Anisotropic friction behaviour of highly oriented pyrolytic graphite

Friction tests on two surfaces of highly oriented pyrolytic graphite that are parallel and perpendicular to the direction of the graphite basal plane were performed under various test conditions. The average friction coefficient value of the perpendicular surface (0.25) is higher than that of the parallel surface (0.1). The friction coefficient of both parallel and perpendicular surfaces decline when the environment is changed from wet air to wet nitrogen and the trend is more pronounced for the perpendicular surface. The perpendicular worn surface has been observed to be much rougher and owns more edge faces than the parallel worn surface. The amount of dangling bonds and chemisorbed oxygen measured on both surfaces after friction is quite different. The influence of atmosphere on the friction of graphite is attributed to the chemical reaction of dangling bonds with gases, which can affect the adhesive force of the two contact surfaces. The amount of dangling bonds generated on the sliding surface is confirmed to impact the sensitivity of graphite to gases during friction.