Atomistic Modeling of the Oxidation of Titanium Nitride and Cobalt-chromium Alloy Surfaces

Oxidation phenomena on metal surfaces represent a vast field of basic scientific interest and play a key role in many technological and environmental processes, such as in electronic devices and mechanical tools, in heterogeneous catalysis, in corrosion, and in bioactivity. In the latter case, a precise knowledge of the oxidation process and of the chemical and physical properties of the passivating oxide layers is required in order to cope with complex key processes as corrosive attack, ion leaching, and protein adsorption on implant surfaces.This thesis deals with two types of biocompatible materials: titanium nitride (TiN) crystals and cobaltlchromium (CoCr) alloys. TiN and CoCr are known to oxidize with formation of a stable, ultrathin, passivating oxide film acting as a protective layer against further oxidation and corrosion. This work presents a detailed atomistic study of the oxidation mechanisms on the CoCr(OOOl) and the TiN(llO) surfaces, studied by means of first-principles modeling based on density-functional theory (DFf) and Car-Parrinello molecular dynamics (MD). Before considering the oxidation process of a CoCr alloy, a preliminar study of the composition of the alloy surfaces and the interaction among the constituents is indispensable. For this purpose, phenomena such as the oxidation mechanisms of the separate elements, as weIl as surface segregation processes and selective oxidation in the alloy have been analyzed in detail. MD simulations revealed that pure cobalt and pure chromium oxidize according to very different mechanisms, in good agreement with the available experimental data. Oxide nucleation on cobalt occurs via an early place-exchange of metal and oxygen atoms and the growth of an open, pseudo-amorphous oxide structure with evident C0304-like features. Instead, in the case of chromium, perfect oxygen ad-layers tend to form, followed by a layerby-layer growth, which is limited by Cr ion diffusion already in these initial oxidation stages. Notably, the initial formation of chromate-like structures, containing over-oxidized Cr atoms, seems to be precursory for subsequent growth of Cr203 thin films. Despite the higher oxygen affinity of chromium, the oxidation of CoCr proceeds with the initial formation of a cobalt oxide network which facilitates the incorporation of Cr ions into the oxide layer at an earlier stage than on pure chromium. This suggests that the further development of chromium oxides in CoCr, as observed experimentally, may occur via a different mechanism than in the case of pure chromium, with the Co matrix actively promoting the formation of stable amorphous layers.