in capillary drag effects. Again, this tribological phenomenon is independent of the load (Buhl et al., 2001). Lehtovaara (1985) found the coefficient of friction to be constant for varying normal loads at low temperatures but to decrease with increasing normal loads at warm snow temperature. Buhl et al., (2001) also investigated the effect of different loads at different temperatures. They found that the load influences the c.o.f. only in the low temperature region. The c.o.f. remained constant at any temperature between -10 C and -5 C for various loads. However, at colder 23

40 temperatures, below -10 C, the c.o.f. was found to be significantly lower for higher loads Ambient Weather Conditions Temperature: There is a sensitive/unstable energy balance between the ambient air and the ice/snow surface. The ambient temperature has an immediate effect on the ice/snow surface temperature. Thus, ambient temperature influences the coefficient of friction indirectly by affecting the ice/snow surface temperature. This effect has been mentioned in the ice/snow temperature section. Similar effects are expected for the case of ice friction. Humidity: Accordingly, ambient humidity directly influences the ice/snow liquidwater content, therefore indirectly affects the snow coefficient of friction and this is the case for ice Waxes There is very little information available on the chemistry of ski waxes. At the same time there is almost no technical literature regarding the scientific basis governing the wax application techniques. These have been developed mostly empirically during years of practice and they differ depending on the snow and ambient conditions (Lind and Sanders, 2004). Skiers use two main categories of wax-like compounds to optimize glide and grip: glide wax (for alpine, cross country and snowboard) and kick wax or/and klister (strictly used in the classical style of cross country skiing). Typical waxes are paraffinic, fluoropolymers, perfluoropolymers, powders and sprays. Sometimes additives such as graphite and molybdenum disulphide are used in the paraffinic wax concentrations. Their selection depends on the age of the snow (old or new), the liquid-water content (wet or dry) and the temperature (cold or warm). At low temperatures, where lubrication by melt-water films is tenuous and the ice crystals are hard, harder waxes are used and adhesion between the ski and ice 24

41 crystals through the water films is minimal (Colbeck, 1996). On the other hand, at high temperatures, where lubrication by melt-water is plentiful and the ice crystals are softer, waxes with greater hydrophobicity are used to minimize the adhesion through the water films. 2.5 Sliding Base Preparation Techniques The two basic techniques utilized to prepare skis and snowboards for the best performance are stone grinding and wax application Base Stone-Grinding Treatment Stone-grinding (SG) is a technology that has been used on the World Cup for many years and until recent times, has been a bit of a mystery, even to top ski technicians. This ski running surfaces treatment was commonly used from the mid-1980s in alpine skiing. Today, SG is the only accepted method of ski base treatment (Kuzmin, 2006). It has evolved so that top racers have their skis ground multiple times per year. Today s leading stone-ground bases are literally race ready and require an amazingly low amount of waxing work to get them up to top speed. When skis are stone-ground, a very thin layer of base material is removed by the stone and actually exposes a new, fresh layer of base material which is able to absorb a considerable amount of wax. Without regular stonegrinds, most ski bases become rounded, scratched, and unable to absorb wax; all factors which make for poorly performing skis. When selecting the final stonegrind structure, one must consider the snow conditions the skis will be used in. For racers who have a number of skis, ideally they would have different skis to be used in specific conditions with different grinds for those specific conditions. The idea behind the stone-grinding technique is to create different micropatterns. Ski base grinds allegedly influence the gliding of skis on snow, which is a complex and poorly understood tribological interaction. No sufficiently sophisticated models exist to optimize the design of a ski sliding base structure/pattern (Jordan and Brown, 2006). While it is clear that, under most 25

42 conditions, a sliding base should not be as smooth as possible, it is not clear what is the best pattern of the sliding base that minimizes the coefficient of friction, in a particular set of environmental conditions, including snow temperature and snow liquid-water content Wax Application Stone grinding is followed by wax application. The proper wax or wax combination to be used is chosen based on the snow and ambient conditions. The wax application process is as follows: a) wax is applied on the surface and heat it up by using a hot iron (typical temperatures are C), b) wax is then removed from the surface by means of metallic or plastic scrapers and c) steel brushes are used to remove the remaining wax of the surface. By doing this structure created from the stone-grinding equipment is exposed back again while at the same time the grooves contain some wax. Wax application is intended to increase the hydrophobicity of the sliding base. As mentioned in section different waxes are used for different snow and ambient conditions. Initially wax fills up the pores/grooves on the ski base, therefore decreasing the roughness. The scraping and brushing is intended to clear up the pores, thus bring the structure back and the roughness returns to the initial state, but at the same time keep a layer of wax on the base. 2.6 Polymer Surface Modification It is very common in polymer science to aim to improve surface properties of a specific polymer while maintaining the desirable and characteristic properties of the bulk (Egitto and Matienzo, 1994). Properties such as elasticity, elongation, hardness, chemical inertness, hydrophobicity or hydrophilicity among others are very desirable. 26

43 2.6.1 Polymer Additives One proven method for altering the polymer surface characteristics is by blending. Additives that are used to improve hydrophobicity vary from different low molecular weight perfluoropolyether (PFPE) and hydro-fluoropolyether (HFPE) solvents and liquid perfluoropolyalkylether (PFPAE), to ceramic reinforced fluoropolymers and micro-powders of polytetrafluoroethylene (PTFE) and boron nitride (BN) in solid form. Their common characteristic is they exhibit a low coefficient of friction and/or hydrophobicity (Satyanarayana and Sinha, 2005; Puukilainen and Pakkanen, 2005; Puukilainen et al., 2005; 2006; 2007). Puukilainen et al., (2006) prepared composites of UHMWPE with solid and liquid lubricants and found that the solid additives had little effect on the hydrophobicity of UHMWPE. A similar mixing approach was performed by Ebbens and Badyal (2001) who prepared polypropylene films by melt blowing. They found that the surface energy of the polymer was lowered by mixing a small amount of fluorinecontaining material into the polymer melt. Interestingly, they also found that migration of the fluorochemical additive towards the surface occurred. Puukilainen and Pakkanen (2005) and Puukilainen et al., (2006) altered the hydrophobicity of high-density polyethylenes (HDPE) and polypropylenes by melt blending with perfluoropolyethers Plasma Application A plasma is an ionized gas. Plasma enhanced chemical vapour deposition (PECVD) is a process used to deposit thin films from a vapour to a solid state on a substrate. Proper selection of gases from which the plasma is generated can result in deposition of organic or inorganic films. Chemical surface modification results when the species generated in the gas react at a surface to form stable products with physical and/or chemical properties that are different from those of the bulk. In many instances, etching and modification occur simultaneously (Egitto and Matienzo, 1994). 27

44 Surface modification of polymers is required in many applications where particular surface properties such as crystallinity, cross-linking, hydrophobicity, hydrophilicity and roughness amongst others are required (Meichsner et al., 1995; 1998; Hopkins et al., 1996). Lately, there is significant interest in methods that render different surfaces hydrophobic and superhydrophobic without changing the bulk properties (Fresnais et al., 2005; Kim et al., 2006; Tressaud et al., 2007; Kharitonov, 2008). Plasma enhanced chemical vapour deposition (PECVD) is a procedure that is being used to deposit films on different substrates. PECVD in the presence of CF 4 is a method that has been used to render polyethylene based polymers hydrophobic and superhydrophobic (Chen et al., 1999; Olde Riekerink et al., 1999; Fresnais et al., 2006; Kim et al., 2006). The objective behind these techniques is to create a structure in the micro- or nano-level on the surface and then to deposit a fluorine-film that will increase its hydrophobicity (Sigurdsson and Shishoo, 1997; Olde Riekerink et al., 1999; Fresnais et al., 2005; Kim et al., 2006;). It is well known that the wettability, or liquid repellency, of a given surface is a combination of its surface structure in the micro or nano level and its chemical nature (Hopkins et al., 1996; Youngblood and McCarthy, 1999; Woodward et al., 2003; Burton and Bhushan, 2005; Zhu et al., 2006). Therefore, in order to render a surface hydrophobic or superhydrophobic (i.e exhibiting water contact angles greater than 150 ) two factors must come into play; the micro- or nano-pattern of the surface as well as its naturally exhibiting hydrophobicity. 28

45 3 SCOPE AND THESIS OBJECTIVES 3.1 Introduction It is safe to say that based on the literature review the sliding phenomena on ice and snow are very similar. There is additional complexity in snow friction that arises from the nature of snow and the many structural forms it may exist. In this work the experimental work on the measurement of the coefficient of friction was done on ice surfaces in order to obtain reproducible results. Furthermore, the knowledge obtained from these measurements was utilized to develop techniques and methods to minimize friction during competitive snow events (skiing and snowboarding). 3.2 Thesis Objectives The main goal of the project is to develop techniques and methods to minimize snow friction during competitive snow events (skiing and snowboarding). This can be accomplished by satisfying the following objectives: 1. To set up a new experimental device for measuring friction consistently and reproducibly between polymeric surfaces and ice/snow. 2. To develop a methodology to obtain ice coefficient of friction data between polymeric surfaces and ice as a function of the different parameters that influence friction. 3. To develop a ski/snowboard sliding base having a smaller coefficient of friction than the existing commercial bases. 4. To understand the effect of temperature, sliding velocity, surface average roughness and hydrophobicity on the coefficient of friction. 29

46 5. To assess the effect of different additives and coatings, that will be implemented via physical or chemical methods, on the coefficient of friction. 6. To correlate the wettability of different polymeric surfaces (UHMWPE, PTFE, and PMMA) with their ice coefficient of friction. 3.3 Thesis Organization The first chapter of the thesis provides an introduction and describes the motivation for this work. It is followed by a literature review on ice and snow friction. Emphasis is placed on the mechanisms that accompany the sliding motion of a solid material on ice and snow. In addition, an in-depth explanation of the parameters that influence friction is given. It also includes a literature review on ice and snow friction with particular emphasis on the effects of sliding velocity, temperature, surface topography and wettability. Chapter 3 presents the objectives of the thesis and the thesis organization. Chapter 4 includes a detailed description of the experimental equipment and materials used as well as the procedures and methodology followed in the present study (objective 1). Chapter 5 discusses the effect of sliding velocity, temperature, surface roughness and structure on the coefficient of friction of ultra-high molecular weight polyethylene sliding on ice (objectives 1, 2 & 4). Following this, the addition of lubricants into ultra-high molecular weight polyethylene and its impact on surface wettability as well as the behaviour of the coefficient of friction is being presented in Chapter 6 (objectives 3, 4, 5 & 6). Chapter 7 discusses the effect of a plasma enhanced chemical vapour deposition method on the wettability and coefficient of friction of treated polyethylene substrates. Emphasis is given on the effects of hydrophobicity and surface roughness at different friction regimes (objectives 4, 5 & 6). Following this, a comparison of the coefficients of friction of three polymers exhibiting very different wettabilities is presented in Chapter 8. The effect of hydrophobicity alone is being discussed in this chapter (objective 6). Finally, the thesis is concluded in Chapter 9 with a summary of the findings of this work, as 30

