Thermal Insulation Performance of RETaO4 Thermal Barrier Coatings Based on Finite Element Simulation
The temperature of engine components is increasing,and most alloys cannot operate directly in ultra-high temperature en-vironments.Thermal barrier coatings(TBCs),e.g.,yttria stabilized zirconia(YSZ),rare earth tantalite(RETaO4),are widely used as a protective measure in superalloys to enhance gas temperature,efficiency and service life.RETaO4 is regarded as a potential next generation TBCs due to the lower thermal conductivity,suitable thermal expansion coefficient,suitable Young's modulus,no phase change volume difference and ferroelastic toughening,and it is very difficult to estimate the thermal insulation capacity of TBCs by ex-periments,and the measurement results are inaccurate.Thermal insulation capacity of TBCs was descripted by means of finite ele-ments and experiments in this work and the simple model of TBCs,including ceramics(YTaO4 or YSZ),bond coat(NiCoCrAlY),thermal growth oxide(Al2O3)and substrate(Ni-based superalloys),built according to the heat transfer theory.Meanwhile,the effects of thermal conductivity,convective heat transfer coefficient,porosity and coatings thickness on the thermal insulation capacity were estimated.The thermal conductivity of ceramics,bond coat,thermal growth oxide and substrate from experimentand theoretical extrap-olation was used.The results showed that thermal insulation capacity changes with thermal conductivity,i.e.the lower thermal conduc-tivity,the better thermal insulation capacity.The thermal insulation capacity of the TBCs was enhanced from 143 to 520 K when ther-mal conductivity of ceramics was reduced from 2.5 to 0.3 W·m-1·K-1.The thermal conductivity of the coatings was lower,the thermal insulation capacity of the coatings increased faster.So thermal conductivity played an important role in thermal insulation capacity.When the surface temperature of the coating was 1773 K,the temperature of the cooling gas was 873 K,the convective heat transfer coefficient was 1900 W·m-2·K-1,and the maximum temperature of the substrate was below 1273 K(the maximum service temperature of superalloys),the thermal conductivity of TBCs should be less than 0.3 W·m-1·K-1.Secondly,thermal insulation capacity increased linearly with the increase of porosity of ceramics and the larger porosity,the better its thermal insulation capacity,e.g.,when the po-rosity was approximately 10％,the thermal insulation capacity reached about 300 K.When the porosity was approximately 30％,the thermal insulation capacity reached about 390 K.It was noted that increasing porosity of ceramics to improve the thermal insulation ability required taking into account the mechanical properties of TBCs at the same time.Thirdly,convective heat transfer coefficient was another parameter that affects the heat insulation capacity.The higher the convective heat transfer coefficient,the faster the tem-perature drop of TBCs.When the service temperature was 1773 K,the cooling gas temperature was 873 K and requiring the substrate temperature to be lower than 1273 K,the convective heat transfer coefficient was at least 2800 W·m-2·K-1.When the convective heat transfer coefficient was more than 1900 W·m-2·K-1,the increase in TBCs insulation capacity slowed down.Therefore,the method of increasing the convective heat transfer coefficient to improve the thermal insulation capacity of TBCs had limitations.In addition,TBCs had different thermal insulation capacity with different thicknesses of ceramics.Increasing the thickness of the ceramics was ben-eficial to the thermal insulation capacity of TBCs,e.g.,the thickness of coatings was 0.1 mm,and the thermal insulation capacity of TBCs was 293 K,the thickness of coatings was 0.3 mm,and the thermal insulation capacity of TBCs rose rapidly to 512 K.When the thickness of coatings was more than 0.5 mm,the temperature gradient inside the thermal barrier coatings cooled down slowly.The thickness of coatings as less than 2 mm and the thermal insulation capacity of TBCs increased limitedly.Besides,the substrate surface reaching thermal equilibrium spent more time when the thickness of TBCs was thicker,e.g.,the substrate surfacereaching the thermal equilibrium spent 7 s and the temperature of the coating rose rapidly in 0～3 s when the thickness of ceramics was 0.1 mm.The time of thermal equilibrium was more 20 s when the thickness of TBCs was more than 2 mm.TBCs of YSZ and YTaO4 had excellent thermal in-sulation ability.In service at 1373 K,the thermal insulation capacity of YSZ was about 147 K,and the thermal insulation capacity of YTaO4 reached 274 K.In service at higher temperatures,the advantage of YTaO4 was more obvious compared with YSZ.In service at 1573 K,the thermal insulation capacity of YSZ was about 205 K,and the thermal insulation capacity of YTaO4 reached 392 K.The thermal insulation performance of YTaO4 was better than YSZ.This paper simulated the thermal insulation performance of YTaO4,veri-fied the thermal insulation mechanism of the coatings,and provided theoretical support for the engineering application of YTaO4.
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