Water-Based Gelcasting of Surface-Coated Silicon Nitride Powder

A layer of Y₂O₃–Al₂O₃, used as a sintering aid, was coated onto the surface of Si₃N₄ particles by the precipitation of inorganic salts from a water-based solution containing Al(NO₃)₃, Y(NO₃)₃, and urea. The electrokinetic and colloidal characteristics of the Si₃N₄ powder were changed significantly by the coating layer. As a result, dispersion of the Y₂O₃–Al₂O₃-coated Si₃N₄ powder was significantly greater than that of the original powder. Furthermore, the Y₂O₃–Al₂O₃ coating layer prevented the hydrogen-gas-discharging problem that occurred during gelcasting of the original Si₃N₄ powder because of reaction between the uncoated powder and the basic aqueous solution in suspension. Surface coating, as well as the gelcasting process, significantly improved the microstructure, room-temperature bending strength, and Weibull modulus of the resulting ceramic bodies.     

Processing of Concentrated Aqueous Silicon Nitride Slips by Slip Casting

The optimization of concentrated Si₃N₄ powder aqueous slurry properties to achieve high packing density slipcast compacts and subsequent high sintered densities was investigated. The influence of pH, sintering aid powder (6% Y₂O₃, 4% Al₂O₃), NH₄PA dispersant, and Si₃N₄ oxidative thermal treatment was determined for 32 vol% Si₃N₄ slurries. The results were then utilized to optimize the dispersion properties of 43 vol% solids Si₃N₄-sintering aid slurries. Calcination of the Si₃N₄ powder was observed to result in significantly greater adsorption of NH₄PA dispersant and effectively reduced the viscosity of the 32 vol% slurries. Lower viscosities of the optimized dispersion 43 vol% Si₃N₄-sintering aid slurries resulted in higher slipcast packing density compacts with smaller pore sizes and pore volumes, and corresponding higher sintered densities.     

Electrostatic Adsorption of a Colloidal Sintering Agent on Silicon Nitride Particles

Pressureless sintering of silicon nitride requires addition of sintering agents. The main part of this study was done in order to homogenize the distribution of sintering agents, in this case Y₂O₃, in a silicon nitride matrix. Colloidal 10-nm Y₂O₃ Particles were electrostatically adsorbed on Si₃N₄ particle surfaces. The adsorption was studied by X-ray fluorescence analysis and electrophoretic measurements. Addition of Y₂O₃ sol to a Si₃N₄ suspension decreased the viscosity of the suspension. The slip casting properties of Si₃N₄ suspensions with added Y₂O₃ sol were examined, and the homogeneity of Y₂O₃ in the green compacts was compared with conventionally prepared samples. An improved microstructural homogeneity was obtained when Y₂O₃ sol particles were adsorbed on the Si₃N₄ particle surfaces.     

Dispersion Methods of Yttria in Silicon Nitride by Comminution

Sintering additives were incorporated into Si₃N₄ by attrition and ball milling using both Si₃N₄ and Al₂O₃ media. Dispersion of Y₂O₃ was observed by backscattered electron imaging. Attrition milling for only 15 min using an Si₃N₄ medium, was equivalent to 24 h of ball milling. Minimal contamination by the Si₃N₄ was encountered. [Key words: silicon nitride, yttria, comminution, sintering, dispersion.     

Effect of Addition of Silicon on the Microstructures and Bending Strength of Continuous Porous SiC–Si3N4 Composites

The microstructures and mechanical properties of continuous porous SiC–Si₃N₄ composites fabricated by multi-pass extrusion were investigated, depending on the amount of Si powder added. Si powder with different weight percentages (0%, 5%, 10%, 15%, 20%) was added to SiC powder to make raw mixture powders, with 6 wt% Y₂O₃–2 wt% Al₂O₃ as sintering additives, carbon (10–15 μm) as a pore-forming agent, ethylene vinyl acetate as a binder, and stearic acid (CH₃(CH₂)₁₆COOH) as a lubricant. In the continuous porous SiC–Si₃N₄ composites, Si₃N₄ whiskers like the hairs of nostrils were frequently observed on the wall of the pores. In this study, the morphology of Si₃N₄ whiskers was investigated with the nitridation condition and silicon addition content. In composites containing an addition of 10 wt% Si, a large number of Si₃N₄ whiskers were found at the continuous pore regions. In the sample to which 15 wt% Si powder was added, a maximum value of about 101 MPa bending strength and 57.5% relative density were obtained.     