47 well as the contribution to knowledge, practical implications and recommendations for future work. 31

48 4 MATERIALS AND METHODS 4.1 Introduction This chapter describes the materials and the equipment used in this study as well as the methodology and the experimental protocols followed to study microscopic snow friction with various polymeric substrates. In addition, the sample preparation methodology and the equipment for performing surface characterization are discussed. Ultra-high molecular weight polyethylene (UHMWPE) is the main polymer under study. This is the only polymer resin being used for the making of sliding bases for ski and snowboard at competitive and recreational levels. Different additives were mixed with the resin to produce substrates that resulted into a lower coefficient of friction. Liquid perfluoropolyalkylether (PFPAE) was found to be successful and its properties are discussed in this chapter. 4.2 Materials The ultra-high molecular weight polyethylene (UHMWPE) used in this work comes under the commercial name of GUR 4170 and was provided by Ticona (Oberhausen, Germany). GUR 4170 UHMWPE is a linear specialty polyolefin resin in powder form. The extremely high molecular weight of this resin yields several unique properties including superior abrasion resistance and impact strength. Other properties include a low coefficient of friction that results in selflubricating and non-stick surfaces after processing. The resin is normally processed by compression molding and ram extrusion in order to produce smooth substrates. Compression molding is the process that was used in this study to prepare the testing samples. The basic properties of UHMWPE are listed Table

49 Additionally, two more resins, a hydrophilic and a hydrophobic one, poly(methyl methacrylate) (PMMA) and polytetrafluoroethylene (PTFE) respectively were studied. The main purpose was to investigate the effect of wettability, namely the effect of surface energy (static contact angle) of different polymeric substrates on the coefficient of friction with ice. These polymeric samples were provided by McMaster Carr (Chicago, US). Properties of PMMA and PTFE are summarized in Tables 4.2 and 4.3. Table 4.1 Properties of UHMWPE Ultra-High Molecular Weight Polyethylene Properties Density (g/cm 3 ) at 25 C 0.93 Average molecular weight (g/mol) 10.5*10 6 Thermal conductivity {W/(m.K)} Melting temperature ( C) Static contact angle ( ) 90 Table 4.2 Properties of PMMA Poly(methyl methacrylate) Properties Density (g/cm 3 ) 1.18 Average molecular weight (g/mol) ( )*10 6 Thermal conductivity {W/(m.K)} Glass transition temperature ( C) Static contact angle ( ) 65 It is believed that the development of a new ski base made of UHMWPE with hard hydrophobic additives (e.g. fluoropolymers) is the most promising way to 33

50 improve ski glide (Kuzmin, 2006). To achieve this, various lubricious additives, such as micro-powders of polytetrafluoroethylene, boron nitride and liquid perfluoropolyalkylether were impregnated into the sliding base. The idea is to improve the sliding base material tribological characteristics and at the same time retain the excellent mechanical properties of UHMWPE. It is well known that chemicals such as fluoropolymers, graphite, molybdenum disulphide and others are used as additives in waxes to improve gliding under different conditions. According to Holmberg and Wickström (1987) who have performed friction tests with a pin-on-disc friction tribometer, polytetrafluoroethylenes, polyesters and other polymers exhibit low coefficient of friction. In this study, an attempt is made to mix these types of additives with UHMWPE. Different types of additives, in solid and liquid form were used. The solid additives did not mix properly to create a homogeneous mixture, on the contrary liquid additives did. Table 4.3 Properties of PTFE Polytetrafluoroethylene Properties Density (g/cm 3 ) 2.2 Thermal conductivity {W/(m.K)} 0.25 Melting temperature ( C) 328 Static contact angle ( ) 118 Mixing liquid perfluoropolyalkylether (PFPAE) with UHMWPE proved to be very successful. Liquid PFPAE comes under the commercial name Krytox and is clear and colourless fluorinated synthetic oil. It was provided by DuPont (Wilmington, USA) and its chemical structure is depicted in Figure 4.1. On a weight basis, typical Krytox oil contains 21.6% carbon, 9.4% oxygen and 69% fluorine. Table 4.4 summarizes typical properties of PFPAE. 34

51 F (CF-CF 2 -O) n CF 2 CF 3 I CF 3 Figure 4.1 Chemical structure of perfluoropolyalkylether Friction measurements were conducted on ice surfaces. Distilled and de-ionized water were used to make ice in order to produce reproducible and consistent substrates for the friction experiments. Special dishes were used which were filled with the distilled and de-ionized water and placed into a commercial freezer Kenmore ON (T ϵ [-2 C, -15 C]). The dishes containing the ice were then transferred to an environmentally controlled lab ( cold-room ), where the temperatures was set as low as -5 C. Work in the cold room permitted us to load and prepare the ice dishes without exposing them to thermo-shock. Surface irregularities and unevenness were noticed after the ice formation due to the crystallization process of the water molecules. The ice surface was smoothened by a process that is being discussed later in this chapter. Table 4.4 Properties of liquid PFPAE Perfluoropolyalkylether Properties Oil viscosity (cst) Oil density (g/ml) 17.4 (at 20 C) 2 (at 100 ) 1.89 (at 0 C) 1.70 (at 100 C) 35

52 4.3 Experimental Equipment Concentric Parallel-Plate Rheometer/Tribometer The main apparatus to measure the friction coefficient of different polymeric surfaces is called a tribometer (Blaum, 2008). There are different available types of tribometers such as pin-on-disk, four-ball and block-on-ring, and they are mainly used to measure friction and wear of different materials (Bhushan, 2002; Blaum, 2008). The Paar-Physica MCR 501 (Figure 4.2), rheometer has been used as the main apparatus for performing friction measurements in this study. A special friction testing fixture was designed and manufactured for measuring the friction under various loads and various temperatures. The geometry used is that of parallel plate/disk. A schematic is shown in Figure 4.3. In this geometry, two circular plates are mounted on a common axis of symmetry. The lower plate is the ice sample. The sliding ski base samples in the form of thin rings are attached to the upper plate which is rotated at a specified angular velocity, ω. The motion of the upper plate can be programmed to generate in principle any type of motion, and the resulting torque, M, is measured. Figure 4.2 Schematic of the Paar-Physica MCR 501 rheometer and the environmental chamber 36

53 ω(t) R Sliding base sample ice Figure 4.3 Schematic of the concentric parallel plate rheometer/tribometer geometry As discussed above, a specially designed lower fixture was designed to hold the ice dishes, as can be seen in Figure 4.4. This design has bearings and a spring that allow for movement of the lower fixture in order for the two surfaces, the sliding base specimen and the ice surface, to be parallel during testing. Figure 4.4 Specially designed ice dish holder 37

54 The upper plate of the rheometer is moving, during operation, with a constant rotational velocity, ω, and the torque, M, required to accomplish this motion is measured, while the normal load is kept at a specified level during this test. As mentioned herein, the frictional and normal forces are needed to estimate the snow coefficient of friction, which is then calculated from the following equation: F F T μ = [4-1] N where F T is the tangential frictional force and F N is the normal force. For the annular ring used, the coefficient of friction is calculated easily from the torque based on Equation 4.2: M 2 μ = [4-2] FN R1 + R2 where R 1 and R 2 are the outer and inner radii of the ring respectively as can be seen in Figure 4.5. R 1 R 2 Figure 4.5 Schematic of the ring 38

55 The oven of the rheometer is connected to an evapouration unit (EVU) which serves to evapourate liquid nitrogen and control the temperature in the oven. The temperature range covered in these experiments is between -1 C and -25 C. Figure 4.6 depicts the system of the ring and the ice dish inside the oven of the rheometer. Figure 4.6 Image of the ice dish and the rotating ring Field Emission Scanning Electron Microscope (FESEM) A Scanning Electron Microscope (SEM) was used for analyzing and evaluating the modification/deformation of sliding base sample surfaces by comparing the surfaces before and after friction experiments, and before and after various surface treatments. This microscope is able to magnify structures from the microand nanometer scale up to 800,000 times and additionally allows the surface roughness estimation. The microscope used in this study was the Hitachi S FESEM (Tokyo, Japan). 39

56 4.3.3 Contact Angle Measurements Contact angle measurements were performed to study the relationship between wettability (i.e., hydrophobicity) and the friction coefficient of the sliding bases. The measurements were conducted by using the sessile drop technique and they were performed at room temperature (25 C). Additionally, these measurements were performed in order to evaluate the effect of surface patterning on the static contact angle. The static contact angle θ, as defined by Young s equation (Eq. 2.7), can be measured as illustrated in Figure 4.7. air liquid solid surface Figure 4.7 Measurement of the static contact angle θ The contact angle of water, θ, with all substrates with distilled and de-ionized water was measured using images obtained with a high resolution camera Nikon D80, Digital SLR Camera. The camera was attached to a Sigma AF-MF zoom lens (105 mm, F2.8 EX DG Macro) and a Kenko extension tube set for better image magnification and resolution. The FTA32 software from First Ten Angstroms was used to analyze the images and calculate the static contact angles of the various surfaces under study. The volume of the water droplet was maintained constant for all the substrates, 1 μl, in order to prevent possible changes in the droplet due to gravity effects. 40

57 4.3.4 Plasma Enhanced Chemical Vapour Deposition (PECVD) Gas plasma treatment processes are used for chemical modification of polymer surfaces (Chen et al., 1999; Olde Riekerink et al., 1999; Fresnais et al., 2006; Kim et al., 2006). Plasma is said to be the fourth state of matter and plasma techniques are among the most widely studied subjects in modern science. Stated simply, a plasma is an ionized gas that conducts electricity (Egitto and Matienzo, 1994; Hopkins et al., 1996; Woodward et al., 2003). One-step synthesis The one-step treatment consists of a simple fluorination through CF 4 plasma treatment. Under the electrical discharge of CF 4 gas is dissociated by electronic impact (Bretagne et al., 1992): e + CF 4 e + CF + F 3 e + CF 4 e + CF + 2F 2 e + CF 3 e + CF + F 2 e + CF 2 e + CF + F e + CF e + C + F CF x radicals and mostly CF 2 act as functionalization agents and their density should be the highest in the plasma phase, and in opposite manner since fluorine atom corresponds to the etching agent, its density should be the lowest one (Fresnais et al., 2006). The Trion PECVD, with CF 4 gas, was used in this study to create superhydrophobic films on the UHMWPE surfaces. During plasma treatment the surface is being etched while the amorphous phase of the bulk of the polymer is being removed (Youngblood and McCarthy, 1999; Olde Riekerink et al., 1999; Woodward et al., 2003; Milella et al., 2009). This creates a roughness on the 41

