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

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.

Effect of high temperature on the characterization of fretting wear regimes at Ti6Al4V interfaces

“…12 and 13. This is a phenomenon that was studied extensively by Bowers et al [43,44] in the late 1960s and early 1970s for polymers and thin solid films and then again by Singer et al [45] in the early 1990s for MoS 2 coatings. A Hertzian contact model was used to explain why the friction decreases with increasing load, and to predict what the friction would be at a given load.…”

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“…Although Amontons' Law (Le., the friction coefficient is independent of load) holds for many combinations of materials [24], a load-dependent friction behavior is common for many solid lubricants such as polymers [25,26], and thin, inorganic coatings on hard substrates [27,28,29,30].…”

“…4 for In (a soft metal) on steel and for MoS 2 on Cr [4]. A model that has been used to explain this load dependence is the Hertzian contact model [26,27,28,29]. The friction coefficient is derived from Eq.…”

On the wear of brake pads

“…Therefore, in terms of adhesion theory of friction (Eq. 3) [3], the first wear model is derivated as Eq. 4.…”