Two different low friction mechanisms of diamond-like carbon with silicon coatings formed by plasma-assisted chemical vapor deposition

Diamond-like carbon with silicon (DLC-Si) coatings formed by plasma-assisted chemical vapor deposition showed low friction coefficients of the order of 0.01 against steel without a lubricant, not only in dry atmosphere but also in humid atmosphere, where conventional DLC coatings showed higher friction coefficients of 0.1–0.2. DLC-Si coatings with 1 μm thickness deposited on steel were slid against steel using a conventional ball-on-disk type of apparatus to compare with a low friction mechanism of DLC-Si in dry and humid atmospheres. Analyses of wear scars indicated that formation and/or transfer of graphite-like carbon including hydrogen that originated in a DLC-Si coating occurred in dry atmosphere, while oxidation of contained silicon with water vapor formed silica-sol by sliding in humid atmosphere. The latter, peculiar to DLC-Si, was considered to cause the low friction coefficient in humid atmosphere through adsorbed water on silica.