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

• α-AlB 2 , the phase prototype of transition metal diborides, was synthesized as thin film. • Changes of Ar pressure slightly alters the stoichiometry and thus phase composition of AlB y films. • AlB 2.27 thin film exhibits large columnar α-AlB 2 grains with small amorphous B regions. • AlB 2.27 thin film clearly overshadows binary transition metal diborides in oxidation resistance. Despite AlB 2 is the most typical structure prototype of transition metal diborides (TMB 2 ), studies on AlB 2 thin films are scarce. Furthermore, although Al is the primary alloying element for TMB 2 to improve their oxidation resistance, no such data are available for AlB 2 thin films. Here, we develop AlB x thin films through non-reactive sputtering of an AlB 2 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 p Ar = 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 AlB 2.27 thin film exhibits small platelet-like amorphous B regions next to the large columnar α-structured AlB 2 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, and maintains its AlB 2 structure during annealing up to ∼950 °C. The AlB 2.27 thin film also shows exceptional oxidation-resistance with an onset temperature for the formation of oxides (α-Al 2 O 3 and o-Al 18 B 4 O 33 ) between 950 and 1000 °C when exposed to lab-air.