Satyesh Kumar Yadav

Interface by definition is two-dimensional (2-D) as it separates 2 phases with an abrupt change in structure and chemistry across the interface. The interface between a metal and its nitride is expected to be atomically sharp, as chemical gradation would require the creation of N vacancies in nitrides and N interstitials in metal. Contrary to this belief, using first-principles density functional theory (DFT), we establish that the chemically graded Ti/TiN interface is thermodynamically preferred over the sharp interface. DFT calculated N vacancy formation energy in TiN is 2.4 eV, and N interstitial in Ti is −3.8 eV. Thus, diffusion of N from TiN to Ti by the formation of N vacancy in TiN and N interstitial in Ti would reduce the internal energy of the Ti–TiN heterostructure. Diffusion of N is thermodynamically favorable till ∼23% of N has diffused from TiN to Ti, resulting in an atomically chemically graded interface, which we refer to as a 3-D interface. We show gradual variation in lattice parameters and mechanical properties across the Ti/TiN interface. This opens a new way to control properties of metal/ceramic heterostructures, in line with the already established advantage of gradation at interfaces in micrometer length scale.

A gradual variation in chemical composition at an atomic level in a metal/ceramic interface would result in a gradual variation in structural features that can give rise to unusual properties of the heterostructure. One of the ways such atomic-level chemical gradation can be achieved is by diffusion of anions from ceramic to metal. This process is perceived to be unfavorable because it requires the creation of anion vacancies in ceramic and anion interstitials in metal. We use the sum of C, N, or O vacancy formation energy in ceramics and their interstitial formation energy in metals to assess the possibility of migration of anions across the metal/ceramic interface, leading to an atomically chemically graded interface. We use the first-principles density functional theory to calculate the driving force for a few scientifically and technologically important metal/ceramic systems: a combination of 10 metals (M=Ti, Zr, Hf, V, Nb, Ta, Mg, Al, Cr, and Fe) and corresponding 30 ceramics MaXb (X=C, N, and O). The metal/ceramic systems that favor chemical gradation have metals from group IVB and VB mainly due to the large negative interstitial formation energies.