Sintering and Properties of Sialons without Externally Added Liquid Phase

β'-Sialon has the general formula Si_6-ZAl_ZO_ZN₈-Z The synthesis of pure β'-sialon with three different Z values of 0.5, 1.0, and 2.0 in the system Si₃N₄-AlN-Al₂O₃-SiO₂ has been reported without the addition of a foreign sintering aid. A small shift in the composition toward the SiO₂ corner has been made in each case. The Z=1.0 and 2.0 sialon can be sintered almost to full density while that with Z=0.5 sialon is difficult even with a higher amount of excess oxide addition. A packing bed of Si₃N₄ and SiO₂ in a weight ratio of 7:3 was found to be most useful. The coefficient of linear thermal expansion of Z=1.0 sialon is 2.2°10^-6/°C (25° to 1000°C). The room-temperature modulus of rupture value can be retained up to 90% of the value at 1400°C. A similar trend has also been observed in the K_lC value with temperature. The steady-state flexural creep rate varies from 0.5°10^-6 to 2.5°10^-6 h^-1 in the temperature and load ranges of 1200° to 1300°C and 100 to 250 MPa.     

Combustion Synthesis of Sialon Powders (Si6-zAlzOzN8-z, Z = 0.3, 0.6)

Fine sialon powders (Si_6-zAl_zO_zN_8-z, Z = 0.3, 0.6) were prepared by nitriding combustion. Silicon powders was reacted with AlN and Si₃N₄ under 10 MPa of nitrogen gas. The reaction temperature reached above 2000°C and single-phase sialon powders were synthesized within minutes. The obtained powders were fully densified without additives by hot isostatic pressing. The sintered body had a flexural strength of 600 MPa by 4-point bending.     

Processing, Microstructure, and Wear Behavior of Silicon Nitride Hot-Pressed with Alumina and Yttria

Commercial silicon nitride powder with A1₂O₃ and Y₂O₃ additives was hot-pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining α-Si₃N₄. The aspect ratio of the elongated grains increased with increasing soak time at a fixed hot-pressing temperature. X-ray diffraction analysis showed that the crystalline phase in the hot-pressed samples was β-sialon (Si_6−zAl_zO_zN_8−z) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightly as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were conducted on a reciprocating pin-on-disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mechanical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third-body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusively identified. However, TEM examination showed clear evidence of dislocation motion under the wear scar. Grain boundary microstresses caused by the anisotropic thermal expansion and elastic properties of the elongated grains may have contributed to the observed pullout.     

Homogeneity Region and Thermal Stability of Neodymium-Doped α-Sialon Ceramics

Dense sialon ceramics along the tie line between Si₃N₄ and Nd₂O₃·9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated in the temperature range 1300–1750°C and cooled either by turning off the furnace (yielding a cooling rate (T_cool) of ∼50°C/min) or quenching (T_cool≥ 400°C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase α-sialon forms for 0.30 x 0. 51 in the formula Nd_xSi_{12–4S x} Al_{4.5 x} O_{1. 5 x} , N_{16–1.5 x} . The α-sialon is stable only at temperatures above 1650°C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an α-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed α-sialon decomposes to the more stable β-sialon and either the melilite phase or a new phase of the composition NdAl(Si_6-zAl_z)N_10-zO_z. Nd-doped α-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22. 5 GPa) and a fracture toughness ( K _lc= 4.4 MPa·m^1/2) at room temperature. The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials.     

Synthesis of Porous Si3N4 Ceramics with Rod-Shaped Pore Structure

Porous Si₃N₄ ceramics were synthesized by pressureless sintering of green compacts prepared using slip casting of slurries containing Si₃N₄, 5 wt% Y₂O₃+2 wt% Al₂O₃, and 0–60% organic whiskers composed of phenol–formaldehyde resin with solids loading up to 60 wt%. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800°C in air and sintered at 1850°C in a N₂ atmosphere. Porosities ranging from 0% to 45% were obtained by the whisker contents (corresponding to 0–60 vol% whisker). Samples exhibited a uniform pore distribution. Their rod-shaped pore morphology originated from burnout of whiskers, and an extremely dense Si₃N₄ matrix.     

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%.     

Silicon Nitride Joining Using Silica and Yttria Ceramic Interlayers

Dense hot-pressed β-Si₃N₄ blocks were joined using both SiO₂ and SiO₂-Y₂O₃ powder slurries as bonding interlayers. The assembled specimens (Si₃N₄/interlayer/Si₃N₄) were heated in a flowing N₂ atmosphere in the temperature range of 1500°–1650°C. The joining interlayer was clearly distinguished from the Si₃N₄ bulk. The microstructure and the reaction products found in the bonding interlayer were very different in both compositions. Reactions occurring between the Si₃N₄ and the ceramic joining compositions have been explained based on existing diagrams of the YN–Si₃N₄-Y₂O₃-SiO₂ system.     

Formation and Stability of β-Quartz Solid-Solution Phase in the Li-Si-Al-O-N System

The development of crystalline phases in lithium oxynitride glass-ceramics was examined, with particular emphasis placed on the effect of the nitrogen source (AlN or Si₃N₄) on the formation and stability of a β-quartz solid-solution ( ss ) phase. Oxynitride glasses derived from the Li-Si-Al-O-N system were heat-treated at temperatures up to 1200°C to yield glass-ceramics in which β-quartz( ss ) and β-spodumene( ss ) of approximate composition Li₂OAl₂O₃4SiO₂ formed as major phases and in which X-phase (Si₃Al₆O₁₂N₂) and silicon oxynitride (Si₂N₂O) were present as minor phases. The nitrogen-containing β-quartz( ss ) phase that was prepared with AlN was stable at 1200°C; however, the use of Si₃N₄ as the nitrogen source was significantly less effective in promoting such thermal stabilization. Lattice parameter measurements revealed that AlN and Si₃N₄ had different effects on the crystalline structures, and it was proposed that the enhanced thermal stability of the β-quartz( ss ) phase that was prepared with AlN was due to both the replacement of oxygen by nitrogen and the positioning of excess Al³⁺ ions into interstitial sites within the β-quartz( ss ) crystal lattice.     

