Influence of CrAlN layers on the microstructure, thermal stability, oxidation and corrosion resistance of AlN/CrAlN multilayers

Abstract

Recently, the development of AlN-based multilayers has raised interest due to the probability of improved performance from both Al-addition and multilayer architecture. Here we prepared AlN/Cr1-xAlxN (x = 0.44 and 0.64) multilayers, with monolithic AlN and Cr1-xAlxN coatings as reference. The dependence of microstructure, mechanical properties, thermal stability, oxidation and corrosion resistance on Al content of CrAlN layers were investigated in detail. The AlN/Cr0.56Al0.44N (low-Al ML) shows a superlattice feature with single-phase face-centered cubic (fcc) structure and its hardness is 29.7 ± 1.0 GPa, which is ∼4.8 GPa higher than the theoretical value from rule of mixture. While the increase of Al content in CrAlN layer leads to a mixed cubic and wurtzite structure of AlN/Cr0.36Al0.64N (high-Al ML) with semi-coherent interface. Accordingly, the high-Al ML only has a hardness of 16.9 ± 0.4 GPa. During annealing, AlN/Cr1-xAlxN multilayers reveal hindered decomposition of CrN bonds, compared with the corresponding Cr1-xAlxN monoliths. The low-Al ML possesses the best thermal stability, reflected by its high hardness maintained up to 1000 °C, which is 100 °C higher than Cr1-xAlxN monoliths. Additionally, AlN/Cr1-xAlxN multilayers show improved oxidation resistance, with high-Al content in CrAlN layers favorable for such resistance. After oxidation at 1100 °C for 15 h, the low-Al ML and high-Al ML generate oxide scales with thicknesses of ∼1.6 and ∼1.4 μm, respectively, compared to the fully oxidized Cr0.56Al0.44N and ∼1.8-μm-thick oxide scale of Cr0.36Al0.64N. In electrochemical experiments, the corrosion current density (Icorr) of multilayer coatings decreases, compared to their corresponding monolithic coatings.