Effect of bridged cyclotriphosphazenes as lubricants on the tribological properties of a steel-on-steel system

The friction and wear behavior of a steel-on-steel contact lubricated by phosphazene-type additive tetrakis(3-trifluoromethylphenoxy)-bis(4-fluorophenoxy)-cyclotriphosphazene (X-1P) and novel synthetic bridged cyclotriphosphazenes was investigated on a oscillating friction and wear tester. The thermal stability of the lubricants was investigated by means of thermogravimetric analysis. It was found that bridged cyclotriphosphazenes give lower wear volume loss and higher thermal stability than X-1P. In particular, the bridged cyclotriphosphazene with aromatic-ring side branch structures has the best antiwear ability and thermal stability among the investigated lubricants. The worn surfaces were analyzed by means of scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). It was found that tribochemical reaction was involved in the sliding of steel against steel under the lubrication of the synthetic lubricants, with the generation of a protective layer composed of inorganic fluoride FeF₂ and organic compounds consisting of C, F, O, N and P, which contributed to improve the friction and wear behavior.