Synthesis And Properties Of Some Rare Earth Hafnates

Ceramic matrix composites are promising materials used as hot-section components for aero turbine engine with high thrust-to-weight ratio,which will be protected by thermal/environment barrier coatings with a multilayer structure due to the high temperature,complex loads and corrosive environment inside the engine.The top layer is required to possess high-temperature capabilities,including high melting point,good phase stability and sinter resistance at high temperature,also mechanical and thermal properties are important to ensure strain resistance and insulation.The top layer is directly exposed to combustion environmental,needs resistance to corrosion of low melting point oxides deposits(CaO-MgO-Al2O3-SiO2,CMAS).Rare earth hafnates are the candidates for the top layer of T/EBC due to their high melting point and excellent high temperature phase stability.In this thesis,the critical properties of RE4Hf3O12(RE is rare earth element)materials were systematically studied,and the potential of the material as top layer was evaluated.Specifically:Firstly,?-Yb4Hf3O12 and ?-Lu4Hf3O12 were successfully synthesized by solid reaction method.The mechanical and thermal properties of the ? phase rare earth hafnates were systematically studied.It was found that the material exhibited high hardness,excellent high temperature elastic modulus,low thermal expansion coefficient and thermal conductivity.The property data of ?-Yb4Hf3O12 and ?-Lu4Hf3O12 will contribute to the material selection and design of the coating system.The CMAS corrosion behavior of ?-Yb4Hf3O12 and ?-Lu4Hf3O12 at 1300 0C and 1500 0C were studied,respectively.?-Yb4Hf3O12 exhibited better CMAS resistance than ?-Lu4Hf3O12.Also,the water vapor corrosion resistance of the material under the condition of 1400 ? with 9.2%H2O was investigated.Secondly,rare earth hafnates with defect fluorite structure including Ho4Hf3O12,Er4Hf3O12 and Tm4Hf3O12 were synthesized.And the mechanical and thermal properties governing the performance of coatings,such as hardness,elastic modulus,flexural strength,thermal expansion coefficient and thermal conductivity were studied.The effect of defect fluorite structure on the high temperature properties was discussed.The defect fluorite rare earth hafnates exhibited larger coefficient of thermal expansion and lower thermal conductivity than those of ? phase rare earth hafnates.The CMAS corrosion resistance of Ho4Hf3O12,Er4Hf3O12 and Tm4Hf3O12 at 1300? and 1500 0C increased with rare earth ionic radius decrease.The corrosion mechanism of CMAS at 1300 ? was studied.It was found that the CMAS corrosion resistance was related to the kinetics of apatite product formation during the reaction and the more difficult it is to form apatite,the better of the CMAS corrosion resistance.Focus on the requirement of coating for low thermal conductivity and CMAS corrosion resistance,we designed and synthesized two single-phase multi-component solid solutions with defective fluorite structure,the(La0.2Gd0.2Ho0.2Er0.2Tm0.2)4Hf3O12 and(Yb0.2Lu0.2Ho0.2Er0.2Tm0.2)4Hf3O12.The results showed that the solid solution method effectively reduced the thermal expansion coefficient and thermal conductivity of the rare earth hafnates with defect fluorite structure.The CMAS corrosion behavior of solid solutions were greatly affected by the rare earth element,and the rare earth element with larger ionic radius can promote the formation of apatite product,thereby consume the CMAS deposits.