Microstructure and Grain-Boundary Composition of Hot-Pressed Silicon Nitride With Yttria and Alumina

The microstructure of two hot-pressed silicon nitrides containing Y₂O₃ and Al₂O₃ was examined by electron microscopy, electron diffraction, and quantitative, energy-dispersive X-ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y₂O₃+2 wt% Al₂O₃, as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high-resolution dark-field and lattice imaging. The second phase in a material with additives of S wt% Y₂O₃+5 wt% Al₂O₃ was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot-stage experiments in a high-voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si₃N₄ grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y₂O₃/Al₂O₃ fluxing.     

New Strategies for Preparing NanoSized Silicon Nitride Ceramics

We report the preparation of nanosized silicon nitride (Si₃N₄) ceramics via high-energy mechanical milling and subsequent spark plasma sintering. A starting powder mixture consisting of ultrafine β-Si₃N₄ and sintering additives of 5-mol% Y₂O₃ and 2-mol% Al₂O₃ was prepared by high-energy mechanical milling. After milling, the powder mixture was mostly transformed into a non-equilibrium amorphous phase containing a large quantity of well-dispersed nanocrystalline β-Si₃N₄ particles. This powder precursor was then consolidated by spark plasma sintering at a temperature as low as 1600°C for 5 min at a heating rate of 300°C/min. The fully densified sample consisted of homogeneous nano-Si₃N₄ grains with an average diameter of about 70 nm, which led to noticeable high-temperature ductility and elevated hardness.     

Rheology and Consolidation of Colloidal Alumina-Coated Silicon Nitride Suspensions

In an attempt to improve the colloidal processing of Si₃N₄ ceramics, we studied the rheology and consolidation of colloidal suspensions of Si₃N₄ particles (average particle size 0.7 μm) with small Al₂O₃ particles (average particle size 20 nm). It was found that at pH >7, the viscosity of the mixtures increased and then decreased with an increasing concentration of Al₂O₃. λpotential measurements, optical micrographs, and visible light absorptance measurement suggest that such viscosity behavior is due to clustering of Si₃N₄ particles bridged by the small Al₂O₃ particles. This is also supported by the Derjaquin-Landau-Verwey-Overbeek (DLVO) potential calculations that show the barrier height in the DLVO potential between Al₂O₃ and Si₃N₄ is small. The small barrier height under current experimental conditions stems from the small size of the Al₂O₃ particles. The small barrier height allows the thermal motion of the two kinds of particles to overcome the barrier and attach to each other. The adsorption of small Al₂O₃ particles on Si₃N₄ can occur even when both Al₂O₃ and Si₃N₄ carry the same sign of charges. The adsorption of Al₂O₃ on Si₃N₄ also increases the density of consolidated compacts.     

Cyclic Thermal Shock Resistance of Several Advanced Ceramics and Ceramic Composites

The cyclic thermal shock behavior of two Si₃N₄ ceramics, two SiC-whisker-reinforced alumina composites (Al₂O₃/SiC_w), a SiC-particulate-reinforced alumina (Al₂O₃/SiC_p), and an alumina continuously reinforced with SiC fibers (Al₂O₃/SiC_f) composite has been studied. Specimens were repeatedly quenched from 1473 K into a fluidized bed with a heat transfer coefficient of 1400 W/(Km²) [250 Btu/(hft²F)]. The thermal shock damage was assessed by room-temperature flexure strength measurements. Si₃N₄ and Al₂O₃/SiC_p showed no noticeable damage after 100 cycles, whereas Al₂O₃/SiC_w and Al₂O₃/SiC_f degraded substantially. The experimental results are discussed and rationalized in terms of finite element simulations and microstructural observations. Our analysis suggests that the thermal shock performance of other materials may be estimated from comparisons with the present work.     

