Metal matrix composites (MMCs) provide a significant advantage for their tribological properties compared to pure metals. The traditional thought process is that ceramic reinforcements enhance the mechanical properties and load-bearing capacity, resulting in higher wear resistance. However more recent studies of MMC tribology have shown that the formation of tribofilms with enhanced hardness. The importance of the structure and properties of these tribofilms in terms of the tribological performance and wear resistance of MMCs is the topic of this presentation.
There are many methods of manufacture for MMCs in bulk form and a few options for manufacturing them as coatings. Cold spray is one such coating deposition technique that is increasingly being used to fabricate MMC coatings. We have recently fabricated and studied the tribological performance cold spray coatings of Al-Al2O3, Ti-TiC, Cu-MoS2 and Ni-WC. Coatings are tested in sliding wear and sometimes fretting wear test conditions. For both cases, post-characterization of cross-sectioned wear scars reveals microstructural evolution near surface leading to formation of tribofilms that provide wear resistance and friction control. Structure and properties of the tribofilms are determined with SEM, TEM, EDS, Raman spectroscopy and nanoindentation. Generally, tribofilms are found to be mixtures of the two components in the MMC, but with finer microstructure and some level of oxidation that leads to higher hardness. The tribological performance of cold sprayed MMC coatings was found to depend significantly on the nature of the third bodies formed by the wear process. Combining results for all the MMC systems studied, a general trend of decreasing wear rate was found with respect to tribofilm hardness, which while reminiscent of Archard’s Law clearly indicates the properties of third bodies is important for realizing wear resistance for MMCs.