Preparation And Properties Of Hafnium Oxide Coating On The Surface Of Iridium

C/C composites with high strength and low density are suitable to applying to the nozzles and wing leading edges of hypersonic vehicles.But they will be oxidized at the high temperature environment with oxygen.Iridium?Ir?with high melting point and rather low oxygen permeability is considerably conducive to protecting C/C composites as antioxidant coating.However,its thermal emissivity is low,which is hard to cool down substrate through radiating energy to space.When it endures aerodynamic heating,the temperature of its surface may be higher than its melting point.Besides,the surface of Ir coating will volatilize by oxidizing at oxidative and high-temperature environment,which limits its service life.Therefore,it is necessary to prepare a layer of high-thermal-emissivity HfO₂ ceramic coating on the surface of Ir coating.HfO₂ coating with high emissivity can cool down Ir coating and substrate through radiation.What is more,it can also slow down the oxidization and increase its service life.This paper used plasma spraying?PS?method and “electrophoretic deposition?EPD?– sintering” method to produce HfO₂ coating and HfO₂-Y2O3?HYO?composite coating on the surface of Ir coating,respectively.Details are as follow:?1?Manual pelletization method and heat treatment method were applied to prepare HfO₂ powders which were suitable to PS.A layer of HfO₂ coating was prepared on the surface of Q235 low-carbon steel by PS.But because of the high hardness of Ir coating,it is not suitable to prepare HfO₂ coating on its surface directly.?2?“Ball milling mixing–low temperature sintering–ball milling smashing” method was used to prepare HYO composite powders which were instrumental in EPD.Suitable EPD method was electrophoresis at a constant voltage of 60 V in the well-dispersed suspension,which content was 2.4g/L HYO powders and I2 in the acetylacetone.This method obtained even and dense HYO green coating.?3?Appropriate sintering method to HYO green coating was sintering at 1800? for 2h at vacuum environment at low speed to increase temperature.HYO coating with high density and good honding strength with Ir coating was obtained by this method.More times of EPD – sintering could ascent the density of HYO coating and its bonding-strength with Ir coating.?4?After 4 times of EPD – sintering,the average bonding strength of HYO coating was 20.8MPa and it could resist heat shock 3 times from 25? to 1250?.This means that HYO coating meet the operating requirement.The thermal emissivity of HYO coating rised with the increment of temperature,which is 0.69 at 400? and much higher than that of Ir coating.In the ablation experiments,when the distance of ablation was 10 mm,the surface temperature of HYO coating sample was 400? lower than that of Ir coating sample.The wind tunnel experiment showed that the surface temperature of HYO coating sample was 600? lower than that of Ir coating sample.