Hardness evolution of Al–Cr–N coatings under thermal load

Microstructure and hardness evolution of arc-evaporated single-phase cubic Al_0.56Cr_0.44N and Al_0.68Cr_0.32N coatings have been investigated after thermal treatment in Ar atmosphere. Based on a combination of differential scanning calorimetry and x-ray diffraction studies, we can conclude that Al_0.56Cr_0.44N undergoes only small structural changes without any decomposition for annealing temperatures T_a ⩽ 900 °C. Consequently, the hardness decreases only marginally from the as-deposited value of 30.0 ± 1.1 GPa to 29.4 ± 0.9 GPa with T_a increasing to 900 °C, respectively. The film with higher Al content (Al_0.68Cr_0.32N) exhibits formation of hexagonal (h) AlN at T_a ⩾ 700 °C, which occurs preferably at grain boundaries as identified by analytical transmission electron microscopy. Hence, the hardness increases from the as-deposited value of 30.1 ± 1.3 GPa to 31.6 ± 1.4 GPa with T_a = 725 °C. At higher temperatures, where the size and volume fraction of the h-AlN phase increases, the hardness decreases to 27.5 ± 1.0 GPa with T_a = 900 °C.