Study On The Thermophysical Properties And High Temperature Phase Stability Of Doped Zirconia-based Ceramic Materials

With the advent of the era of large aircraft,the aero-engine known as the "heart"of the aircraft needs to develop to higher thrust-to-weight ratio,more efficient and safer.Among the 100 major projects in the“14th Five-Year Plane",the design and manufacture of gas turbines and aero-engines are listed in the first place.Yttriastabilized zirconia（YSZ）ceramic material has excellent mechanical properties,high thermal expansion coefficient,good thermal insulation performance and thermal stability,is the most widely used top layer material of thermal barrier coatings.However,when the service temperature of YSZ ceramic materials exceeds 1200℃,it is prone to high temperature sintering,phase transformation,molten salt corrosion,etc.,which is difficult to meet the needs of gas turbines to develop to higher inlet temperatures.Therefore,improving the phase stability and corrosion resistance of YSZ ceramic materials by doping one or more ions is one of the important research directions to develop new thermal barrier coating materials.In this paper,Yb³⁺,Er³⁺and Ti⁴⁺ are selected to dope 8.5YSZ ceramic materials.By studying the effects of different ion doping on the crystal structure,phase composition,thermophysical properties,high temperature phase stability and corrosion resistance of 8.5 YSZ ceramic materials,the differences between different ion doping and crystal structure,phase composition and macroscopic properties are discussed.Under the premise that the stabilizer content remains unchanged,Yb³⁺is gradually added to the 8.5YSZ ceramic material until Y3+is completely replaced.The xYbYSZ（x=0,2.5,4.5,6.5 and 8.5 mol%）ceramic materials are composed of tetragonal and metastable tetragonal phases.The incorporation of Yb³⁺has a great influence on O-O coupling,which in turn affects tetragonality and phase stability of tetragonal phase.Compared with 8.5YSZ ceramic materials,xYbYSZ samples have higher Vickers hardness and fracture toughness,lower thermal conductivity,better high-temperature phase stability and excellent corrosion resistance.When the addition of Yb³⁺is 6.5 mol%,the Vickers hardness of ceramic gets maximum（12.60 GPa）,which is 26.38%higher than that of 8.5YSZ.And the thermal conductivity(1.90 W·m^-1·K^-1)of 6.5YbYSZ at 1000℃ is the lowest.Which is 5.94%lower than that of 8.5YSZ.In addition,the tetragonal phase of 6.5YbYSZ has stronger resistance to phase transformation at 1300℃.Besides,the addition of Yb³⁺can effectively improve high temperature phase stability of 8.5YSZ at 1300℃,and the monoclinic phase content of the ceramic samples shows an "S"-curve with the heat treatment time.On the basis of 6.5YbYSZ,by adding Er³⁺and Ti⁴⁺ respectively,more new scattering centers are generated to further reduce the thermal conductivity of ceramic materials.The results show that the incorporation of Er³⁺and Ti⁴⁺has an obvious effect on the z-axis direction of the cell,which affects the tetragonality and phase stability.For Er-YbYSZ ceramics,as the doping amount of Er³⁺ with larger ionic radius increases,the t phase in ceramic samples gradually changes to t’ phase and c phase.Besides,the addition of Er³⁺is benefical to improve the transformability of t’→c and t→m.Therefore,Er-YbYSZ ceramics have poor mechanical properties,good high temperature phase stability and resistance to CMAS corrosion.When the addition of Er³⁺is 8 mol%,the elastic modulus（205.4 GPa）,Vickers hardness（11.71 GPa）,fracture toughness(2.90 MPa·m^1/2)and thermal conductivity(1.30 W·m^-1·k-1 at 1000℃)are the lowest.Compared with 6.5YbYSZ ceramic samples,they are 3.57%,7.1%,26.4%and 31.46%lower,respectively.However,for Ti-YbYSZ ceramics,the addition of Ti⁴⁺with smaller ionic radius increases the tetragonality of t phase and enhances the transformability of t→m,which leads to the excellent fracture toughness and poor phase stability of Ti-YbYSZ ceramic materials.When the Ti⁴⁺ doping amount is 8 mol%,the fracture toughness of the ceramic material is the highest at 4.91 MPa·m^1/2,which is 24.6%higher than that of the 6.5YbYSZ ceramic sample.Combined with the effects of Er³⁺ and Ti⁴⁺ doping on the mechanical properties,thermophysical properties,high-temperature phase stability and corrosion resistance of YbYSZ ceramic samples,the mechanical and thermal properties of Er₂O₃-TiO₂-ZrO₂ ceramic materials are further explored.The exploration in this system declares that the Er₂O₃-TiO₂-ZrO₂ ceramic samples concentrate the advantages of Er³⁺ doping on the thermophysical properties and Ti⁴⁺doping on the mechanical properties of YbYSZ ceramic materials.When the doping amount of Er³⁺and Ti⁴⁺are 8 mol%and 8 mol%,the hardness and fracture toughness of this ceramic are 12.68 GPa and 4.83 MPa·m^1/2,respectively.Besides,the thermal conductivity of this ceramic sample is 1.80 W·m^-1·K^-1 at 1000℃.