Low-Frequency Measurements on the Grain-Boundary Internal Friction Peak in Chlorine-Doped Silicon Nitride

A procedure is shown to quantitatively analyze the morphology of the internal friction peak resulting from grain-boundary sliding. A Si₃N₄ polycrystal containing chlorine-doped SiO₂ at grain boundaries is selected as a model system for discussing chemical (e.g., anion) gradients at glassy grain boundaries. In this model material, grain boundaries lodging Cl⁻ anions show nonuniform thickness characteristics, which suggests a non-negligible dependence of the intergranular SiO₂-network structure upon grain misorientation. Both chemical and microstructural inhomogeneities existing in a polycrystalline ceramic body can result in peak broadening. The key for separating broadening contributions of chemical gradients from grain-size/morphology distributions resides in analyzing the peak-width change upon damping frequency. Groups of grain boundaries with different chemical characteristics may produce broadening because different peak components are generated that obey a spectrum of activation energies. On the other hand, microstructural inhomogeneities obey a single activation energy, but they generate a distribution of relaxation times. As a result, when a chemical gradient is present at grain boundaries, the peak may shift upon changing damping frequency with obeying a true activation energy, but its width increases with decreasing damping frequency. When peak broadening results only from microstructural inhomogeneities, the peak width is independent of damping frequency.     

Grain Boundary Films in Rare-Earth-Glass-Based Silicon Nitride

The thickness of the intergranular films in Si₃N₄ densified with lanthanide oxides has been systematically investigated using high-resolution transmission electron microscopy. Four lanthanide oxides—La₂O₃, Nd₂O₃, Gd₂O₃, and Yb2O3—as well as Y₂O₃ are chosen so that the results will reflect the overall trend in the effect of the lanthanide utilized. The film thicknesses increase with increasing ionic radius of the lanthanide. In addition, Si₃N₄ particles flocculated into isolated clusters in the lanthanide-based glasses are also characteristically separated by an amorphous film whose thickness is similar to that in the comparable polycrystalline ceramics, demonstrating that the film thickness is dictated entirely by the composition and not by the amount of the glass phase present.     

Grain Boundaries in Silicon from Zero Temperature through Melting

The results of atomistic simulations of twist grain boundaries in covalent silicon are presented and compared with similar studies in metals. Three aspects are discussed in detail: (i) the zero-temperature structure-energy correlation, (ii) the elastic anomalies near a twist boundary at zero temperature, and (iii) the high-temperature stability of a boundary and its role in thermodynamic melting. In each case striking similarities with studies on metals are found, which are attributed to the important role played by atoms in close proximity. By contrast the covalent nature of bonding in silicon appears to play only a minor role.     

Processing of Concentrated Aqueous Silicon Nitride Slips by Slip Casting

The optimization of concentrated Si₃N₄ powder aqueous slurry properties to achieve high packing density slipcast compacts and subsequent high sintered densities was investigated. The influence of pH, sintering aid powder (6% Y₂O₃, 4% Al₂O₃), NH₄PA dispersant, and Si₃N₄ oxidative thermal treatment was determined for 32 vol% Si₃N₄ slurries. The results were then utilized to optimize the dispersion properties of 43 vol% solids Si₃N₄-sintering aid slurries. Calcination of the Si₃N₄ powder was observed to result in significantly greater adsorption of NH₄PA dispersant and effectively reduced the viscosity of the 32 vol% slurries. Lower viscosities of the optimized dispersion 43 vol% Si₃N₄-sintering aid slurries resulted in higher slipcast packing density compacts with smaller pore sizes and pore volumes, and corresponding higher sintered densities.     

Grain Boundary Film Thicknesses in Superplastically Deformed Silicon Nitride

The thickness of intergranular films in polycrystalline β-Si₃N₄ ceramics, both before and after superplastic deformation, has been systematically investigated using high-resolution transmission electron microscopy. In characterizing the film thickness, care was taken to correlate the grain boundary orientation with the direction of the compressive stress applied during the hot-pressing and the superplastic deformation. The film thickness shows a dependence on the intersection angle between the grain boundary and the applied force direction, typically ranging from around a characteristic value for most of the boundaries to zero for a boundary which has an overall short length and is perpendicular to the applied force direction. The film thicknesses in the deformed material, as compared with those before deformation, are marked by a wider distribution and an increased fraction of boundaries free of films, unequivocally demonstrating that during the superplastic deformation the liquid phase is redistributed within short ranges, a process governed by the local stress level as well as kinetic factors. Possible consequences of the liquid-phase redistribution on the deformation behavior are also discussed.     