58 surface. At the same time the electrostatic charge inside the chamber accelerate the ions which are bombarded on the surface. Figure 4.8 depicts a schematic of the surface fluorination. Fluorination + roughening CF 4 plasma CF x CF x CF x CF x CF x base substrate Figure 4.8 Surface plasma fluorination X-Ray Photoelectron Spectroscopy (XPS) X-Ray Photoelectron Spectroscopy (XPS) is a surface analysis technique with a sampling volume that extends from the surface to a depth of approximately 5-7 nm. XPS is an elemental analysis technique that is unique in providing chemical state information of the detected elements. Film thicknesses of ~ 9nm were analyzed in this study. This technique was mainly utilized to measure the fluorine deposition on the UHMWPE surfaces before and after CF 4 plasma treatment as well as after sliding over ice. Squared samples of 1 cm 2 were carefully prepared, to avoid any type of contamination, cut precisely, and then stored in glass containers during transportation to the lab. The specifications of the equipment used are listed in Table 4.5 (Oerlikon Leybold Vacuum GmbH, Germany). 42

59 Table 4.5 XPS specifications XPS specifications Equipment X-ray source Pass energy of analyzer Leybold MAX200 Monochromatic Al K-alpha, 15 kv and 20 ma 192 ev Optical Surface Profiler (WYKO) Non-contracting profilometer using light interferometry, with computerized XY table capable of measuring surface heights from sub-nm to mm-range; particularly suited for roughness measurements of soft materials like carbon fiber paper, bi-polar plates and fabric for sports apparel was used in this study. This technique was utilized to investigate the surface patterns and measure the average roughness on the different surfaces. The specifications of the equipment used are listed in Table 4.6 (Veeco Instruments). Table 4.6 Profiler specifications Profiler specifications Equipment Vertical resolution Spatial resolution Feature heights WYKO 0.1nm Ra 40 nm to 2 mm 4.4 Methodology The development and evaluation of a base in terms of coefficient of friction involves the following steps; creation of different thin polymeric substrates (with and without additives) of ~2mm in thickness; cutting rings out of the substrate; polishing the rings; fluorination of the rings; friction testing; and data analysis. 43

60 4.4.1 Sample Preparation Ice surfaces: Distilled and de-ionized water was placed in special dishes. The ice dishes were then transferred to the Kenmore freezer and placed into commercial Tupperware containers. After freezing they were carefully transferred to the cold-room a room with controlled temperature which was set at -4 C. A drill press was used for smoothing the ice surfaces. A polymeric disk of diameter of 40 mm is attached to the holder of the drill press. The rotational speed of the drill press was set at 1500 rpm during this process. The polymer disk has a rough surface and acts like silicon carbide sandpaper on the ice surface. An image of the drill press can be seen in Figure 4.9. Polymer rings: Compression molding was used to make the different polymeric substrates. The heating-pressing cycle can be seen in Figure A Carver hydraulic press was used for this process. Subsequently rings, as shown in Figure 4.5, where cut from the substrates from sampes formed by the mold in the hydraulic press. Figure 4.9 Drill press in cold-room 44

61 The polymeric rings were subsequently polished by using different grit sandpaper. Grits of 420, 600 and 1200 were used. The average roughness, static contact angle and coefficient of friction (c.o.f.) of the polished surfaces were measured using the optical surface profiler as explained above Temperature ( C) MPa 5 MPa 10 MPa Time (min) Figure 4.10 Compression-molding cycle for the preparation of the polymeric substrates Plasma Fluorination of Polymer Substrates Disks and rings from the UHMWPE substrates were cut, polished and then transferred to the Trion PECVD device. They were placed in the vacuum chamber and the details of the operational parameters are listed in Table 4.7. Table 4.7 PECVD specifications Trion PECVD Power (W) 100 Pressure (mt) 300 Gas flow (sccm) 10 Time (min) 10 45

62 The static contact angle, average surface roughness and coefficient of friction prior to and after the fluorination process were measured Contact Angle Measurements Measurements of the static contact angle were performed on all polymeric surfaces and surfaces that were exposed to different treatments. Specimens of these surfaces were carefully cut, to avoid any surface contamination, and transferred to the contact angle setup. Figure 4.11 shows typical image analysis of a hydrophilic (θ < 90 ) and a hydrophobic (θ > 90 ) substrate and the measured contact angle values. a) b) Figure 4.11 Static water contact angle analysis of a hydrophilic (a) and a hydrophobic surface (b) Friction Measurements Measurements of the coefficient of friction of different substrates were performed using ring specimens of the substrates as explained above. The rings have an outer diameter (OD) of 25 mm, inner diameter (ID) of 21 mm and a thickness of 46

63 1.5-2 mm. The rings were chosen as the optimum geometry to be used in the rheometer/tribometer. All polymeric rings are attached to a disk holder (Figure 4.12a) which is placed at the lower part of a shaft in the apparatus (Figure 4.12b). The shaft is connected to the head of the rheometer/tribometer. Figure 4.12 a) Disk holder with an attached polymeric ring, b) the shaft with the disk holder All tests were conducted under the same normal force (F N ) value of 3N thus minimizing the possibility of significantly altering the contact area. The load had to be kept at a value less than 5N due to equipment limitations. This limitation is due to the high lateral forces that are being developed during testing, essentially due to solid to solid contact (polymer substrate on ice). The controlled variables are the angular velocity and the oven temperature. A typical torque versus time graph can be seen in Figure The test time was also kept constant at 60s for the surface sliding on the ice. The average of the torque plateau is calculated and then converted to coefficient of friction (c.o.f.) by using Eq Average torque values are determined for different specimens at different temperatures and different angular velocities. To limit variability and experimental uncertainty, at least 5 ice surfaces were used and only two runs per surface were performed 47

64 for a total of 10 runs. Furthermore, the sliding speeds were randomized during testing in order to even out the changes on the slider s surface. Rheoplus 5 mnm 4 N Normal load (F N ) Torque (M) 3 2 B_xx 2 PP25; [d=1 mm] M average F N M F N Torque Normal Force s 80 Time t -1 Anton Paar GmbH Figure 4.13 A typical transient friction experiment indicating the torque versus time graph. Note the constant normal load during the test, essential for these measurements 48

65 5 SLIDING FRICTION OF UHMWPE ON ICE 5.1 Introduction The sliding friction of different solid materials, including ultra-high molecular weight polyethylene (UHMWPE) and other polymeric surfaces as well as steel and other metallic surfaces, on ice and snow has been the subject of study for many years (Bowden and Hughes, 1939; Bowden, 1952; Evans et al., 1976; Calabrese et al., 1980; Lehtovaara, 1985; Glenne, 1987; Akkok et al., 1987; Colbeck, 1992; Buhl et al., 2001; Ducret et al., 2005; Fauve et al., 2005; Bauerle, 2006a, Higgins et al., 2008). The common characteristic system of study is the interface between the slider (solid material) and the ice or snow. The sliding process of the solid material on the ice surface may induce a phase change of the water at the slider/ice contact points which gives rise to melt water (Bowden and Hughes, 1939; Bowden, 1952; Evans et al., 1976; Glenne, 1987; Colbeck, 1988; 1992; 1994; Buhl et al., 2001; Baurle, 2006a). It is difficult to explain all the phenomena occurring under one single mechanism due to the number of different parameters that influence the coefficient of friction which include, temperature, sliding velocity, surface roughness, and load. As Buhl et al., (2001) have stated: The delicate equilibrium between the components of the friction and heat production has not been to date described quantitatively. In this chapter the tribological behaviour of UHMWPE surfaces sliding on ice is studied at different conditions and is compared with previous findings in literature. More specifically, the tribological behaviour of UHMWPE with respect to sliding velocity, temperature, surface roughness and surface energy is assessed. Substrates of UHMWPE were prepared by compression molding and the testing specimens were subsequently cut out from the substrates. Their tribological behaviour, namely the coefficient of friction (c.o.f.), was evaluated at different temperatures, velocities and roughness under the same load (normal force, F N = 49

66 3N). As mentioned in Chapter 4, the testing specimens are rings with an outer diameter (OD) of 25mm and an inner diameter (ID) of 21mm. The resulted normal stress during all tests was 21 kpa. 5.2 Background The tribological behaviour of a typical slider/ice system is as shown in Figure 2.2. At a set temperature (lower than 0 C) a surface is sliding over ice. The phenomenon that dominates this motion is frictional heating. As shown by many researchers (Calabrese et al., 1980; Glenne, 1987; Colbeck, 1988, 1992 and 1994; Buhl et al., 2001; Ducret et al., 2005) the coefficient of friction (c.o.f.) of materials sliding over ice should follow the trend of the graph depicted in Figure 2.2. This graph explains the influence of the melt-water production on the c.o.f. As seen in Figure 2.2, the graph is divided in three main regimes, namely, the dry-to-boundary, mixed, and hydrodynamic friction regimes. In summary, when a slider is moving against ice or snow, initially there is dry friction and as the velocity of the slider increases, and depending on temperature, a melt-water film is being created on the interface. This film gradually covers the asperities on both the slider and the ice and acts as a lubricating layer. At this point the resistance to motion is minimum. Furthermore, as the melt-water film thickness increases it induces the action of the capillary drag force which presents an additional resistance to motion. The environmental conditions surrounding the slider/ice system greatly influence the creation of melt-water at the interface as well as the coefficient of friction. The basic hypothesis in this work is that for temperatures lower than -7 C the map of the friction regimes depicted in Figure 2.2 is modified as shown in Figure 5.1. The c.o.f. increases in the dry friction regime with increasing velocity until point A (in Figure 5.1) where a substantial amount of melt-water is being produced that is enough to create the lubricating film and therefore induce the reduction in 50

67 friction. At this transition point A, the slider velocity reaches its critical point for the onset of lubricated friction. This hypothesis has been confirmed by researchers who have studied the frictional behaviour of different materials on ice under dry conditions (Unal et al., 2004; Bongaerts et al., 2007; Burris, 2008). Similar observations, i.e., increasing friction with increasing velocity at cold and dry conditions have also been reported by Fiorio et al., (2002), Albracht et al., (2004), Nikki et al., (2005) and Baurle (2006a). It should be noted here that melt-water film thickness measurements could not be performed simultaneously with the friction measurements during this present work. Regime I Regime II Regime III Coefficient of friction Dry / Boundary Friction A Substantial melt-water creation Mixed friction / Mixed Lubrication Hydrodynamic friction / Capillary drag Sliding velocity Figure 5.1 The effect of sliding velocity on the ice coefficient of friction Maeno and Arakawa (2003) stated that there are two mechanisms that can explain the frictional phenomena on ice. The first one is the lubrication mechanism at sliding velocities roughly above 0.01 m/s and the second one is 51

