Reactive Hot Pressing of Alumina-Silicon Carbide Nanocomposites

A dense alumina-silicon carbide (Al₂O₃–SiC) nanocomposite was synthesized in situ from the reaction of mullite, aluminum, and carbon by reactive hot pressing (RHP). Transmission electron microscopy investigation showed that in situ -formed, nanometer-sized SiC particles were mainly entrapped in the matrix grains, whereas submicrometer-sized particles were located at the grain boundaries or triple points of the Al₂O₃. In addition, no amorphous phase was observed at the interfaces of the Al₂O₃ and SiC grains, which indicated strong direct bonding. Fracture-surface analysis by scanning electron microscopy revealed an intrafracture mode. The bending strength of the nanocomposite RHP-treated at 1800°C was 795 ± 160 MPa, and the fracture toughness, measured by the indentation method, was 3.1 MPa·m^1/2.     

Tensile Creep of Alumina-Silicon Carbide "Nanocomposites"

The tensile creep behavior of an (Al₂O₃-SiC) nanocomposite that contains 5 vol% of 0.15 μm SiC particles is examined in air under constant-load conditions. For a stress level of 100 MPa and in the temperature range of 1200°–1300°C, the SiC reduces the creep rate of Al2O3 by 2–3 orders of magnitude. In contrast to Al₂O₃, the nanocomposite exhibits no primary or secondary stages, with only tertiary creep being observed. Microstructural examination reveals extensive cavitation that is associated with SiC particles that are located at the Al₂O₃ grain boundaries. Failure of the nanocomposite occurs via growth of subcritical cracks that are nucleated preferentially at the gauge corners. A modified test procedure enables creep lifetimes to be estimated and compared with creep rupture data. Several possible roles of the SiC particles are considered, including (i) chemical alteration of the Al₂O₃ grain boundaries, (ii) retarded diffusion along the Al₂O₃-SiC interface, and (iii) inhibition of the accommodation process (either grain-boundary sliding or grain-boundary migration).     

Threshold Stress in Creep of Alumina-Silicon Carbide Nanocomposites

Tensile creep and the creep ruptures of an alumina-17 vol% silicon carbide nanocomposite at 1200°C were investigated, with a particular interest in the behaviors at low applied stresses. At stresses of <50 MPa, the creep rates were remarkably decayed and the creep lives were substantially prolonged, which suggested the presence of a threshold stress, below which creep stopped, in the creep of the nanocomposite. The stress was estimated to be in the range of 20–35 MPa. This threshold stress was in agreement with that predicted from another model where the motion of grain-boundary dislocations that were responsible for vacancy nucleation and annihilation was considered to be pinned by hard particles.     

Strength and Microstructure of Brazed Alumina-Silicon Carbide Ceramic-Matrix Composites

Alumina-silicon carbide whisker (Al₂O₃-SiC _w ) composite pieces were brazed under high vacuum with Ag-Cu-based active braze alloys. Joined specimens exhibit mean fracture strengths of 480 MPa (4.5 wt% titanium in braze) and 591 MPa (2.0 wt% titanium in braze) in four-point bend tests; these strengths are 78%−96% of the measured strength of the composite. The interfacial phases have been identified, by using transmission electron microscopy and electron probe microanalysis, as being Ti₃(Cu,Al,Si)₃O, Ti₅Si₃, TiC, and γ-TiO, and the relative locations of the interfacial phases are described. The high strengths are believed to result from both good wetting behavior under high vacuum and the composite nature of the reaction product morphology.     

Effects of Zirconium Doping on Grain-Boundary Bonding in Alumina-Silicon Carbide Composites

In this work, 800 ppm of Zr⁴⁺ dopants were added to Al₂O₃-5 vol% SiC particle composite. Zr⁴⁺ doping led to a weak Al₂O₃ grain-boundary bonding so that the fracture mode changed from transgranular in undoped composite to intergranular in Zr⁴⁺-doped composite. The fracture mode change increased the fracture toughness of the composite. Transmission electron microscopy and energy-dispersive spectroscopy examinations revealed that the weak grain-boundary bonding in the doped composite was caused by the segregation of Zr⁴⁺ and Si⁴⁺ ions at the Al₂O₃ grain boundary.     

