Key Engineering Materials Vol. 742

Abstract: Conventional WC-Co hardmetals are widely used in various applications due to their excellent properties. High hardness can be achieved using compositions with little to no content of cobalt or nickel. These binder metals are hazardous to health, making a substitution not only desirable because of availability and cost reasons. A new possibility to manufacture such hard materials is the combination of tungsten carbide with oxides such as Al₂O₃ and ZrO₂. In this way the binder metal content can be replaced. Furthermore the content of the also expensive WC can be reduced. Such metal carbide – oxide composites with oxide contents between 16 vol% and 40 vol% were manufactured. The completely dense composites feature high hardness values of 2000 HV10 to 2400 HV10 while also having an acceptable fracture toughness of up to 7 MPa⋅m^1/2. The improved mechanical properties make the replacement of WC-Co hardmetals and binder free WC ceramics in special areas possible.

Abstract: In this study, it was tried to develop a process chain for ceramic injection molding of Al₂O₃-chopped-fiber reinforced oxide-ceramic-matrix-composite. The feedstocks are compounded at 50 Vol. % filling degree of solid (Al₂O₃ μ-powder (Taimei Chemicals Co. Ltd.) and 3,2 mm chopped fibers (3M)), in which fiber content varies from 0 Vol. % to 100 Vol. %. As binder system, PE + Paraffin Wax + Stearic Acid are used. The ingredients are compounded in a kneader (Brabender) at 125°C and after the viscosity measurement in the high pressure capillary rheometer at 160°C and certain shear rates, the feedstock is injection molded (Battenfeld) at 160°C, which is followed by debinding process, including chemical (in n-Hexane) and thermal steps, and 2h sintering at different temperatures. Flow paths in the machinery parts, rheological properties of binding system, fiber content and the fiber orientation have significant effect on the flow behavior of the feedstock, fiber -orientation, -distribution & -length, which are crucial to understand the properties of end-parts like mechanical reinforcement of the fibers. The fibers in the sintered parts are ca. 200 μm in average length. The fibers in the feedstock show different orientations depending on the part-geometry and the green bodies have different densities depending on sintering temperature, amount of dispersant and fiber orientation.

Abstract: Liquid Silicon Infiltration (LSI) is a technique to manufacture non-oxide ceramic matrix composites such as C/C-SiC or SiC/SiC. In the beginning of this three-step process, fiber preforms are shaped and impregnated with phenolic resins. After curing, the preforms are pyrolyzed to convert the polymer matrix to a porous carbon matrix. This porosity is then used to infiltrate liquid silicon by capillary forces. Simultaneously, an exothermic reaction of silicon and carbon creates a silicon carbide matrix. Generally the liquid silicon reacts with any carbon and even with SiC present in the form of fibers, fiber coatings or matrix. Therefore, especially the fibers must be protected from Si attack effectively. The morphology of silicon carbide was observed to be heavily driven by Ostwald ripening. This can be suppressed by the addition of boron to the melt. The initially formed SiC crystals in C/C-SiC composites are hereby prevented from grain coarsening, resulting in almost completely preserved C/C blocks. For the manufacture of SiC/SiC composites, the silicon boron alloys allow an effective preservation of the nanocrystalline SiC-fibers. Thus, the use of Si based B containing alloys helps effectively to moderate and control the aggressive reaction during LSI process.

Abstract: Carbon fibre reinforced carbon composites (C/C) are characterised by their excellent thermal, chemical and mechanical properties. The intrinsic porosity and fibre reinforcement grant them an excellent damage tolerance. The production of complex structures is time consuming and very expensive. An innovative approach to this topic is the integration of simple geometric ceramic composite materials within complex polymer structures. The motivation of this contribution is to investigate the influence of hexamethylenetetramine as hardener (hardener content: 4, 8, 12 and 16 %) and curing parameters (tempered and non-tempered) on the microstructure and mechanical properties of the porous C/C composites. During the course of this contribution, selected carbon fibre reinforced polymer (CFRP) composites with different porosities were produced while adjusting the resin or hardening agent-ratio, as well as the processing parameters. Subsequent to the curing of the CFRP samples, porous C/C composites were produced by means of a pyrolysis process. The final part of the contribution is comprised of the microstructural analysis by light microscopy and the explanation of the flexural strengths, by utilising a “three-point-bending test”.

