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.     

Long Crack R-Curve of Aligned Porous Silicon Nitride

Long crack R -curve of a porous Si₃N₄ with aligned fibrous grains was investigated, using a chevron-notched beam technique. A crack was constrained to propagate normal to the grain alignment. The crack growth resistance of aligned porous Si₃N₄ was much larger compared with that of dense Si₃N₄ ceramics. Microstructure observations showed that pullouts of fibrous grains in aligned porous Si₃N₄ markedly increased during crack propagation relative to those of dense Si₃N₄, due to the existence of pores. The efficient grain pullouts in porous Si₃N₄ increased the bridging stress at the crack wake.     

High-Temperature Crack Growth in Monolithic and SiCw-Reinforced Silicon Nitride under Static and Cyclic Loads

Experimental results are presented on subcritical crack growth under sustained and cyclic loads in a HIPed Si₃N₄ at 1450°C and a hot–pressed Si₃N₄–10 vol% SiC_w composite in the temperature range 1300°–1400°C. Static and cyclic crack growth rates are obtained from the threshold for the onset of stable fracture with different cyclic frequencies and load ratios. Fatigue crack growth rates for both the monolithic and SiC_w-reinforced Si₃N₄ are generally higher than the crack growth velocities predicted using static crack growth data. However, the threshold stress intensity factor ranges for the onset of crack growth are always higher under cyclic loads than for sustained load fracture. Electron microscopy of crack wake contact and crack–tip damage illustrate the mechanisms of subcritical crack growth under static and cyclic loading. Critical experiments have been conducted systematically to measure the fracture initiation toughness at room temperature, after advancing the crack subcritically by a controlled amount under static or cyclic loads at elevated temperatures. Results of these experiments quantify the extent of degradation in crack–wake bridging due to cyclically varying loads. The effects of preexisting glass phase on elevated temperature fatigue and fracture are examined, and the creep crack growth behavior of Si₃N₄–based ceramics is compared with that of oxide-based ceramics.     

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.     

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.     

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.     

R-Curve Behavior and Crack-Closure Stresses in Barium Titanate and (Mg,Y)-PSZ Ceramics

R -curves, process zones, and shielding stresses of barium titanate (BaTiO₃) and partially stabilized zirconia (PSZ) have been studied using compact-tension (CT) specimens. BaTiO₃ and PSZ exhibited pronounced R -curves that rose over similar crack lengths and showed steady-state toughnesses of 0.7 and 6.4 MPa·m^1/2, respectively. Both steady-state toughnesses were ∼80% larger than the initial fracture toughnesses. Ferroelastic domain switching was the main toughening mechanism in BaTiO₃, whereas, in PSZ, transformation toughening was the main toughening mechanism. The crack process zone and crack-opening-displacement (COD) profile of each material was studied in detail using atomic force microscopy. Crack-closure-stress distributions were extracted from the COD profiles, using weight-function methods. The resulting stress profiles indicated that compressive residual stresses of 40 MPa in BaTiO₃ and 400 MPa in PSZ acted in a limited region behind the crack tip. In the PSZ, crack bridging seemed to be a competing mechanism to transformation toughening.     

Fracture Behavior of Multilayer Silicon Nitride/Boron Nitride Ceramics

The fracture behavior of multilayer Si₃N₄/BN ceramics in bending has been studied. The materials were prepared by a process of tape casting, coating, laminating, and hot pressing. The Si₃N₄ layers were separated by thin, weak BN interlayers. Crack patterns in bending bars were examined with a scanning electron microscope. The weak layers deflected cracks in bending and thus prevented catastrophic failure. In one well-aligned multilayer ceramic A, a main crack propagated through the specimen although along a zigzag path. A second multilayer ceramic B was made to simulate a wood grain structure. Its failure was dominated by shear cracking along the weak BN layers. Besides crack deflection, interlock bridging between toothlike layers in the wake of the main crack appeared also to contribute to toughening.     

Simulation of Cold Compaction Densification Behavior of Silicon Nitride Ceramic Powder

The cold-compaction densification behavior of silicon nitride (Si₃N₄) ceramic powder was analyzed using two models: the Shima model and the Cam-Clay model. Triaxial-compression experimental data were used to evaluate these two models. Shima models that used Si₃N₄ matrix material with different yield stresses were discussed. It is clear that the Cam-Clay model can effectively simulate the cold-compaction densification behavior of Si₃N₄ ceramic powder. The Shima model that used a very high yield stress for the Si₃N₄ matrix material slightly overestimated the experimental data, and the Shima model that used the actual yield stress for Si₃N₄ matrix material largely overestimated that data.     

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.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics

Crack deflection and the subsequent growth of delamination cracks can be a potent source of energy dissipation during the fracture of layered ceramics. In this study, multilayered ceramics that consist of silicon nitride (Si₃N₄) layers separated by boron nitride/silicon nitride (BN/Si₃N₄) interphases have been manufactured and tested. Flexural tests reveal that the crack path is dependent on the composition of the interphase between the Si₃N₄ layers. Experimental measurements of interfacial fracture resistance and frictional sliding resistance show that both quantities increase as the Si₃N₄ content in the interphase increases. However, contrary to existing theories, high energy-absorption capacity has not been realized in materials that exhibit crack deflection but also have moderately high interfacial fracture resistance. Significant energy absorption has been measured only in materials with very low interfacial fracture resistance values. A method of predicting the critical value of the interfacial fracture resistance necessary to ensure a high energy-absorption capacity is presented.     

