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X2-RE 2 SiO 5 orthosilicates are promising candidate environmental/thermal barrier coating (ETBC) materials for silicon-based ceramics because of their excellent durability in high-temperature environments and potential low thermal conductivities. We herein present the mechanical and thermal properties of X2-RE 2 SiO 5 orthosilicates based on theoretical explorations of their elastic stiffness and thermal conductivity, and experimental evaluations of the macroscopic performances of dense specimens from room to high temperatures. Mechanical and thermal properties may be grouped into two: those that are sensitive to the rare-earth (RE) species, including flexural strength, elastic modulus, and thermal shock resistance, and those that are less sensitive to the RE species, including thermal conductivity, thermal expansion coefficient, and brittle-to-ductile transition temperature (BDTT). The orthosilicates show excellent elastic stiffness at high temperatures, high BDTTs, very low experimental thermal conductivities, and compatible thermal expansion coefficients. The reported information provides important material selection and optimization guidelines for X2-RE 2 SiO 5 as ETBC candidates. [ABSTRACT FROM AUTHOR]

β-Yb2Si2O7 is a promising environmental barrier coating ( EBC) material and recently attracted attention for its damage tolerance. To investigate the mechanisms of its damage tolerance and possible plasticity, dense β-Yb2Si2O7 sample was synthesized by in situ reaction/hot-pressing method, and its mechanical properties were measured from room to high temperatures. The low magnitudes of hardness to Young's modulus ratio HV /E, shear modulus to bulk modulus ratio G/B, and high fracture toughness to strength ratio KIC /σ provide evidences of damage tolerance of β-Yb2Si2O7. β-Yb2Si2O7 exhibits extensive plastic deformation in Hertzian contact tests at both room and high temperatures. Transmission electron microscopy ( TEM) observations show that the deformation mechanisms are different at low and high temperatures. Deformation twinning and parallel dislocation arrangement occur in plastic deformation at room temperature. Above the brittle-to-ductile transition temperature (between 1200°C and 1300°C), plastic deformation brings out extensive slip and climb of dislocations, while twinning is seldom observed. Measurement of temperature-dependent dynamic Young's modulus demonstrates excellent elastic stiffness retention up to 1300°C. [ABSTRACT FROM AUTHOR]

Rare earth monosilicate (RE 2 SiO 5) is one of the most promising candidates as an environmental barrier coating (EBC) for SiC f /SiC ceramic matrix composites. But single-component RE 2 SiO 5 is hard to meet the multiple and harsh performance requirements of EBC which brings a significant challenge to their applications. Based on our previous research on single-component RE 2 SiO 5 ceramics, (Ho 0.4 Yb 0.3 Lu 0.3) 2 SiO 5 solid solution was designed and successfully fabricated in this work. Doping of multiple RE elements endows (Ho 0.4 Yb 0.3 Lu 0.3) 2 SiO 5 with excellent thermal insulation properties and matched thermal expansion coefficient with SiC f /SiC substrates. In addition, it exhibits lower elastic modulus and comparable hardness than that of single-component RE 2 SiO 5. (Ho 0.4 Yb 0.3 Lu 0.3) 2 SiO 5 also presents good resistance to calcium-magnesium alumino-silicates (CMAS) corrosion. Rational composition design allows (Ho 0.4 Yb 0.3 Lu 0.3) 2 SiO 5 to retain the merits of single-component RE 2 SiO 5 while taking advantage of the solid solution effect. The results of this work suggest (Ho 0.4 Yb 0.3 Lu 0.3) 2 SiO 5 as a promising EBC candidate. [ABSTRACT FROM AUTHOR]