Design, Fabrication, and Characterization of Silicon Nitride Particle-Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration

Silicon nitride particle-reinforced silicon nitride matrix composites were fabricated by chemical vapor infiltration (CVI). The particle preforms with a bimodal pore size distribution were favorable for the subsequent CVI process, which included intraagglomerate pores (0.1–4 μm) and interagglomerate pores (20–300 μm). X-ray fluorescence results showed that the main elements of the composites are Si, N, and O. The composite is composed of α-Si₃N₄, amorphous Si₃N₄, amorphous SiO₂, and a small amount of β-Si₃N₄ and free silicon. The α-Si₃N₄ transformed into β-Si₃N₄ after heat treatment at 1600°C for 2 h. The flexural strength, dielectric constant, and dielectric loss of the Si₃N_4(p)/Si₃N₄ composites increased with increasing infiltration time; however, the pore ratios decreased with increasing infiltration time. The maximum value of the flexural strength was 114.07 MPa. The dielectric constant and dielectric loss of the composites were 4.47 and 4.25 × 10⁻³, respectively. The present Si₃N_4(p)/Si₃N₄ composite is a good candidate for high-temperature radomes.     

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.     

Dissolution Kinetics of β-Si3N4 in an Mg-Si-O-N Glass

The rate of dissolution of β-Si₃N₄ into an Mg-Si-O-N glass was measured by working with a composition in the ternary system Si₃N₄-SiO₂-MgO such that Si₂N₂O rather than β-Si₃N₄ was the equilibrium phase. Dissolution was driven by the chemical reaction Si₃N₄(c)+SiO₂( l )→Si₂N₂O(c). Analysis of the kinetic data, in view of the morphology of the dissolving phase (Si₃N₄) and the precipitating phase (Si₂N₂O), led to the conclusion that the dissolution rate was controlled by reaction at the crystal/glass interface of the Si₃N₄, crystals. The process appears to have a fairly constant activation energy, equal to 621 ±40 kJ-mol⁻¹, at T=1573 to 1723 K. This large activation energy is believed to reflect the sum of two quantities: the heat of solution of β-Si₃N₄ hi the glass and the activation enthalpy for jumps of the slower-moving species across the crystal/glass interface. The data reported should be useful for interpreting creep and densification experiments with MgO-fluxed Si₃N₄.     

Synthesis and Properties of Porous Single-Phase β'-SiAlON Ceramics

Single-phase β'-SiAlON (Si_{6− z} Al _z O _z N_{8− z} , z = 0–4.2) ceramics with porous structure have been prepared by pressureless sintering of powder mixtures of á-Si₃N₄, AlN, and Al₂O₃ of the SiAlON compositions. A solution of AlN and Al₂O₃ into Si₃N₄ resulted in the β'-SiAlON, and full densification was prohibited because no other sintering additives were used. Relative densities ranging from 50%–90% were adjusted with the z -value and sintering temperature. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses indicated that single-phase β'-SiAlON free from a grain boundary glassy phase could be obtained. Both grain and pore sizes increased with increasing z -value. Low z -value resulted in a relatively high flexural strength.     

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.     

Bond Energetics at Intergranular Interfaces in Alumina-Doped Silicon Nitride

In Si₃N₄ ceramics sintered with Al₂O₃, the interfacial strength between the intergranular glass and the reinforcing grains has been observed to increase with increases in the aluminum and oxygen content of the epitaxial β-Si_{6- z} Al _z O _z N_{8– z} layer that forms on the Si₃N₄ grains. This has been attributed to the formation of a network of strong bonds (cross bonds) that span the glass-crystalline interface. This proposed mechanism is considered further in light of first-principles atomic cluster calculations of the relative stabilities of bridge and threefold-bonded atomic fragments chosen to represent compositional changes at the glass/Si₃N₄ grain interface. Calculated binding energies indicate Al-N binding is favorable at the Si₃N₄ grain surface, where aluminum occupancy can promote the growth of SiAlON, further enhancing the cross-bonding mechanism of interfacial strengthening.     

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.     

Phase Relationships in the Si3N4–SiO2–Lu2O3 System

Phase relationships in the Si₃N₄–SiO₂–Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P 2₁/ c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄–SiO₂–Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄–LuN–Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.     

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.     

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.     

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.     

Processing of Concentrated Aqueous Silicon Nitride Slips by Slip Casting

The optimization of concentrated Si₃N₄ powder aqueous slurry properties to achieve high packing density slipcast compacts and subsequent high sintered densities was investigated. The influence of pH, sintering aid powder (6% Y₂O₃, 4% Al₂O₃), NH₄PA dispersant, and Si₃N₄ oxidative thermal treatment was determined for 32 vol% Si₃N₄ slurries. The results were then utilized to optimize the dispersion properties of 43 vol% solids Si₃N₄-sintering aid slurries. Calcination of the Si₃N₄ powder was observed to result in significantly greater adsorption of NH₄PA dispersant and effectively reduced the viscosity of the 32 vol% slurries. Lower viscosities of the optimized dispersion 43 vol% Si₃N₄-sintering aid slurries resulted in higher slipcast packing density compacts with smaller pore sizes and pore volumes, and corresponding higher sintered densities.