Microstructure, mechanical properties, thermal decomposition and oxidation sequences of crystalline AlB₂ thin films

Abstract

Despite AlB₂ is the most typical structure prototype of transition metal diborides (TMB₂), studies on AlB₂ thin films are scarce. Furthermore, although Al is the primary alloying element for TMB₂ to improve their oxidation resistance, no such data are available for AlB₂ thin films. Here, we develop AlBₓ thin films through non-reactive magnetron sputtering of an AlB₂ compound target and investigate their microstructure, mechanical properties, thermal stability and oxidation resistance. Keeping the substrate temperature at 700 °C and increasing the Ar pressure during deposition from pAr = 0.4 to 0.8 to 1.2, Pa, the films‘ chemistry slightly varies between x = 1.99, 1.97, and to 2.27, respectively. Detailed transmission electron microscopy shows that the highly (0001)-oriented AlB₂.₂₇ thin film exhibits small platelet-like amorphous B regions next to the large columnar α-structured AlB₂ crystals. In the as deposited state, this film exhibits an indentation hardness and elastic modulus of 19.2 ± 1.2 GPa and 331.8 ± 14.4 GPa, respectively. Between 850 and 900 °C, the AlB₂.₂₇ thin film starts to decompose into tetragonal (t-) AlB₁₂, but still maintains dominant α structure up to 950 °C. At 1000 °C, the thin film is completely decomposed into t-AlB₁₂ and hexagonal AlB₁₀. The AlB₂.₂₇ thin film also shows exceptional oxidation-resistance with an onset temperature for the formation of oxides (α-Al₂O₃ and o-Al₁₈B₄O₃₃) between 950 and 1000 °C when exposed to lab-air.