First-principles molecular dynamics (FPMD) simulations are used to explore the tribological behavior of systems consisting of two Al 2 O 3 (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.

First-principles molecular dynamics (FPMD) simulations were used to study static friction, Fs, using model systems based on bulk and hydroxylated forms of Al2O3. The results demonstrate that Fs is significantly affected by adhesive interactions and by changes in the numbers of those interactions through the formation and dissociation of bonds across the slip interface. A model that directly incorporates the strengths of the adhesive interactions during slip is introduced to account for the effects on Fs and is found to perform satisfactorily. A procedure is developed to evaluate the strengths of these interactions using first-principles calculations. As a whole, the work clarifies how adhesive interactions affect Fs, provides a means of reducing the number of fit parameters used in modeling friction, and illustrates the importance of accounting for changes in bonding during simulations of friction.

First-principles molecular dynamics simulations are used to investigate the chemical response of acetaldehyde molecules (MeCHO) to compression and decompression between (0001) surfaces of α-alumina (Al(2)O(3)), with pressures reaching approximately 40 GPa. The results demonstrate that the MeCHO molecules are transformed into other chemical species through a range of chemical processes involving the formation of C-O and C-C bonds between MeCHO monomers as well as proton transfer. The mechanistic details of a representative set of the observed reactions are elucidated through analysis of maximally localized Wannier functions. Analysis of the changes in structure demonstrates that the main role of compression is to reduce the distances between MeCHO molecules to facilitate the formation of C-O bonds. Additional examination of the electronic structure demonstrates that the surface plays a role in facilitating proton transfer by both rendering hydrogen atoms in adsorbed MeCHO molecules more acidic and by acting as a proton acceptor. In addition, adsorption of the MeCHO molecules on the surface renders the sp(2) carbon atoms in these molecules more electrophilic, which promotes the formation of C-C and C-O bonds. It is suggested that the reaction products may be beneficial in the context of wear inhibition. Comparison of the surface structure before compression and after decompression demonstrates that the aldehydes and reaction products are capable of inhibiting irreversible changes in the structure as long as there is at least a monolayer coverage of these species. As a whole, the study sheds light on the chemical behavior of the aldehydes in response to uniaxial compression in nanoscopic contacts that likely applies to other molecules containing carbonyl groups and other metal oxide surfaces.

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