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

We recently showed that sputter-deposited Zr1-xTaxBy 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 ZrB2.4, Zr0.8Ta0.2B1.8, and Zr0.7Ta0.3B1.5 films annealed in Ar atmosphere as a function of annealing temperature T-a up to 1200 degrees C. In-situ and ex-situ nanoindentation analyses reveal that all films undergo age hardening up to T-a = 800 degrees C, with the highest hardness achieved for Zr0.8Ta0.2B1.8 (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 degrees 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. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.

Automated summary

The mechanical properties of Zr1-xTaxBy thin films were studied, showing an increase in hardness during annealing with the highest achieved for Zr0.8Ta0.2B1.8. Despite a decrease in hardness due to factors like recrystallization, all films maintained hardness values above 34 GPa throughout the annealing temperature range.