Processing and Testing of RE2Si2O7 Fiber–Matrix Interphases for SiC–SiC Composites

Rare‐earth disilicates (RE₂Si₂O₇) are investigated for use as oxidation‐resistant alternatives to carbon or BN fiber–matrix interphases in ceramic matrix composites (CMC). Dense α, β, γ‐Y₂Si₂O₇, and γ‐Ho₂Si₂O₇ pellets were formed at 64 MPa and 1050°C–1200°C for 1 h using the field‐assisted sintering technique (FAST). Pellet modulus was measured using nanoindentation, and Vickers hardness was measured at loads of 100, 500, and 1000 g. The sliding stress of SCS‐0 SiC fibers incorporated in α‐, β‐, and γ‐RE₂Si₂O₇ matrices were measured by fiber push‐out. Deformation of RE₂Si₂O₇ after indentation and after fiber push‐out was characterized by TEM. Implications of the results for use of RE₂Si₂O₇ as a fiber–matrix interphase in CMCs are discussed.     

High-temperature mechanical properties of NextelTM 610 fiber reinforced silica matrix composites

Novel Nextel? 610 fiber reinforced silica (N610f/SiO₂) composites were fabricated via sol-gel process at a sintering temperature range of 800–1200?°C. The sintering-temperature dependent microstructures and mechanical properties of the N610f/SiO₂ composites were investigated comprehensively by X-ray diffraction, nanoindentation, three-point bending etc. The results suggested a thermally stable Nextel? 610 fiber whose properties were barely degraded after the harsh sol-gel process. A phase transition in the silica matrix was observed at a critical sintering temperature of 1200?°C, which led to a significant increase in the Young's modulus and hardness. Due to the weak fiber/matrix interfacial interaction, the 800?°C and 1000?°C fabricated N610f/SiO₂ composites exhibited quasi-ductile fracture behaviors. Specially, the latter possessed the highest flexural strength of ≈ 164.5?MPa among current SiO₂-matrix composites reinforced by fibers. The higher sintering-temperature at 1200?°C intensified the SiO₂ matrix, but strengthened the interface, thus resulting in a brittle fracture behavior of the N610f/SiO₂ composite. Finally, the mechanical properties of this novel composite presented good thermal stability at high temperatures up to 1000?°C.     

Oxidation of SiC/BN/SiC ceramic matrix composites in dry and wet oxygen at intermediate temperatures

The oxidation behavior of SiC/BN/SiC ceramic matrix composites (CMCs) was evaluated from 400° to 800 °C in 100% O₂ and 50% H₂O/50% O₂ gas mixtures. Thermogravimetric analysis (TGA) was utilized to measure weight change during controlled environment exposures at elevated temperatures for 1 and 50 hours. Oxidized CMCs and their oxides were studied post-exposure with scanning electron microscopy and energy dispersive spectroscopy. The oxidation onset and composition transition temperatures were evaluated. Key observations include oxide composition, oxide wetting, oxygen solubility in Hi-Nicalon SiC fibers and BN fiber coating oxidation and volatility behavior as a function of temperature. Degradation in wet environments at 600 °C was most extensive due to the formation of a non-wetting, non-protective surface oxide, allowing oxidant access to the BN fiber coatings followed by oxidation and volatilization. Implications of the CMC oxidation behavior are discussed for CMCs in service.     

Effect of filler‐polymer interactions on the crystalline morphology of PEO‐based solid polymer electrolytes by Y2O3 nano‐fillers

A novel lithium solid polymer electrolyte (SPE) based on polyethylene oxide (PEO) matrix and yttrium oxide (Y₂O₃) as nano‐filler was prepared by solution casting technique. The Lewis acid‐based filler‐polymer coordination interactions between the surface of Y₂O₃ and the ether oxygen of PEO were proved by FTIR, which induced an obvious modification of crystalline morphology of the PEO‐based SPEs. Polarized optical microscope (POM) analysis shows that the induced nucleation and steric hindrance effects of Y₂O₃ nano‐filler result in the increased amount as well as decreased size of spherulites in the PEO matrix, respectively. Atomic force microscope (AFM) images indicate the surface morphology of PEO gets rougher as Y₂O₃ content increases. X‐ray diffractomer (XRD) and differential scanning calorimetry (DSC) results demonstrate the crystallinity of SPEs decreases from 51.1% to 32.5% with the Y₂O₃ weight ratio [m(Y₂O₃)/m(PEO+LiI)] increasing from 0 to 0.15. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Characterization and High-Temperature Mechanical Behavior of an Oxide/Oxide Composite

