Abstract

Wear of metals in dry sliding contacts is governed by third body flows leading to dynamic processes such as adhesive transfer, material mixing and oxidation. These phenomena, along with the mechanical deformation of the near-surface, lead to the formation of tribofilms, which are also called mechanically mixed layers. Microstructural and chemical modification of tribofilms lead to significantly different properties from the bulk material that is subjected to dry sliding wear. In some instances, tribofilms can be stable and resist wear and in other cases, there can be persistent wear flow related to tribofilm instability.

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 load-bearing capacity, resulting in higher wear resistance. However, more recent studies of MMC tribology have shown that MMCs can form tribofilms with higher hardness and better wear resistance than metals without reinforcements. This creates opportunities for engineering MMCs and the tribofilms forming during service to better enhance wear resistance and sustainability.

In this presentation, we review recent advances in the understanding of MMC coatings made by cold spray and thermal spray processes. Coatings are tested in sliding wear conditions. 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 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.