Microstructural Control of a 70% Silicon Nitride– 30% Barium Aluminum Silicate Self-Reinforced Composite

The processing response of a 70% silicon nitride–30% barium aluminum silicate (70%-Si₃N₄–30%-BAS) ceramic-matrix composite was studied using pressureless sintering, at temperatures ranging from 1740°C, which is below the melting point of BAS, to 1950°C. The relationship between the processing parameters and the microstructural constituents, such as morphology of the β-Si₃N₄ whisker and crystallization of the BAS matrix, was evaluated. The mechanical response of this array of microstructures was characterized for flexural strength, as well as fracture behavior, at test temperatures up to 1300°C. The indentation method was used to estimate the fracture resistance, and R -curves were obtained from modified compact-tension samples of selected microstructures at room temperature.     

Bimodal Microstructure in Silicon Nitride–Barium Aluminum Silicate Ceramic-Matrix Composites by Pressureless Sintering

A distinct bimodal microstructure has been obtained in a Si₃N₄–BAS (barium aluminum silicate) ceramic-matrix composite by pressureless sintering. It is shown that the addition of coarse β-Si₃N₄ seeds causes abnormal grain growth in this composite, and hence encourages the formation of a bimodal microstructure. This abnormal grain growth is due to the nature of the heterogeneous nucleation mechanism in Si₃N₄α-to-β phase transformation, and is promoted by the transformation. After complete phase transformation, further abnormal grain growth is comparably slow and governed by the Ostwald ripening mechanism. Therefore, a stable bimodal microstructure can be easily achieved by pressureless sintering.     

Preparation and Properties of Porous Aluminum Nitride–Silicon Carbide Composite Ceramics

Microporous two-phase AlN–SiC composites were prepared using Al₄C₃ and either Si (N₂ atmosphere) or Si₃N₄ (Ar atmosphere) as precursors. The reaction mechanisms of the two synthesis routes and the effect of processing conditions on reaction rate and the material microstructures were demonstrated. The exothermic reaction between Si and Al₄C₃ under N₂ atmosphere was shown to be a simple processing route for the preparation of porous two-phase AlN–SiC materials. The homogeneous two-phase AlN–SiC composites had a grain size in the range of 1–5 μm, and the porosity varied in the range of 36%–45%. The bending strength was 50–60 MPa, in accordance with the high porosity.     

Mechanical and Electrical Properties of Silicon Nitride–Silicon Carbide Nanocomposite Material

Mechanical and electrical properties of nanocomposite materials composed of a Si₃N₄ matrix and nanometer-sized SiC particles are described. Composites containing less than 10 vol% SiC particles have the same order of resistivity and dielectricity as the non-SiC material as well as highly improved mechanical properties. The composites are promising materials for use under harsh conditions.     

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al₄SiC₄) with the specific surface area of 15.5 m²·g⁻¹. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m²·g⁻¹, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.     

Boron Nitride–Aluminum Nitride Ceramic Composites Fabricated by Transient Plastic Phase Processing

BN–AlN ceramic composites have been successfully fabricated by a novel process referred to as transient plastic phase processing (TPPP). The process used BN as both the reactant phase and the matrix and Al as the transient plastic phase. The products AlN and AlB₁₂ were regarded as the reinforcing phases. With the addition of Al powder in BN, both the mechanical and thermal properties were improved. Relatively high green density (2.03 g/cm³, 82.0% of theoretical density (TD)) and as-sintered density (2.18 g/cm³, 92.6%TD), high bending strength (106 MPa), and high thermal conductivity (72 W/(m·K)) were attained for one kind of BN–AlN composite. A low thermal expansion coefficient of 2.0 × 10⁻⁶/K was also achieved.     

