How deposition parameters affect corrosion behavior of TiO ₂ -Al ₂ O ₃ nanocomposite coatings

Titania-Alumina coatings were grown on titanium substrates by Eectrophoretic Enhanced Micro Arc Oxidation (EEMAO) technique in electrolytes containing various concentrations of alumina nanoparticles. The effect of concentration of alumina on surface morphology, phase composition, microhardness, and corrosion behavior was investigated at different voltages. It was found that the impact of alumina concentration on microstructure actually depends on the voltage and its effects gets more conspicuous at intermediate and high voltages. A finer morphology was obtained at higher concentrations of alumina at a constant voltage. The coatings mainly consisted of anatase and rutile. Tialite was observed at 450 V and 3 g l ^-1 alumina. The formation kinetics and thermodynamics of the tialite phase was studied. In all voltages, samples prepared in an alumina-rich electrolyte showed a higher surface hardness. The nanocomposite coatings exhibited an enhanced corrosion resistance compared to the titanium substrates where the effect of alumina concentration became more tangible when the voltage exceeded a critical value (350 V). At 450 V, the polarization resistance of the sample prepared in alumina-rich electrolytes was higher by 3 orders of magnitude compared to the coating prepared in the electrolyte containing 1 g l ^-1 alumina. A processing-microstructure-properties correlation is established.