The Composition And Thermal Conductivity Of Modified Lanthanum Zirconate Thermal Barrier Coatings Prepared By Air Plasma Spray

Due to the limitation of temperature capacity,the conventional yttria stabilized zirconia（YSZ）thermal barrier coatings（TBCs）are not suitable for applications when the working temperature of gas turbine engines exceeds 1200^oC.Recently,the rare-earth zirconates have been proposed as a promising high temperature thermal barrier coatings due to their low thermal conductivity,low oxygen permeability,high melting temperature and excellent phase stability.Unfortunately,the radiative thermal conductivity of rare-earth zirconates is quite high at high temperature as a result of their infrared transparent nature.In order to resolve this critical problem,this study proposes three different strategies:the first is to employ yttrium（Y）or cerium（Ce）to dope La2Zr2O7;the second is to introduce a second phase lanthanum phosphate（LaPO₄）so as to reduce infrared transparency of coatings;the third is to apply a MoSi2-based high emissivity coating.After coating deposition by an air plasma spray approach,the structure evolution and heat insulating performance are studied after a static thermal exposure at 1300 ^oC of the targeting service temperature.First,La₂Zr₂O₇,A-site yttrium doped La₂Zr₂O₇ and B-site cerium doped La₂Zr₂O₇coatings have been prepared so as to reduce the minimum lattice thermal conductivity in high temperature plateau area(k_min).The results show that,the phases of yttrium doped La₂Zr₂O₇ and cerium doped La₂Zr₂O₇ coatings can maintain the pyrochlore structure during the entire heat treatment duration,suggesting these modified coatings have excellent high temperature stability.Compared to the undoped La₂Zr₂O₇ coatings,yttrium doped La₂Zr₂O₇ and cerium doped La₂Zr₂O₇ coatings tend to possess a better sintering resistance property.Thermal conductivity versus temperature（k-T）curve of yttrium doped La₂Zr₂O₇ coating is low and flat,which is caused perhaps by a rattling effect of Y3+in the oversized AO8 atomic cages in La₂Zr₂O₇ pyrochlores.In addition,the Y3+rattlers in such oversized cages tend to vibrate incoherently with other atoms in the lattice,which dramatically increase lattice anharmonicity and thus evidently reduce k_min.Further,the introduction of Ce⁴⁺will further decrease k_min by evidently reducing the elastic modulus of the doped La₂Zr₂O₇ solid solutions.Secondly,the lanthanum phosphate（LaPO₄）with thermal conductivity inversely proportional to temperature has been successfully added in the yttrium heavily-doped La₂Zr₂O₇.It turns out that the introduction of LaPO₄ accelerates the precipitation of the fluorite phase.A temperature-indepentent thermal conductivity in the multiple-phased coatings can be found,probably attributing to an increased phonon-grain boundary scattering effect.Finally,a high emissivity coating,MoSi₂,is deposited on the surface of LYZ-LaPO₄coatings.The emissivity of MoSi₂ coating and MoSi₂/LYZ-LaPO₄ coating is higher than single layer coating in near-infrared regions,which suggests the feasibility of reducing radiative thermal conductivity by an outmost high emissivity coating.In summary,the above three strategies can evidently reduce k_min and improve the infrared opacity of coatings,which can potentially improve thermal insulating and sintering resistance properties of coatings at high temperature.The current study offers some new insights into the improvement of thermal insulating property of rare earth zirconates at high temperatures for TBC applications.