Different Mechanisms Leading to Superhard Coatings: Stable Nanocomposites and High Biaxial Compressive Stress

A strong enhancement of the hardness above 40 GPa in thin coatings can be achieved either by the formation of a suitable nanostructure (heterostructures and nanocomposites) or by a large biaxial compressive stress of ≥ 5 GPa. Superhard and thermally fairly stable nanocomposites, such as nc-TiN/a-Si 3 N 4 , nc-TiN/a-Si 3 N 4 /a-TiSi 2 , nc-(Ti 1-x Al x )N/a-Si 3 N 4 retain their hardness after annealing to 900-1100°C whereas coatings with a high stress, such as HfB 2 , Cr x N/Ni, ZrN/Ni and others soften to ≤ 20 GPa already after their annealing to 400-500°C due to the relaxation of that stress.