Tribochemistry of Aldehydes Sheared between (0001) Surfaces of α-Alumina from First-Principles Molecular Dynamics

First-principles molecular dynamics (FPMD) simulations are used to explore the tribological behavior of systems consisting of two Al2O3 (0001) surfaces separated by acetaldehyde molecules. The simulations were performed with normal pressures, P, that ranged from 0 to 20 GPa. The simulations show that sliding occurs with little or no significant changes in the structure of the aldehydes when P is low. Meanwhile, tribochemical reactions between aldehydes to yield oligomers, and between the oligomers and the surfaces, occur at higher P. The occurrence of these reactions leads to slip mechanisms that are dominated by the dissociation of chemical bonds. The different slip mechanisms affect the friction forces required to maintain motion of the surfaces. Slip mechanisms that do not involve bond rupture require low forces, with a friction coefficient of 0.034 to 0.044. The friction forces are much larger for the slip processes involving the rupture of bonds. Interestingly, the results indicate that the friction forces associated with slip mechanisms involving bond rupture are lower when the longer oligomers are involved in the slip process. Overall, this work sheds light on the atomic-level chemical processes that occur when lubricated surfaces move past one another, and may aid in the rational use of tribochemical reactions in functional lubrication.