Various experimental and theoretical explanations have been given for the friction characteristics of diamond and diamond-like carbon（DLC）. Nevertheless, their friction characteristics cannot be clarified solely based on conventional explanations. Particularly, the theory that “the extremely low coefficient of friction of 0.01 order develops because very soft material is produced by friction” seems doubtful.

Therefore, this study assessed the hypothesis that friction-generated gas is a factor causing the low coefficient of friction. Earlier reports of the literature have yielded data showing evidence of CO and CO₂ gas evolution by friction of DLC and AlTiC. For DLC containing a large amount of hydrogen, some reports present a picture of bubbles in the transfer film, which can be regarded as evidence of hydrogen gas and/or hydrocarbon gas evolution.

To elucidate these facts, one can reasonably infer that transfer material exhibiting a low coefficient of friction is a harder material with lower thermal conductivity than the conventional soft material. This hypothesis has been supported by many data related to the load dependence: the frictional heat dependence of the friction coefficient of diamond and DLC. These results suggest that gas evolution causes the low coefficient of friction of diamond and DLC.