Effect of Sintering Aid Composition on the Processing of Si3N4/BN Fibrous Monolithic Ceramics

Si₃N₄/BN fibrous monoliths were prepared with 4 wt% Y₂O₃ added as a sintering aid to the Si₃N₄. Residual carbon, present in the billet before hot-pressing, was shown to influence the final microstructure. The sintering aid glass, known to migrate into the BN cell boundaries during hot-pressing, was not sufficient in quantity to prevent premature shear failure when samples were tested in flexure. Increasing the hot-pressing temperature alleviated this problem. For flexure samples tested at 1400°C, fibrous monoliths fabricated with 4 wt% Y₂O₃ demonstrated linear-elastic loading behavior at a greater stress than fibrous monoliths fabricated with 6-wt%-Y₂O₃/2-wt%-Al₂O₃ sintering aids.     

Pressureless Sintering of Dense Si3N4 and Si3N4/SiC Composites with Nitrate Additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si₃N₄ and a Si₃N₄/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si₃N₄ particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si₃N₄ particles milled with oxide additives. Monolithic Si₃N₄ could be sintered to 94% of theoretical density (TD) at 1500°C with nitrate additives. The sintering temperature was about 100°C lower than the case with oxide additives. After pressureless sintering at 1750°C for 2 h in N₂, the bulk density of a Si₃N₄/20 wt% SiC composite reached 95% TD with nitrate additives.     

Long Crack R-Curve of Aligned Porous Silicon Nitride

Long crack R -curve of a porous Si₃N₄ with aligned fibrous grains was investigated, using a chevron-notched beam technique. A crack was constrained to propagate normal to the grain alignment. The crack growth resistance of aligned porous Si₃N₄ was much larger compared with that of dense Si₃N₄ ceramics. Microstructure observations showed that pullouts of fibrous grains in aligned porous Si₃N₄ markedly increased during crack propagation relative to those of dense Si₃N₄, due to the existence of pores. The efficient grain pullouts in porous Si₃N₄ increased the bridging stress at the crack wake.     

In Situ Synthesis and Microstructures of Tungsten Carbide-Nanoparticle-Reinforced Silicon Nitride-Matrix Composites

A W₂C-nanoparticle-reinforced Si₃N₄-matrix composite was fabricated by sintering porous Si₃N₄ that had been infiltrated with a tungsten solution. During the sintering procedure, nanometer-sized W₂C particles grew in situ from the reaction between the tungsten and carbon sources considered to originate mainly from residual binder. The W₂C particles resided in the grain-boundary junctions of the Si₃N₄, had an average diameter of ∼60 nm, and were polyhedral in shape. Because the residual carbon, which normally would obstruct sintering, reacted with the tungsten to form W₂C particles in the composite, the sinterability of the Si₃N₄ was improved, and a W₂C–Si₃N₄ composite with almost full density was obtained. The flexural strength of the W₂C–Si₃N₄ composite was 1212 MPa, ∼34% higher than that of standard sintered Si₃N₄.     

Fracture Energies of Tape-Cast Silicon Nitride with β-Si3N4 Seed Addition

The fracture energies of the tape-cast silicon nitride with and without 3 wt% rod-like β-Si₃N₄ seed addition were investigated by a chevron-notched-beam technique. The material was doped with Lu₂O₃–SiO₂ as sintering additives for giving rigid grain boundaries and good heat resistance. The seeded and tape-cast silicon nitride has anisotropic microstructure, where the fibrous grains grown from seeds were preferentially aligned parallel to the casting direction. When a stress was applied parallel to the fibrous grain alignment direction, the strength measured at 1500°C was 738 MPa, which was almost the same as room temperature strength 739 MPa. The fracture energy of the tape-cast Si₃N₄ without seed addition was 109 and 454 J/m² at room temperature and 1500°C, respectively. On the contrary, the fracture energy of the seeded and tape-cast Si₃N₄ was 301 and 781 J/m² at room temperature and 1500°C, respectively, when a stress was applied parallel to the fibrous gain alignment. The large fracture energies were attributable primarily to the unidirectional alignment fibrous Si₃N₄ grains.     

Effect of Sintering Additives on Microstructure and Mechanical Properties of Porous Silicon Nitride Ceramics

Porous silicon nitride (Si₃N₄) ceramics with about 50% porosity were fabricated by pressureless sintering of α-Si₃N₄ powder with 5 wt% sintering additive. Four types of sintering aids were chosen to study their effect on the microstructure and mechanical properties of porous Si₃N₄ ceramics. XRD analysis proved the complete formation of a single β-Si₃N₄ phase. Microstructural evolution and mechanical properties were dependent mostly on the type of sintering additive. SEM analysis revealed the resultant porous Si₃N₄ ceramics as having high aspect ratio, a rod-like microstructure, and a uniform pore structure. The sintered sample with Lu₂O₃ sintering additive, having a porosity of about 50%, showed a high flexural strength of 188 MPa, a high fracture toughness of 3.1 MPa·m^1/2, due to fine β-Si₃N₄ grains, and some large elongated grains.     

