Mechanical properties and thermal conductivity of ytterbium-silicate-mullite composites

Various Ytterbium-Silicate-Mullite composites were successfully fabricated by adding Yb2SiO5 into mullite ceramics and then using pressureless sintering at 1550 °C. The influence of Yb₂SiO₅ addition on the microstructure, mechanical properties, and thermal conductivity of ytterbium-silicate-mullite composites was investigated. Results show that the composites mainly consisted of a mullite matrix and some in situ formed Yb₂Si2O₇ and Al₂O₃ phases. By the addition of Yb₂SiO₅, the Vickers hardness of composites reached ~9.0 at an additive concentration of 5 mol %. Fracture toughness increased to ~2.7 MPa m^1/2 at the additive concentration of 15 mol %, owing to the trans-granular fracture and crack deflection of the pinning effect of the Al₂O₃ phase in the composites. With the increase of the Al₂O₃ phase in the composite, the thermal conductivity for the 15YbAM reached around 4.0 W/(mK) at 1200 °C. Although the thermal conductivity increased, it is still acceptable for such composites to be used as environmental barrier coatings.