Materials Science Forum Vols. 587-588

Abstract: The effect of different dispersion states on the rheological and AC conductivity properties of carbon nanofibre/epoxy suspensions was investigated. Both rheological and electrical properties revealed to be good indicators of the fillers’ dispersion state, as confirmed by optical microscopy. It was shown that imposing a low shear deformation to poorly dispersed suspensions leads to agglomerate rearrangement resulting in a worse dispersion quality and, consequently, lower electrical conductivity. On the other hand, the imposition of a high shear deformation improves the original dispersion, resulting in better electrical conductivity. Rheological observations confirm the trends observed.

Abstract: In recent years, industries have increasingly demanded novel materials of low density (and, therefore, weight) and high strength. Such materials are required for all kinds of transport, especially for automobile and aviation industries. Lower weight vehicles would allow for reduced fuel consumption, which increases the transportation efficiency (the economical point of view) and a corresponding decrease of emission of pollutants to the atmosphere (the ecological point of view). This is why the development of materials with decreased densities is of great importance. The aim of this study was to develop novel materials as reinforcing elements for metal matrix composites. Boron crystalline compounds are proposed to be used, including the derivatives of higher boron– hydrogen anions BnXn 2- n = 10,12, X = H (in some cases – halogen, for example, Cl) as their respective densities meet the criteria. The study pursued two goals: (i) to develop a method for producing these boron hydrides and (ii) to investigate the structure and properties of these boron hydrides.

Abstract: Current technology provides means of fabrication of spherical micro-particles, either hollow or compact, for all engineering materials. Such spherical particles can be further embedded into another material to build-up either random dispersions or close-packed arrays, according to the production route and the degree of anisotropy intended for the ultimate composite material. In this study, a simple analytical formula for the composite stiffness is derived from an early micromechanics model, to describe the actual reinforcement of ductile matrices by a random dispersion of uniform spherical ceramic particles. Predictions from this model are checked against some other relevant models, and specific features arising from its theoretical derivation are pointed out.

Abstract: Syntactic functionally graded metal matrix composites (SFGMMC) are a class of metallic foams in which closed porosity results from the presence of hollow ceramic microspheres (microballoons), whose spatial distribution varies continuously between the inner and the outer section of the part, thus resulting in a continuous variation in properties. In this work, aluminiumbased SFGMMC rings were fabricated by radial centrifugal casting. The graded composition along the radial direction is controlled mainly by the difference in the centrifugal forces which act on the molten metal matrix and the ceramic particles, due to their dissimilar densities. In this case where the density of the SiO2-Al2O3 microballoons is lower than that of molten aluminium, the particles show a tendency to remain closer to the inner periphery of the ring. Thus the microballoon volume fraction increases along the radial direction of the ring from the outer to the inner periphery; in other words, the particle-rich zone is limited to an inner layer of the ring. Precursor conventional MMCs were prepared by stir-casting from the constituent materials, by homogeneously dispersing commercial SiO2-Al2O3 microballoons (particle size: 50 µm; particle volume fraction: 5 and 10 %) within a molten commercial Al-7Si-0.3Mg (A356) alloy. The resulting MMCs were then re-melt and centrifugally cast in order to produce the functionally graded composites. Particle gradients in the centrifugally cast composites were investigated by quantitative image analysis of optical micrographs (for the estimation of the particle volume fraction, mean particle diameter and porosity volume fraction).

Abstract: Carboxyalumoxanes have been incorporated into a polyurethane matrix by in situ polymerization. The filler was dispersed in the polyurethane matrix by either both ultrasonic and mechanical mixing or by mechanical mixing alone. The physico-mechanical properties of the composites have been characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Using ultrasound improves the degree of dispersion of the fillers in the matrix, but it also causes changes in the structure of the polyurethane matrix.

Abstract: Rapid tools for injection moulding are often produced by casting of epoxide filled composites. The moulding blocks obtained in this way are likely to undergo wear especially at the surfaces of the moulding cores during the ejection phase. In this study, tribological properties of epoxide composites containing different volume fractions of short steel fibres used in moulding blocks were assessed. Friction tests with the composites and moulded polypropylene were carried out with a prototype equipment that reproduces the actual ejection phase of injection moulding. The friction data were interpreted in terms of the roughness and compared with the microscopic features of the epoxy composite surface.

Abstract: Epoxide filled composites are being increasingly used for mouldings blocks of hybrid injection moulds. The filling is sought for improving both mechanical and thermal properties that are relevant for the mould performance. In spite of several works investigating the particulate filling of resins, there are only a few reports on fibre reinforcement. Composites based on an epoxy system with varying volume fractions of short steel fibres (SSF) were investigated. The mechanical properties were determined for each composite, and the topography of the fracture surfaces was analyzed by SEM. The mechanical properties of the epoxy filled composites were also compared to commercial particulate composites that are used for producing casting moulds. In spite of the SSF being more difficult to mix that the usual metal particulate fillers, it was found that the resulting composites show some improvement in the mechanical properties.

Abstract: Model composites with Polypropylene (PP) as matrix and alfa fibres (cellulose-based fibres obtained from the esparto grass of the subsaarian dry regions of Tunisia) were prepared for fibre/matrix interfacial characterization studies. The matrices tested were PP and PP combined with PP modified with maleic anhydride (PP-g-MA). The surface of the alfa fibres was treated by air plasma treatment. The adhesion between the untreated and treated fibres and both matrices was studied using the fragmentation test method. Composites with 10% weight of fibres were prepared by melt extrusion and injection moulding, and the specimens obtained tested for tensile properties. The fracture surfaces of the composites, obtained at low temperature, were observed by scanning electron microscopy. The presence of a small concentration of maleic anhydride grafted to the polymer matrix was found to be of the utmost importance for the establishment of a good fibre/matrix interface. The air plasma treatment had a cleaning effect of the fibre’s surface.

Abstract: This work reports the effects of nanoclays and aluminium hydroxide (ATH) on the thermomechanical properties of an unsaturated polyester resin. Dynamic mechanical thermal analysis in the temperature range from 25 to 150 °C has indicated the formation of different structures for the different clay (1, 5 and 10 wt. %) and ATH loadings (50 and 100 wt. %) investigated. The rubbery modulus increases with nanoclay and ATH content which indicates that both nanoclays and ATH act positively on the final network density and consequently lead to systems showing higher stiffness at higher temperatures. The mechanical loss peak value decreases with either nanoclay or ATH content which seems to indicate that both nanoclays and ATH improve network density. The glass transition temperature and the mechanical loss peak value changes linearly with ATH content.

Abstract: In this work the actuator performance of a ceramic piezoelectric device is studied. Its ability of deforming a Carbon Fiber Reinforced Plastic (CFRP) laminate is measured in the absence/presence of mechanical constraints. Deformation and bending of the CFRP laminate is obtained when a voltage is applied to the piezoelectric device. This deformation was assessed using ESPI technique and a very good agreement with the expected values for the unconstrained case was observed. When completely embedded a reduction of 86% in the piezoelectric response is expected. Finite Element Analysis confirmed the obtained experimental data and a very good linearity in the response was observed.