Synthesis of Aluminum Nitride–Silicon Carbide Solid Solutions by Combustion Nitridation

AlN–SiC solid solutions were synthesized via a combustion nitridation process. Reactions between powder mixtures of aluminum, silicon, and carbon or aluminum with β-SiC and gaseous nitrogen under pressures of 0.1–8.0 MPa are self-sustaining once they have been initiated. Investigations were made with reactant ratios of Al:Si:C = 7:3:3, 6:4:4, and 5:5:5 and Al:SiC = 7:3, 6:4, and 5:5. For the Al-Si-C system (molar ratio of 6:4:4), the maximum combustion temperature was dependent on the nitrogen pressure, increasing from 2300°C to 2480°C with an increase in pressure, from 0.1 MPa to 6.0 MPa. In all cases, the product contained the solid solution as the primary phase, with minor amounts of silicon. The amount of unreacted silicon decreased as the nitrogen pressure increased; the presence and dependence of unreacted silicon on pressure has been explained in terms of the volatilization of aluminum. The full width at half maximum for the (110) peak of the AlN–SiC solid solution decreased as the nitrogen pressure increased, which indicated the formation of a more homogeneous product.     

In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composite from Silicon Nitride–Boron Oxide–Carbon

This work proposes a new approach, based on the reaction Si₃N₄+ 2B₂O₃+ 9C → 3SiC + 4BN + 6CO, to synthesize an SiC–BN composite. The composite was prepared by reactive hot pressing (RHP), at 2000°C for 60 min at 30 MPa under an argon atmosphere, following a 60 min hold at 1700°C without applied pressure before reaching the RHP temperature. TG-DTA results showed that a nitrogen atmosphere inhibited denitrification somewhat and retarded the reaction rate. The chemical composition of the obtained material was consistent with theoretical values. FE-SEM observation showed that in situ -formed SiC and BN phases were of spherical morphology with very fine particle size of ∼100 nm.     

Mechanical Properties of Three-Layered Monolithic Silicon Nitride–Fibrous Silicon Nitride/Boron Nitride Monolith

A three-layered composite, composed of a strong outer layer (monolithic S₃N₄) and a tough inner layer (fibrous Si₃N₄/BN monolith), was fabricated by hot-pressing. For the inner layer, a Si₃N₄–polymer fiber made by extrusion was coated by dipping it into a 20 wt% BN-containing slurry. The three-layered composite exhibited excellent mechanical properties, including high strength, work of fracture, and crack resistance, because of the combination of a strong outer layer and a tough inner layer. In other words, the strong outer layer withheld the applied stress, while the tough inner layer promoted crack interactions through the weak BN cell boundaries. Also, the residual thermal stress on the surface due to the anisotropy in the coefficient of thermal expansion of BN affected a median/radial crack generation after indentation.     

Reaction-Bonded and Superplastically Sinter-Forged Silicon Nitride–Silicon Carbide Nanocomposites

Silicon nitride–silicon carbide (Si₃N₄–SiC) nanocomposites were fabricated by a process involving reaction bonding followed by superplastic sinter-forging. The nanocomposites exhibited an anisotropic microstructure, in which rod-shaped, micrometer-sized Si₃N₄ grains tended to align with their long axes along the material-flow direction. SiC particles, typically measuring several hundred nanometers, were located at the Si₃N₄ grain boundaries, and nanosized particles were dispersed inside the Si₃N₄ grains. A high bending strength of 1246 ± 119 MPa, as well as a high fracture toughness of 8.2 ± 0.9 MPa·m^1/2, was achieved when a stress was applied along the grain-alignment direction.     

Thermogravimetry, Differential Thermal Analysis, and Mass Spectrometry Study of the Silicon Nitride–Boron Carbide–Carbon Reaction System for the Synthesis of Silicon Carbide–Boron Nitride Composites

Thermogravimetry, differential thermal analysis, mass spectrometry, and X-ray diffractometry were used to study the reaction process of the in situ reaction between Si₃N₄, B₄C, and carbon for the synthesis of silicon carbide–boron nitride composites. Atmospheres with a low partial pressure of nitrogen (for example argon + 5%–10% nitrogen) seemed to inhibit denitrification and also maintain a high reaction rate. However, the reaction rate decreased significantly in a pure nitrogen atmosphere. The experimental mass spectrometry results also revealed that B₄C in the Si₃N₄–B₄C–C system inhibited the reaction between Si₃N₄ and carbon and, even, the decomposition of Si₃N₄. The present results indicate that boron could be a composition stabilizer for ceramic materials in the Si-N-C system used at high temperature.     

