Elastic Moduli and Hardness of Cubic Silicon Nitride

The bulk modulus B ₀= 290(5) GPa and its first pressure derivative B '₀= 4.9(6) were obtained for c -Si₃N₄ from volume versus pressure dependence. Measurements were performed under quasi-hydrostatic conditions in a diamond anvil cell to 53 GPa using synchrotron radiation and energy dispersive X-ray powder diffraction. This combined with nanoindentation measurements determined the shear modulus G ₀ of c -Si₃N₄ to be 148(16) GPa. The Vickers microhardness H _V(0.5) for dense, oxygen-free c -Si₃N₄ was estimated to be between 30 and 43 GPa. Both the elastic moduli and microhardness of c -Si₃N₄ exceed those of the hexagonal counterparts, α- and β-phases.     

Theoretical Prediction of Post-Spinel Phases of Silicon Nitride

New phases of Si₃N₄ that may be stable at higher pressure than spinel have been searched using a first-principles plane-wave pseudopotential method. The CaTi₂O₄-type phase is found to be the prime candidate for the post-spinel phase among six phases selected on the analogy to high-pressure oxides. The phase transformation from the spinel is predicted to occur at 210 GPa. All silicon atoms of the new phase are coordinated by six anions, similar to the case of the high-pressure forms of SiO₂ and SiC. Because of its high energy at zero pressure, this new phase may be difficult to quench. The bandgap increases with an increase of pressure when compared in the same polymorph. However, the bandgap and the net charge decrease in the order of β, spinel, and CaTi₂O₄-type phases at zero pressure. The theoretical bulk modulus of the CaTi₂O₄-type phase is comparable with that of spinel.     

Bond Energetics at Intergranular Interfaces in Alumina-Doped Silicon Nitride

In Si₃N₄ ceramics sintered with Al₂O₃, the interfacial strength between the intergranular glass and the reinforcing grains has been observed to increase with increases in the aluminum and oxygen content of the epitaxial β-Si_{6- z} Al _z O _z N_{8– z} layer that forms on the Si₃N₄ grains. This has been attributed to the formation of a network of strong bonds (cross bonds) that span the glass-crystalline interface. This proposed mechanism is considered further in light of first-principles atomic cluster calculations of the relative stabilities of bridge and threefold-bonded atomic fragments chosen to represent compositional changes at the glass/Si₃N₄ grain interface. Calculated binding energies indicate Al-N binding is favorable at the Si₃N₄ grain surface, where aluminum occupancy can promote the growth of SiAlON, further enhancing the cross-bonding mechanism of interfacial strengthening.     

In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composites

SiC–BN composites were prepared via the proposed in situ reaction, which used Si₃N₄, B₄C, and carbon as reactants. Adding SiC powder to the reactants controlled the BN content in the composite. For comparison, SiC–BN composites with the same phase compositions were produced via conventional processing. The in situ process was advantageous for obtaining better composites with fine grain size and homogeneous microstructures. The in situ composite that had a BN content of 53.71 vol% exhibited considerably high strength (342 MPa) and a very low elastic modulus (107 GPa). The SiC–25-vol%-BN in situ composite had a peak strength of 588 MPa, which was 95% of that of monolithic SiC; however, the elastic modulus was as low as half that of monolithic SiC. These in situ SiC–BN composites can be expected to have excellent thermal shock resistance and mechanical strain tolerance.     

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.     

Synthesis of Porous Silicon Nitride with Unidirectionally Aligned Channels Using Freeze-Drying Process

Porous silicon nitride with macroscopically aligned channels was synthesized using a freeze-drying process. Freezing of a water-based slurry of silicon nitride was done while unidirectionally controlling the growth direction of the ice. Pores were generated subsequently by sublimation of the columnar ice during freeze-drying. By sintering this green body, a porous silicon nitride with high porosity (over 50%) was obtained and its porosity was controllable by the slurry concentration. The porous Si₃N₄ had a unique microstructure, where macroscopically aligned open pores contained fibrous grains protruding from the internal walls of the Si₃N₄ matrix. It is hypothesized that vapor/solid phase reactions were important to the formation mechanism of the fibrous grains.     

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.     

Strength of Silicon Nitride after Thermal Shock

A water-quenching technique was used to evaluate the thermal-shock strength behavior of silicon nitride (Si₃N₄) ceramics in an air atmosphere. When the tensile surface was shielded from air during the heating and soaking process, the quenched specimens showed a gradual decrease in strength at temperatures above 600°C. However, the specimens with the air-exposed surface exhibited a ∼16% and ∼29% increase in strength after quenching from 800° and 1000°C, respectively. This is because of the occurrence of surface oxidation, which may cause the healing of surface cracks and the generation of surface compressive stresses. As a result, some preoxidation of Si₃N₄ components before exposure to a thermal-shock environment is recommended in practical applications.     

