The Friction of Very Thin Solid Film Lubricants on Surfaces of Finite Roughness

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Abstract
The various theoretical hypotheses that have been put forward to explain the effect of the thickness of solid lubricant films on the friction between relatively sliding surfaces are described. The rise in friction that occurs with decreasing thickness when the films are very thin is discussed in detail and a new explanation of this phenomenon based upon an increasing degree of film penetration is put forward. A theoretical model to describe such a process is set up. The predicted variation of the friction made on this basis is in good agreement with existing experimental data. Experiments on a pin disc machine that were carried out to provide further evidence for checking the postulated mechanism are also described. Friction coefficient results obtained from these experiments with films of different thicknesses on surfaces of different roughnesses provide further evidence to support the given interpretation of the friction process. Explanations for certain discrepancies between the theoretical and experimental results are offered.