FASCICLE VIII, 25, ISSN 1221-459 65 EFFECT OF INCORPORATED LUBRICATION ON THE TRIBOLOGICAL PROPERTIES OF POLYAMIDES Mihaly KOZMA Budapest University of Technology and Economics, Budapest, Hungary, kozma.mihaly@gszi.bme.hu ABSTRACT Polyamides are the most common engineering polymer. They are semi-crystalline material possess many useful characteristics. There are a wide variety of PA types allows selecting the desirable properties. The tribological properties of polyamides considered good, but yet different fillers as lubricating oils or solid lubricants are added into polymer matrixes to improve their performance in sliding systems. Experiments were executed to determine the effectiveness of the incorporated lubrication of polymers, reliving some contradictions. Especially MoS 2 content of polyamides proved less effective as expected, and as is published in some literatures. KEYWORDS: tribology, polyamide, incorporated lubrication, MoS 2, friction. 1. INTRODUCTION Polymers are used in mechanical engineering more and more often to produce elements of sliding couples owing to their beneficial properties. Recently maybe polyamides are the most common engineering polymer spread all over the world, in east Europe too. Polyamides are semi-crystalline material with many useful characteristics. There are many types of polyamide, of which the PA6 and PA 66 are often applied to build machines in mechanical engineering. Generally PA 6 has high tensile and good impact resistance. and wear can be low making different types of PA suitable for sliding bearings and plates, gears, seals operating in applications, where loads, speed and temperature are low or moderate. Chemical resistance and electrical insulating properties are also good. It is pity that PA absorbs moisture lowering and changing dimensions of elements causing difficulties in operation and influencing durability. Another setback is the deterioration caused by the UV radiation. 2. IMPROVING POLYAMIDE PROPERTIES Properties of polyamides can be improved by adding into the polyamide matrix different fillers. Glass-fibers and pearls improve tensile and stiffness. TiO 2 helps to develop more crystals. MoS 2, graphite, PTFE and silicon oil can reduce friction and wear, but also influence the mechanical properties, especially the and the impact resistance. By casting a higher molecular weight and degree of crystallinity can be produced resulting in beneficial physical properties and higher heat resistance. Tables 1 and 2 presents some mechanical properties of pure and filled polyamides made by casting or extruding. Table 1. Mechanical properties of cast polyamide. Properties PA6 PA66 PA66 PA66 MoS 2 GF3 Density g/cm 3 1.13 1.14 1.16 1.35 MPa 61 78 85 123 MPa 15 168 182 714 Elongation % 285 3 15 5 MPa 46 46 MPa 84 126 69 Compr. MPa 7 76 128 Compr. MPa 15 168 714 As many properties of polyamides are very dependent on moisture content and temperature, data given in tables are based on normal environment conditions: 5% relative humidity and 2 C temperature. The data in tables 1 and 2 presents the effect of some fillers on the mechanical properties of cast and pressed polyamides.
66 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI FASCICLE VIII, 25, ISSN 1221-459 Table 2. Mechanical properties of extruded polyamide. Properties PA6 PA66 PA66 PA66 oil MoS 2 GF3 Density g/cm 3 1.16 1.14 1.16 1.26 MPa 67 5 68 97 MPa 2429 2 135 2 723 Elongation % 6 85 45 MPa 86 65 89 134 MPa 2282 1813 2 534 6125 Compres. MPa 88 66 9 134 Compres. MPa 196 1 33 2 1 3. TRIBOLOGICAL PROPERTIES OF FILLED POLYAMIDE The variation of mechanical properties of polyamides owing to the fillers can be followed relatively easily. But the tribological properties depend also on the characteristics of application (tribological system), which sometimes alters the beneficial influence of fillers in the opposite direction. Many research works were devoted to investigate the effectiveness of incorporated lubrication, some of which were presented in [1]. It was proved that in some cases the lubricant content in polyamide can lower the coefficient of friction, and the wear, but in other circumstances the lubricants are not enough effective, even increase the friction or wear instead of lower them [5]. John M. Thorp [2] performed many experiments on polyamides containing different lubricants. Some results are presented in figure 1 and 2.,8,6,4,2 v=.1 m/s v=.1 m/s v=1 m/s p=2-6,8 MPa, polymer pin on mild steel disc PA 6, extruded PA66+PTFE, PE Cast PA6+MoS2 Fig. 