A MODEL FOR THE BOUNDARY FILM FORMATION AND TRIBOLOGICAL BEHAVIOR OF A PHOSPHAZENE LUBRICANT ON STEEL

The dynamics of formation and loss of the boundary films formed during the lubricated sliding of steel surfaces were investigated over a range of temperature and applied load. Tests were performed on a cylinder-on-disk machine using a phosphazene lubricant (X-1P), a polyphenyl ether, and mineral oil with and without addition of zinc dialkyldithiophosphate (ZDP). Among these lubricants, X-1P was found to have the best high-temperature, high-load performance. The thickness and refractive index of the boundary films were monitored in situ with an ellipsometer, and the composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). The performance of the lubricants was found to be closely associated with boundary film-forming ability. In order to achieve high load-carrying capacity, a boundary film must be formed. The films formed in X-1P grow more slowly than those in ZDP-containing mineral oil, but they remain thick even at high load and high temperature (250 degrees C). These films are durable and effective in reducing friction and preventing scuffing. The film formed with X-1P consists of a mixture of compounds containing Fe, O, C, F, P, and N. Among the compounds formed, some form of iron fluoride appears to be most important in determining the performance of the film.