Aluminum matrix composites based on preceramic-polymer-bonded SiC preforms

Polymethylsiloxane (PMS) was used as a binder to make self-supporting SiC preforms for pressurized aluminum melt infiltration. The SiC particles were coated with preceramic polymer by spray drying; this ensured a fine and homogeneous distribution coupled with a high yield of the binder. The conditioned SiC powder mixtures were processed into preforms by warm pressing, curing and pyrolysis. A polymer content of 1.25 wt.% conferred sufficient stability to the preforms to enable composite processing. Using this procedure, SiC preforms with various SiC particle size distributions were prepared. The resulting Al/SiC composites with SiC contents of about 60 vol.% obtained by squeeze casting infiltration exhibit a 4-point bending strength of ∼500 MPa and Young’s moduli of ∼200 GPa. These values are comparable to those of compositionally identical, but binder-free composites. It is thus shown that the PMS-derived binder confers the desired strength to the SiC preforms without impairing the mechanical properties of the resulting Al/SiC composites.     

Particulate size effect on the rheology of SiC-glass composites

The rheological behavior of SiC particulate glass composites was investigated in the present study. The nature and extent of flow modifications are addressed with respect to solid content in the suspension, temperature and dispersoid size. A transition from Newtonian to non-Newtonian viscous flow and characteristic shear thinning behavior were observed. With progressive strengthening and deviation from Newtonian flow, a significant loss in rate sensitivity occurred. The apparent viscosity of the composites increased with the concentration and size of reinforcements. The increase in viscosity is explained in terms of hydrodynamic/mechanical interactions between particles in the composites.     

多孔C/SiC复合材料的制备及其性能

以W丝作为成孔剂,采用孔隙预置技术制备了发汗多孔C/SiC复合材料,对其孔隙结构进行表征,研究了材料的力学性能和渗透行为.结果表明:采用孔隙预置技术能够有效的控制多孔C/SiC材料开孔率和孔隙结构,其孔隙主要由W丝去除后形成的直通孔组成,开孔率决定于W丝的体积含量,所制备的材料具有良好的力学性能和渗透性能.其弯曲强度达到358 MPa、弯曲模量达到124 GPa,断裂韧性达到16.7 MPa·m1/2,空隙率为23.5%,渗透率为1.02×10-3mm2,材料表现为韧性断裂模式,其孔隙的存在并没有对材料的力学性能产生明显的影响.     

Analysis of three-dimensional textile preforms for multidirectional reinforcement of composites

A mathematical model has been developed to describe the structural geometry of three-dimensional textile preforms made by the two-step braiding process. These structures consist of parallel yarns, interconnected with braiding yarns, that lie in complex spatial orientations. The model predicts structural features such as fibre orientation, fibre volume fraction, and interyarn voids from the key process variables of braiding pattern, advance rate, and yarn geometry. The limiting geometry was computed by establishing the point at which yarns jam against each other. Using this factor makes it possible to identify the complete range of allowable geometric arrangements for this type of preform. Experiments of several bare and impregnated samples confirmed the theoretical predictions and demonstrated that very high fibre loadings (above 75% fibre volume fraction) could be achieved. The modelling technique used a “unit cell” approach, which can be applied to many other types of preform for advanced composites.     

Highly aligned flax/polypropylene nonwoven preforms for thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from unidirectional yarns and woven fabrics are known to perform significantly better than composites made from random nonwoven mats, but unidirectional yarns and fabrics are much more expensive to manufacture than random nonwoven mats. Here, we report on highly aligned natural fibre nonwoven mats that can be used as a replacement for unidirectional woven fabrics. A drawing operation is added to the conventional nonwoven process to improve fibre alignment in the nonwoven preforms and the final composites. The modified nonwoven manufacturing process is much simpler and cheaper than the unidirectional woven fabric process because of the elimination of expensive spinning and weaving operations. The composites fabricated from the highly aligned nonwoven mats showed similar mechanical strength as the composites made from unidirectional woven fabrics.     

Pressure-pulsed chemical vapour infiltration of TiN to SiC particulate preforms

SiC particulate preforms were infiltrated by TiN matrix from a gas mixture of TiCl₄ (5%), nitrogen (30%) and hydrogen using a repeating pressure pulse between 760 and about 1 torr. SiC particle sizes of 5 and 20 m were used. For matrix packing into deep level, optimum temperature was determined between 800 and 850 °C, and the maximum packing ratio reached 67% after 4 × 10⁴ pulses at 850 °C. The increase of TiCl⁴ concentration to 10% resulted in higher deposition rate and packing ratio. The decrease of nitrogen concentration led to slower deposition, that is, a similar effect to temperature lowering. The maximum flexural strength measured was 140 MPa.     