Stress Relaxation, Creep Recovery, and Newtonian Viscous Flow in Silicon Nitride

A new approach to tensile creep testing and analysis based on stress relaxation is described for sintered silicon nitride. Creep rate data covering up to 5 orders of magnitude are generated in tests lasting less than 1 day. Tests from various initial stresses at temperatures up to 1300°C are analyzed and compared with creep rates measured during conventional constant load testing. It is shown that at least 40% of the creep strain accumulated under all test conditions is recoverable, and that the deformation may properly be described as viscoelastic. A regime which approximated as Newtonian viscous behavior (creep rate directly proportional to stress) was observed during decreasing stress at temperatures between 1200° and 1300°C. This resulted in anomalous behavior at low strains in pseudo stress-strain curves generated from the stress relaxation data. However, the otherwise systematic rate dependence provides a possible basis for design in terms of a secant modulus analysis. The anelastic, recoverable component of creep may lead to complex deformation history-dependent phenomena.     

High-Resolution Electron Microscopy Investigation of Viscous Flow Creep in a High-Purity Silicon Nitride

The redistribution of intergranular amorphous films during creep deformation of Si₃N₄ has been studied by high-resolution transmission electron microscopy. The film thickness distribution of a high-purity Si₃N₄ material before and after creep was measured. A narrow range of film widths in the uncrept material and a bimodal distribution after creep were observed. These observations provided convincing evidence of the occurrence of viscous flow of intergranular amorphous films during creep deformation of Si₃N₄. Moreover, the creep response predicted by a viscous flow model was in good agreement with experimental data.     

Grain Growth Studies of Silicon Nitride Dispersed in an Oxynitride Glass

Isothermal growth of β-Si₃N₄ crystals dispersed in an oxynitride glass (Y-Si-Al-O-N) was studied by electron microscopy after heat treatment at temperatures between 1550° and 1640°C for 1 to 18 h. The β-crystals exhibited growth striations introduced by intermediate coolings and these striations were used for developing a sophisticated technique for analysis of growth. It was determined that α/β-transformation and Ostwald ripening can be treated as different kinetic stages of grain growth, while β-nucleation was found to be insignificant. The mean diameter of the needlelike β-grains was almost constant during phase transformation, indicating negligible growth of the β-prism plane; growth was mainly one-dimensional with the maximum mean length and aspect ratio just at the end of the phase transformation. The growth rate of the β-basal plane was independent of diameter, indicating interface-controlled growth. During Ostwald ripening, the length distribution broadened and the aspect ratio of smaller grains decreased. Dissolution of small grains caused an increase in the mean diameter, while the mean aspect ratio decreased.     

Mechanical Properties of Hot-Pressed Silicon Nitride with Different Grain Structures

During hot-pressing of α-Si₃N₄ powders, the equiaxed α micro-structure gradually transforms into a β structure characterized by needle-shaped prismatic grains which are closely entangled and linked together. With increasing amounts of the β fraction, the bend strength, fracture toughness, and work of fracture increase significantly, then decrease as grain growth occurs. The K _lc, improves by a factor of >2 and the change in γ_F by a factor of >4. The crack resistance to achieve the same crack velocity in materials of different β contents shows a similar trend. The dependence of the mechanical properties on the microstructure is explained by linking and pullout of the β crystals and by grain coarsening.     

Thermodynamic Analysis of Grain Aspect Ratio in Fibrous Microstructures of Silicon Nitride

Needlelike, free-floating ß-crystals of silicon nitride, precipitated within a supersaturated Y-Si-Al-O-N oxynitride glass, apparently approach an equilibrium value of the aspect ratio. An expression that relates this aspect ratio to the anisotropy in the interfacial energies of the longitudinal and transverse interfaces of the crystal is derived. The prediction agrees reasonably well with theoretical estimates of the interfacial energies. Kinetic and stearic effects that often lead to nonequilibrium values of the aspect ratio are discussed.     

Chemical and Structural Widths of Interfaces and Grain Boundaries in Silicon Nitride–Silicon Carbide Whisker Composites

The extent of chemical distributions into crystals bounding whisker/matrix interfaces and matrix grain boundaries and the lateral continuity of the distributions was investigated by analytical electron microscopy methods and compared to their structural widths determined by high-resolution TEM. The extent of the distributions into the bounding crystals, defined as chemical widths, was 10 to 120 times the structural widths; this ratio was larger for grain boundaries than for the interfaces. Further, the chemical distributions were laterally discontinuous at interfaces but continuous at grain boundaries. The elements from the sintering aids, Y₂O₃ and Al₂O₃, were the primary chemical distribution constituents. The distributions were examined by the new position-resolved electron energy loss spectroscopy and Z-contrast scanning TEM methods. Microstructural observations indicated that chemical widths resulted from solid-state diffusion into the bounding crystals and that lateral discontinuities in the distributions resulted from preferential Gibbs–Thompson solution effects of the whisker surfaces at interfaces. These nonequilibrium distributions are process-dependent, and are expected to affect composite properties.     

