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.     

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.     

Effect of Grain Size on the Thermal Conductivity of Si3N4

Polycrystalline Si₃N₄ samples with different grain-size distributions and a nearly constant volume content of grain-boundary phase (6.3 vol%) were fabricated by hot-pressing at 1800°C and subsequent HIP sintering at 2400°C. The HIP treatment of hot-pressed Si₃N₄ resulted in the formation of a large amount of ß-Si₃N₄ grains ∼10 µm in diameter and ∼50 µm long, and the elimination of smaller matrix grains. The room-temperature thermal conductivities of the HIPed Si₃N₄ materials were 80 and 102 Wm⁻¹K⁻¹, respectively, in the directions parallel and perpendicular to the hot-pressing axis. These values are slightly higher than those obtained for hot-pressed samples (78 and 93 Wm⁻¹K⁻¹). The calculated phonon mean free path of sintered Si₃N₄ was ∼20 nm at room temperature, which is very small as compared to the grain size. Experimental observations and theoretical calculations showed that the thermal conductivity of Si₃N₄ at room temperature is independent of grain size, but is controlled by the internal defect structure of the grains such as point defects and dislocations.     

Impurity Phases in Hot-Pressed Si3N4

Impurity phases in commercial hot-pressed Si₃N₄ were investigated using transmission electron microscopy. In addition to the dominant, β-Si₃N₄ phase, small amounts of Si₂N₂O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 Å thick between, β-Si₃N₄ grains and ∼ 30 Å thick between Si₂N₂O and β-Si₃N₄ grains.     

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.     

Grain-Boundary Microstructure and Chemistry of a Hot Isostatically Pressed High-Purity Silicon Nitride

Two high-purity Si₃N₄ materials were fabricated by hot isostatic pressing without the presence of sintering additives, using an amorphous laser-derived Si₃N₄ powder with different oxygen contents. High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) analysis of the Si₃N₄ materials showed the presence of an amorphous SiO₂ grain-boundary phase in the three-grain junctions. Spatially resolved EELS analysis indicated the presence of a chemistry similar to silicon oxynitride at the two-grain junctions, which may be due to partial dissolution of nitrogen in the grain-boundary film. The chemical composition of the grain-boundary film was SiN_xO_y, (x ∼ 0.53 and y ∼ 1.23), and the triple pocket corresponded to the amorphous SiO₂ containing ∼2 wt% nitrogen. The equilibrium grain-boundary-film thickness was measured and found to be smaller for the material with the lower oxygen content. This difference in thickness has been explained by the presence of the relatively larger calcium concentration in the material with the lower amount of SiO₂ grain-boundary phase, because the concentration of foreign ions has been shown to affect the grain-boundary thickness.     

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.     

Characteristics of ZrO2-Dispersed Si3N4 without Additives Fabricated by Hot Isostatic Pressing

Dense, ZrO₂-dispersed Si₃N₄ composites without additives were fabricated at 180 MPa and ∼1850° to 1900°C for l h by hot isostatic pressing using a glass-encapsulation method; the densities reached >96% of theoretical. The dispersion of 20 wt% of 2.5YZrO₂ (2.5 mol% Y₂O₃) in Si₃N₄ was advantageous to increase the room-temperature fracture toughness (∼7.5 MPa˙m^1/2) without degradation of hardness (∼15 GPa) because of the high retention of tetragonal ZrO₂. The dependence of fracture toughness of Si₃N₄–2.5YZrO₂ on ZrO₂ content can be related to the formation of zirconium oxynitride because of the reaction between ZrO₂ and Si₃N₄ matrix in hot isostatic pressing.     

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.     

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.     

Mechanical Properties of Si3N4-SiC Three-Layer Composite Materials

The effects of microstructure and residual stress on the mechanical properties of Si₃N₄-based three-layer composite materials were investigated. The microstructure of each layer was controlled by the addition of two differently sized silicon carbides: fine SiC nanoparticles (∼200 nm) or relatively large SiC platelets (∼20 µm). When the SiC nanoparticles were added, the average grain size of Si₃N₄ was reduced because of the inhibition of grain growth by the particles. On the other hand, when the SiC platelets were added, the microstructure of Si₃N₄ was not much changed because of the large size of the platelets. Three-layer composites were fabricated by placing the Si₃N₄/SiC-nanoparticle layers on the surface of the Si₃N₄/SiC-platelet layer. The residual stress was controlled by varying the amount of SiC added. The mechanical properties of three-layer composites with various combinations of microstructure and residual stress level were investigated.     

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.     

R-Curve Behavior of Si3N4–BN Composites

R -curve behavior of Si₃N₄–BN composites and monolithic Si₃N₄ for comparison was investigated. Si₃N₄–BN composites showed a slowly rising R -curve behavior in contrast with a steep R -curve of monolithic Si₃N₄. BN platelets in the composites seem to decrease the crack bridging effects of rod-shaped Si₃N₄ grains for small cracks, but enhanced the toughness for long cracks as they increased the crack bridging scale. Therefore, fracture toughness of the composites was relatively low for the small cracks, but it increased significantly to ∼8 MPa·m^1/2 when the crack grew longer than 700 μm, becoming even higher than that of the monolithic Si₃N₄.     

