Stress-assisted crystallisation in anodic titania

The relationship between the microstructural and internal stress evolution during Ti anodising is discussed. Samples anodised galvanostatically to 12 V and 40 V, corresponding to different stages of the internal stress evolution, were examined by in-plane and cross-section transmission electron microscopy. Electron diffraction patterns have been complemented with stoichiometry data obtained from energy loss near edge structure spectra. The sample anodised to 40 V was observed to consist of two regions, with a crystallised inner region adjacent to the metal/oxide interface. Crystallisation of this region is associated with the presence of large compressive internal stresses which build up during anodising up to 12 V.

Research highlights
► Correlations between microstructure and internal stress during Ti anodising are established.
► Large internal compressive stresses are accumulated in the film during anodising upto 12 V.
► A transition from compressive to tensile stress is observed when the cell voltage exceeds 12 V.
► At 40 V, the oxide films consist of two regions with different compositions and microstructures. Crystallisation of amorphous to anatase TiO2 contributes to the compressive stress relaxation.