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.     

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.     

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.     

The α–β Transformation in Silicon Nitride Single Crystals

Single crystals of α-Si₃N₄ were annealed at 2000°–2150°C. The β phase was detected after annealing at 2150°C only when the crystals were surrounded by MgO·3Al₂O₃ or Y₂O₃ powders. On the other hand, no evidence of the α–β transformation was found when the crystals were annealed without additives. The solution–precipitation mechanism was concluded to be the dominant factor in the α–β transformation of Si₃N₄.     

Densification and Microstructural Development of Silicon Nitride–Silica During Hot Isostatic Pressing

The influence of SiO₂ addition on the densification and microstructural development of high-purity Si₃N₄ during hot isostatic pressing (HIP) was studied. During HIP, densification was promoted, but the phase transformation from α -Si₃N₄ to β -Si₃N₄ was impeded by SiO₂. Analysis using a simple model shows that the enhanced densification was mainly due to the viscous flow of SiO₂. The microstructure changed remarkably at between 10 and 20 wt% SiO₂ additions. Analysis of the phase transformation kinetics suggests that the diffusion of Si₃N₄ through SiO₂ glass is the ratecontrolling step for the transformation.     

Controlled Crystallization of Amorphous Si3N4by Ti and CI Additives

Thin films of amorphous Si₃N₄ were prepared by the rf-sputtering method, and the effects of titanium and chlorine additives on its crystallization were examined. When Ti-doped amorphous Si₃N₄ was heated, TiN precipitated at >1100°C; the TiN precipitates promoted the conversion of amorphous Si₃N₄ to β-Si₃N₄. Chlorine led to preferential conversion of amorphous Si₃N₄ to α-Si₃N₄.     

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.     

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.     

Experimental Evidence for the Anisotropic Ostwald Ripening of β-Silicon Nitride

A powder mixture of α-Si₃N₄, Y₂O₃, and SiO₂ was heat-treated in a loose powder state in the temperature range of 1750°–1900°C for 2 h; then, the mixture was acid-rinsed to remove the glassy phase. The widths and lengths of the resulting β-Si₃N₄ crystals were analyzed quantitatively. The width–aspect-ratio distribution of the β-Si₃N₄ crystals initially showed a strong negative correlation, and then the aspect ratio of crystals with small widths quickly decreased. After a stage in which aspect ratio was almost constant, regardless of the width, the width-aspect-ratio distribution evolved to show a positive correlation in the final stage. This pattern of morphology evolution of the β-Si₃N₄ crystals was in good agreement with that predicted by the anisotropic Ostwald ripening model.     

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.     

Sintering Silicon Nitride Ceramics in Air

Silicon nitride (Si₃N₄) ceramics, prepared with Y₂O₃ and Al₂O₃ sintering additives, have been densified in air at temperatures of up to 1750°C using a conventional MoSi₂ element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y₂O₃ and 3 wt% Al₂O₃. Densification is accompanied by a small weight gain (typically <1–2 wt%), because of limited passive oxidation of the sample. Complete α- to β-Si₃N₄ transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si₂N₂O is also observed to form below 1750°C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with β-Y₂Si₂O₇ being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β-Si₃N₄ elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si₃N₄-based ceramics.     

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.     

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.     

Formation of α-Si3N4 Solid Solutions in the System Si3N4-A1N-Y2O3

The subsolidus phase diagram of the quasiternary system Si₃N₄-AlN-Y₂O₃ was established. In this system α-Si₃N₄ forms a solid solution with 0.1Y₂O₃: 0.9 AIN. The solubility limits are represented by Y_0.33Si_10.5Al_1.5O_0.5N_15.5 and Y_0.67Si₉A1₃ON₁₅. At 1700°C an equilibrium exists between β-Si₃N₄ and this solid solution.     

Phase Equilibria in the System Si3N4-SiO2-BeO-Be3N2

The 1780°C isothermal section of the reciprocal quasiternary system Si₃N₄-SiO₂-BeO-Be₃N₂ was investigated by the X-ray analysis of hot-pressed samples. The equilibrium relations shown involve previously known compounds and 8 newly found compounds: Be₆Si³N₈, Be₁₁Si₅N₁₄, Be₅Si²N₆, Be₉Si₃N₁₀, Be₈SiO₄N₄, Be₆O₃N₂, Be₈O₅N₂, and Be₉O₆N₂. Large solid solubility occurs in β-Si₃N₄, BeSiN₂, Be₉Si₃N₁₀, Be₄SiN₄, and β-Be₃N₂. Solid solubility in β-Si₃N₄ extends toward Be₂SiO₄ and decreases with increasing temperature from 19 mol% at 1770°C to 11.5 mol% Be₂SiO₄ at 1880°C. A 4-phase isotherm, liquid +β-Si₃N₄ ( ss )Si₂ON₂+ BeO, exists at 1770°C.     

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.     

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.     

Fabrication of Low-Shrinkage, Porous Silicon Nitride Ceramics by Addition of a Small Amount of Carbon

Successful net-shape sintering offers a significant advantage for producing large or complicated products. Porous Si₃N₄ ceramics with very low shrinkage were developed, in the present investigation, by the addition of a small amount of carbon. Carbon powders (1–5 vol%) of two types, with different mean particle sizes (13 nm and 5 μm), were added to α-Si₃N₄−5 wt% Y₂O₃ powders. SiC nanoparticles formed through reaction of the added carbon with SiO₂ on the Si₃N₄ surface or with the Si₃N₄ particles themselves. Such reaction-formed SiC nanoparticles apparently had an effective reinforcing effect, as in nanocomposites. Sintered Si₃N₄ porous ceramics with a high porosity of 50%–60%, a very small linear shrinkage of ∼2%–3%, and a strength of ∼100 MPa were obtained.     

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.     

Silicon Nitride Joining

Hot-pressed Si₃N₄ was joined using an Mgo-A1₂O₃-SiO₂ glass composition chosen to approximate the oxide portion of the grain-boundary phase in the ceramic. After it has been heated at 1550° to 1650°, the interface of the joined ceramic is an interlocking mixture of Si₂N₂O, β-Si₃N₄, and a residual oxy-nitride glass. The kinetics of reactions between Si₃N₄ and the molten joining composition were studied by X-ray diffraction analysis of the phases present in Si₃N4 powder-glass mixtures quenched after varied heat treatments. Analytical transmission electron microscopy of the composition and micro-structure of the reaction zone in joined specimens, together with the X-ray diffraction results, suggests that the driving force for joining is the lowering of the Si₃N₄ interfacial energy when it is wet by the molten silicate, augmented by the negative Gibbs energy for the reaction SiO₂( l ) + Si₃N₄= 2Si₂N₂O.