Grain Growth During Gas-Pressure Sintering of β-Silicon Nitride

The microstructures of gas-pressure-sintered materials from β-Si₃N₄ powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850° to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The rate-determining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from α-Si₃N₄ powder despite the smaller aspect ratio of the grains     

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.     

Nonequiaxial Grain Growth and Polytype Transformation of Sintered α-Silicon Carbide and β-Silicon Carbide

α(6 H )- and β(3 C )-SiC powders were sintered with the addition of AlB₂ and carbon. α-SiC powder could be densified to ∼98% of the theoretical density over a wide range of temperatures from 1900° to 2150°C and with the additives of 0.67–2.7 mass% of AlB₂ and 2.0 mass% of carbon. Sintering of the β-SiC powder required a temperature of >2000°C for densification with these additives. Grains in the α-SiC specimens grew gradually from spherical-shaped to plate-shaped grains at 2000°C; the 6 H polytype transformed mainly to 4 H . On the other hand, grains in the β-SiC largely grew at >2000°C; the 3 C polytype transformed to 4 H , 6 H , and 15 R . The stacking faults introduced in grains were denser in β-SiC than in α-SiC. The rapid grain growth in the β-SiC specimen was attributed to polytype transformation from the unstable 3 C polytype at the sintering temperature.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

Microstructural Changes in β-Silicon Nitride Grains upon Crystallizing the Grain-Boundary Glass

Crystallizing the grain-boundary glass of a liquid-phase-sintered Si₃N₄ ceramic for 2 h or less at 1500° led to formation of δ-Y₂Si₂O₇. After 5 h at 1500°, the δ-Y₂Si₂O₇ had transformed to β-Y₂Si₂O₇ with a concurrent dramatic increase in dislocation density within β-Si₃N₄ grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500° reduced dislocation densities to the levels found in as-sintered material.     

Influence of Pyridine–Borane on the Sintering Behavior and Properties of α-Silicon Carbide

In the present study, α-SiC powder is coated with pyridineborane (BH₃·C₅H₅N), a liquid molecular compound, which forms a boron carbonitride (BC_3.5N) layer by heat treatment at 1000°C under argon. The precipitation method leads to an improved chemical homogeneity in the compacted powder resulting in enhanced densification and significant reduction in grain growth during subsequent sintering at temperatures exceeding 2070°C. Thus, small average grain sizes of d ₅₀= 1.3 μm and a narrow grain size distribution ( d ₁₀= 0.6 μm, d ₉₀= 2.2 μm) are detected in the liquid-phase-processed sample sintered at 2200°C for 0.5 h in argon. Final densities of at least 98% of theoretical could be obtained by pressureless sintering at 2100°C. These results as well as the microstructural distribution of the sintering aids in the densified samples are discussed.     

Characterization of β-Silicon Nitride Whiskers

The physical, chemical, and structural properties of a commercially available β-Si₃N₄ whisker were characterized. Bulk chemical analysis indicated that the whiskers were close to stoichiometric silicon nitride, with oxygen and yttrium as the major impurities. Surface chemistry analysis by XPS analysis revealed that the surfaces consisted primarily of silicon nitride, with the oxygen and yttrium impurities concentrated at the surfaces. SEM and STEM studies indicated that the whiskers were dimensionally straight with relatively featureless surfaces, although some whiskers had Y-rich particles attached. The whiskers were also found to be of extreme crystallographic perfection, as determined by TEM analysis.     

Superplastic Behavior of Fine-Grained β-Silicon Nitride Material under Compression

The deformation behavior of a hot-pressed, fine-grained β-Si₃N₄ ceramic was investigated in the temperature range 1450°—1650°C, under compression, and the results for strain rate and temperature dependence of the flow stress are presented here. The present results show that the material is capable of high rates of deformation (∼10⁻⁴—10⁻³ s⁻¹) within a wide range of deformation temperatures and under a pressure of 5—100 MPa; no strain hardening occurs in the material, even at slow deformation rates, because of its stable microstructure; Newtonian flow occurs, with a stress exponent of approximately unity; and the material has activation energy values for flow in the range 344—410 kJ·mol⁻¹. Grain-boundary sliding and grain rotation, accommodated by viscous flow, might be the mechanisms of superplasticity for the present material.     

Elimination of large Pores During Gas-Pressure Sintering of β'-Sialon

The effect of N₂-gas pressure on the liquid filling of large pores (20 to 120 μm in diameter) is studied in sintered β'-sialon ( z = 1). The initially sintered sialon with large pores is sintered again and infitrated by a liquid (41A1₂O₃-41Y₂O₃-14Si₃Y₄-4AIN (wt%)) at 17800°C for various times under 0.1-, 0.3-, and 0.5-MPa (1-, 3-, and 5-atm) N₂. The liquid fills large interconnected pores; the size of the pores filled with liquid increases with N₂-gas pressure and time. In some liquid pockets, gas bubbles are formed and subsequently disappear during prolonged sintering treatment. The liquid-filling be havior with sintering pressure and time is explained by gas pressure in the pore and thermal decomposition of the material. The benefit of gas-pressure sintering for the elimination of large pores is assessed.     

Pulsed Electric Current Sintering of Silicon Nitride

Pulsed electric current sintering (PECS) has been used to densify α-Si₃N₄ powder doped with oxide additives of Y₂O₃ and Al₂O₃. A full density (>99%) was achieved with virtually no transformation to β-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the α-to-β phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated β-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (∼500 kJ/mol), thereby causing the retention of α-phase during the densification by PECS.     

