Abrasive wear of UHMWPE yarns against ceramic pins

Transcription

1 Abrasive wear of UHMWPE yarns against ceramic pins Juan Pu Ph.D student Mechanical Engineering, UC Berkeley

2 Outline 1. Introduction 2. Experimental setup 3. Surface characterization of UHMWPE fibers and ceramic pins 4. Primary wear test results of UHMWPE yarns against ceramic pins 5. Surface characterization of polished TiN surface 6. Summary 4/13/2013 Juan Pu 2

3 Introduction >200,000 anterior cruciate ligament (ACL) ruptures each year in the United States Long standing interest in synthetic materials for ACL reconstruction Unilimited supply, no harvesting of tissues, reduced rehabilitation, a quick return to physical activity Ultra-high-molecular-weight polyethylene (UHMWPE) is an attractive candidate of artificial ligaments. High strength, low elongation, high abrasion resistance, bio-compatible Abrasive wear is one of the major mechanisms of artificial ligament failure 4/13/2013 Juan Pu 3

4 Experimental setup of wear test F = 50N Reservoir Ceramic pin UHMWPE yarn Distilled water w = 50π rad/s = 1500 rpm UHMWPE yarn: artificial ligament unknitted free bundle of parallel fibers Ceramic pin: anchor of artificial ligament sapphire, TiN on CoCr substrate 4/13/2013 Juan Pu 4

5 SEM images of UHMWPE fibers Diameter ranges from 5 to 20 µm The surface characterization of ceramic pins will be done in the range of one individual fiber 4/13/2013 Juan Pu 5

6 AFM images of sapphire & TiN As-received sapphire 200 nm 200 nm 200 nm 200 nm 200 nm 200 nm As-received TiN (no polishing) 4/13/2013 Juan Pu 6

7 Surface profiles of sapphire and TiN 20 Sapphire Y (µm) Z (nm) Z (nm) TiN Y (µm) 10 5 Z (nm) Z (nm) /13/2013 Juan Pu 7

8 Surface roughness of sapphire & TiN R a (nm) As-received sapphire As-received TiN (no polishing) Based on 10 sampling areas for each case 0 5µm 5µm 10µm 10µm 20µm 20µm Sampling area TiN surface is three times rougher than sapphire The error bar in TiN case decreased as we increased sampling area 4/13/2013 Juan Pu 8

9 Wear test results Life of UHMWPE yarn: Sapphire: ± min TiN: 1.25 ± 0.12 min Based on 8 samples for each case Sapphire: The wear life is inversely proportional to the surface roughness of sapphire pin One possibility for huge scatter of the results for sapphire is the nonuniformity of the surface 4/13/2013 Juan Pu 9

10 Estimation of friction coefficients F 2 F 1 Spring Pin R w UHMPWE yarn μ k = ln (1 ηui/rw F 1 )/θ μ k : kinetic friction coefficient I: current 4/13/2013 Juan Pu 10

11 The effect of polishing on TiN surface As-deposited Area density of pits 3568±431 mm-2 10µm Polished Based on 10 sampling areas for each case Area density of pits 81.8±28.8 mm-2 10µm More than 97% decrease in the density of pits after polishing 4/13/2013 Juan Pu 11

12 The effect of polishing on TiN surface 10 As-received Y (µm) Z (nm) Z (nm) Polished Y (µm) 6 4 Z (nm) Z (nm) /13/2013 Juan Pu 12

13 The effect of polishing on TiN surface 20 As-received TiN Polished TiN R a (nm) Based on 10 sampling areas for each case 0 5µm 5µm 10µm 10µm 20µm 20µm Sampling area Polished TiN surface is 5 times smoother than as-received TiN surface 4/13/2013 Juan Pu 13

14 Summary 1. Unpolished TiN surface is 3 times rougher than sapphire surface, and the protrusions on TiN surfaces may cut UHMWPE yarn quickly. 2. The UHMWPE yarns rubbing against as-received sapphire pins last 150 times longer than those rubbing against unpolished TiN pins. 3. Polishing process has significantly reduced the density of pits on TiN surface and made the surface 5 times smoother. 4. We do not know the limits on polishing of sapphire. This is work in progress. We will evaluate if polishing will decrease the surface roughness and increase surface uniformity of sapphire. 4/13/2013 Juan Pu 14

15 Acknowledgements Prof. K. Komvopoulos Dr. Daniel Martin Kyon Pharma 4/13/2013 Juan Pu 15

16 Thank you! Any questions? 4/13/2013 Juan Pu 16

17 Properties of UHMWPE 15 times stronger than steel Summary of its Benefits High tensile strength 35 cn/dtex (3Gpa) and high modulus 1300cn/dtex (125GPa). High pliability and softness Lower profile with equivalent strength Proven biocompatibility Non-hemolytic Cut resistant Low friction coefficient 4/13/2013 Juan Pu 17

18 Fabrication of UHMWPE fibers Dyneema fibers are made using a DSM patented (1979) method called gel spinning. A precisely-heated gel of UHMWPE is processed by an extruder through a spinneret. The extrudate is drawn through the air and then cooled in a water bath. The end-result is a fiber with a high degree of molecular orientation, and therefore exceptional tensile strength. Gel spinning depends on isolating individual chain molecules in the solvent so that intermolecular entanglements are minimal. Entanglements make chain orientation more difficult, and lower the strength of the final product [1]. 1. A.J. Pennings*, R.J. van der Hooft, A.R. Postema, W. Hoogsteen, and G. ten Brinke, High-speed gel-spinning of ultra-high molecular weight polyethylene, Polymer Bulletin 16, (1986) 4/13/2013 Juan Pu 18

19 Estimation of friction coefficients F 2 Pin R w P e =UI P m = f F Rw=ηUI Capstan equation: F 1 = F 2 e μ k θ f F =F 1 F 2 = F 1 (1 e μ k θ ) Spring F 1 UHMPWE yarn ηu/rw I= F 1 (1 e μ k θ ) U=38 V, η=0.9,r=2 mm, w=50π rad/s, F 1 =50 N Assume F 1 does change during wear tests. μ k : Kinetic friction coefficient 4/13/2013 Juan Pu 19

20 I F 1 (1 e μ k θ ) Transform from boundary lubrication to hydrodynamic lubrication Sapphire 1#: min; Sapphire 2#: 42.5 min; TiN: 1.3 min 4/13/2013 Juan Pu 20