Processing Strategy for Producing Highly Anisotropic Silicon Nitride

Silicon nitride with a preferred orientation of large elongated grains was obtained by tape casting of raw powder slurry seeded with rodlike β-Si₃N₄ particles, followed by a gas pressure sintering under 1 MPa nitrogen pressure. The large elongated grains developed from seeds lay in planes parallel to the casting direction in a two-dimensional distribution. Increased fracture toughness (11.1 MPa·m^1/2) and bending strength (1100 MPa) were achieved in the direction perpendicular to the grains alignment compared to specimens with a random distribution of elongated grains. Morover, the specimens exhibited a high Weibull modulus of 46 due to the uniform distribution of large grains.     

Texture and Anisotropy in Silicon Nitride

In this investigation quantitative texture analysis, including calculation of the orientation distribution function, is used to demonstrate the degree of preferred orientation in β -Si₃N₄ which has been hot-pressed or hot-worked. The results indicate that plane strain compression can produce strong textures. The texture is decided by the processing parameters including temperature, sintering additives, and stress state. Grain rotation and preferred grain growth apparently both contribute to texture development in β -Si₃N₄. Basal (00 l ) pole figures obtained from the orientation distribution function are consistent with microstructural observations and are reflected in indentation fracture toughness anisotropy. In plane strain the ratio of maximum to minimum fracture toughness is greater than 2.     

Further Improvement in Mechanical Properties of Highly Anisotropic Silicon Nitride Ceramics

Si₃N₄ceramics were fabricated by tape casting of a raw-powder slurry seeded with three types of rodlike β-Si₃N₄particles. The effects of seed size on the microstructure and mechanical properties of the sintered specimens were investigated. All the seeded and tape-cast silicon nitrides presented an anisotropic microstructure, where the elongated grains grown from seeds were preferentially oriented parallel to the casting direction. The orientation degree of these grains, f ₀, was affected by seed size, and small-seed addition led to the highest f ₀value. This material exhibited high bending strength (∼1.4 GPa) and high fracture toughness (∼12 MPa^.m^1/2) in the direction normal to the grain alignment, which were attributed to the highly anisotropic and fine microstructure.     

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.     

Electrostatic Adsorption of a Colloidal Sintering Agent on Silicon Nitride Particles

Pressureless sintering of silicon nitride requires addition of sintering agents. The main part of this study was done in order to homogenize the distribution of sintering agents, in this case Y₂O₃, in a silicon nitride matrix. Colloidal 10-nm Y₂O₃ Particles were electrostatically adsorbed on Si₃N₄ particle surfaces. The adsorption was studied by X-ray fluorescence analysis and electrophoretic measurements. Addition of Y₂O₃ sol to a Si₃N₄ suspension decreased the viscosity of the suspension. The slip casting properties of Si₃N₄ suspensions with added Y₂O₃ sol were examined, and the homogeneity of Y₂O₃ in the green compacts was compared with conventionally prepared samples. An improved microstructural homogeneity was obtained when Y₂O₃ sol particles were adsorbed on the Si₃N₄ particle surfaces.     

Silicon Carbide Platelet/Alumina Composites: I, Effect of Forming Technique on Platelet Orientation

SiC-platelet-reinforced Al₂O₃-matrix composites were made by three different forming techniques, i.e., slip casting, tape casting, and dry compaction of a granulated powder. All samples were densified with hot pressing at 1650°C and 25 MPa for 0.5 h. The orientation of SiC platelets in the composites was studied before and after hot pressing using optical microscopy and a pole figure X-ray device. X-ray diffraction of the (0006) plane of silicon carbide (6H) was used to analyze the degree of preferred orientation. It was found that both tape casting and die pressing could give rise to preferred orientation in green bodies with the faces of SiC platelets parallel to the tape faces or perpendicular to the pressing direction, respectively. The preferred orientation in die-pressed samples also showed an increase with the increase of the compaction stress; however, this reached a saturation level at about 70 MPa in a similar way to the green density. Samples formed by slip casting gave a platelet orientation close to a random one in the green body. After hot pressing, preferred orientation was observed in both slip-cast and tape-cast samples with the faces of SiC platelets perpendicular to the direction of hot pressing. The effect of platelet size on the orientation was also investigated. The preferred orientation in platelet composites was found to yield higher toughness than the random state.     

Fracture Behavior of Multilayer Silicon Nitride/Boron Nitride Ceramics

The fracture behavior of multilayer Si₃N₄/BN ceramics in bending has been studied. The materials were prepared by a process of tape casting, coating, laminating, and hot pressing. The Si₃N₄ layers were separated by thin, weak BN interlayers. Crack patterns in bending bars were examined with a scanning electron microscope. The weak layers deflected cracks in bending and thus prevented catastrophic failure. In one well-aligned multilayer ceramic A, a main crack propagated through the specimen although along a zigzag path. A second multilayer ceramic B was made to simulate a wood grain structure. Its failure was dominated by shear cracking along the weak BN layers. Besides crack deflection, interlock bridging between toothlike layers in the wake of the main crack appeared also to contribute to toughening.     

