Scuffing initiation in metals sliding against copper under non-lubricated conditions

Metallic components in sliding contact are sometimes subjected to high-loads with little or no lubrication. Such starved conditions can lead to a phenomenon called scuffing. Various definitions exist for this term, but in the present case, three criteria were used to signal its onset: changes in friction, vibrations, and noise, coupled with surface examination. On this basis, scuffing initiation was determined for seven technically pure metals (Al, Mo, Nb, Ta, Ti, W, Cu) and stainless steel, all rubbing against Cu. A flat-ended pin-on-disk test configuration was used with normal loads of 1-3 N, and with step-wise increases in sliding speed from 0.16 to 2.56 m/s. Al was only weakly resistant to scuffing, presumably due to its solubility in Cu, its high ductility and its relatively low elastic modulus. Niobium provided satisfactory sliding behavior at low speeds and loads, presumably due to protective oxides; however, it scuffed at higher loads when the oxide broke through. Stainless steel, Mo, and Ta had higher friction coefficients than Al and Nb, presumably because the relatively high strengths of the former prevented severe wear even when their oxide films failed. Like Al, Ti scuffs on Cu, probably because of its high relative solubility; however, Ti's higher elastic modulus resists the more severe forms of surface damage than does Al. Of all the materials slid against Cu, W displayed the least scuffing, even under maximum speed and load. Tungsten's negligible solubility in Cu may have reduced its adhesion, and W's high elastic modulus resisted shear-deformation, even at high frictional heating. Self-mated Cu couple scuffed when the speed was increased. The oxides on the Cu surface serve as solid lubricant avoiding scuffing at lower speeds.