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.     

Residual α–Si3N4 in O' Crystals in CeO2-Doped O'+β' SiAlON Ceramics

The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO₂ doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si₃N₄ inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO₂-doping and its high doping level as well as the high Al₂O₃ concentration were thought to be the main reasons for accelerating the reaction between the α-Si₃N₄ and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si₃N₄ from the reacting liquid, resulting in a delay in the α→β-Si₃N₄ transformation. The large O' grains and the α-Si₃N₄ inclusions have a pronounced effect on the strength degradation of O'+β' ceramics.     

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.     

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.     

Topotactically Oriented α-Si3N4 Cores in Sintered β-Si3N4

α-Si₃N₄ core structures within β-Si₃N₄ grains have been studied by transmission electron microscopy. The grains were dispersed in an oxynitride glass which was previously melted at 1600°C. The cores were topotactically related to the as-grown β-Si₃N₄ crystallites and are related to epitactical nucleation during heat treatment as the most probable mechanism.     

Influence of Phase Formation on Dielectric Properties of Si3N4 Ceramics

The influence of phase formation on the dielectric properties of silicon nitride (Si₃N₄) ceramics, which were produced by pressureless sintering with additives in MgO–Al₂O₃–SiO₂ system, was investigated. It seems that the difference in the dielectric properties of Si₃N₄ ceramics sintered at different temperatures was mainly due to the difference of the relative content of α-Si₃N₄, β-Si₃N₄, and the intermediate product (Si₂N₂O) in the samples. Compared with α-Si₃N₄ and Si₂N₂O, β-Si₃N₄ is believed to be a major factor influencing the dielectric constant. The high-dielectric constant of β-Si₃N₄ could be attributed to the ionic relaxation polarization.     

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.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.     

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.     

Fatigue Crack Growth of Silicon Nitride at 1400°C: A Novel Fatigue-Induced Crack-Tip Bridging Phenomenon

high-strength Si₃N₄with elongated β-Si₃N₄ and equiaxed α-sialon was tested in cyclic and static fatigue at 1400°C. At low stress intensity factors and high frequencies, the pullout process of the elongated grains was enhanced, which suppressed the crack growth. This provides a possible explanation for the increased lifetime under cyclic leading conditions reported for ceramics by several investigators. While crack-healing by high-temperature annealing was found to greatly reduce the subsequent static fatigue crack growth rate, it had only a modest effecf on cyclic fatigue and none at high frequencies.     

Ionic Conductivity of the System Si-Al-O-N from 850° to 1400°C

Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si₃N₄-Al₂O₃, containing β-sialon, X phase, β-Si₃N₄, and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si⁴⁺ migration for the temperature range studied. Pure X phase shows ionic conduction by Si⁴⁺ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si₃N₄-Al₂O₃ containing β-sialon, β-Si₃N₄ X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively.     

Comparison of the Luminescence Properties of Dy3+ in α-Sialon and Oxynitride Glass

Dy-α-sialon and β-Si₃N₄ materials containing Dy-oxynitride glass were hot pressed at 1800°C for 1 h. The luminescence spectra of Dy³⁺ in these samples were compared when excited at 350 nm. The results showed that two strong emission bands in the region 470–500 nm and 570-600 nm, associated with the ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions of Dy³⁺ ions, were observed in Dy-α-sialon. However, no emission peak was detected from the β-Si₃N₄ sample, despite it containing the same amount of Dy³⁺ cations. This proved that only the Dy³⁺ cations in the α-sialon structure, not those in the oxynitride glass, produce the luminescence spectrum.     

Near-Net Shape β-Si4Al2O2N6 Parts by Hydrolysis Induced Aqueous Gelcasting Process

In this paper, a new net-shaping process, an hydrolysis-induced aqueous gelcasting (GC) (GCHAS) has been reported for consolidation of β-Si₄Al₂O₂N₆ ceramics from aqueous slurries containing 48–50 vol%α-Si₃N₄, α-Al₂O₃, AlN, and Y₂O₃ powders mixture. Dense ceramics of same composition were also consolidated by aqueous GC and hydrolysis assisted solidification routes. Among three techniques used, the GCHAS process was found to be superior for fabricating defect-free thin wall β-Si₄Al₂O₂N₆ crucibles and tubes. Before use, the as purchased AlN powder was passivated against hydrolysis. The sintered β-Si₄Al₂O₂N₆ ceramics exhibited comparable properties with those reported for similar materials in the literature.     

Nucleation and Growth of β'-SiAlON

Elongated β'-SiAlON grains grown from several finegrained Y_m/3Si_12(m+n)Al_m+nO_nN_16–r compositions with α-Si₃N₄, AlN, Al₂O₃, and Y₂O₃ starting materials have been examined. These grains have large aspect ratios and are oriented along the [0001] axis. TEM structural and chemical analysis suggests that they are nucleated from various seed crystals, which can be α-Si₃N₄, β-Si₃N₄, or other β'-SiAlON. The β'-SiAlON seed and the initial precipitation on β-Si₃N₄ show a higher content of Al and O, indicating that a large transient supersaturation of Al and O in the liquid is instrumental for β'-SiAlON formation, whereas subsequent growth proceeds under a much lower driving force. The misfit between phases is accommodated by interfacial dislocations ( c -type and a -type). Fully grown β'-SiAlON grains usually contain several variants independently nucleated from the same seed. In particular, the two alternative α/β phase-matching possibilities result in two [0001] growth habits separated by a twin boundary.     

Combustion Synthesis of Si3N4–SiC Composite Powders

Fine Si₃N₄-SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si₃N₄, β-Si₃N₄, and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si₃N₄, β-SiC, and a few Si₂N₂O.     

Microstructural Development of Calcium alpha-SiAlON Ceramics with Elongated Grains

Silicon nitride (Si₃N₄) and SiAlONs can be self-toughened through the growth of elongated β-Si₃N₄/β-SiAlON grains in sintering. α-SiAlONs usually retain an equiaxed grain morphology and have a higher hardness but lower toughness than β-SiAlONs. The present work has demonstrated that elongated alpha-SiAlON grains can also be developed through pressureless sintering. alpha-SiAlONs with high-aspect-ratio grains in the calcium SiAlON system have exhibited significant grain debonding and pull-out effects during fracture, which offers promise for in-situ -toughened α-SiAlON ceramics.     

Hot-Pressing Behavior of Si3N4

The densification behavior of Si₃N₄ containing MgO was studied in detail. It was concluded that MgO forms a liquid phase (most likely a magnesium silicate). This liquid wets and allows atomic transfer of Si₃N₄. Evidence of a second-phase material between the Si₃N₄ grains was obtained through etching studies. Transformation of α- to β-Si₃N₄ during hot-pressing is not necessary for densification.     

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.     

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.     

Effect of α to β (β') Phase Transition on the Sintering of Silicon Nitride Ceramics

By using α-Si₃N₄ and β-Si₃N₄ starting powders with similar particle size and distribution, the effect of α-β (β') phase transition on densification and microstructure is investigated during the liquid-phase sintering of 82Si₃N₄·9Al₂O₃·9Y₂O₃ (wt%) and 80Si₃N₄·13Al₂O₃·5AIN·5AIN·2Y₂O₃. When α-Si₃N₄ powder is used, the grains become elongated, apparently hindering the densification process. Hence, the phase transition does not enhance the densification.