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The role of reinforcements and their interface withthe matrix is known to be crucial in determining the performance of a polymeric compositematerial. The mechanical properties of composites can be improved by enhancing the load transfer atthe reinforcement/matrix interface, e.g. by tailoring the interfacial shear strength.A novel, multiscale hybrid reinforcement has been developed to this end. The reinforcement consistsof microsized commercial carbon-fibre yarns and fabrics coated with carbon nanostructures.This type of hybrid reinforcement is expected to have great potential for polymeric-matrixcomposite applications, because it combines microscaled and nanoscaled materials.
By ENEA-ITALIAN NATIONAL AGENCY FOR NEW TECHNOLOGIES, ENERGY AND SUSTAINABLE ECONOMIC DEVELOPMENT(1) R. GIORGI, SURFACES TECHNOLOGIES LABORATORY-HEAD,(2) E.SALERNITANO, (3) TH.DIKONIMOS, (4) N.LISI, SURFACES TECHNOLOGIES LABORATORY(5) M.F. DE RICCARDIS, UNIT FOR MATERIAL TECHNOLOGIES(6) V. MARTINA, (7) D. CARBONE, COMPOSITES AND NANOSTRUCTURED MATERIALS LABORATORY(Published on January-February 2011 – JEC Magazine #62)
Polymer-matrix composites reinforced with carbon fibreare one of the most commonly used advanced materialsin a number of sectors, such as aerospace, automotiveindustry, energy production, sports, etc. The possibility tovary fibres architectures, and consequently to tailor the composite’s mechanical properties, widens the spectrum ofapplications. The fibre-matrix interface plays an essential rolein the performance of fibrous polymer composites. Goodinterfacial adhesion determines structural integrity andefficient load transfer from fibre to matrix. The mechanical properties of composites can be improved by enhancing theload transfer at the einforcement/matrix interface by tailoringthe interfacial shear strength. A variety of chemical andphysical surface treatments – such as fibrewhiskerization andsurface roughening – can be performed to this end.
Unfortunately, these treatments inevitably induce defects oradditional phases that could be detrimental to the finalcomposite performance. To overcome this drawback, someresearch efforts have been devoted to the combination ofmicroscaled (i.e. carbon fibres commonly used in traditionalfibrous polymer-matrix composites) and nanoscaled (i.e.carbon nanotubes) materials. As a matter of fact, carbonnanotubes (CNTs) have been attracting great interest sincetheir discovery, due to their outstanding mechanical, thermaland electrical properties, which can exceed those of manyother known materials. CNTs have proved to be an extremelystrong reinforcement for polymeric-matrix nanocomposites,but they unfortunately tend to aggregate and float, which complicates things. Coating carbon-fibre bundles and fabricswith CNTs in an attempt to distribute nanotubes uniformlyacross the composite also resulted in a combination of microandnano-reinforcements. Because CNTs fill in anymicropores at the interface between polymer and carbon fibre,the final mechanical properties of traditional fibrouscomposites are expected to be enhanced.
The new multiscale hybrid reinforcement is made of carbonnanostructures (carbon nanotubes, carbon nanofibres, etc.)grown directly on commercial carbon-fibre yarns and fabricswith a uniform three-dimensional distribution. The directgrowth on the fibres eliminates the problems associated withthe orientation of carbon nanostructures within the polymermatrix. Among the techniques used to grow carbonnanomaterials (arc discharge, laser ablation, etc.), onlychemical vapour deposition (CVD) methods – based on thedissociation of hydrocarbon molecules catalyzed by transitionmetals at high temperature and the dissolution of carbonatoms in the metal catalyst nanoparticles – are capable ofgrowing CNTs directly onto carbon fibres.
