Mechanical Properties of Alumina/Silicon Carbide Whisker Composites

The improvement of mechanical properties of Al₂O₃/SiC whisker composites has been studied with emphasis on the effects of the whisker content and of the hot-pressing temperature. Mechanical properties such as fracture toughness and fracture strength increased with increasing whisker content up to 40 wt%. In the case of the high SiC whisker content of 40 wt%, fracture toughness of the sample hot-pressed at 1900° decreased significantly, in spite of densification, compared with one hot-pressed at 1850°. Fracture toughness strongly depended on the microstructure, especially the distribution of SiC whiskers rather than the grain size of the Al₂O₃ matrix.     

High-Temperature Failure Mechanisms of Hot-Pressed Si3N4 and Si3N4/Si3N4-Whisker-Reinforced Composites

The high-temperature flexural strength of hot-pressed silicon nitride (Si₃N₄) and Si₃N₄-whisker-reinforced Si₃N₄-matrix composites has been measured at a crosshead speed of 1.27 mm/min and temperatures up to 1400°C in a nitrogen atmosphere. Load–displacement curves for whisker-reinforced composites showed nonelastic fracture behavior at 1400°C. In contrast, such behavior was not observed for monolithic Si₃N₄. Microstructures of both materials have been examined by scanning and transmission electron microscopy. The results indicate that grain-boundary sliding could be responsible for strength degradation in both monolithic Si₃N₄ and its whisker composites. The origin of the nonelastic failure behavior of Si₃N₄-whisker composite at 1400°C was not positively identified but several possibilities are discussed.     

Mechanical Properties of β-Si3N4-Whisker/Si3N4-Matrix Composites

Composites containing 30 vol%β-Si₃N₄ whiskers in a Si₃N₄ matrix were fabricated by hot-pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m^1/2, compared to 4.0 MPa · m^1/2 for unreinforced polycrystalline Si₃N₄. The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms.     

Fractography, Mechanical Properties, and Microstructure of Commercial Silicon Nitride–Titanium Nitride Composites

Commercial-grade Si₃N₄–TiN composites with 0, 10, 20, and 30 wt% TiN content have been characterized. Submicrometer grain-size Si₃N₄ was reinforced with fine TiN grains. Density, Young's modulus, coefficient of thermal expansion, and fracture toughness increased linearly with TiN content. Increased strength was observed in the Si₃N₄+20 wt% TiN, and Si₃N₄+30 wt% TiN composites. Fractography was used to characterize the different types of fracture origins. Improvements in toughness and strength are due to residual stresses in the Si₃N₄ matrix and the TiN particles. A threefold improvement in dry wear resistance of the Si₃N₄+30 wt% TiN composite over the Si₃N₄ matrix was observed.     

Mechanical and Microstructural Characterization of Mullite and Mullite-SiC-Whisker and ZrO2-Toughened-Mullite—SiC-Whisker Composites

High-purity mullite-SiC-whisker composites and mullite-ZrO₂-SiC-whisker composites were fabricated in situ by hot-pressing using a matrix prepared by the alkoxide process. Varying degrees of ZrO₂ stabilization were achieved by varying amounts of Y₂O₃ or MgO addition. Microstructural characterization was accomplished using SEM and energy dispersive analysis. Room-temperature flexural strength and fracture toughness were determined as a function of SiC-whisker content (0% to 30%) and ZrO₂-stabilizer content. The flexural strength of mullite varied with composition and was increased ∼50% by the addition of ∼30% ZrO₂ phase. The flexural strength of mullite and mullite + 30% ZrO₂ was increased ∼50% for 30% SiC-whisker additions. The fracture toughness of mullite + 30% ZrO₂ was nearly twice that of mullite. For a 30% SiC-whisker addition, the fracture toughness of mullite was doubled, and the fracture toughness of mullite + 30% ZrO₂ was increased 25% to 50%.     

Spherical-Impact Damage and Strength Degradation in Silicon Carbide Whisker/Silicon Nitride Composites

The silicon carbide (SiC) whisker reinforcement of silicon nitride (Si₃N₄) improves fracture strength and toughness, hardness, and Young's modulus, resulting in higher resistance of the composites to sphere penetration and crack initiation at spherical impact. Sintered Si₃N₄ shows an elastic/plastic response and initiates median/radial cracks at 100 m/s impact velocity. SiC-whisker/Si₃N₄ composites, on the other hand, demonstrate an elastic response, with Hertzian cone crack initiation, only when impact velocity exceeds 280 m/s. The SiC-whisker/Si₃N₄ composites thus exhibit improved strength degradation versus critical impact velocity characteristics because of improved mechanical properties provided by the SiC whiskers.     

