Study On Mechanisms And Methods For The Rare Earth Hexaaluminate Thermal Barrier Coatings Resistant To CMAS Attcks

Thermal barrier coatings（TBCs）are widely used on the surface of the aero-engine hot-section components such as combustion chamber and turbine blade/vane,for thermal insulation and corrosion protections.Nowadays,the operation temperatures of the aero-engine hot-section components continue to grow as the turbine inlet temperature gradually increases.If the hot-section component surface temperature is above 1260 ℃,the ingested dust,volcanic ash and fine sand particles will be melted to for a liquid glass phase and adherent to surface of the TBCs.A liquid-solid reaction will occur between the melted CMAS and ceramic thermal barrier coating,followed by dissolution of the ceramic coating in the melted CMAS pools.Such a process will facilitate the degradation and spallation failure of the TBCs,causing serious problems to the performance and lifetime of the aero-engine.Therefore,the developments of new TBC candidate materials and methods with much improved resistances to CMAS degradation is a very important research direction for further developments of high-temperature TBCs.The main research objects of this paper are the rare earth hexaaluminate new high temperature thermal barrier coating materials La MgAl₁₁O₁₉（LaMA）and GdMgAl₁₁O₁₉（GdMA）with excellent thermo-mechanical properties.First,we comparatively studied the CMAS corrosion degradation mechanism of Atmospheric Plasma Spraying（APS）YSZ（6～8 wt.%Y₂O₃ stabilized zirconia）,LaMA and GdMA coatings at 1350 ℃.Secondly,dense barrier layers were constructed on the surface of APS LaMA and YSZ coatings by in-situ laser-glazing,and the mechanisms of their resistance to CMAS penetration corrosion at 1350 ℃ were compared.Results show that at 1350 ℃,the molten CMAS evenly covered the surface of the YSZ coating.After interaction for 1 h,the molten CMAS has completely penetrated the～1 mm thick YSZ coating.With the reaction time is extended to 12-48 h,the YSZ coating undergoes a more and more serious transition from the metastable tetragonal phase（t’）to the monoclinic phase（m）.The resulting spherical monoclinic Zr O₂ particles further intensify the penetration of liquid CMAS into the inner YSZ coating,which will seriously reduce the service life of the coating.However,at 1350 ℃,the molten CMAS does not evenly cover the surface of the APS LaMA and GdMA coatings,but present a dispersed local distribution.A larger wetting angle is beneficial to prevent the spread of liquid CMAS on the surface of the LaMA and GdMA coatings and the penetration into the coatings.Due to the high porosity of the APS LaMA coating,the corrosion degradation products of CMAS are mainly the porous CaAl₂Si₂O₈ anorthite surface layer and the thinner MgAl₂O₄ inner layer within the reaction time of 1-48 h.Because the liquid CMAS is dispersed in the porous coating,the dissolved La³⁺concentration is low.After corrosion,the surface and cross section of the LaMA coating do not form the Ca₂La₈（SiO₄）₆O₂ apatite phase,only the La-rich fine white spot-like grains.After the GdMA coating is corroded at 1350 ℃ for 1-48 h,the reaction products are distributed in layers.The top layer is CaAl₂Si₂O₈ anorthite phase,the second layer is MgAl₂O₄ phase,and the third layer is Ca₂La₈（SiO₄）₆O₂ apatite.The bottom layer is the un-corroded GdMA coating.Corrosion products can effectively heal a large number of longitudinal microcracks in the GdMA coating,effectively block the penetration of liquid CMAS,and improve the high-temperature service performance of the coating.After laser glazing,a dense barrier layer is formed in situ on the surface of the APS YSZ and LaMA coatings.Due to the large thermal stress during the laser glazing process,the dense barrier layer can have a certain number of penetrating vertical cracks.At 1350 ℃,the molten CMAS penetrates along the vertical cracks and undergoes a rapid dissolution-precipitation degradation reaction of the APS YSZ layer below the dense barrier layer,forming a large number of holes and cracks,seriously destroying the original layered structure of the APS YSZ coating,and causing the dense barrier layer peels off.Since the number of penetrating vertical cracks in the dense barrier layer of LaMA is relatively small,most of the CMAS is blocked on the surface of the sample to undergo continuous dissolution-precipitation reaction,corrosion products CaAl₂Si₂O₈ anorthite,MgAl₂O₄ spinel,Ca₂La₈（SiO₄）₆O₂ apatite forms a dense barrier surface with LaMA.The research shows that APS GdMA coatings have good anti-CMAS corrosion performance,and laser glazing can effectively build a dense barrier layer on the surface of LaMA coatings,significantly improving the high-temperature resistance of this type of coating to CMAS corrosion performance.This study has important guiding significance for the development of a new type of high temperature thermal barrier coatings for aero-engines against CMAS corrosion.