Preparation and Properties of(Yb0.25Lu0.25Tm0.25Y0.25)2Si2O7 Environmental Barrier Coating Materials
Environmental barrier coatings(EBCs)can effectively protect SiC-based ceramic matrix composites(CMC-SiC)from high-temperature water vapor corrosion in aero-engines.Rare-earth pyrosilicates(RE2Si2O7)have become one of the most promising EBC materials owing to their coefficient of thermal expansion(CTE),which is similar to that of CMCs,and their excellent high-temperature performance.However,RE2Si2O7 is prone to cracking during thermal cycling,and its ability to resist high-temperature water vapor corrosion is insufficient.High-entropy silicate materials prepared by mixing multiple rare-earth elements tend to exhibit better thermo-physical properties and high-temperature water vapor corrosion resistance.A new type of high-entropy ceramics(Yb0.25Lu0.25Tm0.25Y0.25)2Si2O7((4RE0.25)2Si2O7)is synthesized using four rare-earth elements for a high-entropy design.In terms of element selection,pyrosilicates with the rare-earth elements Yb or Lu have stable single β crystal polymorphs and excellent resistance to high-temperature water vapor corrosion.In addition,pyrosilicates with rare-earth elements Tm or Y have β crystal polymorph at high temperatures,and β-Y2Si2O7 has lower thermal conductivity.Therefore,(4RE0.25)2Si2O7 is likely to crystallize in a single β crystal polymorph.The microstructure,phase composition,thermal properties,and resistance to water vapor corrosion are systematically investigated.The experimental results show that(4RE0.25)2Si2O7 bulks with monoclinic β-phase are successfully prepared by non-pressure sintering.The doped rare-earth elements are uniformly distributed in the matrix,and no obvious component segregation occurs.In addition,the atomic proportion of each element in(4RE0.25)2Si2O7 is close to the theoretical value.The thermal gravimetric(TG)curve remains horizontal without obvious weight changes from room temperature to 1300 ℃.There are no obvious absorption/exothermic peaks in the differential scanning calorimetry(DSC)curve,indicating that no phase transition or decomposition occurs during heating.It has a low CTE(2.66×10-6-3.84×10-6 ℃-1).The CTE of(4RE0.25)2Si2O7 increases with increasing temperature but is lower than that of Yb2Si2O7 at all temperatures.It can also be applied to substrates with a lower CTE,such as Si3N4(3×10-6-4×10-6 ℃-1).The thermal diffusion coefficient(0.41-0.92 mm2/s)is lower than that of Yb2Si2O7.Below 500 ℃,the thermal conductivity decreases as the temperature increases.When the temperature is higher than 500 ℃,the thermal conductivity increases rapidly with the increase in temperature because of the increased effect of thermal radiation,which causes the thermal conductivity of(4RE0.25)2Si2O7 to exceed that of Yb2Si2O7 above 800 ℃.To compare the performance of the synthesized(4RE0.25)2Si2O7 in a high-temperature water vapor environment,Yb2Si2O7,which has been proven to exhibit good water vapor corrosion resistance,is selected as the control group.In the atmosphere of 90％Air-10％H2O at 1300 ℃ for 200 h,the weight loss of(4RE0.25)2Si2O7 is significantly lower than that of Yb2Si2O7,with negligible weight loss from 180 to 200 h.The weight loss is only 0.625 mg/cm2,which indicates that the(4RE0.25)2Si2O7 bulk is not liable to react with water vapor.After 200 h of water-vapor corrosion,(4RE0.25)2Si2O7 remains single and stable without any phase transition.It contains a small amount of Y2SiO5 impurities before corrosion,which disappear after water vapor corrosion.The slight reaction between Y2SiO5 and H2O,which generates gaseous Y(OH)3 and Si(OH)4,is the main reason for the weight loss in the high-entropy ceramic bulk.There is no significant difference between the EDS energy spectra before and after corrosion.After corrosion,it can be observed that Yb,Lu,and Tm are evenly distributed,whereas Y,Si,and O are concentrated in the areas without pores,which demonstrates that the formation of pores is related to Y2SiO5.Doping multiple rare-earth elements into the lattice of Yb2Si2O7 reduces the diffusion rate of atoms,resulting in better phase stability in high-entropy ceramics.These excellent properties demonstrate that high-entropy silicate ceramics prepared according to the properties of different rare-earth elements have promising potential in terms of their thermal properties,which lays the foundation for their practical application.
environmental barrier coatinghigh entropy pyrosilicatenon-pressure sinterthermal propertieshigh temperature water vapor corrosion
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