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 fim 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.
Amorphous carbon-silicon (a–C–Si) coatings with smooth surface and adhesion to steel substrate were deposited at 550°C by a dc glow discharge method from reactant gases of CH4, SiCl4, H2, and Ar at a deposition rate of 1–3 μm h−1. Diamond-like carbon was detected by laser Raman spectroscopy in the coatings with high hardness of Hv 2000 and more than 70 at. % carbon. Ball-on-disk type and Ohgoshi-type apparatuses were used to measure the tribological properties. Ball-on-disk tests revealed that the a–C–Si coatings showed a friction coefficient as low as 0.04 against steel with no lubricant in an ambient atmosphere of 70% relative humidity, which was one-third that of an i–C coating. The wear rate of the steel ball against the coated disk was an order and three orders of magnitude smaller than that against i–C coated and uncoated disks, respectively. Both types of tests showed that the wear rate of the a–C–Si coating itself was also very small.
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