Effect of Nitrate Salts as Sintering Additives during the Ball-Milling Process of Silicon Nitride Powders

A chemical adsorption method in a Si₃N₄ slurry that contained a nitrate solution was studied during ball milling, with particular interest in increasing the oxide layer in the Si₃N₄ powder and improving the distribution homogeneity of the sintering additives. The nitrate salts Al(NO₃)₃·9H₂O and Y(NO₃)₃·6H₂O were selected as sintering additives. The following characterization techniques were used: oxygen–nitrogen analysis, X-ray photoelectron spectroscopy, high-resolution electron microscopy (coupled with energy-dispersive X-ray spectroscopy), and X-ray imaging (using wavelength-dispersive X-ray spectroscopy). The thickness of the amorphous layer and the oxygen content of the Si₃N₄ powder were greater for samples that were milled with nitrate additives, which were heat-treated at 600°C, than those of powders that were milled with oxide additives. The chemical composition of the oxygen-containing layer—that is, the amorphous layer that formed and/or changed on the Si₃N₄ surface—was similar to Si₂N₂O in heat-treated Si₃N₄ powder with nitrate additives, whereas the composition of heat-treated Si₃N₄ powder with oxide additives was similar to SiO₂. Furthermore, a homogeneous distribution of the additives was achieved via the incorporation of aluminum and yttrium into the amorphous layer on the Si₃N₄ surface. The metal ratio (Y:Al) of the adsorbates was somewhat higher than that of the additives.     

Pressureless Sintering of α-Si3N4 Porous Ceramics Using a H3PO4 Pore-Forming Agent

A new method for preparing high bending strength porous silicon nitride (Si₃N₄) ceramics with controlled porosity has been developed by using pressureless sintering techniques and phosphoric acid (H₃PO₄) as the pore-forming agent. The fabrication process is described in detail and the sintering mechanism of porous ceramics is analyzed by the X-ray diffraction method and thermal analysis. The microstructure and mechanical properties of the porous Si₃N₄ ceramics are investigated, as a function of the content of H₃PO₄. The resultant high porous Si₃N₄ ceramics sintered at 1000°–1200°C show a fine porous structure and a relative high bending strength. The porous structure is caused mainly by the volatilization of the H₃PO₄ and by the continous reaction of SiP₂O₇ binder, which could bond on to the Si₃N₄ grains. Porous Si₃N₄ ceramics with a porosity of 42%–63%, the bending strength of 50–120 MPa are obtained.     

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.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Influence of Planetary High-Energy Ball Milling on Microstructure and Mechanical Properties of Silicon Nitride Ceramics

The influence of ball-milling methods on microstructure and mechanical properties of silicon nitride (Si₃N₄) ceramics produced by pressureless sintering for a sintering additive from MgO–Al₂O₃–SiO₂ system was investigated. For planetary high-energy ball milling, the mechanical properties of Si₃N₄ ceramics were evidently improved and a homogeneous microstructure developed. In contrast, some exaggerated elongated grains were developed due to the local enrichment of sintering additives in the specimen prepared by general ball milling. For Si₃N₄ ceramics produced by planetary ball milling, flexure strength of 1.06 GPa, Vickers hardness of 14.2 GPa, and fracture toughness of 6.6 MPa·m^0.5 were achieved. The differences in the mechanical properties of Si₃N₄ ceramics produced by different processing seem to arise mainly from the changes in microstructural homogenization and sinterability. The planetary high-energy ball-milling process provides a good route to mix starting powders for developing ceramics with uniform microstructure and promising mechanical properties.     

Microstructure and Property Characterization of Sintered Si3N4, SiC, and SiAlON

Commercially produced pressureless sintered Si₃N₄, SiC, and SiAlON were characterized with respect to density, phases present, bend strength, and oxidation resistance. The room-temperature bend strengths of sintered Si₃N₄, SiC, and SiAlON are comparable. However, the room-temperature strengths are much lower (=40 to 50%) than the room-temperature strength of hot–pressed Si₃N₄ (NC-132). The strength loss in Si₃N₄ and SiAlON materials at high temperature was attributed to a viscous grain-boundary phase retained during cooling from the sintering temperature. The oxidation resistance of sintered a-SiC was the best of any materials tested.     

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.     

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.     

Silicon Nitride/Molybdenum Disilicide Composite with Superior Long-Term Oxidation Resistance at 1500°C

The long-term high-temperature cyclic oxidation (100 cycles, 10⁴ h, 1500°C) of a Si₃N₄ material and a Si₃N₄/MoSi₂ composite, both fabricated with Y₂O₃ as a sintering additive, was studied. Both materials exhibited similar oxidation rates because of surface SiO₂ formation described by an almost parabolic law and a total weight gain of 3–4 mg/cm² after 10⁴ h. As a consequence of oxidation processes in the bulk, microstructural damage was found in the Si₃N₄ material. These effects were not observed in the composite. The remarkable microstructural stability observed offers the high potential of Si₃N₄/MoSi₂ composites for long-term structural applications at elevated temperatures up to 1500°C.     

Stability of Si3N4 and Liquid Phase(s) During Sintering

Advanced sintering techniques for consolidation of Si₃N₄ powders in the presence of an oxygen-rich liquid phase(s) require high temperatures and usually high nitrogen pressures. A stability diagram is constructed for Si₃N₄ as a function of the partial pressures of nitrogen (P_N2) and silicon (P_Si). High P_N2 (20 to 100 atm) increases the stability of Si₃N₄ and the oxygen-rich liquid phase by reducing the P_Si and P_Si0, respectively. The region of high sinterability is outlined for submicrometer Si₃N₄ powders containing 7 wt% BeSiN₂ and 7 wt% SiO₂ as densification aids .     

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.     

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.     

Effect of Silicon Activity on Liquid-Phase Sintering of Nitrogen Ceramics

Volatilization resulting from the thermal decomposition of Si₃N₄ causes the large weight loss and desintering phenomenon observed during pressureless sintering of Si₃N₄-5% MgO and sialon (z =2)-5% MgO. The addition of a few weight percent of Si to the powder suppresses this volatilazation and helps to achieve fully dense Si₃N₄ components.