Fine-Grained Silicon Nitride Ceramics Prepared from β-Powder

Fine β-powder with an average particle size of 0.28 μm was prepared by grinding and centrifugal sedimentation of sub-micrometer β-powder. Fine- and uniform-grained ceramics were fabricated from the powder by hot pressing. The average grain size of the ceramic was 0.21 μm. It was shown that this kind of microstructure was desirable for the matrix of in situ composite. It was also shown that the ceramics could be superplastically deformed at a temperature as low as 1500°C.     

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.     

Preparation of β-Silicon Nitride Seeds for Self-Reinforced Silicon Nitride Ceramics

This paper describes a method for the preparation of silicon nitride (Si₃N₄) seeds that have an average aspect ratio of ∼4. The seeds are prepared via heat treatment of a powder mixture that contains alpha-phase-rich Si₃N₄ and 0.5 wt% Y₂O₃ at a temperature of 1800°C and a nitrogen pressure of 35 kPa. A Y-Si-O-N liquid forms during heat treatment; this liquid acts as a flux for seed precipitation. During cooling, the Y-Si-O-N liquid transforms to a thin intergranular grain-boundary phase and causes strong agglomeration of the seeds. The seeds can be isolated by dissolving the grain-boundary phase in hot phosphoric acid, followed by an ultrasonic treatment (for 30 min). The method can be used to produce large quantities of seeds.     

High-Temperature Oxidation Behavior of High-Purity α-, β-, and Mixed Silicon Nitride Ceramics

High-temperature oxidation behavior, microstructural evolution, and oxidation kinetics of additive-free α-, β-, and mixed silicon nitride ceramics is investigated. The oxidation rate of the ceramics depends on the allotropic ratio; best oxidation resistance is achieved for ceramics rich in α-phase. Variations in the oxidation kinetics are directly related to average grain size and glass distribution in the oxidation scale. The oxygen contents incorporated into the Si₃N₄ phase before its dissolution at the oxidation front affects the local glass composition and thereby yields nucleation and growth rates of SiO₂ crystallites within the glass phase and a final oxidation scale microstructure, which depend on the incorporated oxygen contents. For the α-polymorph, the dynamic oxygen solubility is found to remain negligible; therefore, a nitrogen-rich glass forms at the oxidation front, which promotes devitrification and yields a scale with small grain size and thin intergranular glass films. β-Si₃N₄ is observed to form oxygen-rich solid solutions on oxidation, which are in contact with silicon oxynitride or oxygen-rich glass. Nucleation of cristobalite in the latter is sluggish, yielding coarse-grained oxidation scales with thick intergranular glass film.     

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.     

Effects of Free Silicon on the α to βPhase Transformation in Silicon Nitride

Free silicon in silicon nitride compacts can act as suitable liquid solvent for Si₃N₄ and assists in the α to β phase transformation. The rate of transformation depends on the size of the added silicon particles. When submicrometer silicon particles are added, the rate of transformation is more rapid than when large particles are added; the complete transformation can be achieved at 1700°C in 1 h.     

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).     

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.     

Mechanism of Grain Growth and α-β'Transformation During Liquid-Phase Sintering of β'-Sialon

The rates of grain coarsening and α-β'transformation during the liquid-phase sintering of Si₃N₄-β'₆₀-YAG sialon have been measured at varying liquid fractions and z values in order to determine the rate-controlling mechanism. The average β'-grain size after sintering for 16 h at 1650°C shows no variation with the liquid-matrix fraction if the z value is fixed and a marked increase with the z value if the liquid fraction is fixed. Similarly, the amount of untransformed α-phase after sintering for 2.5 or 3.5 min at 1600°C shows no variation with the liquid-matrix fraction if the z value is fixed and a marked decrease with the z value if the liquid fraction is fixed. These results show that the grain coarsening and the α-β'transformation are controlled by the interface reaction. This conclusion is consistent with the observations in carbide-Co systems and with the theoretical predictions that the growth of faceted grains is controlled by interface reaction and that of spherical grains by diffusion. A general rule between the shape and the growth mechanism of grains in a liquid matrix is thus proposed.     

Microstructure of an Extruded β-Silicon Nitride Whisker-Reinforced Silicon Nitride Composite

β-Si₃N₄ whisker-reinforced β'SiAlON composites were fabricated by extrusion and densified, using pressureless sintering. Whisker alignment was observed in both the green and sintered microstructures. SEM analysis of polished, sintered samples showed a microstructure consisting of the original β-Si₃N₄ whiskers in a matrix of fine SiAlON grains. SEM of plasma-etched samples and TEM analysis showed that the whiskers, as a result of grain growth, consisted of two phases, a core and a sheath layer. X-ray mapping and EDS analysis revealed that the core material contained no trace of Al, confirming the presence of original β-Si₃N₄ whiskers. The composition of the sheath was qualitatively identical to that of the fine β' SiAlON grains in the matrix. The sheath was thus formed by the precipitation of the β'SiAlON during liquid-phase sintering and led to substantial growth of the whiskers. Microdiffraction showed that the β'SiAlON grew epitaxially on the β-Si₃N₄ whiskers, resulting in a heavily faulted SiAlON layer.     

