Synthesis of CrN/AlN superlattice coatings using closed-field unbalanced magnetron sputtering process

Synthesis of CrN/AlN superlattice coatings with various composition (Cr/Al at.%) and superlattice period (λ) using closed-field unbalanced magnetron sputtering method was studied in this work. The coatings were characterized in terms of crystal phase, chemical composition, microstructure and mechanical properties by transmission electron microscopy (TEM), X-ray diffractometry (XRD), glow discharge optical emission spectroscopy and nano-indentation test. Results from TEM and XRD analysis showed that the crystal structure of AlN layer in the CrN/AlN superlattice coatings has a metastable cubic lattice structure rather than an equilibrium hexagonal structure (wurtzite-type), matching coherently with the CrN layer, which has a NaCl type lattice structure. The maximum hardness and plastic deformation resistance (H3/E2) of CrN/AlN superlattice film, when the atomic concentration ratio of Cr to Al is 0.98 and the superlattice period (λ) is 4.8 nm, are approximately 37 and 0.48 GPa, respectively. These values are 1.6 and 2.5 times higher than those of the CrN single layer coating (23.5 and 0.17 GPa), respectively. These enhancement effects in superlattice films could be attributed to the resistance to dislocation glide across interface between the CrN and AlN layers.