A. R. Boccaccini

Glass matrix composites from coal flyash and waste glass

Abstract Glass matrix composites have been fabricated from waste materials by means of powder technology. Flyash from coal power stations and waste glass, residue of float glass production, were used. Commercial alumina platelets were employed as the reinforcing component. For flyash contents up to 20% by weight nearly fully dense compacts could be fabricated by using relatively low sintering temperatures (650°C). For higher flyash contents the densification was hindered due to the presence of crystalline particles in the as-received flyash, which jeopardized the viscous flow densification mechanism. The addition of alumina platelets resulted in better mechanical properties of the composites than those of the unreinforced matrix, despite a residual porosity present. Youngs modulus, modulus of rupture, hardness and fracture toughness increase with platelet volume fraction. The low brittleness index of the composites ( B ≈ 3 μm −1 2 ) suggests that the materials have good machinability. A qualitative analysis of the wear behaviour showed that the composite containing 20% by volume platelet addition has a higher wear resistance than the unreinforced matrix. Overall, the results indicate that the materials may compete with conventional glasses and glass-ceramics in technical applications.

Use of electrophoretic deposition in the processing of fibre reinforced ceramic and glass matrix composites: a review

Abstract Electrophoretic deposition (EPD) is a simple and cost-effective method for fabricating high-quality ‘green’ composite bodies which, after a suitable high-temperature treatment, can be densified to a composite with improved properties. In this contribution, we describe the use of EPD technique in the fabrication of fibre reinforced composites, with an emphasis on composites with glass and ceramic matrices containing metallic or ceramic fibre fabric reinforcement. EPD has been used to infiltrate preforms with tight fibre weave architectures using different nanosized ceramic particles, including silica and boehmite sols, as well as dual-component sols of mullite composition. The principles of the EPD technique are briefly explained and the different factors affecting the EPD behaviour of ceramic sols and their optimisation to obtain high infiltration of the fibre preforms are considered. In particular, the EPD fabrication of a model alumina matrix composite reinforced by Ni-coated carbon fibres is presented. The pH of the solution and the applied voltage and deposition time are shown to have a strong influence on the quality of the infiltration. Good particle packing and a high solids-loading were achieved in most cases, producing a firm ceramic deposit which adhered to the fibres. Overall, the analysis of the published data and our own results demonstrate that EPD, being simple and inexpensive, provides an attractive alternative for ceramic infiltration and coating of fibre fabrics, even if they exhibit tight fibre weave architectures. The high-quality infiltrated fibre mats are suitable prepregs for the fabrication of advanced glass and ceramic matrix composites for use in heat-resistant, structural components.

Machinability and brittleness of glass-ceramics

Abstract The relationship between the machinability and the brittleness of glass-ceramic materials is investigated. A brittleness index (B), given by the ratio of the hardness to the fracture toughness, is proposed as a parameter for estimating the machinability. This approach is confirmed by considering experimental data from the literature on turning operations of mica-containing glass-ceramics. It is shown that machinability parameters, such as the slope of the log-log plot of the specific cutting energy versus the cutting rate, or the specific cutting energy at low cutting rates, are in good agreement with the brittleness indices for seven different glass-ceramics. In order for a glass-ceramic to be machinable, it was found that the brittleness index of the material should be lower than B ≈ 4.3 μm−12.

A new approach for the Young's modulus-porosity correlation of ceramic materials

A new approach for determining the Youngs modulus of porous ceramics is presented. The approach is based on a previous equation derived for two-phase composites on the basis of a microstructural description that uses the contiguity and continuous volume as topological parameters. The derived equation shows very good agreement with experimental data drawn from the literature on porous glasses and ceramics for a wide range of porosity volume fractions. As a result of the limited geometrical and topological information on the porosity structure supplied in the original studies, however, the full potential of the model for Youngs modulus prediction could not be demonstrated. For a rigorous verification of the equation, therefore, experimental data on materials with well-described porosity structures is required.

Review In Situ high-temperature optical microscopy

High-temperature optical microscopy is an essential in situ characterisation and monitoring technique with wide applications in different areas of materials science. The devices used include commercial available instruments, known as heating microscopes, and custom-made devices, usually called “high-temperature processing microscopes” or “thermo-optical instruments”. The different areas of applications of high-temperature optical microscopy are discussed on the basis of practical examples drawn from the literature. Besides the classical use of the technique to study the melting and softening behaviour of glass, slags, ashes and other silicate and ceramic materials, this review covers alternative applications, in particular the use of heating microscopes as “optical dilatometers” to investigate the sintering kinetics of powder compacts. In this regard, the advantages of the technique over conventional dilatometry are emphasised. A variety of custom-made devices is described, developed to investigate particular problems, such as delamination and curling of laminate composites during densification, cosintering of multilayer metal-ceramic and ceramic-ceramic systems, and wetting behaviour of liquid phases on rigid substrates. As a particular example of such a custom-made equipment, a novel, multi-purpose high-temperature processing microscope is described, and its application potential, which is well beyond that of commercial devices, is outlined. This instrument is unique in that it combines both vertical and horizontal sample observation capability, as well as the possibility to investigate samples of relatively large sizes (65 mm3), i.e. about 10 times larger than those suitable for commercial heating microscopes.

