Preparation and Analysis of a Silicon Carbide Composite Membrane

Silicon carbide (SiC) porous substrates, containing alumina (Al₂O₃) dopant levels of 3, 5, and 8 wt%, are prepared by slip casting and sintering in the temperature range of 1450°–1800°C. The linear shrinkage, bulk density, and pore size of the sintered substrate increase as the sintering temperature and the amount of dopant increase. A large amount of β-phase SiC is transformed to α-phase SiC if the dopant concentration is 5 or 8 wt%. The flexural strength of the substrate doped with 8 wt% Al₂O₃ is higher than that of the substrate doped with 3 wt% Al₂O₃; however, the Weibull modulus of the former is lower. SiC composite membranes of improved selectivity and strength are fabricated by coating the porous substrate with layers of lower Al₂O₃ contents at lower sintering temperatures.     

Mullite Coatings on SiC and C/C–Si–SiC Substrates Characterized by Infrared Spectroscopy

Mullite (3Al₂O₃·2SiO₂) coatings on SiC substrates and SiC precoated carbon/carbon composite (C/C-Si-SiC) substrates were produced by pulsed laser deposition (PLD) using pressed mullite powder targets. The layers can be characterized efficiently by IR reflection spectroscopy in the spectral range between 650 and 5000 cm⁻¹. The deposited coatings turn into mullite upon oxidation in air at temperatures between 1400° and 1600°C. Fabry-Perot interferences indicate a high quality and homogeneity of the mullite coating/SiC substrate interface. Amorphous SiO₂ gradually forms during prolonged heating or at higher temperatures.     

Effects of Wet Oxidation on the Properties of Sub-10-nm-Thick Silicon Nitride Films

Capacitors were prepared by wet oxidation of 8 nm (80 Å) thick silicon nitride films which had been deposited on polysilicon substrates. The capacitance and breakdown field of these capacitors were measured as a function of the wet oxidation conditions. The properties of the polysilicon films were also investigated by varying the preparation conditions to find a better substrate for the deposition of the ultrathin silicon nitride films. These polysilicon films were prepared using low-pressure chemical vapor deposition at 560° and 625°C on silicon wafers. The films were then annealed at 900°C for 30 min after As doping by implantation with a dose of more than 2 × 10¹⁵/cm². The films deposited at 560°C have a smooth surface and a low electrical resistivity and are thus suitable substrates for the ultrathin silicon nitride films as well as suitable contact electrodes. For the capacitors with an oxide nitride composite layer, the capacitance decreases sharply, but the breakdown field increases with an increase in the wet oxidation time at 900°C. This is due to an increase in the thickness of the oxide—nitride layer. For the capacitors with silicon oxynitride layers created by removing the top oxide of the silicon nitride wet-oxidized at 900°C for more than 35 min, however, the values of both the capacitance and the breakdown field are higher than those of capacitors with an unoxidized silicon nitride layer.     

Mechanical Properties, Thermal Shock Resistance, and Thermal Stability of Zirconia-Toughened Alumina-10 vol% Silicon Carbide Whisker Ceramic Matrix Composite

Zirconia-toughened alumina (Al₂O₃–15 vol% Y-PSZ (3 mol% Y₂O₃)) reinforced with 10 vol% silicon carbide whiskers (ZTA-10SiC _w ) ceramic matrix composite has been characterized with respect to its room-temperature mechanical properties, thermal shock resistance, and thermal stability at temperatures above 1073 K. The current ceramic composite has a flexural strength of ∽550 to 610 MPa and a fracture toughness, K _IC , of ∽5.6 to 5.9 MPa·m^1/2 at room temperature. Increases in surface fracture toughness, ∽30%, of thermally shocked samples were observed because of thermal-stress-induced tetragonal-to-monoclinic phase transformation of tetragonal ZrO₂ grains dispersed in the matrix. The residual flexural strength of ZTA–10 SiC _w ceramic composite, after single thermal shock quenches from 1373–1573 to 373 K, was ∽10% higher than that of the unshocked material. The composite retained ∽80% of its original flexural strength after 10 thermal shock quenches from 1373–1573 to 373K. Surface degradation was observed after thermal shock and isothermal heat treatments as a result of SiC whisker oxidation and surface blistering and swelling due to the release of CO gas bubbles. The oxidation rate of SiC whiskers in ZTA-10SiC _w composite was found to increase with temperature, with calculated rates of ∽8.3×10⁻⁸ and ∽3.3×10⁻⁷ kg/(m²·s) at 1373 and 1573 K, respectively. It is concluded that this ZTA-10SiC _w composite is not suitable for high-temperature applications above 1300 K in oxidizing atmosphere because of severe surface degradation.     

