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.     

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.     

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.     

Superplastic Deformation in Silicon Nitride–Silicon Oxynitride In Situ Composites

Starting with a mixture of ultrafine β-Si₃N₄ and a SiO₂-containing additive, a superplastic Si₃N₄-based composite was developed, using the concept of a transient liquid phase. Significant deformation-induced phase and microstructure evolutions occurred in the nonequilibrium, fine-grained Si₃N₄ material, which led to the in situ development of a Si₃N₄–22-vol%-Si₂N₂O composite and strong texture formation. The unusual ductility of the composites with elongated Si₂N₂O grains was attributed to the fine-grained microstructure, the presence of a transient liquid phase, and the alignment of the elongated Si₂N₂O grains. The mechanical properties of the resultant composite were enhanced rather than impaired by superplastic deformation and subsequent heat treatment; the resultant composite exhibited both high strength (957 MPa) and high fracture toughness (4.8 MPa·m^1/2).     

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.     

Fabrication and Characterization of Three-Dimensional Carbon Fiber Reinforced Silicon Carbide and Silicon Nitride Composites

Three-dimensional (3D) carbon fiber reinforced SiC and Si₃N₄ composites have been fabricated using repeated infiltration of an organosilicon slurry under vacuum and pressure. Open porosity of the infiltrated body was reduced from 40% after the first infiltration to approximately 8% after the seventh cycle. Further reduction of open porosity to less than 3% was accomplished by hot-press densification. The maximum values of flexural strength and fracture toughness were, respectively, 260 MPa and 7.3 MPa·m^1/2for C/Si₃N₄ composites, and 185 MPa and 6 MPa·m^1/2 for C/SiC composite.     

Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si₂N₂O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures. α -Si₃N₄ is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si₂N₂O is approximately 19 GPa, comparable with the hardness of Si₃N₄ hot isostatically pressed with 2.5 wt% Y₂O₃, while the fracture toughness level is around 3.1 MPa. m^1/2, being approximately 0.8 MPa.m^1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.     

Mechanical and Thermal Properties of β'-Sialon Prepared by a Slip Casting Method

Various mechanical and thermal properties of β'-sialon ceramics, Si_6–zAl_zO_zN_8–z, prepared from aqueous slurries by a slip casting method were investigated as a function of Z values from 0 to 4. In the present study, Si_6–zAl_zO_zN_8–z occurred in the β'-sialon crystalline phase only near Z = 1. The maximum values for bending strength and fracture toughness, 660 MPa and 5.7 MPa·m^1/2, respectively, were obtained at Z = 0.5. The thermal expansion coefficient exhibited its lowest value, 3.4 × 10⁻⁶· T⁻¹ , at Z = 0.5. In view of the present results, β'-sialon, Si_6–zAl_zO_zN_8–z (Z = 0.5–1), prepared by a slip casting method using aqueous slurries could be adopted, with no problem, for refractory parts.     

Effect of Sintering Additives on Microstructure and Mechanical Properties of Porous Silicon Nitride Ceramics

Porous silicon nitride (Si₃N₄) ceramics with about 50% porosity were fabricated by pressureless sintering of α-Si₃N₄ powder with 5 wt% sintering additive. Four types of sintering aids were chosen to study their effect on the microstructure and mechanical properties of porous Si₃N₄ ceramics. XRD analysis proved the complete formation of a single β-Si₃N₄ phase. Microstructural evolution and mechanical properties were dependent mostly on the type of sintering additive. SEM analysis revealed the resultant porous Si₃N₄ ceramics as having high aspect ratio, a rod-like microstructure, and a uniform pore structure. The sintered sample with Lu₂O₃ sintering additive, having a porosity of about 50%, showed a high flexural strength of 188 MPa, a high fracture toughness of 3.1 MPa·m^1/2, due to fine β-Si₃N₄ grains, and some large elongated grains.     

