Role of tribochemistry in nanowear of single-crystalline silicon.

The effects of counter-surface chemistry, relative humidity, and applied normal load on nanowear of single-crystalline silicon were studied with atomic force microscopy. In the absence of humidity, the silicon surface can resist mechanical wear as long as the contact pressure is lower than the hardness of silicon regardless of the counter-surface chemistry (diamond or SiO(2)) and ambient gas type (vacuum, N(2), O(2), air). In these conditions, the sliding contact region is protruded forming a hillock. However, when the relative humidity is higher than ~7%, the hillock formation is completely suppressed and, instead, tribochemical wear of the silicon surface takes place even at contact pressure much lower than the hardness. The tribochemical wear increases drastically in the relative humidity regime where the adsorbed water layer assumes the "solid-like" structure; further increase of wear is small in higher relative humidity regime where the "liquid-like" water layer is formed. It is also noted that the humidity-induced wear occurs only when the counter-surface is SiO(2); but not with the diamond counter-surface. This implies that the interfacial shear of the water-adsorbed SiO(2) surface with a chemically inert counter-surface is not sufficient to initiate the tribochemical wear; both substrate and counter-surface must be chemically reactive. A phenomenological model is proposed to explain the experimental observations.