The Investigation About Formation Mechanism of Tribofilm Induces Super-Lubricity of DLC Films Sliding Against ZrO₂ in Vaporized Ethanol Environment

Abstract

Diamond-like carbon (DLC) films have complicated structures that are generally composed of sp²- and sp³-hybridized orbitals of carbon and hydrogen. Owing to their superior properties as a coating material, DLC films have been the subject of great interest within academia and industry since the early 1970s. In our previous studies, we have confirmed super-lubricity of DLC films sliding against ZrO₂ ball in mixed gas environment of H₂ (and/or N₂) and vaporized ethanol. And from the results of surface observation and structure analysis, we have concluded that this super-lubricity was induced by the presence of hydrocarbon tribofilm which was formed at the sliding interface. We found that this tribofilm was generated via catalytic reaction of ZrO₂ ball against vaporized ethanol. It is well known that ZrO₂ has two kinds of catalytic reaction such as “Dehydration” and “Dehydrogenation”. In this study, we performed friction test using ZrO₂ plate and ZrO₂ ball in mixed gas environment of N₂ gas and vaporized ethanol. From the result of Raman spectroscopy on wear track of ZrO₂ ball, we confirmed that “polyethylene” was formed via friction in vaporized ethanol environment. It is considered that polyethylene change to the hydrocarbon tribofilm by any chemical reaction such as polymerization. From these results, we considered that catalytic reaction of ZrO₂ (such as dehydration and dehydrogenation) contributed to form the tribofilm which induce super-lubricity.