An oxide/oxide ceramic fiber-matrix composite (CMC) has been extensively characterized for high-temperature aerospace structural applications. This CMC is called GEN-IV™, and it has a porous and cracked aluminosilicate matrix reinforced by 3M Nextel 610™ alumina fibers woven in a balanced eight harness weave (8HSW). This CMC has been specifically designed without an interphase between the fiber and matrix, and it relies on the porous matrix for flaw tolerance. Stress-strain response is nearly linear to failure and without a well-defined proportional limit in tension and compression. In-plane shear and interlaminar strength increases with increasing temperature. The 1000°C fatigue limit in air at 105 cycles is 160 MPa, and the residual tensile strength of run-out specimens is not affected by the fatigue loading. The creep-rupture resistance above 1000°C is relatively poor, but it can be improved with a more-creep-resistant fiber.     

To drill or not to drill? Creep of an oxide-oxide composite with diamond-drilled effusion holes at elevated temperature

Ceramic matrix composites (CMCs) are prime candidates for use in aircraft engines. Yet even with their high-temperature capabilities, many CMC components will need cooling. Film cooling technique requires rows of small holes through the component surface. Effects of multiple small holes on tensile stress-strain and tensile creep performance of an oxide-oxide CMC consisting of Nextel 720 alumina-mullite fibers in a porous alumina matrix were evaluated at 1200°C. Test specimens included 17 holes with a .5 mm diameter in the gage section. The holes were precision drilled using diamond coated drill bits. The presence of diamond-drilled holes noticeably lowered tensile properties and degraded creep performance of the CMC. Our earlier study considered the effects of small holes drilled using a CO₂ laser on the tensile properties and tensile creep resistance of this composite. The presence of laser-drilled holes also considerably lowered the creep resistance of the Nextel 720/alumina CMC. In both cases the reductions in tensile strength and creep resistance are due to damage caused to composite microstructure by hole drilling. However, different drilling techniques result in different microstructure degradation mechanisms. Damage to the CMC microstructure caused by these two drilling techniques and implications for mechanical performance and durability are discussed.     

Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO₂ coated CMCs at high impact speed showed localized damage without any shattering.     

Electromagnetic characteristics and microstructure stability of Nextel 610 fiber after heat treatment

The thermal and microstructure stability of Nextel 610 fibers has great influence on high-temperature application of Nextel 610 fiber-reinforced ceramic matrix composites. In this work, Nextel 610 fibers were heat treated at 500–700 °C in vacuum and 800–1100 °C in Ar atmosphere, respectively. The sizing agent on Nextel 610 fiber surface could be decomposed into pyrolytic carbon, SiC and gaseous little molecules at lower temperatures, otherwise it was decomposed mainly in the form of gaseous little molecules at higher temperatures, so that the complex permittivity firstly increased and then decreased with the increasing of temperatures. The results showed that the annealed Nextel 610 fiber (T>900 °C) could be regarded as electromagnetic wave transparent fibers, while the tensile strength had declined by half when the temperature increased to 1100 °C. Therefore, Nextel 610 fibers after being annealed at higher temperatures could be further used as reinforcement to prepare high temperature ceramic matrix composites for electromagnetic wave absorption and transparent applications.     

Interaction of fiber matrix bonding in SiC/SiC ceramic matrix composites

Non-oxide ceramic matrix composites (CMC) based on SiC fibers with SiC matrix were fabricated by polymer infiltration and pyrolysis (PIP) and characterized regarding their microstructural features and their mechanical properties. The fiber preform was made using winding technology. During the winding process, the SiC fiber roving was impregnated by a slurry containing SiC powder and sintering additives (Y₂O_3, Al₂O₃ and SiO₂). This already helped to achieve a partial matrix formation during the preform fabrication. In this way, the number of PIP cycles to achieve composites with less than 10% open porosity could be reduced significantly. Additionally, damage-tolerant properties of the composites were obtained by an optimal design of the matrix properties although only uncoated fibers were used. Finally, composites with a strength level of about 500 MPa and a damage-tolerant fracture behavior with about 0.4% strain to failure were obtained.     