1. Coefficients of friction of polyamides at different sliding speed (unlubricated) [2]. As can be seen in figure 1, at very low sliding speed the incorporated lubrication has little effect on the coefficient of friction. Increasing the sliding speed enhanced the coefficient of friction except the polyamide containing PTFE, where the sliding speed,2,15,1,5 v=,1 m/s v=,1 m/s v=1 m/s p=2-6,8 MPa, polymer pin on mild steel disc PA 6, extruded PA66+PTFE, PE Cast PA6+MoS2 Fig. 2. polyamides at different sliding speed (lubricated with mineral oil. ISO VG 68) [2]. did not influence the friction properties. The MoS 2 was not able to lower the coefficient of friction, even yet increased it at sliding speed of 1 m/s comparing to the results measured at pure polyamide. External lubrication with mineral oil ISO VG 68 lowered the coefficient of friction, and in this case the incorporated lubrication also proved to be unbeneficial, as can be seen in figure 2. Neither PTFE nor MoS 2 were able lower the coefficient of friction of polyamide pins sliding on mild steel discs, in spite of the fact that the hydrodynamic lubrication effect was very poor. This is proved by the magnitude of the coefficient of friction. It is interesting that higher sliding speed increased the coefficient of friction of polyamide containing solid PTFE, and the MoS 2 content increased the coefficient of friction at lowest sliding speed (.1 m/s) and at the highest sliding speed (1 m/s) comparing to the results measured on moderate sliding speed (.1 m/s). These phenomena hardly can be explained by the effect of the solid lubricant adhering to the rubbing surfaces of steel discs Our recently performed investigations on short polyamide sliding bearings (bore diameter 32 mm, width 1 mm) also proved the ineffectiveness of the MoS 2 content of PA6 in reducing the coefficient of friction. As can be seen in figure 3 the coefficient of friction of sliding bearing made of PA6 containing MoS 2 was higher at the end of the investigation comparing to the coefficient of friction of pure PA 6. Large-scale friction and wear testing of polyamide 6 block sliding on steel plate at low velocity (.5 m/s) proved that at high contact pressure (55 MPa) the coefficient of friction of pure PA/steel pair is.35, and the silicon oil content of PA was able to decrease the coefficient of friction of PA/steel pair to a level as low as.18 [3]. The same phenomena were found in an experiments in laboratory performed on small-scale polymer specimens [4], where the coefficient of friction of pure cast PA 6 were lowered by the silicon oil content by more then 6% [4].
FASCICLE VIII, 25, ISSN 1221-459 67 1,8,6,4,2 PA6 MoS2 PA6 Temperature p=1 N/mm 2, v=,32 m/s Sliding bearing 32x1 mm 3 6 9 12 unlubricated Sliding distance, m 1 8 6 4 2 Temperature, C Fig. 3. and temperature of polyamide sliding bearings versus sliding distance. Our detailed investigations on polyamide block /steel cylinder pairs at a sliding speed of.32 m/s and line load of 1 N/mm also presented the beneficial effect of silicon oil content of polyamide 6 on the coefficient of friction as can be seen in figure 4. 1,,8,6,4,2, Polymer block/steel cylinder, F=1 N, w=1 N/mm, v=,32 m/s PETP PETP (L) cpa 6 (L) POM PA 66 GF 3 Fig. 4 polymer blocks sliding on hard steel cylinder without lubrication. cpa6(l) contains 2 wt% silicon oil [1]. 4. INVESTIGATION OF COEFFICIENT OF FRICTION OF POLYAMIDE In order to determine the effectiveness of MoS 2 content of PA6 in lowering the coefficient of friction experiments in laboratory were performed at the Department of Machine Design, Budapest University of Technology and Economics. Test rig: pin on disc tribometer. Test parameters: Applied load: 5 N, 15 N and 25 N. Sliding speed:.25 m/s and.1 m/s. The experiments were executed on dry and also on lubricated surfaces. Lubricant: mineral base oil ISO VG 1 without any additives. During the experiments the friction force was measured continuously using an electric measuring system controlled by a PC, which also collected and stored the data. PA 66 cpa 6 PA 6 (T) PA 6 The experiments were executed on the following test pieces: pins possess diameter 1 mm and length 1 mm, materials of pins: PA6 extruded polyamide 6, PA6 GG heat stabilized cast polyamide, PA66 MoS 2 extruded polyamide 66 containing 2 wt% MoS 2, PA66 GF3 extruded polyamide 66 containing 3 wt% short glass fiber. Discs have diameters of 11 mm and thicknesses of 8 mm. Discs were made of carbon steel C45 without hardening. Hardness of rubbing