Low-volume-fraction particulate preforms for making metal-matrix composites by liquid metal infiltration

A preform technology for making particulate metal-matrix composites with a low particulate volume fraction (as low as 18%) by liquid metal infiltration is provided. This technology used a non-combustible reinforcement (SiC) as the primary particulate and combustible particles (carbon) as the secondary particulate in the preform. The secondary particulate was removed from the preform by oxidation prior to liquid metal infiltration.     

Direct measurement of drainage curves in infiltration of SiC particle preforms

Drainage curves (plots of the non-wetting fluid saturation versus applied pressure) for infiltration of SiC particle preforms with Al and Al–12.2 at.% Si are measured at 1023 K (750 °C) with a pressure infiltration apparatus adapted for direct tracking of the metal ingress into the porous preform. From these curves the work of immersion is estimated by integration over the whole range of saturation and pressures from 0.1 MPa to 10 MPa, which in turn is used to deduce the metal contact angle on the ceramic. Drainage curves obtained for powder beds based on monomodal SiC particles of mean diameter from 6.5 μm to 40 μm yield values for the work of immersion and contact angles that are consistent among themselves and are in good agreement with data in the literature determined by the sessile drop method.     

Pulsed chemical vapour infiltration of SiC to three-dimensional carbon fibre preforms

Three-dimensional carbon fibre preforms were infiltrated with silicon carbide from a gas system of CH₃SiCl₃-H₂ using a process of pressure pulsed chemical vapour infiltration. To infiltrate to a deep level, the temperature had to be lowered to 870–900°C, and the hold time per pulse below 1.0 s. Three-dimensional carbon fibre preforms partly filled with SiC fine powder were compared with those without filler. The weight of the preforms increased linearly with increasing number of pulses up to 10⁵ when no filler was present. However, the weight increase slowed down above 8×10⁴ pulses when the filler was used. Preforms with and without SiC filler showed three-point flexural strengths of 160 and 80 MPa after CVI of 10⁵ pulses, respectively. In order to improve the strength, a denser filling of SiC powder is necessary.     

Microstructure and tensile properties of BN/SiC coated Hi-Nicalon, and Sylramic SiC fiber preforms

Batch to batch and within batch variations, and the influence of fiber architecture on room temperature physical and tensile properties of BN/SiC coated Hi-Nicalon and Sylramic SiC fiber preform specimens were determined. The three fiber architectures studied were plain weave (PW), 5-harness satin (5HS), and 8-harness satin (8HS). Results indicate that the physical properties vary up to 10 percent within a batch, and up to 20 percent between batches of preforms. Load-reload (Hysteresis) and acoustic emission methods were used to analyze damage accumulation occurring during tensile loading. Early acoustic emission activity, before observable hysteretic behavior, indicates that the damage starts with the formation of nonbridged tunnel cracks. These cracks then propagate and intersect the load bearing 0° fibers giving rise to hysteretic behavior. For the Hi-Nicalon preform specimens, the onset of ° bundle cracking stress and strain appeared to be independent of the fiber architecture. Also, the 0° fiber bundle cracking strain remained nearly the same for the preform specimens of both fiber types. TEM analysis indicates that the CVI BN interface coating is mostly amorphous and contains carbon and oxygen impurities, and the CVI SiC coating is crystalline. No reaction exists between the CVI BN and SiC coating.     

三维针刺C/C-SiC复合材料预制体工艺参数优化EI北大核心CSCD

基于误差反向传播(BP)神经网络与改进的遗传算法建立三维针刺C/C-SiC复合材料预制体工艺优化的代理模型,获得针刺工艺参数与复合材料刚度性能之间的关系。利用BP网络实现复合材料刚度性能预测,BP网络的预测值与有限元计算结果吻合程度较好,模型训练误差最大为0.526%,测试数据误差最大为0.454%,BP网络预测精度高。对传统遗传算法的遗传策略和优化策略进行改进,利用两种改进的遗传算法对针刺工艺参数进行优化。优化后的工艺参数显著提高了材料的刚度性能,其中面内拉伸模量分别提高了11.07%和11.48%,面外拉伸模量分别提高了49.64%和48.13%,复合材料的综合刚度性能分别提高18.17%和18.21%。     

Oxidation of SiC powders for the preparation of SiC/mullite/alumina nanocomposites

The oxidation behaviour of two types of SiC powder of differing particle size and morphology distribution has been studied in the present work; one submicron-sized and the other micron-sized. It has been observed that the onset-temperature for significant oxidation of the SiC powder of smaller particle size is much lower than that for the SiC powder of larger particle size; namely, about 760 °C as compared with about 950 °C. Furthermore, the rate and extent of oxidation of the former SiC powder is much higher than that of the latter SiC powder. Interestingly, however, the SiC powder of smaller particle size exhibits more controllable oxidation behaviour in the context of the preparation of SiC/mullite/alumina nanocomposites, i.e., in terms of the extent of oxidation, and hence the amount of silica formed as an encapsulating outer layer and the resulting core SiC particle size, than the SiC powder of larger particle size. The SiO₂ layer formed was amorphous when the SiC powders were oxidized below 1,200 °C, but crystalline in the form of cristobalite when they were oxidized above 1,200 °C. Since the presence of amorphous silica can accelerate the sintering of the nanocomposite, oxidation of the chosen SiC powder should thus take place below 1,200 °C.     