Microstructure and Mechanical Properties of Heat-Treated Silicon Carbide–Aluminum Nitride Solid Solutions

Nanophase-structured composites were fabricated by heat treating hot-pressed 2H-wurtzite SiC-AlN solid-solution specimens of 25, 50, and 75 mol% AlN within the spinodal decomposition zone. Heat-treatment conditions were 1750°C for 150 h, in flowing nitrogen gas. The hot-pressed specimens contained 2H-wurtzite equiaxed grains, and the grain size increased with AlN content. Lattice parameters followed Vegard's law. Nanoprecipitates with typical modulated tweed-type structures were observed along the [2 1 1 0] zone axis and were orthogonal to the {01 1 2} planes that make angles of 46.70°, 46.90°, and 47.11° to the [0001] for the three compositions. The microhardness, flexural strength, and fracture-toughness values of the heat-treated specimens were not significantly different from the hot-pressed values.     

Effect of Microstructure on High-Temperature Compressive Creep of Self-Reinforced Hot-Pressed Silicon Nitride

An experimental self-reinforced hot-pressed silicon nitride was used to examine the effects of microstructure on high-temperature deformation mechanisms during compression testing. At 1575–1625°C, the as-received material exhibited a stress exponent of 1 and appeared to deform by steady-state grain-boundary sliding accommodated by solution-reprecipitation of silicon nitride through the grain-boundary phase. The activation energy was 610 ± 110 kJ/mol. At 1450–1525°C for the as-received material, and at 1525–1600°C for the larger-grained heat-treated samples, the stress exponent was >1. Damage, primarily in the form of pockets of intergranular material at two-grain junctions, was observed in these samples.     

冷冻注凝制备氮化硅陶瓷基耐高温复合材料

采用硅溶胶冷冻胶凝陶瓷成型技术制备Si3N4/BAS陶瓷复合材料,分析了硅溶胶冷冻胶凝技术原理和特点,并对Si3N4/BAS陶瓷复合材料性能及微观形貌进行了研究。结果表明:该成型方法所获得的坯体干燥无变形无开裂,收缩率小于1%;陶瓷烧结体密度为2.9 g/cm3时,烧结体抗弯强度、弯曲弹性模量、断裂韧性以及洛氏硬度分别为350 MPa、193GPa、6.2 MPa·m1/2和58。该成型技术实现了陶瓷界多年来对先进陶瓷高效、低成本、原位近净尺寸成型的追求。     

Phase Relations and Microstructural Development of Aluminum Nitride–Aluminum Nitride Polytypoid Composites in the Aluminum Nitride–Alumina–Yttria System

AlN–AlN polytypoid composite materials were prepared in situ using pressureless sintering of AlN–Al₂O₃ mixtures (3.7–16.6 mol% Al₂O₃) using Y₂O₃ (1.4–1.5 wt%) as a sintering additive. Materials fired at 1950°C consisted of elongated grains of AlN polytypoids embedded in equiaxed AlN grains. The Al₂O₃ content in the polytypoids varied systematically with the overall Al₂O₃ content, but equilibrium phase composition was not established because of slow nucleation rate and rapid grain growth of the polytypoid grains. The polytypoids, 24 H and 39 R , previously not reported, were identified using HRTEM. Solid solution of Y₂O₃ in the polytypoids was demonstrated, and Y₂O₃ was shown to influence the stability of the AlN polytypoids. The present phase observations were summarized in a phase diagram for a binary section in the ternary system AlN–Al₂O₃–Y₂O₃ parallel to the AlN–Al₂O₃ join. Fracture toughness estimated from indentation measurements gave no evidence for a strengthening mechanism due to the elongated polytypoids.     

