First-principles study of fracture toughness enhancement in transition metal nitrides

Due to its superior thermophysical properties, ZrN thin film has been studied as a diffusion barrier for a variety of applications. However, cracks and delamination of ZrN coating takes place because of its brittleness. To identify new diffusion barrier materials with improved fracture toughness, we systematically studied the mechanical properties, surface energies and fracture toughness of IVB, VB and VIB transition metal mononitrides using first-principles calculations. TiN and HfN show slightly better toughness than ZrN. To improve the toughness of ZrN, we investigated the fracture toughness of ZrN alloyed with 25% IVB, VB and VIB transition metal nitrides. TiN, HfN and TaN was found to be able to increase the fracture toughness of ZrN through alloying. Meanwhile, we predicted the interfacial fracture toughness of ZrN/TiN and ZrN/HfN interfaces to evaluate the fracture toughness of ZrN/TiN and ZrN/HfN multilayer structures. Both ZrN/TiN and ZrN/HfN films exhibit higher interfacial fracture toughness than that of ZrN single film. Therefore, alloying and multilayer structures are promising approaches to enhance the toughness of brittle ZrN film.