The crucial influence of Al on the high-temperature oxidation resistance of Ti1-xAlxBy diboride thin films (0.36 ≤ x ≤ 0.74, 1.83 ≤ y ≤ 2.03)

The high-temperature oxidation resistance and mechanical properties of Ti_1-xAl_xB_y (0.36 ≤ x ≤ 0.74, and 1.83 ≤ y ≤ 2.03) films grown by hybrid HiPIMS/DCMS co-sputtering from TiB₂ and AlB₂ targets at substrate temperatures (<180 °C) are studied. The air-annealing experiments conducted at temperatures ranging from 700 to 900 °C reveal a strong correlation between the starting Al concentration and the oxidation resistance. Low Al content films (x ≤ 0.49 ± 0.03 in the as-deposited state) show higher oxidation rates and develop B-depleted porous oxide scales as the original film is consumed. In contrast, oxides growing on top of high-Al content films (x ≥ 0.58 ± 0.03) are compact, composed of amorphous alumina (Al₂O₃) and borate (Al₁₈B₄O₃₃), which passivate the surface against oxidation effectively. Oxide scales on films with x ≥ 0.58 are, on average, 60 % harder and have 18 % higher elastic moduli. The hardest scale grew on the Ti_0.42Al_0.58B_1.87 film, with the nanoindentation hardness of 27.3 ± 2.7 GPa, which is comparable to that of as-deposited TiAlN, used widely for high-temperature wear protection. Electron microscopy also shows that for x ≥ 0.58 ± 0.03, the oxide scales adhere well to the bottom unoxidized portions of Ti_1-xAl_xB_y film, which is explained by a better match of the respective thermal expansion coefficients.