High Infrared Emissivity Ceramic Coatings for Metallic Thermal Protection Systems

High infrared emissivity ceramic coatings for protection of metallic thermal protection systems (TPS) are urgently needed in reusable launch vehicles (RLVs). The basic technical requirements for the coating to protect metallic TPS include a spectral emissivity of higher than 0.8 over a broad waveband to maximize re-radiation and good high temperature protection against oxidation of the underlying metallic substrate. However, how to fabricate a high emissivity coating with desirable properties of high adhesion, lightweight, low cost, and easy repairing still remains a serious challenge.

A team of researchers led by Prof. Yaming Wang and Prof. Jiahu Ouyang from Harbin Institute of Technology in China, developed a variety of high emissivity ceramic coatings on metallic surfaces such as TiAl[1], Ti[2], Nb[3] et al by microarc oxidation (MAO) method for thermal protection. Moreover, to further improve the thermal emission of ceramic coatings, a new MAO coating incorporated with SiC particles was developed for the first time on Ti2AlNb alloy in a Na2SiO3 basic electrolyte. The chemical composition, microstructure and emissivity enhancement mechanisms of MAO ceramic coatings incorporated with or without SiC particles were investigated. Their work was recently published in peer-reviewed journal, Applied Surface Science.

In their work, high emissivity ceramic coatings incorporated with SiC have been successfully formed on Ti2AlNb alloy by microarc oxidation. These ceramic coatings are composed of R-TiO2, A-TiO2, Al2SiO5 and SiC phase as well as amorphous phase. The introduction of SiC into the Na2SiO3 electrolyte leads to a significant change of coating microstructure. In addition, the addition of SiC particles reduces the volume of clusters structure and the diameter of micropores, however, increases obviously the number of micro-pores in ceramic coating.

Yaming Wang and his colleagues demonstrated that MAO ceramic coatings improve infrared radiation properties of Ti2AlNb alloy at high temperature. The coating incorporated with 10g/L SiC particles exhibits excellent infrared emissivity property at 600 oC, which has the emissivity of more than 0.80 over the whole waveband of 3-20 μm. Furthermore, they elaborated the emissivity enhancement mechanisms. The SiC particles distributed in the oxide matrix influences the phase constituents and surface morphologies, and finally enhances the infrared emissivity of the coating.

In addition, the team also develops a variety of high emissivity coatings on Al [4] and Mg [5] alloy radiators for enhanced radiating heat dissipation of high power devices. Generally, the MAO technique is a universal and efficient method by tailoring the electrical parameters and electrolyte compositions to enhance the thermal protection of metallic TPS and high efficient thermal dissipation of power devices.