Formation of β-Si3N4 Coatings by Chemical Vapor Deposition

BetaSi₃N₄ coatings were obtained by chemical vapor deposition in a fused-silica reaction tube by outside heating of the system SiCl₄-NH₃-N₂ at a deposition temperature (reaction tube temperature) of 1300°C, whereas α- and α+β-phase coatings were obtained at depositon temperatures of 1150° and 1250°C, respecively. Formation of β-phase coatings at relatively low temperatures is explained in terms of the effect of a catalytic impurity, SiO vapor from the reaction tube. The X-ray diffraction patterns and sulface morphologies of the coatings were studied.     

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.     

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.     

Microstructural Observations on Hot-Pressed Si3N4

The development of microstructure in hot-pressed Si_aN₄ was studiehd for a typical Si₃N₄ powder with and without BeSiN₂ as a densification aid. The effect of hot-pressing temperature on density, α- to β-Si₃N₄ conversion and specific surface area showed that BeSiN₂ appears to increase the mobility of the system by enhancing densification, α- to β-Si₃N₄ transformation, and grain growth at temperatures between 1450° and 1800°. These processes appear to occur in the presence of a liquid phase.     

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.     

Sintering of Si3N4-Y2O3-Al2O3

Sintering kinetics of the system Si₃N4-Y2O₃-Al₂O3 were determined from measurements of the linear shrinkage of pressed disks sintered isothermally at 1500° to 1700°C. Amorphous and crystalline Si₃N₄ were studied with additions of 4 to 17 wt% Y₂O₃ and 4 wt% A1₂O₃. Sintering occurs by a liquid-phase mechanism in which the kinetics exhibit the three stages predicted by Kingery's model. However, the rates during the second stage of the process are higher for all compositions than predicted by the model. X-ray data show the presence of several transient phases which, with sufficient heating, disappear leaving mixtures of β ' -Si₃N₄ and glass or β '-Si₃N₄, α '-Si₃N₄, and glass. The compositions and amounts of the residual glassy phases are estimated.     

Preparation of Silicon Nitride-Titanium Nitride and Titanium-Titanium Nitride Composites from (CH3)3SiNHTiCl3-Coated Si3N4 and Ti Particles

[(Trimethylsilyl)amino]titanium trichloride, (CH₃)₃-SiNHTiClj, was isolated as a red-orange crystalline solid in 58% yield from the reaction of TiCl₄ with [(CH₃)₃Si]₂NH in 1:1 molar ratio in dichloromethane at —78°C. Pyrolysis of (CH₃)₃SiNHTiCl₃ at 600°C furnished titanium nitride. This precursor is suitable for the preparation of composites and was employed to prepare Si₃N₄-TiN and Ti-TiN powders by adding Si₃N₄ particles or titanium powders to a solution of (CH₃), SiNHTiCl₃ in dichloromethane, drying and pyrolyzing the resulting solid. This precursor also has been used as a binder to prepare Si₃N₄-TiN and Ti-TiN bodies. High-resolution transmission electron microscopic studies of the Si₃N₄-TiN composite showed that titanium nitride is concentrated on the surface of the Si₃N₄ particles.     

Solvothermal Synthesis of Si3N4 Nanomaterials at a Low Temperature

Silicon nitride nanowires or nanorods have been synthesized from SiCl₄, NaN₃, and metallic Mg at temperatures ranging from 200° to 300°C. X-ray powder diffraction patterns indicated that the as-obtained products were mainly β-Si₃N₄. Scanning electron microscope and high-resolution transmission electronic microscopy showed that the samples mostly consisted of Si₃N₄ nanowires or nanorods. As metallic iron powder was used, α-Si₃N₄ was mainly formed at 250°C.     

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.     

