In situ observation of the formation of MoDTC-derived tribofilm on a ta-C coating using reflectance spectroscopy and its effects on friction

We clarify the relationships between tribofilm thickness, chemical composition, and friction coefficient by combining in situ and ex situ analyses. An in situ reflectance observation is conducted for MoDTC-derived tribofilm on a ta-C coating during friction tests. The results show that the thicker the tribofilm is, the lower the friction coefficient. In addition, the refractive index increases as friction decreases. XPS analysis shows that the topmost surface of the thick tribofilm contains a higher MoS2 concentration than the deep position. Therefore, the generated MoS2-rich tribofilm is considered to reduce friction. In addition, as the MoS2 tribofilm wears out, friction increases. In conclusion, we have developed a MoDTC-derived tribofilm formation model using a new method of in situ optical measurement. •We conducted friction tests in the sapphire/ta-C system with MoDTC-added oil.•In-situ reflectance observation analyzed the transition of the tribofilm.•A friction coefficient decreased with an increase in the tribofilm thickness.•Thick tribofilm may have a rich MoS2 layer on the top-most surface•XPS analysis and in-situ reflectance observation confirmed the above model.