| Thermal barrier coatings(TBCs)are applied to high-temperature components of gas turbines in thermal power plant and aero engines,playing a role in insulating the heat and protect matrix.The traditional material used in thermal barrier coatings is yttria partially stabilized zirconia(YSZ),which has the advantages of high thermal expansion coefficient,low thermal conductivity and low price.However,the YSZ coating will undergo phase transformation and increased sintering in an environment above 1473K(1200?),resulting in loss of the coating's effectiveness,so searching new materials is imminent.The pyrochlore structural material,represented by lanthanum zirconate La2Zr2O7,has thermal stability at high temperature,sintering resistance,oxygen impermeability,and lower thermal conductivity than YSZ.Due to its good prospects,it is a candidate for a new generation material system of thermal barrier coatings.However,due to the low coefficient of thermal expansion,which is far from the substrate,it is very limited in the practical application of the coating,thus its performance needs to be improved.The discovery of multi-principal high-entropy material provides new ideas for the design of material.Because high-entropy materials have special properties such as high temperature stability and low thermal conductivity,this paper explores the synthesis and properties of high-entropy ceramic materials for thermal barrier coatings.This study used the design principle of high-entropy materials,based on pyrochlore A2B2O7 structure,to design a new type of thermal barrier coating material.Firstly,8 kinds of A2B2O7(A=La0.5Gd0.5,La0.5Y0.5,Y0.5Gd0.5,La or Gd;B=Zr0.25Hf0.25Ce0.25Ti0.25 or Zr0.2Hf0.2Ce0.2Sn0.2Ti0.2)formulations were designed according to the atomic ratio.Then solid-phase synthesis was used to prepare materials.XRD method was used to study the formation characteristics of high-entropy pyrochlore structure materials.And then,one of the formulations,La2(Zr0.2Hf0.2Ce0.2Sn0.2Ti0.2)2O7,was studied in detail.According to X-ray diffraction(XRD)experiment,the new material was a pyrochlore-based system with a small amount of miscellaneous phases in some formulations.The diffraction peak of La2(Zr0.2Hf0.2Ce0.2Sn0.2Ti0.2)2O7 was almost completely consistent with the pyrochlore structured lanthanum zirconate,and there was only a small deviation of the diffraction peak caused by lattice distortion and low intensity peak from second phases.Scanning electron microscope(SEM)was used to observe the surface morphology of the material after sintering.Due to the sintering process,it was observed that the microstructure was sparse and porous.No reunion occurred but there was still a certain amount of pores after the temperature was increased.Energy dispersive spectrometer(EDS)analysis proved that the cationic elements were evenly distributed and there was no segregation occurs.Various methods were used to study its thermophysical properties.By differential scanning calorimetry(DSC)test,the new material was still stable at 1200? with no phase change and decomposition.Laser-flash method was used to study its thermal conductivity,which was 0.961 W·m-1·K-1(1000?),lower than that of pyrochlore lanthanum zirconate and YSZ.Coefficient of thermal expansion of the material measured by the thermal expansion instrument reached 9.67×10-6K-1,which was larger than that of the general pyrochlore structure lanthanum zirconate.From the point of view of thermophysical properties,it has the characteristics of being a candidate material for thermal barrier coatings.Nano-indentation method was used to determine the mechanical properties.Hardness was 0.856Gpa and elastic modulus was 33.694Gpa,respectively.The results show that its mechanical properties are much lower than the general dense materials.Ansys software was used to simulate the high temperature thermal stress in coating application,the maximum stress occured at the interface between the ceramic layer and the bonding layer. |
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