Abrasion of Steel by Diamondlike Carbon Coatings

Abstract

Thin ceramic coatings can increase the fatigue lifetime of bearings and gears, possibly by polishing their counterparts and reducing stresses from asperities. Thus, a coating’s ability to polish or abrade may determine its usefulness. Yet there has been little work examining factors which control the abrasiveness of such coatings. We have analyzed the abrasiveness of diamondlike carbon and boron carbide coatings against steel for this study. We find an extremely steep dependence of abrasiveness on hardness. We show that coating roughness with horizontal features on the nanometer-scale is strongly correlated with abrasiveness, while roughness with horizontal features on the micron-scale is not correlated with abrasiveness. The nano-scale—but not the micro-scale—structure is quickly obliterated by sliding against steel, explaining the drastic reduction with time in the abrasiveness of the coating that we observe. We derive quantitative scaling relationships that show how the time dependence of the abrasion rate varies with important parameters of sliding wear, and we use these relationships to predict abrasion kinetics for new experiments. Detailed modeling of the stresses present during abrasion leaves some important questions unanswered.