Low Friction Property and its Mechanism of DLC-Si Films Under Dry Sliding Conditions

Diamond-like carbon (DLC) films are of significant interest for the automobile field, because they possess the potential to improve friction properties under various sliding conditions. Among the various DLC films, the authors focus on silicon-containing DLC (DLC-Si) films, which exhibit extremely low friction coefficient under dry sliding conditions in an ambient air atmosphere. The aim of this study is to examine the influence of silicon content in DLC-Si films on the friction property of the films, and to clarify the low friction mechanism of the films. The friction test was conducted under dry sliding conditions. It was found that the films have an exceedingly low friction coefficient (about 0.05) ranging in silicon content from 4 at% to 17 at%. In order to examine the low friction mechanism of the films, surface analyses were done on the wear surface of DLC-Si films slid against bearing steel. The formation of a silanol (Si-OH) layer on the films was observed by using derivatization-XPS. To confirm the friction coefficient of the Si-OH layer, the model samples made of a Si-OH layer on the silicon wafer were examined with a friction test. The samples, like DLC-Si films, exhibited a low friction coefficient below 0.1. Furthermore, the Si-OH surface analyzed by ATR-IR showed spectra with a 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 exhibit a low friction property caused by the adsorbed water on Si-OH under the dry sliding conditions.