Rotating Ferroelastic Domains And Toughening Mechanism Of YTZ Ceramics

It is urgent to develop a new generation of thermal barrier coating materials（TBCs）since 7-8YSZ cannot meet the increasing high-performance demands for TBCs.Rare-earth tantalite（RETaO₄）and its modified materials are viewed as a new generation of potential TBCs since they have better thermal properties and ferroelastic domain switch than 7-8YSZ.Nevertheless,the extremely severe service environment proposes higher requirements on performances of TBCs.Specifically,mechanical properties become particular important as the basic property of TBCs.At present,researches on RETaO₄ and its modification materials mainly concentrate in phase structure,thermal performance and simple anti CMAS corrosion performance.However,mechanical properties,possible toughening mechanism and its relations with microstructures of RETaO₄ and its modification materials have not been explored deeply.On this basis,mechanical properties,microstructure and toughening mechanism were studied systematically after Zr O₂ gradient doping modification of the YTaO₄ base.Major research contents are introduced as follows:1.The preparation,phase structure test and fracture toughness characterization of Y_0.5-x/2Ta_0.5-x/2Zr_xO₂ thermal barrier coating block have been completed.New TBCs composed of Y_0.5-x/2Ta_0.5-x/2Zr_xO₂（YTZ;x=0,0.04,0.08,0.12,0.16,0.20,0.24 and0.28）were prepared successfully through chemical coprecipitation method.According to phase structural analysis after slow cooling,it found that the Zr O₂ solid solubility limit in pure M-phase YTZ was 0.15≤x<0.16.According to calculated results of fracture toughness,Y_0.44Ta_0.44Zr_0.12O₂ showed the highest fracture toughness(3.44±0.35 MPa·m^1/2),which was basically equal to and even slightly higher than that of 7-8YSZ(3.33±0.28 MPa·m^1/2).2.The toughening mechanism of Y_0.5-x/2Ta_0.5-x/2Zr_xO₂ thermal barrier coating material was systematically analyzed by the changes of microstructure before and after loading.After observation of atomic sizes of YTaO₄ and Y_0.44Ta_0.44Zr_0.12O₂which had the optimal fracture toughness,it found that M-phase YTZs were all composed of alternating arrangement of two types of equal thick variant lamellas which have different orientations and presented rotating ferroelastic domain structures.Finally,it proved through a theoretical calculation that the mean spontaneous strain of rotating ferroelastic domain structure in M-phase YTZ was the lowest and the mean spontaneous strain energy was the lowest.These explained the reasonability of rotating ferroelastic domain structures.3.The toughening mechanism of Y_0.5-x/2Ta_0.5-x/2Zr_xO₂ thermal barrier coating material was systematically analyzed by the changes of microstructure before and after loading.According to STEM atomic characterization of microstructures of pure YTaO₄ and Y_0.44Ta_0.44Zr_0.12O₂ before and after loading,it found that both materials had ferroelastic domain switch phenomena after loading,thus achieving ferroelastic toughening effect.In the late stage of ferroelastic domain switch pure YTaO₄ and Y_0.44Ta_0.44Zr_0.12O₂,M→M’phase transitions without volume effect occurred,thus producing toughening effect.Since adulteration of Zr O₂ decreased the angle difference（α）between two variants of M-phase YTZ,the crack deflection was strengthened.Moreover,since Zr O₂ adulteration decreases the energy barrier against ferroelastic domain switch and energy barrier against stress-induced M→M’phase transitions,the ferroelastic toughening effect is strengthened and the phase transition zone is expanded.As a result,fracture toughness of materials is increased.