The aim of this study is to examine the effect of silicon in DLC-Si films on friction properties, and to clarify the mechanism of low friction under dry sliding conditions. The friction coefficient of DLC-Si films was measured using a ball-on-disk type tester at a load of 10N and a sliding velocity of 0.2m/s with no lubricant in an ambient air atmosphere. Compared to DLC films without Si, DLC-Si films exhibited an extremely low friction coefficient (about 0.05) ranging from 4 to 17at. % in silicon. On the wear surface of DLC-Si coated disks, the formation of Si-OH was eventually detected using derivatization-XPS. In order to explain the relation between Si-OH formation and low friction behavior, model samples with Si-OH, Si-H and their mixture on the silicon wafer were prepared, and the friction coefficient of the samples was measured with a ball-on-plate type tester. Compared to Si-H, Si-OH had a lower friction coefficient. Furthermore, Si-OH surface analyzed by ATR-IR showed spectra with broad band at around 3400cm−1
, indicating the existence of adsorbed water on the Si-OH surface. From the results obtained, it was suggested that DLC-Si films exhibited a low friction property caused by the formation of Si-OH and adsorbed water in it under the dry sliding condition.