The role of abrasive particle size on the wear of diamond-like carbon coatings

Diamond-like carbon (DLC) coatings exhibit several attractive characteristics such as low coefficients of friction and low wear rates in certain environments. While the performance of DLC coatings under lubricated and dry contact has been a subject of extensive study, the study on the effect of abrasive media and three-body interactions on the wear of DLC coatings is not widespread in the literature. The present study focuses on the wear of DLC coatings in a highly abrasive environment with varying abrasive particle size under the mixed lubrication regime. Abrasive wear behavior of Diamond-like Carbon (DLC) coatings has been evaluated through tests conducted on a Block-on-Ring (BOR) tribometer where a DLC coated ring was rubbed against an uncoated steel block. An oil-water emulsion containing various sizes of sand particles was used as a lubricating fluid. Based upon results from surface analysis and cross-sectional microscopy studies, the lubricating fluid having no sand revealed no delamination or any major damage to the coating; rather two-body or asperity-asperity interactions removed some high spots from the coating surface. In contrast, when sand was introduced in the lubricating fluid as an abrasive medium, three-body abrasive wear was prevalent causing internal cracking within the coating followed by spallation and sequential removal of coating. The three-body wear mechanism is attributed to the entrainment of abrasive (sand) particles into the tribological contact. While the presence of sand particles dramatically influenced the DLC wear, the hardness of DLC coating seemed to surpass the effect of sand on the softer steel counterface wear, i.e. the material appeared to be primarily removed by the harder DLC asperities in the two-body cutting or ploughing wear mechanism.