Friction reduction under mixed and full film EHL induced by hot micro-coined surface patterns

Hemispherical structures with different area densities and depths were fabricated by hot micro-coining on stainless steel samples. The tribological properties were studied using a ball-on-disc tribometer in rotational sliding mode with different sliding velocities in order to study the frictional behaviour under different lubricated conditions. For low sliding velocities (mixed EHL), all coined surfaces demonstrate a significant friction reduction, whereas only some of the structures lead to an improved frictional behaviour under full-film EHL (higher velocities). Furthermore, the influence of an additional pressure build-up and cavitation is discussed. The largest improvement (friction reduction by a factor of 3 for all velocities) is achieved using a pattern with a pocket depth of 25 μm and an area density of 5%.