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.     

Machinability of Silicon Nitride/Boron Nitride Nanocomposites

The machinability and deformation mechanism of Si₃N₄/BN nanocomposites were investigated in the present work. The fracture strength of Si₃N₄/BN microcomposites remarkably decreased with increased hexagonal graphitic boron nitride ( h -BN) content, although machinability was somewhat improved. However, the nanocomposites fabricated using the chemical method simultaneously had high fracture strength and good machinability. Hertzian contact tests were performed to clarify the deformation behavior by mechanical shock. As a result of this test, the damage of the monolithic Si₃N₄ and Si₃N₄/BN microcomposites indicated a classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.     

Tensile Creep Behavior of a Gas-Pressure-Sintered Silicon Nitride Containing Silicon Carbide

The tensile creep behavior of a gas-pressure-sintered silicon nitride containing silicon carbide was characterized at temperatures between 1375° and 1450°C with applied stresses between 50 and 250 MPa. Individual specimens were tested at fixed temperatures and applied loads. Each specimen was pin-loaded within the hot zone of a split-tube furnace through silicon carbide rods connected outside the furnace to a pneumatic cylinder. The gauge length was measured by laser extensometry, using gauge markers attached to the specimen. Secondary creep rates ranged from 0.54 to 270 Gs⁻¹, and the creep tests lasted from 6.7 to 1005 h. Exponential functions of stress and temperature were fitted to represent the secondary creep rate and the creep lifetime. This material was found to be more creep resistant than two other silicon nitride ceramics that had been characterized earlier by the same method of measurement as viable candidates for high-temperature service.     

Fabrication and Microstructure of Silicon Nitride/Boron Nitride Nanocomposites

A chemical process for fabrication of Si₃N₄/BN nanocomposite was devised to improve the mechanical properties. Si₃N₄/BN nanocomposites containing 0 to 30 vol% hexagonal BN ( h -BN) were successfully fabricated by hot-pressing α-Si₃N₄ powders, on which turbostratic BN ( t -BN) with a disordered layer structure was partly coated. The t -BN coating on α-Si₃N₄ particles was prepared by reducing and heating α-Si₃N₄ particles covered with a mixture of boric acid and urea. TEM observations of this nanocomposite revealed that the nanosized hexagonal BN ( h -BN) particles were homogeneously dispersed within Si₃N₄ grains as well as at grain boundaries. As expected from the rules of composites, Young's modulus of both micro- and nanocomposites decreased with an increase in h -BN content, while the fracture strength of the nanocomposites prepared in this work was significantly improved, compared with the conventional microcomposites.     

Rheological Behavior of Slurries and Consolidated Bodies Containing Mixed Silicon Nitride Networks

Two particle networks, each formulated with a different interparticle potential, were mixed to control the rheological properties of ceramic slurries and to develop claylike plasticity in consolidated bodies. A weakly attractive network, containing silicon nitride powder, alkylated with hexadecanol, was mixed with a second slurry containing flocculated (nonalkylated) silicon nitride powder. The elastic modulus and apparent yield stress of concentrated suspensions containing each constituent and their mixtures were found to increase with volume fraction according to a previously reported power law function (exponents of 4.8 and 3.75, respectively). Because of the large difference in the relative strengths of the two networks, the flocculated network overwhelmingly controlled the behavior of the mixed slurries when its volume fraction (relative to total solids) exceeded 0.30. Slurries were consolidated by pressure filtration, and the saturated bodies were tested in uniaxial compression. Bodies containing only alkylated powder packed to a high volume fraction and deformed at a low flow stress. The addition of small amounts of the flocculated network increased the flow stress to produce a body with rheological properties similar to clay.     

Si3N4及其复合材料强韧化研究进展

简述了氮化硅陶瓷的结构、性能和制备工艺，并分别通过自增韧补强、纤维／晶须强韧化、层状结构强韧化、相变强韧化以及颗粒弥散强韧化等方法对氮化硅陶瓷的强韧化研究进行了分类叙述。     

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.     

Elevated-Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

A unique, all-ceramic material capable of nonbrittle fracture via crack deflection and delamination has been mechanically characterized from 25° through 1400°C. This material, fibrous monoliths, was comprised of unidirectionally aligned 250 μm diameter silicon nitride cells surrounded by 10 to 20 μm thick boron nitride cell boundaries. The average flexure strengths of fibrous monoliths were 510 and 290 MPa for specimens tested at room temperature and 1300°C, respectively. Crack deflection in the BN cell boundaries was observed at all temperatures. Characteristic flexural responses were observed at temperatures between 25° and 1400°C. Changes in the flexural response at different temperatures were attributed to changes in the physical properties of either the silicon nitride cells or boron nitride cell boundary.     

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.     

