Annealing Induced Atomic Structure Alternation In YSZ

Thermal Barrier Coatings(TBCs)can reduce the metal surface temperature and result in a significant enhancement of performance and efficiency of gas turbine engines.Consequently,TBCs have been a key technique for heat protection of aeroengines.Up to now,7YSZ thermal barrier coating technology has been relatively mature and widely applied in aeroengines.The working temperature of aeroengine is very high.Complex phase transformation will occur in YSZ because of the segregation of yttrium cation at high temperature.According to the concentration of Y in the phase transition products,these phases can be divided into rich-yttrium phase and lean-yttrium phase.The structure of YSZ has an important influence on its mechanical and physical properties.It is closely related to the failure of the thermal barrier coatings.Therefore,it is important to obtain the changes of the structure of YSZ after annealing,which can provide direct evidence for us to understand the failure mode of the thermal barrier coating at high temperature.In this thesis,the structural evolution of YSZ after annealing at 1200 ? for 48 h is studied.SEM is used to the characterize microstructures of YSZ before and after annealing.The structural characteristic of the rich-yttrium region is analyzed by TEM and a L12 superlattice structure is obtained.STEM is used to analyze the atomic structure information of L12 superlattice.The crystal structure model of L12 superlattice structure is constructed and analyzed by combining experiment and computer simulation.The results obtained in this study are as follows:Firstly,it is shown that the columnar crystals of YSZ fuse with each other and the columnar crystal gaps become smaller after annealed by SEM analysis.The formation of vacancy clusters in YSZ is observed in YSZ by TEM.EDS analysis confirms that they are rich-yttrium regions.A superlattice with L12 type structure is acquired in a rich-yttrium area,and it produces forbidden spots along the[100]direction.In the HAADF-STEM mode,the lattice distortion of cations in the L12 superlattice structure region is observed,where the space between two cations shows the change of width and width alternately along the[100]direction.Secondly,the atomic structure model of Zr6Y2O15 is built by MS.The structure model constructed by 1/4[001]displacement of the oxygen atoms plane parallel to the(100)lattice plane has been proposed to describe the cation distortion in YSZ.Through the relaxation of the structure model,it is found that the characteristics of cationic lattice distortion conform to the characteristics of HAADF image in experiment.According to the characteristics of the atomic arrangement in YSZ,two shear models in the oxygen planes are proposed.Thirdly,the electron diffraction pattern and the HAADF image of the Zr6Y2O15 model are simulated.It is found that the electron diffraction pattern produces the forbidden diffraction spots along the[100]direction,which is the same as the experiment results.The HAADF image simulation is also matched with the HAADF image in the experiment.The mechanical properties of the Zr6Y2O15 model are calculated and compared with the c-Zr02 phase.It is found that Zr6Y2O15 is prone to shear deformation.