Growth, microstructure and mechanical properties of microarc oxidation coatings on titanium alloy in phosphate-containing solution

Ceramic coatings were fabricated by microarc oxidation in galvanostatic regime on Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution. The growth, microstructure and phase composition of coatings were investigated using scanning electron microscope and X-ray diffraction. With increasing treatment duration, coating growth varies from rapidness to tardiness accompanied by gradually roughening in appearance. Meanwhile, phase transformation of anatase to rutile occurs. The crystalline AlPO4 is involved in the coatings via high-temperature thermolysis of hydrated aluminium polyphosphates in the nearby discharging channels. The stepped current regime enables coating structure to be controllable. The mechanical properties distribution across the coating thickness and the adhesion strength were determined by nanoindentation and shear test, respectively. A similar evolution profile of hardness and elastic modulus across the coating thickness is found: remaining high values (5.5 and 69.1 GPa) in the compact region before finally declining to low values (5.1 and 65.6 GPa) in the looser region. The adhesion strength of substrate/coating interface is about 40 MPa.