Oxidational wear modelling: I

Abstract

This paper is the first of a series in which increasingly complex oxidational wear models are applied to a tribosystem involving the unlubricated sliding of high chromium ferritic steel pins against austenitic stainless steel discs at speeds between 0.23 and 3.3 m s −1. The most simple oxidational wear model assumes that there is no significant oxidation of the contacting surfaces at the general surface temperature of the pin nor at the general surface temperature of the disc. The model also assumes that a very small number of dominant large plateaux of contact oxidize (at a contact temperature, which is normally much higher than the general surface temperatures of either the pin or the disc) until they reach critical thicknesses which are related to the Archard K factor. The contact temperature is itself related to the division of heat at the plateaux interfaces, a quantity which can be estimated in terms of a surface model in which N circular contacts (each of radius A) are assumed to occur at each of the contacting plateaux of thickness on the pin and on the disc. Since the simple oxidational wear theory also involves both the number N of contacts and the thicknesses of the oxide film on either the pin or the disc, it can be seen that it should be possible to find suitable values of N that will satisfy both the experimentally measured division of heat and the experimental wear rate coefficient, provided that the oxide thicknesses are known. The main thrust of this paper is to show that, by measuring the critical oxide thicknesses on the pin and on the disc in the scanning electron microscope, it is possible to compute tentative tribological oxidation constants for these particular steels from associated measurements of the wear rate coefficient and the division of heat, using a computer spreadsheet technique which is particularly user friendly. The importance of computer modelling for finding definitive values for the tribological oxidation constants of a tribosystem, and hence for predicting the magnitudes of the contact temperature and the wear rate of a triboelement undergoing oxidational wear in that system, is also discussed.