Relationship between Microstructure and Mechanical Performance of a 70% Silicon Nitride–30% Barium Aluminum Silicate Self-Reinforced Ceramic Composite

The abnormal grain growth of β-Si₃N₄ was observed in a 70% Si₃N₄–30% barium aluminum silicate (70%-Si₃N₄–30%-BAS) self-reinforced composite that was pressureless-sintered at 1930°C; Si₃N₄ starting powders with a wide particle-size distribution were used. The addition of coarse Si₃N₄ powder encouraged the abnormal growth of β-Si₃N₄ grains, which allowed microstructural modification through control of the content and size distribution of β-Si₃N₄ nuclei. The mechanical response of different microstructures was characterized in terms of flexural strength, as well as indentation fracture resistance, at room temperature. The presence of even a small amount of abnormally grown β-Si₃N₄ grains improved the fracture toughness and minimized the variability in flexural strength.     

Highly Texturing β-Sialon Via Strong Magnetic Field Alignment

This paper reports the texturing behavior of β-sialon by strong magnetic field alignment (SMFA) during slip casting, followed by reaction pressureless sintering, using either α or β-Si₃N₄, Al₂O₃, and AlN as the starting materials. It is found that the β-Si₃N₄ crystal exhibits a substantially stronger orientation ability than the α-Si₃N₄ crystal regardless of the Si₃N₄ raw powders in the magnetic field of 12 T. The β-raw powder produces a highly a , b -axis-oriented β-Si₃N₄ green body with a Lotgering orientation factor of up to 0.97. During sintering, the β-raw powder allows the a , b -axis-oriented β-sialon to retain the Lotgering orientation factor similar to and even higher than that of β-Si₃N₄ in the green body. In contrast, the α-raw powder leads to a faster transformation rate of α/β-Si₃N₄ to β-sialon but a substantially lower texture in β-sialon. The results indicate that the use of the β-raw powder is more efficient for producing highly textured β-sialon via SMFA than that of the α-raw powder as well as the prolonged sintering.     

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.     

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.     

Texture Development in Silicon Nitride–Silicon Oxynitride In Situ Composites via Superplastic Deformation

Silicon nitride–silicon oxynitride (Si₃N₄–Si₂N₂O) in situ composites have been fabricated via either the annealing or the superplastic deformation of sintered Si₃N₄ that has been doped with a silica-containing additive. In this study, quantitative texture measurements, including pole figures and X-ray diffraction patterns, are used in conjunction with scanning electron microscopy and transmission electron microscopy techniques to examine the degree of preferred orientation and texture-development mechanisms in these materials. The results indicate that (i) only superplastic deformation can produce strong textures in the β-Si₃N₄ matrix, as well as Si₂N₂O grains that are formed in situ ; (ii) texture development in the β-Si₃N₄ matrix mainly results from grain rotation via grain-boundary sliding; and (iii) for Si₂N₂O, a very strong strain-dependent texture occurs in two stages, namely, preferred nucleation and anisotropic grain growth.     

Plasma Etching of α-Sialon Ceramics

Plasma etching of β-Si₃N₄, α-sialon/β-Si₃N₄ and α-sialon ceramics were performed with hydrogen glow plasma at 600°C for 10 h. The preferential etching of β-Si₃N₄ grains was observed. The etching rate of α-sialon grains and of the grain-boundary glassy phase was distinctly lower than that of β-Si₃N₄ grains. The size, shape, and distribution of β-Si₃N₄ grains in the α-sialon/β-Si₃N₄ composite ceramics were revealed by the present method.     

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.     

Microstructural Development During Gas-Pressure Sintering of α-Silicon Nitride

Gas-pressure sintering of α-Si₃N₄ was carried out at 1850 ° to 2000°C in 980-kPa N₂. The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si₃N₄ powder was similar to that in the materials from β-Si₃N₄ powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si₃N₄ ceramics.     

Effect of Grain Growth of β-Silicon Nitride on Strength, Weibull Modulus, and Fracture Toughness

β-Si₃N₄ powder containing 1 mol% of equimolar Y₂O₃–Nd₂O₃ was gas-pressure sintered at 2000°C for 2 h (SN2), 4 h (SN4), and 8 h (SN8) in 30-MPa nitrogen gas. These materials had a microstructure of " in-situ composites" as a result of exaggerated grain growth of some β Si₃N₄ grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa-m^1/2 because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas-pressure sintering of β-Si₃N₄ powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.     

Microstructure and Mechanical Properties of the in situβ-Si3N4/α'-SiAlON Composite

The in situ β-Si₃N₄/α'-SiAlON composite was studied along the Si₃N₄–Y₂O₃: 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β-Si₃N₄/α'-SiAlON composite were found to vary with composition. Because of the cooperative formation of α'-Si AlON and β-Si₃N₄ during its phase development, this composite had equiaxed α'-SiAlON (∼0.2 μm) and elongated β-Si₃N₄ fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30–40%α', which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m^1/2. α'-SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°-SiAlON grains were affected by the sintering conditions.     

