Structure Control And High Temperature Failure Behaviors Of Microarc Oxidation Multifunctional Coatings On Niobium Silicide Layer ↑

Niobium alloys tend to be the key candidate structural materials for new gas-turbine engines and metal thermal protective system of hypersonic flight vehicle,due to the combined advantages of high melting point,low density and the outstanding mechanical properties in high temperature.The poor oxidation resistance is the key obstacle that limits the extended applications.The single niobium silicide coatings have reached a plateau in improving the oxidation resistance.Besides,the thermal radiation properties and corrosion resistance to melten salt properties are low,which has been unable to meet the application demands of thermal radiation protection and molten corrosion salts resistance when applying in combustion turbine engine/hypersonic flight vehicle.Based on the problems mentioned above,this thesis proposed a series of multifunctional coatings fabricated by a duplex method.Firstly,a NbSi₂ layer is prepared by pack cementation as anti-oxidation bottom layer.Then,a ceramic outer layer is prepared on surface of NbSi₂ bottom layer by microarc oxidation/nanoparticles deposition synchronous microarc oxidation.The NbSi₂/Nb₂O₅-SiO_2（0.46）,NbSi2/Nb2O5-SiO2（0.64）and NbSi₂/Nb₂O₅-SiO₂-Al₂O₃ multilayer coatings are prepared by controlling the eletrolyte composition（（NaPO₃）₆,Na₂SiO₃ and NaAlO₂）;the NbSi₂/Nb₂O₅-SiO₂-SiC and NbSi₂/Nb₂O₅-SiO₂-MoSi₂ multilayer coatings are prepared by nanoparticles deposition synchronous microarc oxidation.The effect of the microstructure of the Nb₂O₅-SiO₂-X ceramic outer layer on the high temperature oxidation resistance,the molten salt corrosion resistance and the surface thermal radiation are determined,and the acitve mechanism of high temperature oxidation resistance,molten salt corrosion resistance and the surface radiation of the multilayer coatings are explored.The outer ceramic layers are prepared on NbSi₂ layer surface by microarc oxidation in the unique Na₂SiO₃,NaAlO₂ and（NaPO₃）₆ electrolytes,respectively.The Nb₂O₅-SiO_2（0.46）ceramic outer layer,mainly consisting of Nb₂O₅ and SiO₂phases transformed from NbSi₂ bottom layer,was formed in（NaPO₃）₆ electrolyte,and the PO₃^–ion in（NaPO₃）₆ electrolyte does not produce the obvious effect on phases composition.The AlO₂^–and SiO₃^2–ions dispersed in NaAlO₂ and Na₂SiO₃electrolyte,were induced to react and enter into the ceramic outer layer as Al₂O₃ and SiO₂ phase during plasma reactions in micro discharge areas,in this way the Nb₂O₅-SiO₂-Al₂O₃ and Nb₂O₅-SiO_2（0.64）ceramic outer layer was obtained on NbSi₂bottom layer surface,respectively.The oxygen diffusion mechansim controls the high temperature oxidation process for the NbSi₂/Nb₂O₅-SiO_2（0.46）,NbSi₂/Nb₂O₅-SiO_2（0.64）and NbSi₂/Nb₂O₅-SiO₂-Al₂O₃ multilayer coatings.The NbSi₂/Nb₂O₅-SiO₂-Al₂O₃ coating decreases the oxidation rate in the initial 15 h,attributing to the formation of the compact SiO₂ scales layer.In fact,the Al₂O₃ reacts with Nb₂O₅ and transforms into NbAlO₄ promotes the formation of the compact SiO₂ scales layer.The NbSi₂/Nb₂O₅-SiO_2（0.64）coating shows no obvious effect on the oxidation resistance,which is attributed to the fact that the porous structure can not transform into compact scale despite with high content of SiO₂.The NbSi₂/Nb₂O₅-SiO_2（0.46）coating shows a poor oxidation resistance because of the high content of Nb₂O₅,which promotes the rod-like grains growth and increases the oxygen diffusion paths.The oxidation resistance（in accordance with mass gain）of