Nonequiaxial Grain Growth and Polytype Transformation of Sintered α-Silicon Carbide and β-Silicon Carbide

α(6 H )- and β(3 C )-SiC powders were sintered with the addition of AlB₂ and carbon. α-SiC powder could be densified to ∼98% of the theoretical density over a wide range of temperatures from 1900° to 2150°C and with the additives of 0.67–2.7 mass% of AlB₂ and 2.0 mass% of carbon. Sintering of the β-SiC powder required a temperature of >2000°C for densification with these additives. Grains in the α-SiC specimens grew gradually from spherical-shaped to plate-shaped grains at 2000°C; the 6 H polytype transformed mainly to 4 H . On the other hand, grains in the β-SiC largely grew at >2000°C; the 3 C polytype transformed to 4 H , 6 H , and 15 R . The stacking faults introduced in grains were denser in β-SiC than in α-SiC. The rapid grain growth in the β-SiC specimen was attributed to polytype transformation from the unstable 3 C polytype at the sintering temperature.     

Side-Surface Structure of a Commercial β-Silicon Carbide Whisker

The side surfaces of a commercial β-SiC whisker were analyzed by calculating the surface energy and observing the microstructure of the whiskers. The results indicated that the side surfaces displayed a type of zigzag structure and were composed of {111}, {110}, and {100} crystal planes.     

Effect of β-to-α Phase Transformation on the Microstructural Development and Mechanical Properties of Fine-Grained Silicon Carbide Ceramics

Ultrafine β-SiC powders mixed with 7 wt% Al₂O_3, 2 wt% Y₂O₃, and 1.785 wt% CaCO₃ were hot-pressed and subsequently annealed in either the absence or the presence of applied pressure. Because the β-SiC to α-SiC phase transformation is dependent on annealing conditions, the novel processing technique of annealing under pressure can control this phase transformation, and, hence, the microstructures and mechanical properties of fine-grained liquid-phase-sintered SiC ceramics. In comparison to annealing without pressure, the application of pressure during annealing greatly suppressed the phase transformation from β-SiC to α-SiC. Materials annealed with pressure exhibited a fine microstructure with equiaxed grains when the phase transformation from β-SiC to α-SiC was <30 vol%, whereas materials annealed without pressure developed microstructures with elongated grains when phase transformation was >30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mechanical properties could be achieved through annealing with or without pressure.     

Thermoelastic Micromechanical Stresses Associated with a Large α-Silicon Carbide Grain in a Polycrystalline β-Silicon Carbide Matrix

The thermoelastic micromechanical stresses associated with a single large hexagonal α-SiC grain within a fine-grain-size cubic (3C) β-SiC matrix were calculated. The naturally occurring residual stresses which are created during cooling from the processing temperatures and the effects of superimposed applied external stresses are both considered. A significant effect of the shape or geometry of the α-SiC grain is revealed, with the largest residual stresses associated with the naturally occurring tabular or platelet structure. The stresses are compared with the published strength results for these materials, which suggests that the residual stresses assume a significant role in the strength reduction that is observed.     

Microstructural Development During Gas-Pressure Sintering of α-Silicon Nitride

Gas-pressure sintering of α-Si₃N₄ was carried out at 1850 ° to 2000°C in 980-kPa N₂. The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si₃N₄ powder was similar to that in the materials from β-Si₃N₄ powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si₃N₄ ceramics.     

Morphology and Stacking Faults of β-Silicon Carbide Whisker Synthesized by Carbothermal Reduction

The main formation reaction for whisker that has been synthesized from SiO₂ and carbon black (CB) in a hydrogen-gas atmosphere was a solid–gas reaction between SiO and CB. The synthesized whiskers were classified into three types, in terms of the morphology, growth direction, and stacking-fault planes: (i) type A, which has a relatively flat surface and the stacking-fault planes are perpendicular to the growth direction; (ii) type B, which has a rough surface and the stacking-fault planes are inclined at an angle of 35° to the growth direction; and (iii) type C, which has a rough sawtooth surface and the stacking faults exist concurrently in three different {111} planes. The observed angles in the deflected and branched whiskers were 125°, 70°, and 109°. These whiskers were composed of mixtures of type A and type B, type A only, or parallel growth by two pairs of type A and type B whiskers. The whisker deflection was closely related to the difference in the growth speed of each type of whisker.     