Experimental and mathematical modelling of corrosion behaviour of CMAS coated oxide/oxide CMCs

CMAS corrosion of turbine blades is a crucial failure in turbine engines and their components. In this study, oxide/oxide CMCs (AS-N610), which are candidates for gas turbine (GT) applications, are investigated for its corrosion behaviour at different temperatures and time in presence of CMAS. The corrosion studies using CMAS coating of the CMCs reveal that CMCs had a weight gain of ～6% owing to formation of α-Al₂O₃ at 1000 °C. The SE image indicated the penetration of CMAS into the porous CMC. At 1000 °C, CMAS degraded to form a black glassy substance (Calcium alumino silicate) with traces of Mg which led to corrosion of the matrix. Indentation fracture toughness of the oxide/oxide CMCs was 7.78 ± 0.5 MPa m^0.5 which degraded by ～12% at 1000 °C after 10 h in the presence of CMAS. A mathematical model derived through diffusion equation indicated weight gain of ～0.3 g which was closer to experimental data.     

Resistance of 2ZrO2·Y2O3 top coat in thermal/environmental barrier coatings to calcia‐magnesia‐aluminosilicate attack at 1500°C

Internally cooled, hollow SiC‐based ceramic matrix composites (CMCs) components that may replace metallic components in the hot section of future high‐efficiency gas‐turbine engines will require multilayered thermal/environmental barrier coatings (T/EBCs) for insulation and protection. In the T/EBC system, the thermally insulating outermost (top coat) ceramic layer must also provide resistance to attack by molten calcia‐magnesia‐aluminosilicate (CMAS) deposits. The interactions between a potential candidate for top coat made of air‐plasma‐sprayed (APS) 2ZrO₂·Y₂O₃ solid‐solution (ss) ceramic and two different CMASs (sand and fly ash) are investigated at a relevant high temperature of 1500°C. APS 2ZrO₂·Y₂O₃(ss) top coat was found to resist CMAS penetration at 1500°C for 24 hours via reaction products that block CMAS penetration pathways. In situ X‐ray diffraction (XRD) studies have identified the main reaction product to be an Ca‐Y‐Si apatite, and have helped elucidate the proposed mechanism for CMAS attack mitigation. Ex situ electron microscopy and analytical spectroscopy studies have identified the advantageous characteristics of the reaction products in helping the CMAS attack mitigation in the APS 2ZrO₂·Y₂O₃(ss) coating at 1500°C. Finally, the Y³⁺ solubility limit and transport behavior are identified as potential comparative tools for assessing the CMAS resistance ability of top‐coat ceramics.     

Nonisothermal crystallization behaviors and conductive properties of PEO‐based solid polymer electrolytes containing yttrium oxide nanoparticles

Lithium solid polymer electrolytes (SPE) composed of polyethylene oxide (PEO) and yttrium oxide (Y₂O₃) nanoparticles were prepared. The influence of the Y₂O₃ nanoparticles on the non‐isothermal crystallization behaviors, crystal structure, and conductive properties of the SPE were investigated. The peak temperature, crystallinity, and crystallization half‐time (t_1/2) of the SPE were strongly dependent on the concentration of Y₂O₃ and the cooling rate. The non‐isothermal crystallization data were analyzed by the modified Avrami model, which successfully described the nonisothermal crystallization process of the SPE. The Avrami exponents suggested that the Y₂O₃ nanoparticles significantly affected both the nucleation mechanism and crystal growth of the PEO matrix. The nucleating and crystallization activation energies (ψ and E_c) estimated with different theories indicated that the Y₂O₃ nanoparticles were inclined to serve as heterogeneous nucleating agents to benefit the crystallization at lower concentration whereas as physical hindrance to inhibit the crystal growth at relatively higher concentration. The maximum conductivity (σ) of 5.95 × 10^?5 S cm^?1 at room temperature for the SPE was obtained at the Y₂O₃ weight ratio of 0.10. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Oxidation and thermal shock resistant properties of Al-modified environmental barrier coating on SiCf/SiC composites

SiC_f/SiC ceramic matrix composites (CMCs) are being widely used in the hot-sections of gas-turbines, especially for aerospace applications. These CMCs are subjected to surface recession if exposed to heat-corrosion. In this research, an alternative environmental barrier coating (EBC) is introduced to protect the SiC_f/SiC CMC from high temperature degradation that is, Al film was deposited on the surface of SiC_f/SiC CMC followed by heat-treatment in a vacuum. After that, a dense Al₂O₃ overlay was in-situ synthesized on the surface of CMC, and in this process the microstructure evolution of SiC_f/SiC CMC was analyzed. The oxidation and thermal shock resistance were characterized, showing that the Al-modified SiC_f/SiC CMC has a better oxidation resistance, because the dense Al₂O₃ overlay can hinder oxygen diffusion from environment. What is more, the water-quenching testes show that the Al-modified SiC_f/SiC CMC has a good spallation resistance.     

