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.     

Combustion Synthesis of Si3N4 by Selective Reaction of Silicon with Nitrogen in Air

Silicon nitride (Si₃N₄) was synthesized by a selective combustion reaction of silicon powder with nitrogen in air. The α/β-Si₃N₄ ratio of the interior product could be tailored by adjusting the Si₃N₄-diluent content in the reactant mixtures. The synthetic β-Si₃N₄ showed a well-crystallized rod-like morphology. Mechanical activation greatly enhanced the reactivity of silicon powder, and the slow oxidation of silicon at the sample surface promoted the combustion reaction in air. The formation mechanism of Si₃N₄ was analyzed based on a proposed N₂/O₂ diffusion kinetic model, and the calculated result is in good agreement with the experimental phenomenon.     

Simulation of Cold Compaction Densification Behavior of Silicon Nitride Ceramic Powder

The cold-compaction densification behavior of silicon nitride (Si₃N₄) ceramic powder was analyzed using two models: the Shima model and the Cam-Clay model. Triaxial-compression experimental data were used to evaluate these two models. Shima models that used Si₃N₄ matrix material with different yield stresses were discussed. It is clear that the Cam-Clay model can effectively simulate the cold-compaction densification behavior of Si₃N₄ ceramic powder. The Shima model that used a very high yield stress for the Si₃N₄ matrix material slightly overestimated the experimental data, and the Shima model that used the actual yield stress for Si₃N₄ matrix material largely overestimated that data.     

Seamless Joining of Silicon Nitride Ceramics

High-strength joining of Si₃N₄ ceramics has been achieved by developing a process that effectively eliminates the seam, and may allow for fabrication of large or complex silicon nitride bodies. This approach to joining is based on the concept that when sintering aids are effective in bonding individual grains, they could be equally effective in joining bulk pieces of Si₃N₄. Optimization of the process led to Si₃N₄/Si₃N₄ joints with room-temperature bend strengths as high as 950 MPa, corresponding to more than 90% of the bulk strength of the Si₃N₄. At elevated temperatures of 1000° and 1200°C joint strengths of 666 and 330 MPa, respectively, were obtained, which are the highest values reported to date for these temperatures. These bend strengths are also more that 90% of the strength of bulk Si₃N₄ measured at these temperatures.     

Design, Fabrication, and Characterization of Silicon Nitride Particle-Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration

Silicon nitride particle-reinforced silicon nitride matrix composites were fabricated by chemical vapor infiltration (CVI). The particle preforms with a bimodal pore size distribution were favorable for the subsequent CVI process, which included intraagglomerate pores (0.1–4 μm) and interagglomerate pores (20–300 μm). X-ray fluorescence results showed that the main elements of the composites are Si, N, and O. The composite is composed of α-Si₃N₄, amorphous Si₃N₄, amorphous SiO₂, and a small amount of β-Si₃N₄ and free silicon. The α-Si₃N₄ transformed into β-Si₃N₄ after heat treatment at 1600°C for 2 h. The flexural strength, dielectric constant, and dielectric loss of the Si₃N_4(p)/Si₃N₄ composites increased with increasing infiltration time; however, the pore ratios decreased with increasing infiltration time. The maximum value of the flexural strength was 114.07 MPa. The dielectric constant and dielectric loss of the composites were 4.47 and 4.25 × 10⁻³, respectively. The present Si₃N_4(p)/Si₃N₄ composite is a good candidate for high-temperature radomes.     

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.     

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.     

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.     

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 .     

Chemical Degradation of Si3N4-Bonded SiC Sidelining Materials in Aluminum Electrolysis Cells

A thorough analysis of a silicon nitride (Si₃N₄)-bonded SiC sidelining material from a Hall-Heroult electrolysis cell is reported. Phase composition before and after chemical degradation of the material is obtained by quantitative analysis using Rietveld refinement of X-ray diffraction data and chemical analysis. The main degradation products as a result of the oxidation of Si₃N₄ binder phase are Si₂ON₂ in the upper part and Na₂SiO₃ in the lower part of the sidelining. The microstructure of α-Si₃N₄ (needle) and β-Si₃N₄ (shell) as well as the degradation products Si₂ON₂ (fiber) and Na₂SiO₃ (flake) were revealed by electron microprobe analysis. Chemical reactions and degradation mechanisms are proposed based on the presented findings. The degradation in the lower part is more severe than that in the upper part because Na diffusion from the cathode enhances the oxidation of Si₃N₄. The degradation changes the physical properties of Si₃N₄-bonded SiC such as density and porosity.     

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.     

