Influence of coating thickness and substrate on stresses and mechanical properties of (Ti,Al,Ta)N/(Al,Cr)N multilayers

The build-up of residual stresses in physical vapour deposited hard coatings can lead to delamination and crack formation in thin films when a critical stress value is exceeded, which typically limits the possible coating thickness. Because for some applications rather thick coatings are desired, the stress evolution with coating thickness is of uttermost importance. Therefore, we systematically studied the residual stresses of arc evaporated 0.8, 1.6, 2.4, 4, 8, and 16 μm thick Al0.62Cr0.38N/Ti0.44Al0.41Ta0.15N multilayer coatings (having a bilayer period of 24 nm), grown on monocrystalline Si and Al2O3 as well as on polycrystalline hard metal and austenitic stainless steel. Only the thinner coatings exhibit a pronounced stress-thickness dependence, whereas coatings with thicknesses ≥4 μm exhibit similar stresses (within the error of measurement). The measured indentation modulus of the coatings on flexible substrates (Si and austenitic stainless steel) increases to ~490 GPa, but those on stiff substrates (Al2O3 and hard metal) decreases to ~490 GPa, with increasing coating thickness to 16 μm. Contrary, the measured hardness of the coatings slightly increases with increasing thickness to 4 μm, and stays constant for thicker coatings, because the substrates are softer than the coating. The hardness values of the thicker coatings (≥4 μm) are 37.3 ± 0.7, 38.2 ± 0.6, 35.8 ± 0.8, and 34.0 ± 1.2 GPa on Si, austenitic stainless steel, Al2O3, and hard metal, respectively. Consequently, even the rather thick coatings still show lower hardness values on stiffer substrates.