Inversion Domain Boundaries in Aluminum Nitride

Inversion domain boundaries (IDBs) have been identified in undoped, hot-pressed AIN. The microstructure and micro-chemistry of the IDBs have been studied using conventional transmission electron microscopy, convergent-beam electron diffraction, and analytical electron microscopy. Two distinct IDB morphologies are present: a planar variant which lies on the basal plane (0001), and a curved variant which does not possess a particular habit plane, but portions of the boundary are often seen lying on one of the {1011} planes. The boundaries exhibit α-like fringe contrast, indicating that a translation exists across the boundary. The displacement vectors R_F for the planar and curved boundaries have been investigated using both two-beam and multiple-beam techniques. Microchemical analysis has revealed oxygen segregation to the planar IDB; when present on the curved IDB, oxygen is at a lower concentration than in the planar case. Lattice fringe imaging and long-exposure selected area electron diffraction patterns have indicated the presence of thin, platelike precipitates at the planar IDBs. Sintering and annealing studies indicate that oxygen is necessary for the formation of the planar IDBs and that oxygen is not uniformly distributed along the curved IDBs.     

Anisotropy of Silicon Nitride with Aligned Silicon Nitride Whiskers

A model based on anisotropic sintering shrinkage of silicon nitride with aligned silicon nitride whisker seeds was built in order to provide an easy way to obtain information on how the large elongated grains were aligned. The method requires a simple measuring device for the information. XRD analysis showed a good correlation with predictions of the model. Both predictions of the model and experimental results indicated that the fraction of aligned large elongated grains increased as the whisker content increased.     

Elevated-Temperature Creep of Silicon Carbide-Aluminum Nitride Ceramics: Role of Grain Size

Samples containing 50 mol% SiC and 50 mol% AIN were fabricated to neartheoretical density by hot-pressing in graphite dies in N₂ atmosphere. Grain size was varied by varying the hot-pressing conditions. Bar-shaped samples cut from the billets were subjected to creep deformation in four-point bending. Creep was found to depend upon the grain size with coarse-grained material exhibiting lower creep rate. The stress exponent was ∼2.0.     

Coating of Fine-Grained Silicon Nitride Whiskers on Fiber-like Silicon Nitride

Fine-grained silicon nitride (Si3N4) whiskers were coated on Si3N4 fibers through a vapor-liquid-solid mechanism under the following process. After the oxide glass of a Si-Al-Y-O system was coated on seed Si3N4 fibers, the coated fibers were heated at 1490°C and 700° in the vapor of the Si-N system which was generated by decomposition of amorphous Si3N4. The resulting specimens looked just like rose twigs. The Si3N4 whiskers were precipitated by nucleation in a liquid phase generated by melting of the oxide glass layers on the Si3N4 fibers.     

Rapid Nondiffusional Penetration of Oxide Melts along Grain Boundaries of Oxide Ceramics

Interaction of the liquid phase with the grain boundaries of solids and a rapid mass transport in solids were studied in the present work with a CuO ceramic melt of the Bi₂O₃system as an example. Anomalously rapid penetration of the liquid phase along the grain boundaries of a solid ceramic material was observed. The velocity and activation energy of the process were each determined. A mechanism for the development of a grain-boundary liquid channel in the kinetic mode, when the channel growth rate is reaction-limited, was proposed. This mechanism leads to the formation of a liquid-channel grain-boundary structure (LGBS) in solids. It was demonstrated that liquid can interact with one-dimensional as well as two-dimensional defects in solids.     

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.     

Silicon Nitride Joining

Hot-pressed Si₃N₄ was joined using an Mgo-A1₂O₃-SiO₂ glass composition chosen to approximate the oxide portion of the grain-boundary phase in the ceramic. After it has been heated at 1550° to 1650°, the interface of the joined ceramic is an interlocking mixture of Si₂N₂O, β-Si₃N₄, and a residual oxy-nitride glass. The kinetics of reactions between Si₃N₄ and the molten joining composition were studied by X-ray diffraction analysis of the phases present in Si₃N4 powder-glass mixtures quenched after varied heat treatments. Analytical transmission electron microscopy of the composition and micro-structure of the reaction zone in joined specimens, together with the X-ray diffraction results, suggests that the driving force for joining is the lowering of the Si₃N₄ interfacial energy when it is wet by the molten silicate, augmented by the negative Gibbs energy for the reaction SiO₂( l ) + Si₃N₄= 2Si₂N₂O.     

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。