Thermal stability of AlN/CrN superlattice hard coatings

Thermal stability of multilayered coatings is essential for maintaining the desired properties. Structural and chemical analysis of nanoscale AlN/CrN superlattices were predominantly studied by transmission electron microscopy and atom probe tomography after exposure to elevated temperatures. Good thermal stabilities are observed for temperatures below 800°C, whereas chemical and microstructural changes occur at higher temperatures. The AlN layers were found to initiate the dissolution of the multilayered structure by interrupting at grain boundaries. A model for the layer dissolution is introduced showing that such AlN layer interruptions lead to nanocrystalline hexagonal AlN (NX h-AlN) grains at grain boundary junctions. A poor chemical stability was observed for the CrN layers which were subject to N outdiffusion at 800 and 900°C until a stable Cr2N composition was reached. The present coherency strains of the nanoscale multilayers prevent the transformation of the CrN to the hexagonal crystal structure resulting in cubic Cr2N layers. The AlN layers show a very good chemical stability. Hence, they are found to act as diffusion barriers for Cr and Fe atoms retarding e. g. the formation of a Cr2O3 surface oxide. The microstructural changes lead the degradation of the mechanical properties in terms of hardness and elasticity. The resistance to plastic deformation was found to be improved after heat treatment for 60 min at 700°C. Ab initio calculations of the young's modulus of the superlattice are in good agreement with the experimental data thus enabling a way for theory-guided materials design.