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.     

Fabrication of Transparent γ-Al2O3 from Nanosize Particles

The compaction and heat-treatment behavior of nanosize γ-Al₂O₃ powder (average diameter = 20 nm) was studied. A diamond anvil high-pressure cell was used to compact the powder at pressures up to 3 GPa, both in air at room temperature and under liquid nitrogen, followed by pressureless heat treatment at 800°C. For all conditions studied, the fabricated compacts were optically transparent. X-ray diffraction confirmed retention of the γ-phase. The compacts were also characterized before and after heat treatment by microhardness measurements and by transmission electron microscopy. For both ambient and cryogenic compaction, sample hardness increased with pressure, and heat treatment resulted in about a 50% increase in hardness independent of the initial green-state value. Samples compacted in LN₂ were significantly harder (up to 9.6 GPa) than those compacted in air. TEM examination revealed a random-dense-packed particle structure and interconnected porosity; interstitial void dimensions, however, were always less than the average particle diameter (20 nm). Observed effects on the increase in hardness could not be explained by microstructural changes normally attributed to increased compaction pressure or heat treatment, most notably densification. Alternative explanations are proposed.     

Synthesis of Silicon Nitride/Silicon Carbide Nanocomposite Powders through Partial Reduction of Silicon Nitride by Pyrolyzed Carbon

An alternative method to incorporate nanometer-sized silicon carbide (SiC) particles into silicon nitride (Si₃N₄) powder was proposed and investigated experimentally. Novolac-type phenolic resin was dissolved in ethanol and mixed with Si₃N₄ powder. After drying and curing, the resin was converted to reactive carbon via pyrolysis. Si₃N₄ powder was partially reduced carbothermally using the pyrolyzed carbon, and nanometer-sized SiC particles were produced in situ at 1530°-1610°C in atmospheric nitrogen. At temperatures <1550°C, the reduction rate was low and the SiC particles were very small; no SiC whiskers or barlike SiC was observed. At 1600°C, the reduction rate was high and the reaction was close to completion after only 10 min, with the appearance of SiC whiskers as well as curved, barlike, and equiaxial SiC, all of which were dozens of nanometers in diameter; this size is greater than that at observed temperatures <1550°C. A longer soaking time at 1600°C led to agglomerates. SiC particles were close to the surface of the Si₃N₄ particles. The SiC content could be adjusted by changing the carbon content before reduction and the reduction temperature. A reaction mechanism that involved the decomposition of Si₃N₄ has been proposed.     

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.     

Carbothermic Synthesis of Monodispersed Spherical Si3N4/SiC Nanocomposite Powder

The synthesis and structure of a monodispersed spherical Si₃N₄/SiC nanocomposite powder have been studied. The Si₃N₄/SiC nanocomposite powder was synthesized by heating under argon a spherical Si₃N₄/C powder. The spherical Si₃N₄/C powder was prepared by heating a spherical organosilica powder in a nitrogen atmosphere and was composed of a mixture of nanosized Si₃N₄ and free carbon particles. During the heat treatment at 1450°C, the Si₃N₄/C powder became a Si₃N₄/SiC composite powder and finally a SiC powder after 8 h, while retaining its spherical shape. The composition of the Si₃N₄/SiC composite powder changed with the duration of the heat treatment. The results of TEM, SEM, and selected area electron diffraction showed that the Si₃N₄/SiC composite powder was composed of homogeneously distributed nanosized Si₃N₄ and SiC particles.     

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.     

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.     

Porous 2H-Silicon Carbide Ceramics Fabricated by Carbothermal Reaction between Silicon Nitride and Carbon

Porous SiC ceramics were synthesized by sintering pressed and pressed/CIPed powder compacts of α-Si₃N₄, carbon (Si₃N₄:C = 1:3 mol as ratio), and sintering aids, at 1600°C for few hours to achieve a reaction, and subsequently sintering at a temperature range of 1750°–1900°C, in an argon atmosphere. High porosities from 45%–65% were achieved by low shrinkage with large weight loss. Formation of pure 2H-SiC phase via a reaction between Si₃N₄ and carbon can be demonstrated by X-ray diffractometry. The resultant porous SiC samples were characterized by SiC grain microstructures, pore-size distribution, and flexural strength. This method has the advantage of fabricating high-porous SiC ceramics with fine microstructure and good properties at a relatively low temperature.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

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.     

Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si₂N₂O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures. α -Si₃N₄ is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si₂N₂O is approximately 19 GPa, comparable with the hardness of Si₃N₄ hot isostatically pressed with 2.5 wt% Y₂O₃, while the fracture toughness level is around 3.1 MPa. m^1/2, being approximately 0.8 MPa.m^1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.     

Decomposition Suppression of Silicon Nitride by Hexagonal Boron Nitride Nanocoatings

We report a stabilized Si₃N₄ simply with nanocoatings of h-BN. Very thin BN coatings are enough for suppressing the decomposition of Si₃N₄ particles. This approach should open up a new potential way to prepare stabilized Si₃N₄. Reduced nitridation of H₃BO₃-coated Si₃N₄ powder at 1050°C in a flowing mixed 40% N₂+60% H₂ atmosphere, and then following heat-treatment at 1500°C in a flowing N₂ atmosphere can realize the nanocoating of BN on Si₃N₄ particles. Compared with the Si₃N₄ powder without nanocoatings of h-BN, TG and XRD analysis showed that the obtained h-BN nanocoated Si₃N₄ powder demonstrated obviously improved stability in argon atmosphere.     

