Evidence of water chemisorption during graphite friction under moist conditions

The influence of water on the tribological properties of carbon materials is often explained by the lubricating properties of water. In this study, the interactions between water and the active sites created by the friction, and their influences on the friction coefficient and on the wear are analysed. In order to investigate the chemical reactions under a mechanical stress, experiments were performed under a controlled atmosphere in a planetary ball mill. The chemical reactions between water and graphite were examined by using isotopic labelled water, by doing a chemical balance of the gas phase in the jar and by analysing the surface of the milled material by temperature programmed desorption coupled with mass spectrometry. From the experimental results, a chemisorption mechanism could be proposed to explain how the graphite lubricating properties depend on water. Since the phenomena in the ball mill are similar to those observed with a classical pin-on-disc tribometer, the results obtained could therefore be extrapolated to the graphite behaviour in a conventional tribometer system.