The effects of surface texture in reciprocating contacts – An experimental study

Abstract The effect of surface texture on friction has been investigated experimentally for a convergent-divergent bearing, operating under different lubrication regimes. This was achieved using recently developed test apparatus that closely simulates an automotive piston ring-liner contact, by sliding patterned plane fused silica surfaces against a convex steel pad. Textured patterns, consisting of pockets of varying shapes and orientations, were assessed on their ability to reduce frictional losses, and results compared with those from a reference smooth surface. These tests were performed under a range of lubricant viscosities and applied normal loads in order to vary lambda values (i.e. the ratio of minimum film thickness to composite roughness) and hence highlight the beneficial or detrimental effects of texture in boundary, mixed and full film regimes. In the boundary and mixed regimes – the regimes where surface texture was beneficial – grooves normal to the sliding direction were the most effective patterns among those investigated, reducing friction by up to 62%. The results suggest that pockets act to increase fluid entrainment and hence reduce any asperity contact that is present. However, pockets at reversal were shown to increase friction dramatically. In the full film regime, when a liquid film fully separates sliding surfaces, texture was shown to increase friction compared to the smooth case. In addition to this, the experimental setup allowed cavitation in the ring-liner pairing to be imaged. These results may suggest that the increase in full film friction due to surface texture may be due to the interaction of pockets with the cavitated region. Furthermore, imaging results confirm previous research in that the number of cavitation streamers increase as the film thickness decreases.