Tribological behaviors of DLC film deposited on female die used in strip drawing

With the miniaturization of parts, dimensions of parts have obvious effects on friction in microforming processes when liquid lubricants are applied. To eliminate size effects of friction, diamond-like carbon (DLC) film was deposited on the surface of female die and blank holder for its superior tribological characteristics. Strip drawing tests were carried out on conditions of different lubricants, and tribological behaviors of DLC film at female die surface were evaluated using punch load. The results reveal that the specimen dimensions and surface topography of specimen have a little effect on tribological behaviors of DLC film than castor oil. DLC film shows lower coefficient of friction (COF) in large deformation and small dimension. With the increasing of test times, wear of DLC film occurs. Raman spectra indicates that the ratio between integrated intensities of D and G peaks (ID/IG) increases with the increasing of tests times, which means that graphitization of DLC film occurs for the rise of temperature induced by the friction at contact surface. The interesting thing is that H content increases with the times of tests especially for high contact load, which is evaluated by the ratio of the slope m of fitted linear background to the intensity of the G peak (m/IG). The increase of H content means that the number of free hydrogen atoms becomes larger for the breakage of CH bonds induced by the friction work. The graphitization and breakage of CH bonds may have an obvious effect on the tribological behaviors of DLC film in microforming.

► Decreasing of dimension and surface polishing have a little effect on its tribological behaviors of DLC film.
► Graphitization of DLC film becomes more clearly with the increasing of test times.
► A model is developed to analyze the graphitization of DLC film by calculate the friction work.
► DLC coating still contains a certain amount of free hydrogen atoms.