68 the adhesion and plastic deformation of ice at the friction interface at sliding velocities below 0.01 m/s. The effect of the melt-water film thickness depends on the structure and the height of the asperities on the sliding surfaces. If the surfaces are smooth at the nano-level, the film of similar thickness will affect the c.o.f. If the surfaces are smooth at the micro-level, the film thickness at the microlevel might affect the c.o.f. (Liu et al., 2008). Kennedy et al., (2002) have attributed the initial increase of the c.o.f., at temperatures lower than -10 C, to the additional force required to break the bonds between ice crystals that causes the plastic deformation of ice. Even though their work focuses on ice on ice friction, their justification can be further used to explain the force required to shear the irregularities on the ice surface where it is known that the dominant phenomenon is the interlocking of the asperities between the ice and slider surfaces. A very interesting observation from Baurle (2006a) is that there is an initial increase in the friction force due to the increase in real contact area during the sliding motion. This explains the initial increase in the c.o.f. that is being hypothesized here and is depicted in Figure 5.1. This means that initially the rotating motion of the slider causes the shearing of the ice asperities until they become of uniform height. This is the case until point A in Figure 5.1. That is the point where the frictional heating creates a film of melt-water that is responsible for the initial decrease of the c.o.f. in Figure Effect of Velocity Figure 5.2 depicts the friction curves as functions of the sliding velocity at different temperatures. It can be seen that at -1.5 and -4 C the coefficient of friction (c.o.f.) initially increases and reaches a plateau with a tendency to decrease at higher sliding velocities. This behaviour resembles that depicted in Regimes I and II in Figure 5.1. This means that at the lower velocities there is not enough melt-water production to induce the decrease in the c.o.f. The melt-water production that is sufficient to induce friction reduction occurs at velocities greater 52

69 than m/s at -1.5 C and 0.79 m/s at -4 C. Beyond which point the c.o.f. decreases with increasing velocity at both temperatures. Marmo et al., (2005) found that for a given load, as frictional heating increases, the thickness of the melt-water film increases with velocity which results into a decrease of the coefficient of friction. They performed their experiments with steel on ice. Baurle et al., (2006b) have found that the c.o.f. increases with increasing velocity for polyethylene sliding on ice close to melting point UHMWPE 0.14 Coefficient of friction C -4 C -7 C -10 C -15 C -25 C Sliding velocity (m.s -1 ) Figure 5.2 The effect of sliding velocity on the c.o.f. for UHMWPE substrates sliding on ice at different temperatures It can also be seen from Figure 5.2 that for the temperatures below -4 C the c.o.f. increases with increasing velocity. It seems that at lower temperatures [from -7 to -25 C] the dominant mechanism is dry friction (Regime I in Figure 5.1). The heat produced on the surface from friction is not sufficient to melt the ice in order to 53

70 create a substantial melt-water film that will induce a decrease in the c.o.f. due to frictional force. Burris (2008) has investigated the tribological behaviour of polytetrafluoroethylene (PTFE) at cryogenic temperatures. He performed experiments of PTFE against steel at very low temperatures and very low relative humidity (2 and 6%). The tribometer was placed in an environmental glove-box where they used liquid nitrogen to control the temperature (an apparatus very similar to the one used in this study). He also found that the c.o.f. increased with increasing sliding velocities. Furthermore, Fiorio et al. (2002) found that increasing the sliding velocity leads to an increase in the induced strain rate of the ice asperities against the actual contact with the slider. This furthermore increases the tangential forces at the interface which leads to an increase of the coefficient of friction. Albracht et al. (2004) found that the coefficient of friction (c.o.f.) increases with increasing sliding velocity for a high alloy steel pin on ice disk at the temperature of -7 C. This finding is in agreement with the frictional behaviour for temperatures lower than -7 C shown in the Figure 5.2. They have also suggested that the increase in sliding velocity leads to an increase in the strain rate at the frictional contact which, therefore, increases the frictional force. Baurle (2006a) recognized that three friction regimes with respect to melt-water film thickness can be identified. The first, where the increase of the film thickness leads to a sub-proportional growth of the real contact area (the effect of the surface lubrication is more pronounced than the increase of the surface area) and thus the overall friction decreases. The second regime is obtained where the increase of film thickness leads to an over-proportional growth of the real contact area (the effect of the surface lubrication is less pronounced than the increase of the surface area) and thus the overall friction increases. Finally the third regime is obtained where the real area of contact reaches almost 100%. Baurle s interpretation of the ice frictional behaviour is in a good agreement with the observations of this present work. More specifically, this is shown in Fig. 5.2 for the results at -1.5 and -4 C. A further increase in sliding velocity, beyond 2 m/s, 54

71 would initially cause the c.o.f. to reach a minimum. Further increase of the sliding velocity would cause the c.o.f. to increase due to capillary bridges. Unfortunately, due to equipment limitations, higher velocities could not be reached and therefore this hypothesis could not be proved. The sliding velocity of a slider at point A in Figure 5.1 (the velocity at which the heat generated from friction is enough for a substantial melt-water formation), is defined as the critical velocity, υ c. This is the velocity at which the c.o.f. reaches its maximum value and the mechanism of friction shifts from dry to mixed friction regime. It can be seen from Figure 5.2 that υ c (-1.5 C) = m/s and υ c (-4 C) = 0.79 m/s. It can also be assumed from the available data that υ c (-7 C) = 1.96 m/s, the minimum possible value. Thus υ c -1.5 C < υ c -4 C < υ c -7 C which is an expected behaviour, that is the lower the temperature the higher the critical sliding velocity for the onset of the lubricated water-film formation. Unfortunately the experimental set up in this work does not allow for friction measurements at higher velocities. It is reasonable to assume that the critical velocities at temperatures lower than -7 C are greater than the value of 1.96 m/s. Table 5.1 summarizes the critical velocities for all temperatures for the occurrence of lubricated friction. Table 5.1 Critical sliding velocities for the onset of lubricated friction TEMPERATURE ( C) υ c (m/s) , -15, It should be noted at this point that the thermal conductivity of the two materials in contact, namely the ice and the UHMWPE surfaces is very important in the 55

72 estimation of the critical velocity. It is safe to hypothesize that most (if not all) of the heat being generated during the sliding process remains on the ice/slider interface. The thermal conductivities of ice and polyethylene slider are 2.3 and 0.42 W/mK, respectively. Therefore, most of the produced heat is being transported to the ice surface which induces the formation of the melt-water film. Higgins and co-workers (2008) performed pin-on-disk friction tests of rubber on ice. They have reported that for the lower velocities there was not enough time required for the ice to melt. Ho and co-workers (2002) used Equation 5.1 to calculate the time necessary for the ice to reach the melting by specified heat flux on the surface: t c ( ΔT ) 2 2 κc p ρ = [5-1] 2 3q Where, T is the temperature rise to reach the melting temperature (0 C), κ is the thermal conductivity of ice (2.3 W/m.K), C p is the heat capacity of ice, ρ is the density of ice and q is the heat flux at the interface. For the heat flux calculation Higgins and co-workers (2008) used Equation 5.2: q = υμσ n [5-2] where υ is the sliding velocity, μ the coefficient of friction and σ n the normal stress at the interface. Equations 5.1 and 5.2 are also used here in order to estimate the time for the ice surface to melt during the ice friction experiments performed in this work. The objective of this exercise is to investigate if the slider s running time of each friction experiment performed (60s) at the critical velocity agrees with the time required to produce a water film as estimated from Equations 5.1 and 5.2. In a few words, if the hypothesis is correct, the wetting time calculated from Equation 5.1 should be equal or less than 60s. It should be noted here that the average c.o.f. values plotted in Figure 5.3 are averages (averages of 10 different ice 56

73 surfaces) over 60s sliding times. The results are listed in Table 5.2. The results at the lower velocities ( and m/s) based on the findings agree well with the observations by Higgins et al., (2008). Table 5.2 Estimated time for the onset of lubricated friction in s. TEMPERATURE ( C) υ c (m/s) The results listed in Table 5.2 suggest that the frictional behaviour for the sliding velocity of 1.96 m/s at temperatures equal or higher than -15 C should be at the onset of the mixed lubrication zone. This is because the calculated wetting time is lower that the actual running time of the experiment which is 60 s. It means that the coefficient of friction would drop with increase of sliding velocities greater than 1.96 m/s at -7, -10, and -15 C, and that 1.96 m/s is possibly the wetting velocity. Unfortunately and due to equipment limitations higher sliding velocities could not be achieved in order to further investigate. However, it seems to exist good agreement between the calculated values and the experimental observations. There is an excellent agreement for the -4 C curve; the calculated wetting velocity and the experimental observation are both at 0.79 m/s. For the case of the -1.5 C curve, the calculated wetting velocity is at 0.79 m/s and the experimental observation is one order lower, at m/s. Most likely the wetting velocity lies between those two velocities. Another possibility is that the ice might 57

74 melt but that the produced melt-water film thickness is not enough to cover the asperities of the ice and slider and thus have an impact on friction. Strausky and co-workers (1998) showed that the melt-water thickness for sliding velocity below 0.1 m/s was below 100 nm. 5.4 Effect of Temperature Several research groups have studied the effect of snow and ice temperature on the sliding coefficient of friction (Bowden and Hughes, 1939; Evans et al., 1976; Calabrese et al., 1980; Akkok et al., 1987; Buhl et al., 2001; Albracht et al., 2004; Higgins et al., 2008). Bowden (1953) found that there is a minimum value for the c.o.f. at around 0 C. He has performed his experiments with real skis at different snow temperatures; from wet, known as slush, to dry at -24 C. Buhl et al., (2001) found that there is a minimum value for the c.o.f. at around -3 C. The c.o.f. increased for temperatures lower than -5 C and close to 0 C. They performed their experiments with polyethylene (PE) sliding on snow and ice and for their measurements they utilized a tribometer that was placed in a freezing chamber. Fiorio et al., (2002) and Albracht et al., (2004) mentioned that for temperatures lower or equal to -7 C the dominant phenomenon during the sliding process on ice is the plastic deformation on ice. Figure 5.3 depicts the friction curves with respect to temperature at different sliding velocities. The frictional behaviour for the low sliding velocities ( and m/s) and for temperatures equal or higher than -10 C resembles the one depicted in Regime I (from Figure 5.1) before point A. For the higher velocities (0.79 and 1.96 m/s) it is just after point A where there is a drop in the coefficient of friction. This means that there is a minimum value for the c.o.f. at the low sliding velocities and on the other hand for the higher sliding velocities the coefficient of friction keeps decreasing. Theoretically, the higher the sliding velocity is, the more heat should be generated at the slider/ice interface from friction, and therefore more melt-water should be produced. This means that the mixed lubrication zone should be reached faster for the higher velocities than the 58

75 lower ones. Based on the interpretations of Figures 2.2 and 5.1, we can conclude that for the lower velocities there is not any, or perhaps not enough, melt-water production to induce the shift towards the mixed lubrication zone. Therefore, the observed minimum is due to the deformations occurring on the ice surface. Coefficient of friction UHMWPE m/s 0.079m/s 0.79m/s 1.96m/s Temperature ( C) Figure 5.3 The effect of temperature on the ice c.o.f. of UHMWPE substrates sliding on ice at different sliding velocities Baurle (2006a) has shown that the most critical parameter determining friction between the sliding base and snow (and ice) is the real area of contact as well as the thickness of the melt-water film. Friction decreases when the dominant phenomenon is the sub-proportional growth of the real contact area, whereas it increases with over-proportional growth of real contact area. Strausky and coworkers (1998) showed that the melt-water film thickness for sliding velocities less than 0.1 m/s must be below 100 nm. Calabrese et al., (1980) have shown 59

