高熵稀土单硅酸盐环境障涂层局域结构的原子对分布函数解析

Neutron Pair Distribution Function Analysis to the LocalStructure of High-Entropy RE2SiO5 EnvironmentalBarrier Coating

调控稀土单硅酸盐(RE_(2)SiO_(5))环境障涂层热膨胀系数,优化其与碳化硅纤维增强碳化硅(SiC_(f)/SiC)陶瓷基复合材料基体热膨胀系数匹配性,是其应用于高推比航空发动机的关键。本工作基于RE_(2)SiO_(5)/Yb_(2)Si_(2)O_(7)/Si三层结构涂层体系,对比研究了采用与基体热膨胀系数匹配优的(Y_(1/4)Ho_(1/4)Er_(1/4)Yb_(1/4))_(2)SiO_(5)面层与典型Yb_(2)SiO_(5)面层涂层体系的抗热冲击性能,发现Yb_(2)SiO_(5)面层热膨胀系数偏大造成的应力累积,是导致三层结构涂层失效的主要原因。依据中子衍射数据,分析中子对分布函数,对2种单硅酸盐局域结构特征进行解析。结果表明,高熵稀土单硅酸盐中多种稀土元素协同作用,降低了[ORE_(4)]四面体畸变程度,表现为热膨胀系数减小。提出通过合理选择稀土元素组合,扰动[ORE_(4)]局域结构畸变,有望实现稀土单硅酸盐热膨胀系数的定向调控。

Environmental barrier coatings(EBCs)enable SiC_(f)/SiC ceramic matrix composites(CMC)to operate under high-temperature combustion conditions.They reduce the oxidation rate of SiC_(f)/SiC,the volatilization of the composites due to reaction with water vapor,and the surface temperature of the composites.Rare-earth monosilicates(RE_(2)SiO_(5)),owing to their excellent high-temperature durability,low thermal conductivity,and good phase stability,are used as the top layer of EBCs.However,they exhibit a high coefficient of thermal expansion(CTE),leading to thermal mismatch and inducing tensile residual stress(with a magnitude of several hundred MPa)in the coating,resulting in the formation of vertical cracks,which act as extremely-high-diffusivity paths for oxidation species transportation and silica volatilization.Therefore,regulating the CTE of RE_(2)SiO_(5) EBCs and minimizing the CTE mismatch among constituent RE_(2)SiO_(5) layers with SiC_(f)/SiC CMCs are critical for multilayered EBCs.Through atmospheric plasma spraying,a typical Yb_(2)SiO_(5)/Yb_(2)Si_(2)O_(7)/Si coating system and a tri-layer structured multicomponent(Y_(1/4)Ho_(1/4)Er_(1/4)Yb_(1/4))_(2)SiO_(5)/Yb_(2)Si_(2)O_(7)/Si coating system with better matched CTEs were manufactured.Both the coatings remained adhered to the substrate during deposition and after annealing,and no mud cracks that would compromise the coating gas-tightness quality and delamination cracks were observed at any of the coating interfaces.In thermal cycling tests,(Y1/4Ho1/4Er1/4Yb1/4)2SiO_(5)/Yb_(2)Si_(2)O_(7)/Si coatings showed a lifetime that is three times longer than that of conventional Yb_(2)SiO_(5)/Yb_(2)Si_(2)O_(7)/Si coatings.The failure mechanisms in thermal cycling were investigated via the finite element simulation of stress.It was found that the stress in the substrate was low,and the residual thermal stress was mainly concentrated on the top,inter,and bond layers and increased with an increase in temperature.Compared with that of the(Y1/4Ho1/4Er1/4Yb1/4)2SiO_(5) top coat,the Yb_(2)SiO_(5) top coat showed obviously higher residual tensile stress,which contributed to a higher tendency for mud-crack formation and higher energy release rate,substantially reducing the coating's thermal cycling lifetime.Through neutron powder diffraction and pair distribution function(PDF)analysis,the average and local structures of RE_(2)SiO_(5) were studied.Overall,the average and local structures did not differ significantly,both of which can be described using the C2/c structure.Nevertheless,the PDF results demonstrated some differences in the disorder degree of Si—O and RE—O coordination environments.In particular,Rietveld refinement results of the PDF showed lower local distortion degree of[ORE_(4)]tetrahedrons when compared with that of the average structure.It is effective to reduce the distortion degree of[ORE_(4)]tetrahedrons by introducing Y^(3+),Ho^(3+),and Er^(3+)into the Yb^(3+)sites of Yb_(2)SiO_(5),and smaller distortion degrees lead to lower CTE values.Coordinative local disturbances introduced by strategic high-entropy design have been proposed as the key method for CTE regulation.