OBSERVATION OF THE WEAR SURFACE BY SCANNING TUNNELING MICROSCOPY

Wear tests were conducted on hot-pressed silicon nitride under lubricated sliding contact against AISI 4620 steel using a block-on-ring test apparatus. The worn surfaces of the silicon nitride, coated with 10 nm gold film, and the steel counterface were examined by the scanning tunneling microscope and a scanning electron microscope. The scanning tunneling microscope was operated in air using tungsten tips. Examination of worn surfaces of the silicon nitride and the steel counterface revealed patches of thin film oriented in the direction of sliding, the thickness of which varied between 25 and 40 nm. Closer examination of 350 nm X 350 nm areas revealed the structure of the film. The analysis of the film by energy-dispersive spectroscopy in the scanning electron microscope showed that it consisted of a mixture of iron transferred from the steel counterface and organic deposits from the lubricant. The wear surface of silicon nitride also appeared much smoother than the unworn surface, possibly because of material removal by fine-scale polishing. Evidence of microplowing was also observed.