Abstract: The development of innovative bio-based composites with efficient manufacturing processes is the purpose of the current project C4 in the framework of the Excellence Cluster MERGE EXC 1075, funded by DFG (Deutsche Forschungsgemeinschaft). Efficiency in terms of mass-production, reproducibility and flexibility requires the performance of successive steps in the manufacture of semi-finished and final bio-based products. About bio-based materials, natural fibres composite (NFC) prepregs have been recently investigated as a potential cost-efficient semi-finished product. By means of continuous production processes, prepreg rolls can be manufactured with unidirectional natural fibres (flax) fabrics as reinforcement and thermoplastic biopolymers films as matrix. The used natural fibre non-crimp fabrics are made of high twisted yarns. For a better impregnation and higher stiffness properties, non-crimp fabrics with non-twisted yarns, which have been lastly developed by natural fibres suppliers, represent an appropriate solution. A second suitable option is the substitution of the biopolymer films, whose impermeability does not facilitate the release of humidity from the natural fibres while the impregnation, by produced low cost thermoplastic spunlace fabrics with a higher permeability and lower reachable surface weights. With these material developments and innovative process optimizations suitable to natural fibres, NFC prepreg properties tend to be improved. From prepregs to finished parts can be implemented by discontinuous processes, with compression molding and back-injection molding, or by continuous processes, with devices gathering several stages such as cutting, stacking, points welding, pre-heating and back injection molding. By stacking, a multi-axial orientation of prepregs can be performed in order to optimize the placement of reinforcing yarns according to the possible load path of future products. The mechanical properties profile of the combination of non-crimp natural fibres fabrics with thermoplastic films or thermoplastic spunlace fabrics has been here studied in detail with press-engineered samples and has confirmed the potential as an alternative to glass fibre-reinforced composite.

Abstract: In the present work it is shown that the resin transfer molding (RTM) is a beneficial technique to manufacture natural fibers into high-performance natural fiber composites. At first, three different types of weaves were produced by using low-twist flax yarns and standard-twisted flax yarns. Laminates based on the weaves and a petrochemical derived epoxy thermoset were fabricated by RTM process. For each laminate different numbers of plies (4, 5, 6, and 7) were used to achieve a broad range of v_f (from 32 % up to 55 %) which are having a pore volume fraction, v_p, as low as possible (min. 0.7 % - max. 2.7 %). For the laminates, flexural properties in warp and weft direction were determined (ISO 14125) and the effect of respective yarn type on flexural properties was investigated. The best properties were achieved for the laminate based on weave2 with v_f = 55 % (strength=303 MPa, modulus=19.3 GPa). When laminates were tested again after half of the year the modulus and strength were reduced, but the strain increased. The laminates were immersed into a water bath (ASTM D570) to test the influence of v_f and v_p on the water absorption behavior. The maximum water uptake (4-7 wt.-%) and the maximum thickness swelling (3-12 %) were observed for the samples with higher v_f. Laminates based on weave1 were immersed again into the water bath to investigate the extent of deterioration of flexural properties with respect to water absorption at various time intervals. The laminates were tested immediately after removing from the water bath and after re-drying.

