Study On Thermophysical Properties And CMAS Corrosion Behavior Of La_1-xYb_xPO₄ Ceramic For Thermal Barrier

Thermal barrier coating is one of the core technologies to improve the performance of aero-engine turbine blades and prolong the service life.The 8wt.%Y₂O₃ stabilized Zr O₂（8YSZ）coating material that has been in service cannot meet the growing demand for high thrust-to-weight ratio and high-efficiency aero-engines due to phase change and sintering（≥1200^oC）issues.However,rare earth phosphate materials are considered as one of the candidate materials for a new generation of ultra-high temperature thermal barrier coatings due to their advantages of lower thermal conductivity and better high temperature phase stability.In this paper,sol-gel method was used to prepare five kinds of ceramic powders of La_1-xYb_xPO₄（x=0,0.05,0.1,0.15,0.2）,and then the ceramic blocks were prepared by pressing and sintering.The tissue structure,micro-morphology,thermophysical properties,mechanical properties and CMAS（Ca O-Mg O-Al₂O₃-Si O₂）corrosion behavior of five kinds of ceramic blocks were studied.The main contents of the research and the results obtained are as follows:The tissue structure and micro-morphology of five kinds of ceramic powders and ceramic blocks of La_1-xYb_xPO₄（x=0,0.05,0.1,0.15,0.2）were studied.As the calcination temperature increases,the crystallinity of the ceramic powder gradually increases.When the doping content is x=0,0.05,0.1,the XRD pattern of the ceramic powder is La PO₄ with monazite structure,and there is no Yb PO₄ diffraction peak of xenotime structure in the XRD pattern.When the doping content x=0.15,0.2,the XRD patterns are not only La PO₄ of monazite structure,but also Yb PO₄ diffraction peak of xenotime structure.Five kinds of ceramic powders were pressed and sintered to prepare corresponding ceramic blocks.The XRD patterns of the ceramic blocks are basically the same as those of the ceramic powder.With the increase of Yb³⁺doping content,the average grain size of the main phase La PO₄ of the ceramic blocks decreases from 2.818μm to 1.687μm,which decreases by about 67.04%.The relative density of ceramic blocks increases from 89.5%to 98.4%,with an increase of about 9.94%.The thermophysical and mechanical properties of five kinds of ceramic blocks of La_1-xYb_xPO₄（x=0,0.05,0.1,0.15,0.2）were studied.At 800^oC,the thermal conductivity value of the ceramic blocks decreases first and then increases with the increase of the Yb³⁺doping content.When the doping content is x=0.05,the thermal conductivity value is the lowest,0.99 W/（m·K）,it is about 9.09%higher than undoped ceramic blocks.At 1400^oC,the thermal expansion coefficient of the ceramic block with a doping content of x=0.2 increased from 10.118×10^-6 K^-1 of undoped ceramic block to10.871×10^-6 K^-1,an increase of about 7.44%.The hardness and fracture toughness of ceramic blocks gradually increase with the increase of Yb³⁺doping content.The hardness of ceramic bulk materials increases from 2.95 GPa to 3.86 GPa without doping,which increases by about 30.85%.The fracture toughness of ceramic block increases from 0.996 MPa·m^1/2 when undoped to 1.052 MPa·m^1/2,with an increase of about 5.62%The CMAS corrosion behavior of five kinds of ceramic blocks of La_1-xYb_xPO₄（x=0,0.05,0.1,0.15,0.2）at 1250^oC was studied.The results show that the products of reaction between the undoped ceramic block and CMAS are anorthite Ca Al₂Si₂O₈ and spinel Mg Al₂O₄.After doping,the products of reaction between ceramic blocks and CMAS are not only the above several substances,Ca₈Mg Yb（PO₄）₇,Yb₂Si₂O₇ and a small amount of apatite Ca₂RE₈（Si O₄）₆O₂（RE=La,Yb）are also produced.The cross-section of the ceramic blocks is composed of three areas after corrosion,which are the CMAS residual layer,the corrosion penetration layer and the part that has not been penetrated and corroded.After 5 h and 10 h of CMAS corrosion,the corrosion penetration depth of Yb³⁺-doped ceramic blocks is significantly reduced compared with that of undoped ceramic block.This is the result that Yb³⁺doping can effectively promote the formation of high melting point substances of Ca₈Mg Yb（PO₄）₇ and Yb₂Si₂O₇,which consumes CMAS and fills the corrosion penetration channels of molten CMAS,effectively inhibiting further corrosion penetration of CMAS.