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.     

Highly Transparent Lu-α-SiAlON

Novel Lu-α-SiAlON ceramics were produced by hot pressing mixtures of Si₃N₄, Lu₂O₃, AlN, and Al₂O₃ at 1950°C for 2 h in a nitrogen atmosphere. The resultant SiAlON was fully dense and possessed a uniform, equiaxed microstructure with a grain size of ∼1 μm, which resulted in a high hardness of >19 GPa. In addition to high hardness, the sample showed very high optical transparency in the visible light region, with >70% transmission at higher wavelengths. This high transparency was attributed to the uniform, dense microstructure and lack of residual grain-boundary phase.     

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.     

Change of Crystal Phases and Microstructure of Amorphous Si-C-N Powder by Hot Pressing

An amorphous Si-C-N powder with Y₂O₃ and Al₂O₃ powder as sintering additives was hot-pressed at 1900°C for 120 min in a nitrogen atmosphere. Changes in the crystalline phases and microstructure of the amorphous Si-C-N powder during sintering were investigated by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The defects at the fracture origins of the sintered bodies after bending tests also were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). XRD showed that alpha-Si₃N₄ was formed initially from the amorphous Si-C-N by 1530°C, which then transformed to ß-Si₃N₄ at 1600°C. Also, a slight formation of crystalline SiC occurred during the transformation from alpha- to ß-Si₃N₄, and it increased after the transformation was completed at 1900°C. TEM revealed that many SiC nanoparticles were incorporated into ß-Si₃N₄ grains after the transformation from alpha- to ß-Si₃N₄ at 1600°C. They were located at the triple points of the grain boundaries of ß-Si₃N₄ after continued Si₃N₄ grain growth at 1900°C. Besides the SiC nanoparticles, large agglomerations of carbon or SiC particles of 20-60 µm size were observed by SEM and EPMA at the fracture origins of the sintered bodies after the bending tests.     

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.     

Effect of Silicon Carbide Whisker and Titanium Carbide Particulate Additions on the Friction and Wear Behavior of Silicon Nitride

A Si₃N₄/TiC composite was previously demonstrated to exhibit improved wear resistance compared to a monolithic Si₃N₄ because of the formation of a lubricious oxide film containing Ti and Si at 900°C. Further improvements of the composite have been made in this study through additions of SiC whiskers and improved processing. Four materials—Si₃N₄, Si₃N₄/TiC, Si₃N₄/SiC_wh, and Si₃N₄/TiC/SiC_wh— were processed to further optimize the wear resistance of Si₃N₄ through improvements in strength, hardness, fracture toughness, and the coefficient of friction. Oscillatory pin on flat wear tests showed a decrease in the coefficient of friction from ∼0.7 (Si₃N₄) to ∼0.4 with the addition of TiC at temperatures reaching 900°C. Wear track profiles illustrated the absence of appreciable wear on the TiC-containing composites at temperatures above 700°C. Microscopic (SEM) and chemical (AES) characterization of the wear tracks is also included to deduce respective wear and lubricating mechanisms.     

In Situ Polysilane-Derived Silicon Carbide Particulates Dispersed in Silicon Nitride Composite

A dense (97% of theoretical density) Si₃N₄—SiC composite containing 10 wt%β-SiC was prepared by introducing a SiC phase by the pyrolysis of a polymeric SiC precursor. The composite material was produced by mixing an alkyl/aryl-substituted polysilane with Si₃N₄ powder and, by subsequently forming green compacts, pyrolyzing the polymeric species, and finally sintering the sample. Synthesis and characterization of the polymeric compound was described. Its transformation reactions to SiC and the characterization of the ceramic residue were also studied. High ceramic yields were obtained by curing the as-synthesized polysilane at 500°C in an Ar atmosphere. The heat treatment had no effect on the good solubility of the polymeric precursor in organic solvents. This was important for processes such as infiltration, sealing, and coating and for the mixing of the polymer with powders for the preparation of homogeneous composite ceramics. The dense microstructure of the pyrolyzed and sintered Si₃N₄ powder–polysilane mixture exhibited reduced grain growth of the Si₃N₄ particles and a very homogeneous distribution of the in situ-formed β-SiC phase.     

Self-Reinforced Nitrogen-Rich Calcium–α-SiAlON Ceramics

Nitrogen-rich Ca–α-SiAlON ceramics with nominal compositions Ca _x Si_{12−2 x} Al_{2 x} N₁₆ and 0.2≤ x ≤2.6, extending along the Si₃N₄–1/2Ca₃N₂:3AlN tie line, were prepared from Si₃N₄, AlN, and CaH₂ precursors by hot pressing at 1800°C. The x values attained were determined by energy-dispersive X-ray (EDX) microanalysis and X-ray powder diffraction (XRPD) data using the Rietveld method. The results show that Ca–α-SiAlONs form continuously within the compositional range x =0 to at least x =1.82. Phase assemblages, lattice parameters, Vickers hardness, and fracture toughness were determined and correlated to the calcium content, x . Owing to a high sintering temperature and the use of CaH₂ as a precursor, grain growth was kinetically enhanced, resulting in self-reinforced microstructures with elongated grains. The obtained Ca–α-SiAlON ceramics demonstrate a combination of both high hardness ∼21 GPa, and high fracture toughness ∼5.5 MPa·m^1/2.