Effect of HfO2 coatings on the oxidation behavior of ZrB2.

Abstract

Zirconium diboride (ZrB2) is a highly investigated material of the class of ultra-high temperature ceramics (UHTCs) due to its combination of a high melting point (3246 °C), low density (6.09 g/cm³), and a high electrical/thermal conductivity. Accordingly, ZrB2 has a high potential for the space- and aerospace industry as thermal protective shields (TPS) or leading edges of hypersonic vehicles. Unfortunately, the low oxidation resistance in oxidative atmospheres above 1400°C limits sustained usability and is a drawback for such kind of applications, forming unprotective porous zircon oxide (ZrO2) and liquid boria-glass (B2O3). The formation of a mixed oxide scale would slowing oxygen diffusion to the reaction front due to densification and provide an effective barrier. A potential candidate is hafnium oxide (HfO2), which forms a continuous series of high refractory solid solutions with ZrO2. Therefore, this study focuses on the application of thin HfO2 ceramic coatings by means of magnetron sputtering to improve the oxidation resistance of ZrB2. As an initial step, HfO2-coated specimens were annealed in vacuum at 1800°C to improve the adhesion of hafnia with ZrB2 at elevated temperatures. Subsequently, the oxidation studies were performed at 1500°C in a tube furnace under constant air flow (0.4 l/min) for 1 h and 4 h exposure times. After the annealing the HfO2-coatings were still intact. The influence of the annealing as well as the results of the oxidation experiments will be discussed.