Research On Microstructure And Hot Corrosion Behavior Of LaMgAl₁₁O₁₉ Thermal Barrier Coating

Thermal barrier coatings（TBCs）are widely used in the hot-end parts of aerospace engines to reduce the substrate temperature,and enhance the oxidation resistance and thermal corrosion resistance of products.However,with the development of aviation gas turbine towards higher thrust-weight ratio and aerospace engine towards higher Mach number,these conventional yttria stabilized zirconia（YSZ）thermal barrier coatings are no longer sufficient for the development of aerospace engines.The novel TBC technology has gradually become the main factor restricting the development of the high-performance engine.Among them,the rare earth hexaaluminate has various excellent thermophysical properties,such as low thermal conductivity and excellent sintering resistance.The LaMgAl₁₁O₁₉coating,as a typical representative of the new thermal barrier coating material,has become one of the research highlights at home and abroad.Unfortunately,there is few of systematical researches on the microstructure and high-temperature corrosion properties of the LaMgAl₁₁O₁₉coatings.Besides,the theoretical support for the engineering application is also waited to be proceeded.Therefore,it is necessary to systematically carry out the novel design and preparation of rare earth hexaaluminate thermal barrier coatings,which has important academic value and broad engineering application prospect.The LaMA coatings were prepared by an atmospheric plasma spraying techniques.The mapping relationship between microstructure and macroscopic properties of the coatings under different spraying current was studied.The microstructures of the annealed coatings at different temperatures were thoroughly analyzed.It was indicated that the volume fraction of the amorphous structures in LaMA coating and the density of the coating both increased with the increase of the spraying current from 350A to 650A.Meanwhile,it was found that the micro-hardness and elastic modulus of these coatings increased first and then decreased with the spraying current.The coating presented the largest micro-hardness and elastic modulus as the spraying current was 600A.In addition,the coating with higher spraying current coating possessed better high-temperature thermal-shock life and bonding strength.Moreover,the crystallization reaction,volume shrinkage and fracture damage of the LaMA were further aggravated with the increase of annealing temperature.The study of LaMA coatings’high temperature oxidation shows that the scale and distribution density of the defects in LaMA layer together with the damage degree of the YSZ layer affect the oxygen diffusion and the growth of interface TGO layer significantly.The as-sprayed LaMA coating contained a large amount of amorphous structure and the crystallization reaction was sluggish,which resulted in a slight structure fracture,as well as a smaller size and a lower distribution density of the crack.Besides,the sluggish crystallization reaction also caused the slight fracture damage and barrier ability degradation of YSZ layer.In the process of high temperature oxidation,the diffusion rate of oxygen atoms in sprayed LaMA was slow,and the growth rate of interface TGO in sprayed LaMA was slow,which resulted in a relatively uniform thickness.In contrast,the annealed LaMA layer experienced a prominent crystallization reaction,which suffered from a severe structural fracture.Therefore,it caused the formation of the high-density and large-scale micro-cracks.Meanwhile,the tensile stress induced from the crystallization reaction further aggravated the fracture damage of YSZ and weakened its barrier ability.The large-scale micro-cracks in LaMA and YSZ eventually resulted in the fast oxygen diffusion,the rapid increase in weight of the bonding layer and the fast growth of the TGO layer,which consequently aggravated the fracture damage of the structure.Obviously,the crack size and the defect density both increased induced from the crystallization reaction in LaMA with the annealing temperature,which led to the more serious the fracture damage and barrier ability degradation of the YSZ layer,as well as the faster growth rate of the interface TGO layer and the more serious fracture damage of the structure.During high temperature sulfate corrosion research,the density of LaMA layer and internal infiltration of molten salt affected the oxygen diffusion and the growth kinetics of interface TGO layer significantly.There are a large amount of defects（e.g.,holes and cracks）and amorphous structures in the sprayed LaMA coating,which provided the channels for the internal diffusion of oxygen and corrosive media.Moreover,it destroyed the stability of the interface TGO layer and accelerated the growth of（Ni,CO）Cr₂O₄spinel.Eventually,it resulted in the rapid growth of thermal growth stress at the interface and serious fracture at the YSZ-Co Cr Ni Al Y interface.In contrast,the volume fraction of the amorphous structure in the annealed LaMA layer decreased significantly and the crystalline structure showed high density and barrier ability.In the process of high-temperature molten salt corrosion,these crystalline structure effectively inhibited the diffusion of oxygen atoms and corrosion medium.Although the corrosion medium and oxygen atoms mainly diffused through micro-cracks induced from crystallization reaction,their diffusional speed and region were significantly reduced.Therefore,the element diffusion of interface TGO layer and the growth of（Ni,CO）Cr₂O₄spinel phase were effectively suppressed.Clearly,The volume fraction of the crystalline structure in the LaMA layer increased with the annealing temperature.Besides,the barrier-inhibition effect on the diffusion of corrosion medium and oxygen was stronger,which led to the slower the diffusion of interface element and the growth of（Ni,CO）Cr₂O₄spinel phase.The CMAS corrosion failure behaviors of the sprayed and annealed LaMA coatings under different doses and temperatures were investigated.The results showed that the structural damage and fracture of all coatings were intensified with the dose of CMAS increased from 10mg/cm²to 20mg/cm²,which was mainly attributed to the more intense internal infiltration and degradation reaction during the corrosion process of high-dose CMAS.In addition,the structural damage and fracture of all coatings were further intensified with the increase of the test temperature from 1200℃to 1250℃,which was mainly originated from the more intense multi-scale diffusion and degradation reaction of the CMAS salt at high temperature.Meanwhile,the growth rate of interface TGO layer was increased.Therefore,the synergistic effect of the abovementioned factors induced a higher tissue stress and thermal growth stress,which eventually led to more serious structure damage and fracture failure of the as-prepared coatings.