Development of Reaction-Bonded Electroconductive Silicon Nitride-Titanium Nitride and Resistive Silicon Nitride-Aluminum Oxide Composites

Reaction-bonded Si₃N₄· TiN and Si₃N₄· Al₂O₃ composites were successfully fabricated by heating mixed powder compacts of Si and TiN or Si and Al₂O₃ in a nitrogen atmosphere. The former showed electrical conductivity, owing to the presence of TiN. An electrical resistivity of 2.6 × 10⁻⁵Ω· m was obtained for the Si₃N₄· TiN composite with 70 vol% TiN. The composite with 20 vol% TiN showed an electrical resistivity of 0.22 Ω· m and a bending strength of 460 MPa. On the other hand, the Si₃N₄· Al₂O₃ composite had insulating properties. The use of an appropriate amount of resin binder resulted in a higher green density and, consequently, a higher bending strength. Moreover, electroconductive Si₃N₄· TiN/resistive Si₃N₄· Al₂O₃ complex ceramics could be fabricated by heating green compacts composed of two different portions, one composed of mixed powders of Si and TiN and the other of Si and Al₂O₃. Attainment of such complex ceramics was attributed to the small dimensional change at the nitriding stage, under 0.3% and the similarity of the thermal expansion coefficients of the two composites.     

Synthesis of Porous Si3N4 Ceramics with Rod-Shaped Pore Structure

Porous Si₃N₄ ceramics were synthesized by pressureless sintering of green compacts prepared using slip casting of slurries containing Si₃N₄, 5 wt% Y₂O₃+2 wt% Al₂O₃, and 0–60% organic whiskers composed of phenol–formaldehyde resin with solids loading up to 60 wt%. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800°C in air and sintered at 1850°C in a N₂ atmosphere. Porosities ranging from 0% to 45% were obtained by the whisker contents (corresponding to 0–60 vol% whisker). Samples exhibited a uniform pore distribution. Their rod-shaped pore morphology originated from burnout of whiskers, and an extremely dense Si₃N₄ matrix.     

Microstructure of Y2O3 Fluxed Hot-Pressed Silicon Nitride

Detailed microstructural analysis of a 10 mol% Y₂O₃ fluxed hot-pressed silicon nitride reveals that, in addition to the yttrium-silicon oxynitride phase located at the multiple Si₃N₄ grain junctions, there is a thin boundary phase 10 to 80 Å wide separating the silicon nitride and the oxynitride grains. Also, X-ray microanalysis from regions as small as 200 Å across demonstrates that the yttrium-silicon oxynitride, Y₂Si(Si₂O₃N₄), phase can accommodate appreciable quantities of Ti, W, Fe, Ni, Co, Ca, Mg, Al, and Zn in solid solution. This finding, together with observations of highly prismatic Si₃N₄ grains enveloped by Y₂Si(Si₂O₃N₄), suggests that densification occurred by a liquid-phase "solution-reprecipitation" process.     

Effect of α to β (β') Phase Transition on the Sintering of Silicon Nitride Ceramics

By using α-Si₃N₄ and β-Si₃N₄ starting powders with similar particle size and distribution, the effect of α-β (β') phase transition on densification and microstructure is investigated during the liquid-phase sintering of 82Si₃N₄·9Al₂O₃·9Y₂O₃ (wt%) and 80Si₃N₄·13Al₂O₃·5AIN·5AIN·2Y₂O₃. When α-Si₃N₄ powder is used, the grains become elongated, apparently hindering the densification process. Hence, the phase transition does not enhance the densification.     

Influence of Phase Formation on Dielectric Properties of Si3N4 Ceramics

The influence of phase formation on the dielectric properties of silicon nitride (Si₃N₄) ceramics, which were produced by pressureless sintering with additives in MgO–Al₂O₃–SiO₂ system, was investigated. It seems that the difference in the dielectric properties of Si₃N₄ ceramics sintered at different temperatures was mainly due to the difference of the relative content of α-Si₃N₄, β-Si₃N₄, and the intermediate product (Si₂N₂O) in the samples. Compared with α-Si₃N₄ and Si₂N₂O, β-Si₃N₄ is believed to be a major factor influencing the dielectric constant. The high-dielectric constant of β-Si₃N₄ could be attributed to the ionic relaxation polarization.     

Suspension Dry Spinning and Rheological Behavior of Ceramic-Powder-Loaded Polymer Solutions

Green ceramic fibers of Al₂O₃, Si₃N₄, Ce-ZrO₂, and SiC were prepared by suspension dry spinning of powderloaded polymer solutions. The fiber-forming polymer was ethyl methacrylate dissolved in methyl ethyl ketone. The experimental conditions for fiber formation were defined for the alumina system. Thread lengths measured as a function of extrusion rate showed a transition from capillary-limited fracture to continuous fibers in agreement with Ziabicki's theory of thread formation. The conditions for spinnability were correlated to rheological parameters. Spinnable Al₂O₃ dopes were elastic fluids with a high viscosity (60 Pa·s at 0.46 s⁻¹) under the conditions of this study.     