Effects of Additives on the Pressure-Assisted Densification and Properties of Silicon Carbide

The densification of silicon carbide (SiC) was studied using a variety of additives (Al, AlN, Al₂O₃, B₄C, C, Si₃N₄, and Y₂O₃). The onset of densification of SiC with small amounts of additives occurred at temperatures between 1500° and 1900°C with 28 MPa applied pressure. Al, B₄C, and C promoted densification, while N (added as AlN or Si₃N₄) retarded sintering. A 96.75 wt% SiC–2 wt% Al–1 wt% C–0.25 wt% B₄C starting composition yielded the same percent of theoretical density (in the range of 70%–90% theoretical density) 400°C lower than a 95 wt% SiC–5 wt% AlN material. Yttria additions promoted intergranular fracture, which increased the single-edged precracked beam fracture toughness. The appropriate selection and amount of additives allowed for the tailoring of grain size and intergranular fracture, thus controlling the mechanical properties. While oxygen was present in all materials containing aluminum, the incorporation of additional oxygen as alumina resulted in reduced sintering activity compared with Al metal. Corrosion resistance decreased in both HF and NaOH solutions at 80°C for materials containing a grain boundary phase.     

Effects of Yttrium on the Sintering and Microstructure of Alumina–Silicon Carbide "Nanocomposites"

Alumina and alumina-based "nanocomposites" with 2 and 5 vol% silicon carbide and varying amounts of yttria (0–1.5 wt%) have been prepared by pressureless sintering in the temperature range 1450°–1650°C. The effects of composition and sintering temperature on density and microstructure are reported. Yttria inhibited sintering in alumina, but enhanced the sinterability of the nanocomposites. It also induced abnormal grain growth in both alumina and nanocomposites, but strongly bimodal grain size distributions could be prevented by careful choice of the composition and the sintering temperature. Fully dense (>99%), fine-grained alumina–5 vol% SiC–1.5 wt% yttria nanocomposites were produced from uniaxially pressed powders with a yttria content of 1.5 wt% and a sintering temperature of 1600°C. Reasons for this behavior are discussed, and it is suggested that the enhancement of sintering in the alumina–SiC materials is because of the reaction of silica on the surface of the silicon carbide particles with alumina, yttria, and possibly magnesia, modifying the grain boundary composition, resulting in enhanced grain boundary diffusion. scanning transmission electron microscopy/energy-dispersive X-ray data show that such co-segregation does occur in the yttria-containing nanocomposites.     

Densification of Alumina-Silicon Carbide Powder Composites: I, Effects of a Polymer Coating on Silicon Carbide Particles

One possible approach to improving the densification of powder composites containing a major crystalline phase which densifies (e.g., Al₂O₃) and a difficult-to-sinter phase (e.g., SiC) is to accommodate the matrix volume shrinkage with a "disappearing" polymer coating. A polymer coating prevents contact between the nonsinterable particles and the surrounding matrix. The coating can be burned off before sintering, allowing the matrix phase to "shrink-fit" around the nonsinterable particles during sintering. The effects of a polymer coating on the densification of a two-phase particle system were tested using SiC powder dispersed in an Al₂O₃ matrix. The composites processed with a polymer coating showed more densification during equivalent firing cycles than did those processed without a polymer coating. Densification during sintering was approximately proportional to the amount of polymer adsorbed on SiC, suggesting that the Al₂O₃ matrix did shrink-fit into the gaps between the SiC particles and the surrounding Al₂O₃ matrix. Differences in the pore-size distributions of polymercoated green compacts and uncoated compacts indicated a perturbation of the green microstructure by the gaps. The estimated average thickness of the gap is approximately 20 nm, ∼8% of the average radius of the SiC powder used in this study.     

