Thermal stability of CrN/AlN superlattice coatings

CrN/AlN superlattices are favored as protective coatings for various production tools and automobile and aerospace components. Recently, we showed that for thicknesses below 3 nm, the AlN layers crystallize in their metastable cubic structure if the CrN layers are at least as thick as these AlN layers. By optimizing the CrN/AlN layer thickness ratios to 2/1, 3.5/2, and 3/3 nm highest as-deposited hardnesses of ~ 31 GPa can be obtained. Unfortunately, this is combined with a low thermal stability, whereas coatings with CrN/AlN layer thickness ratios below 1 (i.e., thinner CrN than AlN layers) are able to remain their as-deposited hardnesses of ~ 24 GPa even after annealing at 1100 °C. Complementary differential scanning calorimetry and structural investigations after various annealing steps by transmission electron microscopy and X-ray diffraction show, that for these coatings the properties are mainly determined by the higher overall content of AlN. Contrary, for CrN/AlN layer thickness ratios above 1, the thermal stability is mainly determined by the dissociation of the CrN layers to Cr and N2 via the formation of Cr2N. Our results clearly show that especially for CrN/AlN superlattices higher as-deposited hardnesses are balanced with lower thermal stability.