Combustion Synthesis of Silicon Nitride-Silicon Carbide Composites

The feasibility of synthesizing silicon nitride-silicon carbide composites by self-propagating high-temperature reactions is demonstrated. Various mixtures of silicon, silicon nitride, and carbon powders were ignited under a nitrogen pressure of 30 atm (∼ 3 MPa), to produce a wide composition range of Si₃N₄-SiC powder products. Products containing up to 17 vol% of SiC, after being attrition milled, could be hot-pressed to full density under 1700°C, 3000 psi (∼ 21 MPa) with 4 wt% of Y₂O₃. The microhardness and fracture toughness of these composites were superior to those of the pure β-Si₃N₄ matrix material and compared very well with the properties of "traditionally" prepared composites.     

Properties of Silicon Nitride–Molybdenum Disilicide Particulate Ceramic Composites

The physical and mechanical properties of hot-pressed Si₃N₄–MoSi₂ particulate composites containing 15 and 30 vol% MoSi₂ were studied. The average room-temperature four-point bend strength, fracture toughness, and electrical resistivity are 522 MPa, 3.6 MPa·√m, and 6.3 × 10⁵Χ·cm for the 15 vol% MoSi₂ composite, and 487 MPa, 5.3 MPa·√m, and 0.31 Ω·cm for the 30 vol% MoSi₂ composite. The mechanical properties of the composites are very close to those of hot-pressed Si₃N₄ ceramics. The high electrical conductivity of the 30 vol% MoSi₂ composite was attributed to the percolation effect of MoSi₂ particles. Parabolic oxidation behaviors were observed for the 30 vol% MoSi₂ composite during the 1200°C long-term oxidation experiments.     

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.     

Mechanical Properties of Si2N2O/SiC-Whisker Composites

The mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were studied with emphasis on the effect of matrix composition and of whisker content. The fracture toughness of Si₂N₂O was remarkably improved by 90% with a concomitant 70% strength improvement by addition of SiC whiskers of only 10 vol%. Optimum mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were obtained at an equimolar ratio of Si₃N₄/SiO₂, which is the stoichiometric composition for Si₂N₂O. Additional investigation concerning the Si₂N₂O-matrix/SiC-whisker interface by controlling sintering additives is necessary for further improvement of mechanical and thermal properties of Si₂N₂O/SiC-whisker composites.     

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.     

Mechanical Behavior of MoSi2 Reinforced–Si3N4Matrix Composites

The mechanical behavior of MoSi₂ reinforced–Si₃N₄ matrix composites was investigated as a function of MoSi₂ phase content, MoSi₂ phase size, and amount of MgO densification aid for the Si₃N₄ phase. Coarse-phase MoSi₂-Si₃N₄ composites exhibited higher room-temperature fracture toughness than fine-phase composites, reaching values >8 MP·am^1/2. Composite fracture toughness levels increased at elevated temperature. Fine-phase composites were stronger and more creep resistant than coarse phase composites. Room-temperature strengths >1000 MPa and impression creep rates of ∼10⁻⁸ s⁻¹ at 1200°C were observed. Increased MgO levels generally were deleterious to MoSi₂-Si₃N₄ mechanical properties. Internal stresses due to MoSi₂ and Si₃N₄ thermal expansion coefficient mismatch appeared to contribute to fracture toughening in MoSi₂-Si₃N₄ composites.     

Development of Reaction-Bonded Electroconductive Silicon Nitride-Titanium Nitride and Resistive Silicon Nitride-Aluminum Oxide Composites

Reaction-bonded Si₃N₄· TiN and Si₃N₄· Al₂O₃ composites were successfully fabricated by heating mixed powder compacts of Si and TiN or Si and Al₂O₃ in a nitrogen atmosphere. The former showed electrical conductivity, owing to the presence of TiN. An electrical resistivity of 2.6 × 10⁻⁵Ω· m was obtained for the Si₃N₄· TiN composite with 70 vol% TiN. The composite with 20 vol% TiN showed an electrical resistivity of 0.22 Ω· m and a bending strength of 460 MPa. On the other hand, the Si₃N₄· Al₂O₃ composite had insulating properties. The use of an appropriate amount of resin binder resulted in a higher green density and, consequently, a higher bending strength. Moreover, electroconductive Si₃N₄· TiN/resistive Si₃N₄· Al₂O₃ complex ceramics could be fabricated by heating green compacts composed of two different portions, one composed of mixed powders of Si and TiN and the other of Si and Al₂O₃. Attainment of such complex ceramics was attributed to the small dimensional change at the nitriding stage, under 0.3% and the similarity of the thermal expansion coefficients of the two composites.     

