Tribological behaviour of a graphite–phenolic resin solid lubricant under high mechanical load

Graphite can be considered as a suitable solid lubricant in highly loaded contacts and extreme environmental conditions, where conventional liquid lubricants or greases are of restricted use (e.g. in rolling bearings). Indeed, graphite as a solid lubricant, has been reported to exhibit good lubricating properties even at high mechanical loads quite well, though it yielded a limited lifetime. The use of binders, (i.e. resins) stems in a continuance of the adequate lubricating behaviour, as well as, an improved wear resistance. This work aims to understand the friction and wear behaviour of solid lubricating coatings consisting of a graphite–phenolic resin blend. For this investigation various coatings, with three different graphite contents, applied using the simple airspray method on 100Cr6 bearing steel plates of comparable thicknesses were examined. When tested on a ball-on-flat reciprocating motion, we find that compared to pure graphite coatings, the lifetime indeed is significantly enhanced reaching up to 10 000 cycles, whilst keeping a coefficient of friction around 0.1. During the experiments with the higher graphite content specimen a transfer layer on the counterbody was observed as well as a thin compressed layer of material. Raman spectroscopy reveals that the observed carbonaceous tribofilm consists of what could be a rather amorphous graphite-like material which is responsible for the favourable lubricity.