A hot filament CVD (HFCVD) reactor and a DC plasmaenhanced CVD (PECVD) reactor were used to grow thecarbon nanostructures (nanotubes and nanofibres). The first system consists of a water-cooled stainless-steel chamber, a turbomolecular pumping group, and digitally controlled flowmeters, as schematically shown in Figure 1. The second reactor consists of a vacuum-sealed quartz chambercoaxial to a tube furnace, containing a sample holder and a twinelectrode system, as represented in Figure 2.
Nickel clusters were used as catalysts to grow the CNTs. PAN- and pitch-based carbon-fibre bundles and fabrics (the typical reinforcement of traditional polymeric fibrous composites) were used as substrate. Nickel clusters were obtained on the fibre surface by electrochemical deposition (ELD), a simple and inexpensive method widely used to produce thin and thick metal coatings with a very good adherence. Different deposition conditions (duration, applied electrode overpotential, applied passed charge) were tested in order to obtain a uniform and dense distribution of Ni clusters on the carbon-fibre surface, with a narrow diameter distribution.
The morphology and surface distribution of the catalyst nanoparticles were found to be dependent on electrodeposition parameters. By varying the operating conditions, it is possible to obtain clusters characterized, for example, by a round shape, small dimensions and a uniform spatial distribution (Fig. 3), or by a very dense distribution of nanoparticles larger than 100nm in diameter with an evident surface feature (Fig. 4).
High-quality carbon nanostructures, with a low impurity content and rather smooth walls, were grown on catalyzed PAN- and pitch-based carbon fibres, both in the form of bundles (Fig. 5) and fabrics (Fig. 6), regardless of the cluster size and morphology. Carbonfibre bundles and fabrics appeared fully infiltrated by a network of homogeneously and densely distributed carbon nanostructures. Carbon nanomaterials surrounded the fabric’s fibre bundles uniformly, even in the critical overlapped regions. This new hybrid reinforcement could enlarge the field of application to fibrereinforced composite materials, providing a way to distribute carbon nanomaterials in products with a wide planar extension while avoiding their aggregation. Obviously, when the surface distribution of the catalyst was low, the carbon nanostructures that originated from the metal nanoparticles present on the fibre surface had a low distribution density (Fig. 7).
The nanostructures on the fibre surface can be made more or less entangled by acting on the activation system and precursor flow rates (Fig. 8). Moreover, the morphology of carbon nanostructures can be tailored by varying the growth process parameters (temperature and pressure), yielding different alignment of the graphene layers that constitute the carbon fibres. Graphene layers are perpendicular to the growth axis in platelet carbon nanofibres and parallel to the growth axis in tubular carbon nanofibres (Fig. 9). The properties of graphitic structures being anisotropic, the orientation of graphene layers strongly affects the performance of the resulting composite material. Therefore, controlling the morphology and layer stacking of carbon nanofibres is very important for their applications.
As good anchorage of carbon nanostructures to the fibre surface is essential for the embedding process in a polymeric matrix, some pseudo-adhesion tests were performed on the hybrid reinforcement. The three-component material (carbon fibre/catalyst cluster/carbon nanostructure) was immersed in different liquids and processed with various stressing treatments (immersion, magnetic stirring, centrifugation and sonication) to test the adhesion of both catalyst clusters to carbon fibres and carbon nanostructures to catalyst clusters. The tests revealed a strong anchorage between the components of the multiscale composite reinforcement, i.e. between carbon nanostructures and carbon fibres and fabrics, via metallic clusters. The homogeneous distribution of catalytic clusters did not change significantly and no mechanical damage was observed. The carbon nanostructure coating still surrounded the carbon fibre, without any significant morphological change.
The above results suggest it is possible to use the carbon fibres and fabrics coated with nanotubes/nanofibres as reinforcement for low-weight structural composites. In principle, this material allows the dispersion of a high volume fraction of nanostructures throughout the composite without the problems associated with their aggregation. Moreover, it is also expected that the combined use of microscaled and nanoscaled reinforcements enhances the mechanical performance of polymeric-matrix composites.
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