76 that the c.o.f. increased with increasing temperature and that was due to the fact that the ice surface became softer and therefore it was easier for the slider to rub-in the ice surface. The asperities on the slider s surface were essentially penetrating the ice surface. Therefore, the longer the sliding time on the ice surface, the more the asperities penetrate the ice. Based on these findings, for and m/s and for temperatures from - 25 to -10 C there is a slight decrease in friction possibly due to the creation of a very thin melt-water film while real area of contact negligibly increases. As the temperature increases (greater than -10 C), there is an increase of the real area of contact, a phenomenon which is more dominant than the creation of a very thin melt-water film and as a result friction increases. On the other hand and at the higher velocities the increasing temperature induces surface lubrication and therefore the coefficient of friction constantly decreases. The friction regime should be in the mixed lubrication regime or in the onset of mixed lubrication and this is the reason why the c.o.f. constantly decreases with increasing temperature. The more sudden drop of the c.o.f. for the 1.96 m/s velocity after - 7 C is due to the fact that the behaviour is clearly in the mixed lubrication regime. This phenomenon has also been shown by Ducret et al., (2005). 5.5 Effect of Surface Roughness The effect of surface roughness on the coefficient of friction (c.o.f.) has also been studied. Three different grit sandpapers as a final finishing were used to prepare the testing specimens. These are 400, 600 and 1200 grits which resulted in surfaces exhibiting average roughness of 1300, 1000 and 760 nm respectively. The average roughness was measured by means of an optical surface profiler as discussed in Chapter 4. The contact angle measurements performed on the substrates with different average roughness showed no significant change on the static water contact angle values. The average roughness and contact angle data are summarized in Table 5.3. Contact angle images and surface topographies 60

77 from the three different average roughness substrates are shown in Figures a) b) Figure 5.4 Topography (a) and contact angle (b) images of an UHMWPE substrate prepared with 400 grit sandpaper a) b) Figure 5.5 Topography (a) and contact angle (b) images of an UHMWPE substrate prepared with 600 grit sandpaper 61

78 a) b) Figure 5.6 Topography (a) and contact angle (b) images of an UHMWPE substrate prepared with 1200 grit sandpaper Table 5.3 The effect of preparation method, on the average roughness and the static contact angle of UHMWPE substrates PREPARATION METHOD AVERAGE ROUGHNESS, R a (nm) STATIC CONTACT ANGLE ( ) 400 grit ± grit ± grit ± 3 The effect of surface roughness on the coefficient of friction has also been investigated in the past. Glenne (1987) mentioned that at low temperatures the coefficient of friction increases with increasing runner roughness but at high temperatures (closer to the melting point) it initially decreases until it reaches a minimum value and then increases again. Fiorio et al., (2002) determined that the average roughness is the parameter that determines the actual contact between the ice surface and the slider. They found that higher roughness results in decreasing number of contact points. This means that there is more penetration on the ice surface. Therefore the tangential stress in ice near the real contacts should be higher for a higher average surface roughness which in turn 62

79 increases the c.o.f. Similar findings have been reported by Calabrese et al., (1980). The effect of the melt-water film thickness depends on the structure roughness of the sliding surfaces. If the surfaces are smoothened at the nano-level, thicknesses in the nano-level might affect the c.o.f. If the surfaces are smoothened at the micro-level, thicknesses in the micro-level might affect the c.o.f. (Liu et al., 2008). Bongaerts et al., (2007) mentioned that roughness does not affect the c.o.f. at the hydrodynamic friction regime. The c.o.f. increases in the mixed regime and decreases in the boundary region as roughness increases. There is a presence of a very soft but elastic layer on the ice (Petrenko, 1997). At high rates and/or low temperatures ice frictional sliding appears to be the result of elastically deforming asperities that undergo shear failure (Rist, 1997). During sliding of solid materials on ice, the slider/ice interface always fails plastically because of the high compressive stresses around the regions of contact (Raraty and Tabor, 1958). Figure 5.7 plots the corresponding results at -1.5, -4, and -7 C as a function of surface average roughness. The c.o.f. is independent of roughness with the exception at the smallest sliding velocity at -1.5 and -4 C. This observation is in agreement with the findings from Bongaerts et al., (2007), based on the hypothesis that the results are at the onset of the mixed lubrication regime. This is true based on the results of Figure 5.2 where it was seen that the frictional behaviour for those two temperatures lies at the borders of the mixed lubrication regime. Furthermore, at -4 and -7 C the c.o.f. slightly increases with increasing average roughness (R α ). This observation is in agreement with the findings reported by Fiorio et al., (2002) who reported that the ice friction coefficient increases with the increase of R a at a given sliding speed and temperature. 63

80 T = -1.5 C m/s m/s 0.79 m/s 1.96 m/s Coefficient of friction T = -4 C m/s Average roughness, R a (nm) m/s m/s 1.96 m/s Coefficient of friction T = -7 C Average roughness, R a (nm) m/s m/s m/s 1.96 m/s Coefficient of friction Average roughness, R a (nm) Figure 5.7 The effect of roughness on the c.o.f. for different sliding velocities at T = -1.5, -4, and -7 C 64

81 Figures 5.8 and 5.9 re-plot the data for all the roughnesses studied as a function of temperature and sliding velocity respectively. The c.o.f. exhibits the same trend for the three substrates of different roughness. A deviation from this trend is observed at the lowest sliding velocity and this is because the friction regime is in the dry region where the increased roughness results into more asperity shearing and therefore higher c.o.f. values. This frictional behaviour was also discussed above. UHMWPE substrates having the lowest average roughness exhibit the lowest coefficient of friction. This observation is consistent at all temperatures and sliding velocities υ = m.s nm 1000 nm 760 nm υ = m.s nm 1000 nm 760 nm Coefficient of friction Coefficient of friction υ = 0.79 m.s -1 Temperature ( C) 1300 nm υ = 1.96 m.s -1 Temperature ( C) 1300 nm 1000 nm 1000 nm 760 nm 760 nm Coefficient of friction Coefficient of friction Temperature ( C) Temperature ( C) Figure 5.8 The effect of temperature on the ice c.o.f. for substrates of different roughnesses sliding on ice at , 0.079, 0.79, and 1.96 m/s 65

82 T = -1.5 C 1300 nm 1000 nm 760 nm Coefficient of friction T = -4 C Sliding velocity (m.s nm ) 1000 nm 760 nm 0.10 Coefficient of friction T = -7 C Sliding velocity (m.s ) Coefficient of friction Sliding velocity (m.s -1 ) Figure 5.9 The effect of sliding velocity on the c.o.f. for substrates of different roughnesses at -1.5, -4 and 7 C 66

83 5.6 Surface Patterns: Concentric and Cross-Sectional As mentioned previously the system under study in this work is the slider/ice where the slider is a ring having and an outside diameter of 25 mm and an inside diameter of 21mm. The average thickness of this ring is 2 mm. Due to the fact that the geometry of the surface is circular it was decided to create two distinctly different surface patterns through polishing. The polishing technique used is described in more detail in Chapter 4. The two structures are the concentric one where the created grooves are parallel to the rotational motion of the ring and the cross-sectional one, where the grooves are more randomly distributed over the ring surface. A schematic of the two surface patterns and their microscopic (magnification of x150) images are shown in Figures 5.10 and 5.11 respectively. a) b) Figure 5.10 Surface patterns created by different polishing techniques: a) concentric grooves and b) cross-sectional grooves 67

84 a) b) Figure 5.11 Microscopic images of surface patterns created by different polishing techniques: a) concentric grooves and b) cross-sectional grooves Two samples labelled base A and base B for each type of surface pattern were used to investigate the effect of structuring on the coefficient of friction. All samples (four in total) for both structures had an average roughness value of 760 nm. The results from friction experiments were conducted at -1.5 C at several sliding velocities are shown in Fig Sliders with the cross-sectional pattern exhibited a very similar frictional behaviour with that of UHMWPE (same surface pattern) that was presented in Fig More asperity shearing takes place for the case of cross-sectional pattern which results into more resistance at the low sliding velocities ( and m/s). At the same time more water is being produced at the interface which results into the c.o.f. drop at the higher velocities. On the other hand for the sliders with the concentric surface structure, the c.o.f. exhibits a different behaviour and increases with increasing sliding velocity. The asperity shearing is less at the low sliding velocities, and therefore the c.o.f. is lower than that of the cross-sectional, and as the velocity increases the produced melt-water is trapped in the concentric grooves, which adds more resistance to the sliding motion due to capillary drag. 68

85 T = -1.5 o C A - cross-sectional B - cross-sectional A - concentric B - concentric Coefficient of friction Sliding velocity (m.s -1 ) Figure 5.12 The effect of sliding velocity on the ice c.o.f. of two (A and B) commercial ski base samples having a cross-sectional and concentric surface patters sliding on ice at -1.5 C 5.7 Summary Experiments were conducted to investigate the effect of sliding velocity, temperature, average surface roughness and surface structural patterns on the coefficient of friction of ultra-high molecular weight polyethylene substrates sliding on ice. The frictional behaviour and the shape of the friction curves on the friction map were found to be very different for the low and high sliding velocities. A wide temperature range was covered throughout this work. The objective was to investigate the frictional behaviour from the so-called wet conditions (close to 0 C) to the so-called dry conditions (lower than -15 C). The effect of the sliding velocity on the coefficient of friction with respect to temperature was found to depend on the production of a melt-water film that is 69

86 sufficient to cover the ice and slider asperities and create a lubricating layer. It was also found that the effect of temperature on the coefficient of friction with respect to the sliding velocity greatly depends on the magnitude of the sliding velocities. The friction curve for the low sliding velocities is dominated by the changes that occur on the ice surface and more specifically on the deformation of the ice surface. On the other hand and for the higher sliding velocities, the dominant mechanism was the surface lubrication due to the melt-water production. The change in slider s average roughness did not have a significant effect in the wettability of the UHMWPE. On the other hand it was shown that the c.o.f. slightly increases with an increase in average roughness. The effect of surface patterning was also investigated. Grooves parallel to the sliding direction resulted to significant lower c.o.f. at lower sliding velocities and significantly higher c.o.f. at higher sliding velocities compared to the cross-sectional grooves. 70

87 6 EFFECT OF LIQUID ADDITIVES ON UHMWPE PROPERTIES 6.1 Introduction One of the objectives of this present work was to identify additives that would improve the surface characteristics, namely increase the hydrophobicity and decrease the coefficient of friction, and at the same time retain the excellent mechanical properties of ultra-high molecular weight polyethylene (UHMWPE) during sliding on ice/snow. Different types of additives were mixed with the UHMWPE resin to produce substrates that exhibit lower coefficient of friction. These additives varied from different low molecular weight perfluoropolyether (PFPE) and hydro-fluoropolyether (HFPE) solvents and liquid perfluoropolyalkylether (PFPAE), to ceramic reinforced fluoropolymers and micro-powders of polytetrafluoroethylene (PTFE) and boron nitride (BN) in solid form. Their common characteristic is that they exhibit a low coefficient of friction and/or hydrophobicity (Satyanarayana and Sinha, 2005; Puukilainen and Pakkanen, 2005; Puukilainen et al., 2006). The solid additives did not mix and mold properly in order to form a homogeneous substrate and the results are not included in this work. From the liquid additives, liquid PFPAE was the one that most successfully mixed and produced satisfactory results. Similarly, Puukilainen and co-workers (2006b) prepared composites of UHMWPE with solid and liquid lubricants and found that the solid additives had little effect on the hydrophobicity of UHMWPE. Liquid PFPAE was mixed with the UHMWPE resin at four different concentrations. Subsequently, the mixture was compression molded to produce substrates of an average thickness of about 2 mm. These substrates were furthermore processed as discussed in Chapter 4 and analyzed to determine their surface characteristics. 71

