Improved oxidation resistance of B-overstoichiometric Hf0.33-xAlxB0.67+y compared to Ti0.33-xAlxB0.67+y thin films: The role of point defects on grain boundary composition

• B-rich Hf 0.13 Al 0.18 B 0.69 films form dense, passivating oxide scales at 700 °C, unlike Ti 0.10 Al 0.19 B 0.71. • Enabled by avoiding B segregation to grain boundaries, unlike in Ti 0.10 Al 0.19 B 0.71 . • Calculations show excess B likely accommodated by Al vacancies in Hf 0.13 Al 0.18 B 0.69. • Easing need for strict compositional control improves processability of Hf-based diboride films. Oxidation of B-overstoichiometric Hf 0.13 Al 0.18 B 0.69 and B-understoichiometric Hf 0.13 Al 0.23 B 0.64 films at 700 °C reveals that both compositions form Al-rich and passivating oxide scales. The superior oxidation resistance of Hf 0.13 Al 0.18 B 0.69 compared to previously reported B-overstoichiometric Ti 0.10 Al 0.19 B 0.71 films is attributed to the absence of B-enrichment at grain boundaries, as evidenced by STEM-EDX mapping. These observations align with ab initio calculations, predicting that excess B in Hf 0.13 Al 0.18 B 0.69 is preferentially incorporated into the lattice as point defects. Formation energies of Al vacancies, transition metal vacancies, and B interstitials are lower on average in the Hf-based system by 43%, 9%, and 19%, respectively. The most plausible scenario involves the formation of Al vacancies, which, among the considered defects, display the lowest energetic barrier and best agreement with experimental lattice parameter data. Consequently, Hf-based films can accommodate higher B concentrations within the grain lattice without developing B-rich grain boundaries, in contrast to Ti-based overstoichiometric films. This significantly improves the processability of Hf-based diboride films by easing the need for strict control of the B-to-metal ratio during synthesis.