Reaction mechanisms in organophosphate vapor phase lubrication of metal surfaces

This paper discusses the surface chemistry of a number of reactants and species that are probable intermediates in vapor phase lubrication chemistry. In order to understand the mechanism by which arylphosphates [(RO)3PO] such as tricresylphosphate [R=CH3C6H4–] react to form lubricating films on metal surfaces, several of the likely elementary steps are illustrated using experiments with model compounds on the Cu(111) surface. Trimethylphosphite [(CH3O)3P] has been used as a simple model for the phosphates and experiments suggest that the initial step in the decomposition mechanism is P–O bond breaking to form alkoxy groups. Other evidence suggests that under some conditions the decomposition of arylphosphates may be initiated by either P–O or the C–O cleavage to produce adsorbed aryloxy or aryl intermediates. The decomposition of aryloxy groups or aryl groups then leads to the deposition of carbon into the lubricating films. The chemistry of aryl, alkyl, aryloxy, and alkoxy species has been investigated on the Cu(111) surface. In either the alkoxy groups produced by P–O cleavage or the alkyl groups produced by C–O cleavage the presence of β-CH bonds has a strong influence on the rate of carbon deposition onto the surface. In the arylphosphates which would produce either aryl or aryloxy groups, the lack of β-CH bonds leads to greater rates of carbon deposition onto the surface than for either alkyl or alkoxy groups.