Fracture Energies of Tape-Cast Silicon Nitride with β-Si3N4 Seed Addition

The fracture energies of the tape-cast silicon nitride with and without 3 wt% rod-like β-Si₃N₄ seed addition were investigated by a chevron-notched-beam technique. The material was doped with Lu₂O₃–SiO₂ as sintering additives for giving rigid grain boundaries and good heat resistance. The seeded and tape-cast silicon nitride has anisotropic microstructure, where the fibrous grains grown from seeds were preferentially aligned parallel to the casting direction. When a stress was applied parallel to the fibrous grain alignment direction, the strength measured at 1500°C was 738 MPa, which was almost the same as room temperature strength 739 MPa. The fracture energy of the tape-cast Si₃N₄ without seed addition was 109 and 454 J/m² at room temperature and 1500°C, respectively. On the contrary, the fracture energy of the seeded and tape-cast Si₃N₄ was 301 and 781 J/m² at room temperature and 1500°C, respectively, when a stress was applied parallel to the fibrous gain alignment. The large fracture energies were attributable primarily to the unidirectional alignment fibrous Si₃N₄ grains.     

Friction and Wear Properties of Si3N4/Carbon Fiber Composites with Aligned Microstructure

The friction and wear properties of silicon nitride/carbon fiber composites have been assessed and compared with monolithic Si₃N₄. Three different types of composites have been produced; one in which both the Si₃N₄ grains and the carbon fibers were aligned, one in which only the fibers had alignment, and a third where both the grains and fibers had random orientation. The friction coefficients of all of the composites, following running in, were around 0.2–0.3, typically less than one-third of that of the monolithic material. However there was no significant difference in friction coefficient between the three different types of composite. The specific wear rates of all the materials decreased with sliding distance and those of the composites were lower than the monolithic material. Among the composites, the wear rate of the one with aligned fibers in a randomly oriented Si₃N₄ matrix showed no dependence on sliding direction relative to the fiber alignment, and the specific wear rates of these samples were similar to that of the randomly oriented fiber composite, indicating little effect of fiber alignment alone on the wear properties under the present testing conditions. However, the specific wear rate of the composite with both fiber and grain alignment showed directional dependence. Grain cracking was observed perpendicular to the sliding direction, and the Spara specimen, in which the sliding direction was parallel to the Si₃N₄ grain alignment, showed higher wear rates than the Sperp and N samples of this composite. Such cracks are perpendicular to the major axis of the grains in the Spara sample and are thought to lead to easier removal of grains following their cracking under the tensile stresses induced particularly during the running in period.     

Processing and Tribological Properties of Si3N4/Carbon Short Fiber Composites

Si₃N₄/carbon fiber composites were fabricated using several types of fiber. All the composites had higher fracture toughness compared with monolithic Si₃N₄ ceramics. Tribological properties were investigated by a ball-on-disk method under unlubricated conditions. The composite containing fibers with a high orientation of graphite layers and high graphite content indicated a low friction coefficient. It was identified, by Raman spectroscopy, that graphite was transferred from the composite to the Si₃N₄ ball of the counterbody during the wear test. This transferred layer was effective for producing the low friction behavior of the composite.     

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.     

Relation Between Strength, Fracture Energy, and Microstructure of Hot-Pressed Si3N4

A fracture mechanics approach was used to investigate the high strength of hot-pressed Si₃N₄. Room-temperature flexural strengths, fracture energies, and elastic moduli were determined for material fabricated from α- and β-phase Si₃N₄ powders. When the proper powder preparation technique was used, α-phase powder resulted in a high fracture energy (69,000 ergs/cm²), a high flexural strength (95,000 psi), and an elongated (fiberlike) grain morphology, whereas β-phase powder produced a low fracture energy (16,000 ergs/cm²), a relatively low strength (55,000 psi), and an equiaxed grain morphology. It was hypothesized that the high strength of Si₃N₄ hot-pressed from α-phase powder results from its high fracture energy, which is attributed to the elongated grains. High-strength Si₃N₄ has directional properties caused, in part, by the elongated grain structure, which is oriented preferentially with respect to the hot-pressing direction.     

Tribological Behavior of Si3N4 and Si3N4/Carbon Fiber Composites Against Stainless Steel Under Water Lubrication for a Thrust-Bearing Application

The tribological behavior of Si₃N₄ ceramics and Si₃N₄/carbon fiber composites sliding against stainless steel under water lubrication was investigated using a thrust-bearing-type test method with normal applied loads varying from 0 to 1000 N in 100 N increments. In the case of the monolithic Si₃N₄, the friction coefficient was found to increase up to 0.4 the first time the applied load was increased from 100 to 200 N, and sudden failure of the ceramic ring specimen occurred. In the case of the Si₃N₄/carbon fiber composite, a low friction coefficient was maintained up to the maximum normal load of 1000 N. The addition of the carbon fibers to the silicon nitride ceramics effectively restricts material transfer from the stainless steel to the Si₃N₄ worn surface due to reduction of solid–solid contact through the solid lubricating effect of the carbon fibers.     

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.     

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.     

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 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.     

