Tribofilm formation on boron carbide in sliding wear

Abstract

A hot isostatically pressed boron carbide (B 4C) containing some free carbon was evaluated in self-mated sliding tests. Special attention was paid to the formation of tribofilms and their effect on the friction and wear properties. Sliding tests were performed in a pin-on-disc configuration pressing pins with rounded ends towards a rotating disc. The effects of load, sliding speed and relative humidity (RH) of the surrounding air were particularly investigated. The friction was continuously measured and the wear volumes of the pins were estimated after different sliding distances. The worn surfaces were characterised utilising scanning electron microscopy and X-ray diffraction. It was confirmed that B 4C forms tribofilms of oxides that have pronounced effects on the wear and friction properties. Depending on the load conditions and the surrounding humidity, transitions to low friction levels occur. At suitable conditions of high RH and unexcessive loads, the material shows friction coefficient values around 0.1, small wear rates and smooth tribosurfaces. The conditions for these transitions are described and discussed.