the three kinds of multilayer coatings and the single NbSi₂ coating are evaluated in the order:NbSi₂/Nb₂O₅-SiO₂-Al₂O₃（5.8mg/cm²）>NbSi₂/Nb₂O₅-SiO_2（0.64）（6.34mg/cm²）≈NbSi₂（6.49mg/cm²）>NbSi₂/Nb₂O₅-SiO_2（0.46）（6.96mg/cm²）.The corrosion behaviors of the three kinds of multilayer coatings and the single NbSi₂ coating indicate that the main corrosion products are NaNbO₃ and Na₂O?m SiO₂.The formation of a compact SiO₂ corroded scale layer is the key factor of corrosion resistance.More concentration of SiO₂ promotes the formation of a compact SiO₂scale layer,while Al₂O₃ in the ceramic outer layer grows into hollow rod-like grains,which decreases the compactness of the corroded scale layer.The corrosion resistance of the three kinds of multilayer coatings and the single NbSi₂ coating are ranked in the order NbSi₂/Nb₂O₅-SiO_2（0.64）>NbSi₂/Nb₂O₅-SiO_2（0.46）>NbSi₂>NbSi₂/Nb₂O₅-SiO₂-Al₂O₃.In order to further improve the oxidation resistance and the radiation emissivity of the outer ceramic,the NbSi₂/Nb₂O₅-SiO₂-SiC and NbSi₂/Nb₂O₅-SiO₂-MoSi₂multilayer coatings were fabricated by nanoparticles deposition synchronous microarc oxidation during microarc discharge process.The thickness of multilayer coatings significantly increased up to 50μm.The inner ceramic layer mainly consists of Nb₂O₅+SiO₂ mixture oxides,and gradually transit to the outer ceramic layer with a thickness of 10³0μm,mainly dominated with nano SiC or MoSi₂phase.The oxidation behaviors of the NbSi₂/Nb₂O₅-SiO₂-SiC and NbSi₂/Nb₂O₅-SiO₂-MoSi₂ multilayer coatings indicate that SiC and MoSi₂ are oxidized into SiO₂ after oxidation for 2 h and 30 h.The ceramic outer layers transform into the compact oxide scale layer mainly consisting of SiO₂,which efficiently decrease the oxygen diffusion rate towards the substrate.The mass gain per unit area are 2.49 mg/cm² and 0.37 mg/cm² respectively for the NbSi₂/Nb₂O₅-SiO₂-SiC and NbSi₂/Nb₂O₅-SiO₂-MoSi₂ multilyaer coating,which are lower than that of the single NbSi₂ coating.The molten salt corrosion behaviors of the NbSi₂/Nb₂O₅-SiO₂-SiC and NbSi₂/Nb₂O₅-SiO₂-MoSi₂ multilayer coatings indicate that the ceramic outer layers transform into the corroded scales,and SiC and MoSi₂ are oxidized into SiO₂ after oxidation for 2 h and 30 h,respectively.The corrosion salts permeate into the micropores,preferentially react with Nb₂O₅transforming to NaNbO₃.The mass change is–0.56 mg/cm² and–0.52 mg/cm²,respectively for the NbSi₂/Nb₂O₅-SiO₂-SiC and NbSi₂/Nb₂O₅-SiO₂-MoSi₂multilayer coating,which shows better corrosion resistance than the single NbSi₂coating with the mass change of 0.70 mg/cm².The emissivity of the single NbSi₂ coating is as low as 0.3⁰.5 in the wavelength of 3²0μm.While the emissivity of the NbSi₂/Nb₂O₅-SiO_2（0.46）、NbSi₂/Nb₂O₅-SiO_2（0.64）and NbSi₂/Nb₂O₅-SiO₂-Al₂O₃ multilayer coatings in 3⁸μm and 8²0μm is raised to 0.6⁰.7 and 0.75⁰.85,respectively.The introducing of SiO₂ or Al₂O₃ phase increases the surface radiation of Nb₂O₅-SiO₂ based coating.The emissivity value of NbSi₂/Nb₂O₅-SiO₂-SiC and NbSi₂/Nb₂O₅-SiO₂-MoSi₂multilayer coating is significantly promoted up to higher than 0.9 in the extensive wavelength of 3²0μm.The increased high emissivity contributes to the strong infrared vibration absorption bands of Si-C and Mo-Si,especially in the wavelength of 3⁸μm.To summarize,the oxidation resistance,the molten salt corrosion resistance and the thermal infrared radiation properties are significantly improved for the NbSi₂/Nb₂O₅-SiO₂-SiC and NbSi₂/Nb₂O₅-SiO₂-MoSi₂ multilayer coatings,which extend the better potential for engineering applications.