Phase Transformation and Texture in Hot-Forged or Annealed Liquid-Phase-Sintered Silicon Carbide Ceramics

Starting from three powder mixtures of 80 vol% SiC (100α, 50α/50β, 100β) and 20 vol% YAG, liquid-phase-sintered silicon carbide ceramics were prepared by hot pressing at 1800°C for 1 h under 25 MPa, and then by hot forging or annealing at 1900°C for 4 h under an applied stress of 25 MPa in argon. The phase transformation and texture development in the as-hot-pressed, hot-forged, and annealed SiC ceramics were investigated via X-ray diffraction (XRD) and the pole figure measurements. The 6H → 4H polytypic transformation was observed in samples consisting of both α- and β-SiC phases when subjected to compressive deformation but absent in the case of annealing, suggesting the deformation-enhanced solubility of aluminum in SiC. Deformation was also found to enhance the 3C → 4H transformation in the sample containing entirely β-phase, which is due to the accelerated solution-precipitation process assisted by grain boundary sliding. The current study showed that the β- →α-phase transformation had little effect on texture development in SiC. Hot forging generally produced the strongest texture, with the calculated maximum of 2.2 times random in samples started with pure α-SiC phase. The mechanism for texture development was explained based on the microstructural observations.     

Single-Step Synthesis of Chemically Cross-Linked Polysilastyrene and Its Conversion to β-Silicon Carbide

A new method for chemically cross-linking polysilastyrene using divinylbenzene as the cross-linking agent is reported. The procedure involves a single-step synthesis using the alkali-metal sodium to promote the polymerization of dimethyldichlorsilane in the presence of the comonomers phenylmethyldichlorosilane and divinylbenzene. The cross-linked polymer can be readily converted to β-SiC on pyrolysis at 1500°C. The β-SiC obtained by this procedure is nanocrystalline and has a grain-size distribution of 8–20 nm.     

Formation of Silica-Coated Carbon Powder and Conversion to Spherical β-Silicon Carbide by Carbothermal Reduction

Spherical β-SiC powders that are a few micrometers in size have been prepared by heating a mixture of phenolic resin powder and fine-grained fumed silica at 1600°C in argon. The overall process is composed of two consecutive steps: (i) the formation of silica-coated spherical carbon powder and (ii) carbothermal reduction. The irregularly shaped resin powder transforms to a spherical-shaped morphology in the first step, and the resulting silica-coated spherical carbon powder is converted to β-SiC in the second step. The key factor in the first step is the utilization of fumed silica that has hydrophobic surface functional groups. Hydrophobic interactions at the point of intimate contact between the resin powder and the silica likely reduce the surface energy of the resin powder, thereby discouraging interparticle coalescence. The resulting β-SiC powder exhibits a radially developed columnar microstructure. Hollow β-SiC spheres also can be prepared by controlling the reaction conditions in the carbothermal reduction step.     

Synthesis of Monodispersed Spherical β-Silicon Carbide Powder by a Sol-Gel Process

Monodispersed spherical β-SiC powder was synthesized by heating spherical gel powder derived from the hydrolysis of a mixture of phenyltriethoxysilane and tetraethyl orthosilicate. The solution was prepared in a beaker and hydrolyzed by NH₄OH without stirring. The monodispersed spherical gel powder of submicrometer size was obtained when the added amount of NH₄OH was greater than 16 moles per mole of silane plus alkoxide, and it became monodispersed spherical β-SiC powder by heat-treating at 1500°C for 4 h in an Ar atmosphere. The SiC content of the powder was 92.6 wt%.     

Microstructural Changes in β-Silicon Nitride Grains upon Crystallizing the Grain-Boundary Glass

Crystallizing the grain-boundary glass of a liquid-phase-sintered Si₃N₄ ceramic for 2 h or less at 1500° led to formation of δ-Y₂Si₂O₇. After 5 h at 1500°, the δ-Y₂Si₂O₇ had transformed to β-Y₂Si₂O₇ with a concurrent dramatic increase in dislocation density within β-Si₃N₄ grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500° reduced dislocation densities to the levels found in as-sintered material.     

Grain Growth During Gas-Pressure Sintering of β-Silicon Nitride

The microstructures of gas-pressure-sintered materials from β-Si₃N₄ powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850° to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The rate-determining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from α-Si₃N₄ powder despite the smaller aspect ratio of the grains     

Effect of Annealing Conditions on Microstructural Development and Phase Transformation in Silicon Carbide

The effect of annealing with and without applied pressure on the microstructural development and phase transformation was investigated in fine-grained β-SiC ceramics containing α-SiC seeds. Materials annealed without pressure had a microstructure consisting of elongated grains, while materials annealed with pressure showed a duplex microstructure consisting of small matrix grains and some of elongated grains. However, annealing with pressure (25 MPa) was found to greatly retard phase transformation from β→α polytypes and inhibit grain growth. This change in lattice parameter suggests that the retardation of phase transformation and grain growth might be attributed to a reduced mass transport rate, which is the result of Al being introduced into the SiC by the annealing pressure.     

