The Effect of Carbon Structure of DLC Coatings on Friction Characteristics of MoDTC-derived Tribofilm by Using an in situ reflectance spectroscopy

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In this paper, we prepared 6 types of DLC coatings with varying carbon structures (amorphous ta-C coating and GNC coatings that include nano graphite crystals) and different doped Ta amounts to study friction characteristics. Results from the friction test with MoDTC-added lubricant revealed that DLC coatings with a higher ID/IG ratio exhibited a lower friction coefficient. Furthermore, in situ observations using a reflectance spectroscopy showed that the tribofilm formed on the DLC coatings with a higher ID/IG ratio maintained a higher MoS2 / (MoS2 + MoO3) ratio. This ratio strongly correlated with its friction coefficient. From the measurement of a work function of each DLC coating, the DLC coatings with a higher ID/IG ratio had a higher work function, which indicated to include larger amount of the defect of graphite structure. This active defect of graphite structure was considered to enhance the friction reduction effect of MoDTC. The findings of this paper suggest a material design concept for a DLC coating that amplifies the effects of lubricant additives in reducing friction.