SOME FRICTIONAL FEATURES ASSOCIATED WITH THE SLIDING WEAR OF THE NICKEL-BASE ALLOY N80A AT TEMPERATURES TO 250-DEGREES-C

The time-dependent variations of friction coefficient and the contact resistance of a nickel-base high-temperature alloy, N80A, during like-on-like sliding in pure oxygen at temperatures of 20-250-degrees-C were simultaneously recorded and the tribological behaviour correlated with the nature of the sliding contact. A transition to a positive contact resistance always occurred after some time of sliding in the temperature range investigated. However, the time-dependent variations of coefficient of friction showed quite different features at the various temperatures. Corresponding to the times of the transitions in contact resistance, at 20-degrees-C, the friction coefficient increased to a higher value from the initial value while, at 250-degrees-C, it decreased to a lower value; however, at 150-degrees-C, the coefficient of friction remained unchanged after the transition. Scanning electron microscopy observations have shown that the predominant factor for such different tribological responses at the various temperatures is the nature of the contact between the surfaces. At 20-degrees-C, the real contact areas mainly consisted of loosely compacted particles while, at 250-degrees-C, the load-bearing areas were smooth wear-protective oxide layers. At the intermediate temperature, 150-degrees-C, the load-carrying areas comprised both types of contact. A mathematical model has been proposed and is used to relate the frictional behaviour to morphological features of the wear surfaces developed at the various temperatures.