Morphology, mechanical properties, and oxidation behavior of stoichiometric Ti0.33-xAlxB0.67 coatings (x = 0.04, 0.15, 0.21, and 0.28)

Stoichiometric Ti0.33-xAlxB0.67 coatings with x = 0.04, 0.15, 0.21, and 0.28 were synthesized by magnetron sputtering and characterized regarding phase formation, mechanical properties, and oxidation behavior. By increasing the Al concentration from 4 to 28 at. %, the measured elastic modulus (496 ± 19 GPa) and unit cell volume (25.646 Å3) decreased by 33 and 0.8 %, respectively. The Al concentration induced changes in measured elastic modulus and unit cell volume are in very good agreement with ab initio predictions, as the maximum deviations between experiment and theory, observed here, are 12 and 1.1 %, respectively. The corresponding hardness values decreased by 45 % from 22 ± 1 to 12 ± 1 GPa. The oxidation experiments were performed in ambient air at 700, 800, and 900°C for 1, 4, and 8 hours. Analysis by scanning transmission electron microscopy (STEM) revealed a bimodal, strongly Al concentration-dependent oxidation behavior where films containing ≤ 15 at. % of Al form a porous, non-passivating crystalline oxide scale containing Ti-rich as well as Al-rich oxide regions, while the formation of a passivating, dense, X-ray amorphous oxide scale was observed for films containing ≥ 21 at. % of Al. Coincident with the passive scale formation for Al concentrations ≥ 21 at. %, the elastic modulus decreases by ≥ 32.6 % compared to TiB2 and can be rationalized based on Al concentration induced bond weakening as revealed by the concomitant cohesive energy reduction of ≥ 22 %.