Electrical Joining of Silicon Nitride Ceramics

The electrical joining of sintered Si₃N₄ ceramics by Joule heating was studied. A mixture of CaF₂/kaolinite (70/30 wt%) with excellent electroheating characteristics and reactivity with Si₃N₄ ceramics was selected as a joining agent. The optimum conditions for electrical joining were determined using this joining agent. Analysis of the joint obtained under optimum conditions revealed that joining was accomplished by the formation of reaction zones and a joining layer through the mutual diffusion of the components in the joining agent and the sintering aids in the Si₃N₄. The joint layer was composed of a glassy substance consisting of Ca─Al─Si─Y─O─(F)─(N) and contained a few particles of β─Si₃N₄. Four-point bend tests indicated that joined bodies could be obtained which maintained a strength of about 300 MPa up to 800°C. Finally, a comparative study was made with a joint obtained using furnace heating. These results indicated that the joints obtained using electrical joining were superior to those produced in the furnace.     

Synthesis of Silicon Nitride by a Combustion Reaction under High Nitrogen Pressure

Fine Si₃N₄ powders were prepared by the combustion reaction of an Si powder compact undez 10 MPa nitrogen pressure. Addition of Si₃N₄ powder to the starting Si promoted conversion of the reactants to homogeneous Si₃N₄ particles. Submicrometer SisN₄powders with a uniform size distribution around 0.5 μm were obtained from a 1.8Si-0.4Si₃N₄ mixture (molar ratio); they were free of residual Si.     

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.     

Preparation of Ultrafine Silicon Nitride, and Silicon Nitride and Silicon Carbide Mixed Powders in a Hybrid Plasma

Ultrafine Si₃N₄ and Si₃N₄+ SiC mixed powders were synthesized through thermal plasma chemical vapor deposition (CVD) using a hybrid plasma which was characterized by the superposition of a radio-frequency plasma and an arc jet. The reactant, SiCl₄, was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH₄ and/or NH₃, was injected into the tail flame through multistage ring slits. In the case of ultrafine Si₃N₄ powder synthesis, reaction effieciency increased significantly by multistage injection compared to single-stage injection. The most striking result is that amorphous Si₃N₄ with a nitrogen content of about 37 wt% and a particle size of 10 to 30 nm could be prepared successfully even at the theoretical NH₃/SiCl₄ molar ratio of ∼ 1.33, although the crystallinity depended on the NH₃/SiCl₄ molar ratio and the injection method. For the preparation of Si₃N₄+ SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH₃ and CH₄ reactant gases and the H₂ quenching gas, but also by adjusting the reaction space. The results of this study provide sufficient evidence to suggest that multistage injection is very effective for regulating the condensation process of fine particles in a plasma tail flame.     

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.     

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.     

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.     

Subsolidus Phase Relationships in Part of the System Si,Al,Y/N,O: The System Si3N4─AIN─YN─Al2O3─Y2O3

The subsolidus phase relationships in the system Si,Al,Y/N,O were determined. Thirty-nine compatibility tetrahedra were established in the region Si₃N₄─AIN─Al₂O₃─Y₂O₃. The subsolidus phase relationships in the region Si₃N₄─AIN─YN─Y₂O₃ have also been studied. Only one compound, 2YN:Si₃N₄, was confirmed in the binary system Si₃N₄─YN. The solubility limits of the α'─SiAION on the Si₃N₄─YN:3AIN join were determined to range from m = 1.3 to m = 2.4 in the formula Y _{m /3}Si_{12- m} Al _m N₁₆. No quinary compound was found. Seven compatibility tetrahedra were established in the region Si₃N₄─AIN─YN─Y₂O₃.     

Fractography, Mechanical Properties, and Microstructure of Commercial Silicon Nitride–Titanium Nitride Composites

Commercial-grade Si₃N₄–TiN composites with 0, 10, 20, and 30 wt% TiN content have been characterized. Submicrometer grain-size Si₃N₄ was reinforced with fine TiN grains. Density, Young's modulus, coefficient of thermal expansion, and fracture toughness increased linearly with TiN content. Increased strength was observed in the Si₃N₄+20 wt% TiN, and Si₃N₄+30 wt% TiN composites. Fractography was used to characterize the different types of fracture origins. Improvements in toughness and strength are due to residual stresses in the Si₃N₄ matrix and the TiN particles. A threefold improvement in dry wear resistance of the Si₃N₄+30 wt% TiN composite over the Si₃N₄ matrix was observed.     

