Effects of bilayer thickness and post-deposition annealing on the mechanical and structural properties of (Ti,Cr,Al)N/(Al,Si)N multilayer coatings

Heterostructural (Ti,Cr,Al)N/(Al,Si)N multilayer coating with various multilayer periods (λ) between 10 and 60 nm was deposited and the effects of bilayer thickness and post-deposition annealing on the mechanical and structural properties of the coating were investigated. From the X-ray diffraction results, it was found that the multilayer coating had both crystal structures of cubic from (Ti,Cr,Al)N and hexagonal from (Al,Si)N for all multilayer periods. The hardness for all multilayer periods was higher than the calculated value from rule of mixture and the highest hardness was attained at the coating with the smallest multilayer period. Transmission electron microscopy (TEM) observation confirmed that deposited coatings formed a sharp interface and uniform multilayer structure throughout the coating. In addition, for coating that showed hardness increase, partial transformation of hexagonal (Al,Si)N to cubic phase was observed at the interface of (Ti,Cr,Al)N and (Al,Si)N. The thermal stability of the (Ti,Cr,Al)N/(Al,Si)N multilayer coating was evaluated by annealing the coatings in vacuum furnace for 2 h at temperatures ranging from 800 to 1100 °C. The multilayer coatings did not form a new crystal structure nor transform its as-deposited crystal structure after high temperature annealing. Uniform multilayer structure was observed even after vacuum annealing and the hardness drop was also suppressed for multilayer coatings with small periods. High thermal stability was attained by forming a multilayer structure with immiscible interface.