Structure And Properties Of Er₂O₃ Doped New Thermal Barrier Coating Materials

With the increase of aero engines' thrust ratio,the temperature of their combustors becomes higher and also working conditions become harsher.Under the circumstances,higher requirements should be fulfilled in structure stability,thermal and mechanical properties of thermal barrier coatings(TBC).Nowadays,Y2O3-stabilized ZrO2 ceramics(YSZ)have been widely used in TBC field while rare earth zirconates have received much attention as a novel type of TBC.But some intrinsic flaws exist in the above both TBC materials.YSZ coatings possess relatively high thermal conductivity,low absorptivity in high temperature radiation and undesired phase transition which will lead to cracking,spallation and even failure.And low thermal expansion coefficient and poor fracture toughness of rare earth zirconates impede their extensive application.The modification of the existing TBC by doping with rare earth oxides has become one of the main research directions.Among rare earth oxides,Er2O3 has manifested certain advantages in terms of modifying crystal structure stability and thermal conductivity of ZrO2 and La2Zr2O7 ceramic materials.However,the effects of Er2O3 on other properties in different systems have been rarely reported and also lacked comprehensive,systematic and deep research.The thesis made the study of both 9YSZ and Sm2Zr2O7 TBC systems by doping with Er2O3.Er2O3-stabilized ZrO2(ESZ)and(Sm1-xErx)Zr2O7 ceramics were prepared through solid phase synthesis.The effects of Er2O3 content were investigated on crystal structure,phase stability,microscopic morphology,thermal expansion,thermal conductivity,thermal radiation and mechanic properties.EB-PVD process was adopted to obtain ESZ thermal barrier coatings on metal substrate and the reasons for thermal shock failure at 1100? were also analyzed.On the basis of experimental results and comprehensive analysis,the underlying mechanisms on which Er2O3 doping exerts its influence on the structure and properties have been revealed.The main research results are listed as follows:(1)Tetragonal phase structure stability of ESZ ceramic materials and coatings obtained by EB-PVD has been improved through Er2O3 doping.Phase transition does not occur below 1300? while few monoclinic phase starts to appear at 1400?.With the increase of Er2O3 doping,high temperature structure stability becomes enhanced.After Er2O3 doping,the lattice distortions of(Sm1-xErx)2Zr2O7 ceramic materials arise and lattice vibrations become weaker.Furthermore,the(Sm1-xErx)2Zr2O7 structures show pyrochlores with low order.(2)Er2O3 doping has certain restrictions on crystal growth.Growth orientations about columnar crystals of ESZ obtained by EB-PVD change,the gaps between columnar crystals narrow and surface roughness of the coatings also decrease.The above structure changes increase surface infrared reflectance.Er segregation at grain boundaries suppresses the grain growth,which endows TBC materials with resistant-sintering.(3)Due to the atomic mass and the ionic radius difference between Er3+ and Y3+/Sm3+,Er3+doping enhanced phonon scattering in crystal lattice and decrease the thermal conductivity of the two types of materials.In particular,the thermal conductivity of 10ESZ decreases?10%and the thermal conductivity of(Sm1-xErx)2Zr2O7 reduces?8%at 1200? compared with 9YSZ.(4)Er3+ doping has a small effect on lattice energy and ionic bond strength and even reduces them.Thus,thermal expansion coefficients of both ESZ and(Sm1-xErx)2Zr2O7 are greater than11×10-6K-1,and fracture toughness of(Sm1-xErx)2Zr2O7 improves from 1.1 MPa·m1/2 to1.4 MPa·m1/2.(5)At high temperature,radiative thermal conductivity in ESZ system is proportional to T3.The changes of Er2O3 spectral absorption and lattice vibrations increase infrared absorption from 0.65 to 0.75,especially in short wavelength below 6?m.Radiative thermal conductivity in(Sm1-xErx)2Zr2O7 system has similar trend as infrared emissivity(absorption).The infrared emissivity first increases and then decreases with Er2O3 doping.And the optimal doping amount is x=0.1 when the infrared emissivity reached to above 0.75.(6)The main factors the failure of ESZ thermal barrier coatings include the diffusion of Al,Ni atoms in bond coats,the growth and evolution of TGO and the resulting crack propagation.After Er2O3 doping,the gaps between columnar crystals decrease which reduces the strain tolerance.ESZ thermal barrier coatings begin to fail after 3500 thermal shock at 1100?.With the increase of Er2O3 doping,the sizes of columnar crystals become even and dense,which impedes the diffusion of oxygen atoms and restrains the growth and evolution of TGO layers.Besides,the life of thermal shock resistance can reach to 4000 cycles.