Preparation And Study Of Infrared Radition Coating With High Emissivity At High Temperature On SiO_2f/SiO₂ Composites

Thermal protection system（TPS）,protecting aerospace craft with high velocity in the aerosphere from destruction or burning,is a significant structure of aerospace craft.Based on the background of thermal protection structure,high emissivity infrared radiation coating was prepared on the surface SiO_2f/SiO₂ composite to improve thermal insulating efficiency of thermal protection structure.In this paper,SiO₂ with low thermal expansion coefficient was chosen as base material,Cr₂O₃ and Fe₂O₃ were chosen as the main infrared radiation component,and MnO₂,NiO and Co₂O₃ were chosen as high infrared radiation additive.The mixed material was pelleted by slurry spray drying.The high emissivity infrared radiation coating was prepared by plasma spraying.The optimal proportion between the base material and the main infrared radiation component and between the different main infrared radiation components,which can enable the infrared radiation coating with high emissivity and high thermal shock resistance at the same time,was confirmed by our study.The powders prepared by slurry spray drying were simple physical bonding of raw particles,so the powders need to be baked at high temperature in order to obtain the spinel phases which have high emissivity.According to the experiments,the proper baking temperature is 1250℃,the proper baking time is 30 min.The shape of powders prepared by slurry spray drying is spherical before and after baking.After baking process,the diameter of powders is increased,and the flowability is decreased.But the processing properties of the baked powders can still meet the requirement of plasma spraying.After plasma spraying process,the coatings and powders have the same highemissivity spinel phase structure,but the crystallinity of the spinel phases wasdecreased.For the samples A,B and C,of which the main infrared radiationcomponent is only Cr₂O₃,the emissivity at 600℃ is in the range of 0.71⁰.79 at the3⁵μm infrared band;the emissivity is high at the 1²2μm infrared band,which is inthe range of 0.80⁰.87.With the proportion of Cr₂O₃ decreasing,the emissivity at allinfrared band will decrease.For the samples D,E and F,of which the main infraredradiation components are Cr₂O₃ and Fe₂O₃,the emissivity at 600℃ is in the range of0.80⁰.83 at the 3⁵μm infrared band;the emissivity is high at the 1²2μm infraredband,which is in the range of 0.86⁰.88.With the proportion between Cr₂O₃ andFe₂O₃ decreasing,the emissivity at the 3⁵μm infrared band will has a little increase,but at the 3⁵μm infrared band,the emissivity is essentially constant.With thetemperature increasing,the emissivity of all samples will be increased.The plasma spray coating has high bonding strength and good thermal shock resistance property.Among all coating samples,sample B and E have high emissivity and high thermal shock resistance at the same time.