Mechanical Properties of Unidirectionally Oriented SiC-Whisker-Reinforced Si3N4 Fabricated by Extrusion and Hot-Pressing

SiC-whisker-reinforced Si₃N₄ was fabricated by extrusion and hot-pressing. A unidirectional alignment of the whiskers was achieved through sheet forming by extrusion. The degree of whisker orientation changed with the thickness of the green sheets. Unidirectionally oriented whiskers increased fracture strength and toughness compared to samples with more randomly oriented whiskers. Anisotropy of fracture strength was observed. Bridging by whiskers impeded crack propagation when the whisker orientation was perpendicular to the crack plane.     

Si3N4/SiC-Whisker Composites without Sintering Aids: I, Quantitative Evaluation of Microstructure

A microstructural evaluation of Si₃N₄ with 20 vol% SiC whiskers, fully densified by hot isostatic pressing (HIP) without sintering aids, is presented. The grain size and morphology of the matrix, the whisker aspect ratio after sintering, interfacial bonding, and the structural stability of reinforcement up to 2000°C are discussed. Image analysis provides quantitative information about whisker dispersion and orientation. It is pointed out that a whisker dispersion and orientation. It is pointed out that a whisker composite with a high degree of homogeneity and isotropy can be obtained by optimizing the mixing procedure and using HIP.     

SiC-Whisker-Reinforced Al2O3 Composites by Free Sintering of Coated Powders

SiC whiskers were coated with a thick cladding of finegrained Al₂O₃ powder by controlled heterogeneous precipitation in a concentrated suspension of whiskers. After calcination, the coated whiskers were compacted by cold isostatic pressing and sintered at a constant heating rate of 5°C/min in a helium atmosphere. The parameters which control the coating process and the sintering characteristics of the consolidated powders are reported. Starting with an initial matrix density of 40–45% of the theoretical, composites containing up to ≅20 vol% whiskers (aspect ratio ≅15) were sintered freely to nearly theoretical density below 1800°C. By comparison, a similar composite formed by mechanical mixing of the whiskers and the precipitated Al₂O₃ powder reached a density of only 68% of the theoretical after sintering under identical conditions. For a fixed whisker content, the sinterability of the composites formed from the coated whiskers shows a fairly strong dependence on the whisker aspect ratio.     

Slip Casting of SiC-Whisker-Reinforced Si3N4

Si₃N₄ composite materials containing up to 20 vol% SiC whiskers were slip cast and pressureless sintered at 1820°C and 0.13 MPa of N₂. Viscosimetry showed no influence of whisker loading on the rheology of the highly concentrated aqueous slips up to 15 vol% whiskers. During casting the whiskers were preferentially aligned parallel to the mold surfaces. Depending on the whisker loading, green densities of 0.64 to 0.69 fractional density could be achieved. Strong anisotropic shrinkage occurred during sintering with a maximum linear shrinkage of 21% perpendicular but only 7% parallel to the whisker plane. With increasing whisker content from 0 to 20 vol% sintered densities decreased from 0.98 to 0.88, respectively.     

Investigation of whisker orientation in SiC whisker-reinforced alumina composites using neutron diffraction

Alumina composites with 10, 20 and 30 vol.% SiC whiskers were fabricated using colloidal processing methods followed by uniaxial hot pressing. Orientation of whiskers was quantified using neutron diffraction methods that allow one to probe the bulk of the composite samples. The results indicate that significant whisker alignment was present even in the green samples and that hot pressing further enhanced this alignment but not to a substantial extent. Although slightly enhanced whisker rotation was observed with increase in whisker content in the alumina matrix, the overall degree of whisker alignment in the final products was not strongly dependent on pH of the slurry and whisker volume fraction.     

Fracture of Si3N4–SiC Whisker Composites under Cyclic Loads

This communication demonstrates the role of cyclic compressive loads in inducing mode I fatigue crack growth at room temperature in Si₃N₄ matrix–SiC whisker composite materials containing stress concentrations. The characteristics of stable, cyclic fracture are examined for several volume fractions of the SiC whisker and are compared with those of the matrix material. It is found that the composites with higher volume fractions of SiC whiskers exhibit an inferior resistance to fracture under cyclic compressive loads despite improvements in fracture toughness values.     

