Preparation And Investigation Of Graphene Doped SiO₂-based High Emissivity Coatings

Hypersonic vehicle plays a very important role in the space strategy,and its design flight speed is getting faster and faster.It means that the aerodynamic heating is more intense and will cause great harm to the surface materials of the aircraft.High emissivity coating can radiate the heat accumulated on the surface of the aircraft rapidly,reduce the surface temperature of the aircraft,and improve the thermal protection capability of the aircraft.In this study,SiO₂ is the basic coating material.The graphene oxide is mixed into the coating slurry by mechanical stirring and ultrasonic dispersing.Graphene-doped SiO₂-based high emissivity coating is prepared by sprayed the coating slurry doped with graphene oxide on the kovar alloy and sintered at high temperature.The morphology of the coating is observed by Scaning Electron Moicroscope（SEM）and laser confocal microscope（LCM）.The distribution of graphene oxide in coating and the effect of grapheme oxide on microstructure morphology of coating are investigated.The microstructure of coating is studied by FTIR.The structure of graphene oxide doped in coating is characterized by Raman spectroscopy.XRD is used to analyze the coating phase.The emissivity of coating at 500°C and 800°C is measured by radiation energy method and the enhancement mechanism of emissivity of coating is discussed.Thermal cycling test is carried on from room temperature to 500 ℃,the morphology,microstructure,phase composition and infrared emission of the coating are studied.The surface of the coating with porous structure prepared by spray sintering is smooth,and the coating is well combined with the alloy matrix.Graphene oxide is sufficiently dispersed in the coating without agglomeration.A large number of coating material particles are adsorbed on the surface of graphene oxide,forming sheet structure in the coating.The porous structure and the sheet structure will improve the emissivity of the coating effectively.Doping graphene oxide into the coating increases the emissivity,and the emissivity increases with the increase of doped graphene oxide content.At 500°C and 800°C,the emissivity of coating without graphene oxide reaches 0.84 and 0.86 respectively,and the emissivity of coating doped with 5wt% graphene oxide reaches 0.9 and 0.94 respectively.The emissivity of coating doped with 3wt% graphene oxide shows good stability during thermal cycling test,and the emissivity are above 0.9 at 500°C and 800°C.The coating without graphene oxide showed excellent thermal shock resistance and no shedding occurs even the coating suffered 480 thermal cycles.Coatings doped with 1 wt%,3 wt%,and 5 wt% graphene oxide showed shedding on the surface of the coatings after 360,270,and 180 thermal cycles,respectively.The doping of graphene oxide reduces the thermal shock resistance of the coatings,and the greater the doping amount of graphene oxide,the more severe the damage against thermal shock performance.