Fabrication of SiC-Whisker-Reinforced MoSi2 Composites by Tape Casting

In this paper, we describe a procedure for the processing of SiC-whisker-reinforced MoSi₂ composites via tape casting. Based on the characteristics of SiC whiskers and MoSi₂ powder in aqueous and nonaqueous solvents, a slip formulation (solvent, dispersant, binder, etc.) has been developed. The formulation developed allows for a uniform distribution of SiC whiskers in the matrix, easy separation of the tapes from the polymeric carrier, convenient control of both tape thickness and orientation of SiC whiskers, and a low binder burnout temperature. The latter is important for the prevention of the oxidation of MoSi₂ powder during the binder burnout in an oxidizing atmosphere.     

Processing of a Novel Multilayered Silicon Nitride

A new type of silicon nitride with a layered structure of alternating dense and porous layers was obtained by addition of β-Si₃N₄ whiskers to the porous layers. The materials consisted of dense layers 60 μm thick and porous layers 40 μm thick with a final porosity of about 30%. Highly anisotropic shrinkage behavior was observed during sintering. A large addition of whiskers to the porous layers resulted in layers with well-oriented and tightly tangled elongated grains, where porosity is represented by anisotropic shaped pores.     

Porous 2H-Silicon Carbide Ceramics Fabricated by Carbothermal Reaction between Silicon Nitride and Carbon

Porous SiC ceramics were synthesized by sintering pressed and pressed/CIPed powder compacts of α-Si₃N₄, carbon (Si₃N₄:C = 1:3 mol as ratio), and sintering aids, at 1600°C for few hours to achieve a reaction, and subsequently sintering at a temperature range of 1750°–1900°C, in an argon atmosphere. High porosities from 45%–65% were achieved by low shrinkage with large weight loss. Formation of pure 2H-SiC phase via a reaction between Si₃N₄ and carbon can be demonstrated by X-ray diffractometry. The resultant porous SiC samples were characterized by SiC grain microstructures, pore-size distribution, and flexural strength. This method has the advantage of fabricating high-porous SiC ceramics with fine microstructure and good properties at a relatively low temperature.     

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.     

Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites

Based on a biomimetic design, Si₃N₄/BN composites with laminated structures have been prepared and investigated through composition control and structure design. To further improve the mechanical properties of the composites, Si₃N₄ matrix layers were reinforced by SiC whiskers and BN separating layers were modified by adding Si₃N₄ or Al₂O₃. The results showed that the addition of SiC whiskers in the Si₃N₄ matrix layers could greatly improve the apparent fracture toughness (reaching 28.1 MPa·m^1/2), at the same time keeping the higher bending strength (reaching 651.5 MPa) of the composites. Additions of 50 wt% Al₂O₃ or 10 wt% Si₃N₄ to BN interfacial layers had a beneficial effect on the strength and toughness of the laminated Si₃N₄/BN composites. Through observation of microstructure by SEM, multilevel toughening mechanisms contributing to high toughness of the laminated Si₃N₄/BN composites were present as the first-level toughening mechanisms from BN interfacial layers as crack deflection, bifurcation, and pull-out of matrix sheets, and the secondary toughening mechanism from whiskers in matrix layers.     

High Thermal Conductivity in Silicon Nitride with Anisotropie Microstructure

Silicon nitride was fabricated by tape casting of α-Si₃N₄ powder with 5 wt% Y₂O₃ and 5 vol% rodlike β-Si₃N₄ seed particles, followed by tape stacking, hot pressing under 40 MPa, and annealing at 1850°C for 2-66 h under a nitrogen pressure of 0.9 MPa. Silicon nitrides fabricated by this procedure exhibited a highly anisotropic microstructure with large elongated grains (developed from seed particles) uniaxially oriented parallel to the casting direction. Thermal conductivities parallel to the grain alignment were much higher than those measured in other directions and exhibited high values of up to 120 W/(m.K). The anisotropic thermal conductivity of the specimen could be explained by the rule of mixture, considering that large elongated grains developed from seeds have higher thermal conductivity than a small-grained matrix.     

Templated Grain Growth of Textured Bismuth Titanate

Textured bismuth titanate, Bi₄Ti₃O₁₂ (BiT), was produced by templated grain growth (TGG). Molten-salt-synthesized BiT platelets were dispersed in a matrix of 200 nm BiT powder and aligned by tape casting. Highly textured BiT was obtained with the use of only 5 vol% template particles by sintering at 1000°C for 1 h. The uniformity of the through-thickness texture is much higher than reported in the literature for BiT tapes cast with 100% platelets. Initial platelet alignment is shown to increase because of frequent interaction with the fine powder particles during tape casting. By avoiding pressure densification techniques and using only a small portion of anisometric particles, TGG is a low-cost option for fabricating textured ceramics.     