surfaces of disc was 18-19 HB. These surfaces were grounded to a surfaces roughness of R a =.3-.35 µm. Before experiments the rubbing surfaces of test pieces were cleaned and degreased by alcohol. Tests were executed 3 times one after another using the same disc and polyamide pin. Test duration with the same parameters was 3 min. First experiments without lubrication were performed using the three applied load at sliding speed of.25 m/s. Thereafter the experiments with the three applied load at the sliding speed.1 m/s followed. At the end, tests applying lubricating oil were performed at three load level at sliding speed of.1 m/s 4.1. Test Results (without Lubrication) During the experiments the friction force was determined in every second, so the variation of friction force could be detailed evaluated. In this paper only the steady state friction conditions developed at the end of the experiments are discussed. Using the measured friction force and the applied load the PC calculated the steady state coefficients of friction, which are summarized presented in figures 5-1. All figures presented the coefficients of frictions of four polyamide types at all three applied load. PA6 G PA6 GG PA66 MoS2 PA66 GF3,4 v=,25 m/s, t=3 min, d=1 mm,3,2,1 5 15 25 Unlubricated, test 1 Fig 5 of polyamide pin sliding on steel disc at sliding speed.25 m/s (unlubricated test 1).
68 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI FASCICLE VIII, 25, ISSN 1221-459 As can be seen in figure 5, in first tests, where at the beginning the polymer pin contacted with clean grounded steel surfaces, the coefficients of friction were relatively low after 3 min operation. Repeated two times the tests at the same operational conditions, the steady state coefficients of friction have been changed (compare the values presented in the figure 5 and 6). PA6 G PA6 GG PA66 MoS2 PA66 GF3,4 v=,25 m/s, t=3 min, d=1 mm,3,2,1 5 15 25 Unlubricated, test 3 Fig. 6. of polyamide pin sliding on steel disc at sliding speed.25 m/s (unlubricated test 3). The coefficient of friction of pure PA6 decreased at all load level while the coefficient of friction of heat stabilized PA6, and of the PA66 3G (3 wt% glass fiber content) have not changed considerably. At the same time the coefficients of friction of PA6 MoS 2 increased during the longer running. This can be explained by the changing of the steel rubbing surfaces during the experiments. Continuous thin polyamide film developed in the friction process of polyamides except PA MoS 2, where solid lubricants hinder it. The thin polyamide film filled the valleys of surface roughness and brought about lower coefficient of friction. During sliding of a pin made of PA MoS 2 on the steel disc the developed polymer transfer film were not continuous, so the deformation process at the contacting surfaces were more intensive increasing the coefficient of friction. PA6 G PA6 GG PA66 MoS2 PA66 GF3,4,3,2,1 v=,1 m/s, t=3 min, d=1 mm 5 15 25 Unlubricated, test 1 Fig. 7. of polyamide pin sliding on steel disc at sliding speed.1 m/s (unlubricated test 3). It is interesting to see that the glass fiber were able decrease the coefficient of friction at lower sliding speed comparing to pure polyamide. At higher sliding speed the tendency of variation of coefficient of friction was opposite: the coefficient of friction of polyamide containing glass fiber was higher then the coefficient of friction of pure polyamide. At larger sliding speed (.1 m/s) the coefficients of friction of investigated polyamides were lower then at speed of.25 m/s, except the PA MoS 2, too. Comparing the data presented in figures 7 and 8 shows that at all applied load the polyamides without solid lubricant have lower coefficient of friction then.1 except in one experiment (the coefficient of frictions of PA66 GF3 at load of 25 N was.12). PA6 G PA6 GG PA66 MoS2 PA66 GF3,4,3,2,1 v=,1 m/s, t=3 min, d=1 mm 5 15 25 Unlubricated, test 3 Fig 8. of polyamide pin sliding on steel disc at sliding speed.1 m/s (unlubricated test 3) After three experiment cycles the coefficients of friction of pure PA6 and PA6 GG containing no MoS 2 were also lower at sliding speed.1 m/s then after the first cycle. The coefficients of friction of PA66 GF3 were a little higher after the third cycle but the coefficient of friction of PA6 MoS 2 increased enormously, especially at lower applied load. Comparing the data presented in figures 7 and 8 proves these observations. It can be stated that the MoS 2 content of polyamides has not lowered the coefficient of friction in the conditions of our investigation. Even yet, while the coefficients of friction of PA6, PA6 GG, and PA66 GF3 were lower at higher speed level, in the case of the fourth polyamide the tendency is opposite, the coefficient of friction of PA66 MoS 2 was higher at higher speed of.1 m/s. 4.2. Test Results (with External Lubrication) Beside the experiments without lubrication investigations were executed at sliding speed of.1 m/s at the above mentioned three load level on lubricated rubbing surfaces. The aims of these tests were to determine the effect of lubrication with low
