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.     

Reaction Hot Pressing of α'- and β'- SiAlON Ceramics

Hot pressing kinetics of α-Si₃N₄, AIN, Al₂O₃, and Y₂O₃ powder mixtures forming α'- and β'-SiAlONs have been studied. Densification proceeds in two steps, first by a small shrinkage upon ternary eutectic oxide melting (SiO₂–Al₂O₃–Y₂O₃) at 1340°C, followed by a massive particle rearrangement and further shrinkage at higher temperature when nitride dissolution begins. With better wettability, AIN initially traps the oxide melt and delays densification. In addition, the preferential dissolution of AIN at 1450°C enriches the melt composition in AI, triggering transient precipitation of supersaturated β'-SiAlON. Full densification is readily achieved at 1550°C without complete α-Si₃N₄ conversion.     

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.     

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.     

Novel Two-Step Sintering Process to Obtain a Bimodal Microstructure in Silicon Nitride

A two-step sintering process is described in which the first step suppresses densification while allowing the α-to-β phase transformation to proceed, and the second step, at higher temperatures, promotes densification and grain growth. This process allows one to obtain a bimodal microstructure in Si₃N₄ without using β-Si₃N₄ seed crystals. A carbothermal reduction process was used in the first step to modify the densification and transformation rates of the compacts consisting of Si₃N₄, Y₂O₃, Al₂O₃, and a carbon mixture. The carbothermal reduction process reduces the oxygen:nitrogen ratio of the Y-Si-Al-O-N glass that forms, which leads to the precipitation of crystalline oxynitride phases, in particular, the apatite phase. Precipitation of the apatite phase reduces the amount of liquid phase and retards the densification process up to 1750°C; however, the α-to-β phase transformation is not hindered. This results in the distribution of large β-nuclei in a porous fine-grained β-Si₃N₄ matrix. Above 1750°C, liquid formed by the melting of apatite resulted in a rapid increase in densification rates, and the larger β-nuclei also grew rapidly, which promoted the development of a bimodal microstructure.     

Microstructure Characterization of Gas-Pressure-Sintered β-Silicon Nitride Containing Large β-Silicon Nitride Seeds

Fine β-Si₃N₄ powders with or without the addition of 5 wt% of large β-Si₃N₄ particles (seeds) were gas-pressure sintered at 1900°C for 4 h using Y₂O₃ and Al₂O₃ as sintering aids. The microstructures were examined on polished and plasmaetched surfaces. These materials had a microstructure of in situ composites with similar small matrix grains and different elongated grains. The elongated grains in the materials with seeds had a larger diameter and a smaller aspect ratio than those in the materials without seeds. A core/rim structure was observed in the elongated grains; the core was pure β-Si₃N₄ and the rim was β-SiAION. These results show that the large β-Si₃N₄ particles acted as seeds for abnormal grain growth and the rim was formed by precipitation from the liquid containing aluminum.     

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.     

Effect of Sintering Additives on Superplastic Deformation of Nano-Sized β-Silicon Nitride Ceramics

The effect of sintering additives on superplastic deformation of nano-sized β-Si₃N₄ ceramics has been studied by compression tests at 1500°C. The sintering additives were (i) Y₂O₃+Al₂O₃; (ii) Y₂O₃+MgO; and (iii) Y₂O₃. Nano-sized Si₃N₄ ceramics with different sintering additives had similar microstructures. For the first two sintering additives, the stress exponents were determined to be ∼2 at a lower stress region and ∼1 at a higher stress region, where the strain rate was dependent on sintering additives only at the higher stress region, and was independent at the lower stress region. Nano-ceramics with Y₂O₃ additives had only one region, which had a stress exponent of ∼1 within the stress range that we studied. The results could be explained by the different deformation mechanisms at the higher and lower stress regions and the influence of viscosity of liquid phase on the transition stress.     

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.     

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.     

Influence of Yttria–Alumina Content on Sintering Behavior and Microstructure of Silicon Nitride Ceramics

The present study investigates the influence of the content of Y₂O₃–Al₂O₃ sintering additive on the sintering behavior and microstructure of Si₃N₄ ceramics. The Y₂O₃:Al₂O₃ ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering-additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β-Si₃N₄ grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction.     

Effect of Nitrogen on the α/β Phase Conversion in Silicon Nitride

The effect of nitrogen on the α/β phase conversion of Si₃N₄-based, pressure-sintered bodies was investigated using nitrides and oxides as additives. When stable oxides are used, the α/β conversion is promoted, but when stable nitrides are present, the conversion is inhibited. α-Sialon can also be made from Si₃N₄ and AIN without the use of oxide additives (e.g., Y₂O₃).     

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.     

Densification Behavior and Properties of Y2O3-Containing α-SiAlON-Based Composites

Different SiAION composites based on α'-SiAION are investigated, with respect to the phase relationships, densification behavior, and mechanical properties. The compositions are located on a phase-diagram line parallel to the Si₃N₄-Y₂O₃ 9AIN compound in the Si₃N₄-SiO₂-AlN-Al₂O₃-Y₂O₃-YN system. Analysis of the reaction sequences shows that the formation of the composites is associated with the transient appearance of Y₄A1₂O₉ (YAM), yttrium-aluminum-garnet (YAG), melilite, and a nitrogen-rich liquid phase. The small shift of compositions on the Si₃N₄-Y₂O₃-9AIN compound phase-diagram line toward the Al₂O₃-rich side offers the advantage of a higher sinterability and the removal of the melilite phase from a wide range of compositions containing α'-SiAlON and polytypes. The α'/β'-SiAlON composites show better mechanical properties in comparison to pure α'-SiAlON and composites of α'-SiAION and polytypes. A post-heat-treatment causes the crystallization of YAG as a grain-boundary phase and leads to excellent strength retention up to temperatures of 1350°C.     

Hot-Pressing of Si3N4 with Y2O3 and Li2O as Additives

The rates of densification and phase transformation undergone by α-Si₃N₄ during hot-pressing in the presence of Y₂O₃, Y₂O₃−2SiO₂, and Li₂0−2Si0₂ as additives were studied. Although these systems behave less simply than MgO-doped Si₃N₄, the data can be interpreted during the early stages of hot-pressing as resulting from a solution-diffusion-reprecipitation mechanism, where the diffusion step is rate controlling and where the reprecipitation step invariably results in the formation of the β-Si₃N₄ phase.     

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.     

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.     

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.     

Thermal Conductivity of β-Si3N4: II, Effect of Lattice Oxygen

Dense β-Si₃N₄ with various Y₂O₃/SiO₂ additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y₂O₃/SiO₂ ratio increased. The oxygen contents in the β-Si₃N₄ crystal lattice of these samples were determined using hot-gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain-boundary phase, lattice oxygen content, and thermal conductivity of β-Si₃N₄ that was sintered at various Y₂O₃/SiO₂ additive ratios has been clarified.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.