The effect of partial pressure of oxygen on the tribological behaviour of a nickel-based alloy, N80A, at elevated temperatures

Abstract

In this study, the frictional behaviour and wear of a nickel-based alloy, N80A, as a function of partial pressure of oxygen in a mixture of oxygen and argon at temperatures to 600 °C, were investigated on a pin-on-disk reciprocating sliding wear machine. At temperatures below 250°C, a maximum was observed in the relationship between wear and partial pressure of oxygen in the environment. However, at temperatures above 250°C, wear was almost independent of the partial pressure of oxygen. A transition in friction and contact resistance with sliding time was usually observed within the total sliding time of 6 h, except in argon below 400 °C, where the contact between the pin and the disk was always of metal on metal. The general features of variations in friction coefficient with sliding time at the various partial pressures of oxygen were similar to those in pure dry oxygen at a given temperature, although the transition time increased dramatically at partial pressures of oxygen below 0.1 atm. Scanning electron microscopy observations showed that compact wear debris particle layers were developed at partial pressures of oxygen above 0.1 atm, corresponding to the transition in friction coefficient with time. However, only very small areas of oxidized smooth load-bearing areas were observed at low partial pressures of oxygen and at lower temperatures. The observed experimental results are explained on the basis of a wear model for metals at elevated temperatures. The effect of partial pressure of oxygen on the tribological behaviour of the alloy has been incorporated into the model by derivation of relationships for the size and generation rate of wear debris particles as a function of partial pressure of oxygen. Reasonably good agreement between the theory and experiment results has been obtained.