Failure Mechanism Of Rare Earth Hexaaluminate Thermal Barrier Coatings

Thermal barrier coating（TBC）has been widely used on the surface of the hot-section components in gas turbine engines to improve the service lifetime and thermal efficiency by increasing the turbine inlet temperature and reducing the amount of cooling air.Zirconia partially stabilized with 7-8 wt.%yttria（7-8 YSZ）as the commonly used TBC material performs quite well up to 1200°C,which cannot match the requirements of the gas turbine operating at a higher gas temperature.LaMgAl₁₁O₁₉（LMA）with the magnetoplumbite structure has essential characteristics of low thermal conductivity,high fracture toughness,high thermal expansion coefficient（TEC）and outstanding thermal stability,which has been one of the most promising new TBC materials.However,a large amount of amorphous phase often presents in the plasma-sprayed LMA coating.During thermal cycling tests,the crystallization of the amorphous phase may put questions on the reliability of the plasma-sprayed LMA coating.This paper focused on reducing the amorphous phase content in LMA coatings by optimizing the synthesis conditions of LMA powder,preparation parameters and the following heat treatment process of the coating,resulting in the improvement for the thermal cycling behavior of the coating.In addition,the failure mechanism of the coating and the crystallization mechanism of the amorphous phase were studied.LMA powders were synthesized at different temperatures by solid-state reaction method using La₂O₃,MgO and Al₂O₃ powders as raw materials.The purity,crystalline grain size and crystal morphology of the LMA powder were dependent on its preparation temperature.In the temperature range of 1300¹550°C,LMA showed platelet grain and the average crystalline grain size increased with the increase in temperature.At 1600°C,if the powder was sintered for two times,the equiaxed grain could be found as the decrease in grain space,resulting in the reduction of the crystalline grain size.The melting degree of the feedstock was influenced by the amount of the second phase LaAlO₃ and density of the powders.The higher the amorphous phase content,the higher the volume shrinkage of the coating during the crystallization process and the shorter the thermal cycling lifetime were.However,when the amorphous phase contents were in the same level,the thermal cycling lifetime differences of these coatings seemed to be dependent on the grain size.LMA coating sprayed with powders synthesized at 1450°C had relatively low amorphous phase content and moderate grain size,showing the longest thermal cycling lifetime.LMA coatings were prepared by plasma spraying with different spraying powers.The amorphous phase content of the coatings increased with the spraying power increasing,while the porosity of the coatings exhibited an opposite trend.Bonding strength and microhardness of the coatings increased with the spraying power increasing from 22 to 32 kW and then decreased from 32 to 42 kW.Spallation of the TBC was mainly correlated with the formation of thermally grown oxides（TGO）and mismatch of TECs between the topcoat and the substrate.The longest lifetime was achieved on LMA coating deposited at 32 kW,owing to its relatively low amorphous phase content and the highest bonding strength.LMA/graphite powder mixtures with different weight fractions were used to prepare porous LMA coatings.Following the deposition of the topcoat,a post heat treatment was conducted at 700°C to remove the graphite within the topcoat to obtain the porous microstructure.However,graphite in the powder mixtures led to a better molten state of the particles and high amorphous phase content in the coating.With the increase of the graphite content in the powder mixtures,coatings prepared exhibited high porosity and weak bonding strength,resulting in a poor thermal cycling behavior of the coating.The as-sprayed LMA coatings were heated at 900°C,1000°C and 1100°C for20h,respectively.The amorphous phase content decreased with the increase of temperature,while the porosity and the TEC value exhibited an opposite trend.Heat treatment at 900°C improved the lifetime of LMA coatings,whereas their thermal cycling performance did not be improved significantly when the aging temperature increased to 1000°C or 1100°C.The crystallization of amorphous LMA occurred in two stages at about 900°C and 1170°C,respectively.Highly charged La³⁺in LMA magnetoplumbite structure controlled the crystallization rate.The higher the La content in the composition the harder the crystallization occurred.