From High-Temperature Tribology to Thermal Spraying: Engineering Surfaces Through Wear Mechanisms

Over the years, tribologists have dedicated substantial effort to the engineering and optimization of surfaces. The selection of materials, alongside surface processes, will depend heavily on the demands of the environment. Extreme environments, such as high temperatures, are among the most difficult scenarios, where many variables must be accounted for in the design phase. When dealing with high temperatures, the formation of oxides is expected, and in some cases even desired. This is because many alloys were developed with the specific intention of forming certain types of oxides that act as solid lubricants at such temperatures. Classic examples include cobalt-based alloys, which can form a compact and dense oxide layer at the tribological interface, commonly referred to as a glaze layer, thereby further reducing wear and friction. Nonetheless, the formation of such layers is dependent on other factors, which can limit their performance. The running-in phase prior to the formation of such glaze layers is usually accompanied by intense wear rates. Also, the stability of such layers, as well as their need to retain the generated debris at the interface, can further decrease their performance. Therefore, we have been developing coatings that mimic the composition of such surfaces, bypassing their formation phase during running-in and eliminating the need for their formation conditions. Several coatings based on cobalt and chromium oxides, which are the major constituents of such layers, were developed using suspension plasma spraying and tested under representative environmental conditions. It has been observed that such surfaces have the potential to reduce overall wear as running-in is minimized. Moreover, these coatings can be developed on different substrates, such as nickel-based superalloys, which display superior creep performance compared to cobalt-based ones. This makes such surfaces attractive for environments where glaze layers are usually unstable, such as cyclic applications like gas turbine engines, and for surfaces where the wear mode usually does not develop a glaze layer, such as sliding interfaces.