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 17 at. % 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 3400 cm"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.%ダイヤモンドライクカーボン（Diamond-Like Carbon：rnDLC）は，非晶質で硬質な膜として，Aisenbergらによりrn1970年代に初めて合成された。DLC膜の主な機能としては，rn耐摩耗性，低摩擦，耐食性，光透過性，ガス遮断性等が挙げrnられる。現在，耐摩耗性と低摩擦が要求される工具·rn金型および機械部品，光透過性を利用した赤外線用ガラスのrn保護膜，プラスチック容器へのガスバリアコーティングを初rnめとし，実用にも供されつつある。それらの中でも，適用対rn象の幅広さ，効果の大きさなどからしゅう動部に用いられるrn機械部品への応用に最も大きな期待が寄せられている。
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