88 6.2 Hydrophobicity The first goal of using different additives in the UHMWPE resin was to alter its surface wettability, rendering it more hydrophobic. It is widely accepted that the more hydrophobic a surface is, the less adhesion it exhibits with water and therefore the resistance decreases (Jung and Bhushan, 2006). Shimbo (1971) showed that the coefficient of friction of ski sliding base decreased with a hydrophobic fictionalization on its surface. As mentioned in the introduction, solid and liquid types of additives were introduced in the UHMWPE matrix. The liquid additives proved to mix more homogeneously with the UHMWPE. Fluorine based compounds were chosen as additives based on their hydrophobic nature (Ebbens and Badyal, 2001, Puukilainen and Pakkanen et al., 2001, Satyanarayana and Sinha, 2005). Liquid PFPAE was the most successful additive. Four additive mass concentration resins were prepared and tested, namely 1, 2.5, 5 and 7.5%. Following the substrate preparation (discussed in detail in Chapter 4) the static water contact angle of the different surfaces was measured, and the values are listed in Table 6.1 and depicted in Figure 6.1. It can be concluded that the higher the lubricant concentration the higher the contact angle. Table 6.1 Static contact angle of different surfaces SAMPLE STATIC CONTACT ANGLE ( ) UHMWPE 88 ± 6 UHMWPE + 1% liquid PFPAE 96 ± 2 UHMWPE + 2.5% liquid PFPAE 102 ± 1 UHMWPE + 5% liquid PFPAE 106 ± 2 UHMWPE + 7.5% liquid PFPAE 112 ± 4 72

89 A similar mixing approach was performed by Ebbens and Badyal (2001) who prepared polypropylene films by melt blowing. They found that the surface energy of the polymer was lowered, thus the polymers became more hydrophobic (i.e., they exhibited a less wettability) by mixing a small amount of fluorine-containing material into the polymer melt. Interestingly, they also found that migration of the fluorochemical additive towards the surface occurred. Puukilainen and Pakkanen (2005) and Puukilainen et al., (2006a) altered the hydrophobicity of high-density polyethylenes (HDPE) and polypropylenes (PP) by melt blending with perfluoropolyethers. The FTA32 software (version 2.1) was used to analyze the static water contact angle images. Images of the static contact angles of all the surfaces can be seen in Figures Static water contact angle ( ) Lubricant concentration (%) Figure 6.1 The effect of lubricant concentration on the UHMWPE static contact angle 73

90 Figure 6.2 Static contact angle image and water droplet analysis for a substrate of pure UHMWPE Figure 6.3 Static contact angle image and water droplet analysis for a substrate of UHMWPE + 5% PFPAE Figure 6.4 Static contact angle image and water droplet analysis for a substrate of UHMWPE + 7.5% PFPAE 74

91 6.3 Effect of Additive Concentration on the C.o.F. The c.o.f. was measured for the four substrates containing various additive concentration resins, namely 1, 2.5, 5 and 7.5%. The two main parameters were the sliding velocity and the temperature. The sliding velocities, were kept the same as before namely at , 0.079, 0.79 and 1.96 m/s. The experiments were run at -1.5, -4 and -7 C. The friction measurements show that there is a dependence of the c.o.f. on the amount of additive used in the UHMWPE matrix. Figures show that the coefficient of friction shows a minimum in most cases. This minimum defines the optimum lubricant concentration for the best sliding performance. The additive concentration that proves to be optimum in most cases was between 1 and 2.5% (Figure 6.5-8). Each data point represents the average of 10 measurements. The error bars in the graphs represent the corresponding 95% confidence interval. Additional results for other temperatures and sliding velocities performed in this study are included in Appendix A T = -1.5 C υ = 0.79m/s 0.07 Coefficient of friction Lubricant concentration (%) Figure 6.5 The effect of liquid additive (PFPAE) concentration on the c.o.f. of UHMWPE sliding on ice at 0.79 m/s and temperature of -1.5 C 75

92 T = -1.5 C υ = 1.96m/s Coefficient of friction Lubricant concentration (%) Figure 6.6 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at 1.96 m/s and temperature of -1.5 C 0.09 T = -4 C υ = 0.79m/s 0.08 Coefficient of friction Lubricant concentration (%) Figure 6.7 The effect of liquid additive (PFPAE) concentration on the c.o.f. of UHMWPE sliding on ice at 0.79 m/s and temperature of -4 C 76

93 T = -7 C υ = 0.79m/s Coefficient of friction Lubricant concentration (%) Figure 6.8 The effect of liquid additive concentration (PFPAE) on the c.o.f. of UHMWPE sliding on ice at 1.96 m/s and temperature of -7 C As mentioned before, a minimum coefficient of friction has been observed in all graphs at an additive concentration between 1 and 2.5%. It was also seen that the higher the amount of the additive in a substrate, the more hydrophobic it becomes (Figure 6.1 and Table 6.1). Tominaga et al., (2008) found that the higher the water contact angle on a substrate the lower the frictional stress, at the higher velocity regimes. Interestingly enough, this is not the case here where a minimum value of the c.o.f. with respect to the amount of additive is observed. This might be due to the fact that the addition of another element at high concentration in the main UHMWPE matrix affects its surfaces characteristics such as porosity and hardness, and this might have an effect on the c.o.f. Therefore, hydrophobicity is not the only parameter directly influencing the c.o.f. on ice and/or snow. Similarly, Rogowski et al., (2005) found that there is a limit in the improvement of paraffin wax properties by the addition of higher amounts of fluorine-based additives. 77

94 Based on the previous findings, the substrate containing 2.5% liquid PFPAE was chosen for subsequent testing. More friction tests were conducted at lower temperatures in order to generate data for a complete comparison with friction data for pure UHMWPE. The results are shown in Figures Each data point represents the average of 10 measurements. The error bars in the graphs represent the corresponding 95% confidence interval. The UHMWPE substrates containing 2.5% PFPAE exhibit lower c.o.f. than the pure UHMWPE for temperatures greater or equal to -7 C at all sliding velocities, except at 1.96 m/s at -4 C. At temperatures closer to 0 C liquid water is easier created and the presence of fluorine-based additives augments water repellency which furthermore decreases capillary drag and therefore the c.o.f. At lower temperatures, it seems that capillary drag does not play a role and perhaps other factors such as hardness and porosity of the substrate might play a role. For example, liquid PFPAE and its particles might affect roughness at a nano-level and this causes the increase in the c.o.f υ = m.s -1 UHMWPE UHMWPE+2.5%PFPAE 0.08 Coefficient of friction Temperature ( C) Figure 6.9 The effect of temperature on the ice c.o.f. of UHMWPE and UHMWPE + 2.5% PFPAE substrates sliding on ice at m/s 78

95 υ = 0.79 m.s -1 UHMWPE UHMWPE+2.5%PFPAE 0.14 Coefficient of friction Temperature ( C) Figure 6.10 The effect of temperature on the ice c.o.f. of UHMWPE and UHMWPE + 2.5% PFPAE substrates sliding on ice at 0.79 m/s υ = 1.96 m.s -1 UHMWPE UHMWPE+2.5%PFPAE 0.14 Coefficient of friction Temperature ( C) Figure 6.11 The effect of temperature on the ice c.o.f. of UHMWPE and UHMWPE + 2.5% PFPAE substrates sliding on ice at 1.96 m/s 79

96 6.4 Effect of Velocity and Temperature on the C.o.F. Figures 6.12 and 6.13 depict the effect of sliding velocity and temperature on the coefficient of friction, respectively, for the substrates of UHMWPE containing 2.5% liquid PFPAE. The overall frictional behaviour is similar to that of pure UHMWPE as it was shown in Figures 5.2 and 5.3 respectively (discussed in Chapter 5). This is possibly due to the fact that they are both hydrophobic and the main difference is that the c.o.f. values for UHMWPE containing 2.5% liquid PFPAE were higher at temperatures equal or greater than -7 C UHMWPE + 2.5% liquid PFPAE Coefficient of friction C -4 C -7 C -10 C -15 C -25 C Sliding velocity (m.s -1 ) Figure 6.12 The effect of sliding velocity on the ice c.o.f. of UHMWPE + 2.5% PFPAE substrates sliding on ice at different temperatures 80

97 UHMWPE + 2.5%PFPAE m/s 0.079m/s 0.79m/s 1.96m/s Coefficient of friction Temperature ( C) Figure 6.13 The effect of temperature on the ice c.o.f. of UHMWPE + 2.5% PFPAE substrates sliding on ice at different sliding velocities 6.5 Field Testing The motivation and drive of the present work was to improve the performance of racing skis and snowboards by means of discovering an efficient additive for the running bases. The additive that was used here has proven that it reduces the coefficient of friction (c.o.f.) of the ultra-high molecular weight polyethylene substrates. This additive was also used for manufacturing sliding bases for real skis (alpine and cross-country) and snowboards. The amount that was used was 2%. The additive was sent to a ski base manufacturer in the US and the produced base was subsequently sent to different ski and snowboard manufacturers. It was thereafter tested by the Canadian National Team. From personal communication (MacLean, 2009; Coyne, 2009; and Trottier, 2009) it has become known that the sliding bases containing 2% liquid PFPAE proved to be faster 81

98 than all other sliding bases for fresh and/or wet snow and also for old snow at temperatures higher than -4 C. The feedback that was received from the Canadian Olympic Team personnel has been very positive. MacLean (2009) mentioned that we have tested the base in a number of different conditions and for the most part, the feedback from the skiers and the results of the tests have been positive. Some of the test skis were even used by our athletes at a race this August in New Zealand (Winter Games, New Zealand Championship, August 2009). Ivan Babikov won the race using the test skis with this new base. It seems that the ski base runs very well in snow that has relatively high liquid moisture content. The feedback has been good in both humid and wet new snow and in wet old snow. The only condition that I have observed a negative result is in dry, granular old snow. Although it is not a revolutionary breakthrough with the speed of the skis, we are optimistic and excited that this base will give us some small percentage advantage at the Olympics. In fact the tested base was thereafter used with great success by the Canadian athletes competing in snowboard, cross-country, and biathlon at the Vancouver 2010 Olympic Games. The outcome was two gold medals and one silver in four events in snowboard and excellent placements in nordic ski events. 6.6 Summary It was shown that the addition of liquid perfluoropolyalkylether (PFPAE) in the ultra-high molecular weight polyethylene matrix rendered UHMWPE more hydrophobic. The contact angle increased from 88 to 114. An optimum amount of lubricant, which was between 1 and 2.5%, was found to be the best concentration that decreased the c.o.f. and thus improved the sliding properties of the substrates. The improvement was observed at temperatures close to 0 C in the range [-7 to 0 C] where liquid water is present. Application of this finding into making real competitive skis was proven beneficial to the Canadian Olympic 82

