Thermal-induced decomposition and mechanical properties of AlN/CrN superlattice coating

Abstract

Here, the thermal-induced decomposition and mechanical properties of AlN/CrN superlattice coating with modulation period of ∼8 nm are investigated in detail. Introduction of CrN template layers stabilizes the cubic growth of AlN layers, where the coherent AlN/CrN coating exhibits the higher hardness value of ∼24.1 GPa than CrN (∼16.9 GPa) and AlN (∼18.5 GPa) monolithic coatings. After thermal exposure, the AlN/CrN coating decomposes into stable body-centered cubic Cr and wurtzite AlN via intermediate phases of hexagonal Cr2N and cubic AlN. Especially, the formation of metastable cubic AlN is observed due to the destruction of coherent interfaces. Meanwhile, the thermal load leads to the grain deformation, which causes the change of AlN and CrN sublayers from the crossover to pinch-off to finally dissociation. The driving force of grain deformation mainly arises from the N-loss of CrN sublayers and the high-density stacking faults formation of AlN sublayers. Furthermore, the occurrence of stacking faults and h-Cr2N phase offset the softening effect caused by the thermal decomposition. Correspondingly, the hardness almost remains unchanged between 900 and 1100 °C.