Influence of Al2O3 coatings on the oxidation behavior of ZrB2

Kurzfassung

Properties such as a high melting point >3000°C and a low density of 6.09 g/cm3 project ZrB2 as a suitable candidate for sharp leading edges or the thermal protective system (TPS) of hypersonic missiles and re-entry vehicles. The formation of a porous ZrO2 scale and volatile B2O3 glass at temperatures above 1500°C offers no long-term protection against oxidation. Protective overlay coatings such as Nb and HfO2 have shown an improvement in oxidation resistance of ZrB2. In this study, Al2O3 coatings have been investigated as a new potential candidate for protective overlay coatings on ZrB2. The coatings were applied by means of slurry deposition (Reactive Bonded Aluminum Oxide (RBAO)). Subsequently, the coated specimens were oxidized at 1500°C, 1600°C, and 1700°C for up to 60 min in a tube furnace under controlled atmosphere. On one hand, the Al2O3 scale reacted with the liquid B2O3 and formed a solid solution Al18B4O33 at the surface. The stabilized glass prevented the B2O3 evaporation and reduced the oxygen diffusion to the oxidation front of ZrB2 as long as it remained at the surface. On the other hand, the precipitation of secondary ZrO2 induced the formation of a dense ZrO2 oxide scale at the surface. The dense oxide scale provided protection against oxygen diffusion to the oxidation front of ZrB2 even though the protective Al18B4O33 and B2O3 left the surface. The results will be presented and discussed in detail.