Microstructural characterisation of a glass and a glass-ceramic obtained from municipal incinerator fly ash

Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) analyses were used to characterise the microstructure and chemical composition of a glass and a glass-ceramic material obtained from incinerator filter fly ash. Although the as-quenched material (vitrified fly ash) was amorphous under the detection limits of XRD, a dispersion of droplets indicating glass-in-glass phase separation was observed. In the glass-ceramic material (crystallised vitrified fly ash), crystals belonging to the pyroxene group and spinels were identified. The microstructure of the glass-ceramic consisted of crystals embedded in an amorphous glassy phase. The crystalline phases contain a higher amount of metallic elements (e.g. Al, Cr, Fe, Ni and Zn and most probably also other heavy metals) than the residual glassy phase. A change of composition of the residual glass phase in the glass-ceramic product, in comparison with the parent glass, is considered to explain, in comparative terms, the higher toxic potential of the glass-ceramic over the glass. The present results demonstrate that for an accurate assessment of the correlation between toxicity, release of hazardous compounds and microstructure, high-resolution characterisation techniques must be employed. In this context, the effect of crystallisation on the chemical durability of the products remains as an important area for further research.

Densification and crystallization of glass powder compacts during constant heating rate sintering

Abstract Heating microscopy was used to study the interaction between the processes of densification and crystallization of glass powder compacts under constant heating rate sintering conditions without the application of external loads. For barium magnesium aluminosilicate (BMAS) glass powder compacts sintered between 800–1100 °C, it has been shown that the relative rates of crystallization and densification can be controlled by changing the heating rate. Samples sintered at a high heating rate of 15 K min−1 could be fully densified in the amorphous state, delaying the onset of crystallization. In the samples sintered at a low heating rate of 1 K min−1, the onset of crystallization coincided with the termination of densification at ∼ 1000 °C. Since the experiments were performed without an application of external loads, the results are applicable for the manufacturing of dense BMAS glass-ceramics via a pressureless sintering route.

Toughening and strengthening of glass by A1203 platelets

The aim of this study was to investigate the toughening and strengthening capability of Al 2 O 3 -platelets dispersed in a dense glass matrix. The mechanical behaviour of composites fabricated via a hot-pressing route was evaluated in terms of the dependence of the elastic modulus, fracture strength and fracture toughness on the platelet inclusion content and composite microstructure. The toughening and strengthening mechanisms were assessed qualitatively

Ceramic coatings on carbon and metallic fibres by electrophoretic deposition

The electrophoretic deposition (EPD) technique was applied to coat carbon and stainless steel fibres with alumina and titania nanoparticles, respectively. Non-aqueous colloidal suspensions suitable for EPD experiments were prepared by mixing ultrasonically a given amount of the ceramic particles in ethanol. After ultrasonic mixing, the suspensions were clear and stable and, therefore, suitable for the EPD experiments. The EPD parameters, e.g. voltage between electrodes, deposition time and pH of the suspensions, as well as the drying conditions were optimised in order to obtain crack-free, high-quality ceramic coatings on individual fibres. The coated fibres are intended as reinforcing elements for the fabrication of ceramic composites and as oxidation resistant fibrous bodies for applications in filters and membranes.

Preparation, microstructure and mechanical properties of metal-particulate/glass-matrix composites

Abstract Composites with a borosilicate glass matrix containing different concentrations of vanadium particles were fabricated by powder metallurgy and hot-pressing. The mechanical properties and fracture behaviour of the composites were assessed by a range of techniques. Youngs modulus, fracture strength in bending, and fracture toughness increased with vanadium content. By virtue of the good interfacial bonding and low residual internal stresses, an effective crack-particle interaction during fracture was achieved. The fracture toughness of composites containing 30 vol. % of vanadium inclusions was approximately 65 % higher than that of the unreinforced glass. Experimental values for the fracture toughness increment were in good qualitative agreement with the predictions of theoretical models in the literature. Extensive plastic deformation of the vanadium inclusions was not found, however. This was attributed mainly to the constraint imposed by the rigid matrix surrounding the particles and to possible embrittlement of the particles during composite fabrication at high temperatures. The brittleness index (B) of the composites was calculated and its relevance for characterisation of the ductile versus brittle behaviour of brittle-matrix composites is discussed.