Influence of the crystallographic orientations of Ti–6Al–4V substrate on the morphology and optical properties of oxide thin films obtained by anodizing

Abstract

The origin of color inhomogeneity in anodized titanium at the microscopic scale is investigated. Oxide layers were produced by galvanostatic anodizing in sulfuric acid on commercial Ti–6Al–4V substrates, with and without HF/HNO3 etching prior to anodizing. The crystallographic orientations of the Ti–6Al–4V substrates were determined using Electron Backscatter Diffraction (EBSD) before anodizing. The chemical composition of the oxide thin films was characterized using X-ray Photoelectron Spectroscopy (XPS). The oxide thin films were observed optically using a digital microscope. The internal morphology of the oxide layers was observed through a Scanning Transmission Electron Microscope (STEM) on Focused Ion Beam (FIB) lamella preparations, taken at the boundary of two differently colored areas on anodized Ti–6Al–4V samples. By characterizing both the substrate and the oxide layer, a correlation was established between the crystallographic orientations of the underlying Ti–6Al–4V substrate and the morphology of the oxide layer. It is shown that crystallographic orientations of the α phase of the substrate close to the basal plane (0001) lead to a porous oxide layer, resulting in a blue color. For other α grains and the β grains of the substrate, the oxide layer is dense and exhibits a yellow color. The development of an optical model to simulate color, accounting for the porosity of the oxide layer, confirms the correlation between porosity and color.