Applied Composite Materials 5: 207222, 1998. 1998 Kluwer Academic Publishers. Printed in the Netherlands. 207

Alternative Uses of Waste Glasses: Issues on theFabrication of Metal Fibre Reinforced Glass MatrixComposites

P. A. TRUSTYIRC in Materials for High Performance Application, School of Metallurgy and Materials,The University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K.

A. R. BOCCACCINI?Institute for Mechanics and Materials, University of California at San Diego, La Jolla,CA 92093-0404, U.S.A.

(Received 5 November 1997; accepted 4 December 1997)Abstract. In order to investigate a potential use for recycled speciality glasses (specifically thosecontaining hazardous elements), a processing route has been developed for the fabrication of metallicfibre mat reinforced glass matrix composites. Commercially available 3-dimensional stainless steel316L fibre mats were used as the metal reinforcement, and a borosilicate glass which had been usedpreviously in radiation experiments was used as the glass matrix. The fibre mats were infiltratedwith a commercially available silica sol using electrophoretic deposition (EPD), and the glass matrixwas laid in between infiltrated fibre mats prior to consolidation using uniaxial cold pressing. It wasfound that composites with sufficient integrity could be obtained from this recycled waste glass aftersintering in air at 850C for 1 h. The deposited silica remained amorphous at the processing temper-ature, providing a porous interface between the metallic reinforcement and the waste glass matrix.The processing issues involved in composite fabrication, namely, the EPD infiltration parameters forthe silica sol, the quantity and subsequent effect of the impurities present in the waste glass, and thedensification of the composite material on sintering, have been discussed.

Key words: recycling, waste glass, composites, metal fibre reinforcement, electrophoretic deposi-tion.

1. Introduction

The development of metal-reinforced glass matrix composites (GMCs) is a promis-ing alternative to monolithic glasses for achieving structural materials combiningintermediate temperature capability, toughness and damage tolerance [16]. Theincorporation of the reinforcing metallic phase into the glass matrix provides theglass-metal composite body with a degree of pseudoductility, preventing catastro-phic crack growth by such mechanisms as plastic deformation, debonding, pull-? On leave from: Technische Universitt Ilmenau, Fachgebiet Werkstoffkunde, D. 98689, Ilme-

nau, Germany.

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out and crack bridging [58]. The main potential toughening characteristic im-parted to a brittle matrix by a metallic second phase is the dissipation of energyby plastic deformation [9, 10]. In order to exploit this toughening mechanism, thebest results are obtained when using continuous metal reinforcements, includingboundary networks [9], foils [10] or continuous fibres [47, 11], as there is forcedcrack/secondary phase interaction in these cases. Examples of continuous ductilemetal reinforcements for glass matrix systems which have been studied to dateinclude: lead [7]; Ni [1]; Cu and Ni-Si-B alloys [4, 5] and stainless steel [11].When considering continuous fibre reinforcement, however, the properties of thecomposite material are invariably anisotropic, which can be a disadvantage. Re-cent advances in ceramic processing have attempted to alleviate the problem ofanisotropy by incorporating woven reinforcements into a matrix material [1214].These 2D and 3D reinforcements reduce material property anisotropy and improvethe inter- and intra-laminar strengths. Apart from the present authors own recentwork [14], we are only aware of the work of Russian researchers who have used2D metallic fibre mats to reinforce glasses [15]. Interest in these reinforcementsmay increase due to the fact that, in recent years, more woven metallic fibreshave become available through their use in the production of anti-static/conductivetextiles and for the fabrication of filters [16]. Preliminary studies have been un-dertaken on the use of such woven fibres in the fabrication of a model glass matrixcomposite [14] and this work has led to further research efforts being applied to thedevelopment of GMCs, with particular emphasis being placed on the use of wasteglass for the matrix material.

The growing interest in ecological preservation has led to an increase in therecovery of solid wastes, including glass cullet from the hollow and float glassproduction processes as well as the silicate residues from combustion processes,such as slag and flyash from coal power stations or incinerators [1719]. Besidesthe traditional use of slag and flyash as a filler in the fabrication of cements [18],alternative uses of the recovered silicates have been proposed including the fabri-cation of glass-ceramics with interesting mechanical properties [19, 20], or the useof mixtures of cullet glass and flyash to form the matrix of particle reinforced glassmatrix composites [17]. In this context, the development of alternative techniquesto reuse (recycle) speciality glasses e.g. glasses from fluorescent tubes, TV-setcones and screens, and cathode-ray tubes is of paramount importance becausesuch glasses cannot be directly re-used in the glass production plant via the meltingprocess [21]. This is due to the significant variation in their composition whichdepends on the manufacturer, year of production, etc. In addition, many of theseglasses contain significant amounts of hazardous elements, e.g. heavy metals suchas Pb, Ba and Sr, which could be leached out by environmental factors, for exam-ple, whilst in contact with water, which would then lead to ecological pollutionwhen the glass cullet is buried in normal landfill sites [22]. Hence, in order toovercome these drawbacks, new concepts are continuously sought to recycle thesespeciality glasses by forming new products.

