Probing the anodic growth mechanism of titanium oxide

Hunter Feltham, Western University

Abstract

This thesis describes the study of the oxidation growth mechanism of thin film titanium oxide on the surface of titanium metal under aqueous saline conditions relevant to highly corrosive environments. Complimentary electrochemical and physical methods were applied, both in-situ and ex-situ, to differentiate between High-Field Model (HFM) and Point Defect Model (PDF) mechanisms of titanium oxide growth. These methods included using implanted Kirkendall markers, electrochemistry, and ion beam analysis.

Achieving a low-density gold marker layer without influencing the oxide's growth was a significant challenge. This was addressed using modification of the outer layer (mOO) with ion implantation, followed by chemical etching. This allowed precise control over the implantation energy and dose to ensure minimal impact on the growth process. Our results demonstrate the efficacy of this ultra-shallow ion implantation mOO technique and provide evidence supporting the point defect model as the dominant growth mechanism of TiO₂ passive film growth.

In-situ ion beam analysis like Rutherford backscattering spectroscopy (RBS) to determine oxide growth mechanisms is challenging due to the need for a liquid electrolyte in ultra-high vacuum (UHV). Our research involves using a specially designed in-situ cell with an ion-permeable silicon nitride window to provide a barrier between the UHV needed for RBS and the electrolyte solution required for electrochemical techniques. In-situ RBS results show a significant increase in the oxidation rate of titanium compared to equivalent ex-situ measurements, as well as spontaneous TiO₂ film growth without applied potential. Direct and indirect alpha radiation exposure measurements determine the enhanced titanium oxide thickness generated via radiation and radiolysis effects. Quantification of these effects allows for reliable comparison of in-situ and ex-situ anodization experiments.

The mechanism of titanium electrochemical oxidation was examined for ultra-thin Ti films sputtered onto Si (001) substrates and in-situ exposed to H₂¹⁸O, and then anodized in D₂¹⁶O. The effects of this isotopic labelling procedure were studied using medium energy ion scattering (MEIS) and nuclear reaction profiling (NRP). Both MEIS and NRP results show that the titanium oxide layer is composed of two distinct regions, Ti¹⁶O₂/Ti¹⁸O₂/Ti/Si(001), suggesting that PDM is a likely mechanism of anodic titanium oxidation.