99 team. They have reported that the use additive at 2% and under wet conditions improves ski performance significantly. This was also proven in practice during the 2010 Olympic Games, where Canadian athletes won several medals in snowboard and have some of their best results ever in cross-country skiing and biathlon. 83

100 7 PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION (PECVD) TREATMENT OF UHMWPE 7.1 Introduction Another objective of this work was to identify different techniques, chemical and/or physical, that could be utilized to improve the surface characteristics, namely the hydrophobicity and the coefficient of friction, of the UHMWPE substrates on ice. Lately, there is a considerable interest on methods that can be used to render different polymeric surfaces hydrophobic and superhydrophobic without changing the bulk properties (Fresnais et al., 2005; Kim et al., 2006; Tressaud et al., 2007; Kharitonov, 2008). One of these methods is the plasma treatment. More specifically, the plasma enhanced chemical vapour deposition (PECVD) is a procedure that is being used to deposit films on different substrates. PECVD in the presence of CF 4 is a method that has been used to render polyethylene based polymers hydrophobic and superhydrophobic (Chen et al., 1999; Olde Riekerink et al., 1999; Fresnais et al., 2006; Kim et al., 2006). The rationale behind the utilization of these techniques was first to create a structure in the micro- or nano-level on the surface and second to deposit a fluorine-film that will increase its hydrophobicity (Sigurdsson and Shishoo, 1997; Olde Riekerink et al., 1999; Fresnais et al., 2005; Kim et al., 2006;). It is well known that the wettability, or liquid repellency, of a given surface is a combination of its surface structure in the micro- or nano- level and its chemical nature (Hopkins et al., 1996; Youngblood and McCarthy, 1999; Woodward et al., 2003; Burton and Bhushan, 2005; Zhu et al., 2006). Therefore, in order to render a surface hydrophobic or superhydrophobic (i.e exhibiting water contact angles greater than 150 ) two factors must come in play: the micro- or nano-pattern of the surface as well as it naturally exhibiting hydrophobicity. The PECVD method will be utilized here to render UHMWPE and other sliding ski bases hydrophobic. 84

101 Subsequently, and after measuring the water contact angle of those surfaces, their coefficient of friction (c.o.f.) on ice will be measured. 7.2 Background A plasma is an ionized gas. An electrical field must be applied to a supplied gas in a specific volume to generate plasma. In a vacuum chamber, where the ions and electrons have long lifetimes, it is relatively easy to do this. Radio frequency (RF) power is usually applied in the chamber (Figure 7.1) to create a capacitive discharge. More specifically, plasma enhanced chemical vapour deposition (PECVD) is a process used to deposit thin films from a vapour to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by RF (AC) or DC discharge between two electrodes, the space between which is filled with the reacting gases. The nature of the interaction of the plasma constituents with the polymeric surfaces is determined by the configuration and processing parameters that can be adjusted to modify (chemically and/or physically) the various polymer surfaces. PECVD occurs when precursors to the deposited film are generated by fragmentation of the gas in the plasma and transported to a surface where they react to form a solid layer. Proper selection of gases from which the plasma is generated can result in deposition of organic or inorganic films. Chemical surface modification results when the species generated in the gas react at a surface to form stable products with physical and/or chemical properties that are different from those of the bulk. In many instances, etching and modification occur simultaneously (Egitto and Matienzo, 1994). 85

102 Figure 7.1 PECVD chamber Figure 7.2 explains what happens on the substrate when it undergoes plasma treatment in a CF 4 gas environment. Under the electrical discharge, the excitation of CF 4 gas gives rise to high concentrations of fluorine atoms, both radicals and ions (Woodward et al., 2003). The constituent fluorine atoms can lead to direct surface fluorination (Hopkins et al., 1996). Since the plasma exists at a greater electrical potential than any surface with which it is in contact, positive ions from the plasma are accelerated toward the substrate surface, in a direction normal to that surface (Egitto and Matienzo, 1994). Fluorination + roughening CF 4 plasma CF x CF x CF x CF x CF x base substrate Figure 7.2 Illustration of the surface treatment under CF 4 plasma application 86

103 The application of plasma treatment on polymeric surfaces provides superhydrophobicity in short treatment times. Fresnais and co-workers (2005) demonstrated that the synthesis of superhydrophobic polymer surface is possible by the simple plasma surface modification, either in one- or two-step process. The O 2 plasma allows the creation of a variable roughness while CF 4 plasma emphasizes this roughness and creates the apolar layer. By the two-step treatments, several plasma parameters were found to elaborate stable transparent superhydrophobic surfaces with a controlled roughness and whose chemical structure is close to a polytetrafluoroethylene-like structure. The resulted static contact angle was around 160. Milella and co-workers (2009) deposited nanostructured films from modulated C 2 F 4 plasmas. Deposition time as long as 90 min was necessary in order to obtain superhydrophobicity. Moreover, the slippery functionality was achieved in a very narrow window of time around 90 min. Although the time-scale for this process was very long, superhydrophobic films were obtained on different commercial polymeric substrates. The second route to superhydrophobic surfaces was a simultaneous roughening and fluorination process on polyethylene. Results have indicated that with this process and in the presence of CF 4 /O 2 mixtures, superhydrophobic and slippery surfaces were achieved in a short time (4 min). 7.3 Sample Preparation PECVD treatment was applied on pure ultra-high molecular weight polyethylene (UHMWPE) substrates as well as on ski and snowboard running bases. The substrates were placed in the Trion PECVD chamber and underwent plasma treatment in CF 4 gas. The duration of this procedure was 5 and 10 min, following suggestions by Fresnais et al., (2006). The static water contact angle and the average surface roughness were the same after both treatment times. It was decided to use 10 min as the treatment time in all experiments. Prior to the PECVD treatment all substrates were polished with the same procedure as 87

104 described in Chapters 4 and 5. The surface properties of all substrates, namely wettability, average roughness and the ice coefficient of friction were measured after the PECVD treatment, and these are presented below. 7.4 SEM, Roughness Analysis, and Hydrophobicity Ultra-high molecular weight polyethylene (UHMWPE) is a linear semi-crystalline polymer which consists of both crystalline and amorphous phases (Lin and Argon, 1994; Sobieraj and Rimnac, 2009). The crystalline phase contains chains folded into highly oriented lamellae, with the crystals being orthorhombic in structure. For semi-crystalline polymers during the plasma treatment the amorphous phase is being removed at a much higher degree than the crystalline (Meichsner et al., 1995; 1998; Herbert et al., 1996; Olde Riekerink et al., 1999). This process is called etching. The surfaces of the polymer samples that undergo plasma treatment exhibit higher average roughness after the treatment. The surface after treatment does not have a well-defined structure. Its morphology depended on the arrangement of the two-phases (amorphous and crystalline) of the bulk structure. Based on this, surfaces can be created which contain spherical holes, ridges, tubular or spherical protuberances. The common characteristic of all the surfaces are the irregular patterns and structures after undergoing plasma treatment. The same effects and results from the plasma treatment of the UHMWPE substrates were observed in this present work. Figures show scanning electron microscope (SEM) images of the UHMWPE substrates before and after treatment. The magnification of the images examined varied from x100 to x30k. The ones presented here are of x2.5k and x10k magnification. The effect of the plasma treatment on the substrates can clearly be seen. Before treatment the surfaces are smooth while after treatment they become rougher. Ripples and waves, peaks and valleys appeared on the surface. The plasma environment causes the domains of amorphous phase of the bulk polymer to be etched and therefore what is left are only the crystalline domains. Since the amorphous 88

105 phase was striped away in a much higher degree the remainder surface structure is very random (Youngblood and McCarthy, 1999; Olde Riekerink et al., 1999; Woodward et al., 2003; Milella et al., 2009). The average roughness of the surface was measured before and after PECVD treatment and these are listed in Table 7.1. The average roughness of the surface was increased from 760 nm to 1500 nm with the PECVD treatment. Figures 7.7 and 7.8 illustrate topography images as well as the static water droplet images before and after PECVD treatment. CF 4 plasma treatment also resulted into significant decrease of water wettability, rendering the plasma treated surfaces hydrophobic. Static water contact angles of 138 were measured for the UHMWPE treated surfaces as well as ski sliding base samples (see Figure 7.9). Olde Riekerink et al., (1999) reported static water contact angles on CF 4 plasma treated polyethylene (PE) films between 130 and 140. Fresnais et al., (2006) reported static water contact angles close to 160. Similar results have been reported by Woodward et al., (2003). Figure 7.3 SEM image of a UHMWPE substrate before PECVD treatment. Magnification: x 2.5K 89

106 Figure 7.4 SEM image of a UHMWPE substrate before PECVD treatment. Magnification: x 10K Figure 7.5 SEM image of a UHMWPE substrate after PECVD treatment. Magnification: x 2.5K 90

107 Figure 7.6 SEM image of a UHMWPE substrate after PECVD treatment. Magnification: x 10K a) b) Figure 7.7 Topography (a) and contact angle (b) images of a UHMWPE substrate prepared with 1200 grit sandpaper 91

108 a) b) Figure 7.8 Topography (a) and contact angle (b) images of a CF 4 plasma treated UHMWPE substrate prepared with 1200 grit sandpaper Table 7.1 Effect of various surface preparation methods on the average roughness of UHMWPE substrates and ski running bases PREPARATION METHOD AVERAGE ROUGHNESS, R a (nm) 400 grit grit grit grit / CF 4 -PECVD 1500 Table 7.2 Effect of various surface preparation methods on the static contact angle of UHMWPE substrates PREPARATION METHOD STATIC CONTACT ANGLE 400 grit 84 ± grit 84 ± grit 82 ± grit / CF 4 -PECVD 138 ± 4 92

109 7.5 XPS Analysis It is important to know the elemental analysis of the surface before and after PECVD treatment. The chemical analysis of the surfaces of UHMWPE substrates as well as those of ski sliding bases was performed by using X-ray photoelectron spectroscopy (XPS). Tables 7.3 and 7.4 show the elemental analysis of UHMWPE substrates and ski sliding bases respectively. Table 7.4 includes the elemental analysis of PECVD treated samples before and after friction experiments. The intention was to determine whether or not the fluorine atoms still remain on the substrates after sliding on ice for about an hour. Table 7.3 XPS analysis of pure UHMWPE substrates before and after PECVD in a CF 4 environment treatment PRIOR TO PLASMA TREATMENT AFTER PLASMA TREATMENT Element Composition (%) Element Composition (%) C 87.8 C 48.2 O 8.8 O 6.7 N 2.0 N 1.5 F 0.5 F 41.1 SI 0.7 Al 2.5 S 0.2 As expected, the XPS analysis revealed that all surfaces were highly fluorinated after the CF 4 PECVD treatment. There was an incorporation of 48% fluorine atoms on the surfaces after treatment. Similarly, Olde Riekerink et al., (1999) reported fluorination of more than 50% after treatment; Hopkins et al., (1996) reported a fluorine to carbon (F/C) ratio of around 1.3 after treatment; and Sigurdsson and Shishoo (1997) reported and F/C ration between 1.2 and 1.6 for different polymers after CF 4 plasma treatment. Interestingly enough, there was still a significant amount of fluorine atoms on the ski sliding bases after the ice 93

