Phase transformation of coatings produced with micro-arc oxidation on plastically deformed titanium through hydrothermal treatment

In this study, the effect of hydrothermal treatment on the microstructure and residual stresses of micro-arc oxidation (MAO)-produced coatings on hydrostatically deformed commercially pure titanium was investigated. A detailed microstructure study of both as-deposited and hydrothermally treated coatings was conducted with scanning and transmission electron microscopy supported by ASTAR orientation mapping and chemical analysis via energy dispersive X-ray spectrometry. Macro-scale measurements of residual stresses were carried out using the classical sin²ψ method in an X-ray diffractometer (XRD), while the novel multi-HKL method, based on grazing-incidence XRD, was used to determine of the stresses at a specific depths. The results demonstrate that hydrothermal treatment is a highly efficient method allowing to fully transform the MAO coatings which initially contained a fraction of amorphous phase, into a fully crystalline structure, while preserving high mechanical properties. The grains of TiO₂-anatase crystallographic variant grow in various forms: equiaxed, elongated or nanorods, both at the expense of amorphous phase and due to coarsening of original ultra-fine grains. The low solubility of phosphorus and sodium in TiO₂-anatase resulted in a decrease in their content in hydrothermally treated coatings. For all the coatings, a favorable change in the stress field, to compressive stresses, was observed.