Tribo-Oxidation of Steels

Abstract

The present study is concerned with the oxidational wear of EN8 low alloy steel under unlubricated, unidirectional sliding conditions. Wear tests were conducted at various loads and sliding speeds using a pin on disc wear machine in which wear rate, frictional force and pin and disc temperatures were monitored. Wear debris and worn surfaces were examined by means of x-ray diffraction. Scanning electron microscopy was used to gain topographical information of the worn surfaces and also to directly measure the thickness of surface oxide films. Particular emphasis was placed on the analysis of surfaces by Auger electron spectroscopy and x-ray photoelectron spectroscopy, both techniques having been used in conjunction with argon ion etching. Results of the analyses indicated that the oxide films on the worn surfaces were physically homogeneous films which grew separately on the pin and disc surfaces. Furthermore, transitions in the wear rate versus load curves which were observable at certain critical loads were found to be associated with a change in oxide composition: oxide films consisting predominantly of Fe3O4 were found to have greater wear protective properties than those which consisted mainly of αFe2O3 or FeO. A theoretical surface model was used which enabled the contact temperature and the number and size of the asperity contacts to be deduced. Excellent correlation between contact temperatures and the composition of the surface oxides indicated that the contact temperature, and not the much lower general surface temperature, governed the growth of oxide films during wear. A method was devised for calculating values of activation energy (Qₚ) and Arrhenius constant (Aₚ) applicable to tribo-oxidation. Comparison of these values with those relevant to oxidation under static oxidizing conditions indicated that Qₚ was approximately the same in both cases, but Aₚ values were much greater in the case of tribo-oxidation.