Hydrocarbon materials, such as oil, have traditionally been used to prevent the friction and wear of mechanical components in sliding contact. The advent of chemical vapor deposition technology has piqued interest in the use of solid hydrocarbons as lubricants in systems such as microelectromechanical devices. A detailed knowledge of the molecular-scale mechanisms responsible for lubrication would be invaluable in the design of novel solid lubricants. We are using molecular dynamics to examine the atomic-scale phenomena governing the tribology of hydrocarbon-containing systems. Because boundary layer lubricants, such as self-assembled monolayers, and liquid hydrocarbons are to be studied, the potential energy function must include intermolecular interactions. The new adaptive intermolecular reactive empirical bond-order potential (AIREBO)1 can simulate reactive and non-reactive processes in the gas, liquid, and solid phases. We have conducted extensive simulations that have examined the friction of alkane monolayers (model SAMs) and amorphous carbon films attached to diamond surfaces. Friction as a function of chain length,2 packing density3, and sliding direction have been examined in the model SAM systems. Recent AFM results of Perry and coworkers4 unambiguously demonstrate that decreasing the packing density, or the disorder of the film, increases the friction. Simulations reproduce this trend and provide an atomic-scale explanation for this observation. Friction as a function of film thickness, sp2-to-sp3 ratio, and hydrogen content in the amorphous carbon film systems has also recently been examined. These simulation results will also be discussed.5
*Supported by The Office of Naval Research and The Air Force Office of Scientific Research.
Coworkers: P. T. Mikulski, G. Gao, G. M. Chateauneuf
Stuart, Tutein, and Harrison, J. Chem. Phys.112, 6472-6486 (2000).
Tutein, Stuart, and Harrison, Langmuir16, 291-296 (2000); Ibid. , J. Phys. Chem. B 103, 11357-11365 (1999).