Effect of Simple Lubricants on Deformation and Wear in Concentrated Sliding Contact

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Abstract
Friction measurements were performed on the ball-on-cylinder machine at a low sliding speed and high loads to establish conditions of isothermal boundary lubrication. Friction coefficients, as well as the topology of the wear tracks, were measured under different conditions of lubrication: dry contact in laboratory air, lubricated contact with pure n-hexadecane, and a saturated (1 percent) solution of palmitic acid in hexadecane. Dry sliding produced a friction coefficient of 0.45 and wear characterized by heavy fissuring. The palmitic acid solution provided a low friction coefficient of 0.12 and a burnished, crack-free sliding track, as expected. When highly purified n-hexadecane was used as the lubricating medium, the recorded friction coefficient was as high as 0.75 but the sliding track was at first similar to that produced in 1 percent palmitic acid; wear occurs in the translation of elongated, thin platelets parallel to the surface. This mode of wear particle formation has apparently not been reported elsewhere and differs markedly from the protruding shear plates observed in dry sliding. It is concluded that the action of pure hexadecane is to redistribute part of the normal and shear loads to regions between those of metallic conjunction in the sliding contact. This redistribution of stresses prevents the occurrence of stress reversal fatigue and leads to reduced wear even if the friction coefficient is larger.