Layered Silicon Nitride-Based Composites with Discontinuous Boron Nitride Interlayers

The influence of a strong/weak interface ratio on the mechanical properties of Si₃N₄/BN-based layered composites was studied. The ratio was controlled by the number of BN spots between the adjacent Si₃N₄ layers. By increasing the BN interface area from 0% to 72%, fracture toughness increased from 7.7 to 10.9 MPa·m^1/2, and bending strength decreased from 1275 to 982 MPa. Fracture toughness was improved from 8.6 to 10.1 MPa·m^1/2 by additional heat treatment of samples containing 2 vol%β-Si₃N₄ seed particles. The bending strength of samples with 35% weak BN interfaces, measured perpendicular and parallel to layer alignment, was 1260 and 1240 MPa, respectively. This confirmed the two-directional isotropy of layered samples.     

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.     

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.     

Molecular Dynamics Simulations of Calcium Silicate Intergranular Films between Silicon Nitride Crystals

Molecular dynamics simulations were used to study the structure of calcium silicate intergranular films (IGFs) formed between the basal planes of silicon nitride crystals. A multibody potential was used to describe the interactions between ions. Samples with different film thickness and CaO contents were studied. Epitaxial adsorption of Si and O atoms from the intergranular films onto N and Si, respectively, in the crystal surface was observed. This epitaxial order induced a structural order into the nominally amorphous IGF that decreased as a function of distance from the IGF/crystal interface. A higher concentration of strained siloxane bonds was observed at the IGF/crystal interface in comparison to the amorphous interior of the IGF. While Ca ions were observed to segregate to the IGF/crystal interface in simulations of calcium silicate glass IGFs between alumina crystals, no segregation of calcium to the first adsorbed layer on the nitride was found in these simulations using silicon nitride crystals. Planar alignment of Ca ions parallel to the IGF/crystal interface occurred with either the largest concentrations of CaO or with the thinnest IGFs studied here. This alignment creates localized nonbridging oxygen that would affect the stability of the IGF/crystal system.     

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.     

Reactivity of Aluminum Nitride Powder in Aqueous Silicon Nitride and Silicon Carbide Slurries

The reactivity of AlN powder with water in supernatants obtained from centrifuged Si₃N₄ and SiC slurries was studied by monitoring the pH versus time. Various Si₃N₄ and SiC powders were used, which were fabricated by different production routes and had surfaces oxidized to different degrees. The reactivity of the AlN powder in the supernatants was found to depend strongly on the concentration of dissolved silica in these slurries relative to the surface area of the AlN powder in the slurry. The hydrolysis of AlN did not occur if the concentration of dissolved silica, with respect to the AlN powder surface, was high enough (1 mg SiO₂/(m² AlN powder)) to form a layer of aluminosilicates on the AlN powder surface. This assumption was verified by measuring the pH of more concentrated (31 vol%) Si₃N₄ and SiC suspensions also including 5 wt% of AlN powder (with respect to the solids).     

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si₃N₄ was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si₃N₄. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si₃N₄ did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.     

Hot Isostatic Pressing of Silicon Nitride with Boron Nitride, Boron Carbide, and Carbon Additions

Si₃ N₄ test bars containing additions of BN, B₄C, and C, were hot isostatically pressed in Ta cladding at 1900° and 2050°C to 98.9% to 99.5% theoretical density. Room-temperature strength data on specimens containing 2 wt% BN and 0.5 wt% C were comparable to data obtained for Si₃ N₄ sintered with Y₂O₃, Y₂O₃ and Al₂O₃, or ZrO₂. The 1370°C strengths were less than those obtained for additions of Y₂O₃ or ZrO₂ but greater than those obtained from a combination of Y₂O₃ and Al₂O₃. Scanning electron microscope fractography indicated that, as with other types of Si₃N₄, roomtemperature strength was controlled by processing flaws. The decrease in strength at 1370°C was typical of Si₃N₄ having an amorphous grainboundary phase. The primary advantage of non-oxide additions appears to be in facilitating specimen removal from the Ta cladding.     

氮化硅粉体水基悬浮液电动特性的研究

通过测量氮化硅粉体在水溶液中的Zeta电位和颗粒大小j研究了不同条件下氮化硅水基悬浮液的电动特性。结果表明,溶液的pH值以及引入的聚合电解质不同均会使氮化硅颗粒表面的荷电状态发生变化,进而导致粉体在水溶液中的分散状况发生改变．在碱性条件下添加适量的阴离子聚电解质有利于氮化硅水基悬浮液的稳定分散。     

In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composites

SiC–BN composites were prepared via the proposed in situ reaction, which used Si₃N₄, B₄C, and carbon as reactants. Adding SiC powder to the reactants controlled the BN content in the composite. For comparison, SiC–BN composites with the same phase compositions were produced via conventional processing. The in situ process was advantageous for obtaining better composites with fine grain size and homogeneous microstructures. The in situ composite that had a BN content of 53.71 vol% exhibited considerably high strength (342 MPa) and a very low elastic modulus (107 GPa). The SiC–25-vol%-BN in situ composite had a peak strength of 588 MPa, which was 95% of that of monolithic SiC; however, the elastic modulus was as low as half that of monolithic SiC. These in situ SiC–BN composites can be expected to have excellent thermal shock resistance and mechanical strain tolerance.     

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.