Elucidating tribochemical reaction mechanisms: insights into tribofilm formation from hydrocarbon adsorbates coupled with tribochemical substrate wear†

Tribochemical reactions, chemical processes that occur by frictional shear at sliding interfaces, lead to tribofilm formation or substrate wear that directly affect the efficiency of machinery. Here, we report tribofilm growth through tribopolymerization and tribochemical wear of a silica surface due to reactions with organic precursors methylcyclopentane, cyclohexane, cyclohexene, and α-pinene. The activation volume determined from the stress dependence of reaction yield is correlated to the chemical reactivity of the precursor molecules. The molecules with higher tribochemical reactivity exhibited smaller activation volume, implying that less mechanical energy was required to initiate tribochemical reactions. Nudged elastic band calculations for the hypothetical pathways for the observed tribochemical reactions suggested that the smaller activation volume could be related to smaller thermal activation energy at the rate-limiting step. The tribofilm formation yield was found to increase with load whereas the load dependence of tribochemical wear was negligible. The environment dependence of the sliding processes was also analyzed. Results showed that, compared to a dry N₂ environment, the tribopolymerization reaction yield increased in dry air but decreased in N₂ with 40% relative humidity, while the wear rate remained unchanged. This finding suggested that during sliding, the reactive sites exposed at the worn surface could be re-oxidized by even trace amounts of oxygen or water vapor in the environment. This analysis of tribofilm yield and substrate wear in various environments showed that ambient gas can change the tribochemical reactivities of the reactant, which leads to different load dependencies of tribopolymerization and tribochemical wear.