Effect Of Ion Substitution On Infrared Radiation And Lifetime Of Magnetite Rare Earth Hexaaluminate Thermal Barrier Coatings

Thermal barrier coating（TBC）is a kind of ceramic coating applied to the surface of hot end components.It can provide thermal insulation and corrosion protection for the surface of hot end components.One of the key properties of thermal barrier coatings is thermal conductivity.The heat conduction of inorganic solid materials can be regarded as phonon conduction procedures in low temperature conditions,while photon conduction makes a large contribution at high temperature.Therefore,how to reduce phonon conduction at low temperature and photon conduction at high temperature is the important way to keep the low thermal conductivity of materials.The ion substitution of La or Me in LaMeAl₁₁O₁₉ crystal structure（Me=Mg,Cu,Zn）is expected to reduce phonon conduction at low temperature and photon conduction at high temperature,thus reducing thermal conductivity and achieving better thermal insulation effect on the matrix.LaMeAl₁₁O₁₉ ceramic material was synthesized by solid state reaction method,and its thermodynamic properties,thermal shock resistance and CMAS corrosion resistance were studied.The results show that the fracture toughness and elastic modulus of LaMeAl₁₁O₁₉ increase with the increase of the atomic number of Me ions.La Cu Al₁₁O₁₉（LCA）has the highest infrared emissivity in the range of room temperature to 600℃,ranging from 0.867 to 0.931.La Zn Al₁₁O₁₉（LZA）is the second,ranging from 0.554 to 0.691.La Mg Al₁₁O₁₉（LMA）has the lowest infrared emissivity,ranging from 0.486-0.600.Therefore,LCA has excellent infrared radiation performance.The average thermal conductivity of LMA is 3.044 W/（m·K）in the range of room temperature to 1000℃.The LZA is the second with 2.802 W/（m·K）.LCA has the lowest average thermal conductivity of 2.681 W/（m·K）.At the same temperature,the coefficient of thermal expansion of the three materials has little difference.LMA and LZA still maintain magnetite phase structure after thermal shock failure,while LCA is decomposed during thermal shock.The CMAS corrosion products of LaMeAl₁₁O₁₉ include Ca（Mg,Al）（Si,Al）O₇（diopside）and Ca Al₂Si₂O₈（anhydrite）,which can be explained by"dissolution-precipitation"mechanism.In conclusion,LZA and LCA have good comprehensive properties and are expected to be applied in thermal barrier coatings.LaMeAl₁₁O₁₉/YSZ thermal barrier coatings were prepared by APS process.The thermal properties and thermal cycling of LaMeAl₁₁O₁₉/YSZ thermal barrier coatings were studied.The results show that LaMeAl₁₁O₁₉ layer exfoliates from YSZ layer due to the mismatch of thermal expansion coefficient between LaMeAl₁₁O₁₉ layer and YSZ layer and volume shrinkage caused by recrystallization of amorphous phase.The YSZ layer was exposed to high temperature,which accelerated the sintering and TGO growth,and promoted the spalling of YSZ layer.In the process of plasma spraying,LZA and LCA powders produce more decomposition than LMA powders,which leads to the difference of magnetoplumbite phase content in the three coatings,and thus affects the thermal cycling lifetimes of the three coatings.The volume shrinkage of the three coatings is related to the contents of LaMeAl₁₁O₁₉,La Al O₃ andγ-Al₂O₃amorphous phases.At low temperature,the thermal conductivity of LaMeAl₁₁O₁₉coating decreases with the increase of Me²⁺atomic number.However,at high temperature,LCA coating has better infrared radiation performance than LMA and LZA coating,which weakens the contribution of photon conduction to thermal conductivity,leading to the decrease of thermal conductivity.Ca_0.75La_0.5Mg Al₁₁O₁₉（CLMA）,La(Mg_0.5Zn_0.5)Al₁₁O₁₉,La(Mg_0.5Cu_0.5)Al₁₁O₁₉ and La(Zn_0.5Cu_0.5)Al₁₁O₁₉（LZCA）were synthesized by solid phase reaction.The thermodynamic properties of ceramic materials were studied.The results show that the average thermal expansion coefficient of the doped bulk material is lower than that of the undoped bulk material.In the range of room temperature to 1000℃,CLMA has the highest thermal conductivity.LMZA,LMCA and LZCA have better thermal insulation performance than undoped bulk materials.The infrared emissivity test shows that the infrared emissivity of bulk materials is related to the element composition,and LCA still has the highest infrared emissivity between room temperature and 600℃.