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The successful implementation of such techniques will also act to reduce theamount of cullet glass deposited in landfill sites, which is an unsatisfactory practiceboth from an economical and ecological point of view. For glass residues contain-ing heavy metals, techniques have been developed to reuse colour-mixed culletglass or waste glass from incinerators together with clays, to form composite ma-terials [23]. Another approach suggested and experimentally verified has been thefabrication of glass fibres from waste TV glass [24]. Recently, the development ofsintered products from Ba- and Pb-containing glasses was demonstrated also [25].This approach exploits the low sintering temperatures of these glasses, allowingfor processing temperatures in the region of 650900C. In the work here, a newalternative is presented for the secondary recycling of speciality glasses, such asthose used in the manufacture of fluorescent tubing for radiation experiments [26].The approach involves the use of such glasses as the matrix material for a metallicwoven fibre reinforced GMC. The metallic fibre mats were infiltrated with a silicasol using electrophoretic deposition [1214] prior to composite manufacture, inorder to minimise intra-tow porosity. The main issues involved in the fabricationof such composites are presented and it is envisaged they may find applicationsas structural materials for low to moderate load bearing applications, provided theenvironment where they are used guarantees a low leachability of the contaminantelements they may contain.

2. Experimental

The metallic reinforcement used in this work comprised 316L stainless steel fibres(14 m diameter) laid-up in a 3D labyrinth architecture and sintered to form adurable, pleatable material (Bekipor ST, Bekitherm FA, Belgium). It was chosen,on the basis of its suitability, to be infiltrated by sol particles using electrophoreticdeposition (EPD), as described below. The maximum use temperature in air forthis grade of stainless steel is 900C and the thermal expansion coefficient (20700C) is 16.8 106 1/C [27]. Due to this relatively low thermal capability andthe oxidation resistance of the metallic fibre used, one must ensure that any glassmaterial used for the matrix can be consolidated at temperatures of (or below)900C. The waste glass used was predominantly a borosilicate glass [26], whichhad been recycled after use as fluorescent tubing in radiation experiments. Thedensity of the glass was measured as 2.19 g/cm3 and it was available as a pow-der with an average particle size of 25m. Simultaneous differential thermalanalysis (DTA)/thermogravimetric analysis (TGA) was employed on the powderusing a Stanton-Redcroft STA 781 thermoanalyser to chart the possibility of crys-tallisation of phases. Dilatometry was also performed on pellets of matrix materialto ascertain the point of maximum rate of densification. The pellets used were drypressed uniaxially using a pressure of 300 MPa and each pellet had a diameterof 12 mm and a height of 10 mm. From the dilatometry results, an opti-mum sintering temperature was ascertained and used subsequently to sinter pellets

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of pure matrix. The matrix was then analysed by scanning electron microscopy(SEM) using a JEOL 6300 scanning electron microscope in both secondary andbackscattered electron imaging modes. Any contaminant phases observed in theglass matrix were analysed using electron energy dispersive X-ray analysis (EDX).A commercially available silica sol (K342, Degussa Aerosil MOX80, Degussa Ltd,Germany) was used for the electrophoretic infiltration of the fibre mats. This sol isstable in the pH range 46, has a solids content of 42 wt.-% and the sol particlesare spherical with a broad particle size distribution (10100 nm) and an averageparticle size of 30 nm. A dried sample of this powder was analysed using TGA toasses the amount of expected weight loss at the composite sintering temperature.

The processing of the composites can be divided into two major steps: (i) infil-tration of the reinforcement fibre mats and (ii) green body formation and compositeconsolidation.

2.1. FIBRE MAT INFILTRATION

Electrophoretic deposition was used to infiltrate the metallic fibre preforms. Aschematic diagram of the basic EPD cell is shown in Figure 1. The process relieson the presence of small charged particles within a colloidal suspension, which, onthe application of an electric field, will migrate toward and deposit on an electrode[1214]. With an aqueous sol, the movement of the suspended particles will begoverned mainly by factors such as the field strength, the pH of the solution and theionic strength of the solution [28]. In the present experiments, the silica particleswere negatively charged, and thus the fibre preforms (20 mm 20 mm) were used

Figure 1. Schematic diagram of an electrophoretic deposition cell incorporating a wovenstainless steel fibre mat as the deposition electrode.

