Pressure Effects on the Friction Coefficient of Thin-Film Solid Lubricants

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Abstract
Thin solid films on harder backings are now widely used as dry‐film lubricants. An early extension by us of the Bowen‐Tabor adhesion theory of friction showed that the coefficient of friction of a coated backing was equal to only a fraction of that of the coating material, and this fraction was simply the ratio of the mean yield pressure of the coating material to that of the backing. However, we pointed out that the effect of pressure on the shear strength of the coating material was neglected. We have since analyzed and applied to this problem the data on the effect of pressure on shear strength of paraffin, gold, and molybdenum disulfide, using the experimental data of Bridgman as well as of Boyd and Robertson. The principal complication in applying these data to the frictional problem is to allow properly for the elastic (or plastic) deformation of the two sliding solids in calculating the pressure exerted on the coating material during sliding. Our calculated coefficients of friction are in good agreement with the recent measurements of Takagi and Liu on gold‐coated hard steel and the earlier data by Haltner and Oliver on molybdenum‐disulfide‐coated steel. Recently we measured coefficients of friction of thin coatings of paraffin on steel and obtained results which were in good agreement with our calculated values. It is concluded that a sound basis now exists for treating the subject of dry film lubricants. Further research on such systems requires more experimental data on the effect of pressure on shear strength in a variety of indicated polymers and inorganic solids.