The Tribooxidative Behavior of Rutile-Forming Substrates

Abstract

AbstractAn assessment of the literature indicates that in high temperature air not only titanium,but also titanium carbide (TiC) and titanium nitride (TiN) react to form rutile, the mostthermodynamically stable polymorph of titanium dioxide. It has been recently hypothesizedthat slightly oxygen-deficient rutile (TiO2-x) behaves as a low shear strength, lubricious oxide.Although the oxidation kinetics-driven reaction of the less stable TiC results in thickeroxide layers than on TiN, the most prevalent film morphology and oxide structure on both TiC and TiN are the same as those generated on the much softer Ti and its alloys. In mostcases of static oxidation and tribooxidation at high temperatures, the oxide layers grow in extremely thin (1 to 5 μm) sheets, which periodically delaminate from the substrate. The kinetics of delamination are highly dependent on substrate properties. These sheets form an array of loosely cohering (under static conditions) or tightly cohering (under triboconditions) scales, similar to the morphology of multilayered pastry dough. The most thermodynamically stable [(110)] cleavage planes of rutile tend to align preferentially in the plane of the peeling flakes, but without any azimuthal order. It is suggested that rutile can be rendered lubricious (a) by this unique structure, (b) by keeping its shear strength minimal by intrinsic (i.e., environment-independent) control of the number of oxygen vacancies within the oxide layers, and (c) by tailoring the load-carrying substrate used to form the rutile films in-situ. Accordingly, the removal rate ofrutile and the friction coefficient provided by the oxide/substrate tribosystem may be further controlled by taking advantage of (a) the high, but still significantly different hardnesses and ideal habit plane orientation of the TiC and TiN mother matrix underlays, and (b) the differences in the reaction kinetics of layered oxide growth on these highly oriented, hard metal coating matrices at elevated temperatures, in air.