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as the anode in the EPD cell. Two stainless steel plates placed either side of thefibre preform served as the negative electrodes. The silica particles migrated underthese conditions towards the metal fibre preform and infiltrated the labyrinth struc-ture. Experiments were performed using varying applied voltages and depositiontimes in order to find the optimum conditions for the complete infiltration of thesol into the preforms. Infiltrated preforms were dried in air and then prepared forexamination via SEM using standard ceramographic techniques.

2.2. GREEN BODY FORMATION AND COMPOSITE CONSOLIDATION

The EPD-infiltrated fibre mats were used to prepare a glass matrix composite bya simple uniaxial cold-pressing and sintering route. Typically, two fibre mats weresandwiched between layers of the waste glass matrix powder in a die (24 mm di-ameter) to form the green bodies. The fabrication of similar laminated composites,but using a model soda-lime glass as matrix, has been presented in an earlier report[14]. Uniaxial pressures of 100 MPa were employed to obtain samples of sufficientstrength to be handled. The specimens were heated at a rate of 10C/min up to thesintering temperature ascertained from the dilatometry studies of the glass matrix,and the dwell time at temperature was 1 h before furnace cooling. Selected sin-tered composite specimens were cut, polished and observed using SEM. Compositeconsolidation by this simple, pressureless densification route was performed at thisstage to investigate the presence of any reaction at the silica/metal or the silica/glassmatrix interfaces and to assess whether pressureless sintering is capable of yieldingdense, i.e. porous-free, materials.

3. Results and Discussion

3.1. MATRIX RESULTS

The graph in Figure 2 shows the dilatometry results for the matrix glass. The curveshows that the rate of densification of the glass increases rapidly above 600C andreaches a maximum at 850C. This maximum rate of densification temperature isbelow the 900C threshold of the stainless steel matrix and therefore this matrixmaterial can be feasibly used to fabricate stainless steel reinforced GMCs withoutundue oxidation of the reinforcement. The XRD trace of this matrix in the temper-ature range 700 to 900C is shown in Figure 3 and it shows that crystallinity of thematrix begins at 800C. In addition, with reference to the relative peak intensities,it is evident that there is an increase in the level of crystallinity in this glass withtemperature. This increase in crystallinity reduces the propensity for the glass todensify via viscous flow mechanisms. From the dilatometry result, however, it isclear that although the glass begins to crystallise at 800C, the extent of crystallinityat this temperature does not significantly hinder the matrix densification process.

The optimum sintering temperature of 850C attained from dilatometry andXRD was used subsequently to sinter a powder compact of the matrix material.

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Figure 2. Graph of the dilatometry results for the waste glass matrix showing a maximum rateof densification at 850C.

A backscattered electron micrograph of a sintered section the glass is shown inFigure 4(a). The main impurities can be seen as light, regular-shaped specks distrib-uted evenly throughout the microstructure. This impurity phase was analysed usingEDX, and the trace in Figure 4(b) shows that it contains gadolinium. Gadoliniumhas been shown to be used with glass borate hosts for applications in the semicon-ductor industry [29]. The microstructure of the glass is shown to be very dense butit also contains many cracks which may be due to the inadequate particle packingfrom the dry pressing consolidation technique, or microcracking induced from theresidual stresses generated between the glassy matrix and the gadolinium-basedphase on cooling of the material.

The graph in Figure 5 shows the DTA/TGA trace for the matrix glass. The DTAcurve shows that there is a sharp endotherm at around 100C which corresponds tothe loss of residual water. There were no other significant exotherms or endothermsabove this temperature. The TGA curve in Figure 5 shows that significant weightloss of the material occurs from room temperature up to a temperature of 600Cand that by 800C, the glass has lost approximately 6% of its weight. No significantfurther weight loss is observable up to a temperature of 1400C. This weight losscan be attributed to the loss of impurities in the glass material. In order to reducethe amount of matrix material lost due to impurities during the sintering process,therefore, it would be beneficial to calcine the glass powder at the highest possibletemperature below that of the onset of densification. This pre-sintering calcinationstage would be necessary for most of the candidate waste glasses used in thisprocess and the calcination temperature used would depend on the TGA resultsfor each system.