Fabrication of Hard and Strong α/β-Si3N4 Composites With MgSiN2 as Addivitives

α/β-Si₃N₄ composites with various α/β phase ratios were prepared by hot pressing at 1600°–1650°C with MgSiN₂ as sintering additives. An excellent combination of mechanical properties (Vickers indentation hardness of 23.1 GPa, fracture strength of about 1000MPa, and toughness of 6.3 MPa·m^1/2) could be obtained. Compared with conventional Si₃N₄-based ceramics, this new material has obvious advantages. It is as hard as typical in-situ-reinforced α-Sialon, but much stronger than the latter (700 MPa). It has comparable fracture strength and toughness, but is much harder than β-Si₃N₄ ceramics (16 GPa). The microstructures and mechanical properties can be tailored by choosing the additive and controlling the heating schedule.     

Pressureless Sintering of Si3N4 Ceramic Using AlN and Rare-Earth Oxides

Using intermediate, liquid-forming compositions in the (Y,La)₂O₃-AlN system as additives, fully dense Si₃N₄ ceramics with high strength at high temperature have been obtained by pressureless sintering. The ceramics contain rod-shaped β-Si₃N₄ with M' or K' solid solutions as grain-boundary phases. The strength of these ceramics is 1150 MPa at 1200°C, and the room-temperature toughness is maintained at }7 MPa·m^1/2. Phase relations that are pertinent to the new additive compositions are delineated to rationalize their beneficial effects on sinterability and mechanical properties.     

Processing and Mechanical Properties of Zirconium Diboride-Based Ceramics Prepared by Spark Plasma Sintering

Zirconium diboride (ZrB₂) reinforced by nano-SiC whiskers has been prepared by spark plasma sintering (SPS). Of most interest is the densification of ZrB₂–SiCw composites accomplished by SPS at a temperature as low as 1550°C. The relative density of ZrB₂–SiCw composites could reach to 97% with an average grain size of 2–3 μm. Both flexural strength and fracture toughness of the composites were improved with increasing amount of SiCw. Flexural strengths ranged from 416 MPa for monolithic ZrB₂ to over 545 MPa for ZrB₂–15 vol% SiCw composites. Similarly, fracture toughness also increased from 5.46 MPa·m^1/2 to more than 6.81 MPa·m^1/2 in the same composition range. The relative density of ZrB₂–SiCw composites could be further improved to near 100% by adding some sintering aids such as AlN and Si₃N₄; however, the effects of different sintering additives on the mechanical properties of the composites were different.     

Microstructure and Properties of Self-Reinforced Silicon Nitride

Problems associated with manufacturing Si₃N₄/SiC-whisker composites have been overcome by developing selfreinforced Si₃N₄ with elongated β-Si₃N₄ grains formed in situ from oxynitride glass. This Si₃N₄–Y₂O₃–MgO–SiO₂–CaO-based material has a flexure strength >1000 MPa and fracture toughness >8 MPa·m^½. The optimum combination of mechanical properties has been obtained with Y₂O₃:MgO ratios ranging from 3:1 to 1:2, CaO contents ranging from 0.1 to 0.5 wt%, and Si₃N₄ contents between 90 and 96 wt%.     

Barium Aluminosilicate Reinforced In Situ with Silicon Nitride

Advanced ceramic composite materials that exhibit high strength and toughness with good thermal shock resistance are needed for emerging high-temperature engineering applications. A recently developed in situ reinforced barium aluminosilicate glass-ceramic shows promise of meeting many of the requirements for these types of applications with the added benefit of low-cost fabrication through densification by pressureless sintering. The material is toughened through in situ growth of rodlike β-Si₃N₄ grains resulting from the α–β silicon nitride phase transformation. Microstructural development and material properties for temperatures up to 1400°C are discussed. When compared to monolithic barium aluminosilicate, barium aluminosilicate reinforced with 70% by volume of Si₃N₄ shows a significant increase in flexural strength (from 80 to 565 MPa) and fracture toughness (from 1.8 to 5.74 MPa·m^1/2) with a high resistance to thermal shock.     

