Friction and counterface wear influenced by surface profiles of plasma electrolytic oxidation coatings on an aluminum A356 alloy

To reduce the fuel consumption and emission of passenger vehicles, aluminum engines have been increasingly used throughout the last 30 years. Since most conventional aluminum alloys have poor wear resistance, various technical solutions have been developed to generate a wear-resistant cylinder bore surface against the sliding piston ring. In this work, the plasma electrolytic oxidation (PEO) process was employed to produce oxide coatings on an Al alloy A356 for Al engine blocks, to protect against the wear attack. The surface morphology and coating thicknesses were tailored by polishing two PEO coatings. A reciprocating sliding tribometer was used to investigate the tribological and wear behavior of the PEO coatings, counterface materials, and that of a state-of-the-art plasma transferred wire arc coating (as a benchmark) under two lubricated conditions. The results show that the PEO coatings have a low coefficient of friction and minimal wear. The variation in tribological behavior and counterface wear among the tested materials was likely due to different topographic features such as skewness and kurtosis caused by microbump distribution, porosity, and valleys on as-prepared, sanded, and polished coating surfaces.