Study On Prepration And High-Temperature Oxidation Behavior Of Aluminizing Iridium Coating

Iridium?Ir?has been applied as a high-temperature oxidation-resistant coating because of its extremely low oxygen permeability.Nevertheless,there are some problems which reduce the service life of Ir coating in the application including volatilization due to high-temperature oxidation,“cold–end”corrosion by propellants,diffusion of micropore,segregation along grain boundaries and low emissivity.In this dissertation,the Ir coating was aluminized by pack cementation.The effects of processing parameters of the aluminizing and heat treatment conditions on the microstructure of the aluminized coating were studied to summarize the formation and evolution rules of the infiltration layer for the purpose of microstructure control of the infiltration layer.The high-temperature failure mechanisms of the aluminized coatings with different structures were revealed by studying the high-temperature oxidation behaviors of different aluminized coatings.The effects of aluminizing temperature,time and the composition of infiltration agent on the microstructure of the infiltration layer were studied.The results show that the infiltration layer changes from monolayer structure of IrAl to double layer structure of IrAl_2.7?3?/IrAl with the increase of aluminizing temperature or aluminizing time.The thickness of infiltration layer has a linear relationship with the aluminizing temperature and the parabolic relationship with aluminizing time,respectively.The diffusion of aluminum?Al?in the aluminizing process follows the Fick's second law.The diffusion activation energy is different when aluminizing in different infiltration agent.The lower weight ratio of catalyst to aluminum agent,the higher the diffusion activation energy.The diffusion activation energy is 69.65KJ/mol when the infiltration agent is composed of 2%Al and 2%NH₄Cl.The results of the mechanical property test and emissivity measurement of the infiltration layer show that the microhardness of Ir layer,IrAl layer and IrAl_2.7?3?layer is429,1417 and 1225?in HV?,respectively.The elastic modulus of Ir layer,IrAl layer and IrAl_2.7?3?layer has a gradient change,which resulting in a pretty good mechanical coMPatibility.IrAl_2.7?3?layer the is the brittlest with a fracture toughness of about 25.8MPa·m^1/2.The bond strengthes among the Ir substrate,IrAl layer and IrAl_2.7?3?layer are all higher than 36.4 MPa.The surface emissivity of the coating increased form 0.114to 0.411 in the wave band of 2.5-25?m after aluminizing,and further increased to 0.728after oxidation.The influence of heat treatment conditions on the microstructure of infiltration layer was investigated under vacuum and argon atmosphere.The results show that in vacuum,25.6%of Al in the IrAl_2.7?3?layer volatilizes outwards,which causes the weight loss of the infiltrated layer.The rest of Al in the IrAl_2.7?3?layer diffuses inwards to form IrAl,which increases the thickness of the IrAl layer.In Argon,almost all the Al in the IrAl_2.7?3?layer diffuses inwards to thicken the IrAl,resulting in an IrAl monolayer structure.IrAl monolayer samples and IrAl_2.7?3?/IrAl double-layer samples were oxidized in static atmosphere at 1400?,1600?,1800?and 2000?,respectively.The results show that IrAl layer is directly oxidized to form a porous Al₂O₃,while the IrAl_2.7?3?layer in IrAl_2.7?3?/IrAl sample undergoes"melting-oxidation"process to form a coMPact Al₂O₃.The weight loss of the pure Ir sample was 45.72 mg/cm² after oxidation at 1800?for 3h,while the net weight losses of the single layer sample and the double layer sample were-0.13 mg/cm² and 1.88 mg/cm²,respectively,indicating a significant increase in the oxidation resistance of the aluminized samples.The net weight loss of the monolayer sample is obviously smaller than that of the double layer sample.Although the Al₂O₃ coating formed from the double-layer coating is denser,the Al₂O₃coating is not uniform due to the“melting-oxidation”process of the outer layer,which accelerates the failure of the coating.Therefore,the oxidation resistance of IrAl_2.7?3?/IrAl double layer is poorer than IrAl monolayer.The evolutions of the structures of the IrAl_2.7?3?/IrAl double layer samples with the oxidation time at 1400?and 1800?were investigated.The results show that a portion of Al in IrAl_2.7?3?out layer diffuses outwards to react with oxygen to form Al₂O₃and Ir*?the newly formed Ir distinguished from the substrate Ir?and the rest of Al in the IrAl_2.7?3?layer diffuses inwards to thicken the IrAl layer.IrAl_2.7?3?layer becomes thinner with increasing time,and the higher the temperature is,the faster the IrAl_2.7?3?is consumed.When the IrAl_2.7?3?is completely consumed,the Al in the IrAl layer continues to diffuse outward to form the Al₂O₃ and the Ir*until the IrAl is exhausted.However,the consumption rate of the IrAl layer is much less than that of the IrAl_2.7?3?layer.Depending on the difference of the temperature and time,the final structure of the coating is double layer of Al₂O₃/Ir*or Al₂O₃/?Ir*+Al₂O₃?.The results of the oxidation assessment in simulated service environment show that under the same test condition?heat flux of 2.03MW/m²,stationary pressure of 1.8kPa and total enthalpy of 19MJ/kg?,the temperature of the stagnation point of the pure Ir/graphite cap is stable at 2494?,resulting in melting of the Ir coating near the stagnation point,obviously higher than that of the aluminizing modified Ir/graphite cap,which is about 1330?.The obvious decrease in the temperature of the stagnation point is ascribed to the high emissivity and low catalyst activity of the aluminized Ir coating.