Microstructure and Short-Term Oxidation of Hot-Pressed Si3N4/ZrO2(+Y2O3) Ceramics

Composite ceramic materials based on Si₃N₄ and ZrO₂ stabilized by 3 mol% Y₂O₃ have been formed using aluminum isopropoxide as a precursor for the Al₂O₃ sintering aid. Densification was carred out by hot-pressing at temperatures in the range 1650° to 1800°C, and the resulting micro-structures were related to mechanical properties as well as to oxidation behavior at 1200°C. Densification at the higher temperatures resulted in a fibrous morphology of the Si₃N₄ matrix with consequent high room-temperature toughness and strength. Decomposition of the ZrO₂ grains below the oxidized surface during oxidation introduced radial stresses in the subscalar region, and from the oxidation experiments it is suggested that the ZrO₂ incorporated some N during densification.     

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.     

Characteristics of ZrO2-Dispersed Si3N4 without Additives Fabricated by Hot Isostatic Pressing

Dense, ZrO₂-dispersed Si₃N₄ composites without additives were fabricated at 180 MPa and ∼1850° to 1900°C for l h by hot isostatic pressing using a glass-encapsulation method; the densities reached >96% of theoretical. The dispersion of 20 wt% of 2.5YZrO₂ (2.5 mol% Y₂O₃) in Si₃N₄ was advantageous to increase the room-temperature fracture toughness (∼7.5 MPa˙m^1/2) without degradation of hardness (∼15 GPa) because of the high retention of tetragonal ZrO₂. The dependence of fracture toughness of Si₃N₄–2.5YZrO₂ on ZrO₂ content can be related to the formation of zirconium oxynitride because of the reaction between ZrO₂ and Si₃N₄ matrix in hot isostatic pressing.     

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.     

Controlled Surface Flaws in Hot-Pressed Si3N4

Surface flaws of controlled size and shape were produced in high-strength hot-pressed Si₃N₄ with a Knoop microhardness indenter. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a 4-point-bend specimen, with attendant reduction in the measured magnitude and scatter of the fracture strength. The stress required to propagate the controlled flaw was used to calculate the critical stress-intensity factor, K _IC, from standard fracture-mechanics formulas for semielliptical surface flaws in bending. After the bend specimen had been annealed, the room-temperature K _IC values for HS-130 Si₃N₄ increased to a level consistent with values obtained from conventional fracture-mechanics tests. It was postulated that annealing reduces the residual stresses produced by the microhardness indentation. The presence of residual stresses may account for the low K _IC, values. Elevated-temperature K_IC values for HS-130 Si₃N₄ were consistent with double-torsion data. Controlled flaws in HS-130 Si₃N₄ exhibited slow crack growth at high temperatures.     

Microstructural Characterization and Mechanical Properties of Si3N4 Formed by Fused Deposition of Ceramics

We present processing (green and sintered), part shrinkage and warping, microstructural characterization, and mechanical properties of Si₃N₄ made by fused deposition of ceramics (FDC), using optical microscopy, scanning electron microscopy, and X-ray diffraction. The mechanical properties (fracture strength, fracture toughness, and Weibull modulus) are also reported. Proper FDC build parameters resulted in dense, homogeneous, near-net-shape Si₃N₄, with microstructures and mechanical properties similar to conventionally processed material. Mechanical properties are shown to be isotropic, while there is some degree of microstructural texturing (preferred β-Si₃N₄ grain orientation) in sintered components.     

Pressureless Sintering of α-Si3N4 Porous Ceramics Using a H3PO4 Pore-Forming Agent

A new method for preparing high bending strength porous silicon nitride (Si₃N₄) ceramics with controlled porosity has been developed by using pressureless sintering techniques and phosphoric acid (H₃PO₄) as the pore-forming agent. The fabrication process is described in detail and the sintering mechanism of porous ceramics is analyzed by the X-ray diffraction method and thermal analysis. The microstructure and mechanical properties of the porous Si₃N₄ ceramics are investigated, as a function of the content of H₃PO₄. The resultant high porous Si₃N₄ ceramics sintered at 1000°–1200°C show a fine porous structure and a relative high bending strength. The porous structure is caused mainly by the volatilization of the H₃PO₄ and by the continous reaction of SiP₂O₇ binder, which could bond on to the Si₃N₄ grains. Porous Si₃N₄ ceramics with a porosity of 42%–63%, the bending strength of 50–120 MPa are obtained.     

Impurity Phases in Hot-Pressed Si3N4

Impurity phases in commercial hot-pressed Si₃N₄ were investigated using transmission electron microscopy. In addition to the dominant, β-Si₃N₄ phase, small amounts of Si₂N₂O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 Å thick between, β-Si₃N₄ grains and ∼ 30 Å thick between Si₂N₂O and β-Si₃N₄ grains.     

Reaction-Bonded and Superplastically Sinter-Forged Silicon Nitride–Silicon Carbide Nanocomposites

Silicon nitride–silicon carbide (Si₃N₄–SiC) nanocomposites were fabricated by a process involving reaction bonding followed by superplastic sinter-forging. The nanocomposites exhibited an anisotropic microstructure, in which rod-shaped, micrometer-sized Si₃N₄ grains tended to align with their long axes along the material-flow direction. SiC particles, typically measuring several hundred nanometers, were located at the Si₃N₄ grain boundaries, and nanosized particles were dispersed inside the Si₃N₄ grains. A high bending strength of 1246 ± 119 MPa, as well as a high fracture toughness of 8.2 ± 0.9 MPa·m^1/2, was achieved when a stress was applied along the grain-alignment direction.     

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.