Flexural Stress Rupture and Creep of Selected Commercial Silicon Nitrides

Four commercial Si₃N₄ compositions were compared with regard to flexural stress rupture and creep in ambient air as functions of temperature from 1100° to 1400°C and stress from 200 to 350 MPa. One Si₃N₄, SN252, was found to be more resistant to time-dependent deformation in both stepped-temperature stress rupture tests and creep tests than a very similar Si₃N₄ composition and two other dissimilar Si₃N₄ compositions. Materials were compared on the bases of percent final strains, creep rates, and posttest microscope examinations. The latter revealed tensile face transverse cracking, and slow crack growth. The superior behavior of the SN252 Si₃N₄ was related to its microstructure.     

Particle Impact Damage in Silicon Nitride

The evolution of particle-impact-induced fracture damage in hot-pressed (HP) silicon nitride was established by accelerating single 2.4-mm-diameter tungsten carbide spheres against polished HP Si₃N₄ surfaces. Threshold velocities for ring, cone, and radial cracks were determined and the corresponding threshold stress for ring cracking was obtained from an elastic stress analysis. Particle size had significant effects on the threshold velocities for the inelastic impression and the various crack types. Loading rate had little effect on the threshold stress for ring cracks; rate effects on other crack types could not be assessed because the quasistatic indenter failed at stresses less than those required to invoke other crack types. A 20-μm-thick oxide scale had little influence on morphology and extent of damage but was removed easily at low velocities, suggesting higher erosion rates for Si₃N₄ in oxidizing environments. Damage phenomenology in 85% dense reaction-bonded Si₃N₄ was similar to that in HP material; however, all stages of damage occurred at substantially lower velocities.     

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.     

Nanoindentation of a Polymer-Derived Amorphous Silicon Carbonitride Ceramic

Nanoindentation has revealed significant scatter of the microscale hardness and elastic modulus of an amorphous SiCN ceramic, because of structural inhomogeneities (nanopores and clusters of free carbon within the material). As a consequence of the common feature, an amorphous nature, SiCN, in regard to its mechanical properties, resembles SiO₂ glass more than SiC or Si₃N₄. However, because of the stronger Si—C and Si—N covalent bonding, SiCN is harder and stiffer than SiO₂. The mean hardness—13 ± 2 GPa, measured at a load of 250 mN for SiCN—is approximately half that of polycrystalline Si₃N₄ (24.9 ± 0.6 GPa) but higher than that of SiO₂ glass (8.9 ± 0.04 GPa). The elastic modulus of the SiCN, measured at a load of 250 mN, is 121 ± 10 GPa.     

Grain-Boundary Viscosity of Calcium-Doped Silicon Nitride

Internal-friction data of calcium-doped Si₃N₄ polycrystalline materials that otherwise contain only SiO₂ at grain boundaries were examined and compared with those collected on the same polycrystal in the undoped state or doped with anions (i.e., fluorine and chlorine). Precise microstructural characterizations previously performed on these materials enabled us to quantitatively evaluate the inherent viscosity of the intergranular SiO₂ film through the analysis of the anelastic internal-friction-peak components. The intergranular glass viscosity and its scaling with increasing calcium addition followed the same trend as bulk SiO₂ glasses with the same chemical composition. Broadening of the internal-friction peak with increased calcium content in the material has also been rationalized according to the reduction of the activation energy for the viscous flow of bulk glasses. The present analysis, which is in agreement with our previous studies on undoped and anion-doped Si₃N₄, has demonstrated that the overall viscoelastic response of the polycrystal is mainly dictated by the chemistry of the intergranular glass.     

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.     

Fabrication and Mechanical Properties of Silicon Carbide-Silicon Nitride Composites with Oxynitride Glass

A microstructure that consisted of uniformly distributed, elongated β-Si₃N₄ grains, equiaxed β-SiC grains, and an amorphous grain-boundary phase was developed by using β-SiC and alpha-Si₃N₄ powders. By hot pressing, elongated β-Si₃N₄ grains were grown via alpha right arrow β phase transformation and equiaxed β-SiC grains were formed because of inhibited grain growth. The strength and fracture toughness of SiC have been improved by adding Si₃N₄ particles, because of the reduced defect size and the enhanced bridging and crack deflection by the elongated β-Si₃N₄ grains. Typical flexural-strength and fracture-toughness values of SiC-35-wt%-Si₃N₄ composites were 1020 MPa and 5.1 MPam^1/2, respectively.     

Relation between Strength, Microstructure, and Grain-Bridging Characteristics in In Situ Reinforced Silicon Nitride

The grain size of in situ Si₃N₄ is varied, and its effects on strength-flaw size relations are related to the behavior of a bridging zone behind the crack tip. The bridging-zone properties are calculated from a Dugdale model assuming that the bridging zone has a constant bridging stress ( p *) and length ( D _b) at the moment of the critical fracture. The results show that as grain size increases, p * decreases while D _b and the critical bridging zone opening ( u *) first increase and then decrease, resulting in a maximum for short-crack fracture toughness at an intermediate grain size. The initial increase of u * and D _b with grain size is attributed to an increase in debonding length, while the decrease of p * is attributed to a decrease in strength for bridging grains due to a statistical effect which also causes D _b and u * to drop in the large-grain regime. Implications on microstructure design are discussed.