Grain Size Effect on Creep Deformation of Alumina-Silicon Carbide Composites

A study of the flexural creep response of aluminas reinforced with 10 vol% SiC whiskers was conducted at 1200° and 1300°C at stresses from 50 to 230 MPa in air to evaluate the effect of matrix grain size. The average matrix grain size was varied from 1.2 to 8.0 μm by controlling the hot-pressing conditions. At 1200°C, the creep resistance of alumina composites increases with an increase in matrix grain size, and the creep rate (at constant applied stress) exhibits a grain size exponent of approximately 1. The stress exponent of the creep rate at 1200°C is approximately 2, consistent with a grain boundary sliding mechanism. On the other hand, the creep deformation rate of 1300°C was not sensitive to the alumina grain size. This was seen to be a result of enhanced nucleation and coalescence of creep cavities and the development of macroscopic cracks as the grain size increases. Observations also indicated that the prevalent site for nucleation and growth of creep cavities in coarsegrained materials is at two-grain junctions (grain faces), whereas in fine-grained materials cavities nucleate primarily at triple-grain junctions (grain edges). Electron microscopy studies revealed that the content of any amorphous phase present at whisker-alumina interfaces is independent of alumina grain size (and hot-pressing conditions). In addition, the alumina grain boundaries are quite devoid of amorphous phase(s). This variation in amorphous phase content does not appear to be a factor in the present creep results.     

SEBS-g-MAH对聚丙烯/氢氧化镁纳米复合材料微观结构与流变性能的影响

以马来酸酐接枝的SEBS(SEBS-g-MAH)作为界面改性剂制备了PP/SEBS-g-MAH/氢氧化镁(MH)纳米复合材料,采用SEM、DSC和毛细管流变仪研究了SEBS-g-MAH对复合材料的微观结构、结晶行为和流动性能的影响。结果表明,SEBS-g-MAH的加入充当了MH粒子与PP基体的界面层,提高了纳米MH粒子在PP中的均匀分散性,改善了两者的界面相互作用和PP/MH复合体系的流动性能。     

碳化硅木质陶瓷的显微结构及力学性能

以汉麻秆芯碳化后的碳粉为原料,分别采用注浆和干压成型工艺制备素坯,通过反应烧结制备出碳化硅木质陶瓷.研究了注浆成型工艺中悬浮稳定剂的种类和添加量对浆料性能的影响.采用激光共聚焦显微镜、扫描电子显微镜和X射线衍射仪等分析了碳化硅木质陶瓷的显微结构、物相组成及力学性能.结果表明:采用注浆成型制备的碳化硅木质陶瓷力学性能优异,实测的游离硅含量同理论计算结果一致,说明渗硅过程中硅碳反应充分,烧结体显微硬度、弯曲强度、弹性模量和断裂韧性分别为22.3 GPa、397 MPa、290 GPa和3.0 MPa·m1/2.     

High-Temperature Stability of Titanium Boride Reinforced Alumina-Silicon Carbide Based Composite

Silicon - Alumina and Silicon Carbide based composites are routinely used in mechanical, automobile, ceramic and aerospace industries with numerous reinforcements. Reinforcements increase the...     

Effects of Elemental Additives on Electrical Resistivity of Silicon Carbide Ceramics

Effects of various elemental additives on the electrical resistivity of hot-pressed SiC ceramics were studied. The electrical resistivity at room temperature of dense SiC ceramics varied greatly depending on the additives used. SiC ceramics with added Be had an extremely high electrical resistivity of 3 × 10¹²°.cm. On the other hand, SiC ceramics with added B and Al had electrical resistivities of 2 × 10 and 0.8 °cm, respectively. The differences in the electrical resistivity of the dense SiC ceramics were considered to be due to different solubilities of the additives in SiC grains. SiC ceramics with added Be had a low level of impurities in the SiC grains as a result of the low solubility of Be in these grains.     

