Structure–Property Correlation in EEMAO Fabricated TiO₂–Al₂O₃ Nanocomposite Coatings

Abstract

We grew TiO2-Al2O3 nanocomposite coatings on titanium substrates by electrophoretic enhanced microarc oxidation (EEMAO) technique under several voltages and established a correlation between microstructure, surface hardness, and corrosion resistance of the coatings in sulfuric acid and sodium chloride solutions. Structural analysis revealed that the coatings contained anatase, rutile, alumina, and tialite phases. Formation kinetics of tialite phase was studied. It was found that increasing the voltage gives rise to a coarser morphology, i.e., larger pore size, and incorporation of more alumina nanoparticles into the layers. It is shown that surface hardness of the titanium substrates increased by a factor of 4 following EEMAO treatment. Corrosion resistance of titanium was enhanced significantly. Resistance against pitting corrosion was improved as well. We proposed a formation mechanism for the TiO2-Al2O3 composite coatings at different voltages based on the chemical and electrochemical foundations.

How deposition parameters affect corrosion behavior of TiO2-Al2O3 nanocomposite coatings

“…Results show that only anatase phase forms at 250 V. Since this voltage was not strong enough to compel alumina particles to incorporate into coatings in large volumes, the alumina characteristic peaks remained latent in this voltage. Anatase forms due to the electrochemical oxidation of titanium substrates in alkali media (sodium phosphate electrolyte) through to the following reactions [18,19,49,54]:…”

“…The applied voltage in MAO process usually is in the order of hundreds of volts resulting in a strong electric field between anode and cathode (about 10 2 V m −1 between the anode and cathode and it may increase up to 10 6 V m −1 across the interface metal/oxide/electrolyte) [41][42][43]46] forcing insoluble charged particles in the electrolyte to move towards electrodes with opposite charge due to the electrophoretic effects [11,29,41,47]. For example, if the electrolyte contains insoluble negatively charged particles, a composite coating consisted of an oxide matrix with the dispersed ceramic particles forms on the surface of anode [11,29,48,49]. This new fabrication technique is called Electrophoretic Enhanced Micro Arc Oxidation (EEMAO) which has the advantage of both micro arc oxidation and electrophoretic deposition methods [10,48,49].…”

“…For example, if the electrolyte contains insoluble negatively charged particles, a composite coating consisted of an oxide matrix with the dispersed ceramic particles forms on the surface of anode [11,29,48,49]. This new fabrication technique is called Electrophoretic Enhanced Micro Arc Oxidation (EEMAO) which has the advantage of both micro arc oxidation and electrophoretic deposition methods [10,48,49]. It should be noted that oxide particles assume a negative charge when pH of the electrolyte exceeds the PZC (Point of Zero Charge) of particles.…”

“…It should be noted that oxide particles assume a negative charge when pH of the electrolyte exceeds the PZC (Point of Zero Charge) of particles. PZC is defined as a pH at which the particle has zero charge [49]. Fabrication of composite coatings containing nano/microparticles of TiO 2 [36], ZrO 2 [12,29,48], Si 3 N 4 [50], CNT [34], and Cr 2 O 3 [47] was previously reported where the wear and corrosion resistance of a metallic substrate have been improved significantly [34].…”

“…In our previous work [49], TiO 2 -Al 2 O 3 composite coatings were fabricated on titanium substrates at several voltages and corrosion behavior of the composite coatings was evaluated [49]. The aim of the current study is to investigate the effect of concentration of ceramic nanoparticles in the electrolyte on the growth mechanism, phase evolution, and morphology of TiO 2 -Al 2 O 3 composite coatings.…”