High temperatures and harsh operating conditions lead to wear in aero-engine components such as the disc-blade interface. Under dry sliding conditions, metal-to-metal contact leads to adhesive transfer and wear because of their similar chemical properties. To reduce this, dissimilar material combinations can be used which reduces friction between the surfaces. However, implementing this solution can be challenging because it requires materials capable of providing both strength and resistance to heat/oxidation. This means incorporating similar elements into the alloys to meet these requirements.
An additional way to avoid metal-to-metal contact is to have an oxide layer on the surface. As sliding begins, the transient oxide layers on the surface break down due to adhesive wear. This exposes fresh metal, which then oxidises, generating wear debris in the contact area and resulting in abrasive wear. These debris particles collect in the grooves created by the abrasive wear and form a compacted oxide layer, which appears smooth and glossy under visual inspection, known as 'glaze'. The glaze layer serves as a protective coating, enhancing the durability of the interface by reducing oxidation, friction and wear, However, the effectiveness of glaze in reducing surface degradation is highly dependent on the rate of generation of glaze and the breakdown of glaze, so the balance between these is essential. Moreover, it is influenced by various operating and environmental conditions such as temperature, load, speed, humidity, and composition of materials etc. Temperature has been widely studied, however, there is limited research available on other factors like load, speed, environmental factors etc. Therefore, this study aims to investigate the influence of different factors on the formation of glaze, including contact pressure, sliding time, and historical dependence, to determine the optimal conditions for maintaining an effective glaze.
In order to carry out the above-mentioned aims, wear tests were carried out using a ball-on-flat test configuration. This study primarily investigated the effect of operating temperature and the load conditions. Initially, a similar material combination was used for both disc and the pin, then a dissimilar material combination to understand the influence of alloying elements on the glaze generation process. Finally, the history dependence of glaze was studied to understand whether having a pre-glazed surface reduces severe wear. Both friction data and surface examinations were conducted post-test. The results have shown that temperature has a higher influence on glaze generation compared to pressure. At lower temperatures, severe adhesion and abrasions were predominant, while at higher temperatures, mild oxidation was observed. The temperature thresholds vary for different materials depending on their chemical composition. For example, nickel alloys exhibited low wear rates above 250°C, whereas cobalt-based alloys required temperatures above 400°C. The chemical composition of the layers formed at different temperatures were different and this determined the effectiveness of these layers in providing wear resistance. Dissimilar material tests indicated that a certain combination (i.e. Cobalt with Nickel based alloys) can have a positive influence by reducing surface degradation and wear rate especially when there is a higher variation among the alloying elements. The history dependence of glaze has a positive effect by reducing the severe wear seen at low temperatures to an extent however, there is no indication of no glaze layers forming. This suggests that further tests are required to confirm whether this is valid for all conditions and materials.
Finally, a wear map was produced with the input parameters such as temperature, load and the wear rate for like-on-like material tests. This allows the prediction of wear behaviour at various test combinations.
