Effect Of Methanol On Plasma Electrolytic Oxidation And Preparation Of TiO₂/ZrO₂ Binary Metal Oxide Film By Plasma Electrolytic Oxidation

Plasma electrolytic oxidation(PEO) is a novel technology for surface modification of light metals. It can be used in the fabrication of in-situ catalytic films on alloys. Formation of vapor sheath by the evaporation of solvent is the key point of PEO process. The addition of methanol can improve the formation of vapor sheath and the growth of PEO coatings. Fabrication of in-situ Ti O2/Zr O2 binary metal oxide films on alloys by PEO is a new attempt to prepare binary metal oxide catalyst.The PEO films on AZ31 magnesium alloys were prepared in alkaline silicate electrolytes with the addition of different volume concentrations of methanol. The influence of methanol on PEO process was studied. The experimental results showed that thickened vapor sheath from methanol addition increased the density of micro-arc discharges, which improved the growth of ceramic films. The thickness of the PEO films increased with the methanol concentration and reached a maximum value at 12%; subsequently, the thickness was reduced with further addition of methanol. The energy consumption of PEO was decreased with the addition of methanol to the electrolytes. The energy consumption reached a minimum value when the methanol concentration was at 16%.The Ti O2/Zr O2 binary metal oxide films were prepared in the alkaline phosphate electrolytes by PEO, using Ti-6Al-4V titanium alloy as the substrate. The effects of oxidation time, discharge voltage and the amount of Zr O2 nanoparticles on the surface morphology and composition of the oxide coatings were investigated. The results showed that as the oxidation time, discharge voltage and the amount of Zr O2 nanoparticles increased, the oxide coatings had a larger pore size but a smaller pore number, as well as a rougher surface. The Ti O2/Zr O2 binary metal oxide films were mainly composed of Zr O2 and anatase and rutile phase Ti O2. The content of Zr O2 first increased with the oxidation time and reached a maximum value at 30 min, then reduced with the extended time. The content of rutile phase increased while the anatase phase decreased. With the increase of discharge voltage, the content of Zr O2 increased gradually. The content of rutile phase increased while the anatase phase reduced. When the amount of Zr O2 nanoparticles increased, the content of Zr O2 increased, while the content of rutile phase and anatase phase decreased.To prepare the Ti O2/Zr O2 binary metal oxide coatings with advanced surface morphology, the PEO technology was combined with the impregnation and calcination method. The influence of immersion time, concentration of H2SO4, annealing time and annealing temperature on the surface morphology and composition of the oxide coatings was studied. It was found that the adhesion of PEO coatings to the substrate was excellent. The oxide films treated by impregnation and calcination exhibited agglomerated cubic grains and the specific surface area increased significantly. With the increase of H2SO4 concentration, the grains changed from cubic structure to spindle shape. A large number of tiny crystals with polyhedral structure appeared at the bottom of cubic grains when the annealing time was extended. As the annealing temperature increased, the grains changed from the cubic structure to the polyhedral structure, and then became sheet-like. The binary metal oxide films treated by impregnation and calcination were mainly composed of anatase and rutile phase Ti O2. The contents of anatase and rutile phase first increased with the H2SO4 concentration and reached a maximum value at 2 mol/L, then reduced. With the increase of annealing temperature, the content of rutile phase increased while the anatase phase reduced.