Oxidation Behavior of Silicon Carbide

The oxidation of purified green silicon carbide in controlled atmospheres was studied by weight-gain measurement and by observation of the surface reaction products, including optical measurement of the thickness of the oxide surface film. The rate of oxidation was much greater for silicon carbide in contact with fluid silicate glasses than for silicon carbide alone. In a vacuum of 0.1 mm., oxidation proceeded with loss of weight, because of the formation of volatile SiO₂, and at a greater rate than at atmospheric pressure. It is postulated that the rate-controlling process in the normal oxidation of silicon carbide is the formation of solid SiO₂ on the surface.     

Highly Creep-Resistant Silicon Nitride/Silicon Carbide Nano–Nano Composites

A high creep resistance at specified temperature and compressive stress was obtained in this investigation in the silicon nitride/silicon carbide composite with a nano–nano structure (nanosized SiC and Si₃N₄ in dual-phase mixture) by a novel synthesis method. Starting from an amorphous Si–C–N powder derived from pyrolysis of a liquid polymer precursor, nanocomposites with varied grain size were achieved. With yttria additive amount decreasing from 8 to 1 wt% and eventually to zero, the structure underwent a transition from micro-nano (nano-sized SiC included in sub-micron Si₃N₄) to nano–nano type. Nanocrystalline silicon nitride/silicon carbide ceramic composite with 30–50 nm grain size was synthesized without using sintering additive.     

Surface Modification of Silicon Nitride Powder with Aluminum

Surface modification of Si₃N₄ with alumina was tried. It was achieved by simply mixing Si₃N₄ powder with an alumina sol up to ∼2 wt% as alumina in an aqueous medium, dried, and followed by calcination at 400°C for 1 h. A TEM micrograph showed a coating layer of ∼15 nm thickness. The isoelectric point of the modified Si₃N₄ powder with porous alumina was at pH 7.8, which is different from 5.8 and 8.6 for Si₃N₄ and amorphous alumina, respectively.     

Oxidation of Zirconium Diboride–Silicon Carbide at 1500°C at a Low Partial Pressure of Oxygen

The oxidation behavior of zirconium diboride containing 30 vol% silicon carbide particulates was investigated under reducing conditions. A gas mixture of CO and ∼350 ppm CO₂ was used to produce an oxygen partial pressure of ∼10⁻¹⁰ Pa at 1500°C. The kinetics of the growth of the reaction layer were examined for reaction times of up to 8 h. Microstructures and chemistries of reaction layers were characterized using scanning electron microscopy and X-ray diffraction analysis. The kinetic measurements, the microstructure analysis, and a thermodynamic model indicate that oxidation in CO–CO₂ produced a non-protective oxide surface scale.     

Growth of Tabular α-Al2O3 Grains on Porous Alumina Substrate

Gradient, porous alumina ceramics were prepared with the characteristics of microsized tabular α-Al₂O₃ grains grown on a surface with a fine interlocking feature. The samples were formed by spin-coating diphasic aluminosilicate sol on porous alumina substrates. The sol consisted of nano-sized pseudo-boehmite (AlOOH) and hydrolyzed tetraethyl orthosilicate [Si(OC₂H₅)₄]. After drying and sintering at 1150°–1450°C, the crystallographic and chemical properties of the porous structures were investigated by analytical electron microscopy. The results show that the formation of tabular α-Al₂O₃ grains is controlled by the dissolution of fine Al₂O₃ in the diphasic material at the interface. The nucleation and growth of tabular α-Al₂O₃ grains proceeds heterogeneously at the Al₂O₃/glass interface by ripening nano-sized Al₂O₃ particles.     