Fabrication of Si3N4/SiC Composite by Reaction-Bonding and Gas-Pressure Sintering

Si₃N₄/SiC composite materials have been fabricated by reaction-sintering and postsintering steps. The green body containing Si metal and SiC particles was reaction-sintered at 1370°C in a flowing N₂/H₂ gas mixture. The initial reaction product was dominated by alpha-Si₃N₄. However, as the reaction processed there was a gradual increase in the proportion of β-Si₃N₄. The reaction-bonded composite consisting of alpha-Si₃N₄, β-Si₃N₄, and SiC was heat-treated again at 2000°C for 150 min under 7-MPa N₂ gas pressure. The addition of SiC enhanced the reaction-sintering process and resulted in a fine microstructure, which in turn improved fracture strength to as high as 1220 MPa. The high value in flexural strength is attributed to the formation of uniformly elongated β-Si₃N₄ grains as well as small size of the grains (length = 2 μm, thickness = 0.5 μm). The reaction mechanism of the reaction sintering and the mechanical properties of the composite are discussed in terms of the development of microstructures.     

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.     

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.     

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.     

Insolubility of Mg in β-Si3N4 in the System Al-Mg-Si-O-N

Solid-solution formation of magnesium in β-Si₃N₄ containing AlN:Al₂O₃ was investigated. Samples were hot-pressed at 1700°C. Under the condition studied, very little or no magnesium entered the β-Si₃N₄ lattice.     

The Role of Cerium Orthosilicate in the Densification of Si3N4

The existence of compounds between Si₃N₄-CeO₂ and Si₃N₄-Ce₂O₃ was investigated for firing temperatures of 1600° to 1700°C. The two new monoclinic compounds found were Ce₂O₃·2Si₃N₄ with lattice parameters a = 16.288, b = 4.848, and c =7.853 Å and β=91.54° and Ce₄Si₂O₇N₂ with lattice parameters a = 10.360, b = 10.865, and c =3.974 Å and β=90.33°. Cerium orthosilicate (Ce 4.67 (SiO₄)₃O) is present during firing as a glassy intermediate phase which promotes sintering and densification and then reacts with silicon nitride to form cerium silicon oxynitrde (CeSiO₂N).     

Solid-Liquid Equilibria in the System Si3N4-AlN-Si02-A12O3

Solid-liquid equilibria at 1750°C and subsolidus phase relations in the system Si₃N₄−AlN-SiO₂−Al₂O₃ were determined for the composition region bounded by the β-Si₃N₄ solid solution line and silica-alumina join X-ray diffraction and optical microscopy were used to determine the phases present in specimens cooled rapidly after equilibration. The extent of a single liquid-phase region and the tie lines for the βsolid solution + liquid field at 1750°C were established from quantitative X-ray diffractometry and lattice parameter measurements of βsolid solutions in equilibrium with liquid. The results were corroborated by optical microscopy and melting behavior observations. A new composition, Si₁₂Al₁₈O₃9N₈, is suggested for the x₁ phase. The lowest melting temperature in the system is ≅ 1480°C and the corresponding composition is 10 eq% Al-90 eq%O.     

Low-Pressure Chemical Vapor Deposition of α-Si3N4 from SiF4 and NH3: Kinetic Characteristics

Deposition of α-Si₃N₄ from SiF₄ and NH₃ was systematically studied using an axisymmetric, vertical hot-wall reactor in the temperature range of 1340° to 1490°C. The relationship between process variables and deposition behavior was identified. The deposition process was most strongly influenced by temperature. In general, deposition rate increased exponentially with increased deposition temperature, although reagent depletion in the axial direction caused a rapid decrease in the deposition rate. The deposition rate increased moderately with increased flow rate or decreased NH₃/SiF₄ molar ratio. The decomposition characteristic of pure NH₃ and SiF₄ were studied utilizing mass spectroscopy and compared to thermodynamic predictions in order to assess their influences on the Si₃N₄ deposition process. Finally, the crystallography of Si₃N₄ deposits was correlated as a function of temperature and deposition rate.     

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.     

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.     

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.