Analysis of Wear Track on DLC Coatings after Sliding with MoDTC-Containing Lubricants

Abstract

Wear of DLC coating has been great concern in DLC/Steel sliding surface lubricated by molybdenum dithiocarbamate (MoDTC) containing lubricants. However, in order to design lubrication system with DLC and MoDTC, the mechanism of this issue is needed to be clarified. In this study, the roller-on-disc oscillation friction test by SRV friction tester was conducted to provide the hypothesis about the mechanism of wear of DLC with MoDTC. In friction test, DLC coatings (a-C:H and ta-C) were coated on discs. Counterpart was the SUJ2 bearing steel roller. It was found that MoDTC in lubricants accelerated the wear of the both two DLC coatings and it was dramatic especially for a-C:H coating. In order to clarify the enhancement mechanism of MoDTC for wear of DLC coatings, the surface analysis on DLC coatings and steel roller was conducted with XPS (X-ray Photoelectron Spectroscopy) and surface profile meter. As a result, sp2 ratio increased layer was observed on the a-C:H coating with XPS and the surface roughness of wear track was larger than out of wear track after sliding with MoDTC containing lubricant. On the other hand, ta-C did not show the increasing of sp2 ratio and the surface roughness of wear track decreased after sliding with and without MoDTC containing lubricant. For both DLC coatings, tribofilm generated from MoDTC was also observed on the counter SUJ2 roller surface with XPS. Depth profile of chemical elements of the tribofilm on the steel roller with XPS contributed to clarify the distribution of Molybdenum compound as MoO3 and Mo2C. Top surface was MoO3 and the interface between the SUJ2 roller and tribfilm was Mo2C. The authors provide the hypothetical model about a tribo-chemical reaction between both DLC coatings as ta-C or a-C:H and tribofilm from MoDTC.

Keywords

• dlc
• wear
• lubrication
• xps