In an earlier report, the author proposed hypotheses to explain mechanism for DLC film and diamond film to develop a low friction coefficient. This study investigated whether the results of many friction tests on CN_x film can also be explained by these hypotheses. Results of the investigation demonstrated that the frictional properties of the CN_x film could not be explained sufficiently by these hypotheses alone. To explain the frictional properties of CN_x satisfactorily, some hypotheses were added to the conventional one. Results show not only that the frictional properties of CN_x film but also that the frictional properties of carbon films of many kinds, including DLC film and diamond film, are explainable by these new hypotheses.

These hypotheses are explained hereinafter:（1） Transferability of CN_x film to the friction surface and its surroundings is higher than that of the DLC film not containing H.（2） The transfer layer formed by friction of the carbon-based film is hard, and it has low thermal conductivity.（3） The CN_x film produces N₂ by friction. In the same manner as diamond film and DLC film, it produces a graphite layer and releases CO and CO₂ in an atmosphere, where trace amounts of O exist, suppress adhesion and develop a low friction coefficient.（4） O₂ and H₂O are adsorbed onto the carbon film surface, which contributes to the development of a low friction coefficient in vacuum and in inert gas,（5） the hard carbon film surface layer is graphitized by formation of －C＝O group or its release as CO.