The Thermophysical And CMAS Corrosion Properties Of The SrZrO₃-La₂Ce₂O₇ Composite Ceramics

Thermal barrier coatings?TBCs?are typically deposited on the hot-end components surface of the gas turbine engines with a consequence of increased lifetime and efficency.The state-of-the-art TBC material,Y₂O₃-stabilized ZrO₂?YSZ?,can not be long-term used above 1200°C due to its phase transitions and sintering effect.It is urgent to develop new ceramic TBC materials to meet the demands of the next generation advanced turbine engines.SrZrO₃ is considered as a new TBC candidate material due to its good thermophysical properties,however,the phase transitions of SrZrO₃ occurred during heating are not desireable.La₂Ce₂O₇ with fluorite structure has extremely low thermal conductivity,high thermal expansion coefficients?TECs?and better high-temperature phase stability compared with YSZ.In order to take advantages both of SrZrO₃ and La₂Ce₂O₇,as well as overcome their shortcomings,?1-x?SrZrO₃-x La₂Ce₂O₇?x=0,SZ;x=0.3,S7L3;x=0.5,S5L5;x=0.7,S3L7;x=1,LC?composite ceramic powders and bulks were prepared by coprecipitation and pressureless sintering method,respectively.The mechanical properties,thermophysical properties and CaO-MgO-Al O_1.5-Si O₂?CMAS?corrosion behavior of the ceramic bulks were investigated.The X-ray diffraction analysis?XRD?was used to study the phase compositions of composite ceramic powders and bulks.The microhardness tester and nanoindenter were used to study the mechanical properties of the ceramic bulks.The TECs and thermal diffusivities of the ceramic bulks were studied by a high-temperature dilatometer and a laser flash method,respectively.The scanning electron microscope?SEM?,energy spectroscopy?EDS?and electron probe micro analysis?EPMA?were used to analyze the microstructures and elements distribution of the ceramic bulks.The particle size of the SZ-LC composite ceramic powders synthesized by the coprecipitation method is approximately 1-2?m with less agglomeration.SZ-LC consists of SZ with perovskite structure and LC with fluorite structure.Compared with single-phase SZ and LC,the diffraction peaks of SZ phase in SZ-LC shifted to small angle?except S7L3?,while the diffraction peaks of LC shifted to large angle.This can be attributed to the SZ in the SZ-LC powders formed solid solutions.The results of mechanical tests showed that,the mechanical properties of the SZ-LC bulks are superior to single-phase ceramics.S7L3 has the highest hardness of 6.9±0.4 GPa gradually with an increase of LC content,due to the lower fracture toughness of LC bulk.And the hardness and fracture toughnes of the SZ-LC bulks increase to some extent after long-term heat-treatment due to the decrease of porosity.The S5L5 bulk has the lowest thermal diffusivity of 0.3 mm²/s?1200??,which is?55%lower than that of SZ,followed by S3L7 with 0.4 mm²/s?1200??.The S5L5 has the lowest thermal conductivity of 0.9 W·m^-1·K^-1?1200??,which is?58%lower than that of SZ,followed by S3L7 with 1.0 W·m^-1·K^-1?1200??.The S5L5 has the highest TECs of S3L7 bulks did not fluctuate significantly during the entire heating process,indicating that the addition of LC makes the composite ceramics good thermal stability.The 33CaO-9MgO-13Al O_1.5-45Si O₂(C₃₃M₉A₁₃S₄₅)powder was uniformly coated on the SZ-LC bulks at a concentration of 30 mg/cm² to investigate their CMAS corrosion behavior at 1250 ^oC for 1 h,4 h and 12 h,respectively.Meanwhile,the SZ-LC composite ceramic powder and CMAS powder were mixted with mass ratio of 1:1 to investigate the corrsion products of the SZ-LC bulks under the same conditions.The results showed that the molten CMAS continuously wetted the ceramics at high temperatures,reacted with the ceramic surface and gradually penetrated into the ceramics,promoting the ceramics to decompose into oxides.And the dense corrosion layer composed of the high-melting melting crystals Ca₂?La_xCe₁-_x?₈?Si O₄?₆O₆-_4x and Ce₂Zr₂O_7.04developed,preventing the SZ-LC bulks from CMAS penetration.At the same time,high-melting point crystals Ca₂Al₂SiO₇ and MgAl₂O₄ also generated inside the molten CMAS,resulting in increasing the viscosity of the CMAS and decreasing the CMAS penetration rate.In addtion,Ce₂Zr₂O_7.04 adn LC developed due to diffusion of the elements in the SZ-LC bulks into the molten CMAS.