Preparation Of Micro-Arc Oxidation Ceramic Coatings On Ti Alloy And High Temperature Oxidation Behavior

Ceramic coatings were prepared in situ on the surface of Ti-6Al-4V (TC4) alloy by means of micro-arc oxidation (MAO). The phase composition, microstructure and high temperature resistance of the ceramic coatings were studied in detail; the process of high temperature resistant coatings was optimized; the coatings were post-sealed; the high temperature oxidation property and mechanism of the coatings in deferent ambiences were also studied. The phase composition and morphology of the coatings were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) respectively. The element contents and their influences on the surface state of the alloys were examined by energy dispersive spectroscopy (EDS), X-ray fluorescence spectroscopy (XRF) and transmission electron microscopy (TEM). The studies on the high temperature resistance properties of the coatings and the substrate alloy were carried out by the high-temperature oxidation weight gain tests.Compound ceramic coatings with the main crystalline phase of Al2TiO5 were prepared on TC4 alloy by pulsed bi-polar MAO in a NaAlO2 solution. Compound ceramic coatings were composed of Al2TiO5,α-Al2O3 and TiO2 when the ratio of both current densities was high. With the increasing current density, the content of Al firstly increased and then decreased while the changing trends of Ti and V contents were opposite to those of Al, P increased with the increasing current density all the time. Compound ceramic coatings with the main crystalline phase of TiO2 were prepared by pulsed bi-polar MAO in a Na2SiO3 solution. With the increase of reaction time, the number of grains on the surface of the coating deceased, but the holes of the coating became bigger. The Si content increased all the time with the increase of reaction time. The contents of Al, Ti and P deceased gradually while that of V firstly increased and then decreased.Under the condition of single-polar pulse, the voltage and duty ratios were inversely proportional to the concentration and were proportional to the current density. With the increase in the concentration, the surface holes of coating firstly increased, and then decreased in size. With the increase in the current density, the size of holes and surface roughness increased, but the number of the holes decreased. When the MAO time was increased, the holes became bigger and the coating surface roughness also increased. When the frequency was increased, the coating became smooth. With the decreasing current density, the surface roughness of the coating decreased. The hardness of the coating decreased with the increase of the concentration. In an aluminate-hypophosphite solution, dense coatings were formed, of which the main crystalline phase was Al2TiO5 when the concentrations of the two electrolytes were 4 and 0.5g/L, respectively, the frequency of the power source was 2000 Hz, the current density was 4A/dm2, and the duty ratio was 10%. The thickness of coatings increased with the increasing current density or increasing reaction time. The weight gains of the coatings prepared under the optimized technique parameters was only 1/11 of the TC4 substrates under the high temperature oxidation of 700oC.The samples that were sealed in sodium silicate had a small porosity and good high temperature resistance property. Compared with the TC4 alloy and the samples without being treated with, the sealed samples had lower weight gains and a better high temperature resistance property.The coating prepared by bipolar pulsed MAO decomposed very quickly in air and the ratio ofα-Al2O3 to TiO2 was 44:55 after a complete decomposition process, and the weight gains of the coated samples were relatively lower than that of TC4. After the coating sample was calcined in argon at 1000oC for four hours, the coatings turned into thin, uniform, and smooth pins which were arranged tightly with small pores. The content ofα-Al2O3 decreased from outside to inside and in the meantime Ti2O3 was formed in the coating. When the samples were calcined in argon for four hours, Al2TiO5 was almost decomposed completely intoα-Al2O3. The curves of weight gains of both coatings show a shape of parabola, and the weight gains of the coated samples treated in argon were relatively lower than those of the as-coated samples.