Mechanical and thermal conductive properties of fiber‐reinforced silica‐alumina aerogels

We report the formation of Al₂O₃‐SiO₂ fiber‐reinforced Al₂O₃‐SiO₂ aerogels with the content of fibers in the range from 40 wt% to 55 wt% by sol‐gel reaction, followed by supercritical drying. The structure and physical properties of fiber‐reinforced Al₂O₃‐SiO₂ aerogels are studied. We find that the fiber‐reinforced Al₂O₃‐SiO₂ aerogels can be resistant to the temperature of 1200°C. The integration of fibers significantly improves the mechanical properties of Al₂O₃‐SiO₂ aerogels. We find that the bending strength of fiber‐reinforced Al₂O₃‐SiO₂ aerogels increases 0.431 MPa to 0.755 MPa and the elastic modulus increases from 0.679 MPa to 1.153 MPa, when the content of fibers increases from 40 wt% to 50 wt%. The thermal conductivity of the fiber‐reinforced Al₂O₃‐SiO₂ aerogels is in the range from 0.0403 W/mK to 0.0545 W/mK, depending on the content of fibers.     

Transformation Plasticity in (GdxDy1−x)PO4 Fiber Coatings During Fiber Push Out

The reduction in fiber push‐out stress by transformation plasticity in xenotime rare‐earth orthophosphate fiber–matrix interphases was demonstrated. Processing methods for transformable xenotime coatings were explored. For conversion to xenotime during processing, (Gd_xDy_1?x)PO₄ solid solutions had to be more Dy‐rich than those for pellets. Single‐crystal alumina fibers were coated with 10–20 μm of (Gd_0.4Dy_0.6)PO₄ xenotime and incorporated into polycrystalline alumina matrices. Coated fiber push‐out stresses were between 10 and 80 MPa, significantly lower than those for fibers with other rare‐earth orthophosphates coatings. Phase transformations and deformation mechanisms were characterized by SEM and TEM in fiber coatings after push out. Bands of deformed coating several micrometers in width formed during fiber push out. Cataclastic flow with fracture, granulation, translation, rotation, and intense plastic deformation of coating grains was observed. Three phase transformations may occur in heavily deformed particles in the deformation band: xenotime → monazite, xenotime → anhydrite, and anhydrite → monazite. Anhydrite was abundant as a fine lamellar phase on (100) planes in xenotime. Selected area electron diffraction and high‐resolution TEM confirmed formation of monazite in a variety of heavily deformed particles. Issues for use of rare‐earth orthophosphate transformation plasticity to lower fiber pull‐out stress in ceramic matrix composites are discussed.     

Yttrium Bearing Silicon Carbide Matrices for Robust Ceramic Composites

Application of SiC‐based ceramic matrix composites (CMCs) in combustion environments demands the use of an environmental barrier coating (EBC) to prevent volatilization of the protective SiO₂ scale in flowing water vapor. The EBC only provides protection while present on the surface; cracking and spallation of the coating leaves the underlying SiC vulnerable to the oxidation–volatilization processes. A robust matrix material chemically tailored to regrow a yttrium silicate scale in the event of EBC loss has been developed by incorporating yttrium bearing species including YB₂, Y₂O₃, and Y₅Si₃ into the SiC. During oxidation a borosilicate glass helps seal cracks while Y₂O₃ and SiO₂ react to form Y₂Si₂O₇ for environmental protection. Candidate compositions were oxidized for 10 min to 100 h at 1400°C and for 24 h at 1500°C to understand the scale growth. The prospects for effectively applying this approach in CMCs are discussed.     