Si3N4及其复合材料强韧化研究进展

简述了氮化硅陶瓷的结构、性能和制备工艺，并分别通过自增韧补强、纤维／晶须强韧化、层状结构强韧化、相变强韧化以及颗粒弥散强韧化等方法对氮化硅陶瓷的强韧化研究进行了分类叙述。     

硅粉直接氮化反应合成氮化硅研究

研究了硅粉直接氮化反应合成氮化硅粉末的工艺因素(包括硅粉粒度、氮化温度、成型压力、稀释剂含量等),借助XRD,SEM等测试手段测定和观察了氮化产物的物相组成和断口形貌.研究结果表明:硅粉在流动氮气氛下,高于1200℃氮化产物中氮含量明显增加;在氮化反应同时还伴随着硅粉的熔结过程,它阻碍硅粉的进一步氮化,其影响程度与氮化温度、氮化速度,素坯成型压力及硅粉粒度等工艺因素有关.在硅粉素坯中引入氮化硅作为稀释剂,提高了硅粉的氮化率,使产物中残留硅量降低;同样在实际生产中可以通过控制适当热处理制度(如分段保温、慢速升温),达到硅粉的完全氮化.在生产中批量合成了含氮量为32.5%,残留硅量为0.05%,主要为α相,含少量β相的针状、柱状的氮化硅.     

Carbothermal Synthesis of Silicon Nitride: Effect of Reaction Conditions

Conditions for carbothermal synthesis of α-Si₃N₄ are presented with special emphasis on the reaction temperature, C:SiO₂ ratio, and precursor mixing. With pure precursors, the conversion temperature is 1500° to 1550°C. An excess of C is necessary for complete conversion, and a simple sol–gelmixing technique provides excellent intermixing of the precursors. Copious flow of N₂ gas throughout the reactor bed is essential if pure Si₃N₄ is to be produced; small concentrations of CO and O₂ promote SiC and Si₂N₂O, respectively.     

自蔓延高温合成制备单相氮化硅镁粉体

通过自蔓延高温合成技术制备了以MgSiN2为主相的粉体,然后利用酸洗工艺除去杂质得到单相MgSiN2粉体.研究了原料配比、稀释剂MgSiN2的添加量和N2压力对燃烧合成产物相组成的影响,并探讨了酸洗条件对洗除MgO杂质的影响.研究结果表明,以化学计量比配制的原料,难以通过自蔓延高温合成法直接合成单相的MgSiN2粉体;...     

Reactivity of Aluminum Nitride Powder in Aqueous Silicon Nitride and Silicon Carbide Slurries

The reactivity of AlN powder with water in supernatants obtained from centrifuged Si₃N₄ and SiC slurries was studied by monitoring the pH versus time. Various Si₃N₄ and SiC powders were used, which were fabricated by different production routes and had surfaces oxidized to different degrees. The reactivity of the AlN powder in the supernatants was found to depend strongly on the concentration of dissolved silica in these slurries relative to the surface area of the AlN powder in the slurry. The hydrolysis of AlN did not occur if the concentration of dissolved silica, with respect to the AlN powder surface, was high enough (1 mg SiO₂/(m² AlN powder)) to form a layer of aluminosilicates on the AlN powder surface. This assumption was verified by measuring the pH of more concentrated (31 vol%) Si₃N₄ and SiC suspensions also including 5 wt% of AlN powder (with respect to the solids).     

Machinability of Silicon Nitride/Boron Nitride Nanocomposites

The machinability and deformation mechanism of Si₃N₄/BN nanocomposites were investigated in the present work. The fracture strength of Si₃N₄/BN microcomposites remarkably decreased with increased hexagonal graphitic boron nitride ( h -BN) content, although machinability was somewhat improved. However, the nanocomposites fabricated using the chemical method simultaneously had high fracture strength and good machinability. Hertzian contact tests were performed to clarify the deformation behavior by mechanical shock. As a result of this test, the damage of the monolithic Si₃N₄ and Si₃N₄/BN microcomposites indicated a classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.     

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si₃N₄ could be completely converted to SiC with molar ratio Si₃N₄:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.     

