Environmental effects on the friction of hydrogenated DLC films

We have investigated environmental effects on hydrogenated diamond-like carbon (H-DLC) films under various pressures of H2O, O-2, and N-2 by ultrahigh vacuum (UHV) tribometry. The H-DLC film exhibits an ultralow coefficient of friction (mu = 0.004 in UHV). The mu value increases with increasing pressure of H2O and O-2. Specifically, mu increases up to 0.07 under 10 Torr of H2O, and up to 0.03 under 150 Torr of O-2; these are typical H2O and O-2 contents respectively in ambient air. Our results are consistent with similar environmental effects previously reported. But, we have also discovered that these friction changes are reversible, returning to the ultralow value when UHV is restored. The reversibility of the friction behavior in both environments, coupled with the lack of evidence of tribochemical changes by Auger electron spectroscopy, suggest that the observed friction changes are due to the weakly adsorbed gas molecules that influence the friction property by physically separating the H-DLC interface. Speed-dependent tribometry also supports this argument. In addition, two DLC films with different hydrogen contents and with widely different friction coefficients in UHV are shown to exhibit identical mu values under humid environments, further demonstrating that the frictional properties of these DLC films are essentially determined by the surface layer of adsorbed gas molecules.