Densification and Grain Growth of Porous Alumina Compacts

Densification and grain growth of porous alumina compacts during various high-temperature processes were investigated. Experimental data were obtained for densification and grain growth of alumina powder during hot pressing. A set of constitutive equations was proposed based on the constitutive equations by Helle et al. ¹ for hydrostatic response and by Rahaman et al. ² for deviatoric response. Theoretical results from the proposed constitutive equations were compared with various experimental data for alumina powder compacts in the literature, including pressureless sintering, sinter forging, and hot pressing. The proposed model well predicts the densification and grain growth of alumina compacts.     

Early-Stage Thermal Oxidation of Carbothermal β-Sialon Powder

Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized β-sialon powder was monitored by X-ray powder diffraction, solid-state ²⁹Si and ²⁷Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but ²⁹Si and ²⁷Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJmol⁻¹, suggesting the rate-limiting step to be due to dissolution of O₂ in an amorphous silica surface layer on the β-sialon particles.     

Modeling Electrophoretic Deposition on Porous Non-Conducting Substrates Using Statistical Design of Experiments

Statistical design of experiments was used to model electrophoretic deposition of yittria-stabilized zirconia (YSZ) particles on porous, non-conducting NiO–YSZ substrates. A 2³–full-factorial matrix with three repetitions of the centerpoint was augmented with six axial runs and two additional centerpoints to form an inscribed central composite design. Fixed ranges of substrate firing temperature (1100°–1300°C), deposition voltage (50–300 V), and deposition time (1–5 min) were used as the independent design variables to model responses of YSZ deposition thickness, area-specific interfacial resistance (ASR), and power density. Regression equations were determined, which were used to optimize deposition parameters based on the desired responses of low interfacial polarization resistance and high-power density. Low substrate firing temperature (1100°C) combined with a low voltage (50 V) and minimal deposition time (1 min) resulted in a 6 μm-thick YSZ film, a power density of 628 mW/cm², and an ASR of 0.21 Ω·cm². Increasing the substrate firing temperature, voltage, and time to 1174°C, 215 V, and 3 minutes, respectively, reduced the ASR to 0.19 Ω·cm², increased YSZ film thickness to 25 μm, but had only a negligible effect on power density (600 mW/cm²).     

In Situ Preparation of Si3N4/SiC Nanocomposites for Cutting Tools Application

Silicon nitride–silicon carbide nanocomposite has been prepared by an in situ method that utilizes the formation of SiC nanograins by carbothermal reduction of intentionally added fine SiO₂ during the sintering process. The mean value of room-temperature four-point bending strength is 675 MPa with the Weibull modulus of 6.4 and an indentation fracture toughness of 7.4 MPa·m^1/2. A significantly enhanced creep resistance was achieved by the incorporation of SiC nanoparticles into the matrix up to 1400°C. The tribological properties of the material were tested using a ball-on-disk configuration and showed a friction coefficient of about 0.7. The cutting inserts machined from this composite had three times longer lifetime compared with those available on the market. On the other hand, the scatter of results is much larger compared with those measured for the commercial inserts.     

Preparation of Highly Dense PZN–PZT Thick Films by the Aerosol Deposition Method Using Excess-PbO Powder

Lead zinc niobate–lead zirconate titanate thick films with a thickness of 50–100 μm were deposited on silicon and alumina substrates using the aerosol deposition method. The effects of excess lead oxide (PbO) on stress relaxation during postannealing were studied. Excess PbO content was varied from 0 to 5 mol%. The as-deposited film had a fairly dense microstructure with nanosized grains. The films deposited on silicon were annealed at temperatures of 700°C, and the films deposited on sapphire were annealed at 900°C in an electrical furnace. The annealed film was detached and cracks were generated due to the high residual compressive stress and thermal stress induced by thermal expansion coefficient mismatch. However, the film deposited using powder containing 2% of excess PbO showed no cracking or detachment from the substrate after the postannealing process. The PbO evaporation at elevated temperature during the postannealing process seemed to have reduced the residual compressive stress. The remanent polarization and relative dielectric constant of the 50 μm thick films annealed at 900°C were 43.1 μC/cm² and 1400, respectively, which were comparable with the values of a bulk specimen prepared by a powder sintering process.     

