Study On Microstructure And Properties Of Silicide Composite Gradient Coating Prepared By Laser Cladding On Nickel-based Alloy

Nickel-based alloys are widely used in aviation,aerospace engines and other fields because of their excellent high-temperature oxidation resistance and excellent comprehensive mechanical properties.However,with the development of spacecraft,there are higher requirements for the comprehensive performance of rocket engine materials,and traditional nickel-based alloys can no longer meet the requirements of engine operating conditions.The preparation of high-temperature anti-oxidation coating on the surface is an effective way to meet the more demanding requirements of engine components.MoSi₂ has the properties of metal and ceramics.During high temperature oxidation,a dense,continuous and self-repairing protective film will be formed on the surface.Due to these advantages,MoSi₂ is a kind of ideal high temperature oxidation resistant coating material.However,the brittleness of MoSi₂ at room temperature greatly limits its application as a coating material.In this dissertation,laser cladding technology is used to prepare MoSi₂ coating on the surface of the nickel-based alloy,and the process method and influence law of laser cladding Mo-Si mixed powder are studied.The toughening of the coating is achieved by adding SiC particles and Ni alloying.A composite gradient coating with a gradient of SiC content is prepared by multi-layer cladding to reduce the interlayer stress and cracks.The influence of SiC content on the structure and properties of laser cladding SiC/Mo-Si-Ni composite gradient coatings was investigated,which can provide the experimental and theoretical basis for laser cladding of high-performance silicide composite coatings on nickel-based alloys.The main conclusions are as follows:（1）The MoSi₂ coating is prepared on the surface of nickel-based alloy by laser cladding technology.The structure of the cladding layer is mainly composed of plane crystals at the interface,cell crystals and dendrites in the junction zone,and equiaxed crystals in the middle and upper parts.The microhardness of the cladding layer and the high temperature oxidation resistance is significantly improved compared to the substrate.The performance of the MoSi₂coating obtained by laser secondary remelting is better.The oxidation rate constant of oxidation at 800°C for 12 hours is 4.21×10^-5,and the hardness of the cladding layer is up to 4.3 times that of the substrate.（2）The SiC/MoSi₂ composite coating on the surface of nickel-based alloy by laser cladding technology,and adding Ni to reduce the defects of cracks.With the increase of SiC content,the microstructure grain refinement,the dilution rate of the cladding layer gradually decreases,and the microhardness,high-temperature wear resistance and oxidation resistance are enhanced.When the SiC content is 15%,the high temperature oxidation rate constant of the coating is only 3.77×10^-5,and the hardness can reach 5.4 times that of the substrate.But when the SiC content reaches 20%,the pores and cracks are serious,and the formation of the cladding layer is difficult.（3）The gradient coating without cracks and defects can be obtained by changing the content of SiC in the preset powder layer by layer to carry out laser multilayer cladding experiments.The structure of the cladding layer changes uniformly and continuously,and forming a good metallurgical bond with the substrate.The microhardness of the coating has a gradient distribution,the maximum value is nearly 6 times higher than that of the substrate,and the high temperature friction and wear performance is also improved,but the high temperature oxidation resistance of the gradient coating changes little compared with a single coating.