Low frictions of self-mated CNx coatings in dry and humid inert gas environments

The friction behavior of CNx coated Si₃N₄ disk sliding against CNx coated Si₃N₄ ball (denoted as CNx/CNx) in dry and humid inert gas environments (i.e. nitrogen, argon, and helium) is investigated in this study. The sliding contact of self-mated CNx coatings has been realized thanks to the promising two-step ball-on-disk friction test, where the rubbed Si₃N₄ ball in the running-in stage (step 1) is replaced by a new CNx coated Si₃N₄ ball in the subsequent stage (step 2). As a result, friction coefficients of less than 0.05 are obtained for self-mated CNx coatings in all three dry inert gas environments with relative humidity under 5%RH. Moreover, friction coefficients of less than 0.10 are obtained for the sliding contact of CNx/CNx in inert gas environments with relative humidity of larger than 35%RH. Especially, the lowest friction coefficient of 0.020 is achieved in the nitrogen gas environment with relative humidity of 37%RH. Furthermore, the sliding contact of self-mated CNx coatings survives after friction tests in humid nitrogen and argon gas environments. The low frictional performance and low sensitivity of friction to water vapor for the self-mated CNx coatings in inert gas environments are mainly attributed to the formation of a sp² rich carbon tribo-layer on the mating surface. The pronounced frictional performance of self-mated CNx coatings has made them good candidates for the demanding industrial applications.