Structure Design,Preparation And Failure Mechanism Of High Thermal Insulation And Thermal Shock Thermal Barrier Coatings On TiAl Alloys

The application of materials with lighter,higher-strength,and higher-temperature resistant on aero-engines,can significantly improve its performance.The theoretical density of TiAl alloys is about 3.9 g/cm³,less than half of that of nickel-based superalloys,and high specific strength,which attracts widespread attention in the aerospace fields.Nevertheless,the service temperature of TiAl alloys ranges from 700to 900℃,which cannot meet the temperature requirements of advanced aero-engines.Therefore,it needs to prepare the thermal barrier coatings on the surface of substrate.The plasma spraying technology is a main technical means for the preparation of thermal barrier coatings,which can further increase the thermal insulation performance of substrate.In this work,the preparation of new thermal barrier coatings with high thermal insulation and high temperature resistance on TiAl alloys surface were studied through plasma spraying technology.Through the design and optimization of composition and structure of ceramic top layer and bonding layer of thermal barrier coatings,different ceramic top layer and bonding layer structures of YSZ and Gd₂Zr₂O₇thermal barrier coatings were prepared;and the thermal shock resistance,high temperature insulation and long-term service performance of the prepared YSZ and Gd₂Zr₂O₇ thermal barrier coatings systems were studied.The service performance and failure mechanism of the prepared thermal barrier coatings were investigated by observing and analyzing the macro and micro structure evolution,residual stress changes,the bonding between the coating and the substrate,and the mechanical and thermophysical properties of the coating.The simulation model of thermal barrier coatings on the surface of TiAl alloys was modified,and the stress evolution and residual stress distribution in the system during thermal shock were calculated.The thermal barrier coatings with excellent high temperature properties on the surface of TiAl alloys was successfully prepared,which laid an experimental foundation for the application of TiAl alloys in aero-engines.The main contents and conclusions are as follows:（1）The structure design for the bonding coating of the TiAl alloys substrates was carried out,and a bonding layer with good compatibility with TiAl alloys substrates was designed.When using TiAlCrY as the bonding coating,the residual tensile stress was smaller than that of NiCrAlY coating,and no new phase was formed at the interface between TiAlCrY and TiAl.Combined with conventional YSZ insulation,the TiAlCrY/YSZ double-layer thermal barrier coating has excellent thermal shock resistance at 1100℃.Moreover,the thermal shock resistance of TiAlCrNb/TiAlCrY/YSZ three-layer structure thermal barrier coating with stress relief layer was further improved.The thermal shock cycle lifetimes at 1000℃,1100℃and1200℃are 425,245 and 75 times,which is about 70%higher than that of double-layer structure coating.（2）Through using plasma spraying technology,TiAlCrY/Gd₂Zr₂O₇ double-layer structure thermal barrier coating was prepared on the surface of TiAl alloys substrates.At high temperature environment of 1100-1300℃,the thermal insulation temperature of TiAlCrY/Gd₂Zr₂O₇ coating ranged from 230 to 390℃,which was more than 70-90℃higher than those of the typical TiAlCrY/YSZ coating with the same thickness.It’s of great significance for the application of TiAl alloys to advanced aero-engines in the future.（3）The studies found that the failure mode of the thermal barrier coatings with YSZ and Gd₂Zr₂O₇ top coatings was different.For the YSZ top coating,the failure mode was the oxidative cracking of the bond coating;while for the Gd₂Zr₂O₇,the degradation was presented by the gradual spallation of the Gd₂Zr₂O₇ top coating.（4）The thermal shock cycle lifetimes of TiAl/TiAlCrY/Gd₂Zr₂O₇ system with high thermal insulation performance were about 300,100 and 50 cycles at 1000℃,1100℃and 1200℃,which were almost 25 times than that of Ni/NiCrAlY/Gd₂Zr₂O₇system with the same thermal insulation effect under the same test conditions.（5）The finite element simulation model of the thermal barrier coating on the TiAl alloys was constructed and modified,and the plasma spraying and thermal shock processes were simulated.By comparing the simulation and experimental results,the feasibility of the finite element model was demonstrated.The failure mechanism of the coating system was theoretically analyzed in depth,and guidance was provided for the material selection and structure design of the thermal barrier coating on the TiAl alloys surface.