Ultrasonic Velocity Technique for Nondestructive Quantification of Elastic Moduli Degradation during Creep in Silicon Nitride

The ultrasonic velocity technique was used for nondestructive quantification of creep damage during interrupted tensile creep tests at 1400°C in an advanced silicon nitride to investigate the possibilities of this technique for creep damage monitoring in ceramic components. The longitudinal and shear wave velocities, Poisson's ratio, and Young's, shear, and bulk moduli linearly decreased with strain. Precise density change measurements indicated a linear relationship with a coefficient of proportionality of 0.69 between the volume fraction of cavities and tensile strain. Cavitation was identified as the main creep mechanism in the studied silicon nitride and the reason for ultrasonic velocity and elastic moduli degradation. The measurement of just the longitudinal wave velocity changes was found to be sufficient for quantification of cavitation during creep. The capability of the ultrasonic velocity technique for simple, sensitive, and reliable nondestructive monitoring of creep damage during intermittent creep was demonstrated in silicon nitride.     

Shock Synthesis of Cubic Silicon Nitride

The phase transitions of α-Si₃N₄ and β-Si₃N₄ have been investigated by shock compression through the recovery technique and Hugoniot measurements. α- and β-Si₃N₄ are transformed into a cubic spinel structure ( c -Si₃N₄). The yield of c -Si₃N₄ increases with increasing shock pressure and reaches 100% at 63 GPa. The shock-synthesized c -Si₃N₄ powders are nanocrystals and display a high-temperature metastability up to about 1620 K. c -Si₃N₄ is one of the hard materials based on the measured equation of state. c -Si₃N₄ powders have been characterized by electron microscopy and ²⁹Si magic angle spinning NMR spectroscopy. The purification and separation method has been developed to obtain pure c -Si₃N₄ powders.     

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.     

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。     

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).     

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.     

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.     

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.     

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.     

Mechanisms of Dopant-Induced Changes in Intergranular SiO2 Viscosity in Polycrystalline Silicon Nitride

Mechanical spectroscopic methods and first-principles density functional calculations were applied to attempt a quantitative analysis of both atomic structure and viscous behavior of Si₃N₄ grain boundaries. In particular, the effect on the intergranular structure/viscosity of small fractions of selected anion/cation dopants was examined in comparison with the undoped polycrystal. From the point of view of mechanical spectroscopy, emphasis was placed on the morphologic analysis, as a function of frequency of oscillation, of a relaxation peak that originates from grain-boundary sliding. The morphologic characteristics of the grain-boundary peak clearly revealed the presence of significant chemical gradients among different grain boundaries for particular dopants (e.g., Cl and Ba). On dopant addition, a reduction in activation energy for viscous intergranular flow was observed which broadened the grain-boundary peak. Chemical inhomogeneities also broadened the peak shape by generating a spectrum of activation energies. First-principles density functional calculations were conducted for cluster fragment models representative of the amorphous SiO₂ intergranular film. The results explicitly showed the mechanism by which the respective dopants break bonds in the host, an action that directly reduces the viscosity of the SiO₂ film. These complementary theoretical studies assist understanding and atomic-scale rationalization of the differences in segregation behavior of different dopants incorporated into the SiO₂ film.     

Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites

Based on a biomimetic design, Si₃N₄/BN composites with laminated structures have been prepared and investigated through composition control and structure design. To further improve the mechanical properties of the composites, Si₃N₄ matrix layers were reinforced by SiC whiskers and BN separating layers were modified by adding Si₃N₄ or Al₂O₃. The results showed that the addition of SiC whiskers in the Si₃N₄ matrix layers could greatly improve the apparent fracture toughness (reaching 28.1 MPa·m^1/2), at the same time keeping the higher bending strength (reaching 651.5 MPa) of the composites. Additions of 50 wt% Al₂O₃ or 10 wt% Si₃N₄ to BN interfacial layers had a beneficial effect on the strength and toughness of the laminated Si₃N₄/BN composites. Through observation of microstructure by SEM, multilevel toughening mechanisms contributing to high toughness of the laminated Si₃N₄/BN composites were present as the first-level toughening mechanisms from BN interfacial layers as crack deflection, bifurcation, and pull-out of matrix sheets, and the secondary toughening mechanism from whiskers in matrix layers.     

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.     

