Prepration And Characterization Of SiC High Emissivity Coating

Thermal protection coatings are one of the important constituents of thermalprotection,coatings with high emissivity can efficiently enhance the system. In this paperthe high emissivity coatings used A1₂O₃ sol as binder,SiC,Co304 naaaoparticles as fillerswere made onto the surface of nickel-based superalloy. The microstructure of thecoatings w-as studied by SEM,XRD and FT-IRï¼›the emissivity of coatings were measuredunder different temperatures and the variations and relevant factors were analyzedï¼›integrated heating-water cooling w-as used to evaluate the thermal shock resistance ofthecoating,the failure mode was investigated.Coatings system composed of A1₂O₃ as bufferlayer and SiC,Co304 mixed intoA1₂O₃ to be radiation layer By studying the preparation,drying process and sinteringprocess ofthe coating,parameters number were obtained. The dosage of A1₂O₃ was 90ï¼…radiation paint was prepared by mixing nanopowder into A1₂O₃ sol milling 8h. To adopta certain thickness,the coating was made by dipping several times,each time after dryingthe coating got a 300C sintering to avoid dipping dried coating. The coating was driedunder 60C. The heating rate during sintering as 2C／min before 520C,10C／min after520C. The coating got density quantity within a certain thickness:when thickness w-asover the critical value,the coating got cracks on the surface. After sintering lh under 900℃particles of the surface of coating grew-up and got a increased roughnessThe sintering treatment of the coating did not change the phase composition,heattreatment completed the crystallization of coating,high temperature annealing made thegrains growThe variations of thickness,sintering temperature and ambient temperature tospectral emissivity of coating were discovered. The emission rate increased with theincreasing thickness until it came to the critical value because of the cracks. Afterannealed at 900C,the emissivity ofthe coating got a small growth. With the temperatureincreasing,the spectral emissivity of coating were increased gradually and presented thesame variation tendency with wavelength. The study obtained the high emissivity coatingwhich had the emission rate over 0.9.The behaviors of the resistance of the coating to thermal shock have been studiedand the failure modes have put forward. The surface of the coating got the thickness as 19μm spalling after 40 cycles under 700℃, while under 900℃it spalling just after 20 cycles. During the thermal shock cycle, the crack had a process of initiation, staggered growth, at last turned into turtle cracks. When the thickness of coating was 19μm, the failure mode was radiation layer spalling from the coating. When the thickness of coating was 25μm, the failure mode manifested as the outer layer spalling firstly, then peeling off layer by layer until falling off from the substrate eventually and leading to failure. The main reason of thermal shock failure of the emissivity coating was the thermal stress caused by the difference of thermal expansion coefficient between coating and substrate.