The thermal chemistry of oxide films and lubricant overlayers on iron surfaces.

摘要

The chemistry of thin oxide films on iron substrates was investigated during annealing in ultrahigh vacuum (UHV), using x-ray photoelectron spectroscopy (XPS). Clean oxide films formed by both UHV oxygen exposure and air exposure were studied on polycrystalline iron and a Fe(110) single crystal; in addition, the chemistry of oxide layers covered with thin films of poly-{dollar}alpha{dollar}-olefin (PAO), a hydrocarbon lubricant, and zinc dialkyldithiophosphate (ZDP), an antiwear additive, in PAO were investigated. XPS was used to measure the concentrations of surface species and the amount of iron in various oxidation states. Nonlinear least-squares fitting of the iron oxidation state data was used to determine the kinetics of oxide film reactions.; Clean oxide films underwent several thermal reactions, the most important being iron cation diffusion into the oxide from the metallic substrate at temperatures {dollar}geq{dollar}50{dollar}spcirc{dollar}C, resulting in a net reduction of the oxide; and oxygen diffusion from the oxide into the substrate at {dollar}sim{dollar}325{dollar}spcirc{dollar}C on oxide film removal. Little difference was seen between UHV polycrystalline iron and {dollar}sim{dollar}475{dollar}spcirc{dollar}C on Fe(110), resulting in oxide film removal. Little difference was seen between UHV and air oxidized specimens.; PAO overlayers increased the rate of oxide film removal, especially at temperatures below 300{dollar}spcirc{dollar}C. A model for the oxide reactions was developed and the rate constants were measured. The effect of reaction of hydrocarbon lubricants with oxide films in rubbing contacts was estimated from the kinetic data.; Adding ZDP's to the PAO overlayers inhibited both oxide reduction by iron cation diffusion and oxide removal by surface-to-bulk oxygen diffusion. The rates for these processes were decreased by a factor of {dollar}sim{dollar}2. Zn and P from the ZDP were eliminated from the surface at {dollar}sim{dollar}400{dollar}spcirc{dollar}C and {dollar}sim{dollar}600{dollar}spcirc{dollar}C, respectively. Based on these results, the role of ZDP's in lubrication is discussed and compared to previous studies.