Properties Of Thermal Barrier Coatings With Multi-sized Pores After Long-term Service

Thermal barrier coatings(TBCs)are often used to protect high-temperature components of gas turbines and aero-engines.Both high thermal insulation and well mechanical properties are essential to the TBCs.Pores often exist in thermal sprayed TBCs.Closed pores are poor conductors of heat,which can significantly reduce thermal conductivity and improve thermal insulation.Small pores will relieve the stress of the TBCs,thus reduce the stress concentration.Large pores will reduce the deformation resistance in a certain extent.Therefore,multi-sized pores(nanometer,micrometer,and tens of micrometer)were introduced into the TBCs to ensure high thermal insulation performance and improve the coatings'deformation resistance simultaneously in this paper.Meanwhile,the microstructure and thermodynamic properties of the multi-sized porous TBCs were investigated after a long-term service at 1000??1050??1100?according to the real serving conditions.The main research results are as follows:(1)The thermal physical properties of the composite ceramics with 5 wt.%Yb₂O₃and 5wt.%,10 wt.%and 15 wt.%Gd₂O₃added into 8YSZ were investigated.The composite ceramic with 5 wt.%Yb₂O₃and 10 wt.%Gd₂O₃added into 8YSZ was optimized to have the best comprehensive properties,which was selected to prepare TBCs.(2)The thermal conductivities and indentation characteristics of TBCs with multi-sized pores under different loads after serving at different temperatures for different times were simulated through Ansys workbench finite element software.The results showed that the thermal conductivity of the multi-sized porous TBCs decreased with the increase of the test temperature,and decreased with the increase of the porosity.It also increased with the prolongation of the serving times while tended to be stable after 60 days serving.Although the introduction of pores at different scales in the coating reduced its mechanical properties,its fracture toughness was improved.(3)The multi-sized porous structures at micro and nano scales were constructed and prepared by the combination of ceramic powder and polypropylene pore-forming agent.The results showed that the pores exhibited a multi-scale distribution of micro/nano.With the extension of the serving time,the closed small-scale pores gradually disappeared,and large-scale pores were connected.When the serving time reached 60 days,the multi-sized pores of the coating tended to be stable.The addition of Gd₂O₃and Yb₂O₃could improve the stability of 8YSZ.(4)Thermal conductivity of multi-sized porous TBCs after a long-term service at high temperature were investigated through the laser flash method.The thermal conductivity of the TBCs decreased with the increase of the serving temperature and decreased with the increase of the polypropylene powder content.The thermal conductivity of the TBCs increased with the prolongation of the serving times while tended to be stable after 60 days serving.The experimental results of the thermal conductivity evolution were consistent with the finite element simulations.The addition of Gd₂O₃and Yb₂O₃could reduce the thermal conductivity and improve the sintering resistance of the TBCs.(5)The mechanical properties of TBCs with multi-sized pores after a long-term service at high temperature were studied through the nanoindentation method.With the increase of pore former content,the hardness and elastic modulus decreased while the fracture toughness increased.With the prolongation of serving time at high temperature,the hardness and elastic modulus increased,but the fracture toughness decreased.Meanwhile,the mechanical properties tended to be stable after the service time reached 60 days.