The frictional behavior of DLC films against bearing steel balls and Si3N4 balls in different humid air and vacuum environments

The frictional behaviors of DLC films against bearing steel balls and Si3N4 balls in different humid air and vacuum environments were investigated. The results show that the friction coefficients of DLC films against bearing steel balls in steady-state decrease from 0.34 to 0.12 with the increase in the relative humidity (RH) of air from 10 to 95%, respectively, while the friction coefficients of DLC films against the Si3N4 balls increase from 0.02 to 0.12 with the increase in the RH of air from 10 to 95%. Both the friction coefficients of DLC films against the bearing steel balls and against Si3N4 balls become gradually close to each other with the increase in the RH of air and are all the same to 0.12 when the RH up to 95%. In vacuum, the friction coefficient of DLC film against DLC film (which is deposited on the bearing steel ball) is the lowest and only about 0.05; the friction coefficients of DLC films against bearing steel balls is about 0.12 and lower than that of in dry air and humid air, while the friction coefficients of DLC films against Si3N4 balls is about 0.1 and higher than that of in dry air but lower than in humid air a little. The variation of the friction coefficients of DLC films against both bearing steel balls and Si3N4 balls with the increase in different RH of air is associated with the formation of the transfer film and partial graphitized DLC films. The friction coefficients of DLC films against bearing steel balls in dry air exhibited higher lever; perhaps related with the oxidization of DLC films and steel counterpart during sliding in dry air. The friction coefficient of DLC films against Si3N4 balls in vacuum higher than in dry air is due to the dehydrogenation and the graphitizing of the DLC film under the action of friction stress, besides, the hydrogen content in DLC films prepared in this study is only 26.7 at.%. This hydrogen content is lower than ordinal a-C:H films; this probably weakened the oxidization for DLC films during sliding in dry and humid air, thereby decreasing the friction coefficient.