Effects of Oxide Layer Formation during Lubricated Sliding on the Frictional Properties of Titanium-Coated Silicon

The principal goal of the experiments described in this article is to elucidate the effect of oxide layer formation during lubricated sliding on the frictional properties of Ti-coated silicon. It was found that the TiO2 layer is the prime determinant of the frictional properties of sliding surfaces coated with Ti coating. At the initial stage of sliding, Ti coating showed a high (0.55) and unstable frictional response. During this period, the Si substrates and Ti coating were damaged simultaneously, indicating that the wearing of the coating and substrate is attributed to the high and unstable friction. With the formation of the oxide layer, after a few cycles of the onset of sliding, the friction coefficient became stable and decreased to 0.16. By analyzing the oxide layer using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and Raman spectroscopy, it was found that the oxide layer (TiO2 layer) was formed from the debris of the worn Ti coating. Adhesion tests using atomic force microscopy (AFM) revealed that the adhesive component of the friction of sliding surfaces was not lowered by the oxide layer. By comparing the SEM images and frictional response, it was demonstrated that the oxide layer restrained the development of surface damage and the frictional work was converted to the growth of the oxide layer instead of damaging the sliding surfaces. From these results, it was confirmed that the deformation component of Ti coating friction was suppressed by the oxide layer.