Preparation And Properties Of Sol-bonded MoSi₂-based Radiative Thermal Protection Coatings

Radiant thermal protection coating is a key component of thermal protection system（TPS）with thermal control structure.At present,the research of radiation heat-resistant coating mainly focuses on borosilicate glass based coatings.To reduce the oxidation of the emittance MoSi₂in the process of coating preparation,and to improve the impact resistance and thermal shock resistance of the radiation thermal protection coating,the sol-based MoSi₂coating was innovatively applied to the rigid thermal insulation tile,and the impact resistance and thermal shock resistance of the coating were improved by structural optimization.The study is of great significance to expand the rigid insulation tile coating system and reveal the high temperature application potential of sol-based coatings.In this study,MoSi₂@SiO₂with a core-shell structure was constructed to improve the low-temperature oxidation resistance of the emittance.The silica sol bonded MoSi₂-based coating was prepared on the surface of mullite fiber insulating tile.Through the analysis of the morphology,phase,molecular binding,and mechanical properties of the coating at high temperatures,the bonding mechanism of the coating was deeply analyzed.Furthermore,the gradient structure coating with porous transition layer and a dense top layer was prepared by structural design.The infrared radiation properties,high-temperature stability and durability,impact resistance properties and thermal shock resistance properties of the coatings were studied.The main results are as follows:（1）To solve the problem of low-temperature oxidation of MoSi₂,the core-shell structure MoSi₂@SiO₂was constructed by in situ coating and adsorption coating respectively.In-situ coated MoSi₂@SiO₂was obtained by preoxidation of the MoSi₂powders at 800℃.The weight gain of the obtained composite powder was less than1.6 wt%after cyclic oxidation at 400～700℃for 12 h.MoSi₂@SiO₂was achieved by the sol-gel method.The weight gain of the composite powder was less than 1.0 wt%after cyclic oxidation at 400～600℃for 12 h.The formation process and oxidation resistance mechanism of the shell were analyzed,which laid a foundation for the preparation and oxidation resistance research of sol bonded MoSi₂coating.（2）To overcome the existing problems and limitations of glass based radiant thermal protection coating,the silica sol bonded MoSi₂-SiC-Al₂O₃coating was innovatively prepared on the mullite fiber insulation tile by low temperature curing.The average directional emissivity was greater than 0.85 in the wavelength range of1.5～21.0μm.The high-temperature bonding mechanism of the coating was studied in detail.The evolution of the coating at high temperatures can be divided into three stages:stable stage（<1000℃）,crystallization stage（1100～1200℃）,and densification stage（1300～1500℃）.With the increase of temperature,the bonding strength of the coating presents a"V"shape,and the inflection point appeared at1200℃.Under the premise of avoiding long-time heating at 1200℃,the coating had the potential of high-temperature application.（3）To solve the problems of low densification and insufficient mechanical strength of the sol-bonded radiation thermal protection coating,the composition and structure optimization were further carried out.The gradient structure coating with porous transition layer and a dense top layer was obtained,and the in-situ mullite fiber toughened surface structure was formed at high temperatures.The emissivity of the coating after heat treatment at 1300～1500℃was higher than 0.8.The coating can withstand heat exposure with different thermal environment and had long-time high-temperature durability.The impact resistance of the coating was better than that of the reaction cured glass coating（RCG）.The coating has excellent thermal shock resistance.After 30 thermal shock cycles from 1500℃to room temperature,the coating has no crack and spalling.