The Basic Structure of Ti-Si-N Superhard Nanocomposite Coatings: Ab Initio Studies

A new interface structure, the cross double tetrahedron Si-4N-4Ti, is reported in this paper. To find out the atomic structure of Ti-Si-N superhard nanocomposite coatings, total energy calculations for the different configurations of TiN with Si addition were performed with the ab initio method. The calculation results indicate that (a) there is no interstitial solid solution of Si in the TiN crystallite under the equilibrium condition; (b) the basic structure of the Ti-Si-N composite is the TiN crystallite with the cross double tetrahedron Si-4N-4Ti in the boundary of TiN. The cross double tetrahedron Si-4N-4Ti is an intrinsic mismatch to the B1-NaCl structure of TiN and it fills the boundary with the covalent combination. The TiN boundary is strengthened and the inter diffusion through the boundary is restricted by the interface Si-4N-4Ti so that the hardness and the thermal stability of Ti-Si-N coatings are enhanced. As a fixation element to the rock salt structure, the potential value of the cross double tetrahedron is in the mass production of nanometer patterns or the quantum dots. The formation of the Si-4N-4Ti in the film growth process is also studied. The study indicates that Si-4N-4Ti cannot be formed in the island of TiN on the TiN(001) surface. Under some process conditions, Si will stay outside the island of TiN. The investigation of the Si distribution in TiN demonstrates that the congregation of the Si-4N-4Ti structures in TiN will result in an increase in the local strain and a decrease in the cohesive energy of the system.