Nano-structured CrN/AlN multilayer coatings synthesized by pulsed closed field unbalanced magnetron sputtering

CrN/AlN superlattice coatings with bilayer period of 2.5 to 22.5 nm were prepared using a pulsed closed field unbalanced magnetron sputtering system. The Al/(Cr + Al) ratios of the coatings were in a range of 61.5%-66.5%. It was found that the AlN layers exhibit Wurtzite type structure when the AlN layer thickness is larger than 3.3 nm. The structure difference between CrN and AlN layers led to low hardness (23-25 GPa), poor adhesion, and low wear resistance of the coatings. The CrN layers epitaxially stabilize the AlN layers to NaCl-type structure as the thickness of the AlN layer is small enough (≤ 3.3 nm). The NaCl type CrN/AlN coatings exhibit super hardness above 40 GPa at Λ of 3.0-4 nm. The highest hardness of 45 GPa was achieved at Λ = 3 nm. The coatings with Λ of 3.0-4.7 nm also showed improved adhesion, toughness, and excellent wear resistance, in which a low coefficient of friction of 0.32 and excellent wear rates in the low 10^{- 7} × mm³N^{- 1}m^{- 1} range were achieved. To avoid the formation of Wurtzite type AlN structure in CrN/AlN superlattice coatings, the AlN layer thickness should be carefully controlled (e.g. ≤ 3.3 nm in the present study).