Effects of Microstructure on Superplastic Behavior and Deformation Mechanisms in β-Silicon Nitride Ceramics

The influence of grain shape and size on superplastic behavior and deformation mechanisms was investigated in annealed β-silicon nitride materials and compared with the results for hot-pressed material. The microstructure of the annealed materials consisted of fine equiaxed β-grains together with some elongated ones. Similar to the deformation behavior in the hot-pressed material, strain hardening did not occur in these annealed materials. Moreover, in contrast to the deformation behavior under tension, grain alignment under compression resulting from the development of a mild texture did not give rise to strain hardening. An annealed material with small elongated grains had a flow-stress dependency of n = 1, whereas other annealed materials with large elongated grains exhibited a flow-stress dependency of n = 1.6. In terms of texture development and the effect of grain shape on the creep rate when diffusion was the rate-controlling mechanism, a single curve with a stress exponent of ∼1 and a grain-size exponent of 3 were obtained for all materials. This suggests that the deformation mechanism in these annealed materials was the same as that of fine equiaxed β-silicon nitride.     

MgSiN2 Addition as a Means of Increasing the Thermal Conductivity of β-Silicon Nitride

Si₃N₄ powders with the concurrent addition of Yb₂O₃ and MgSiN₂ were sintered at 1900°C for 2–48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content, and thermal conductivity of the sintered specimens were evaluated and compared with Si₃N₄, Yb₂O₃, and MgO addition. MgSiN₂ addition was effective for improving the thermal conductivity of Si₃N₄ ceramics, and a material with high thermal conductivity over 140 W·(m·K)⁻¹ could be obtained. For both specimens, lattice oxygen content was decreased with sintering time. However, the thermal conductivity of the MgSiN₂-doped specimen was slightly higher than the MgO-doped specimen with the same oxygen content.     

Ab Initio Calculations of the Atomic and Electronic Structure of β-Silicon Nitride

The atomic and electronic structure of the β-silicon nitride (β-Si₃N₄) crystal have been determined using the ab initio pseudopotential method based on the density functional theory. We have obtained the stable lattice parameters and the stable positions of 14 atoms in the unit cell for the structure P 6₃/ m for the first time. The electronic structure and the charge distribution indicate that the Si–N bond has both ionic and covalent characters. The band structure is in good agreement with the other first-principles results and consistent with the experiments.     

Elevated-Temperature Fracture Resistance of a Sintered α-Silicon Carbide

The fracture toughness of a dense, sintered commercial α-silicon carbide was determined for temperatures from 20° to 1400°C using both straight- and chevron-notched test specimens and also controlled-surface-microflaw specimens, all in three-point bending. The flexural strengths were also measured for the same range of temperatures and the trend is compared with that of the toughness. Measurements from this study are discussed and also compared with other results in the literature. Analysis reveals the importance of contrasting sharp crack and blunt crack techniques and also the need for addressing the microhardness indentation method separately. It is concluded that the fracture toughness of this silicon carbide is about 3 MPa · m½ and that the crack growth resistance is characterized by a flat R -curve behavior, both of which are independent of temperature from 20° to 1400°C.     

低温无压烧结氮化硅陶瓷的相变及致密化研究

采用氧化铝（Al2 O3）和氧化钇（Y2 O3）为烧结助剂,利用无压烧结工艺在低温下制备氮化硅陶瓷材料。利用XRD和SEM等着重研究了无压烧结氮化硅陶瓷低温阶段时的物相组成及其致密化。结果表明：当添加剂含量为10％,烧结温度高于1430℃时,α→β相转变较快;当烧结温度达到1510℃时,α相全部转变为β相。     

低温放电等离子烧结法制备氮化硅陶瓷

分别以MgO-Al2O3或MgO-AlPO4作为烧结助剂,采用放电等离子体低温快速烧结方法制备了主相为α相的Si3N4陶瓷材料.采用X射线衍射和扫描电子显微镜分析了样品的物相组成和显微结构;研究了烧结助剂及其含量、烧结温度对陶瓷样品的相对密度与力学性能的影响.结果表明:当采用4%质量分数,下同)MgO-4%Al2O3烧...     

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 in Silicon Nitride Containing β-Phase Seeding: III, Grain Growth and Coalescence

The mechanical properties of Si₃N₄ materials depend mainly on the microstructure, which originates during the densification process. The microscopic evidence indicates that β-Si₃N₄ seeds incorporated in the starting powders play an important role in microstructural development, especially in the heterogeneous grain growth of β-Si₃N₄ grains during sintering. The growth of β-grains is initiated from the β-seeds, resulting in a core/shell microstructure. The presence of Moiré fringes and dislocations is attributed to misfit strain and compositional differences between the core and the shell. Coalescence can occur at the final stage of sintering.     

烧结助剂对氮化硅陶瓷显微结构和性能的影响

氮化硅中氮原子和硅原子的自扩散系数很低，致密化所必需的扩散速度和烧结驱动力都很小，在烧结过程中需采用烧结助剂。烧结助剂是影响氮化硅陶瓷的显微结构和性能的关键因素之一。有效的烧结助剂不但可以改善氮化硅陶瓷的显微结构，而且可以提高氮化硅陶瓷的高温性能和抗氧化性能。     

Effect of β-Seed Addition on the Microstructural Evolution of Silicon Nitride Ceramics

When a small amount of β-Si₃N₄ seed particles is added during the preparation of Si₃N₄ ceramics, a bimodal microstructure is obtained by sintering at 1760°C. When the specimen is further heat-treated at 1900°C to enhance the bimodal characteristic, the growth of large β grains is limited. The addition of a controlled amount of β seeds of uniform and large size is suggested to obtain the intended bimodal microstructure of Si₃N₄ ceramics.