Fretting behaviors of self-mated diamond-like carbon films: The evolution of fretting regime and transfer film

Diamond-like carbon (DLC) film is undoubtedly one excellent self-lubricating solid film to improve the fretting behavior due to its ultralow friction as well as high-hardness. Investigating the evolution laws of fretting running state with operating condition and the friction reduction mechanism of DLC film under fretting is vital for reducing friction and wear. Here, the fretting performance of self-mated hydrogen-containing DLC (H-DLC) and Cr-doped DLC (Cr-DLC) was studied under different normal loads and amplitudes. In addition, the mechanism of friction reduction for transfer film and the cause of discrepancy in fretting behavior were clarified through a control experiment. The results demonstrate that as the normal load increases, the relative slip decreases, the elastic deformation increases, and the fretting regime develops towards the partial slip regime, while the increasing amplitude moves the fretting regime towards slip regime. During fretting, the third body layer will transfer to the counterface, and form a uniform and graphitized transfer film. The difference in the length of run-in period and the sp2/sp3 hybrid carbon content of DLC films influences the formation rate and the graphitized degree of the transfer film, which induces the discrepancy of fretting behavior of H-DLC and Cr-DLC films. Thus, this study not only clarifies the evolution on fretting regime of DLC film with changes in working conditions, but also demonstrates the key role of highly graphitized transfer film in fretting performances of self-mated DLC films. This work demonstrates the controllability and potential of self-mated DLC films in the field of fretting.