First-principles density-functional study of metal-carbonitride interface adhesion: Co/TiC(001) and Co/TiN(001)

The energetics and the electronic structure of fec Co(001)/TiC(001) and Co(001)/TiN(001) interfaces, which are of much practical importance in the sintering of hardmetals, are investigated by means of first-principles density-functional calculations. using the plane-wave pseudopotential method. The effects of the large Co/ Ti(C,N) lattice mismatch are incorporated within an approach based on a comparative analysis of a representative set of high-symmetry model interface structures. It is shown that the dominating mechanism of the Co/Ti(C,N) inter-face adhesion is strong covalent sigma, bonding between Co-3d and C(N)-2p orbitals. An extensive analysis of the electronic structure elucidates the interface-induced features of the Co-C(N) bonding and antibonding electronic states that are responsible for the enhanced strength of the interface Co-C(N) compared to bonds in bulk carbonitrides, the effect describable as metal-modified covalent bond. A detailed comparison of the energetics and relaxation effects at the Co/TiC and Co/TiN interfaces shows a weaker bonding and less pronounced relaxation effects in the Co/TiN case, which can be connected to the experimentally observed difference in the stability of those interfaces. The weaker Co/TiN adhesion is explained in terms of the relative position of the energy region of the N-2p states with respect to the Co-3d states. The calculated adhesion strength is consistent with the available data from wetting experiments with liquid Co on a TiC surface.