Preparation of Silicon Nitride Whiskers from Diatomaceous Earth: I, Reaction Conditions

Whiskers and powder of silicon nitride were prepared by the carbothermal reduction and nitridation of diatomaceous earth in the presence of flowing N₂ and NH₃. The optimum temperature for the formation of Si₃N₄ whiskers was 1350°C and the yield reached almost 20% after 24 h. The α-Si₃N₄ content decreased with increasing nitridation temperature. Yields of the whiskers were dependent on NH₃ concentration and the total gas feed rate. The maximum yield of inside whiskers was obtained for a 25 vol% NH₃/N₂ mixture, while the maximum quantity of outside whiskers was produced for 75 vol% NH₃/N₂. The sum of the yield of the inside and outside whiskers increased with decreasing total gas feed rate. However, no nitridation of SiO₂ was observed at a feed gas rate below 0.18 mmol·min⁻¹. The yield of the inside whiskers increased gradually with increasing reaction time up to 36 h, whereupon a constant value was attained. Although the amount of outside whiskers produced was relatively small, the quantity seemed to increase until 60 h.     

Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride. I: Effect of Si Powder Characteristics

This paper will present sintered reaction-bonded silicon nitride (SRBSN) material with a high thermal conductivity of 121 W·(m·K)⁻¹, which has been successfully prepared from a coarse Si powder with lower levels of oxygen and aluminum impurities, using a mixture of Y₂O₃ and MgSiN₂ as sintering additives, by nitriding at 1400°C for 8 h and subsequent post-sintering at 1900°C for 12 h at a nitrogen pressure of 1 MPa N₂. This thermal conductivity value is higher than that of the materials prepared from high-purity α-Si₃N₄ powder (UBE SN-E10) with the same additive composition under the same sintering conditions. In order to study the effects of Si powder characteristics on the processing, microstructure, and thermal conductivity of SRBSN, the other type of fine powder with higher native oxygen and metallic impurity (typically Al and Fe) contents was also used. The effects of Si particle size, native oxygen, and metallic impurities on the nitriding process, post-sintering process, and thermal conductivity of the resultant SRBSN materials were discussed in detail. This work demonstrates that the improvement in thermal conductivity of SRBSN could be achieved by using higher purity coarse Si powder with lower levels of oxygen and aluminum impurities. In addition, this work also shows that the nitriding temperature has no significant effect on the microstructure and thermal conductivity of SRBSN during post-sintering, although it does affect the characteristics of RBSN formed during nitridation.     

Tensile Strength of Silicon Nitride Whiskers Synthesized by Reacting Amorphous Silicon Nitride and Titanium Dioxide

The tensile strength of α-Si₃N₄ whiskers synthesized by reacting amorphous Si₃N₄ and TiO₂ at 1490°C under a N₂ pressure of 700 torr was measured using a microbalance, and the diameter dependence of the strength was investigated. The Si₃N₄ whiskers had diameters of 0.04 and 0.8 μm and dominant [1011] and [1010] growth directions. Chemical analysis showed that they contained Ti and O impurities. The tensile strength of six Si₃N₄ whiskers increased from 17 to 59 GPa with decreasing whisker diameter.     

Densification Behavior in Microwave-Sintered Silicon Nitride at 28 GHz

Si₃N₄ powders were sintered using a 28 GHz gyrotron source, with Y₂O₃, Al₂O₃, and MgO as sintering aids, in an attempt to investigate the effect of microwave radiation on densification behavior. The microwave-sintered samples were compared with identical samples produced by conventional pressureless sintering. The effect of sintering on the microstructural development and grain growth of the samples was assessed using scanning electron microscopy. Phase transformation behavior was assessed using X-ray diffractometry. In the microwave-sintered samples, densification and α→β transformation occurred at temperatures ∼200°C lower than those of the conventionally sintered samples. More importantly, at comparable stages of densification, the microstructures of the microwave-sintered and conventionally sintered samples were significantly different, with the microwave-sintered samples showing the development of elongated β grains at a much earlier stage of the α→β transformation. It was concluded that the effect of microwave radiation on sintering was not simply a decrease in sintering temperatures, but in possibly a different sintering mechanism, clearly related to localized heating within the grain-boundary phase.     

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.     

Silicon Carbide Whisker Stability During Processing of Silicon Nitride Matrix Composites

The effects of two different sources of SiC whiskers on the chemistry and microstructure of the SiC-whisker—Si₃N₄ composites were evaluated using scanning transmission electron microscopy. Analyses were performed after presintering in N₂ and after encapsulated hot isostatic pressing. Significant differences in the porosity, α- to β-Si₃N₄ conversion, and whisker degradation were observed after presintering. It was also noted that whiskers containing surface iron impurities were converted to Si₃N₄ during processing. Whiskers from the source having low surface iron exhibited little reaction. After hot isostatic pressing, some oxidation of the cleaner whiskers was observed.