Study On Structures And Properties Of Low Infrared Emissivity AlCr(Si)N/Cr/AlCr(Si)N Coatings

With the continus development of infrared seeking technology,improving the infrared stealth ability is of great strategic significance for developing the survival ability and combat effectiveness of the target.The research,development and application of infrared materials are mostly based on the additional coating deposited on the target surface to achieve the goal of reducing the infrared emissivity of the target surface.Due to the outstanding thermal stability and low infrared emission of the multi-layered coating,it has become the most promising novel infrared stealth materials.The composition and structure of low infrared emissivity multi-layered composited coating（ceramic layer/metal layer/ceramic layer）were designed through theoretical caclution and analysis.However,the multi-layered composite coating is faced with many difficulties,such as the element diffusion from substrate and oxidation of surface at high temperature,the microstructure of nanocrystalline and amorphous matrix will change at high annealing temperature.These factors will cause the failure of infrared emission performance in multi-layered coating,which will lead to the target exposure.Therefore,it is of great practical value and theoretical significance to conduct research on the structures and properties of ceramic layer/metal layer/ceramic layer composite coating based on the multi-layered structure,which mainly focused on the problems such as element diffusion,oxidation and infrared emissivity increase of the multi-layered coating at high temperature.Aim at these problems hereinbefore,the multi-layered AlCrN/Cr/AlCrN and AlCrSiN/Cr/AlCrSiN composite coatings were prepared by multi arc ion plating technology on K424 nickel-based superalloy substrate.The suitably optimum thickness for multi-layered composite coating was explored by studying the influence of metal layer thickness on its infrared properties.Combined with the principle of multi arc ion plating technology,the coating forming mechanism was investigated.The effects of ceramic layer on the thermal stability and infrared emission of multi-layered composite coating were studied by regulating the reaction gas flow rate.In addition,the effect of high temperature heat-treatment process on the chemical composition and microstructure of multi-layer composite coatings was systematically analyzed;the failure mechanism of their infrared emission characteristics was revealed,and the effective working temperature was determined.The main research contents and results are shown as follows:The minimum low infrared emission performance of Cr layer was selected with a deposition time of 30 min（264 nm）by studying the microstructure and infrared emission performance of metal Cr layers with different thicknesses.The effect of coating thickness on grain size and lattice distortion was considered by establishing a model for calculating the grain size of Cr layers.At the same time,the relationship between infrared emissivity and resistivity of samples was established based on free electron theory.Then,the mechanism of coating formation was investigated by combining with the principle of multi arc ion plating technology.Through discussion and analysis,it was found that the phase structure of AlCrN layer changed from metallic to ceramic coating,and from face-centered cubic structure to amorphous structure with the increase of N₂ flow rate.At the same time,the AlCrN layer also changed from conductor characteristics to semiconductor characteristics,and finally displayed as fully non-conductor electrical properties.In addition,the infrared emissivity of multilayer composite coatings increased with the increase of nitrogen partial pressure.After heat-treatment,the multilayer composite coating with amorphous AlCrN layer showed good thermal stability and low infrared emissivity property.The amorphous AlCrN matrix in multi-layered AlCrN/Cr/AlCrN composite coating would transform into a Na Cl fcc-Cr（Al）N structure after heat-treatment at750℃.A large number of grain boundaries existed in the ceramic layer of AlCrN after crystallization would be used as diffusion channels,which would deepen the inward or outward penetration distance of O elements in the atmosphere and Ni elements in the substrate.The nanocrystalline-amorphous AlCrN layer showed better thermal stability than crystal structure samples through modeling and analysis.Such a mass of grain boundaries was responsible for the failure of infrared emission performance of samples due to the formation of crystal phase at high temperature,which was suitable for low infrared emissivity applications below the temperature of 750℃.The nanocrystalline structure of AlCrSiN ceramic layer presented an hcp-Al N structure from hcp-Cr₂N by adding Si element.The crystallization temperature of amorphous AlCrN matrix in the ceramic layer was increased from 750℃to 850℃due to the uniform distribution of Si₃N₄ in the coating.The thermal stability of multi-layered sample could be improved by adding appropriate ratio of Si element,and the effective operating temperature of multi-layered AlCrSiN/Cr/AlCrSiN composite coating in thermal low infrared emission performance applicationgs could be increased to 850℃.