Phase stability behavior of (Ti,Al,Si)N multi-principal nitrides regulated by the Si content: Thermodynamics and first principles calculations

Abstract

The cubic (B1) (rock salt-type) Ti(1−x)AlxN system is one of the most important protective coatings in industry due to properties such as hardness and thermal stability. Moreover, different alloying elements have been studied to increase its working temperature. Silicon has been demonstrated as an interesting option, since it can improve the oxidation resistance and stabilize the system in the hexagonal (B4 wurtzite-type) phase. Different experimental studies have been developed to understand the role of Si in the conformation of Ti(1−x-y)AlxSiyN pseudo-ternary coatings. However, some results seem to be contradictory. Here, we used ab initio and CALPHAD calculations to understand the Si incorporation effects on the phase stability and the critical Si composition. We found that the maximum mixing enthalpy for B1 falls on the TiN-AlN side of the ternary diagram and continues until almost equimolar composition. The B1-B4 phase transition point shifted significantly from x(AlN) = 0.71 without Si to x(AlN) = 0.21 with x(SiN) = 0.4. The critical composition of x(SIN) = 0.06 was found, where the miscibility gap region increased, demonstrating unfavorable results if going higher in Si content.The ab-initio calculations showed that the inclusion of silicon reduced the mechanical properties of the B1 phase, but at the critical threshold of 10 at% of silicon content, the mechanical properties did not significantly diminished. Calculated diagrams at equilibrium conditions mapped out the phase evolution to determine the phase transition paths and phase dependence on common coating deposition parameters. These results could serve as a reference for the experimental obtaining of improved hard coatings based on the (Ti,Al,Si)N multi-principal nitride.