Influence of Planetary High-Energy Ball Milling on Microstructure and Mechanical Properties of Silicon Nitride Ceramics

The influence of ball-milling methods on microstructure and mechanical properties of silicon nitride (Si₃N₄) ceramics produced by pressureless sintering for a sintering additive from MgO–Al₂O₃–SiO₂ system was investigated. For planetary high-energy ball milling, the mechanical properties of Si₃N₄ ceramics were evidently improved and a homogeneous microstructure developed. In contrast, some exaggerated elongated grains were developed due to the local enrichment of sintering additives in the specimen prepared by general ball milling. For Si₃N₄ ceramics produced by planetary ball milling, flexure strength of 1.06 GPa, Vickers hardness of 14.2 GPa, and fracture toughness of 6.6 MPa·m^0.5 were achieved. The differences in the mechanical properties of Si₃N₄ ceramics produced by different processing seem to arise mainly from the changes in microstructural homogenization and sinterability. The planetary high-energy ball-milling process provides a good route to mix starting powders for developing ceramics with uniform microstructure and promising mechanical properties.     

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.     

Mode I Fracture Toughness of a Small-Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects

The Mode I fracture toughness ( K _{I C} ) of a small-grained Si₃N₄ was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si₃N₄ grains was <0.4 µm, with aspect ratios of 2–8. K _{I C} at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m^1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in K _IC was observed through 1200°C, to ∼4.1 MPa·m^1/2, before increasing rapidly to 7.5–8 MPa·m^1/2 at 1300°C. The decrease in K _IC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in K _IC. Crack healing was observed in specimens annealed in air. No R -curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.     

In Situ Reaction of B2O3 with AlN and/or Si3N4 to Form BN-Toughened Composites

BN-toughened oxide matrix composites were formed by the in situ reaction of B₂O₃ with Si₃N₄ and/or AlN. A lowtemperature transient liquid phase aids densification at <1000°C, and the process tends to produce an intrinsically homogeneous microstructure. Mechanical properties and microstructure of the composites formed in situ were compared to those of composites prepared by conventional means from oxide and BN powders. Fracture toughness and flexural strength of the nearly isotropic in situ formed composites ranged from 2.82 to 3.66 MPa · m^1/2 and 130 to 320 MPa, respectively, with Young's moduli of 100 to 110 GPa. Densities achieved ranged from 90% to 97% of estimated theoretical densities. The strength and toughness values are intermediate to the extreme values for the anisotropic composites formed by hot-pressing mixed powders.     

Relationship between Microstructure, Toughening Mechanisms, and Fracture Toughness of Reinforced Silicon Nitride Ceramics

Different microstructures in Si₃N₄ ceramics containing Y₂O₃and Al₂O₃ as sintering additives were prepared by two-step sintering. Pull-out and elastic bridging were most frequently observed as the toughening mechanisms in samples with fine-grained microstructures having needlelike β-Si₃N₄grains with diameters of <1 μm. Crack deflection was the main toughening mechanism observed in samples with coarse-grained microstructures having grains with diameters of >1 μm. The values of fracture toughness were varied from 6.1 to 8.2 MPa·m^1/2 with respect to the microstructural characteristics, characterized by the volume fraction of needlelike grains and their diameter.     

Machinability of Silicon Nitride/Boron Nitride Nanocomposites

The machinability and deformation mechanism of Si₃N₄/BN nanocomposites were investigated in the present work. The fracture strength of Si₃N₄/BN microcomposites remarkably decreased with increased hexagonal graphitic boron nitride ( h -BN) content, although machinability was somewhat improved. However, the nanocomposites fabricated using the chemical method simultaneously had high fracture strength and good machinability. Hertzian contact tests were performed to clarify the deformation behavior by mechanical shock. As a result of this test, the damage of the monolithic Si₃N₄ and Si₃N₄/BN microcomposites indicated a classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.