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.     

Mechanochemical-Activation-Assisted Combustion Synthesis of α-Si3N4

Combustion synthesis (CS) of α-silicon nitride (Si₃N₄) powders was accomplished at a nitrogen pressure lower than 3 MPa. The combination of mechanical activation and chemical stimulation was effective in enhancing the reactivity of Si powder reactants, which was responsible for the reduction of the minimum nitrogen pressure normally required for the CS of Si₃N₄. This breakthrough indicates that nitriding combustion of silicon in pressurized nitrogen could be promoted by activating the solid reactants instead of by increasing the nitrogen pressure. The phase content of α-Si₃N₄ in the as-synthesized product is over 90 wt%. Scanning electronic microscopy observation showed that the combustion-synthesized Si₃N₄ powders are submicron-sized particles with spherical morphologies.     

Formation of Reaction-Bonded Silicon Nitride from Silane-Derived Silicon Powders: Nucleation and Growth Mechanisms

The nucleation and growth of Si₃N₄ on silane-derived Si powders was investigated with transmission electron microscopy and FTIR spectroscopy. Thermogravimetric analysis (TGA) was also used to monitor the process through different stages of the reaction. The FTIR and TEM results provide clear evidence that the nucleation of crystalline Si₃N₄ coincides with the onset of rapid nitridation. Electron diffraction indicates that Si₃N₄ forms heteroepitaxially on the Si powder surfaces, with Si (111) || Si₃N₄(0001) and Si     || Si₃N₄     . Also, flat interfaces between the Si and Si₃N₄ (compared to the initial spherical surface of the Si powders) indicate that a significant rearrangement of the particle surface occurs during the initial stages of nitridation. The results reported here demonstrate that the rapid, low-temperature nitridation observed with silane-derived powders is possible because the Si/vapor surfaces are not covered with a continuous Si₃N₄ product layer. The measured nitridation rates are comparable to Si evaporation rates, which suggests that Si vaporization is rate limiting. This is significantly different from conventional RBSN, where nitridation is limited by solid-state diffusion through a Si₃N₄ product layer.     

Reaction Synthesis of Magnesium Silicon Nitride Powder

The synthesis of magnesium silicon nitride (MgSiN₂) by direct nitridation of a Si/Mg₂Si/Mg/Si₃N₄ powder mixture is described. A nucleation period at 550°C and stepwise heat-treatment schedule up to 1350°C was adopted for the synthesis of MgSiN₂ powder, based on TG-DTA measurements. The influence of the ratio of constituents on the final phase composition also has been studied. The content of magnesium and silicon in the starting powder should fulfill the conditions Mg₂Si/Mg ≥ 3 and Si₃N₄/Si_tot≥ 0.5 to obtain single-phase MgSiN₂. The silicon particle size of <0.5 μm is preferable to decrease the time of nitridation. The oxygen content of as-synthesized powders is in the range 0.9–1.2 wt%. However, the oxygen content of MgSiN₂ powder decreases further by the addition of 2 wt% CaF₂ or 0.75 wt% carbon and reaching the lowest value of 0.45 wt% oxygen after carbothermal reduction in an alumina-tube furnace.     

Fabrication of Transparent Silicon Nitride from Nanosize Particles

Compaction of ultrafine silicon nitride (Si₃N₄) powder at high pressures and various temperatures followed by pressureless sintering was investigated. The powder, consisting of nearly spherical particles (16 nm in diameter) of amorphous stoichiometric Si₃N₄, was pressed in a diamond anvil cell under pressures up to 5 GPa and temperatures ranging from liquid nitrogen to 500°C. Quality of compaction, evaluated by visual transparency and hardness of the produced compacts, depended on the amount of adsorbed gases on the surface of the particles and on the temperature of compaction. Visually transparent compacts were produced by pressing the starting powder without outgassing in liquid nitrogen under 5 GPa. The transparent compacts exhibited a hardness of 1200 kg/mm² after pressing in the diamond anvil cell at 500°C for 3 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 1400°C in a tube furnace under nitrogen, the hardness of these samples increased to over 2000 kg/mm² and the visual transparency was maintained. The results demonstrated that transparency was maintained. The results demonstrated that transparent compacts of nanosize amorphous Si₃N₄ particles could be sintered to high hardness at relatively low temperatures without using sintering aids or applying pressure during sintering.     

