Sintering and Microstructure Formation of β-Silicon Carbide

The effect of additions of B, Al, and B + Al on the pressureless sintering of β-Sic was examined. The influence of the sintering atmosphere and heating schedule on densification behavior, polytype transformation, and microstructure development was also studied. High densities were obtained at 1940°C by the simultaneous addition of B and Al. The decrease in the sintering temperature is attributed to the presence of a liquid phase which results in the formation of platelets (up to 200 # in size) of an α-polytype, predominantly 4H and 6H. Polytype transformation and exaggerated grain growth could be prevented by annealing the compact at 1650° to 18500°C for 0.5 to 1 h. This procedure results in a better redistribution of the sintering aids, giving a fine-grained microstructure, constituted primarily of the cubic 3C polytype.     

Growth of Twinned β -Silicon Carbide Whiskers by the Vapor-Liquid-Solid Process

Whiskers of twinned SiC in the cubic form were produced by the vapor-liquid-solid process as a byproduct of the corrosion of silicon nitride. These whiskers were characterized and their properties related to their use in reinforced materials.     

Characterization of β-Silicon Nitride Whiskers

The physical, chemical, and structural properties of a commercially available β-Si₃N₄ whisker were characterized. Bulk chemical analysis indicated that the whiskers were close to stoichiometric silicon nitride, with oxygen and yttrium as the major impurities. Surface chemistry analysis by XPS analysis revealed that the surfaces consisted primarily of silicon nitride, with the oxygen and yttrium impurities concentrated at the surfaces. SEM and STEM studies indicated that the whiskers were dimensionally straight with relatively featureless surfaces, although some whiskers had Y-rich particles attached. The whiskers were also found to be of extreme crystallographic perfection, as determined by TEM analysis.     

Structural Analysis of Inclusions in β-Silicon Carbide Whiskers Grown from Rice Hulls

Microcrystalline inclusions in the core of β-SiC whiskers derived from the pyrolysis of rice hulls have been studied by transmission electron microscopy using conventional brightfield and dark-field imaging. The electron diffraction patterns from the whiskers show extra reflections arising from these inclusions. Dark-field images from these reflections are consistent with the presence of three different variants of inclusions, all of which are oriented with their [001] axes parallel to the heavily faulted [111] growth axis of the whiskers. A structural model for these inclusions is proposed which accounts satisfactorily for the extra reflections in the electron diffraction patterns.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

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.     

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.     

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.     

Origin of Density Gradients in Sintered β-Silicon Carbide Parts

Sintered densities of β-silicon carbide compacts were observed to decrease as the thickness of the compact increased. The chemistry and density gradients of these compacts were analyzed, as well as the weight loss and the CO evolution during sintering. The sintering cycle was altered to include a vacuum hold between the temperatures of 1400° and 1700°C, which resulted in increased density. Depending on the temperature of the vacuum soak, densities of 98% could be achieved on tiles with a thickness greater than 2.5 cm. The density gradients within the compact were significantly reduced, but not totally eliminated. It is suggested that these gradients are due to the effect of microstructural coarsening which occurs simultaneously with CO evolution.     

Standard Gibbs Energy of Formation of β-Silicon Nitride

Experimental thermochemical data (temperature, pressure) corresponding to the equilibrium conditions between finegrained β-SiC and β-Si₃N₄ for carbon activity a (C) = 1 are presented. Based on these data, the temperature dependence of ΔG°_f(β-Si₃N₄) has been expressed for standard states Si( s ), C( s ), and p(N₂) = 0.1 MPa by the equation ΔA°_f(β-Si₃N₄) = (-995.9 + 0.4547 T/K) kJ mol for T/K ε〈1650; 1968〉.     

Synthesis of Spherical β-Silicon Carbide Particles by Ultrasonic Spray Pyrolysis

Fine agglomerate-free spherical β-SiC powder was synthesized from a dispersion of colloidal silica, saccharose, and boric acid, by means of an ultrasonic spray pyrolysis method. Droplets of 2.2 μm were formed with an aerosol generator, operated at 2.5 MHz, and carried into a reaction furnace at 900°C with argon. Spherical X-ray amorphous gel particles of 1.1 μm were obtained. β-SiC particles with a mean diameter of 0.79 μm and spherical shape resulted when the SiC gel precursor particles were heated at 1500°C in argon.     

Microstructure Characterization of Gas-Pressure-Sintered β-Silicon Nitride Containing Large β-Silicon Nitride Seeds

Fine β-Si₃N₄ powders with or without the addition of 5 wt% of large β-Si₃N₄ particles (seeds) were gas-pressure sintered at 1900°C for 4 h using Y₂O₃ and Al₂O₃ as sintering aids. The microstructures were examined on polished and plasmaetched surfaces. These materials had a microstructure of in situ composites with similar small matrix grains and different elongated grains. The elongated grains in the materials with seeds had a larger diameter and a smaller aspect ratio than those in the materials without seeds. A core/rim structure was observed in the elongated grains; the core was pure β-Si₃N₄ and the rim was β-SiAION. These results show that the large β-Si₃N₄ particles acted as seeds for abnormal grain growth and the rim was formed by precipitation from the liquid containing aluminum.     

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%.     

Characterization of β-Silicon Carbide Powders Synthesized by the Carbothermal Reduction of Silicon Carbide Precursors

Boron-doped and nondoped ultrafine β-silicon carbide (β-SiC) powders were synthesized via the carbothermal reduction of SiC precursors at temperatures of 1773–1973 K. Although the reaction rate of carbothermal reduction was generally higher when a boron-doped precursor was used, the reaction rate for the boron-doped precursor was reduced considerably at 1873 K. For boron-doped and nondoped precursors, the reaction rates were almost the same. Powder characterization via transmission electron microscopy indicated that the suppression of the reaction rate for boron-doped precursor at 1873 K was due to the formation of a special coexistent system with two types of particle agglomerates. As expected, boron doping inhibited the particle growth in the synthesis of SiC powder.     

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.     

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.     

Densification and Properties of α-Silicon Carbide

Densification of a-Sic powders with no premixed sintering aids (type 1) and with premixed B and C (type 2) was investigated by sintering them at 2150° to 2200°C for 30 min. Flexure strengths, Weibull moduli, and fracture flaws were characterized for type 2 α-SiC only. The results were compared with those for a state-of-the-art sintered a-Sic material.     

Plasma-Induced Morphological Changes in α-Silicon Carbide

Significant alterations in the surface morphology of α-silicon carbide powders have been observed after exposure to a low-temperature Ar plasma. Surface alterations were observed in 3 min at 1900°C and 30 min at 1600°C. Powders heated with plasma exposure were characterized by contact development and particle rounding whereas without the plasma substantial faceting was observed at higher temperatures.