Mechanochemical Reactions of Adsorbates at Tribological Interfaces: Tribopolymerizations of Allyl Alcohol Coadsorbed with Water on Silicon Oxide.

Abstract

Mechanochemical reactions of adsorbed molecules at tribological interfaces can benefit or impede lubrication, depending on the type of reactions induced by the interfacial shear or friction. Shear-induced polymerization of oxidatively chemisorbed organic species can occur at tribological interfaces, and their products can mitigate the wear of the surface in the case of the intermittent cessation of the lubricant supply. In contrast, tribochemical reactions involving water molecules impinging from the ambient air could facilitate surface wear. In this study, we investigated how such processes are affected when a silicon oxide surface is exposed to the environment containing both water and polymerizable organic molecules. For the polymerizable organic moiety, allyl alcohol was chosen because it is known to have a good tribopolymerization activity and can compete with water for surface adsorption sites. The adsorbate composition can be divided into two regimes: water-rich and alcohol-rich. The tribopolymerization yield was found to be significantly enhanced, compared to the alcohol-only case, in both water-rich and alcohol-rich regimes. The coadsorbed water molecules appeared to be incorporated into the tribopolymerization product of allyl alcohol. The friction coefficient qualitatively correlated with the tribopolymerization yield. Surprisingly, a small degree of surface wear was observed in the alcohol-rich regime, although wear was completely suppressed in the water-rich regime and the alcohol-only condition. These results suggested that the wear prevention effect does not necessarily correlate with the tribopolymerization effects.