Reaction Synthesis of Aluminum Nitride–Boron Nitride Composites Based on the Nitridation of Aluminum Boride

Aluminum nitride–boron nitride (AlN–BN) composites were prepared based on the nitridation of aluminum boride (AlB₂). AlN powder was added to change the BN volume fraction in the obtained composites. Thermogravimetry–differential thermal analysis (TG-DTA), X-ray diffractometry, and the nitridation ratio were used to investigate the nitridation process of AlB₂. At ∼1000°C, a sharp exothermic peak occurred in the DTA curve, corresponding to the rapid nitridation of aluminum in AlB₂. On the other hand, the nitridation of the transient phase, Al_1.67B₂₂, was very slow when the temperature was <1400°C. However, the nitridation speed obviously accelerated at temperatures >1600°C. The pressure of the nitrogen atmosphere was also an important factor; high nitrogen pressure remarkably promoted nitridation. Treatment at 2000°C was disadvantageous for nitridation, because of the rapid formation of a dense surface layer that inhibited nitrogen diffusion into the specimen interior. Three specimens, with 5 wt% Y₂O₃ additive and different BN contents, were prepared by pressureless reactive sintering, according to the determined sintering schedule. Electron microscopy (scanning and transmission) observations revealed that the in-situ -formed BN flakes were homogeneously and isotropically distributed in the AlN matrix. A schematic mechanism for microstructural formation was developed, based on the results of nitridation and the microstructural features of the obtained composites. The obtained composites, with a low BN content, exhibited a high bending strength, comparable to that of reported hot-pressed AlN–BN composites.     

Impact Damage and Strength Degradation in a Silicon Carbide Reinforced Silicon Nitride Composite

Adding SiC particles to Si₃N₄ and subjecting the mixture to a sinter-hot-isostatic-pressing process increases both the strength and elastic modulus. It also decreases the hardness but maintains the fracture toughness, which results in a higher resistance to crack initiation and propagation during spherical particle impact. Sinter-hot-isostatically-pressed composites exhibit elastic response as their dominant behavior. They also display a high resistance to Hertzian cone crack initiation and extension. This is due to the increased degree of inelastic deformation of sinter-hot-isostatically-pressed composites.     

Fracture Toughness and Thermal Shock Behavior of Silicon Nitride–Boron Nitride Ceramics

Fracture toughness behavior, stress–strain behavior, and flaw resistance of pressureless-sintered Si₃N₄-BN ceramics are investigated. The results are discussed with respect to the reported thermal shock behavior of these composites. Although the materials behave linear-elastic and exhibit no R -curve behavior, their flaw resistance is different from that of other linear-elastic materials. Whereas the critical thermal shock temperature difference (Δ T _c) is enhanced by adding BN, the content of BN has no influence on the strength loss during severe thermal shocks.     

High-Temperature Strength and Cavitation Threshold of Silicon Nitride–Silica Ceramics

The role of high-purity silica in the fracture of Si₃N₄ at high temperatures has been investigated. The flexural strength at 1400°C was found to be greater than that at room temperature. Little plastic deformation was observed even when 10 wt% SiO₂ was added and the strain rate was decreased 2 orders of magnitude from that for a standard bend test. Microstructural observations revealed that the glassy phase was localized at intergranular pockets when SiO₂ additions were ≤ 10 wt%. High strength at 1400°C despite the presence of a fairly large amount of glassy phase is attributed to a high cavitation threshold in such glassy pockets consisting of high-purity SiO₂. However, the deformation behavior changed abruptly for SiO₂ additions of 10 and 20 wt%, which is explained by the morphological change of the glassy phase to thicker intergranular layers which allow macroscopic viscous flow.     

Microstructure and Mechanical Properties of Mullite–Silicon Carbide Composites

Mullite-SiC-whisker composites were prepared by powder processing using two commercial SiC whiskers. These composites were prepared by sintering rather than hot-pressing. A mulliteSlC-powder composite and a base line mallite material were also prepared for comparison with the two whisker composite materials. Fracture toughness measurements showed significant enhancement in only one of the whisker composite materials. The microstructure of the four materials was examined by scanning electron microscopy and transmission electron microscopy to assist in the explanation of the mechanical behavior of these composites. The examinations suggested that most of the toughening results from second-phase particles, with only limited toughening from effects associated with whiskers per se. In one case, higher toughness was partially associated with the formation of sialon phase by reaction with the whiskers and the furnace environment.