Thermal and Electrical Properties in Plasma-Activation-Sintered Silicon Carbide with Rare-Earth-Oxide Additives

This study investigates the thermal and electrical properties of SiC ceramics with a combination of Y₂O₃ and rare-earth-oxide additions as sintering additives, by comparing four types of SiC starting powders varying in particle size and chemical composition. The powder mixtures were plasma-activation sintered to full densities and then annealed at high temperatures for grain growth. The thermal conductivity and electrical resistivity of the SiC ceramics were measured at room temperature by a laser-flash technique and a current–voltage method, respectively. The results indicate that the thermal conductivity and electrical resistivity of the SiC ceramics are dependent on the chemical composition and particle size of the starting powders. The thermal conductivities observed for all of the annealed materials with a rare-earth La₂O₃ sintering additive were >160 W·(m·K)⁻¹, although low electrical resistivity was observed for all materials, in the range 3.4–450 Ω·cm. High thermal conductivity, up to 242 W·(m·K)⁻¹, was achieved in an annealed material using a commercial 270 nm SiC starting powder.     

Processing and Properties of Sol–Gel-Derived Alumina/Silicon Carbide Nanocomposites

Dense alumina/5 vol% SiC nanocomposites were prepared by sol–gel processing using nanosized (180 nm) precoated SiC powders and a commercial boehmite sol. The SiC powder was precoated with boehmite by a controlled heterogeneous precipitation from an aluminum nitrate solution. The coated SiC powder was then dispersed in a boehmite sol, gelled, calcined, and densified by gas pressure sintering under argon atmosphere at 7–8 MPa pressure. The dependence of the calcination conditions on densification, the effect of seeding on the microstructural development, as well as the mechanical behavior of the sintered specimens, are presented and discussed.     

Effect of Aspect Ratio and Liquid-Phase Content on Densification of Alumina-Silicon Carbide Whisker Composites

Densification of alumina-SiC whisker composites by pressureless sintering is inhibited as a result of whisker interference with particle rearrangement and composite shrinkage. Reduction of the aspect ratio improves densification by improving particle/whisker packing for increased green densities and enhances the ability of the whiskers to rearrange themselves during sintering. Increasing the amount of liquid phases present during sintering also improves densification by aiding whisker rearrangement.     

PBT/改性坡缕石纳米复合材料的结构和微观力学性能

采用熔融共混法制备了PAS改性AT/PBT纳米复合材料.结果表明:PBT/AT复合材料中的AT在低含量时可以达到纳米级的良好分散,而高含量时出现AT的团聚体;与PBT复合后.AT的典型衍射峰未变化,且AT对基体PBT的晶型也几乎没有影响.AT的加入对PBT的热分解过程影响不大.微观力学结果表明:作为纳米级填料的AT在含量较低时可以对复合材料的黏度和蠕变性能产生明显影响,而随着AT含量的进一步增加,由于AT本身的团聚,其影响作用减弱.AT作为纳米级填料的纳米效应是其使PBT微观力学性能改变的机理.     

添加剂对MgTiO3陶瓷性能的影响

研究了H3BO3,CaO-SiO2-B2O3玻璃料及V2O5的添加对MgTiO3陶瓷的烧结和介电性能的影响,并对影响机理作了初步探讨。结果表明:合适的添加剂能够使MgTiO3陶瓷在1240～1300℃之间烧结;添加质量分数为3%的H3BO3,V2O5或1%的CaO-SiO2-B2O3玻璃料的MgTiO3陶瓷的介电常数分别为20.8,17.5和19.8,在5～20MHz下,介电损耗低,多为10-4数量级;在10kHz下,介电常数的温度系数在-66×10-6/℃左右,是一种性能良好的微波介质材料。     

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.