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.     

SiC晶须对反应合成多孔TiB_2-TiC复合材料的影响

以Ti、B_4C和SiC晶须(SiC_w)为原料,采用自蔓延高温合成法制备了多孔TiB__2-TiC复合材料。讨论了SiC_w含量对TiB__2-TiC复合材料物相、组织形貌、孔隙率和抗压强度的影响。结果表明:不添加SiC_w时,复合材料中主要物相为贫硼相TiB和Ti_3B_4以及TiC和少量TiB__2;在5Ti+B_4C体系中加入SiC_w后,贫硼相TiB和Ti_3B_4逐渐减少直至消失,而出现富硼相TiB__2和TiC的含量增加。随着SiC_w含量的增加,复合材料的孔隙率逐渐增加,由38.46%增加至5_2.78%。当SiC_w含量小于1.0时,随着SiC_w含量的增加,多孔TiB_2-TiC复合材料的抗压强度明显增加,当SiC_w含量为1.0时,复合材料的抗压强度达到最大值56.04MPa。Ti与SiC_w反应会生成TiC、Ti_3SiC_2和TiSi_2等物相,消耗一定量的Ti,使得与B4C反应的Ti量减少,从而促进富硼相TiB_2形成和TiC的增多。并且在SiC_w表面形成颗粒状TiC或者层片状Ti_3SiC_2,增加SiC_w与TiB_2-TiC基体之间的结合,更有利于发挥SiC_w的强化作用。     

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.     

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.     

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.     

Study on dual binders for fabricating SiC particulate preforms using selective laser sintering

The SiC preforms were successfully produced by selective laser sintering and thermal treatment for fabricating the near-net-shape composites with high SiC volume fraction. The effects of dual binders on the forming accuracy, microstructure and mechanical properties of SiC preforms were investigated. Results show that the SiC preforms with forming accuracy of 98.89% were fabricated by using the dual binders of nylon 6 + NH₄H₂PO₄, which fits the requirement of subsequent near-net-shape manufacturing compared with using single binder of nylon 6 after thermal treatment, the tensile and bend strength were significantly improved by using the dual binders of nylon 6 + NH₄H₂PO₄, which are strong enough to support the external load during infiltration. The bonding among SiC particulates primarily depends on nylon 6 after laser sintering, but after the decomposing of nylon 6, the reaction product of SiP₂O₇ phases can provide effective bonding for maintaining the forming accuracy and supporting mechanical properties of SiC preforms.     

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.     

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.     

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%。     

连续碳纤维/Mg复合材料异形薄板件预制体制备与成型

针对连续碳纤维增强镁基(Cf/Mg)复合材料异形薄板件预制体制备成本高、性能不稳定等问题,提出了"仿形编织-环向缠绕-铺放定位-缝合增强"的低成本组合制备工艺,设计开发了与之相配套的预制体制备装置。通过理论与实验研究,确定薄板与凸台预制体分体制备:薄板预制体采用径向编织、环向缠绕,仿形无纬布织物与环向纤维层针刺合成,其中纬纱引纬张力控制在1.7~2.3N;凸台预制体采用无纬布横向叠层,再与环状薄板预制体缠绕缝合而成。所制备预制体外形完整,形态良好,环向碳纤维分布均匀、与径向增强碳纤维呈规则排列。经液固挤压制备的Cf/Mg复合材料异形薄板件表明,凸台与薄板连接强度良好,为成形高质量Cf/Mg复合材料异形薄板件奠定了基础。     

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.     

不同预制体结构炭/炭复合材料烧蚀性能

采用电弧驻点烧蚀实验方法, 测试了分别以细编穿刺毡和针刺无纬布整体毡为增强体的2种C/C复合材料的烧蚀率, 并用电子扫描显微镜观察了烧蚀表面形貌。结果表明: C/C复合材料的烧蚀由化学烧蚀和机械剥蚀共同控制, 以机械剥蚀为主; 细编穿刺毡结构C/C复合材料由于Z向纤维束的存在, 加速了材料烧蚀表面粗糙度的变化, 烧蚀率略高于针刺无纬布整体毡结构C/C复合材料; 针刺无纬布整体毡结构C/C复合材料中无纬布层与烧蚀气流垂直, 具有良好的烧蚀性能。      

Mechanical performance of carbon fibre-reinforced composites based on stitched preforms

A comparative assessment of the influence of pure assembly seams based on a thin (11 tex) polyester yarn in a zigzag geometry on the resulting mechanical performance of a non-crimped fabric (NCF) carbon fibre-reinforced epoxy composite manufactured by vacuum-assisted resin transfer moulding is presented. This study was aimed at generating a solid foundation regarding the overall performance level of stitched NCF composites and at identifying critical property changes. The comprehensive evaluation of the mechanical composite properties includes static as well as dynamic tests of the in-plane properties as well as a characterisation of the interlaminar properties such as apparent interlaminar shear strength (ILSS) and compression after impact (CAI). It is demonstrated that mechanical properties such as the tensile and compression stiffness and CAI strength are not degraded by the chosen stitching parameters, whereas the tensile and compression strength, ILSS as well as the tensile fatigue behaviour are reduced as a result of pronounced localised fibre ondulations. A direct comparison to properties of a commonly used 5H satin woven fabric composite verifies that the overall performance of these particular stitched NCF composites must be enhanced with regard to the identified key criteria to meet the level required for aircraft applications and in order to maintain the performance advantage of NCF composites as compared to standard woven fabrics in general. Promising approaches include the use of different yarn materials based on soluble thermoplastics and/or modified stitching parameters.