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.     

Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

High-Resolution Electron Microscopy of Chemically Vapor-Deposited β-Si3N4-TiN Composites

Microstructures of Si₃N₄-TiN composites prepared by chemical vapor deposition (CVD) were investigated by the multibeam imaging technique using a 1 MV electron microscope. High-resolution images showed a number of fibrous TIN crystallites dispersed in the matrix of CVD β-Si₃N₄. Crystallographic orientation relations between β-Si₃N₄ and TiN were determined directly from the observed images in the subcell scale. The fibrous axis of TiN is parallel to the (110) direction of the NaCl structure and lies along the c axis of the hexagonal β-Si₃N₄ crystal. Domain boundaries, planar faults, nonplanar faults, and dislocations were found in the CVD β-Si₃N₄ matrix near the TiN crystallites. The origin of the structure defects is briefly discussed in connection with the formation of TiN crystallites in the matrix.     

Processing Strategy for Producing Highly Anisotropic Silicon Nitride

Silicon nitride with a preferred orientation of large elongated grains was obtained by tape casting of raw powder slurry seeded with rodlike β-Si₃N₄ particles, followed by a gas pressure sintering under 1 MPa nitrogen pressure. The large elongated grains developed from seeds lay in planes parallel to the casting direction in a two-dimensional distribution. Increased fracture toughness (11.1 MPa·m^1/2) and bending strength (1100 MPa) were achieved in the direction perpendicular to the grains alignment compared to specimens with a random distribution of elongated grains. Morover, the specimens exhibited a high Weibull modulus of 46 due to the uniform distribution of large grains.     

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.     

Shock Synthesis of Cubic Silicon Nitride

The phase transitions of α-Si₃N₄ and β-Si₃N₄ have been investigated by shock compression through the recovery technique and Hugoniot measurements. α- and β-Si₃N₄ are transformed into a cubic spinel structure ( c -Si₃N₄). The yield of c -Si₃N₄ increases with increasing shock pressure and reaches 100% at 63 GPa. The shock-synthesized c -Si₃N₄ powders are nanocrystals and display a high-temperature metastability up to about 1620 K. c -Si₃N₄ is one of the hard materials based on the measured equation of state. c -Si₃N₄ powders have been characterized by electron microscopy and ²⁹Si magic angle spinning NMR spectroscopy. The purification and separation method has been developed to obtain pure c -Si₃N₄ powders.     

Calculation of Grain-Boundary Bonding in Rare-Earth-Doped β-Si3N4

First-principles molecular orbital calculations are performed by the discrete variational Xalpha method using model clusters of rare-earth-doped β-Si₃N₄ and the interface between prismatic planes of β-Si₃N₄ and intergranular glassy films. On the basis of the total overlap population of each cluster, the rare-earth ions are implied to be stable in the grain-boundary model, while they are not stable in the bulk model. These results are consistent with experimental observations showing significant segregation of Ln³⁺ ions at the grain boundary and no solubility of Ln³⁺ into bulk β-Si₃N₄. Grain-boundary bonding is weakened with an increase of the ionic radius of the rare-earth ions, which provides a reasonable explanation for the ionic size dependence of the crack propagation behaviors as well as the growth rate of the prismatic plane in the rare-earth-doped β-Si₃N₄ during liquid-phase sintering.     

Additive-Assisted Nitridation to Synthesize Si3N4 Nanomaterials at a Low Temperature

Starting from Si powder, NaN₃ and different additives such as N -aminothiourea, iodine, or both, Si₃N₄ nanomaterials were synthesized through the nitridation of silicon powder in autoclaves at 60°–190°C. As the additive was only N -aminothiourea, β-Si₃N₄ nanorods and α-Si₃N₄ nanoparticles were prepared at 170°C. If the additive was only iodine, α-Si₃N₄ dendrites with β-Si₃N₄ nanorods were obtained at 190°C. However, when both N -aminothiourea and iodine were added to the system of Si and NaN₃, the products composed of β-Si₃N₄ nanorods and α, β-Si₃N₄ nanoparticles could be prepared at 60°C.     

Electron Microscopy Study of the Effect of Iron in Reaction-Sintered Silicoa Nitride

The microstructure, crystal structure, and chemical composition of reaction-sintered Si₃N₄ containing iron were studied using conventional and scanning transmission electron microscopy. It was found that the grains of β -Si₃N₄ were large and blocklike with well-developed facets, a series of voids along some grain boundaries, a subgrain of iron silicide near the periphery, and penetration of iron silicide into the three-grain junctions and grain boundaries. At some distance from each β -Si₃N₄ grain was a region of small α-Si₃N₄ grains, with no evidence of iron silicide. Between this region and the β -Si₃N₄ grain was a zone containing both α- and β -Si₃N₄ and iron silicide. These observations suggest that the large β -Si₃N₄ grains grow in liquid iron silicide, that the smaller α-Si₃N₄ grains grow from the vapor, and that the latter are converted to the β form by solution in, and reprecipitation from, liquid iron silicide.