Study On Effects Of Asperity On Tribological Properties Of PDMS Surface

Rubber is a very important raw material among national economy and mankind daily life. Theapplication in friction components making by rubber material keeps the rising trend. So it is veryimportant to study the tribological performance of rubber. Surface texture and oxygen plasmatreatment have been recognized as viable means to improve the tribological performance of slidingsurface. However, the research of the effect of surface texture on soft material such as rubber is toofew. In this paper, the patterns of positive asperity with different diameters, heights and area densitywere fabricated on the surface of poly(dimethylsiloxane)(PDMS) by Lithography&Replicatechniques. An oxygen plasma treatment was used to hydrophilize the surface of PDMS. The resultsindicated good uniformity and repeatability of positive asperity and stability of surface wettability.Friction tests were carried out to evaluate tribological influence of positive asperity using apin-on-disk tester on different lubrication conditions. The effects and the lubrication mechanism ofsurface texture on PDMS with different wettabilities were studied in this research. The main resultsare showed as follows:1. Under dry sliding, the frictional force on the interface of PDMS is consisted of adhesivefriction, deformation and elastic hysteresis. Compared to the untextured PDMS, textured PDMS withhigher area density and smaller diameter can reduce adhesive friction and has lower deformation. Soit shows a better effect of friction reduction. Positive asperity with the diameter of50μm and the areadensity of40.1%presents a reducing density of48.26%at sliding speed of200mm/s.2. Under water lubrication, hydrophobic friction components are performed on the regime ofboundary lubrication at low speed. The textured surface with higher column has better water retentionand drainage capacity. Water which has been squeezed out can be supported to the contact region.Sothe friction coefficient is reduced. Positive asperity with the diameter of100μm, the height of11μmand the area density of2.6%presents a reducing rate of57.46%at sliding speed of40mm/s. However,hydrophobic friction components are performed on the regime of mixed lubrication at high speed.Perhaps because the water film flowing on PDMS surface is destroyed by positive asperity, thesurface texture has higher friction coefficient than that of untextured surface. Positive asperity showsa friction increasing effect at high speed. Positive asperity with the diameter of200μm, the height of5μm and the area density of29.9%presents an increasing rate of889.44%at sliding speed of200mm/s.3. Under methyl silicone oil lubrication, hydrophobic friction components are performed on theregime of mixed lubrication at low speed. The textured surface has higher friction coefficient than thatof untextured surface. Positive asperity with the diameter of50μm and the area density of40.1%presents an increasing rate of374.5%at sliding speed of3mm/s. However, hydrophobic frictioncomponents are performed on the regime of hydrodynamic lubrication at high speed. The frictioncoefficient is mainly effected by the viscosity of lubricant. Positive asperity has little effects onfriction.4. Under water lubrication, the water film which is flowing on hydrophilic reduced adhesive force. So the friction coefficient of the untextured PDMS is very low. But the water film wasdestroyed by the existence positive asperity, the surface texture has higher friction coefficient thanthat of untextured surface. Positive asperity with the diameter of200μm and the area density of40.1%presents an increasing rate of1499.1%at sliding speed of200mm/s.