Oxidation‐led decomposition of hexagonal boron nitride coatings on alloy substrates at 900°C: Ti‐6Al‐4V

Hexagonal boron nitride (h‐BN) coatings on Ti‐6Al‐4V substrates undergo complete decomposition in air at 900 °C. This fate is similar to that of this ceramic material on chromia‐former alloys, and unlike that of a mass of powder treated in isolation. As the ceramic and alloy oxidize concurrently, outwardly diffusing aluminum (III) ions but not the predominant titanium (IV) ions react with the boron trioxide that forms around the h‐BN basal plane peripheries. Resultant aluminum borate is incorporated into the growing scale and the boron trioxide diffusion barrier is depleted. By this mechanism, the oxidation of h‐BN is maintained at an enhanced rate, until both this material and its oxide completely decompose. Liberated nitrogen from the oxidation of h‐BN can enter the underlying scale as a randomly distributed solute in rutile solid solution. The post‐coating oxide‐atmosphere interface comprises elongated aluminum borate crystallites protruding through at the boundaries between 3–5 at% nitrogen‐doped rutile grains. It differs significantly from that of oxidized, uncoated Ti‐6Al‐4V, which is occupied by a thin α‐alumina layer atop rutile. This interface does not change with an additional 72 h of heat‐treatment.