Grain Boundaries In Silicon Nitride Research Articles

Continuum modelling and representations of interfaces and their transitions in materials

Evidence for internal and external interface transitions in ceramics and metals is copious. The work described here lays out a framework for predicting interface transitions and including them on bulk phase diagrams. Advances in understanding possible grain boundary transitions in single- and binary-component materials have been made with energetics modelled with the phase-field model due to Kobayashi, Warren and Carter. Analyses inspired by Cahn’s critical point wetting work allowed the exploration of the stable, metastable, and unstable grain boundary compositions and structures. A general phase-field model has also been developed that allows the couplings between chemistry, structure, and electrostatics to be explored. This model has been applied to grain boundaries in silicon nitride and indicates that intergranular films can be stabilized below the eutectic temperature. Predictive information from the models presented here could be used to inform material processing routes in order to design for interface structures and so bulk properties.

A diffuse interface model of interfaces: Grain boundaries in silicon nitride

A diffuse-interface model for interfaces in multi-component systems with energetic contributions from chemistry, defects, structure, orientation, electrostatics and gradients is proposed. The energy minimizing profiles of planar grain boundaries in the pseudo-binary SiO 2–SiN 4/3 system are calculated in the SiN 4/3-rich single-phase field. Intergranular films are found to be stable below the eutectic temperature. Evidence of first-order grain boundary order–disorder transitions is found in misorientation and chemical potential space. Interface transitions predicted with the model can be plotted on equilibrium phase diagrams to produce “interfacial phase diagrams.” These could be a tool for designing processing routes to optimize bulk, polycrystalline material properties through control of grain boundary characteristics.

Three-dimensional organization of rare-earth atoms at grain boundaries in silicon nitride

Used in the preparation of Si3N4 components, rare-earth elements promote the growth of needlelike grains essential to elevated toughness; evidently, La is significantly more effective than Lu. To explore this difference, we determine the three-dimensional organization of rare-earth atoms in the amorphous phase near prismatic interfaces in La- and Lu-containing Si3N4 using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and image processing. Evidence is presented for substantial atomic structure in notionally amorphous volumes. While the atomic arrangement in the amorphous phase conforms to the periodicity of the terminating crystal plane in both cases, the attachment sites are very different.

Arrangement of rare-earth elements at prismatic grain boundaries in silicon nitride

The arrangement of rare-earth atoms at {100} prism planes of La- and Lu-containing polycrystalline Si3N4 specimens is studied using high-angle annular dark-field scanning transmission electron microscopy. For both systems, the attachment sites of rare-earth atoms are well-defined and largely conform to the periodicity of the terminating plane of the Si3N4 grain. We observe significant differences between the structural arrangement of La and Lu atoms at the interface.

Composition and chemical width of ultrathin amorphous films at grain boundaries in silicon nitride

Two different electron energy loss spectroscopy (EELS) quantitative analytical methods for obtaining complete compositions from interface regions are applied to ultrathin oxide-based amorphous grain boundary (GB) films of ∼ 1 nm thickness in high-purity HIPed Si3N4 ceramics. The first method, 1, is a quantification of the segregation excess at interfaces for all the elements, including the bulk constituents such as silicon and nitrogen; this yields a GB film composition of SiN0.49±1.4O1.02±0.42 when combined with the average film thickness from high resolution electron microscopy (HREM). The second method, II, is based on an EELS near-edge structure (ELNES) analysis of the Si–L2,3 edge of thin GB films which permits a subtraction procedure that yields a completeEELS spectrum, e.g., that also includes the O–K and N–K edges, explicitly for the GB film. From analysis of these spectra, the film composition is directly obtained as SiN0.63±0.19O1.44±0.33, close to the one obtained by the first method but with much better statistical quality. The improved quality results from the fewer assumptions made in method II; while in method I uniform thickness and illumination condition have to beassumed, and correction of such effects yields an extra systematic error. Method II is convenient as it does not depend on the film thickness detected by HREM, nor suffer from material lost by preferential thinning at the GB. In addition, a chemical width for these films can be deduced as 1.33 ± 0.25 nm, that depends on an estimation of film density based on its composition. Such a chemical width is in good agreement with the structural thickness determined by HREM, with a small difference that is probably due to the different way in which these techniques probe the GB film. The GB film compositions are both nonstoichiometric, but in an opposite sense, this discrepancy is probably due to different ways of treating the surface oxidation layers in both methods.

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.

Plasma Etching of TEM Samples for Observing Grain Boundaries in Silicon Nitride

Plasma Etching of TEM Samples for Observing Grain Boundaries in Silicon Nitride

Grain boundaries in silicon nitride

Research on whisker/matrix interfaces and matrix grain boundaries in Si3N4 and Al2O3 composites reinforced with SiC whiskers by HREM imaging have shown that disordered layers exists in these regions. The disordered interfacial regions are often considered amorphous layers, particularly when they are thicker than ∼1nm. They appear to be discontinuous in the whisker/matrix interfaces and continuous in matrix grain boundaries. Thin amorphous layers in composites and conventionally synthesized ceramics, particularly those based on Si3N4, are expected to contain oxygen from sintering aids and powder particle surface impurities. In this paper we report the results of an investigation of silicon nitride matrix grain boundaries in Si3N4/SiC(w) composites (CMC) and polycrystalline CVD silicon nitride, using very high spatial resolution position resolved EELS and HREM imaging. Interfaces between SiC and silicon nitride in CMC materials have been discussed elsewhere.The CMC's were prepared by pressing and presintering Toyo-Soda α-silicon nitride (TS10) powder with 5.5 wt% Yttria and 1.1 wt% Alumina as sintering aids and 20 vol% β-SiC Huber whiskers at 1500°C in 0.1MPa Argon.