Study On High Temperature Antioxidation Coating Of Metal Iridium By Molten Salt Electroplating

With the development of aerospace technology,the working temperature of the engine combustion chamber of aerospace vehicles has gradually increased.The noble metal iridium（Ir）has extremely low oxygen permeability which is a promising oxygen barrier material for thermal protection components in high-temperature propulsion systems.The United States has used rhenium-based iridium plating for engines.The molten salt electrodeposition method has a fast deposition rate and can adapt to complex components.The study of the influence of the Ir coating on the microstructure and other properties of the molten salt electrodeposition process can provide certain technical support for related research and applications in our country.In this paper,the melting point and thermal stability of NaCl-KCl-CsCl molten salt,IrCl3 and NaCl-KCl-CsCl-IrCl3 molten salt were studied.The influence of time on the microstructure of iridium coating was investigated,the feasibility of plating iridium on different substrates was explored,and the nucleation and growth mechanism of iridium coating were summarized.The main findings were as follows:The thermal stability of molten salt was tested and verified by TG-DSC technology.The lowest eutectic point of the NaCl-KCl-CsCl ternary molten salt system was 483℃,and the ternary molten salt volatilized at 750℃;IrCl3 decomposed at about 707.88℃ under argon atmosphere.The Cs3IrCl6 compound phase was found in the cooling material of the IrCl3-NaCl-KCl-CsCl quaternary molten salt.It was speculated that IrCl3 and CsCl would be complexed in the molten state,which was conducive to the stable existence of IrCl3 in molten salt.The stable use temperature of IrCl3-NaCl-KCl-CsCl quaternary molten salt should not exceed 700℃.The research on electroplating iridium coating on different substrates showed that iridium could realize molten salt electrodeposition of iridium on Mo substrate,MoSi2 substrate and Re substrate,and obtained continuous,dense and flat iridium coating on rhenium substrate.In the case of DC electroplating,the electrodeposited iridium in the range of 600℃-700℃ showed that the grain size of the coating increased gradually with the increase of temperature.Compared with the coating prepared at low temperature（600～650℃）,when the temperature was higher than 700℃,the coating and grains grown rapidly and unevenly,the surface quality of the coating was deteriorated,and there existed many holes.Therefore,the preferred plating temperature was 600～650℃.Electrodeposition of iridium at 0.1 A/cm2,0.2 A/cm2,and 0.5 A/cm2 showed that the grain size of the coating decreased gradually with increasing current density,and that too high current density（0.5 A/cm2）would lead to iridium nucleation.With rapid nucleation and growth,the texture orientation of the coating changed as（111）→（220）→（311）with thegradual increase of the current density.The electroplating time was 60 minutes,90 minutes and 120 minutes respectively.The electroplating test showed that the grain size of the coating increased gradually with the prolongation of the electroplating time,and there was a certain thickness of the interdiffusion transition layer between the coating and the substrate.The law of coating from interface nucleation to growth was divided into three stages:in the interfacial incubation period,iridium nuclei were dominated by growth,forming larger grains;in the rapid nucleation period,fine iridium grains rapidly nucleated and covered the matrix;The growth phase was characterized by simultaneous grain growth and coating thickening.An iridium coating with a thickness of about 50 μm could be obtained by pulse plating at 650℃ for 150 min,and the coating deposition rate was 20 μm/h,which was significantly greater than the thickness and deposition rate of the iridium coating by DC plating;Compared with constant current electroplating,the surface of the coating coated by pulse commutation electroplating tended to be flat,the grains of the coating were uniform,the film layer was continuous and dense,and the preferred orientation growth was not obvious.