Carbothermal Reaction of Silica–Phenol Resin Hybrid Gels to Produce Silicon Nitride/Silicon Carbide Nanocomposite Powders

A carbothermal reaction of silica–phenol resin hybrid gels prepared from a two-step sol–gel process was conducted in atmospheric nitrogen. The gels were first pyrolyzed into homogeneous silica–carbon mixtures during heating and subsequently underwent a carbothermal reaction at higher temperatures. Using a gel-derived precursor with a C/SiO₂ molar ratio higher than 3.0, Si₃N₄/SiC nanocomposite powders were produced at 1500°–1550°C, above the Si₃N₄–SiC boundary temperature. The predominant phase was Si₃N₄ at 1500°C, and SiC at 1550°C. The Si₃N₄ and SiC phase contents were adjustable by varying the temperature in this narrow range. The phase contents could also be adjusted by changing the starting carbon contents, or by its combination with varying reaction temperature. A two-stage process, i.e., a reaction first at 1550°C and then at 1500°C, offered another means of simple and effective control of the phase composition: the Si₃N₄ and SiC contents varied almost linearly with the variation of the holding time at 1550°C. The SiC was nanosized (∼13 nm, Scherrer method) formed via a solid–gas reaction, while the Si₃N₄ has two morphologies: elongated microsized crystals and nanosized crystallites, with the former crystallized via a gaseous reaction, and the latter formed via a solid–gas reaction. The addition of a Si₃N₄ powder as a seed to the starting gel effectively reduced the size of the Si₃N₄ produced.     

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.     

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.     

Stability of Phases in the Si-C-N-O System

The stability of the phases in equilibrium is calculated for the Si-C-N-O system in order to analyze and predict the reactions in ceramic whisker formation and sintering of silicon nitride composites. Equilibria among SiC, Si₃N₄, Si₂N₂O, SiO₂, Si, and the gas phase are evaluated at different carbon activities, nitrogen pressures, and temperatures. Phase stability diagrams are constructed as a function of nitrogen and oxygen pressures for two levels of carbon activity. Silicon nitride becomes a stable phase with increasing nitrogen pressure or decreasing carbon activity and temperature, whereas silicon carbide becomes a dominant phase at lower nitrogen pressures or at higher temperatures when carbon activity is unity. The maximum sintering temperature of the SiC/Si₃N₄ composite is higher with an elevated nitrogen pressure or a reduced carbon activity.     

Superplastic Deformation in Silicon Nitride–Silicon Oxynitride In Situ Composites

Starting with a mixture of ultrafine β-Si₃N₄ and a SiO₂-containing additive, a superplastic Si₃N₄-based composite was developed, using the concept of a transient liquid phase. Significant deformation-induced phase and microstructure evolutions occurred in the nonequilibrium, fine-grained Si₃N₄ material, which led to the in situ development of a Si₃N₄–22-vol%-Si₂N₂O composite and strong texture formation. The unusual ductility of the composites with elongated Si₂N₂O grains was attributed to the fine-grained microstructure, the presence of a transient liquid phase, and the alignment of the elongated Si₂N₂O grains. The mechanical properties of the resultant composite were enhanced rather than impaired by superplastic deformation and subsequent heat treatment; the resultant composite exhibited both high strength (957 MPa) and high fracture toughness (4.8 MPa·m^1/2).     

Tribological Behavior of Mo5Si3-Particle-Reinforced Silicon Nitride Composites

The tribological behavior of Mo₅Si₃-particle-reinforced silicon nitride (Si₃N₄) composites was investigated by pin-on-plate wear testing under dry conditions. The friction coefficient of the Mo₅Si₃–Si₃N₄ composites and Si₃N₄ essentially decreased slowly with the sliding distance, but showed sudden increase for several times during the wear testing. The average friction coefficient of the Si₃N₄ decreased with the incorporation of submicrometer-sized Mo₅Si₃ particles and also as the content of Mo₅Si₃ particles increased. When the Mo₅Si₃–Si₃N₄ composites were oxidized at 700°C in air, solid-lubricant MoO₃ particles were generated on the surface layer. Oxidized Mo₅Si₃–Si₃N₄ composites showed self-lubricating behavior, and the average friction coefficient and wear rate of the oxidized 2.8 wt% Mo₅Si₃–Si₃N₄ composite were 0.43 and 0.72 × 10⁻⁵ mm³ (N·m)⁻¹, respectively. Both values were ∼30% lower than those for the Si₃N₄ tested in an identical manner.