Abstract
Among DLC (Diamond-Like Carbon), ta-C (tetrahedral amorphous Carbon) coatings have been applied to improve fuel efficiency in automobile engines. Furthermore, it has been reported that a ta-CNx (tetrahedral amorphous Carbon Nitride) coating showed more than 50% lower friction coefficient in base oil lubrication compared to ta-C coating. On the other hand, the low friction mechanism of the ta-CNx coating has not been clarified. From the previous research, low friction mechanism of the ta-CNx coating in oil lubrication is considered that the increment of oil film thickness in accordance with increasing the oiliness of the transformed layer is important. However, the effects of oil film thickness and oiliness of the transformed layer on friction coefficient have not been quantitatively clarified, because oil film thickness and oiliness of the transformed layer have not been measured during friction. Therefore, in this research, the objective was set to clarify the low friction mechanism of the ta-CNx coating in oil lubrication by in-situ observation with reflectance spectroscopy, and oil film thickness and physical properties (polarizability, chemical bonding fraction) of the transformed layer was measured by in-situ observation with reflectance spectroscopy. Using in-situ observation with reflectance spectroscopy, film thickness, refractive index n and extinction coefficient k can be measured during friction test. And polarizability and chemical bonding fraction of the transformed layer were estimated from refractive index and extinction coefficient. From in-situ observation with reflectance spectroscopy, low friction mechanism of the ta-CNx coating in oil lubrication was proposed as shown below. The transformed layer was formed during friction and the polarizability of the transformed layer increased in accordance with decreasing C-Csp2 bonding fraction of the transformed layer, and PAO4 oil molecules were physically adsorbed on the transformed layer. As a result, oil film thickness increased and low friction occurred.
