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Functionally multilayered environmental barrier coatings (EBCs) with the desired layer sequence of Yb2O3/Yb2SiO5/Yb2Si2O7/SiO2/Si were obtained using an unconventional yet simple technique of reactive sintering. Yb2O3 slurry was sprayed on polycrystalline silicon (used as a bond coat for SiCf/SiC composites) and reactive sintered to generate graded layers that provide a combination of mitigated thermal expansion mismatch as well as silica activity. The Yb2O3‐terpineol slurry produced a uniform deposit of the Yb2O3 topcoat. Reactive sintering of exploratory coatings (t ∼ 10–20 µm) was carried out in air by varying the temperature (1100–1400°C) and time (2–24 h). X‐ray diffraction and scanning electron microscopy‐energy‐dispersive spectroscopy technique confirmed the in situ formation of X2‐Yb2SiO5 and β‐Yb2Si2O7. Rietveld refinement was performed to know the volume fractions of the respective phases. The set of process parameters identified from the preliminary work was employed to fabricate EBCs with thickness ∼ 250 µm. The X2‐Yb2SiO5 and β‐Yb2Si2O7 phases formed next to the Si/SiO2 interface as two separate layers. Post‐sintering heat treatments in air at 1300, 1350, and 1400°C for 3–50 h elucidated the growth characteristics of the oxide layers to be a function of both temperature and time. The theoretical considerations of diffusional reactive phase formation of the EBC multilayer growth rates are discussed. [ABSTRACT FROM AUTHOR]

SiCf/SiC composites are replacing superalloy components in hot sections of gas turbines. The high-velocity combustion gases in gas turbine hot sections contain water vapor. The mechanism of oxidative water vapor attack on SiC is vastly different from that of oxidation in static dry air. Therefore, it is of paramount importance to study the oxidation of these composites in high-velocity combustion environments. This study focuses on the high-temperature oxidation behavior of SiCf/SiC composites with a BN interfacial coating in both static air and combustion environments. The composite was prepared by the chemical vapor infiltration process (CVI). To elucidate the impact of water vapor in a combustion environment, an oxyacetylene flame apparatus was used. Oxidation of the SiCf/SiC composite was investigated at 1200, 1300 and 1400 °C, for up to 100 h in static dry air, and for up to 24 h in a combustion environment. The microstructure evolution and recession of the composite under combustion environment were investigated by scanning electron microscopy. The oxidation kinetics of the SiCf/SiC composite showed a parabolic nature in static air; however, in the combustion environment the composite showed accelerated mass loss owing to the concurrent effects of oxidation and recession. [ABSTRACT FROM AUTHOR]

Multicomponent rare earth oxide (REO) nanocrystalline powders containing up to seven equiatomic rare earth elements were successfully synthesized in a single-phase CaF2-type (Fm-3 m) structure. The addition of more than six elements resulted in the formation of a secondary phase. Annealing at 1000°C for 1 h led to the formation of a single-phase (Ia-3) even in the 7-component system. In the absence of cerium (Ce4+), secondary phases were observed irrespective of the number of cations or the extent of thermal treatment indicating that cerium cations played a crucial role in stabilizing the multicomponent REOs into a phase pure structure. IMPACT STATEMENT Multicomponent equiatomic rare earth oxides pioneer a new group of materials that crystallize into a single-phase structure with the dominant role of a single element instead of entropy. [ABSTRACT FROM PUBLISHER]

This study examined pressure consolidation of amorphous Al2O3–15 mol% Y2O3 powders prepared by co-precipitation and spray pyrolysis. The two amorphous powders had similar true densities and crystallization sequences. Uniaxial hot pressing was carried out at 450°–600°C with a moderate pressure of 750 MPa. The co-precipitated powder could be hot pressed to a maximum relative density of 98% and remained amorphous. Pressure adversely affected the densification of the spray-pyrolyzed powder by favoring an early crystallization of γ-Al2O3 phase at 580°C. Plastic deformation of the amorphous phase is believed to be responsible for the large densification of the amorphous powders. [ABSTRACT FROM AUTHOR]

This study examined pressure consolidation of amorphous Al2O3–15 mol% Y2O3 powders prepared by co-precipitation and spray pyrolysis. The two amorphous powders had similar true densities and crystallization sequences. Uniaxial hot pressing was carried out at 450°–600°C with a moderate pressure of 750 MPa. The co-precipitated powder could be hot pressed to a maximum relative density of 98% and remained amorphous. Pressure adversely affected the densification of the spray-pyrolyzed powder by favoring an early crystallization of γ-Al2O3 phase at 580°C. Plastic deformation of the amorphous phase is believed to be responsible for the large densification of the amorphous powders. [ABSTRACT FROM AUTHOR]

Al2O3-Y2O3 powders were synthesized in the range of 25-55% Y2O3 by reverse coprecipitation of nitrate solutions. All starting powders were amorphous and formed primary yttrium aluminum garnet (YAG) upon crystallization. X-ray diffraction detected only garnet in compositions of 30-40% Y2O3 after heat treatment at 1250 °C. Compositions of 45-55% Y2O3 established a metastable YAG + Y4Al2O9 microstructure. The YAG phase field was metastably extended away from its stoichiometry, as indicated by a systematic increase in lattice parameter with Y2O3 content. Although some Al2O3 enrichment was achieved, YAG appears to tolerate greater off-stoichiometry on the Y2O3-rich side. Possible defect structures accommodating the solubility extension were examined. Phase selection results indicate that compositional inhomogeneity is not the only reason behind the appearance of hexagonal YAlO3, which is frequently reported during YAG synthesis. [ABSTRACT FROM AUTHOR]

Densification behavior of alpha silicon carbide (SiC) during vacuum hot pressing was studied up to 1900ºC with sintering additives based on AlN and Y2O3 in different proportions. Near theoretical density was obtained with a total sintering additive content of < 4 vol.%. The microstructure of SiC sintered with AlN+Y2O3 revealed fine equiaxed grains against the additional elongated grains exhibited by SiC sintered with AlN alone. The SiC having high density exhibited very good strength, elastic modulus, high thermal conductivity, low coefficient of thermal expansion and excellent polishability for telescope mirror applications. [ABSTRACT FROM AUTHOR]