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The thermochemical behavior of EBC candidate materials yttrium disilicate (Y2Si2O7) and ytterbium disilicate (Yb2Si2O7) was evaluated with three calcium-magnesium-aluminosilicate (CMAS) glasses possessing CaO:SiO2 ratios relevant to gas turbine systems. Pellet mixtures of 50 mol% EBC powder to 50 mol% CMAS glass powder were heat treated at 1200°C, 1300°C, and 1400°C. The products of these interactions were evaluated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Above glass melting temperatures, exposure of the disilicates primarily resulted in dissolution into the molten glass followed by precipitation of a Ca2RE8(SiO4)6O2 (RE = Yb3+, Y3+) apatite-type silicate and/or rare earth disilicate. In glasses with high CaO concentrations, apatite readily forms while the disilicate material is consumed by the reaction. As CaO content decreases, the disilicate phase becomes the main reaction product. Overall, reactions with yttrium disilicate favored more apatite crystallization than ytterbium disilicate. The viability of using these disilicates in various operating environments is discussed.

Aerogels can be used in spaceflight applications as thermal insulators due to their low density, low thermal conductivity, and tortuous path for solid conduction. However, the aerogel formulation must be optimized to increase the thermal stability of the aerogels, as a drastic densification and decrease in surface area often is exhibited after high-temperature exposure. This work considers yttria-stabilized zirconia (YSZ) aerogels, which have low thermal conductivity and are expected to be used for temperatures between 600 °C and 1000 °C. It is anticipated that new sol-gel chemistries will yield YSZ aerogels that exhibit retained mesoporous structure, increased surface area, and minimized shrinkage upon exposure to higher temperatures in the expected use range. This work investigates the addition of surfactant to 20 mol% YSZ aerogels. The cationic surfactant, cetrimonium bromide (CTAB), was used as the templating agent, preventing the collapse of the mesoporous structure upon gelation and drying. It was determined that adding one-half times the critical micelle concentration of CTAB increased the surface area of the aerogels by 72% and 41% and increased the pore volume following exposure to 600 °C and 1000 °C, respectively. This level of CTAB had the greatest increase in surface area and pore volume as compared to aerogels without CTAB or with twice the critical micelle concentration of CTAB. By optimizing the concentration of CTAB, the thermal stability of YSZ aerogels can be enhanced to make these materials more efficient when used as thermal management systems.

Advanced thermal barrier coating materials are necessary to improve the efficiency of next-generation gas turbine engines. As such, different TBC chemistries must be developed with enhanced temperature stability above that of 7YSZ (~ 1200 °C) while maintaining thermal and mechanical reliability. The present study investigates the effect of rare earth content on the mechanical properties of ZrO2 TBCs. Various cubic compositions in the ZrO2-GdO1.5 system were investigated in the form of monolithic pellets and coatings with stoichiometric GZO serving as reference. The fracture toughness and erosion durability were evaluated, and it was found that fracture toughness decreased with increasing rare-earth content, 15.66 mol% GdO1.5 yielding 1.25 MPa m1/2, compared to 1.04 MPa m1/2 for GZO (50 mol% GdO1.5). It was observed that the erosion performance of the coatings was much more sensitive to the changes in mechanical properties than the bulk specimens and showed a 125% increase through the fluorite region to GZO whereas the dense pellets only exhibited a 33% increase in erosion behavior. These results indicate that cubic ZrO2 phase with reduced rare-earth content show promise as TBC materials with improved durability over GZO as well as temperature stability in this region of the phase diagram.