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Figure 3. X-ray diffraction traces of the waste glass matrix showing the onset of crystallisationof the glass to silicate hydrate (SiO2 xH2O) at 800C.

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(a)

(b)Figure 4. (a) A backscattered electron micrograph of the waste glass material. Note the pres-ence matrix cracks and regularly-shaped impurities (in white) distributed evenly throughoutthe matrix. (b) EDX analysis trace for one such impurity, showing it to be gadolinium-based.

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Figure 5. A DTA/TGA trace for the waste glass matrix material.

Figure 6. A DTA/TGA trace for the K342 silica sol used to infiltrate the fibre mats.

3.2. SOL INFILTRATION

The DTA/TGA curves of the K342 silica sol used are shown in Figure 6. TheDTA curve shows that there is a small peak at 1400C which can be attributedto crystallisation [12]. This crystallisation temperature is much higher than theintended dwell temperature of 850C and the deposited silica in the final compositewill therefore still be amorphous after sintering. The TGA curve in Figure 6 showsthat there is less than 1% weight loss in the deposited silica up to temperatures of1400C. This sol is therefore ideal for use as the infiltrating medium as there will

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Figure 7. Graph of the thickness of excess silica deposited on the outer surfaces of wovenfibre mats as a function of deposition time and at different applied deposition voltages.

be negligible weight loss on sintering at 850C after forming a dense green bodyusing EPD.

The aim of the EPD stage is to achieve a fully infiltrated preform with the min-imum amount of excess material being present in the outer regions of the preform.Thinner excess silica layers are beneficial because they reduce the propensity forlarge drying cracks to form on sintering. These cracks occur due to the differentialshrinkage of the sol network on drying, which generates tensile stresses at thesurface and leads to the initiation and growth of cracks [30]. These tensile stressesare much lower with thinner sections, and hence the propensity for the initiationand growth of drying cracks is also reduced. In addition, thinner infiltrated silicasections allow for the use of more waste glass in the layers in between the silica-infiltrated preforms. The graph in Figure 7 shows the results of the infiltration ofmetallic preforms using EPD in terms of the thickness of excess silica deposited onthe outside of the preforms under different applied voltages. When using an appliedvoltage of 2 V, the thickness of excess silica did not change significantly with time,indicating that the applied electric field was too low to influence the movement ofthe silica sol particles and cause adequate deposition. Higher voltages of 4 V and5 V produced deposition thicknesses which appeared linear over the time periodstudied.

It was also necessary to avoid significant electrolysis of the water as this ledto gas evolution at the electrodes and subsequent bubble entrapment/pores in theinfiltrated preforms. Each infiltrated preform was examined using SEM and thebest result was achieved using 4 V for 60 s. An example of two infiltrated preformsis shown in Figure 8. Figure 8(a) shows a preform infiltrated using a voltage of 5 V

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(a)

(b)Figure 8. Scanning electron micrograph of EPD-infiltrated woven metal preforms. (a) A pre-form infiltrated using an applied voltage of 5 V for 480 seconds (note the excessive amount ofsilica and the presence of porosity in the deposited layer). (b) A preform infiltrated using anapplied voltage of 4 V for 60 seconds exhibiting a more ideal microstructure.

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Figure 9. Scanning electron micrograph of a woven metal fibre reinforced waste glass matrixcomposite. CTE mismatch-induced cracks can been seen in the central sol-infiltrated region.These cracks run perpendicular to the dry pressing direction.

for 480 s and it can be seen that, although the fibre region is fully infiltrated, thereis an excessive amount of deposited silica on the outer edges of the preform. Thepresence of a large pore produced due to gas bubble entrapment should be notedalso. Figure 8(b) shows a preform infiltrated using EPD parameters of 4 V for 60 s.The fibre tow regions are fully infiltrated, there is an acceptable level of excesssilica in the outer preform regions, and there are no entrapped gas bubbles. Thereare some drying cracks present, however, which will not be eliminated on sinteringdue to the low processing temperature employed. In the present system, the con-solidation temperature is too low for the densification of the silica via a viscousflow led mechanism and hence these cracks will not be able to heal on sinteringat 850C. It has been found by experiment that the sol used in this work requirestemperatures in excess of 1170C to densify via a viscous flow mechanism [31].

