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Plasma electrolytic oxidation (PEO) has become an effective surface modification method to form ceramic coatings on orthorhombic Ti 2 AlNb alloy due to its high productivity, economic efficiency and ecological friendliness. In the present work, different contents of Al 2 O 3 additive were introduced into a NaAlO 2 basic electrolyte with the aim of improving the high-temperature properties of PEO ceramic coatings on Ti 2 AlNb alloy. The influence of Al 2 O 3 addition in NaAlO 2 electrolyte on microstructure, adhesive strength, isothermal oxidation behavior and tribological properties of PEO coatings at elevated temperatures was investigated by means of X-ray diffraction, scanning electron microscopy, the pull-off method, isothermal oxidation tests as well as ball-on-disc friction and wear tests. The introduction of Al 2 O 3 into the basic electrolyte promotes the formation of Al 2 TiO 5 phase in the PEO coatings. Isothermal oxidation tests at 800 °C up to 150 h showed that PEO coatings with Al 2 O 3 addition exhibit better oxidation resistance than those formed in the basic electrolyte. The PEO coating formed in the electrolyte with 4 g L−1 Al 2 O 3 additive exhibits the least mass gain of only 1.33 mg cm−2. The micro-hardness of the PEO coatings prepared with Al 2 O 3 additive is improved due to the presence of Al 2 TiO 5 phase, however, the adhesive strength of PEO coating is slightly weakened due to the formation of defects inside ceramic coating. The wear resistance of Ti 2 AlNb alloy is improved significantly by PEO coatings at room temperature due to high hardness and porous surface morphology. The friction coefficient and wear rate of the PEO coatings prepared with Al 2 O 3 addition is comparable to those formed in the basic electrolyte at both room temperature and 600 °C. • Al 2 O 3 additive was used to prepare PEO coatings and the influences were discussed. • Coatings with Al 2 O 3 additive exhibit better high temperature oxidation resistance. • Wear resistance of Ti 2 AlNb alloy is improved by PEO coatings at room temperature. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti2AlNb alloy in various NaAlO2 electrolytes containing 2 g L−1, 4 g L−1, and 6 g −1 Na2CrO4 additive, respectively. The influence of Na2CrO4 additive in NaAlO2 electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na2CrO4 additive in the NaAlO2 electrolyte not only promotes the formation of γ-Al2O3 phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr3O and (Al0.948Cr0.052)2O3 phases. The PEO ceramic coatings formed in NaAlO2 electrolytes with 2 g L−1 and 4 g L−1 Na2CrO4 additive show better oxidation resistance than those PEO coatings formed in a NaAlO2 basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO2 electrolyte with 4 g L−1 Na2CrO4 additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO2 basic electrolyte without any Na2CrO4 additive. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mechanism of phase transition from X2-RE 2 SiO 5 to apatite is revealed. • The formation of apatite in corroded Y 2 SiO 5 is inhibited by introducing Yb. • Silica-rich grain boundary is considered to reduce the corrosion degree. Environmental barrier coatings (EBCs) are usually coated on SiC ceramic or ceramic matrix composites (CMCs), in order to prevent the attack from corrosive media like water vapor and molten oxides. Solid-solutionizing strategy containing high-entropy engineering is considered as an effective way to mediate corrosion resistances of rare-earth silicates, which are promising candidates for the topcoats of EBCs. In this work, high-temperature corrosion behaviors of hot-pressed (Y 1− x Yb x) 2 SiO 5 ceramics have been investigated in a 50 %H 2 O–50 %O 2 environment at temperatures of 1573/1673/1773 K for 100 h. The main corrosion product of Y 2 SiO 5 is apatite phase, whose formation can be distinctly inhibited by introducing Yb. Consequently, introducing Yb avoids the tensile stress and cracks effectively, which are induced by the volume reduction due to the phase transition to apatite at elevated temperatures. The inhibition of apatite formation by introducing Yb is attributed to the relatively lower volatilization pressure of Yb 2 O 3 than Y 2 O 3 and the smaller cationic radius of Yb3+ than Y3+. Meanwhile, the composition deviations between grain boundary and grain surface results in the grain surface corrosion and ridge-like corrosion characteristics. [ABSTRACT FROM AUTHOR]

This present work concentrates on the influence of microarc oxidation ceramic coatings on microstructure and tribological properties of Ti 2 AlNb alloy. A variety of ceramic coatings are formed on Ti 2 AlNb alloy in NaAlO 2 and Na 2 SiO 3 electrolytes, respectively. A two-step voltage-controlled technology was also used to tailor the microstructure of MAO coatings by adjusting the oxidation time. Microstructure was investigated by using a glancing incidence X-ray diffractometer, a scanning electron microscope, an eddy current thickness meter and a contact surface profiler. The adhesive strength of ceramic coatings to the substrate was tested by a direct pull-off method, and reaches up to 35±3 MPa for the two-step voltage-controlled ceramic coatings. Tribological properties of coatings were tested by using a ball-on-disk high temperature friction and wear tester in dry sliding against sintered Si 3 N 4 ball at both room temperature and 600 °C. Tribological properties of ceramic coatings are distinctly better than those of Ti 2 AlNb alloy at both room temperature and 600 °C. The dominated wear mechanisms of ceramic coatings are mechanical polishing and brittle microfracturing at room temperature, however, fatigue wear and adhesive wear are also responsible for the material removal at 600 o C. The ceramic coatings prepared in the NaAlO 2 electrolyte by a two-step voltage-controlled technology have a relatively high wear resistance, and their wear rates are 8.28×10 –6 mm 3 N –1 m –1 at room temperature and 6.07×10 –6 mm 3 N –1 m –1 at 600 °C. However, the ceramic coatings doped with SiC prepared in the Na 2 SiO 3 electrolyte exhibit low friction coefficients in dry sliding against Si 3 N 4 ball, and their average friction coefficients are 0.45±0.03 at room temperature and 0.55±0.05 at 600 °C. [ABSTRACT FROM AUTHOR]

Highlights: [•] MAO coatings on Ti2AlNb alloy consist of Al2TiO5, R-TiO2, Al2O3 and Nb2O5. [•] Two-step voltage technique enhances the densification of MAO coatings. [•] MAO coating improves the thermal radiation property of Ti2AlNb alloy. [•] MAO coatings protect Ti2AlNb substrate from the high temperature oxidation. [•] Two-step MAO coatings exhibit better high temperature properties. [ABSTRACT FROM AUTHOR]