Reactively Grown Al/Si-based Top Coatings Protecting TM-diborides (TM = W, Ti, Hf) Against High-Temperature Oxidation

Architectural designs involving Al- and/or Si-containing top coatings present an effective approach to protect oxidation-sensitive materials such as binary transition metal (TM) diborides against high-temperature oxidation. Using reactive arc evaporation, Al-Si-N, Al-Si-O, and Si-O top coatings were applied on sputter-deposited WB1.9, TiB2.7, and HfB2.4 thin films, respectively. A pure Si adhesion layer was introduced prior to the deposition of both oxide-based top coatings (Al-Si-O & Si-O) to prevent in-situ oxidation of the diboride base layers. During thermal annealing up to 1200 °C, Al-Si-O and Si-O provided outstanding oxidation resistance on top of TiB2.7 and HfB2.4, while limited adhesion was observed on WB1.9 at elevated temperatures. Contrary, Al-Si-N protected architectures suffered from accelerated oxidation beyond 800 to 1000 °C. Isothermal annealing in ambient air at 1200 °C for 30 h confirmed the long-term stability of the oxide-based top coatings, revealing no decohesion or oxygen inward diffusion beyond the Si interlayer. Complementary TEM analysis showed that the Si-O top coating forms a dense SiO2 scale above the columnar HfB2.4, while the formation of a mullite scale (3Al2O3·2SiO2) and pronounced recrystallization is recorded for the Al-Si-O protected HfB2.4. In summary, the Al-Si-O- and Si-O-based top coatings demonstrated exceptional high-temperature oxidation resistance, fully protecting the TM-diborides from oxidative attack.