CeO₂-based Ceramics For Thermal Barrier Coating

Thermal Barrier Coating(TBC),a ceramic coating widely deposited on high temperature components of gas turbines,can effectively improve the thermal efficiency and prolong the lifetime of gas turbines.As an important TBC candidate,lanthanum cerate(La₂Ce₂O₇,LC)shows extremely low thermal conductivity and large thermal expansion coefficient(TEC),and excellent phase stability at 1673 K.However,the sudden TEC decrease at low temperatures(low-temperature contraction)and the low fracture toughness greatly limit its application.Therefore,to eliminate the contraction at low temperatures and to improve the fracture toughness,especially the high-temperature fracture toughness,will help promote the application of LC coatings in the environment higher than 1473 K.In this paper,LC-based coatings prepared by air plasma spraying(APS)are systemically studied.La₂Ce_1.7Ta_0.3O_7.15(LCT)solid solution was synthesized by partial substitution of Ce⁴⁺in LC lattice with higher valence cation of Ta⁵⁺,and LCT coating was developed by APS technology.Results show that LCT coating with defective fluorite structure has excellent high temperature stability between room temperature and 1573 K,and the low-temperature contraction is effectively suppressed but not completely eliminated.LCT TBC exhibits relatively better thermal shock resistance than LC TBC at 1273 K.Considering that the different saturated vapor pressures of the constitutions of spray powder will lead to the coating composition deviation,the near stoichiometric LCT coating was successfully deposited by regulating the powder composition with excess Ce O₂ and La₂O₃,which further improved the comprehensive performance of LCT coating,but ultimately LCT TBC failed in the form of spallation within the ceramic top coat near the bond coat due to the thermal expansion mismatch stress,poor fracture toughness and weak bonding strength.Three kinds of LCT coatings were fabricated by controlling APS spray power.Results show that with the increase of spray power from 30 k W to 42 k W with interval of 6 k W,the coating structure is more denser,the La/(Ce+Ta)ratio in the as-sprayed coatings increases slowly first and then sharply,the average thermal conductivity of the coating increases gradually,and the average TEC of the coating decreases gradually,while the Vickers hardness,Young's modulus and fracture toughness of the coating increase first and then decrease,and the LCT coating acquired at 36 k W shows the best comprehensive mechanical properties.Spray power has a significant impact on the thermal cycling performance of LCT TBC,the thermal cycling lifetime of LCT TBC sprayed at low power(30 and/or 36 k W)shows better thermal shock resistance,and the low fracture toughness of ceramic top coat is the main reason for the premature failure.The toughening effect and toughening mechanism of 8 wt.%Y₂O₃ partially stabilized Zr O₂(YSZ)on LC coatings were investigated.Results show that LC coating with uniformly dispersed t'-Zr O₂ cannot be manufactured by using the"mixing-granulating-spraying"process.The solution reaction between YSZ and LC occurred during the spraying process,which could increase the fracture energy and thus the fracture toughness.Besides,YSZ doping also improves the thermophysical properties and thermal shock resistance of the composite coatings.Among them,20mol%YSZ doped LC coating presents the best comprehensive properties.Under the same condition,its thermal cycle lifetime is about 8 times as long as that of LC TBC and thermal expansion mismatch stress is the main cause of TBC failure.LCT coating with uniformly dispersed t'-Zr O₂ was successfully produced at 30k W by using the"granulating-mixing-spraying"process.Results show that YSZ-LCT composite coating exhibits relatively large TECs(7.2-10.1×10^-6K^-1,473-1673 K),low thermal conductivity(0.37-0.52 W/(m·K),298-1273 K)and relatively good fracture toughness(1.44±0.21 MPa·m^1/2)due to the combination of ferroelastic toughening,compressive stress field and crack deflection.However,its thermal cycling lifetime at 1323 K and 1373 K is respectively 53 cycles and 9 cycles,and thermal expansion mismatch stress is responsible for the TBC failure.