Powder Processing for the Fabrication of Si3N4 Ceramics: I, Influence of Spray-Dried Granule Strength on Pore Size Distribution in Green Compacts

The effect of spray-dried granule strength on the micro-structure of green compacts obtained by isostatic pressing was quantitatively analyzed. The fracture strength of single granules of Si₃N₄ powder made with ultrafine A1₂O₃ and Y₂O₃ powders was measured directly by diametral compression. It was found that fracture strength increased notably with the increasing relative density of the granule and the decreasing size of agglomerates in suspension before spray-drying. Even when green bodies were prepared at an isostatic pressure of 200 MPa, intergranular pores, which negatively affected densification of the sintered bodies, occurred between unfractured granules. The volume and size of these pores in the green compacts increased with the increasing fracture strength of the granules. In the case of closely packed granules, an isostatic pressure of 800 MPa was required to completely collapse the intergranular pores. A simple equation was derived to calculate the isostatic pressure necessary for complete collapse of intergranular pores in the green compacts, and it was determined that granule strength must be kept as low as possible to obtain uniform green compacts.     

Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites

Based on a biomimetic design, Si₃N₄/BN composites with laminated structures have been prepared and investigated through composition control and structure design. To further improve the mechanical properties of the composites, Si₃N₄ matrix layers were reinforced by SiC whiskers and BN separating layers were modified by adding Si₃N₄ or Al₂O₃. The results showed that the addition of SiC whiskers in the Si₃N₄ matrix layers could greatly improve the apparent fracture toughness (reaching 28.1 MPa·m^1/2), at the same time keeping the higher bending strength (reaching 651.5 MPa) of the composites. Additions of 50 wt% Al₂O₃ or 10 wt% Si₃N₄ to BN interfacial layers had a beneficial effect on the strength and toughness of the laminated Si₃N₄/BN composites. Through observation of microstructure by SEM, multilevel toughening mechanisms contributing to high toughness of the laminated Si₃N₄/BN composites were present as the first-level toughening mechanisms from BN interfacial layers as crack deflection, bifurcation, and pull-out of matrix sheets, and the secondary toughening mechanism from whiskers in matrix layers.     

Silicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites

SiC-monofilament-reinforced SiC or Si₃N₄ matrix composites were fabricated by hot-pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si₃N₄ matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si₃N₄ monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si₃N₄ matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si₃N₄ matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.     

Processing, Microstructure, and Wear Behavior of Silicon Nitride Hot-Pressed with Alumina and Yttria

Commercial silicon nitride powder with A1₂O₃ and Y₂O₃ additives was hot-pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining α-Si₃N₄. The aspect ratio of the elongated grains increased with increasing soak time at a fixed hot-pressing temperature. X-ray diffraction analysis showed that the crystalline phase in the hot-pressed samples was β-sialon (Si_6−zAl_zO_zN_8−z) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightly as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were conducted on a reciprocating pin-on-disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mechanical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third-body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusively identified. However, TEM examination showed clear evidence of dislocation motion under the wear scar. Grain boundary microstresses caused by the anisotropic thermal expansion and elastic properties of the elongated grains may have contributed to the observed pullout.     

Chemical-Vapor-Infiltrated Silicon Nitride, Boron Nitride, and Silicon Carbide Matrix Composites

Composites of carbon/chemical-vapor-deposited (CVD) Si₃N₄, carbon/CVD BN, mullite/CVD SiC, and SiC yarn/CVD SiC were prepared to determine if there were inherent toughness in these systems. The matrices were deposited at high enough temperatures to ensure that they were crystalline, which should make them more stable at high temperatures. The fiber-matrix bonding in the C/Si₃N₄ composite appeared to be too strong; the layers of BN in the matrix of the C/BN were too weakly bonded; and the mullite/SiC composite was not as tough as the SiC/SiC composites. Only the SiC yarn/CVD SiC composite exhibited both strength and toughness.     

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.     

R-Curve Behavior of Si3N4–BN Composites

R -curve behavior of Si₃N₄–BN composites and monolithic Si₃N₄ for comparison was investigated. Si₃N₄–BN composites showed a slowly rising R -curve behavior in contrast with a steep R -curve of monolithic Si₃N₄. BN platelets in the composites seem to decrease the crack bridging effects of rod-shaped Si₃N₄ grains for small cracks, but enhanced the toughness for long cracks as they increased the crack bridging scale. Therefore, fracture toughness of the composites was relatively low for the small cracks, but it increased significantly to ∼8 MPa·m^1/2 when the crack grew longer than 700 μm, becoming even higher than that of the monolithic Si₃N₄.