Abstract: Composite materials do offer freedom to design a material fitting best to the requirements of a given application. In case of fiber reinforced polymers especially the low weight in combination with other favorable properties, e.g. high mechanical performance, are the driving force for their application. Materials from renewable resources are of high interest if sustainability is aimed. In this paper, in a holistic approach a green composite is aimed to be used in a rotor blade for wind energy production. The challenging topic for this approach is to identify a possibility to gain a thermoset resin being really green, i.e. based on renewable resources and being not critical, e.g. toxic, at any stage of the whole processing chain. For this purpose several different approaches are studied and compared with other solutions based on green resin systems from other resources and conventional petrochemical based resin systems. A hemp seed oil based epoxy resin has been tested successfully. But to be completely free of petrochemicals, bio-based hardener and catalysts are still an open topic. For manufacturing of a rotor blade an infusion process has been used and it was found, a through thickness impregnation of the natural fiber yarn based textile structure results in entrapped air. Only in-plane saturation delivered completely impregnated structures.

Abstract: Self-reinforced polymer composites (SRPCs) are receiving increasing attention by the industry for lightweight applications. The polymers used for SRPCs today are derived from fossil resources. However, due to a limitation of resources, interest is growing regarding the use of new alternatives for petrol based polymers in form of bio-based ones. SRPCs combine high stiffness, high impact and high durability with impairing recyclability. In SRPCs the same polymer is used for the reinforcing and matrix phases. SRPCs can be manufactured by commingling reinforcing and matrix fibres with different melting temperatures. The use of commingled yarns allows the combination of a large variety of fibres and therefore a wide range of material properties.

Abstract: Fiber-Metal-Laminates (FML) show superior dynamic mechanical properties combined with low densities. The mechanical performance of for example commercially available fiber-metal-laminate, glass laminate aluminum reinforced epoxy, can be improved by the substitution of glass fibers with carbon fibers. However, carbon fiber reinforced aluminum laminate introduces a mismatch of coefficients of thermal expansion and the possibility of galvanic corrosion. The fiber-metal-laminate is altered by the integration of an elastomer interlayer which is desired to solve both problems. The high electrical resistance is supposed to inhibit the corrosion. This study focuses on the effect of galvanic corrosion caused by neutral salt spray tests on fiber-metal-laminates, the influence of an elastomer interlayer and the quantification of the residual mechanical properties. The galvanic corrosion affects the interfaces of the laminates, therefore in this study edge shear tests and flexural tests were carried out to quantify the residual properties and thereby the corrosive damage. The elastomer interlayer was found to inhibit galvanic corrosion in the salt spray chamber, whereas the fiber-metal-laminate without interlayer showed corrosive damage. Furthermore, the mechanical properties of the fiber-metal-laminate with elastomer interlayer remained constant after the corrosion tests, whilst the fiber-metal-laminate’s properties decreased with corrosive loads.

Abstract: In consideration of environmental aspects and limited availability of resources, the focus of automotive and aerospace industry lies on significant weight optimisations especially for moving loads. In this context, innovative lightweight materials as well as material combinations need to be developed. A considerable potential for lightweight structures can be found in fibre- or textile-reinforced semi-finished products. Due to their specific characteristics and extraordinary structural diversity, thermoset and thermoplastic matrix systems can be used. In particular, carbon fibres as reinforcing components combined with a thermoplastic matrix polymer are able to create new high-performance applications. Besides the significant lightweight characteristics of the fibre-plastic-composites, in some instances contrary requirements must be satisfied in many areas, such as strength and ductility. In this field, the combination of fibre-reinforced polymers with aluminium or titanium sheets creates unique composite materials, so called hybrid laminates, which fulfil the unusual expectations.The focus of the current study lies on the development of a new thermoplastic hybrid laminate named CATPUAL (CArbon fibre-reinforced Thermoplastic PolyUrethane/ALuminium laminate). The structure of the laminate is an alternating sequence of thin aluminium sheets (EN AW 6082-T4) and fibre-reinforced thermoplastic polyurethane (TPU). The individual layers are consolidated to each other by using a hot pressing process. First results showed that the impregnation capability of thermoplastic polyurethane surpasses any other commercially available hybrid laminates. Furthermore, the mechanical properties regarding bending strength and interlaminar shear strength are exceeding the state of the art drastically.