Study On The Stability Of LaMgAl₁₁O₁₉ Thermal Barrier Coating At High Temperature

Thermal barrier coatings（TBCs）are ceramic coatings with well thermal insulation at high temperature.TBCs have important application in internal combustion engine,ramjet,gas turbine,rocket engine and other high temperature thermal protection.Aeroengine is a kind of gas turbine.Owing to the deposition of TBCs on the substrate surface,aeroengine can operate at higher temperatures,increasing the engine’s thermal efficiency and thrust-to-weight ratio.Therefore,the requirements for TBCs are becoming higher and higher with the continuous increase of aeroengine operating temperature.At present,7～8 wt.%Y₂O₃ partially stable ZrO₂（YSZ）,the most widely used TBC,cannot be used above 1200℃for a long time and has low CMAS corrosion resistance.LaMgAl₁₁O₁₉（LMA）thermal barrier coating with magnetoplumbite structure has well phase stability and sintering resistance at high temperature.As a TBC,LMA is promising applications at high temperatures.However,the LMA coating prepared by atmospheric plasma spraying（APS）contains lot of amorphous phases,and its service life is affected due to recrystallization and grain growth of amorphous phases during thermal cycling.LMA coating was prepared by APS.The microstructure evolution and grain growth mechanism of LMA coating were investigated by heat treatment at 900-1600℃,and the effect of microstructure on coating properties was also studied.LMA coating prepared by APS contains a large number of amorphous phases and has the lowest porosity（9%）.The amorphous phase recrystallized at 900℃and 1160℃.The LMA coating after heat treatment at 900℃has the highest porosity（19%）and the lowest thermal diffusion coefficient（0.53 mm²/s at 1000℃）.After heat treatment at 1100-1300℃,the crystallinity of LMA coating increases,and there are a lot of nano-flake crystals.The stress and strain tolerance of LMA coating increases,and the mechanical properties of LMA coating improve.After heat treatment at 1100℃,LMA coating has the highest Vickers hardness（12.1 GPa）.When the heat treatment temperature exceeds1400℃,the temperature exceeds the nucleation barrier during the two-dimensional grain growth,which is beneficial to the step growth of the coated grain.In addition,the probability of grain parallel stacking is increased and the grain thickness is significantly increased,which leads to the decrease of strain tolerance and mechanical properties of the coating.The Vickers hardness of LMA coating after 1600℃heat treatment is only6.8 GPa.The thermal shock resistance of sprayed LMA coating and heat-treated LMA coating at 900℃,1300℃and 1600℃for 12 h were tested.The average thermal cycle life of LMA coating was 544,524,588 and 387 times,respectively.CMAS corrosion was carried out on LMA coating after heat treatment at different temperatures,and it was found that the corrosion thickness of spray LMA coating was the smallest.Comprehensive analysis shows that porosity and amorphous phase can effectively reduce the thermal diffusion coefficient of the coating,the existence of small size flake crystal can improve the stress and strain tolerance of the coating,and low porosity can effectively prevent the infiltration of CMAS.In APS process,LMA coating with high crystallinity can be obtained by increasing the temperature of substrate which can reduce the cooling rate of LMA coating.The microstructure,thermal and mechanical properties of LMA coatings prepared by the two methods were characterized respectively.It is found that the LMA coating prepared by increasing the temperature of substrate has lots of recrystallized fine grains on the surface,higher porosity（9%,the coating of APS is 6%）,higher hardness（7.8 GPa）,higher fracture toughness(10.4 MPa·m^0.5),and lower thermal diffusion coefficient（0.82 m²/s at 1000℃,the coating of APS is 1.46 m²/s）.Increasing substrate temperature,suitably,is beneficial to decrease amorphous phase content and increase thermal cycle times of LMA coating.