110 experiments; that was decreased from 48% to 32%. The significance of this observation is that CF 4 plasma treatment, provided that it induces a friction reduction, can be used to incorporate a film on a ski base that lasts long. Table 7.4 XPS analysis of the surface of a ski base sample before and after PECVD in a CF 4 environment treatment as well as after sliding on ice for about an hour PRIOR TO PLASMA TREATMENT AFTER PLASMA TREATMENT PRIOR TO SLIDING AFTER SLIDING Element Composition (%) Element Composition (%) Composition (%) C 64.4 C O 23.8 O N 3.1 N F 0 F Ca 0.8 Ca Na 3.7 Na K 1.2 K Si 1.0 Al S 1.2 Cl Effect of PECVD Treatment on the C.o.F. The coefficient of friction (c.o.f.) of CF 4 treated UHMWPE was measured and compared to that of the untreated samples. The friction measurements were 94

111 carried out at -1.5, -4 and -7 C and the results are shown in Figures , respectively. At all temperatures the treated surfaces show a decrease of the c.o.f. at the high sliding velocities. The c.o.f. of the treated surfaces starts becoming lower than that of the untreated after attaining a maximum value. As discussed in Chapter 5, when the melt-water film thickness is still lower than the height of the asperities the c.o.f. increases and reaches point A in Figure 5.1. At that point the film builds beyond the asperities. As a result lubrication starts and the c.o.f. starts decreasing towards the mixed lubrication regime (regime II, in Figure 5.1). At -1.5 and -4 C (Figures ) and for the sliding velocities less or equal to m/s the dominant mechanism is the resistance from the interlocking asperities of the ice and polymeric substrates (dry friction). The frictional heat produced from the sliding motion is not enough to produce a melt-water film that is thicker than the surface asperities. As stated in Chapter 5, the greater the height of the asperities, the greater the force required for the ice surface to be sheared as the slider moves along its surface. This causes the initial increase in the c.o.f. At higher velocities there is enough melt water produced and therefore the frictional behaviour is in the boundary friction (regime II after point A in Figure 5.1). In this case the surface with less wettability (more hydrophobic) should also exhibit a lower c.o.f. This behaviour can clearly be seen in Figures A very interesting observation can be made from Figure At -7 C, the transition point A (with reference to Figure 5.1) for the untreated UHMWPE occurs at a different velocity compared to that of the PECVD treated UHMWPE. In fact, it is not clear where exactly appears for the untreated surface. On the other hand it is clear that for the CF4 treated surface it appears at 0.79 m/s. There is a sudden drop in the c.o.f. that implies that the melt-water film is thick enough to induce the c.o.f. decrease. Each data point represents the average of 10 measurements. The error bars in the graphs represent the corresponding 95% confidence interval. 95

112 T = -1.5 C UHMWPE CF 4 plasma treated UHMWPE Coefficient of friction Sliding velocity (m.s -1 ) Figure 7.9 The effect of sliding velocity on the ice c.o.f. of a UHMWPE substrate sliding on ice before and after CF 4 treatment at -1.5 C T = -4 C UHMWPE CF 4 plasma treated UHMWPE Coefficient of friction Sliding velocity (m.s -1 ) Figure 7.10 The effect of sliding velocity on the ice c.o.f. of a UHMWPE substrate sliding on ice before and after CF 4 treatment at -4 C 96

113 T = -7 C UHMWPE CF 4 plasma treated UHMWPE Coefficient of friction Sliding velocity (m.s -1 ) Figure 7.11 The effect of sliding velocity on the ice c.o.f. of a UHMWPE substrate sliding on ice before and after CF 4 treatment at -7 C In conclusion, it has been shown that the average surface roughness dictates the c.o.f. when the friction curve is in the dry friction regime. For example, the transition point A in terms of the sliding velocity increases with decrease of temperature. This is expected since at a lower temperature a higher velocity is required to produce enough water to form a film thicker than the average height of the asperities. Beyond this transition point the surface wettability is the parameter that controls the friction as shown by the PECVD treated surfaces that exhibited lower c.o.f. in spite of the fact they have higher roughness. 7.7 Effect of PECVD on Commercial Ski Bases The motivation and drive behind the work was to improve the performance of racing skis and snowboards. Applying the PECVD treatment in CF 4 proved to significantly decrease the coefficient of friction of UHMWPE at the highest sliding velocities. The reduction in the c.o.f. was 18%, 6% and 12% at -1.5, -4 and -7 C 97

114 respectively. This is significant in ski and snowboard competitions where even tenths of a second can play an important role for the athlete s final placement. The same treatment was applied on commercial ski base samples. The ski base samples were provided by the Canadian National Teams (Coyne; McLean; Perks; Trottier, ) and were prepared for testing at the polymer processing laboratory at the University of British Columbia. The coefficient of friction for the ski base samples was therefore measured after PECVD treatment under CF 4 gas. Figures 7.12 and 7.13 show the c.o.f. as a function of the sliding velocity of the CF 4 PECVD treated commercial ski running bases. These results are similar to the previous observations. As the frictional behaviour moves past the point of substantial melt-water production (point A in Figure 5.1) the more hydrophobic surface exhibits a smaller coefficient of friction. There was a high reduction in the c.o.f. which was amounted to 26% and 28% at -1.5 and -4 C respectively. The frictional behaviour of the ski base samples presented here is very similar to the behaviour of the UHMWPE (presented in Chapter 5) and the UHMWPE substrates containing 2.5% liquid PFPAE (presented in Chapter 6). This furthermore supports the hypothesis made in Chapter 5 that there is critical sliding velocity above which there is a substantial melt water production which induces a drop in the c.o.f. (point A in Figure 5.1). 98

115 0.10 T = -1.5 C Ski base Ski base / CF Coefficient of friction Sliding velocity (m.s -1 ) Figure 7.12 The effect of sliding velocity on the ice c.o.f. of commercial ski bases sliding on ice before and after CF 4 treatment at -1.5 C 0.10 T = -4 C Ski base Ski base / CF Coefficient of friction Sliding velocity (m.s -1 ) Figure 7.13 The effect of sliding velocity on the ice c.o.f. of commercial ski bases sliding on ice before and after CF 4 treatment at -4 C 99

116 7.8 Summary CF 4 gas plasma enhanced chemical vapour deposition (PECVD) was utilized for the treatment of ultra-high molecular weight polyethylene (UHMWPE) substrates and commercial ski running bases. It was found that the PECVD treated samples exhibited higher average roughness and hydrophobicity after treatment. The treated samples also exhibited lower coefficient of friction on ice at higher sliding velocities (0.79 and 1.96 m/s), where there is enough melt-water to lubricate the slider s surface. The opposite is true at lower sliding velocities where dry friction conditions exist and the interactions between the asperities play a crucial role. To summarize, average roughness and hydrophobicity are two important parameters with respect to the ice friction at two different regimes. Roughness is important in the dry friction regime and hydrophobicity in the limit of the mixed friction regime and higher. This was shown experimentally with both the UHMWPE substrates and the commercial ski running bases, PECVD treated and untreated ones. 100

117 8 FRICTIONAL PROPERTIES OF DIFFERENT POLYMERIC SURFACES ON ICE 8.1 Introduction It is well known that coefficient of friction of a surface sliding over snow and ice depends on different parameters such as temperature, sliding velocity, surface average roughness and wettability amongst others (Bowden and Hughes, 1939; Evans et al., 1976; Akkok et al., 1987; Lehtovaara, 1989; Colbeck, 1992; Buhl et al., 2001; Albracht et al., 2004; Kuzmin, 2006; Baurle, 2006a). The objective of the work presented in this chapter was to investigate the coefficient of friction of different polymeric substrates on ice. Three polymers of different wettabilities were chosen for this work: a hydrophilic, poly(methyl methacrylate); a polymer exhibiting limiting hydrophobicity, ultra-high molecular weight polyethylene (UHMWPE); and a hydrophobic, polytetrafluoroethylene (PTFE). Their respective water static contact angles are listed in Table 8.1. Although this was addressed before, the objective of this part of the work was to examine the effect of hydrophobicity on the c.o.f. in a more systematic way. To do so, all the other parameters influencing ice friction were kept constant. The experimental set-up and parameters, such as temperature and sliding velocity range, were similar to those used previously. The average roughness of the polymers substrates under study was also kept constant by using the same surface preparation methodology as described in Chapter 4, which is of 760 nm. 8.2 Hydrophobicity The static contact angle (SCA) of substrates made out of the three polymers was measured. PMMA is a hydrophilic polymer with a contact angle of 66. UHMWPE exhibits limiting hydrophobicity with a contact angle of 90 and PTFE is hydrophobic with a contact angle of 118. These results are in agreement with 101

118 results reported in the literature (Chen et al., 1999; Olde Riekerink et al., 1999; Jung and Bhushan, 2006; Guruvenket et al., 2008). Their contact angle images along with the images analysis are shown in Figure Table 8.1 Static contact angle of different polymeric surfaces SAMPLE (1200 GRIT PREPARATION) STATIC CONTACT ANGLE ( ) UHMWPE 88 ± 6 PMMA 66 ± 8 PTFE 118 ± 2 Figure 8.1 Static contact angle image and water droplet analysis for a substrate of pure UHMWPE Figure 8.2 Static contact angle image and water droplet analysis for a substrate of pure PMMA 102

119 Figure 8.3 Static contact angle image and water droplet analysis for a substrate of pure PTFE 8.3 Effect of Sliding Velocity on the C.o.F. The effect of the sliding velocity on the ice coefficient of friction of UHMWPE (Figure 5.3) was studied and discussed in Chapter 5. Here, it will be compared to that of PMMA and PTFE, a hydrophilic and hydrophobic surface respectively. The c.o.f. of PTFE exhibits a very similar behaviour to that of UHMWPE. This can be seen clearly in Figure 8.4. The c.o.f. increases with increasing velocity until it reaches its maximum value ( point A with reference to Figure 5.1) and then it decreases. This resembles the behaviour seen in the boundary lubrication friction regime. The maximum c.o.f. value at -1.5 and -4 C occurs at about m/s. At -7, -10, -15 and -25 C it occurs at about a higher velocity of 0.79 m/s as expected. PTFE exhibits smaller c.o.f. values than UHMWPE at all velocities and temperatures. This is due to the fact that PTFE is more hydrophobic, thus it repels water. This observation is in agreement with Colbeck (1994) where he stated that a hydrophobic surface would appear to be advantageous at all temperatures. Figure 8.5 presents the c.o.f. curves of PMMA substrates with respect to sliding velocity at different temperatures. At -1.5, -4 and -7 C and with increasing velocity the c.o.f. decreases. This decrease is characteristic of the transition from the boundary lubrication to the mixed lubrication friction regime. The motion of 103