Oxidation of Chemically-Vapor-Deposited Silicon Nitride and Slnglem-Crystal Silicon

Oxidation of {111} single-crystal silicon and dense, chemically-vapor-deposited silicon nitride was done in clean silica tubes at temperatures of 1000° to woo°C. The oxidation rates of silicon nitride under various atmospheres (dry O₂, wet O₂, wet inert gas, and steam) were several orders of magnitude slower than those of silicon under the identical conditions. The activation energy for the oxidation of silicon nitride decreased from 330 to 259 kJ/mol in going from dry O₂ to steam while that for Si decreased from 120 to 94 kJ/mol. The parabolic rate constant for Si increased linearly as the water vapor pressure increased. However, the parabolic rate constant for silicon nitride showed nonlinear dependency on the water vapor pressure in the presence of oxygen. The oxidation kinetics of silicon nitride is explained by the formation of nitrogen compounds (NO and NH₃) at the reaction interface and the counterpermeation of these reaction products.     

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O₂, CO, and N₂. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.     

Elevated-Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

A unique, all-ceramic material capable of nonbrittle fracture via crack deflection and delamination has been mechanically characterized from 25° through 1400°C. This material, fibrous monoliths, was comprised of unidirectionally aligned 250 μm diameter silicon nitride cells surrounded by 10 to 20 μm thick boron nitride cell boundaries. The average flexure strengths of fibrous monoliths were 510 and 290 MPa for specimens tested at room temperature and 1300°C, respectively. Crack deflection in the BN cell boundaries was observed at all temperatures. Characteristic flexural responses were observed at temperatures between 25° and 1400°C. Changes in the flexural response at different temperatures were attributed to changes in the physical properties of either the silicon nitride cells or boron nitride cell boundary.     

Silicon Nitride and Related Materials

Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot-zone components made entirely from ceramics. The ceramic engine programs have had only partial success, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties, which in many respects is more advanced than of more widely used technical ceramics. This review examines from the historical standpoint the development of silicon nitride and of its processing into a range of high-grade ceramic materials. The development of understanding of microstructure–property relationships in the silicon nitride materials is also surveyed. Because silicon nitride has close relationships with the SiAlON group of materials, it is impossible to discuss the one without some reference to the other, and a brief mention of the development of the SiAlONs is included for completeness.     

Dissolution and Deagglomeration of Silicon Nitride in Aqueous Medium

Silicon nitride undergoes hydrolysis and dissolution when subjected to an aqueous environment. Molecular dynamics simulations suggest that hydrolysis proceeds through nucleophilic attack of water with the formation of an intermediate molecular complex involving a pentacoordinated silicon. We found that the dissolution of an oxidized silicon nitride powder resembles that of silica; the dissolution rate could be described using a simple kinetic equation with a dissolution activation energy of 52 kJ·mol⁻¹. The deagglomeration of a fine silicon nitride powder under mild agitation was evaluated; we show that the peptization kinetics at room temperature is dominated by the breakup of particle–particle bonds due to hydrodynamic friction and cluster attrition. For breakup of hard agglomerates of small particles the dissolution of interparticle necks will play an important role.     

Mechanical Properties of Three-Layered Monolithic Silicon Nitride–Fibrous Silicon Nitride/Boron Nitride Monolith

A three-layered composite, composed of a strong outer layer (monolithic S₃N₄) and a tough inner layer (fibrous Si₃N₄/BN monolith), was fabricated by hot-pressing. For the inner layer, a Si₃N₄–polymer fiber made by extrusion was coated by dipping it into a 20 wt% BN-containing slurry. The three-layered composite exhibited excellent mechanical properties, including high strength, work of fracture, and crack resistance, because of the combination of a strong outer layer and a tough inner layer. In other words, the strong outer layer withheld the applied stress, while the tough inner layer promoted crack interactions through the weak BN cell boundaries. Also, the residual thermal stress on the surface due to the anisotropy in the coefficient of thermal expansion of BN affected a median/radial crack generation after indentation.     

氮化硅陶瓷材料的热力学分析

运用热力学方法分析了氮化硅陶瓷在制备和使用过程中涉及的物理化学过程,包括Si3N4烧结、Si3N4在真空中的挥发、Si3N4的稳定性、Si3N4与金属固体的作用4个方面,结合实际论述了反应发生、发展的条件、反应的优先性、生成化合物的稳定性及热力学方法的适用性.