Effect of atomic oxygen erosion on mechanical and tribological properties of Cr and B doped hydrogen-containing diamond-like carbon films

To improve the mechanical and tribological properties of hydrogen-containing diamond-like (H-DLC) films under typical space environment-atomic oxygen (AO) erosion, H-DLC films with multilayer structure of Cr-doped and B-doped were prepared on the surface of GCr15 steel by non-equilibrium magnetron sputtering technique. The changes of mechanical properties such as hardness, elastic modulus and residual stresses of the three DLC films, H-DLC, Cr-H-DLC and B-H-DLC, before and after AO erosion were investigated contrastively. It was found that AO erosion reduced the sp3 hybrid bonds content in the three DLC films, leading to a decrease in film hardness and elastic modulus, and it was also found that the residual stresses in the films were all increased after AO erosion. Compared with the friction test carried out only in a vacuum condition without AO erosion, the coef-ficient of friction (COF) of the films after AO erosion changed slightly, but the wear life showed a notably decrease and a large number of pear grooves appeared in the wear scars. Although wear life of the three DLC films decreased after AO erosion, the decrease of Cr-H-DLC film was much smaller than the remaining two. According to the Raman spectra results, it was found that the ID/IG values at the wear scars of H-DLC film became larger after AO erosion, and the ID/IG values at the wear scars of Cr-H-DLC and B-H-DLC films did not change much, indicating that the AO erosion induced more sp2 C--O bonds with high binding energy in the H-DLC films, which, together with the degradation of the films' mechanical properties, led to the decrease in the wear life of the H-DLC films. The doping of Cr element produced less C--O bonds and thus the decrease of film's wear life was smaller. The effect of doping with B elements on film's wear life was not significantly, but achieving the lowest COF.