Relationship between molecular structures and tribological properties of phosphazene lubricants

Cyclotriphosphazene lubricants were synthesized and the relationship between their structures and tribological properties was investigated using an optimol SRV oscillating friction and wear tester and one-way reciprocating friction tester. The elemental composition and chemical nature of the antiwear films generated on steel surface were analyzed on a scanning electron microscope with a Kevex energy dispersive X-ray analyzer attachment (SEM–EDS) and X-ray photoelectron spectrometer (XPS). It was found that aryloxyphosphazene with polar substituent as a lubricant of steel–steel and steel–aluminum pair gave low wear, while aryloxyphosphazene with nonpolar group on the phenyl pendant led to high wear. Phosphazene provides poor lubricity for the steel–aluminum system under low load (0.5–3 N). The XPS analytical results of the antiwear films generated on the steel and aluminum surface indicate that phosphazene reacted with steel or aluminum counterface and formed a surface protecting film consisting of fluoride and organic compounds containing O, C, F, N, and P during friction. This contributes to reduce the friction and wear of steel–aluminum system.