FASCICLE VIII, 25, ISSN 1221-459 69 viscosity mineral base oil on the coefficient of friction of different polyamide/steel pairs. At the beginning of the investigation, after cleaning the test pieces, the surfaces were covered with thin oil film. During tests the surfaces were not lubricated once more. The results of these experiments are summarized presented in figures 9 and 1. As can be seen in these figures, the filling materials in polyamide influenced the coefficient of friction of lubricated sliding pairs. At external lubrication with mineral base oil the influence of MoS 2 content on the coefficient of friction was smaller then at the tests without lubrication, but the tendency was similar. At the beginning, in the test 1, the MoS 2 content lowered the coefficient of friction. But after longer sliding distance, in the test 3, the coefficients of friction of PA66 MoS 2 become higher than the coefficient of friction of pure PA6 at every load level. PA6 G PA6 GG PA66 MoS2 PA66 GF3,1,8,6,4,2 Test 1 v=,1 m/s, t=3 min, d=1 mm, lubricated 5 15 25 Fig. 9. of polyamide pin sliding on steel disc at sliding speed.1 m/s (lubricated test 1). Test 3 PA6 G PA6 GG PA66 MoS2 PA66 GF3,1,8,6,4,2 v=,1 m/s, t=3 min, d=1 mm, lubricated 5 15 25 Fig. 1. of polyamide pin sliding on steel disc at sliding speed.1 m/s (lubricated test 3). The glass fiber content has not beneficial effect on the friction in the case of external lubrication. Except one test condition (load 5 N, speed.25 m/s), the coefficients of friction of PA66 GF3 were always higher then the coefficient of friction of all other investigated types of polyamides. It seems that the glass fibers came out on the rubbing surfaces and decreased the effectiveness of the lubricating oil. However, the reinforcing with glass fiber is not always unbeneficial as was proved earlier by us in experiments on high performance polymers as PEEK with and without lubrication [6]. 5. CONCLUSIONS Our experiments on some types of polyamides proved the following: The coefficients of friction of polyamides at low sliding speed are relatively low. The MoS 2 content of polyamide did not decrease but rather increase the coefficients of friction of polyamides, especially after longer running distance. Even polyamide containing glass fiber has lower coefficient of friction than the polyamide PA66 MoS 2. External lubrication with mineral base oil decreased the coefficient of friction of polyamides, but also in this case, the MoS 2 was not able to ensure lower coefficients of friction than the coefficients of friction of pure polyamide. We are following the investigations on the effectiveness of incorporated lubrication of engineering polymers on the tribological properties. ACKNOWLEDGMENTS The studies of lubrication of sliding pairs are part of a research program OTKA T 37244 sponsored by the Hungarian Government. REFERENCES 1. Kozma M., Kislinder E., 22, Pecularities of friction and wear of polyamides. INTERTRIBO 22, The High Tatras, Stara Lasná Slovak Republik, pp. 76-81. 2. Thorp J.M., 1986, Tribological Properties of Selected Polymeric Matrix Composites against Steel Surfaces. In Friction and Wear of Polymer Composites by Klaus Friedrich, Elsevier, pp. 89-135. 3. De Baets P., 1996, Large scale friction and wear testing of polyamide 6 sliding against steel at low velocity under very high contact pressures, Proceedings VI Tribological Conference, Budapest, pp. 46-51. 4. Zsidai L., Kalácska G. et al., 21 Investigation of tribological properties of engineering plastics with small scale specimens. Proceedings of the 2 nd World Tribology Congress Vienna, pp. 417-42. 5. Kozma M., 1975, Die ohne Schmierung durchgeführte Untersuchungen über Reibungseigenschaften von Thermoplasten. XVI: VEDECKA Konference,. Brno, pp. 1-8. 6. Kislinder E., Kozma M., 23, Nagyteljesítményű műanyagok tribológiai jellemzőinek vizsgálata. Műanyag és Gumi, 4. 2, pp. 55-59.