Age hardening in superhard ZrB2-rich Zr1-xTaxBy thin films

We recently showed that sputter-deposited Zr_1-xTa_xB_y thin films have hexagonal AlB₂-type columnar nanostructure in which column boundaries are B-rich for x < 0.2, while Ta-rich for x ≥ 0.2. As-deposited layers with x ≥ 0.2 exhibit higher hardness and, simultaneously, enhanced toughness. Here, we study the mechanical properties of ZrB_2.4, Zr_0.8Ta_0.2B_1.8, and Zr_0.7Ta_0.3B_1.5 films annealed in Ar atmosphere as a function of annealing temperature T_a up to 1200 °C. In-situ and ex-situ nanoindentation analyses reveal that all films undergo age hardening up to T_a = 800 °C, with the highest hardness achieved for Zr_0.8Ta_0.2B_1.8 (45.5±1.0 GPa). The age hardening, which occurs without any phase separation or decomposition, can be explained by point-defect recovery that enhances chemical bond density. Although hardness decreases at T_a > 800 °C due mainly to recrystallization, column coarsening, and planar defect annihilation, all layers show hardness values above 34 GPa over the entire T_a range.