Influence of Pyridine–Borane on the Sintering Behavior and Properties of α-Silicon Carbide

In the present study, α-SiC powder is coated with pyridineborane (BH₃·C₅H₅N), a liquid molecular compound, which forms a boron carbonitride (BC_3.5N) layer by heat treatment at 1000°C under argon. The precipitation method leads to an improved chemical homogeneity in the compacted powder resulting in enhanced densification and significant reduction in grain growth during subsequent sintering at temperatures exceeding 2070°C. Thus, small average grain sizes of d ₅₀= 1.3 μm and a narrow grain size distribution ( d ₁₀= 0.6 μm, d ₉₀= 2.2 μm) are detected in the liquid-phase-processed sample sintered at 2200°C for 0.5 h in argon. Final densities of at least 98% of theoretical could be obtained by pressureless sintering at 2100°C. These results as well as the microstructural distribution of the sintering aids in the densified samples are discussed.     

Compression Deformation Mechanism of Silicon Carbide: I, Fine-Grained Boron- and Carbon-Doped β-Silicon Carbide Fabricated by Hot Isostatic Pressing

The deformation behavior of boron- and carbon-doped β-silicon carbide (B,C-SiC) with an average grain size of 260 ± 18 nm containing 1 wt% boron was investigated by compression testing at elevated temperatures. Extensive grain growth during deformation was observed. The stress–strain curves were compensated for grain growth by assuming power-law type of dependence on grain size and strain rate. The stress exponent n was ∼1.3 and the grain size exponent p was ∼2.7 at temperatures ranging from 1593° to 1758°C. The apparent activation energy of deformation Q _d was ∼760 kJ/mol, which was lower than the activation energy for lattice diffusion of silicon and carbon in SiC and higher than that for grain-boundary diffusion of carbon in SiC. These results suggest that the deformation mechanism of the fine-grained B,C-SiC is grain-boundary sliding accommodated by the grain-boundary diffusion.     

Elevated-Temperature Fracture Resistance of a Sintered α-Silicon Carbide

The fracture toughness of a dense, sintered commercial α-silicon carbide was determined for temperatures from 20° to 1400°C using both straight- and chevron-notched test specimens and also controlled-surface-microflaw specimens, all in three-point bending. The flexural strengths were also measured for the same range of temperatures and the trend is compared with that of the toughness. Measurements from this study are discussed and also compared with other results in the literature. Analysis reveals the importance of contrasting sharp crack and blunt crack techniques and also the need for addressing the microhardness indentation method separately. It is concluded that the fracture toughness of this silicon carbide is about 3 MPa · m½ and that the crack growth resistance is characterized by a flat R -curve behavior, both of which are independent of temperature from 20° to 1400°C.     

Effect of Sintering Atmosphere on Grain Shape and Grain Growth in Liquid-Phase-Sintered Silicon Carbide

We investigated the effects of the sintering atmosphere on the interface structure and grain-growth behavior in 10-vol%-YAG-added SiC. When α-SiC was liquid-phase-sintered in an Ar atmosphere, the grain/matrix interface was faceted, and abnormal grain growth occurred, regardless of the presence of α-seed grains. In contrast, when the same sample was sintered in N₂, the grain interface was defaceted (rough), and no abnormal grain growth occurred, even with an addition of α-seed grains. X-ray diffraction analysis of this sample showed the formation of a 3C (β-SiC) phase, together with a 6H (α-SiC) phase. These results suggest that the nitrogen dissolved in the liquid matrix made the grain interface rough and induced normal grain growth by an α→β reverse phase transformation. Apparently, the growth behavior of SiC grains in a liquid matrix depends on the structure of the grain interface: abnormal growth for a faceted interface and normal growth for a rough interface.     

Microstructural Changes in Liquid-Phase-Sintered Silicon Carbide during Creep in an Oxidizing Environment

The knowledge of the microstructural evolution during exposure to high temperatures is important to understanding the mechanisms responsible for the creep resistance of silicon carbide (SiC) ceramics. This includes not only the phase transformation of the SiC grains, but also the phase transformations of the oxynitride grain-boundary phases. For this study, a series of SiC specimens were prepared with varying molar ratios of AlN-Y₂O₃ additives. Increased creep resistance was observed in specimens with an additive system containing a 2:3 molar ratio or 60 mol% Y₂O₃. A continuous oxide layer of Y₂Si₂O₇ formed at the surface during elevated temperature testing in air. No blistering or cracking was observed in this oxide coating. Further increase of the creep resistance was achieved by a post-sintering nitrogen anneal.     

High-Resolution Electron Microscopy Observations of Stacking Faults in β-SiC

Structural images of the stacking faults in β-SiC were obtained with a high-resolution electron microscope. Stacking faults initially present in β-SiC powder particles were eliminated as grain growth proceeded at elevated temperatures.