Investigation On LaPO₄/La₂Zr₂O₇ Composite Ceramics For Thermal Barrier Coatings

Thermal barrier coatings (TBCs) is a thermal protective coating widely used on the surface of the components in aero engine and gas turbine which are applied in high temperature. The coatings play an important role in heat insulation and corrosion resistance, which can increase the working temperature of the parts to prolong their service life. Yttria-stabilized zirconia (YSZ) as a traditional material is widely applied in thermal barrier coatings now, because it processes high temperature stability, low thermal conductivity, high thermal expansion coefficient. However, the YSZ can be sintered at more than 1200℃,which can cause the phase transition and failure of the coating.it’s very important to study a new generation of thermal barrier coatings materials. In this paper, the composite ceramics were investigated. The main contents are stated as following:LaPO4/La2Zr2O7 composite ceramics were calcined at high temperature. The characterization of XRD shows that the composite ceramics are composed of LaPO4 and La2Zr2O7 phases, so there is no chemical reaction and phase transformation during the sintering time, which means that it has high temperature stability. The micro-structures of the composite ceramics observed by SEM contain LaPO4,La2Zr2O7 and pores structures. The number of pores increases with the increase of LaPO4 content, and the smaller grains of LaPO4 were surrounded by the larger grains of La2Lr2O7.The thermal conductivity of LaPO4/La2Zr2O7 composite ceramics was tested by the laser flash method. The results show that the thermal conductivity of composite ceramic materials (1.527～1.554W/m,1000℃) is lower than that of YSZ (2.12W/m·K,1000℃) and La2Zr2O7 (1.56W/m·K,1000℃), which possesses the advantages of low thermal conductivity of La2Zr2O7.Thermal expansion instrument made by German Netzsch was used to measure the thermal expansion coefficient of composite ceramics. The coefficient of thermal expansion of the samples (9.97～10.13×10-6 K-1,1000℃) is 10% higher than that of La2Zr2O7(9.0×10-6 K-1,1000℃),and close to LaPO4(10×10-6 K-1,1000℃). The composite ceramic material possesses the high coefficient of thermal expansion of LaPO4.The hardness and modulus of elasticity of composite ceramics were measured by nanoindentation test. The results reveal that the composite ceramic materials performance good in mechanical properties.The hardness and elastic modulus of composite ceramic materials are between LaPO4 and La2Zr2O7. The hardness is higher than that of LaPO4,so its wear resistance is improved. The elastic modulus is lower than that of La2Zr2O7, which contributes to increasing of toughness.Finally, in order to study the corrosion mechanism of the composite ceramics by glassy materials (CaO-MgO-Al2O3-SiO2,CMAS) and S、V, corrosion test of LaPO4 and CMAS, La2Zr2O7 and Na2SO4/V2O5 mixture were carried out. LaPO4 does not react with CMAS,Na2SO4 and V2O5. There is no reaction between and Na2SO4. But Ca2La8(SiO4)6O2 and ZrO2 are produced by reaction of and CMAS.The reaction of La2Zr2O4 and V2O5 produces LaVO4 that also can be produced by reaction of La2Zr2O7 and Na2SO4/V2O5 mixture. Composite ceramic materials react with CMAS or Na2SO4/V2O5 mixture, because it contains more La2Zr2O7 with poor anti-corrosion and less LaPO4 with good corrosion resistance.