Preparation And Properties Of Magnetoplumbite-type Rare Earth Hexaaluminate Thermal Barrier Coating

TBC is a kind of surface thermal protective coating,which can improve the thermal efficiency of gas turbine and the service life of hot-section metal component.At present,typical 6?8 wt.%Y₂O₃ partially Stabilized Zr O₂(YSZ)can only operate for a long time under 1200°C.YSZ shows infrared transmission,and photon conduction plays a major role at high temperature,which cannot meet development of the next generation advanced gas turbines.Magnetoplumbite-type La MgAl₁₁O₁₉(LMgA)is a promising material for TBC application with its excellent thermal stability and anti-sintering.However,LMgA shows poor stability at 1500°C in humid environments caused by H⁺/Mg²⁺ionic exchange.Besides,LMgA coating prepared byAtmosphere Plasma Spraying(APS)contains amorphous phases.Amorphous phases will recrystallize during high temperature services,which affects the service life of coating.It is a potential way that replacing Mg²⁺with other alkaline earth metal ions to modify the shortcomings of LMgA,such as poor resistance to high temperature water vapor,and further improve the performance of materials.In this paper,La MeAl₁₁O₁₉(Me=Mn,Fe,Co,Ni,Cu,Zn)was prepared by solid-reaction,and the effects of Me²⁺on mechanical properties,high temperature thermal conductivity and infrared emissivity of La MeAl₁₁O₁₉(LMeA)were studied.The results show that Vickers hardness of LMeA ceramics is between 8-14 GPa,fracture toughness of La ZnAl₁₁O₁₉(LZnA)ceramic is the highest(?3.16 MPa·m^1/2),and that of La CuAl₁₁O₁₉(LCuA)ceramic is the second(?2.88 MPa·m^1/2).With the increase of temperature between room temperature and 600°C,the thermal diffusivity of LMeA ceramics decreases,and above 600°C,the thermal diffusivity of LMeA ceramics increases significantly due to photon heat transfer.Due to the high infrared emissivity and low infrared transmittance,the average thermal diffusivity between600°C and 1400°C of LCuA ceramic is the lowest.Comprehensive analysis shows that LZnA and LCuA have a better performance and are expected to be applied in high temperature thermal protection materials.LaZnAl₁₁O₁₉(LZnA)and La CuAl₁₁O₁₉(LCuA)coatings were prepared byAPS,and their mechanical,thermophysical and thermal cycling properties were studied.The results show that as-sprayed LZnA coating contains amorphous phase with a content of 28%,with an average thermal expansion coefficient of 5.97×10^-6 K^-1.The thermal conductivities of LZnA coating is between 1.24-1.46 W?m^-1?K^-1.During exposure to 1300°C,a total porosity of?10.5%can be maintained even after 1000 h aging,indicating a high sintering resistance.In 1100°C furnace cyclic testing,thermal cycle life of LZnA/YSZ DCL-TBC increase significantly when YSZ is used as the intermediate layer.LZnA/YSZ DCL-TBC exhibits a thermal cycling lifetime of 669cycles,which is about 13 1.7 and 1.16 times as long as the LZnA TBC the conventional YSZ TBC and the LMgA/YSZ DCL-TBC,respectively.As a result,LZnA is a promising candidate material for advanced thermal barrier coating at high temperature application.The thermal conductivity of LCuA coating close to that of LZnA coating,which is between 1.36-1.79 W?m^-1?K^-1.The average coefficient of thermal expansion of LCuA coating is 4.13×10^-6 K^-1.Similar to LZnA coating,LCuA coating also has excellent phase stability and good sintering resistance.However,the content of amorphous phase in the as-deposited LCuA coating is 97.9%,which is much higher than that of LZnA coating(28%).Too much amorphous phase will have an adverse effect on the thermal shock resistance of the coating,so the thermal cycling lifetime of LCuA/YSZ DCL-TBC is significantly lower than that of LZnA/YSZ DCL-TBC,only 339 times.One of the outstanding advantages of LCuA coating is the excellent infrared radiation.From room temperature to 600°C,the infrared emissivity of LCuA coating is 0.78?0.82,which is obviously superior to LZnA coating(0.663?0.736)and LMgA coating(0.713?0.755).In addition,among the LCuA,LZnA and LMgA coatings,the UV-vis-NIR reflectivity of LCuA coating is the lowest,so LCuA coating can effectively reduce the photon conduction at high temperature.In addition to its good application prospect in the field of ultra-high temperature thermal barrier coating,LCuA is also expected to be used to the surface thermal protection system of hypersonic aircraft.