NEW INSIGHTS INTO ADHESION AND LUBRICATING PROPERTIES OF GRAPHITE-BASED TRANSFER FILMS

Thin transfer films were formed on steel surfaces during friction against graphite-based pins. Sodium thiosulfate (Na2S2O3) was added to graphite to improve the film formation. The lubricating behaviour of these transfer films was tested during friction experiments against a steel block. Microstructural analyses were performed using classical theta/2theta and grazing incidence X-ray diffraction, Raman and micro-Raman spectroscopy, and transmission electron microscopy. Transformations of the graphite structure were observed. In particular, a turbostratic phase was identified in the thinner layers of the transfer films. To study the influence of the gaseous environment, friction tests were performed in ambient air, dry air and dry nitrogen. The friction coefficient was shown to increase dramatically from 0.03 to more than 0.11 in the presence of water vapor as the endurance life decreased. The apparent shear resistance increased with humidity from 10 MPa up to about 18 MPa. An explanation in terms of interfacial thin film shear is given. In ambient air, blisters and fractures were seen, revealing that the entire film has reacted to accommodate velocity.