Analysis of interfacial strengthening from composite hardness of TiN/VN and TiN/NbN multilayer hard coatings

Multilayer coatings consisting of very thin layers of nitride materials deposited by magnetron sputtering (sometimes called superlattices) exhibit hardnesses over 50 GPa, much higher than that of single layers. Several explanations for this hardness increase have been proposed, including dislocation blocking by layer interfaces, Hall–Petch strengthening, strain effects at layer interfaces, and the supermodulus effect. Among these, dislocation blocking has been generally accepted as a dominant factor in large hardness enhancement. However, the interface between two materials consisting of multilayers is broadened due to interdiffusion. The magnitude of hardness enhancement by the interface has not yet been quantified for real systems. In this study, the concept of composite hardness is introduced to quantify the interface-related hardness enhancement. We suggest a composite hardness equation and quantify the magnitude of hardness increase by using an equation based on the interface hardness and the interface thickness, derived by comparing results derived from this equation and those determined in nanoindentation tests. Two multilayer systems, such as TiN/VN and TiN/NbN, were used in the equation.