Wear of metals in dry sliding contacts is a dynamic process that leads to adhesive transfer, material mixing and oxidation. All of 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.
Recent advances in nanomechanical testing has made it possible to study mechanical properties of small volumes of materials. Tribofilms present an interesting and challenging vista for nanomechanical testing. In recent years, researchers have used nanoindentation and pillar compression testing to determine the properties of tribofilms. While a limited number of studies of this type exist, and measurements are difficult to quantify, there is clearly a trend with tribofilm properties and the wear resistance of a metals and metal-matrix composites (MMCs). In this study, nanoindentation testing of tribofilms revealed that for a wide range of metals (Al, Cu, Ti, Ni) and their MMCs (Al-Al2O3, Ti-TiC, Cu-MoS2 and Ni-WC) that there is a trend of decreasing wear rate with tribofilm hardness. Metals and MMCs 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. Using a wider range of test conditions (e.g. sliding speed, normal load), one can establish additional relationships between tribofilm properties and wear resistance. Furthermore, using recently developed techniques for toughness measurements using nanomechanical methods, a better picture of the overall mechanical properties required for tribofilm stability and wear resistance can be established.