J.-F. Vanhumbeeck

Current Understanding of Ti Anodisation : Functional, Morphological, Chemical and Mechanical Aspects

This paper provides an overview of the current understanding of the Ti anodisation process. As compared to other valve-metals, Ti exhibits several significant differences regarding its anodisation behaviour, the most important one being the marked semiconducting character of its anodic oxide. In the first part of this paper, a general introduction to anodisation processes is given and theoretical models describing the growth kinetics, breakdown and internal stress development in anodic oxide films are presented. In the second part, the peculiarities of Ti regarding the anodisation behaviour are discussed extensively. In particular, the influence of the main experimental parameters (metal substrate, electrolyte and growth rate) on both the anodisation process and the functional, morphological, chemical and mechanical characteristics of the anodic films is reviewed.

On the use of a multiple beam optical sensor for in situ curvature monitoring in liquids

Several methods have been developed since the early 1900 to extract thin film stresses from the curvature of the substrate to which it is attached. One robust method particularly suitable for in situ curvature monitoring is the multiple beam optical sensor, which consists in measuring the change in relative spacings between parallel laser beams reflecting off the curved substrate. Although the technique is already well established for curvature monitoring in low pressure, gaseous environments, its use in liquid media has not yet received similar attention. Moreover, in the majority of the published work so far, spot spacings have been assumed to depend linearly on curvature. In this paper, it is first shown that this assumption may induce significant errors, particularly at large curvature. A more accurate set of equations is proposed. Next, the relationship between spot spacings and curvature is established when the substrate of interest is in a liquid, and a constitutive formula is proposed in that case as well. Finally, some practical aspects of the multiple beam technique for performing curvature measurements in a liquid are discussed. Various factors disturbing the measurement resolution are identified, with a specific interest for thin film anodizing, and a cell design is proposed to minimize their effect.

On the relation between growth instabilities and internal stress evolution during galvanostatic Ti thin film anodization

Sputtered Ti thin-film electrodes have been anodized, both continuously and in an interrupted way, under galvanostatic conditions in 1.0 M H2SO4 up to forming voltages in the range of 5-40 V. The internal stresses developing in the anodic oxide film have been followed in situ during anodizing using a high-resolution curvature measurement technique. The observed stress evolution is discussed in relation to the oxygen evolution reaction and other growth instabilities previously reported in the literature. The onset of the oxygen evolution reaction, which significantly reduces the growth efficiency, is observed to induce very large compressive stresses. The latter are likely to result from oxygen evolution inside the oxide film. At some stage during film growth, the growth efficiency is observed to return to its initial high value, and the instantaneous stress in the film changes from compressive to tensile. A tentative explanation is proposed for the observed phenomena, taking into account the kinetics of the oxygen evolution reaction inside the film

Effect of Current Density on the Internal Stress Evolution during Galvanostatic Ti Thin Film Anodizing

A high resolution curvature measurement technique was used for investigating in situ the internal stresses developing during galvanostatic anodization of titanium thin films. The titanium electrodes were anodized in a 0.1 M H3PO4 electrolyte, with current densities ranging from 0.5 to 4.1 mA/cm(2). Two distinct stages were observed in the evolution of the cell voltage with time: a first, low efficiency growth stage and a second near 100% efficiency growth stage. The transition between both stages systematically occurred at 6.0 V, independent of current density. In terms of internal stresses, these two stages correspond to two distinct regimes as well. In the first stage, compressive internal stresses were observed on the order of several gigapascals, which increased with decreasing current density. In the second stage, a tensile instantaneous stress component was found with an average value of 237 MPa, independent of current density. For the first stage, oxygen evolution inside the oxide film was responsible for both the low efficiency growth and the large compressive stresses. The tensile instantaneous stress evolution in the second stage was rationalized in terms of the rate of net free volume generation at the metal/oxide interface.