Fabrication of Low-Shrinkage, Porous Silicon Nitride Ceramics by Addition of a Small Amount of Carbon

Successful net-shape sintering offers a significant advantage for producing large or complicated products. Porous Si₃N₄ ceramics with very low shrinkage were developed, in the present investigation, by the addition of a small amount of carbon. Carbon powders (1–5 vol%) of two types, with different mean particle sizes (13 nm and 5 μm), were added to α-Si₃N₄−5 wt% Y₂O₃ powders. SiC nanoparticles formed through reaction of the added carbon with SiO₂ on the Si₃N₄ surface or with the Si₃N₄ particles themselves. Such reaction-formed SiC nanoparticles apparently had an effective reinforcing effect, as in nanocomposites. Sintered Si₃N₄ porous ceramics with a high porosity of 50%–60%, a very small linear shrinkage of ∼2%–3%, and a strength of ∼100 MPa were obtained.     

Nanosilver-Coated Porous SiC–Si3N4 Composite Using Microwave-Assisted Process

Using a novel microwave-assisted process, nano-Ag-coated continuous porous SiC–Si₃N₄ substrate was fabricated from a solution containing AgNO₃ salts and ethylene glycol. The detailed microstructure of the fabricated substrate was investigated depending on the amount of AgNO₃ salts in the starting solution and the microwave irradiation time. From a solution containing 0.4 g of AgNO₃ for 60 s irradiation time, the Ag nanoparticles, ∼25 nm in diameter, were homogeneously coated on the continuous porous SiC–Si₃N₄ matrix as well as on the surface of the Si₃N₄ whiskers. However, the Ag nanoparticles (∼15 nm) deposited from a solution containing 0.6 g of AgNO₃ for 60 s irradiation time showed maximum homogeneity and narrow size distribution. The components of Si, N, and Ag were homogeneously distributed on the deposited layer. The deposited Ag nanoparticles covered with a thin (∼2 nm), amorphous layer had nanocrystallinity and adhered well to the surface of the Si₃N₄ whiskers.     

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.     

Low-Pressure Chemical Vapor Deposition of α-Si3N4 from SiF4 and NH3: Nucleation and Growth Characteristics

The crystal structure and surface morphology of Si₃N₄ prepared by LPCVD were characterized as a function of processing conditions. Temperature was the most dominant variable which affected the coating microstructure. Strongly faceted crystalline Si₃N₄ was deposited at temperatures above ∼ 1410°C. In the temperature range of 1300° to 1410°C, crystalline and amorphous phases were codeposited. The content of the crystalline phase rapidly decreased with decreased temperature. In this temperature range, the coating crystallinity was also influenced by kinetic factors such as deposition rate and reagent depletion. For example, Si₃N₄ became more crystalline as the deposition rate was decreased by either decreasing the flow rate or increasing the NH₃/SiF₄ molar ratio. At ∼ 1300°C, the coating surface appeared fully botryoidal, and the coatings were mostly amorphous. Changes in the orientation and size of Si₃N₄ crystallites were parametrically documented. As the temperature was increased, the Si₃N₄ grains generally became more preferentially oriented to the (102) and/or ( l 0 l ) where l = 1,2,3,., directions. The average facet size increased with coating thickness.     

Mechanical Properties of Joined Silicon Nitride

A technique is described to join Si₃N₄ ceramics using oxide glasses. The technique involves a glazing step, followed by a pressureless reaction treatment of 30 to 60 min at 1575° to 1650°C. Reactions between the glasses and Si3N4 are reported. Important events are dissolution of Si₃N₄ and the growth of Si₂N₂O crystals into the joint. The strength of joined bars depends on joint thickness. Two strength regimes are identified, and two corresponding fracture mechanisms are described. A maximum strength of ∼460 MPa is achieved for a joint thickness of ∼30 μm.     

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.     

α→β Reverse Phase Transformation in Silicon Carbide in Silicon Nitride-Particulate-Reinforced-Silicon Carbide Composites

The α→β reverse transformation in SiC is observed in Si₃N₄-particulate-reinforced-SiC composites made from as-received α-SiC and α-Si₃N₄ powders. However, the transformation does not occur to any great extent in composites made from deoxidized Si₃N₄-SiC powder compacts. Detailed transmission electron microscopy shows that most interfaces are covered with an ∼10 Å thick amorphous intergranular film in the composites made from as-received powders, whereas most interfaces are free of such films in the composites made from deoxidized powder compacts. These observations indicate that the α→β reverse transformation in SiC is encouraged by a nitrogen-containing liquid phase that occurs at high temperature in the composites made from the as-received powders. A mechanism is proposed to account for the experimental observations.