Dependence of the infrared emissivity on SiC content and microstructure of microarc oxidation ceramic coatings formed in Na2SiO3 electrolyte

High emissivity coatings have been widely used in thermal protection systems and high heat efficiency devices in furnaces. In the present work, high emissivity ceramic coatings incorporated with different SiC contents have been prepared on Ti2AlNb alloy by microarc oxidation (MAO) method. The addition of different SiC contents into the Na2SiO3 electrolyte has a distinct influence on the microstructure and normal spectral emissivity of composite coatings. Microstructure of MAO coatings was investigated by using a glancing incidence X-ray diffractometer, a scanning electron microscope, an eddy current thickness meter and a contact surface profiler. A two-step voltage-controlled technology was also used to tailor the microstructure by adjusting the microarc oxidation duration. The composite coatings formed on Ti2AlNb alloy exhibit a porous structure, and consist of R-TiO2, A-TiO2, Al2SiO5, SiC as well as amorphous phase. The normal spectral emissivity of both coated and bared Ti2AlNb samples was measured at 600 °C in the infrared wavelength range of 3-20 μm. The S-10C composite coating incorporated with a 10 g/L of SiC in the Na2SiO3 electrolyte has a much higher emissivity in the wavelength range of 3-20 μm than the bared Ti2AlNb alloy. High emissivity of the S-10C coating is attributed to its porous structure and chemical composition, especially SiC content.