Crack Deflection as a Toughening Mechanism in SiC-Whisker-Reinforced MoSi2

Crack propagation in SiC-whisker-reinforced MoSi₂ was studied. In particular, the deflection angles of the cracks were examined to determine the degree to which they are affected by the whisker reinforcement. The composite studied was hot-pressed MoSi₂ with 20 vol% vapor-liquid-solid β-SiC whiskers. A substantial difference was found between the deflection angles of cracks formed in the reinforced MoSi₂ and those in a control sample with no whiskers, showing the process of crack deflection as an important, but not the only, toughening mechanism.     

Synthesis and Characterization of Ti0.33Ta0.33Nb0.33C and Ti0.33Ta0.33Nb0.33CxN1-x Whiskers

Ta_0.33Ti_0.33Nb_0.33C and Ta_0.33Ti_0.33Nb_0.33C _x N_{1− x} whiskers were synthesized via a carbothermal vapor-liquid-solid growth mechanism in the temperature range 900°-1450°C in Ar or N₂. The optimum temperature was 1250°C. Whiskers were obtained in a yield of 70-90 vol%. The whiskers were 0.5–1 µm in diameter and 10–30 µm in length. The starting materials that produced the highest whisker yield were: TiO₂, Ta₂O₅, Nb₂O₅, C, Ni, and NaCl. C was added to reduce the oxides, and Ni to catalyze whisker growth. NaCl was used as a source of Cl for vapor-phase transportation of Ta and Nb oxochlorides and Ti chlorides to the catalyst. The catalyst metal was recycled several times during the synthesis and was transported as NiCl₂( g ) according to thermodynamic calculations. The rate of formation and the chemical composition of the whiskers depended on the synthesis temperature, the choice of catalyst, and the atmosphere. At low temperatures, the whiskers were enriched in Nb and Ta, whereas the Ti content increased with increased synthesis temperature.     

Effects of Temperature and Whisker Volume Fraction on Average Residual Thermal Strains in a SiC/Al2O3 Composite

Residual thermal strains in a SiC-whisker-reinforced/Al₂O₃-matrix composite were measured as functions of temperature and volume fraction of whiskers by means of a neutron diffraction technique. The residual strains in both phases decrease with increasing temperature. At room temperature, the residual strains in the whiskers decrease with increasing whisker volume fraction, but the tensile residual strains in the matrix increase. The results agree well with those predicted by an analytical method.     

Synthesis of Porous Si3N4 Ceramics with Rod-Shaped Pore Structure

Porous Si₃N₄ ceramics were synthesized by pressureless sintering of green compacts prepared using slip casting of slurries containing Si₃N₄, 5 wt% Y₂O₃+2 wt% Al₂O₃, and 0–60% organic whiskers composed of phenol–formaldehyde resin with solids loading up to 60 wt%. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800°C in air and sintered at 1850°C in a N₂ atmosphere. Porosities ranging from 0% to 45% were obtained by the whisker contents (corresponding to 0–60 vol% whisker). Samples exhibited a uniform pore distribution. Their rod-shaped pore morphology originated from burnout of whiskers, and an extremely dense Si₃N₄ matrix.     

Suspension Processing of Al2O3/SiC Whisker Composites

Homogeneous Al₂O₃ powder/SiC whisker compacts were prepared by suspension processing. By optimizing the conditions for particle/whisker codispersion, castable suspensions could be prepared at total-solids concentrations 50 vol%. Green bodies with high relative density (∼66% to 70%) were obtained with SiC whisker contents in the range of 5 to 30 vol%. Although densification was severely inhibited by the SiC whiskers, significantly higher sintered densities were obtained by suspension processing compared to dry processing.     

Microstructure and Densification of Pressureless-Sintered Al2O3/Si3N4-Whisker Composites

Composite densification was studied by performing slip casting and sintering experiments on an Al₂O₃ matrix and Si₃N₄ whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si₃N₄ affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.     

Si3N4/SiC-Whisker Composites without Sintering Aids: II, Fracture Behavior

The fracture behavior of an Si₃N₄/SiC-whisker composite fabricated without sintering aids is investigated using a double approach based on the examination of R -curve behavior and a statistical analysis of crack propagation. In the composite with 20 vol% whisker, a 30% increase in toughness over the matrix value can be attributed to crack-tip phenomena. Strong interfacial bonding prevents any contribution to toughening by mechanisms operating in the wake region of the crack. Based on experimental observations of microfracture in both SiC whiskers and Si₃N₄ grains, toughening caused by crack-tip phenomena is quantitatively discussed in terms of fracture energy and whisker-distribution parameters.     

Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface-Induced Coating

Al₂O₃–ZrO₂–SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO₂ phase on the kinetically stable colloid particles of Al₂O₃ and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO₂ and SiC whisker phases throughout the Al₂O₃ matrix. The fracture toughness values of the Al₂O₃–15 vol% ZrO₂–20 vol% SiC whisker composites (∼12 MPa.m^1/2) are substantially greater than those of comparable Al₂O₃–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites.     

定向排布的SiC晶须补强Si3N4复合材料的制备

通过挤制成型和热压烧结成功地制备了具有很好的一维定向排布的ＳｉＣ晶须补强Ｓｉ３Ｎ４复合材料，并从力学上分析了挤制成型过程中流体的受力状态，使晶须定向排布的原因以及影响晶须定向度的因素。     

Two-Dimensional Whisker Percolation in Ceramic Matrix–Ceramic Whisker Composites

A computer model that treats ceramic-powder matrix–ceramic whisker composites as a percolative network of whiskers has been developed. The model calculates the critical fraction of whiskers at the percolation threshold for a two-dimensional random network of whiskers. The computed critical fraction was found to display an inverse dependence on whisker aspect ratio. In addition, the computed critical fraction (27 vol% for a whisker aspect ratio of 7) agreed well with the zero-shrinkage whisker fraction of 30 vol% in the densification of a colloid-pressed alumina–silicon carbide composite that exhibited a two-dimensional orientation of such whiskers.     

Nitridation of Silicon Containing MgO and CeO2 Powders and SiC Whiskers

Processing and nitridation of MgO/CeO₂-doped silicon containing SiC whiskers was studied. The reaction time was dramatically reduced by incorporating MgO/CeO₂ dopants and SiC whiskers in silicon powders prior to nitridation. The decomposition of SiC whiskers depends on the nitriding temperature, sintering aids, and whisker surface composition.     

Separate Growth of α- and β-Si3N4 Whiskers on or near a Carbon Substrate by Carbothermal Reduction

Whiskers of α- and β-Si₃N₄ were grown on or near a carbon black substrate, respectively, 10 mm downstream from a mixed starting powder of low-grade silica and carbon, in flowing nitrogen gas at 1400°C. The parameters (flowing nitrogen gas rate, growth time, grade of silica, and type of carbon) that promoted growth of the whiskers were examined in view of increasing the whisker yield. The shapes and sizes of both types of whiskers were observed by scanning electron microscopy (SEM). The separate growth of the whiskers is discussed here, based on X-ray diffraction analysis and SEM observation with energy-dispersive X-ray analysis.     

Effects of Boron, Carbon, and Iron Content on the Stacking Fault Formation during Synthesis of β-SiC Particles in the System SiO2-C-H2

Through the systematic addition of B, C, and Fe, additive effects on the stacking fault formation and the morphology of the particles formed during reaction synthesis of β-SiC were investigated in the present study. The whisker content of the synthesized product and the formation mechanism of whiskers were closely related to the stacking fault density. The addition of B inhibited whisker formation probably because of isotropic imperfection and the suppression of surface diffusion. Increase in the reduction force by using an active carbon source and also adding excess carbon led to an increase in stacking fault density through enhanced whisker formation. In the presence of Fe, the synthesized β-SiC whiskers appeared to possess only a small amount of stacking faults. The growth mechanism was different with Fe; i.e., isotropic growth occurred via a dissolution-precipitation reaction through a liquid phase in the Fe-Si system.     

Effect of the size of Sic whiskers on the toughness of alumina-based composite materials

The effect of the dimensions of SiC whiskers on the strength and toughness of hot-pressed composites with a 20% volume fraction of whisker reinforcement was studied. It was established that the mechanical properties of the composites were improved only on introducing whiskers with the diameters, within 1.5–2.5 μm. These materials also exhibited a higher resistance to crack propagation. Crystals of smaller diameter had a negative effect on the mechanical properties, which was explained by bundle formation. There is a definite critical content of whiskers ensuring their uniform distribution in the matrix, and this value decreases on reducing the whisker diameter. A correlation was established between composite toughness and the level of average residual compressive stresses in the whiskers.