Thermal And Mechanical Properties Characterization And Fracture Mechanism Analysis Of ZrO₂ Doped YTaO₄ Ceramics At High Temperature

Since the concept of thermal barrier coating（TBCs）was put forward in 1953,researchers have been constantly exploring it.With the development of aviation industry,the working environment of thermal barrier coating has become worse,which makes the limitations of the widely used 7-8 wt.%Y₂O₃ stabilized ZrO₂（YSZ）ceramic materials increase.For example,8YSZ is prone to T-M transformation when the temperature is low,and its corrosion resistance to CMAS is weak,etc.People have made a series of explorations for the new generation of thermal barrier coating materials and structures,such as doping modified 8YSZ and new zirconate ceramics,and found that their fracture toughness and thermal shock resistance are not as good as 8YSZ,so they have not been applied in practice.This topic is to explore the new zirconium tantalum series thermal barrier coating materials with ferroelasticity.By preparing（2x=0,10%,15%,20%mol）ZrO₂ doped YTaO₄ ceramic block,on the microstructure,thermal properties（thermal conductivity,thermal expansion coefficient）,The fracture toughness,fracture strength and Young’s modulus of ceramics at high temperature were characterized in situ by three-point bending method combined with digital image speckle method,and the evolution law of mechanical properties of ceramic materials with temperature was analyzed,providing the basis of thermal-mechanical experimental data for the study of zirconium and tantalum series coating materials.The main research contents are as follows:1.Determine doping composition through phase diagram.High temperature solid phase reaction was used to synthesize（2x=0,10%,15%,20%mol）ZrO₂ doped YTaO₄ceramic blocks were characterized by XRD、SEM、EDS and other basic characterization techniques.It is found that the XRD patterns of the ceramic powder block can coincide with the diffraction peaks of the standard card（PDF#24-1415）,which proves that the ceramic is M-phase structure.By comparing the surface microstructure of ceramic with four different doping ratios,we found that the grain size of ceramic gradually decreased with the increase of Zrcontent.EDS spectra showed that the four elements of Ta,Y,Zrand O were evenly distributed.When the doping content of ZrO₂ reached 15%mol,the second phase precipitated phenomenon existed in ceramic samples.2.The thermal expansion coefficient and thermal conductivity of ceramics were measured by thermal expansion meter and laser thermal conductivity meter respectively.The results show that when the temperature ranges from room temperature to 1000℃,Different content（2x=0%,10%,20%mol）element Zrceramic thermal expansion coefficient were 2.8×10^-6K^-1-10.9×10^-6K^-1,1.7×10^-6 K^-1-10.4×10^-6K^-1,0.4×10^-6K^-1-10.3×10^-6K^-1,with the increase of Zrcontent,the thermal expansion coefficient of the material gradually decreases.Different content of Zrelement ceramic thermal conductivity at room temperature to 1200℃,respectively is 1.9W/（m·K）～4.1 W/（m·K）,1.5 W/（m·K）～2.6 W/（m·K）,1.6 W/（m·K）～2.7 W/（m·K）,1.6W/（m·K）～2.7 W/（m·K）.The results show that the thermal conductivity of pure YTaO₄ ceramic blocks decreases with the temperature increasing below 800℃,and the thermal conductivity of pure YTaO₄ ceramic blocks increases gradually from800℃ to 1200℃ due to the increasing proportion of thermal radiation.The variation trend of the thermal conductivity of doped Zrceramic blocks is roughly the same as that of pure YTaO₄ ceramics.Compared with pure YTaO₄ ceramics,the thermal conductivity of YTaO₄ ceramics decreases significantly below 800℃,while the thermal conductivity of YTaO₄ ceramics is greater than that of pure YTaO₄ ceramics at 800℃ to 1200℃.3.The mechanical properties of ceramic materials at room temperature,400℃,600℃ and 800℃ were characterized by self-built three-point bending test platform in high temperature environment combined with digital image speckle method,and the evolution law of ceramic Young’s modulus,fracture toughness and fracture strength with temperature was analyzed.The results show that the fracture toughness of the four ceramic samples with different doping ratios decreases gradually with the increase of temperature.The mechanical properties of the 10%mol ZrO₂ doped ceramics have little change with the increase of test temperature.The fracture toughness decreases from 2.6 MPa·m^1/2at room temperature to 2.3 MPa·m^1/2.The change law of fracture strength and Young’s modulus is consistent with fracture toughness,and both show a gentle downward trend.By comparing the SEM images of pure YTaO₄ Young’s modulus samples tested at different temperatures,it was found that the microstructure of the section did not change significantly with the increase of temperature.By comparing the TEM images of pure YTaO₄ Young’s modulus samples tested at room temperature and 800℃,it was found that the path of microcrack propagation at room temperature was more tortuous than that at high temperature,which required more fracture energy consumption.