Interfaces in arc evaporated Al-Cr-N/Al-Cr-O multilayers and their impact on hardness

All multilayers are single-phase cubic structured but the individual nitride and oxide layers can only be detected by XRD for the coatings composed of 12 bilayers, hence with bilayer periods λ of at least 170 nm. Especially these multilayers (with the largest bilayer periods) show a significant dependence of their hardness on the interface-type, which decreases from 19 to 16 to 14 GPa when composed of distinct, hybrid, or blurred interfaces, respectively. Multilayers composed of 25 or 50 bilayers exhibit overlapping XRD peaks of their nitride and oxide layers, due to the rather close lattice parameters of 4.08 and 4.00 Å for cubic AlxCr1-xN and (AlxCr1-x)2O3, respectively. These multilayers with bilayer periods below 40 nm, hence with a significantly higher interface fraction than those composed of only 12 bilayers, do not show a dependence of their hardness (~ 22.5 GPa) on the interface types investigated. These studies suggest that for mechanical properties of oxide/nitride multilayers, the interface fraction is more important than their thickness-within the investigated conditions.