3.3. COMPOSITE FABRICATION

An example microstructure of the composite material pressureless sintered at 850Cis shown in Figure 9. The silica-infiltrated fibre tow region and the matrix wasteglass material are clearly discernible and there is no reaction between these tworegions. The low sintering temperature of 850C means that the infiltrated silicawill remain as a porous, amorphous interphase between the metal fibres and thewaste glass matrix. With reference to the waste glass matrix region, it can beseen that it has densified via a viscous-flow-assisted, pressureless densificationmechanism. The presence of the Gd-based impurities can also be seen. Comparing

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the waste glass matrix with the infiltrated silica region, it has been observed thatduring sintering the waste glass shrinks more than the silica due to its higher CTEin this temperature range. As a result of this differential shrinkage, coupled withthe laminar nature of the composite, tensile stresses are generated parallel to thedry pressing direction. These stresses are large enough to initiate and propagatecracks in the weaker, porous, infiltrated silica region, and these cracks can be seenrunning perpendicular to the pressing direction in Figure 9. This CTE mismatchproblem would be present to a greater or lesser extent depending on the type ofwaste glass used as a matrix material and the type of silica used to infiltrate thefibres. The problem can be minimised, however, by first obtaining a waste glass,and then synthesising a silica source that has a CTE which is similar to that of thematrix, as described below.

It must be stated at this point that no mechanical data were generated for thiscomposite as it was processed to demonstrate the feasibility of using a waste glassto fabricate a composite material. Nevertheless, it is envisaged that in mechanicaltests, the porous silica layer in this composite would promote delamination of thefibre region from the glass matrix region, thus affording some type of debonding ondeformation. In order to promote true composite behaviour, however, the presentsilica sol should be used as a fibre coating which would be porous and weak at850C and could be applied as a coating to the metal fibres using a recently devel-oped dip-coating technique for 3D woven fibre architectures [32]. The remaininginter-fibre region would then be infiltrated with a borosilicate sol which densifiesviscously in the 850C temperature region. This sol would need to be synthe-sised using sol gel/hydrothermal processing techniques. With these techniques itis possible to synthesise glasses of many different compositions, and in this waythe physical properties of the final glass could be tailored to match the CTE ofthe matrix region, thereby reducing CTE mismatch problems highlighted earlier.Consolidation of the preform could be improved also by incorporating wet powdergreenbody methods, for example, pressure filtration. This wet greenbody formationtechnique has been used successfully with other woven fibre reinforced compositesystems e.g. alumina/alumina [33], mullite/mullite [34] and alumina/mullite [35].

The processing issues involved in recycling a waste glass material in order tofabricate a woven metal reinforced GMC have been discussed in this work. Ideally,a composite material of this nature should contain in the region of 40 vol.-% metal-lic reinforcement in order to exploit fully the toughening capability of this phase.Although more studies are required in order to fabricate and mechanically testsuch a material, the main fibre infiltration/composite fabrication issues which needto be addressed in order to reach this stage have, however, have been demonstratedexperimentally.

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4. ConclusionsThe recycling of waste glass is a viable option for use in composite materialsfrom both a cost-effective point of view and also from an environmental aspect.A processing route has been presented for the fabrication of woven metallic fibremat reinforced waste glass matrix composites which involves the use of a silica solto infiltrate the intra tow regions of the metallic fibre mats using electrophoretic de-position. The composites were consolidated by uniaxial cold pressing and sinteringof impregnated fibre mats sandwiched between layers of the matrix glass powder.By pressureless sintering for 1 h at 850C, composite materials of sufficient in-tegrity could be obtained. The deposited silica provides a porous interface betweenthe glass matrix and the metallic reinforcement region thus imparting pseudocomposite behaviour to the material. The system needs to be developed further,however (e.g. a nanosized borosilicate-based sol for the infiltration of preforms,and the incorporation of a weak fibre/matrix interface), in order to achieve truecomposite behaviour, and this matter is the subject of ongoing studies.

Acknowledgements

Professors M. H. Loretto and I. R. Harris are acknowledged for the provision oflaboratory facilities at the IRC in Materials for High Performance Applications andthe School of Metallurgy and Materials, at the University of Birmingham, respec-tively. The authors would like to acknowledge Andrew Barton for his experimentalassistance. N. V. Bekaert S. A. (Belgium), is acknowledged for supplying the fibremats and Degussa Ltd (Germany) are acknowledged for supplying the silica sol.

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35. Kaya, C., Trusty, P. A., and Ponton, C. B., Manufacture of Alumina Fibre/Mullite Multi-layer Nanoceramic Matrix Composites Using Electrophoretic Filtration Deposition (EFD) andOptimisation of the Process Parameters, in Proc. 9th International Metallurgy & MaterialsCongress, Istanbul, Turkey, 1997, pp. 657662.

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