Preparation of MoSi2-SiC-Al2O3-SiO2 coating on mullite fibrous insulation with silica sol as binder by non-firing process

Abstract High-emissivity MoSi2-SiC-Al2O3-SiO2 coating with MoSi2 and SiC as emittance agents, Al2O3 as base skeleton, and silica sol as both dispersive medium of the coating slurry and binder of the coating, was prepared on mullite fibrous insulations via slurry coating and non-firing process. The non-firing coatings were calcined at high temperatures to reveal their reusability after high temperature service. Microstructure, phase composition, interface bonding strength and impact resistance of the non-firing coating and the calcined coatings were investigated comprehensively. Thermal shock resistance, thermal endurance and infrared radiation property of the coatings were studied. The results show that the non-firing coating was dense and flat with all particles bonded together by silica sol; no clear interface existed between the coating and the substrate due to the penetration of the coating slurry; the whole coating was composed of a porous inner layer of about 300 µm in thickness and a top layer of about 100 µm in thickness. The calcined coatings exhibited ideal microstructure with newly formed high temperature phases. Both interface bonding strength and impact resistance of the calcined coatings increased with elevated calcination temperatures, benefited from the good binding effect of silica sol, the gradient structure and the formation of mullite. The coatings remained intact without peeling and spalling after thermal shock tests, and the main defects in the coating surface after thermal shock tests at 1400 °C and 1500 °C were cracks and holes, respectively. The coatings exhibited better long-term thermal endurance at 1400 °C than at 1500 °C, resulted from the weaker oxidation of emittance agents at lower temperature. The emissivity of the non-firing coating at the wavelength range of 3–5 µm and 8–12 µm was as high as 0.9, benefited from the non-firing process, and the emissivity of the calcined coatings were higher than 0.8.