Shear Strength and Tribological Properties of Stearic Acid Films—Part II: On Gold-Coated Glass

This paper reports measurements of the shear strength of dry and lubricated Au/Au, Al/Au and glass/Au couples from measurements of the frictional force during sliding of a hemispherical pin in contact with a flat side. The Au and Al surfaces are generated by vacuum deposition of thin metal films on glass. Shear strength is measured at a sliding speed of 60 μm s⁻¹ and at contact stresses ranging from ~ 0.05 to 0.8 GPa. Lubrication is achieved by depositing a stearic acid on the slide. The shear strength of dry glass/Au sliding interfaces is found to increase linearly with contact stress but decreases slightly with increasing thickness of the gold film. The shear strength of dry Au/Au interfaces is larger than that of dry glass/Au. Stearic acid does not protect gold from mechanical wear during sliding because the acid is expelled from the gold/gold interface, even at the lowest compressive stresses used. These observations are interpreted in terms of the weak adhesion of stearic acid to gold. In contrast, stearic acid sustains mechanical shear in interfaces consisting of gold sliding on either bare glass or aluminum, to which the acid adheres. These observations suggest that a boundary lubricant film protects against mechanical wear if it adheres only to one of the contacting surfaces.