Characterization of microporous oxide layer synthesized on Ti–6Al–7Nb alloy by micro-arc oxidation

In this work a microporous oxide layer was formed on two phase (α+β) Ti–6Al–7Nb titanium alloy by the micro-arc oxidation process in an electrolyte containing (CH3COO)2CaH2O and Na3PO4. The thickness of the surface layer was in the range of 2.7 μm–3.6 μm.Microstructure of the surface layer and the substrate alloy was characterized with use of scanning- and transmission electron microscopy as well as by X-ray diffractometry. The microstructure of the surface layer consisted of TiO2 rutile and anatase nanocrystals as well as of amorphous regions containing mainly Ti, Ca and O atoms and a minority of P, Al and Nb ones. The surface layer was highly porous. The open pores, with diameter up to 6 µm, were homogenously distributed in the specimen surface. Electron tomography was used to investigate the pores morphology and spatial distribution. It was found that open pores exhibited a complex geometry. The closed pores had nearly spherical shape.Adhesion of the surface layer to the titanium alloy substrate was investigated by means of the scratch-test. The value of critical load LC2=14 N indicates a good layer adhesion to the underlying substrate.