| In order to solve the oxidation of the Ti alloy under high-temperature service condition, microarc oxidation(MAO) was used to prepare ZrO2/TiO2 composite coatings on Ti6Al4 V alloy. The composition of electrolyte were optimized in two zircornate systems. The electric parameters were optimized by orthogonal design optimization method, and the optimized processing parameters were obtained. The phase composition, microstructure, elemental distribution were characterization by XRD, SEM, EDS. The oxidation behavior and mechanism at different temperatures were investigated.The investigation on phase composition and growth characteristic of Zr-1 and Zr-2 coatings indicates both of the two electrolyte have common features: The applied voltage influences the phase composition, growth rate and crystallization extent of the coating; With increasing the concentration of zircornate, the content of Zr increases within limits. The coating grows with a growth kinetics of slowing down after accession. With increasing MAO time, the surface content of Zr decreases while the content of Ti increases. The differences between Zr-1 and Zr-2 system: Zr-1 coating is composed of ZrO2 and ZrTiO4 while Zr-2 coating is composed of ZrO2, TiO2 and ZrTiO4. Zr-1 coating with a high content of Zr is mainly structured by elements from electrolyte and grows inward, which results in a porous structure with poor adhesion. Zr-2 with a relatively equivalent content of Zr and Ti is structured by elements both from electrolyte and grows inward and outward simultaneously, which results in a compact structure with good adhesion.The optimized electric parameters for preparing the coating were obtained by orthogonal design optimization method. The influence of coating thickness on high-temperature oxidation resistance was investigated by cyclic oxidation test. The results showed the relationship between coating thickness and oxidation resistance is not a linear fashion. The optimum value is when the thickness matches the compactness well. Thus, with the increase of the coating thickness, no obvious change of the thickness of the oxide layer is observed. The thermal shock test indicates the thermal shock resistance decreases with the increase of coating thickness and the optimum value is 420V/30 min. The oxidation kinetic curves at 500℃~800℃ indicates coated samples present various oxidation law oxidized at different temperatures: the oxidation kinetic curves at 500℃~800℃ indicates coated samples present various oxidation law oxidized at different temperatures: it presents a pseudo-parabolic law 500℃~700℃ while it is in accordance with the parabolic law at 800 oC. The oxidation kinetic parameters indicates the composite ceramic coatings improved the high temperature oxidation resistance of Ti6Al4 V for 2~10 times, which is attributed to the microstructure and phase composition of the coating. The microstructure characteristic of the coated samples at 500℃~800℃ indicates coated samples present various oxidation behaviors oxidized at different temperatures. A double-layer structure forms due to the diffusion of oxygen inward at 500℃ and 600℃. A multilayer structure of MAO coating /transitional layer / TiO2 layer/TiN layer formed on Ti6Al4 V with the combined effect of interface migration of MAO coating and air diffusion inward, the diffusion of substrate elements outward. It was found that a TiN layer with preferential growth towards(111) lattice planes formed in the multilayer structure in the coated samples, which was much different from the oxidation behavior of Ti6Al4 V at 700℃ and 800℃. The thicknesses of TiN layer formed at 700℃and 800℃ are basically the same, while the thickness of TiO2 layer formed at 800℃ is much thicker than that at 700℃.The presence of multilayer structure provided an efficient diffusion barrier. As a result, the growth of TiO2 and TiN dramatically decreased and the oxidation tended to be stable, which was in good agreement with the oxidation kinetic curves. |
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