Microstructure And CMAS Corrosion Behaviour Of Rare-earth Zirconate Thermal Barrier Coatings

Rare–earth zirconates have generated substantial interest as the next generationof TBC （Thermal barrier coatings） based primarily on their intrinsically lowerthermal conductivity, higher phase stability and resistance to sintering than thestate–of–the–art TBC materials YSZ. Sm₂Zr₂O₇, Sm_4/3Yb_2/3Zr₂O₇and La1.6Eu0.4–Zr2O7powders were synthesized by solid reaction. Free–stand coatings of YSZ,Sm₂Zr₂O₇,Sm_4/3Yb_2/3Zr₂O₇and La_1.6Eu_0.4Zr₂O₇were prepared by atmosphereplasma spray. YSZ and double ceramic layered Sm₂Zr₂O₇/YSZ,（Sm2/3Yb1/3）2–Zr2O7/YSZ and La_1.6Eu_0.4Zr₂O₇/YSZ were sprayed on GH99super–alloy substratewith NiCoCrAlY as bond–coat.Sm₂Zr₂O₇and La_1.6Eu_0.4Zr₂O₇exhibit pyrochlore–type structure while Sm4/3–Yb2/3Zr2O7consists of most defect fluorite–type phase and a little of pyrochlore–type phase.Energy Diffraction Spectrum imply that a part of rare–earth oxide in rare–earthzirconates was lost during plasma spray where the most loss happened in Sm₂Zr₂O₇and the least in Sm_4/3Yb_2/3Zr₂O₇. The content of loss increased in step with spraypower. XRD results show that spray power have a significant impact on phasecompositions of rare–earth zirconate coatings. Sm_4/3Yb_2/3Zr₂O₇coatings sprayed onGH99super–alloy substrate at40kW exhibit defect fluorite–type structure whileSm₂Zr₂O₇and La_1.6Eu_0.4Zr₂O₇coatings present a mixture of defect fluorite–typephase and a small amount of tetragonal ZrO₂.CMAS （Calcium–Magnesium Alumino Silicate） attacks on TBC wereinvestigated with free–standing coatings. Results show that a dense sealing layerbetween rare–earth zirconate coatings and molten CMAS is quickly produced assoon as they contact each other. The sealing layer comprised of grains of the apatitetype phase and fluorite type phase can arrest molten CMAS. The smaller ionicradius of rare–earth apply, the more quickly reaction become between correspondingrare–earth zirconate and molten CMAS, consequently the higher resistance coatingspossess to the penetration of molten CMAS.The thermal shock resistance performances of the four as–sprayed coatingswith matrix at1250oC were tested by heating–water method. The results indicatethat the thermal shock resistance ability of Sm_4/3Yb_2/3Zr₂O₇/YSZ andLa_1.6Eu_0.4Zr₂O₇/YSZ coatings are equal to YSZ coatings while Sm₂Zr₂O₇/YSZcoatings is much higher than YSZ coatings. The existences of tetragonal ZrO₂which can toughen Sm₂Zr₂O₇coatings facilitate the higher thermal shock resistance abilityof Sm₂Zr₂O₇/YSZ coatings.