Variation of Width and Composition of Grain-Boundary Film in a High-Purity Silicon Nitride with Minimal Silica

Contrary to the widely accepted observation that grain-boundary amorphous films for a given Si₃N₄ composition have common (equilibrium) widths and compositions, a significant variation for both parameters from film to film was observed in an undoped high-purity Si₃N₄ prepared using a hot isostatic pressing method. This material previously has been reported to have an equilibrium film width of 0.6 nm, as measured using a high-resolution electron microscopy (HREM) method; this value is significantly different from that which is typical for other high-purity Si₃N₄ ceramics (1.0 nm). A total of four boundaries were analyzed, using spatially resolved electron energy-loss spectroscopy methods, which can give the chemical width and composition for the film. Widths of these grain-boundary films were substantially different from each other; only the thinnest matches the previous HREM observations. The nitrogen content in the film decreased concurrently as the film thickened. This material had many cavities and complicated configurations at triple pockets, because of the very low total-SiO₂ content (0.55 vol%). They created locally different equilibrium conditions for grain-boundary films, in comparison with other fully densified Si₃N₄, causing such strong variation in both film structure and chemistry. This observation reveals the importance of triple pockets in equilibrium film structures, providing new insight in evaluating the absorption and wetting models. The thinnest film may correspond to the amorphous structure that is required to bind two randomly oriented Si₃N₄ grains under greater local stress.     

Synthesis of Cubic Boron Nitride by Decomposition of Magnesium Boron Nitride

Cubic boron nitride ( c -BN) was synthesized by the decomposition of Mg₃BN₃ under high pressure and high temperature. The minimum pressure for c -BN synthesis was 4 GPa, which was 1 GPa lower than that of the conventional catalytic process. The decomposition of Mg₃BN₃ was observed only when H₂O was added. Therefore, the reaction was as follows: Mg₃BN₃+ 3H₂O = 3MgO + c -BN + 2NH₃. The c -BN crystals obtained were tetrahedron in shape and about 10 μ m in diameter.     

Fabrication and Microstructure of Silicon Nitride/Boron Nitride Nanocomposites

A chemical process for fabrication of Si₃N₄/BN nanocomposite was devised to improve the mechanical properties. Si₃N₄/BN nanocomposites containing 0 to 30 vol% hexagonal BN ( h -BN) were successfully fabricated by hot-pressing α-Si₃N₄ powders, on which turbostratic BN ( t -BN) with a disordered layer structure was partly coated. The t -BN coating on α-Si₃N₄ particles was prepared by reducing and heating α-Si₃N₄ particles covered with a mixture of boric acid and urea. TEM observations of this nanocomposite revealed that the nanosized hexagonal BN ( h -BN) particles were homogeneously dispersed within Si₃N₄ grains as well as at grain boundaries. As expected from the rules of composites, Young's modulus of both micro- and nanocomposites decreased with an increase in h -BN content, while the fracture strength of the nanocomposites prepared in this work was significantly improved, compared with the conventional microcomposites.     

Thermogravimetry, Differential Thermal Analysis, and Mass Spectrometry Study of the Silicon Nitride–Boron Carbide–Carbon Reaction System for the Synthesis of Silicon Carbide–Boron Nitride Composites

Thermogravimetry, differential thermal analysis, mass spectrometry, and X-ray diffractometry were used to study the reaction process of the in situ reaction between Si₃N₄, B₄C, and carbon for the synthesis of silicon carbide–boron nitride composites. Atmospheres with a low partial pressure of nitrogen (for example argon + 5%–10% nitrogen) seemed to inhibit denitrification and also maintain a high reaction rate. However, the reaction rate decreased significantly in a pure nitrogen atmosphere. The experimental mass spectrometry results also revealed that B₄C in the Si₃N₄–B₄C–C system inhibited the reaction between Si₃N₄ and carbon and, even, the decomposition of Si₃N₄. The present results indicate that boron could be a composition stabilizer for ceramic materials in the Si-N-C system used at high temperature.     

Texture in Silicon Nitride Seeded with Silicon Nitride Whiskers of Different Sizes

Silicon nitride ceramics seeded with 3 wt%β-Si₃N₄ whiskers of two different sizes were prepared by a modified tape casting and gas pressure sintering. The fine whiskers had a higher aspect ratio than the coarse whiskers. Quantitative texture analysis including calculation of the orientation distribution function (ODF) was used for obtaining the degrees of preferred orientation of sintered samples. The maximum multiples of random distribution (mrd) values of samples seeded with the fine and coarse whiskers were large, greater than 15 and 9, respectively. Meanwhile, the mrd value of a sample seeded with fine whiskers was only 9 when it was prepared by conventional tape casting. The microstructures and the XRD data revealed that the well-aligned whiskers grew significantly after sintering and dominated the texture. Differences among the degrees of preferred orientation of the samples were explained using Jeffrey's model on rotation of elliptical particles carried by a viscous fluid.