Effects of structure and interfaces on fracture toughness of CrN/AlN multilayer coatings

In situ scanning electron microscopy compression tests clearly demonstrate that a multilayered arrangement of CrN and AlN – when stabilized by coherency strains to its cubic metastable phase for AlN layer thicknesses below 3 nm – exhibits increased fracture toughness as compared to single-layered CrN. This allows increasing the maximum loading from ∼5.25 to 6.80 GPa. If the AlN layers mainly crystallize in their stable wurtzite structure – for thicknesses above 3 nm – spontaneous and fatal cracking occurs under loads as small as 3.80 GPa.