Effects of Al target power on the mechanical and oxidation resistance of the CrN/AlN multilayer coatings

Synthesis of CrN/AlN superlattice coatings with various composition (Cr/Al at.%) and superlattice period using closed-field unbalanced magnetron sputtering method was studied in this work. The coatings were characterized in terms of crystal phase, chemical composition, microstructure, microhardness using scanning electron microscopy (SEM), X-ray diffraction (XRD), cross-section TEM, glow discharge optical emission spectroscopy (GDOES) and nano-indentation hardness test. In addition an isothermal oxidation test at 900 °C was performed using a TGA for up to 20 h in air to evaluate the oxidation resistance of these films. 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, matching coherently with the CrN layer. The maximum hardness approximately 34 GPa of the CrN/AlN superlattice film was observed when the atomic concentration ratio of Cr to Al is 0.51 and the superlattice period (λ) is 11 nm. This hardness values were 1.4 times higher than that of the CrN single layer coating. These enhancement effects in superlattice films could be attributed to the resistance to dislocation glide across interfaces between the CrN and AlN layers. Isothermal oxidation tests using a TGA showed that the CrN/AlN multilayer films have a superior oxidation resistance to that of CrN films due to the presence of alternating AlN thin films, This oxidation resistance of the thin film increased as the Al content in the film increases up to 12.3 at.% Al above which no further improvement of the oxidation resistance was observed.