The Effect of Anion Vacancies on the Tribological Properties of Rutile (TiO2–x)

Abstract

An assessment of literature data identified the anion-deficient rutile (TiO_2–x) titanium dioxide crystal structures as promising model materials for fundamental studies on lubricious oxides. In certain atmospheric environments and especially at high temperatures, a progressively larger number of oxygen vacancies are generated, both on the surface and in the bulk of the various anion-poor versions of the TiO_2–x lattice. Progressively higher vacancy generation leads to the creation of new crystallographic shear (CS) systems which in turn predominate over the actions of the natural cleavage planes. These CS systems first reduce then increase crystal strength and thus influence the predicted tribological behavior of rutile, where each region may be associated with low friction/high wear and high friction/low wear, respectively. Beyond a brittle-to-ductile transition temperature near 0.7 T_M ≅ 1300°C, the effects of anion vacancies and the high strain energy CS planes are preempted by the increased thermal activity of the lattice. Rutile attains its ultimate softness and ductility due to increased dislocation motion. It is also predicted that the friction of rutile is higher and its wear lower in vacuum or in reducing gases than in air, and its shear strength will be higher on the surface than in the bulk in environments conducive to anion vacancy formation.