Ultra-low friction of ascorbic acid as polyethylene glycol additive in graphite-like carbon film/bearing steel interface

In this work, the tribological properties of polyhydroxy molecule ascorbic acid as lubricating additive was investigated in graphite-like amorphous carbon film/bearing steel interface under polyethylene glycol lubrication. It was shown that ascorbic acid is an excellent friction modifier in graphite-like carbon film solid-liquid lubrication. The friction coefficient decreased from 0.12 of polyethylene glycol to around 0.02 (83.3 % reduction) of polyethylene glycol with >1 wt% ascorbic acid. Ascorbic acid had a little effect on the wear rate of graphite-like carbon film but promoted the steel ball wear. Ascorbic acid inhibited the oxidation of worn surface of steel ball and induced a more oxidized carbon film surface. The oxidized carbon surface and the increased contact area due to steel ball wear facilitated the adsorption of polyethylene glycol molecules at the sliding interface, which helped to establish fluid lubrication film and thereby resulted in ultra-low friction. This suggests that ascorbic acid is a promising candidate friction modifier to design novel ultra-low friction solid-liquid composite lubrication systems based on amorphous carbon films.