| Orthorhombic Ti2AlNb alloy as a new kind of Ti-Al-Nb ternary alloy has been considered as a promising candidate for high temperature structural applications in the aeronautical and aerospace industries due to its high specific strength, low density, high fracture toughness and excellent creep resistance. Ti2AlNb alloy exhibits a better ductility and plasticity at room temperature than traditional α2-Ti3Al, and has attracted more attention. However, practical applications of Ti2AlNb alloy are limited due to its poor oxidation resistance, low thermal radiation property and poor wear resistance at high temperatures. It is of great significance to prepare a thermal protective coating on Ti2AlNb alloy in order to improve its high temperature properties. At present, microarc oxidation (MAO) technique becomes an effective surface modification method to form ceramic coatings on Ti alloys with multi-control parameters, high bond strength, good corrosion resistance and high wear resistance. However, high temperature properties of microarc oxidation ceramic coatings are seldom investigated in the open literature. This present work concentrates on the influence of microarc oxidation ceramic coatings on microstructure and high temperature properties to obtain a better understanding on different mechanisms associated with high temperature oxidation, thermal emissivity and wear process at elevated temperatures.In the present work, Ti2AlNb alloy is used as substrate, and micoarc oxidation method is applied to prepare a variety of ceramic coatings in NaAlO2and Na2SiO3electrolytes, respectively. Orthogonal experiments have been performed to optimize the technological parameters of microarc oxidation, the optimization results are selected as the electrolyte concentration of25g L–1, the voltage of550V, the frequency of600Hz, the duty cycle of8%and the oxidation time of20min. Based on these parameters, a multi-step voltage-controlled technology has been used to tailor the microstructure of coatings by adjusting the fabrication time at different voltages. A new two-step voltage-controlled MAO method has been proposed to produce high oxidation-resistant or high emissivity ceramic coatings at elevated temperatures on Ti2AlNb alloy.Different additives of NaF, Na2CrO4and Al2O3are introduced in a NaAlO2 electrolyte, respectively, to prepare ceramic coatings for the improvement of high temperature oxidation resistance. However, SiC, NH4VO3and Cr2O3are incorporated into a Na2SiO3electrolyte, respectively, to fabricate ceramic coatings with a high emissivity on Ti2AlNb alloy. Microstructures of different ceramic coatings were investigated by using an X-ray diffractometer, a scanning electron microscope, an eddy current thickness meter and a contact surface profiler. The addition of NaF into the NaAlO2electrolyte accelerates the growth of ceramic coating, and promotes the formation of Al2TiO5phase. However, the incorporation of Na2CrO4into the NaAlO2electrolyte increases the surface roughness of ceramic coatings and enhances the content of γ-Al2O3phase. The addition of Al2O3into the NaAlO2electrolyte takes part in the coating growth, and forms a lot of Al2TiO5phase. The incorporation of nano-SiC or Cr2O3particles into the Na2SiO3electrolyte leads to the formation of ceramic composite coatings, while the addition of NH4VO3into the Na2SiO3electrolyte increases the thickness of ceramic coatings.High temperature oxidation behavior of microarc oxidation ceramic coatings prepared in the NaAlO2electrolyte was investigated by isothermal oxidation at800oC up for150h in air. The oxidation resistance of MAO ceramic coatings is much better than that of uncoated Ti2AlNb alloy. The oxidation weight gain of ceramic coatings prepared by a two-step voltage-controlled MAO technology after static oxidation tests at800oC for150h is1.18mg cm–2, while the oxidation weight gains of ceramic coatings formed in a NaAlO2electrolyte doped with4g L–1Na2CrO4or Al2O3are0.91mg cm–2and1.33mg cm–2, respectively, which are less than50%of that of uncoated Ti2AlNb alloy obtained under identical oxidation condition. According to the oxidation kinetics and oxidized products, the microarc oxidation ceramic coatings restrains effectively the oxygen diffusion into the substrate, and forms a dense oxide layer at the interface between ceramic coating and the substrate. Compacted microstructure and high contents of Al-containing oxides are beneficial to the improvement of high temperature oxidation resistance of ceramic coatings.The normal spectral emissivity of microarc oxidation ceramic coatings prepared in the Na2SiO3electrolyte was measured at600oC in the waveband of3~20μm by a method of radiative energy comparison between the samples and a near-blackbody. The normal spectral emissivity of Ti2AlNb alloy keeps at0.2in the waveband of3~20μm, while the emissivity of MAO ceramic coatings is about0.5in the waveband of3~8μm and over0.8in the waveband of10~20μm. The emissivity of MAO cermic coating doped with SiC is above0.8in the whole waveband. The introduction of NH4VO3and Cr2O3into the Na2SiO3electrolyte could increase the emissivity of ceramic coatings at short wavebands of3~8μm and long wavebands of10~20μm up to0.7and0.9, respectively. The thermal radiation property of ceramic coatings is mainly related to the phase constitutents and surface roughness of coatings. The electromagnetic waves can be repeatedly reflected by a rough coating surface, wihle a high absorptivity phase in ceramic coatings can enhance the absorption of electromagnetic waves. The infrared emissivity of ceramic coatings can be improved by increasing the surface roughness or by incorporating a high absorptive character phase.Finally, the adhesive strength of ceramic coatings to the substrate is tested by a direct pull-off method. The tensile failure always occurs between coatings and the mounting resin, which demonstrates that the bonding between ceramic coatings and the substrate is metallurgical bonding. The adhesive strength of microarc oxidation ceramic coatings reaches up to35MPa. Tribological properties of coatings were tested by using a ball-on-disk high-temperature friction and wear tester under dry sliding condition at both room temperature and600oC. Tribological properties of microarc oxidation ceramic coatings are better than that of uncoated Ti2AlNb alloy at both room temperature and600oC. The dominated wear mechanism of ceramic coatings is mechanical polishing and brittle microfracturing at room temperature, however, it is also accompanied with surface fatigue wear and localized delamination at600oC. The ceramic coatings prepared in the NaAlO2electrolyte by a two-step voltage-controlled technology have a relatively higher wear resistance, and their wear rates are8.28×10–6mm3N–1m–1at room temperature and6.07×10–6mm3N–1m–1at600oC. However, the ceramic coatings prepared in the Na2SiO3electrolyte incorporated with SiC exhibit a low friction coefficient in dry sliding against sintered Si3N4ball, and their average friction coefficients are0.45at room temperature and0.55at600oC.The present study proposes a new approach for exploiting a novel thermal protective coating on Ti2AlNb alloy, and provides a new route for the improvement of high temperature oxidation resistance, thermal radiation property and tribological properties of Ti2AlNb alloy, which will definitely expand the industrial applications of microarc oxidation ceramic coatings, and provide a theoretical support for potential applications of Ti2AlNb alloy as high temperature structural materials. |
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