Thermophysical Properties And High-temperature Corrosion Behaviors Of Rare-earth Monosilicate Solid-solutions For Environmental Barrier Coatings Applications

Silicon-based non-oxide ceramics and ceramic matrix composites are considered as the next generation hot-section components of aeroengines.However,the rapid combustion environment containing oxygen,water vapor and volcanic ash（mainly composed of Ca O-Mg O-Al₂O₃-Si O₂,CMAS）usually leads to rapid degradation of high-temperature mechanical properties.It is expected to be solved by adding the environmental barrier coatings（EBCs）on the surface.Rare-earth（RE）monosilicate is a potential top-layer material of EBCs,however its thermal expansion coefficient is not close to that of Si C.Meanwhile its thermal conductivity can be further reduced,and its comprehensive corrosion resistances against water-oxygen and CMAS need to be improved.Therefore,double-component rare-earth monosilicates and multi-component rare-earth monosilicates are designed and fabricated.CTE and thermal conductivity of rare-earth monosilicates have been reduced by rare-earth doping,and the mediation mechanisim of microstructure on thermophysical properties has been investigated.Corrosion thermodynamics and kinetics of rare-earth monosilicates in the high-temperature water-oxygen and CMAS corrosion environments have been investigated to reveal the corrosion mechanism.Meanwhile,the corrosion resistances of rare-earth monosilicates are mediated by rare-earth doping.Pure double-component rare-earth monosilicates(Y_1-xYb_x)₂Si O₅ and multi-component rare-earth monosilicates were synthesized at 1773K for 2h by a solid-state reaction method.The rare-earth monosilicates ceramics with relative density of over 94%were fabricated by hot-pressing at 1773K for 2h under a pressure of 30MPa.The lattice energy of rare earth monosilicates can be improved by rare-earth doping,and thus the coefficient of thermal expansion（CTE）is reduced,and the elastic modulus is improved.The CTE of(Y_0.2Yb_0.8)₂Si O₅ is the lowest,namely 6.79×10^-6K^-1（RT-1473K）,and its elastic modulus is the highest,namely 182.7GPa.RE doping can reduce thermal conductivity of rare-earth monosilicate by increasing mass scattering.The higher the standard deviation of relative atomic mass of rare-earth elements,the more obvious the decrease of thermal conductivity.(Y_0.4Yb_0.6)₂Si O₅ has the lowest thermal conductivity of1.93W·m^-1·K^-1.The microstructural evolution of rare-earth monosilicates in water-oxygen environment of 50vol.%H₂O-50vol.%O₂ at high temperatures of 1573,1673 and 1773K reveals the high-temperature water-oxygen corrosion behaviors.Because the volatilization pressure of rare-earth is higher than twice volatilization pressure of silicon,Y₂Si O₅ and(Yb_0.2Er_0.2Y_0.2Dy_0.2Gd_0.2)₂Si O₅ tend generate the apatite phase,and tensile stress caused by volume shrinkage induces long cracks on the surface.Introducing Yb into Y₂Si O₅ inhibits the phase transition to apatite.(Y_0.8Yb_0.2)₂Si O₅ did not generate apatite phase after being corrosion-tested in water-oxygen environment at 1773K for 100h.When the volatilization pressure of rare-earth is lower than twice volatilization pressure of silicon,rare-earth monosilicate tends to react with gaseous Al（OH）₃ to generate garnet phase.The higher the corrosion temperature and the lower the average rare-earth cation radius,the easier the garnet forms.During the process of water-oxygen corrosion,there arise component deviations between the grain surface and grain boundary.The enrichment of silicon at the grain boundary reduces the total volatilization pressure,resulting in the crystal surface corrosion characteristic.The wetting type of CMAS on the surface of rare-earth monosilicate ceramics is reactive wetting.The CMAS corrosion products of rare-earth monosilicates contain apatite Ca_iRE_10-i（Si O₄）₆O_3-0.5i and garnet ^VII（Ca,RE）₃ ^VI（Mg,Al）₂^IV（Al,Si）₃O₁₂.Apatite phase with a rod-shaped morphology can be formed by the interfacial reaction between CMAS and rare-earth monosilicate,as well as precipitated from the melt with oversaturated rare-earth element concentration.Garnet phase with a globular-like morphology is only formed by the precipitation from oversaturated melt with a high Mg concentration.The dense apatite layer formed by interfacial reaction effectively inhibits the melt infiltration,and thus inhibits the development of CMAS corrosion process.By doping Yb,the reactivity of Y₂Si O₅ is reduced while the dissolution resistance increases,which improve the corrosion resistance to CMAS.By doping Gd,the CMAS corrosion resistance of rare-earth solid-solutions at 1773K is further improved.