Study On Cmas And Water-vapor Corrosion Properties Of (Yb_1-xY_x)₂Si₂O₇ And ((Lu_1-xY_x)₂Si₂O₇ Silicate Coatings At High Temperatures

Ceramic matrix composites（CMCs）have become the main candidate materials for the hot-end components of the next-generation aero-engines due to its characteristics of low density,high temperature resistance,excellent mechanical properties and oxidation resistance.At high temperatures,CMCs usually suffer from the corrosion of the environmental media such as water vapor and molten salts（Ca O-Mg O-Al₂O₃-Si O₂,CMAS）,which leads to a significant decline in mechanical properties and service life,and seriously affects their high-temperature endurance.Thermal/environmental barrier coatings（T/EBCs）prepared on CMCs can effectively improve their environmental durability.Rare earth silicates have the characteristics of low thermal conductivity,excellent phase stability,suitable thermal expansion with CMCs,etc.,as the potential materials of the environmental barrier coatings.In this work,Yb₂Si₂O₇and Lu₂Si₂O₇silicates were selected to form(Yb_1-xY_x)₂Si₂O₇and(Lu_1-xY_x)₂Si₂O₇（x=0,0.25,0.5）by doping Y³⁺ion.(Yb_1-xY_x)₂Si₂O₇and(Lu_1-xY_x)₂Si₂O₇（x=0,0.5）were used as EBC layers to design T/EBCs to investigate their water-vapor and CMAS molten salt corrosion resistance.Y-doped(Yb_1-xY_x)₂Si₂O₇and(Lu_1-xY_x)₂Si₂O₇（x=0,0.25,0.5）silicate powders were synthesized by the high-temperature solid phase reaction method,and the corresponding pellets were prepared by spark plasma sintering（SPS）.Moreover,Yb YSi₂O₇and Lu YSi₂O₇coatings as well as their parent Yb₂Si₂O₇and Lu₂Si₂O₇coatings were prepared by atmospheric plasma spraying（APS）.The thermal-physical properties and CMAS corrosion properties of silicate pellets and coatings were investigated by means of XRD,SEM,EDS and TEM.The results show that both the thermal conductivity and the thermal expansion coefficient（CTE）of Yb₂Si₂O₇and Lu₂Si₂O₇ceramic pellets are reduced by Y³⁺doping.The pores and microcracks in the coatings help to reduce the thermal conductivity,which is much lower than that of the corresponding pellets.The composition segregation phenomenon exists in the as-sprayed coatings and the CTE drops and increases sharply with increasing the temperature.The CTE of the heat-treated coatings is higher than that of the corresponding pellets.Unlike Yb₂Si₂O₇and Lu₂Si₂O₇,Yb YSi₂O₇and Lu YSi₂O₇react more actively with CMAS to form Y-Ca-Si（or Yb-Ca-Si）apatite layers at the interface.The corrosion product between Lu YSi₂O₇and CMAS is only Y-Ca-Si apatite.And the apatite phase in the coatings is randomly oriented,different from the vertical apatite phase in the pellets.The formation of apatite reaction layer can alleviate the further infiltration of CMAS along the pores in the coatings or grain boundaries of the pellets to some extent.Si/(Yb_1-xY_x)₂Si₂O₇/La Mg Al₁₁O₁₉and Si/(Lu_1-xY_x)₂Si₂O₇/La Mg Al₁₁O₁₉（x=0,0.5）three-layer structured T/EBCs were prepared on the surface of Si C or Si C_f/Si C composites by APS.The thermal cycling and water-vapor corrosion properties of T/EBCs composed of different silicates were investigated,respectively.The results show that compared with that of Si/Yb₂Si₂O₇/La Mg Al₁₁O₁₉(or Si/Lu₂Si₂O₇/La Mg Al₁₁O₁₉)T/EBCs,the thermal cycle lifetime of Si/Yb YSi₂O₇/La Mg Al₁₁O₁₉(Si/Lu YSi₂O₇/La Mg Al₁₁O₁₉)T/EBCs is reduced,but their water-vapor corrosion performance is improved.The thermal cycle failure of T/EBCs mainly occurs between La Mg Al₁₁O₁₉-TBCs layer and silicate EBCs layer and between EBCs layer and Si bond layer,which is caused by the CTE mismatch between the layers,the TGO growth stress and the phase transition stress.The main reason for the failure of water-vapor corrosion in T/EBCs is that the diffusion of H₂O and O₂through cracks in La Mg Al₁₁O₁₉-TBCs layer and silicate EBCs layer to Si bond layer,finally leading to decomposition failure by the water-oxygen corrosion.