Fabrication of short carbon fiber preforms coated with pyrocarbon/SiC for liquid metal infiltration

A novel method of fabricating short carbon fiber preforms was proposed for liquid metal infiltration. The preforms were shaped by wet forming and strengthened by pyrocarbon (PyC). SiC layers were prepared on carbon fibers by the reaction of SiO and PyC at 1600 °C. X-ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy were applied in the characterization of the preforms. Gas pressure infiltration was done to demonstrate the feasibility of the preforms for the liquid metal infiltration. The microstructure analysis indicates that carbon fibers are uniformly distributed in the preforms, and fibers are coated with an inner layer of PyC and an outer layer of SiC. The infiltration experiment proves that the prepared preforms are feasible for liquid metal infiltration under low infiltration pressure and temperature.     

Pressure pulsed chemical vapour infiltration of SiC to two-dimensional-Tyranno/SiC-C preforms

Preforms of two-dimensional Tyranno fibre (SiC base) of 7×20×1.3 mm³ were chemically vapour infiltrated with SiC at 850–1050 °C from a gas mixture of CH₃SiCl₃ (6%)-H₂ using pressure pulses between below 0.3 kPa and 0.1 MPa. Above 900 °C, films grew on the macrosurface dominantly. At 850 °C, residual porosity decreased to about 10% after 10⁵ pulses, and three point flexural strength reached about 200 MPa. X-ray diffractograms (XRDs) on the surface showed the deposits to be -SiC only.     

Modeling of electromechanical behavior of woven SiC/SiC composites

A coupled electro-mechanical model was developed to predict the mechanical behavior of woven SiC/SiC ceramic matrix composites and electrical resistance response to mechanical damages in the composites. The matrix is explicitly included in the model such that the matrix cracking and fiber break can be linked to the electrical resistance change during loading. The results show that the electrical resistance increases linearly with an increase of matrix crack density and the number of fiber breaks. The predictions are compared to the experimental results on 2D woven SiC/SiC ceramic composites. With proper materials parameters input, the models can accurately predict the stress–strain curve and electrical resistance change during the loading. The model is further compared to an analytical solution of electromechanical coupling to get an insight into the electrical–mechanical interaction mechanisms in the composites.     

Microstructural investigations of the pyrocarbon interphase in SiC fiber-reinforced SiC matrix composites

The microstructure of the pyrocarbon interphase in SiC fiber-reinforced SiC matrix (SiCf/SiC) composites and its transformations during fiber/matrix debonding were studied. The phenomena of bridging and delamination within the pyrocarbon interphase are found during fiber/matrix debonding. A new phenomenon of ‘stress orientation’ of the basal planes of the pyrocarbon in the bridging region is discovered. It is found that the microstructural features of the pyrocarbon interphase are in favor of the toughness improvement of the SiCf/SiC composites.     

Low-temperature processing of porous SiC ceramics

Porous silicon carbide ceramics were fabricated from SiC, polysiloxane, and polymer microbead (as a pore former) at a temperature as low as 800 °C by a simple pressing and heat-treatment process. The effects of polysiloxane and template contents on the porosity and strength of the ceramics were investigated. During heat treatment, the polysiloxane transformed to an amorphous SiOC phase, which acted as the bonding material between SiC particles, and the polymer microbeads decomposed into gases and left pores. The porosity of porous SiC ceramics could be controlled within a range of 26–56 % with the present set of processing variables. The porous SiC ceramics showed a maximal porosity of 56 % when 10 μm SiC particles and 16 % polysiloxane were used with 20 % polymer microbeads. Flexural strength generally increased with increasing polysiloxane content and decreased with increasing polymer microbead content. Typical flexural strength of the porous SiC ceramics was 53 MPa at 42 % porosity.     

Biotemplated synthesis of novel porous SiC

Amorphous silicon carbide with unique porous structures was synthesised from three biological templates (egg shell membrane, butterfly wing and sea urchin skeleton) using liquid phase infiltration with polycarbosilane at atmospheric pressure followed by heating to 1000 °C under N₂. The structure and porosity of the preform was largely reproduced in the final material, although with egg shell membrane the cellular structure of the preform was compromised after infiltration and heating. The SiC yield of the final material was linearly correlated with the number of infiltration steps in the case of egg shell membrane and butterfly wing. Infiltration of the sea urchin shell was unsuccessful.