Processing of β-Silicon Nitride from Water-Based alpha-Silicon Nitride, Alumina, and Yttria Powder Suspensions

A water-based route for processing ß-Si₃N₄ from alpha-Si₃N₄, Al₂O₃, and Y₂O₃ powder mixtures was established. The surface charges and isoelectric points of the three different powders were investigated within the pH range from pH 3 to pH 12. Citric acid diammonium salt was found to be an effective deflocculant for shifting the isoelectric points to pH 3.5 for Al₂O₃ and to pH 6 for Y₂O₃. Aluminum hydroxide (Al(OH)₃) showed strong interaction with the Si₃N₄ powder, shifting the isoelectric point from pH 7 to pH 5.5. Low-viscosity, high-solids-loading suspensions (60-63 vol%) thus were possible at pH 9.7. The preparation of homogeneous and stable suspensions with a solids content of ≤61 vol% and a viscosity <1 Pa·s was limited to a pH regime between pH 9 and pH 10.5 because of the high solubility of yttria. The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease-catalyzed decomposition of urea at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green-density distribution was homogeneous throughout all bodies, even for complex geometries.     

Sintering Silicon Nitride Ceramics in Air

Silicon nitride (Si₃N₄) ceramics, prepared with Y₂O₃ and Al₂O₃ sintering additives, have been densified in air at temperatures of up to 1750°C using a conventional MoSi₂ element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y₂O₃ and 3 wt% Al₂O₃. Densification is accompanied by a small weight gain (typically <1–2 wt%), because of limited passive oxidation of the sample. Complete α- to β-Si₃N₄ transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si₂N₂O is also observed to form below 1750°C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with β-Y₂Si₂O₇ being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β-Si₃N₄ elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si₃N₄-based ceramics.     

Bond Energetics at Intergranular Interfaces in Alumina-Doped Silicon Nitride

In Si₃N₄ ceramics sintered with Al₂O₃, the interfacial strength between the intergranular glass and the reinforcing grains has been observed to increase with increases in the aluminum and oxygen content of the epitaxial β-Si_{6- z} Al _z O _z N_{8– z} layer that forms on the Si₃N₄ grains. This has been attributed to the formation of a network of strong bonds (cross bonds) that span the glass-crystalline interface. This proposed mechanism is considered further in light of first-principles atomic cluster calculations of the relative stabilities of bridge and threefold-bonded atomic fragments chosen to represent compositional changes at the glass/Si₃N₄ grain interface. Calculated binding energies indicate Al-N binding is favorable at the Si₃N₄ grain surface, where aluminum occupancy can promote the growth of SiAlON, further enhancing the cross-bonding mechanism of interfacial strengthening.     

Silicon Nitride Joining Using Silica and Yttria Ceramic Interlayers

Dense hot-pressed β-Si₃N₄ blocks were joined using both SiO₂ and SiO₂-Y₂O₃ powder slurries as bonding interlayers. The assembled specimens (Si₃N₄/interlayer/Si₃N₄) were heated in a flowing N₂ atmosphere in the temperature range of 1500°–1650°C. The joining interlayer was clearly distinguished from the Si₃N₄ bulk. The microstructure and the reaction products found in the bonding interlayer were very different in both compositions. Reactions occurring between the Si₃N₄ and the ceramic joining compositions have been explained based on existing diagrams of the YN–Si₃N₄-Y₂O₃-SiO₂ system.     

Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid

Titanium diboride (TiB₂) was hot-pressed at a temperature of 1800°C, and silicon nitride (Si₃N₄) was added as a sintering aid. The amount of Si₃N₄ that was added had a significant influence on the sinterability and mechanical properties of the TiB₂. When a small amount (2.5 wt%) of Si₃N₄ was added, the Si₃N₄ reacted with titania (TiO₂) that was present on the surface of the TiB₂ powder to form titanium nitride (TiN), boron nitride (BN), and amorphous silica (SiO₂). The elimination of TiO₂ suppressed the grain growth effectively, which led to an improvement in the densification of TiB₂. The formation of SiO₂ also was deemed beneficial for densification. The mechanical properties-especially, the flexural strength-were enhanced remarkably through these improvements in the sinterability and microstructure. On the other hand, when a large amount (greaterthan equal to5 wt%) of Si₃N₄ was added, the mechanical properties were not improved much, presumably because of the extensive formation of a glassy Si-Ti-O-N phase at the grain boundaries.