Effect Of Doping On The Thermophysical Properties Of Zirconia-Based Thermal Barrier Coating Materials

An increasing operation temperature of the gas turbines and jet engines demands a better and better thermal insulation of the high temperature super-alloy components.Thermal barrier coatings with low thermal conductivity efficiently decrease the temperature at the surface of the super-alloy components and therefore improve the lifetime and efficiency of the engines.The state of the art thermal barrier coating material is the 8% Y₂O₃ stabilized ZrO₂?8YSZ?.In this thesis,multi-component oxide solid solutions are synthesized based on the rare earth oxide and ZrO₂ solid solutions.Effect of defects on the lattice structure and thermophysical properties is investigated.And the potential of these solid solutions as new thermal barrier coating materials is also evaluated.Specifically:First,Ti⁴⁺,Sn⁴⁺ and Th⁴⁺ were chosen to substitute the Zr⁴⁺ in 8YSZ and form the ternary solid solutions.Effect of the radius of the quadrivalent ions on the lattice structure and the t? phase stability was investigated.And the thermophysical properties controlling the performance of thermal barrier coatings,such as hardness,elastic modulus,fracture toughness,thermal conductivity and thermal expansion coefficient were also measured.The relation between the lattice defects or distortions and the thermal conductivity of the materials was discussed in detail.And the potential of the quadrivalent doping 8YSZ as new thermal barrier coating materials was also evaluated.Second,with more rare earth content,Ln₂Zr₂O₇-type rare earth zirconates were prepared.Simultaneously substituting at both Ln and Zr sites,quaternary solid solutions?Ca₂Nb₂O₇?_x?Gd₂Zr₂O₇?_1-x,?Nd₂Sn₂O₇?x?Yb₂Zr₂O₇?_1-x and ?Sm₂Sn₂O₇?_x?Yb₂Zr₂O₇?_1-x were synthesized.Effect of doping on the order-disorder phase transformation between the pyrochlore and fluorite was investigated and the unconventional change of the mechanical properties was also observed at the critical point of the phase transformation.Besides,effect of the lattice structure on both phonon and radiation thermal conductivity was discussed,providing a new perspective to obtain low thermal conductivity through the whole temperature range.Finally,with even more rare earth content,Zr₃Ln₄O₁₂-type solid solutions?Ln=Gd,Y,Er,Yb?were synthesized.Effect of the rare earth ionic radius on both the lattice structure and microstructure was investigated.Thermophysical properties,such as hardness,elastic modulus,fracture toughness,thermal conductivity and thermal expansion coefficient were also measured.Potential of Zr₃Ln₄O₁₂-type solid solutions together with 8YSZ as multilayer or gradient thermal barrier coatings was evaluated.