Fundamental Study On The Modification Of La₂Zr₂O₇ System Materials And Their Thermal Barrier Coatings

Compared to the conventional yttria stabilized zirconia?YSZ?insulation ceramic,the rare-earth zirconate is widely applied in aerospace field because of its low thermal conductivity,low oxygen permeability,high melting temperature,excellent chemical stability and excellent thermal stability.In order to meet the requirements of higher service life of thermal barrier coating,we have prepared various La₂Zr₂O₇-based ceramics employing yttrium?Y?as doping element,and metastable tetragonal yttria stabilized zirconia?t'-YSZ?as the second phase respectively.Then their fracture toughness,thermal expansion coefficient?TEC?and high temperature stability have been studied in detail.Based on the above studies,we have fabricated coatings by the air plasma spraying using the powders with good overall properties and preliminarily studied the basic coating properties,which could hopefully promote the practical application of La₂Zr₂O₇-based ceramics as high-temperature thermal barrier coatings.Firstly,we have studied the influence of element Y on the structure and properties of(La_1-xY_x)₂Zr₂O₇ ceramics.The results show that,the phase in the composition range of x?0.3 exhibits single pyrochlore structure as a result of the complete dissolution of Y³⁺in the La₂Zr₂O₇ host lattice.The Y³⁺solid solubility reaches saturation beyond the composition of 0.3,where the fluorite phase forms.In the composition range of0.4?x?0.8,the pyrochlore and fluorite phase coexist.When x?0.9,the crystal becomes the single fluorite.The conventionally sintered ceramics exhibit rather dense structure,and the grains of the double-phase intermixture shows considerably reduced size.In the pyrochlore corner,the(La_0.7Y_0.3)₂Zr₂O₇ exhibits best overall properties,with an average TEC of 9.84×10^-6/oC which is 11%higher than the undoped La₂Zr₂O₇,a fracture toughness of 1.114 MPa·m¹/2/2 that is 13.09%higher than the undoped La₂Zr₂O₇.In addition,the Y-La₂Zr₂O₇ pyrochlores exhibit no phase transformation up to 1400?,suggesting their excellent high temperature staility.Secondly,we have successfully added the metastable phase t'-YSZ as the second phase in the La₂Zr₂O₇ matrix phase by a combination methods of the plasma spray quenching the commercially available 8wt.%YSZ and the spark plasma sintering.The structure and properties of xt'-YSZ/?1-x?La₂Zr₂O₇ composite ceramics have been studied.The results show that t'-YSZ is thermochemically compatible with the matrix phase La₂Zr₂O₇.There are some tiny particles locating at the grain boundaries and grain interior.With an increase of the second phase t'-YSZ,the average grain sizes decrease noticeably,which is attributed to the the pinning effect by a second phase particles on the grain boundary migration.The fracture toughness of composite ceramics increases with the increase of the second phase,and reaches 2.131?0.144 MPa·m^1/2with a50vol%t'-YSZ addition,which has increased by 140%compared to La₂Zr₂O₇.The primary toughening mechanism is revealed as the ferroelastic domain switching toughening,as no moniclic phase have detected from the fracture surfaces and thus the transformation toughening could be ruled out.The TEC of composite ceramics increased with increasing the content of YSZ phase,which is because the TEC of YSZ is bigger than that of the La₂Zr₂O₇.In addition,it is unveiled that the existence of La₂Zr₂O₇ could somehow retard the phase transformation process of the matestable tetragonal t'-YSZ and the t'-YSZ/La₂Zr₂O₇ double-phase ceramics exhibit decent high-temperature stability.Finally,we have fabricated coatings by the air plasma spraying method by using the 0.5t'-YSZ/0.5La₂Zr₂O₇ powders that exihibit the best overall properties,and preliminarily studied some basic properties of such coatings.The results show that,the La₂Zr₂O₇ and the YSZ phases can coexist reasonably well.The coating shows a typical lamellar morphology exhibited by the APS coatings.The coating has a density of5.15g/cm³ and a porosity of 14.66%.Upon heat treatment at 1300?for various times,the diffusional transition,i.e.t'?t+c,occurs,but the martensitic transformation,i.e.t?m,does not occur,and as a result,no monoclinic phase forms.Therefore,the composite coatings show excellent high-temperature phase stability.