Friction Coefficient of (α+β)-SiAlON Composite against a Steel and Dense Silicon Nitride Tribopair

The friction characteristics of hot-pressed (α+β)-SiAlON, versus those of bearing steel and dense Si₃N₄ under dry sliding conditions, are reported. The coefficient of friction decreases as the alpha-SiAlON content increases and is double that of a metal-ceramic pair, in comparison to that of a ceramic-ceramic pair.     

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.     

Duplex α,β-Sialon Ceramics Stabilized by Dysprosium and Samarium

Duplex αβ,-sialon ceramics with a minimum volume fraction of residual intergranular glass have been prepared using Dy or Sm as the α-sialon stabilizing element. These microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix. A number of the larger α-sialon grains contained dislocations and showed a core/shell structure. Dy gave an α-sialon which was stable over a wide temperature range (1350–1800°C) for long holding times, while the use of Sm resulted in less stable α-sialon structures at medium temperatures (1450°C) and the formation of melilite, R₂Si_3−xAl_xO_3+xN_4−x, β-sialon, and the 21R sialon polytype during prolonged heating. High α-phase contents gave a very high hardness ( H _V10 is approximately 22 GPa) but a comparatively low indentation fracture toughness (around 4.4 MPam^1/2). Duplex sialons fabricated from powder mixtures corresponding to an α-to-β sialon ratio of around 50:50 resulted in a sialon material with a favorable combination of high hardness (around 22 GPa) and increased toughness (to around 5.5 MPam^1/2).     

α→β Reverse Phase Transformation in Silicon Carbide in Silicon Nitride-Particulate-Reinforced-Silicon Carbide Composites

The α→β reverse transformation in SiC is observed in Si₃N₄-particulate-reinforced-SiC composites made from as-received α-SiC and α-Si₃N₄ powders. However, the transformation does not occur to any great extent in composites made from deoxidized Si₃N₄-SiC powder compacts. Detailed transmission electron microscopy shows that most interfaces are covered with an ∼10 Å thick amorphous intergranular film in the composites made from as-received powders, whereas most interfaces are free of such films in the composites made from deoxidized powder compacts. These observations indicate that the α→β reverse transformation in SiC is encouraged by a nitrogen-containing liquid phase that occurs at high temperature in the composites made from the as-received powders. A mechanism is proposed to account for the experimental observations.     

Microstructure of Silicon Nitride Containing β-Phase Seeds: II

The microstructure of silicon nitride containing different percentages of β-seeds was investigated. The average grain size and volume fraction of large grains increased with the incorporation of β-seeds. The length and aspect ratio of large grains in sintered Si₃N₄ initially increased by incorporating β-seeds, and then decreased. Similar trends were observed in the fracture toughness. The toughening mechanisms and fracture behaviors were correlated with the grain morphology measured from image analysis. Plots of measured toughness versus volume fraction of large grains fit very well with the equation derived by Becher and Budiansky, which strongly suggested the essential role of debonding length in the toughness of silicon nitride. Further improvement of fracture toughness in silicon nitride may be possible by tailoring the amount of incorporated β-phase seeds and leading to optimize the volume fraction, grain size, and aspect ratio of the elongated grains.     

Microstructure Control of Silicon Nitride by Seeding with Rodlike β-Silicon Nitride Particles

The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike β-Si₃N₄ singlecrystal particles with a diameter of 1 μm and a length of 4 μm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of β-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation D ⁿ–D₀ⁿ= kt , with growth exponents of 3 and 5 for the c -axis direction and the a -axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4–8.7 MPa·m^1/2, retaining high strength levels of about 1 GPa.     

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.     

Diffusion Bonding of Silicon Nitride Using a Superplastic β-SiAlON Interlayer

A superplastic β-SiAlON was used as an interlayer to diffusionally bond a hot-pressed silicon nitride to itself. The bonding was conducted in a graphite furnace under a constant uniaxial load of 5 MPa at temperatures varying from 1500° to 1650°C for 2 h, followed by annealing at temperatures in the range of 1600° to 1750^oC for 2 h. The bonds were evaluated using the four-point-bend method at both room temperature and high temperatures. The results indicate that strong, void-free joints can be produced with the superplastic β-SiAlON interlayer, with bond strengths ranging from 438 to 682 MPa, and that the Si₃N₄ joints are heat resistant, being able to retain their strength up to 1000°C (635 MPa), and therefore have potential for high-temperature applications.     

29Si MASNMR Spin-Lattice Relaxation Study of α-, β-, and Amorphous Silicon Nitride

The spin-lattice relaxation times, T ₁, in α-, β-, and amorphous Si₃N₄ have been obtained for the first time, using a multiple-pulse saturation recovery method. The saturation recovery of the ²⁹Si magnetization follows exponential behavior under magic-angle spinning conditions, within the limits of experimental error. A rather wide dispersion of T ₁ values is observed for the phases of Si₃N₄: 284 ± 29 min (–46.686 ppm) and 260 ± 23 min (–48.812 ppm) for the α-phase, 36 ± 4 min for the β-phase, and 11 ± 1 min for the amorphous phase, assuming an exponential recovery. The values obtained for the exponent in the power-law fitting are 0.599(9) (–46.686 ppm) and 0.61(1) (–48.812 ppm) for the α-phase, 0.52(2) for the β-phase, and 0.53(3) for the amorphous phase.