Abstract
To enhance the friction reduction characteristics of MoDTC are desired to reduce a friction loss of automobile engines. In this research, we focused on the carbon structures and doped metal of DLC coatings as factors that affect the friction reduction effect of MoDTC. We prepared two types of DLC coatings with different carbon structures: ta-C coating with an amorphous carbon structure and GNC coating with nano-crystals of graphite. Each coating was doped with different concentrations of Ta. The Raman analysis of the as-deposited DLC coatings revealed that GNC:Ta coatings consistently had a higher ID/IG ratio of around 1.2, regardless of the concentration of Ta. On the other hand, the ID/IG ratio of ta-C:Ta coatings increased with the addition of Ta. In order to evaluate the friction characteristics, we conducted friction tests and performed in-situ observations of the tribofilm at the contact point using reflectance spectroscopy. From the reflectance spectrum measured by reflectance spectroscopy, we observed and estimated the thickness and the composition of tribofilm. Interestingly, we found that the tribofilm on the DLC coating with a higher ID/IG ratio contained more MoS2 and exhibited a low friction coefficient of under 0.10. Based on our findings, we proposed that DLC coatings with higher ID/IG ratios contributed more to the reaction of MoDTC, resulting in a larger friction reduction effect of the tribofilms derived from MoDTC.