High-Temperature Failure Mechanisms of Hot-Pressed Si3N4 and Si3N4/Si3N4-Whisker-Reinforced Composites

The high-temperature flexural strength of hot-pressed silicon nitride (Si₃N₄) and Si₃N₄-whisker-reinforced Si₃N₄-matrix composites has been measured at a crosshead speed of 1.27 mm/min and temperatures up to 1400°C in a nitrogen atmosphere. Load–displacement curves for whisker-reinforced composites showed nonelastic fracture behavior at 1400°C. In contrast, such behavior was not observed for monolithic Si₃N₄. Microstructures of both materials have been examined by scanning and transmission electron microscopy. The results indicate that grain-boundary sliding could be responsible for strength degradation in both monolithic Si₃N₄ and its whisker composites. The origin of the nonelastic failure behavior of Si₃N₄-whisker composite at 1400°C was not positively identified but several possibilities are discussed.     

The Role of Cerium Orthosilicate in the Densification of Si3N4

The existence of compounds between Si₃N₄-CeO₂ and Si₃N₄-Ce₂O₃ was investigated for firing temperatures of 1600° to 1700°C. The two new monoclinic compounds found were Ce₂O₃·2Si₃N₄ with lattice parameters a = 16.288, b = 4.848, and c =7.853 Å and β=91.54° and Ce₄Si₂O₇N₂ with lattice parameters a = 10.360, b = 10.865, and c =3.974 Å and β=90.33°. Cerium orthosilicate (Ce 4.67 (SiO₄)₃O) is present during firing as a glassy intermediate phase which promotes sintering and densification and then reacts with silicon nitride to form cerium silicon oxynitrde (CeSiO₂N).     

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.     

Role of Si2N2O in the Passive Oxidation of Chemically-Vapor-Deposited Si3N4

The results of two-step oxidation experiments on chemically-vapor-deposited Si₃N₄ and SiC at 1350°C show that a correlation exists between the presence of a Si₂N₂O interphase and the strong oxidation resistance of Si₃N₄. During normal oxidation, k _p for SiC was 15 times higher than that for Si₃N₄, and the oxide scale on Si₃N₄ was found by SEM and TEM to contain a prominent Si₂N₂O inner layer. However, when oxidized samples are annealed in Ar for 1.5 h at 1500°C and reoxidized at 1350°C as before, three things happen: the oxidation k _p increases over 55-fold for Si₃N₄, and 3.5-fold for SiC; the Si₃N₄ and SiC oxidize with nearly equal k _p's; and, most significant, the oxide scale on Si₃N₄ is found to be lacking an inner Si₂N₂O layer. The implications of this correlation for the competing models of Si₃N₄ oxidation are discussed.     

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.     

Relationship between the Average Coordination Number and Properties of Chalconitride Glasses

Chalconitride glasses obtained by doping Ge-As-Se and Ge-S chalcogenide glass systems with Si₃N₄ (0.35 and 0.50 wt%) have been studied; these glasses have been doped with Si₃N₄ to improve their thermal and mechanical prop-erties. The glasses have been melted under vacuum in sealed silica ampoules, quenched in air, and subsequently annealed. Measured properties include the transition temperature ( T _g), microhardness ( H _v), and thermal expansion coefficient (α). The effect of Si₃N₄ doping is system depen-dent. The concept of average coordination number () is used to explain this dependence. For the glasses in the chal-cogen-rich region, a greater increase in thermal and mechanical properties with Si₃N₄ doping is assigned to the fact that both the introduction of silicon and the substitution of nitrogen for chalcogens enhance the degree of crosslinking within the substructures. However, for the glasses in the chalcogen-deficient region, extra silicon atoms that are in-troduced with Si₃N₄ doping are not helpful in further crosslinking the network, because of the deficiency in the bridging chalcogens. A slight increase in T _g with increasing , in the case of chalcogen deficiency, might result from the "wrong-bond effect," which provides some additional crosslinking in connections of the networked island sub-structural units.     

Texture Development in Silicon Nitride–Silicon Oxynitride In Situ Composites via Superplastic Deformation

Silicon nitride–silicon oxynitride (Si₃N₄–Si₂N₂O) in situ composites have been fabricated via either the annealing or the superplastic deformation of sintered Si₃N₄ that has been doped with a silica-containing additive. In this study, quantitative texture measurements, including pole figures and X-ray diffraction patterns, are used in conjunction with scanning electron microscopy and transmission electron microscopy techniques to examine the degree of preferred orientation and texture-development mechanisms in these materials. The results indicate that (i) only superplastic deformation can produce strong textures in the β-Si₃N₄ matrix, as well as Si₂N₂O grains that are formed in situ ; (ii) texture development in the β-Si₃N₄ matrix mainly results from grain rotation via grain-boundary sliding; and (iii) for Si₂N₂O, a very strong strain-dependent texture occurs in two stages, namely, preferred nucleation and anisotropic grain growth.     

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.