Coherent growth and mechanical properties of AlN/VN multilayers

The growth condition of metastable cubic AlN (c-AlN) in AlN/VN multilayers and the effect of c-AlN on the mechanical properties of multilayers were investigated. A series of AlN/VN multilayers with different modulation periods were prepared by reactive magnetron sputtering. The microstructure and mechanical properties of multilayers were characterized with low-angle x-ray diffraction, high-resolution transmission electron microscopy, and nanoindentation. The results show that AlN exists as a metastable cubic phase in multilayers at small modulation periods due to the “template effect” and forms a superlattice with VN through coherent epitaxial growth. Correspondingly, multilayers show the superhardness effect with the enhancement of hardness and elastic modulus. With the increase of modulation periods, c-AlN transforms to the stable hexagonal structure (h-AlN) and multilayers demonstrate a “brick-wall” structure with nanometer grains. The hardness and elastic modulus of multilayers with large modulation periods are close to the value calculated from the rule of mixtures. The discussion indicates that the prerequisite for the formation of c-AlN is the low coherent interface energy. It is the difference of volume energy between c-AlN and h-AlN that primarily determines the critical thickness of c-AlN. The change of properties with the formation of c-AlN and the alternative strain field resulted from coherent growth of c-AlN and VN are likely reasons for the superhardness effect of AlN/VN multilayers.