Correlation of micromechanical property and microstructure of tribo-layers

Tribological contact often leads to surface deformation, resulting in a substantial increase in dislocation density and a considerable refinement in the microstructural scale. The extensive work hardening associated with this results in significant changes in the mechanical properties of the surface. It is not only the mechanical properties that change, but also the corrosion potential. In some cases, the surface changes enhance the wear resistance of the material. However, in other cases, higher wear rates are found with surface deformation that results in ultra-fine surface structures. Despite the importance of surface deformation, much is unknown about the mechanical properties of the wear induced surface layers. Nanoindentation provides useful information but does not give a good indicator of the ductility. The challenge is to test the mechanical properties of such a fine scale deformed structure. In this work, the micromechanical properties of Ti-6Al-4V worn surfaces after tribocorrosion testing were measured using an in-situ micropillar compression method in the chamber of a scanning electron microscope. Reciprocating tribocorrosion testing was undertaken in 25 vol % Bovine Serum Albumin (BSA) in phosphate-buffered saline (PBS) solution against an alumina counterface, with a load of 0.5N and a speed of 20 mm/s. Tests were conducted under Open Circuit Potential (OCP) conditions and at cathodic and anodic surface potentials, namely at +0.5V and −0.95V. The different test conditions resulted in different extents of surface deformation. This resulted in significant differences in the stress strain curves from the micropillar tests, both in terms of strength and ductility. The microstructure observed by subsequent TEM of the tested micropillars is correlated with the mechanical properties and the reasons for the different mechanical properties are discussed.