Clarification of relationship between friction coefficient and transformed layer of CNx coating by in-situ spectroscopic analysis

• We proposed the method to estimate the thickness and hardness of transformed layer of CNx coating during friction test.
• We estimated the thickness and hardness of transformed layer of CNx coating from optical property with reflectance spectroscopy.
• We verified that experimental friction coefficient could follow thin solid lubricants model with roughness.

Carbon Nitride (CNx) coating shows ultra low friction coefficient (μ<0.01) in dry Nitrogen gas because of existing low sharing strength layer (transformed layer) on bulk CNx coating. In order to use ultra low friction phenomena for industry field, we should establish the friction model of CNx with transformed layer. As a friction model of CNx with transformed layer, we try to use the Halling׳s friction model that is for developed for thin solid lubrication with roughness. For the friction model, we need to know the thickness and hardness of transformed layer of CNx. In this study, we introduce in-situ measurement methods of thickness and hardness of transformed layer with a reflect spectroscopy during sliding test. And we show the possibility to explain the friction coefficient with the Halling׳s friction model after in-situ measurement of important parameters with a reflectance spectroscopy during sliding test. As a result, we could estimate the thickness and hardness of transformed layer by using reflectance spectroscopy. And it was confirmed that the estimated friction coefficient followed observed friction coefficient when we supposed that transformed layer is thin solid lubrication layer in Halling׳s friction model.