Texture,microstructures, and mechanical properties of AlN‐based ceramics with Si3N4–Y2O3 additives

Textured AlN‐based ceramics with improved mechanical properties were prepared by hot pressing using Si₃N₄ and Y₂O₃ as additives. The introduction of Si₃N₄–Y₂O₃ into AlN matrix led to the formation of secondary Y₃AlSi₂O₇N₂ and fiber‐like 2H^δ AlN‐polytypoid phases, the partial texture of all crystalline phases, and the fracture mode change from intergranular to transgranular. Consequently, Vickers hardness, fracture toughness and flexural strength of AlN‐based ceramics by the replacement of Y₂O₃ by Si₃N₄–Y₂O₃ increased significantly from 10.4±0.3 GPa, 2.4±0.3 MPa m^½ and 333.3±10.3 MPa to 14.2±0.4 GPa, 3.4±0.1 MPa m^½ and 389.5±45.5 MPa, respectively.     

Fabrication and Characterization of Glass‐Ceramic Fiber‐Containing Cr3 + ‐Doped ZnAl2O4 Nanocrystals

Glass‐ceramic fibers containing Cr³⁺‐doped ZnAl₂O₄ nanocrystals were fabricated by the melt‐in‐tube method and successive heat treatment. The obtained fibers were characterized by electro‐probe micro‐analyzer, X‐ray diffraction, Raman spectrum and high‐resolution transmission electron microscopy. In our process, fibers were precursor at the drawing temperature where the fiber core glass was melted while the clad was softened. No obvious element interdiffusion between the core and the clad section or crystallization was observed in precursor fiber. After heat treatment, ZnAl₂O₄ nanocrystals with diameters ranging from 1.0 to 6.3 nm were precipitated in the fiber core. In comparison to precursor fiber, the glass‐ceramic fiber exhibits broadband emission from Cr³⁺ when excited at 532 nm, making Cr³⁺‐doped glass‐ceramic fiber a promising material for broadband tunable fiber laser. Furthermore, the melt‐in‐tube method demonstrated here may open a new gate toward the fabrication of novel glass‐ceramic fibers.     

Novel oxide fiber composites by freeze casting

A freeze casting process with gelatine as gelling agent is presented to manufacture novel oxide fiber composites. The growth of ice crystals causes an in situ densification of the green matrix when the composite is frozen after the lamination of 2D-fabrics. This freeze casting process leads to a porous matrix with a cellular morphology, showing no shrinkage during sintering. The porous structure is suitable for a subsequent infiltration with a precursor to densify the matrix. As the matrix is crack-free, the sintered composites have high mechanical properties. The processing is demonstrated by manufacturing laminated Nextel™610/YAG-ZrO₂ oxide fiber composites.     

Er3+ ‐Doped Y2O3 Nanophosphors for Near‐Infrared Fluorescence Bioimaging Applications

Rare‐earth‐doped ceramic nanophosphor (RED‐CNP) materials are promising near‐infrared (NIR) fluorescence bioimaging (FBI) agents that can overcome problems of currently used organic dyes including photobleaching, phototoxicity, and light scattering. Here, we report a NIR–NIR bioimaging system by using NIR emission at 1550 nm under 980 nm excitation which can allow a deeper penetration depth into biological tissues than ultraviolet or visible light excitation. In this study, erbium‐doped yttrium oxide nanoparticles (Er³⁺:Y₂O₃) with an average particle size of 100 and 500 nm were synthesized by surfactant‐assisted homogeneous precipitation method. NIR emission properties of Er³⁺:Y₂O₃ were investigated under 980 nm excitation. The surface of Er³⁺:Y₂O₃ was electrostatically PEGylated using poly (ethylene glycol)‐b‐poly(acrylic acid) (PEG‐b‐PAAc) block copolymer to improve the chemical durability and dispersion stability of Er³⁺:Y₂O₃ under physiological conditions. In vitro cytotoxic effects of bare and PEG‐b‐PAAc‐modified Er³⁺:Y₂O₃ were investigated by incubation with mouse macrophage cells (J774). Microscopic and macroscopic FBI were demonstrated in vivo by injection of bare or PEG‐b‐PAAc‐modified Er³⁺:Y₂O₃ into C57BL/6 mice. The NIR fluorescence images showed that PEG‐b‐PAAc modification significantly reduced the agglomeration of Er³⁺:Y₂O₃ in mice and enhanced the distribution of Er³⁺:Y₂O₃.