Abstract
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·m1/2 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/(m·K) at 1200 °C. Although the thermal conductivity increased, it is still acceptable for such composites to be used as environmental barrier coatings.
Highlights
In order to achieve higher efficiency and stronger thrust of aero engines, demand for higher turbine inlet temperatures is increasing
A small amount of Al2 O3 phase is detected, which might precipitate from the mullite, as reported by others [35,36,37]
One possible reason is that the mullite matrix partially decomposed into Al2 O3 and SiO2 during the sintering process, and SiO2 reacted with Yb2 SiO5 to form Yb2 Si2 O7
Summary
In order to achieve higher efficiency and stronger thrust of aero engines, demand for higher turbine inlet temperatures is increasing. To meet the key requirements of thermal expansion, water vapor stability, chemical compatibility, phase stability, and adherence, multi-layer coating systems have been designed and investigated. Restraining the crack propagation plays an important role in prolonging the lifetime of EBCs. As we know, the thermal shock resistance of the mullite layer is relatively poor, partially due to its low fracture toughness, which limits its application. On top of the mullite, ytterbium monosilicate (Yb2 SiO5 , YbMS) has been extensively used as the top layer in EBCs due to its chemical stability in steam, low CTE, and phase stability at high temperature [23,24,25,26]. Using a candidate of coating material as an additive for mullite to improve its fracture toughness has attracted our interest. The effects of Yb2 SiO5 addition on the mechanical properties and thermal conductivity of the composites were investigated, and associated mechanisms are discussed
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