Microstructure And Thermal Emissive Property Study On La-CeO₂Coatings Prepared By EB-PVD

Hypersonic vehicles have been focused by world powers. The aerodynamicheat increases with the flight speed and present new challenge for the thermalprotection technology. The Metal Thermal Protection Systems （MTPS） with highemissivity protection coatings stand out from several thermal protection systems andis focused in thermal protection engineering for the security, reliability andlong-term effective thermal protection. But the development of MTPS has beenlimited by the low melting point. High emissivity coatings can help to emit a greatdeal of aerodynamic heat, decrease the surface temperature and prolong the servicelife of MTPS. In this paper, a new La-CeO₂thermal protective coating has beenprepared on Haynes214superalloys by electron beam physical vapor deposition（EB-PVD）. The mechanical properties, thermal chock resistance and emissiveproperty of pure CeO₂coatings with different morphology and La doped CeO₂（LDC）coatings with different La concentration have been investigated. The composition,phases, texture, microstructure and chemical states of the thermal protectivecoatings have been investigated by XRD, XRF, XPS, SEM, pole figures and laserconfocal microscopy. The emissivity of the thermal protective coatings at600℃,800℃and1000℃is measured by infrared emissivity tester and the relative factorsof that influence the emissivity of the coatings are discussed. Thermal shockresistance test is also conducted between1000℃and room temperature for480thermal cycles. The residual stress, hardness and interface adhesion have beenstudied by X-ray diffraction, microhardness tester and scratch test.The microstructure and mechanical properties of the coating during thermalcycles have been researched. The deposition power influences the growth rate andmethod, and then influences the microstructure and surface morphology. When thedeposition power is3kW, the coating is feather-like loose columnar structure withporous surface; while the deposition power is4kW, the coating is loose squarecolumnar structure with rough surface distributed by many cauliflower-like grainclusters. When the deposition power is5kW, the coating is dense square columnarstructure with smooth surface. CeO₂coatings have strong titled （200） texture butthis texture weakens with the deposition power. The deposition power has littleeffect to the relative concentration of Ce³⁺and Ce⁴⁺in the CeO₂coatings.Comparing to the smooth and dense CeO₂coating, the emissivity of loose andporous CeO₂coatings reaches0.87at800℃and that of rough CeO₂coatings with cauliflower-like clusters reaches0.9at600℃, both enhanced by50%. The enhancedemissivity attributed to the multiple reflection and absorption in porous surface andresonance absorption in the rough surface. The CeO₂coatings show excellentthermal shock resistance during thermal cycling test and no spallation occurs eventhe coatings suffer480thermal cycles between1000℃and room temperature. Fromthe damage degree in the surfaces, the coatings exhibit better thermal shockresistance with deposition power. The excellent thermal shock resistance of CeO₂coatings can be attributed to three fators: higher thermal expansion coefficient,residual compressive stress and columnar structure. The surface hardness of theCeO₂coatings increases quickly first and then decrease slowly during thermalcycling process. The TGO layer thickness increases with thermal cycles, about16nm per cycle. The deposition power has little influence to the surface hardnessand interface adhesion.In LDC coatings, the dopant La atoms enter into CeO₂lattice and introduceexpansion but not change the cubic fluorite structure. With doped La increasing, thetitled （200） texture became stronger and the texture direction gradually parallel tothe normal of the surface. The doped La has little effect on the Cerium oxidation andthe morphology but increase the oxygen vacancies the coatings.The doping La concentration has important influence on the emissivity of theLDC coatings, especially in the low wavelength band. The emissivity of the LDCcoatings increases with the La content. The40%LDC coating’s emissivity reaches0.9at600℃and0.76at1000℃. With dopant La concentration increasing, in onehand, the free-carrier concentration increase, improving the free-carrier absorption,and in other hand, the lattice distortion increase, improving the phonon absorption.The LDC coatings also show excellent thermal shock resistance in the thermal shocktest and no spallation occurs even the coatings suffer480thermal cycles. Thermalshock resistance has close relationship with the residual stress. Residualcompressive stress is benefit for the thermal shock resistance enhancement. Duringthermal shock progress, the hardness of the LDC coatings increases and thethickness of TGO layers also increase, about16nm per cycle. With the doped Laincreasing, the surface hardness of as-deposited LDC coating increases, but theinterface adhesion becomes weaker.