Atomistic modeling of the oxidation of titanium nitride and cobalt/chromium alloy surfaces

In this work, advanced first-principles molecular dynamics (FPMD) based on density-functional theory are employed to investigate the early oxidation stages of the TiN(110), Co(0001), Cr(110) and CoCr(0001) surfaces. For TiN, I observe selective oxidation of Ti atoms and formation of an ultrathin Ti oxide layer, while Ti vacancies are left behind at the metal/oxide interface. Within the formalism of ab initio thermodynamics I compute the segregation energies of vacancies and vacancy clusters at the metal/oxide interface, comparing the stability of the system obtained by FPMD simulations with ideally reconstructed models. Oxide nucleation on cobalt initially follows a metastable, kinetically driven path that results from the high heat release during the dissociation of O2. The early place-exchange of metal and oxygen atoms leads to the growth of an open, pseudo-amorphous oxide structure with evident Co3O4-like features. Instead, the oxidation of Cr(110) occurs along an energy path close to thermodynamic equilibrium and limited by Cr-ion diffusion already in the earliest oxidation stages. The initial formation of highly oxidized chromate-like structures seems to be precursory for the subsequent growth of Cr2O3 thin films. The oxidation of CoCr alloys occurs via selective oxidation of chromium, which provide vacancies enabling the diffusion of oxygen atoms into inner atomic layers. This outward diffusion of chromium is strongly facilitated by the matrix of amorphous cobalt. In summary, I suggest that superficial oxidation may proceed along two distinct possible pathways: a thermodynamically stable path along the potential energy minimum surface and a metastable, kinetically driven path that results from the high heat release during the dissociation of O2.