Effect of nitrogen atoms included in CNx coatings on friction sliding against Si3N4 ball in nitrogen gas

Carbon nitride (CNx) coatings are thought to have good tribological properties because of low friction and high wear resistance. This material shows a friction coefficient lower than 0.01 when slid against a Si₃N₄ ball in dry N₂ gas. The mechanism of superlow friction was believed to be due to the change in the topmost layer of the CNx surface in the wear track to a low-shear-strength layer by forming a graphite-like structure. However, the effects of nitrogen atoms in the CNx coating and the ambient N₂ gas on the transformation of the surface layers of CNx remain unclear. In this work, we investigated the relationship between the friction coefficient and the nitrogen concentration in the wear track on the CNx coating. We measured the intensity of C KLL and N KLL on the wear track of CNx by Auger electron spectroscopy (AES), and conducted Raman analysis. We compared the N KLL/C KLL ratios of wear tracks from sliding in ambient N₂, Ar, and O₂. Only in the case of the wear track from material slid in N₂, did the N KLL/C KLL ratio decrease to almost zero and the friction coefficient became lower than 0.01. On the other hand, in the Ar environment, the N KLL/C KLL ratio in the wear track was comparable to the as-deposited CNx coating, the friction coefficient became 0.03, and the I_G position of Raman shift increased at most 3 cm^-1. The N KLL/C KLL intensity of the wear track after friction tests in Ar and then in N₂ was approximately 0.2 times as low as the as-deposited CNx. The friction coefficient in N₂ gas was 0.03 when incorporated nitrogen remained in the CNx surface. On the other hand, the friction coefficient was lower than 0.03 when incorporated nitrogen was desorbed.