Effect of asymmetric interfaces on the spinodal decomposition of (Ti,Al)N/ZrN multilayers: First-principles and experimental investigations

Multilayer architectures are effective to tailor the properties of thin films. This study compares the high-temperature structural evolution of (Ti,Al)N and (Ti,Al)N/ZrN by experiments and ab initio calculations. These show that cubic (c-) ZrN layers improve the thermal stability of c-(Ti,Al)N by retarding spinodal decomposition and wurtzite (w-) AlN formation. Correspondingly, (Ti,Al)N and (Ti,Al)N/ZrN exhibit their peak hardness of ∼32.8 and 33.2 GPa upon annealing at 800 and 1000 °C, respectively. Already during growth of c-(Ti,Al)N onto c-ZrN additional Ti-rich c-(Ti,Zr)N interlayers form, but this is not the case when growing c-ZrN onto c-(Ti,Al)N. These different boundaries of c-(Ti,Al)N result in different diffusion activation energies for Al, which is higher at the interface to (Ti,Zr)N than at that to ZrN. Consequently, the interface-directed Al diffusion initiates on the side nearby ZrN, which causes an Al concentration gradient within (Ti,Al)N. This increases the spinodal decomposition period, and thus the critical Al content necessary to trigger w-AlN formation is reached later. Detailed experiments show that at 1000 and 1100 °C also at the c-(Ti,Al)N/ZrN interface, where no c-(Ti,Zr)N developed during growth, a thin c-(Ti,Zr)N layer forms. Only when this layers is completely dissolved into ZrN sublayers at 1200 °C, w-AlN is formed.