Preparation of Silicon Carbide and Aluminum Silicon Carbide from a Montmorillonite–Polyacrylonitrile Intercalation Compound by Carbothermal Reduction

Both silicon carbide and aluminum silicon carbide have simultaneously been obtained directly from naturally occurring aluminosilicate by carbothermal reduction for the first time. A precursor of a montmorillonite–polyacrylonitrile (PAN) intercalation compound was heated at 1700°C in Ar. For comparison, montmorillonite–carbon mixtures were similarly heated. α-SiC, β-SiC, and Al₄Si₂C₅ formed from the montmorillonite–PAN intercalation compound. Mainly α-Al₄SiC₄ was obtained with ternary carbides from the montmorillonite–carbon mixtures in addition to a large amount of β-SiC. Hence, aluminum silicon carbide formation was affected by the mixing condition of the starting materials.     

Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts Following Ground Testing on an F110 Turbofan Engine

Four ceramic-matrix composite flap inserts were evaluated following ground testing on a General Electric F110 turbofan engine. Three of the composites accumulated ∼117 h of engine time. The fourth composite, a Nextel^TM 720 material with aluminosilicate matrix, accumulated ∼40 h. Large through-thickness cracks developed along the longitudinal edges of a Nicalon™/Al₂O₃ insert and the Nextel 720/aluminosilicate insert. The cracks developed because of high tensile stresses caused by the steep in-plane thermal gradients induced across the flap width during afterburner lights. The Nextel 720/aluminosilicate insert also exhibited severe surface wear associated with the acoustic environment and contact with the adjacent divergent seals. Neither a Nicalon/silicon nitrocarbide insert nor a Nicalon/C insert exhibited significant signs of distress.     

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al₄SiC₄) with the specific surface area of 15.5 m²·g⁻¹. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m²·g⁻¹, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.     

High-Temperature Active Oxidation of Chemically Vapor-Deposited Silicon Carbide in an Ar─O2 Atmosphere

Active oxidation behavior of chemically vapor-deposited silicon carbide in an Ar─O₂ atmosphere at 0.1 MPa was examined in the temperature range between 1840 and 1923 K. The transition from active oxidation (mass loss) to passive oxidation (mass gain) was observed at certain distinct oxygen partial pressures ( P _O2^t). The values of P _O2^t increased with increasing temperature and with decreasing total gas flow rates. This behavior was well explained by Wagner's model and thermodynamic calculations. Active oxidation rates ( k _a) increased with increasing O₂ partial pressures and total gas flow rates. The rate-controlling step of the active oxidation was concluded to be O₂ diffusion through the gaseous boundary layer.     

Temperature Dependence of Young's Modulus and Internal Friction in Alumina, Silicon Nitride, and Partially Stabilized Zirconia Ceramics

The temperature dependence of Young's modulus and internal friction (Q⁻¹)in alumina, silicon nitride, and partially stabilized zirconia (Y-PSZ) ceramics was studied. Little change in Q⁻¹ was found for alumina, whereas Q⁻¹ for silicon nitride ceramics increased above 700°C. The Q⁻¹ of Y-PSZ increased markedly with increasing temperature up to a peak at ∼200°C.     

Static Fatigue Limit for Sintered Silicon Carbide at Elevated Temperatures

The modified static loading technique for estimating static fatigue limits was used to study the effects of oxidation and temperature on the static fatigue limit, K ₁₀ for crack growth in sintered silicon carbide. For as-machined, unoxidized sintered silicon carbide with a static load time of 4 h, K ₁₀× 2.25 MPa * m^1/2 at 1200° and ∼1.75 at 1400°C. On oxidation for 10 h at 1200°C, K ₁₀ drops to ∼1.75 MPam^1/2 at 1200° and ∼1.25 at 1400°C when tested in a nonoxidizing ambient. Similar results were obtained at 1200°C for tests performed in air. A tendency for strengthening below the static fatigue limit appears to result from plastic relaxation of stress in the crack-tip region by viscous deformation involving an oxide grain-boundary phase for oxidized material and, possibly, diffusive creep deformation in the case of unoxidized material.