Effect of SiC, TaB₂ and TaSi₂ additives on the isothermal oxidation resistance of fully dense zirconium diboride

The oxidation resistances of ZrB₂ containing SiC, TaB₂, and TaSi₂ additions of various concentrations were studied using isothermal thermogravimetry at 1200, 1400, and 1500 °C, and specimens were further characterized using x-ray diffraction and electron microscopy. Increasing SiC concentration resulted in thinner glassy surface layers as well as thinner ZrO₂-rich underlayers deficient in silica. This silica deficiency was argued to occur by a wicking process of interior-formed borosilicate liquid to the initially-formed borosilicate liquid at the surface. Small (3.32 mol%) concentrations of TaB₂ additions were more effective at increasing oxidation resistance than equal additions of TaSi₂. The benefit of these additives was related to the formation of a zirconium-tantalum boride solid solution during sintering, which during oxidation, fragmented into fine particles of ZrO₂ and TaC. These particles resisted wicking of their liquid/glassy borosilicate encapsulation, which increased overall oxidation resistance. With increasing TaB₂ or TaSi₂ concentration, oxidation resistance degraded, most egregiously with TaB₂ additions. In these cases, zirconia dendrites appeared to grow through the glassy layers, providing conduits for oxygen migration.