Thermal failure mechanism of multilayer brittle TiN/CrAlN films

The thermal failure mechanism of multilayer brittle TiN/CrAlN films at high annealing temperature has been investigated. Firstly, multilayer brittle TiN/CrAlN films (about 2.1 mu m thick) were deposited on high speed steel by using metallic Ti and Al-Cr alloy (Al/Cr at% ratio = 70:30) targets by RF-pulsed magnetron sputtering. Then, the multilayer TiN/CrAlN films were subjected to large heating-cooling shock during the high temperature process. The thermal expansion mismatch between the substrate-film, film-film and oxide-nitride interfaces causes large thermal stress. Finally, large thermal stress results in crack initiation, where the defects are the preferential sites for crack nucleation. During the heating-cooling process, the deformation results in stress concentration and strain mismatch, generates surface cracks and interface cracks due to the different mechanical properties between the substrate and multilayer films. The propagation of the interface cracks ultimately brings about the film delamination and loses thermal protection capability under high temperature service. Additionally, multiple surface cracks, to some extent, cause a reduction of the driving force for interface crack nucleation via relieving some tensile stress, thus inhibiting interface crack propagation and delamination failure.