The Effects of Surface Roughness and Coatings on the Tribological Behavior of the Surfaces of a Piston Skirt

Two methods are proposed to reduce friction losses and the amount of wear in a piston assembly. One is the modification of the surface roughness of the skirt. Surface coating is another method to protect the sliding surfaces. In order to study the roughness of the skirt surfaces, the surfaces were ground to have three different roughness values. Graphite and diamondlike carbon (DLC) were also used to coat the surfaces of the piston skirts, while the specimens used in the test were the skirt and the cylinder bores from a low-friction diesel engine. These were tested with a reciprocating wear tester that used a piece of cylinder block as the reciprocating specimen and a segment of the piston skirt material as the fixed pin. SAE5W40 engine oil was used in the boundary lubrication regime. The friction coefficient of the graphite coating was lower than the DLC coating, but the graphite coating was not effective in protecting the wearing of the surfaces. The DLC coating provided much better tribological performance than the graphite coating.