Influence of CrN and AlN layer thicknesses on structure and mechanical properties of CrN/AlN superlattices

Abstract

Based on their outstanding of chemical, physical and mechanical properties, CrN/AlN multilayer coatings are the topic of many research activities. Our results for CrN/AlN superlattice coatings with AlN layer thicknesses of 1, 2 and 3nm combined with CrN layer thicknesses ranging from 1 to 10nm suggest that the CrN layer thicknesses need to be at least as thick as the targeted AlN layer thicknesses to provide sufficient strength for a fully stabilization of the AlN layers in their metastable cubic structure by epitaxial strain. Otherwise, the AlN layers tend to crystallize in their stable hexagonal (wurtzite type) structure, leading to the loss of coherency between alternating CrN and AlN layers. Hardness maxima of 31.0GPa are obtained at a bilayer period Λ of 3 and 5.5nm, for the fully cubic superlattice coatings composed of 1- and 2-nm-thin AlN layers, respectively. Due to the reduced number of interfaces per coating thickness and hence less obstacles for dislocations, only a hardness maximum of ~28.5GPa at Λ=6.3nm can be obtained for the coatings composed of 3-nm-thin AlN layers.Our study clearly demonstrates that the structural and mechanical properties of superlattice CrN/AlN coatings not just depend on the bilayer period but on the individual layer thicknesses.