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 sin 2 ψ 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 TiO 2 -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 TiO 2 -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. • Conventional heating of MAO coating on cp-Ti causes recrystallization of substrate • Hydrothermal treatment allows to transform it into crystalline TiO 2 -anatase structure • TiO 2 -anatase grows in various forms of grains: equiaxed, elongated or nanorods • Nanorods-like TiO 2 -anatase grows along [001] crystallographic direction • Hydrothermal treatment resulted in formation of compressive stress field