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Optically-transparent oxide fibre-reinforced glass matrix composites
Journal of Non-Crystalline Solids 356 (2010) 2591–2597
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Optically-transparent oxide fibre-reinforced glass matrix composites Deborah Desimone a, Ivo Dlouhy b, William E. Lee a, Dietmar Koch c, Jürgen Horvath c, Aldo R. Boccaccini a,d,⁎ a
Imperial College London, Department of Materials, Prince Consort Road, London SW7 2BP, UK Institute of the Physics of Materials, Zizkova 22, 61662 Brno, Czech Republic c Advanced Ceramics Group, University of Bremen, Germany d Department of Materials Science and Engineering, University of Erlangen-Nuremberg, 91058 Erlangen, Germany b
a r t i c l e
i n f o
Article history: Received 15 October 2009 Received in revised form 2 April 2010 Available online 9 June 2010 Keywords: Fracture toughness; Glass matrix composites; Light transmittance; Optomechanical
a b s t r a c t The development of ‘optomechanical composites’, based on glass matrices reinforced with continuous oxide fibre bundles (Nextel™) or filaments (sapphire) is described. The fibres were arranged unidirectionally between soda-lime silicate or borosilicate glass slides, and heat-treated to achieve densification by viscous flow of the glass matrix between the fibres. Small decreases in light transmittance (up to approximately 20%) upon introduction of fibres indicated that the composites are promising materials for use in optomechanical applications, where high transparency or translucency is required, in addition to improved fracture resistance. Fibre pull-out and crack deflection were identified at fibre/matrix interfaces, and a crack bridging effect was observed, with fibres holding the glass pieces together, thus preventing catastrophic failure. A relatively low fibre/matrix interfacial sliding resistance was measured by the fibre push-in test, although flexural strength measurements indicated the need to improve interfacial bonding. For this purpose, a tetragonal zirconia interfacial layer was introduced by sol–gel dip-coating of the fibres, to modify the fibre/ matrix bonding, providing weaker interfaces that facilitate the occurrence of toughening mechanisms. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Incorporation of stiff and strong ceramic fibres into brittle glass matrices is an effective way to increase the mechanical strength, fracture toughness, impact strength and thermal shock resistance of glass [1,2]. Moreover, when this improvement can be achieved without significant transparency loss (optomechanical composites [3]), the area of application is expected to expand enormously, especially in the building (architecture) and optics industries, as well as for armour, high temperature, electro-electronics and aerospace applications [4]. The selection of appropriate fibres and matrices for optomechanical composites is, however, extremely complex, as both mechanical and optical properties must be considered. The fibres must exhibit higher thermal stability than that of the matrix, besides adequate (e.g. matching) coefficient of thermal expansion and refractive index to that of the matrix. The bonding between matrix and fibres determines to a great extent the mechanical behaviour of the composites. A relatively weak interfacial bonding is required to induce crack deflection at the interface and fibre pull-out during the fracture process, mechanisms which lead to increased fracture toughness [5]. Different types of fibres, with different arrangements and processing routes for the fabrication of
optomechanical composites have been considered [2,6–9]. Oxide fibres show high tensile strength and modulus, as well as high temperature resistance. They are therefore candidates for production of tough glass matrix composites (GMC) [7]. Alumina fibres have been investigated previously as reinforcement in silicate matrix composites, and control of the interface was shown to be critical to improving fracture toughness [2,6–16]. Dericioglu and Kagawa have demonstrated enhanced fracture toughness on incorporation of a ZrO2 interface in a glass matrix/alumina fibre composite, due to the absence of strong chemical bonding between the ZrO2 layer and the alumina fibres. In this case, pull-out is observed at the alumina/zirconia interface, and a load-bearing capability provided by fibre bridging is achieved [8,9]. In the present work, novel GMCs were produced in four different systems: borosilicate glass/sapphire fibre, borosilicate glass/Nextel™ fibre, soda-lime-silica (SLS) glass/basalt fibre and SLS glass/Nextel™ fibre. A novel fabrication technique was investigated, that comprises “sandwiching” the reinforcements (fibres) between glass slides and subsequent thermal treatment to densify the composite. A sol–gel process was investigated for coating the fibres with a ZrO2 layer, to optimize the fibre/matrix interface. This should promote toughening mechanisms such as fibre pull-out and debonding. 2. Experimental
⁎ Corresponding author. Imperial College London, Department of Materials, Prince Consort Road, London SW7 2BP, UK. E-mail addresses: [email protected], [email protected] (A.R. Boccaccini). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.015
Schott Nexterion® glass B (borosilicate) and soda-lime-silica (SLS) microscope glass (Agar Scientific Ltd.) slides were selected for the manufacture of composites reinforced with sapphire (Saphikon®,
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Table 1 Heat-treatment parameters for the consolidation of the GMCs produced. Glass matrix
Ts (°C)
Heating rate (°C/min)
Tmax (°C)
Holding time at Tmax (h)
Cooling rate (°C/min)
Borosilicate Soda-lime silicate
820 720
20 20
1000 850
0.5 0.5
5 5
Laser Components, UK) and Nextel™ (Ceramic Textiles & Composites Europe, 3 M Deutschland GmbH, Neuss) fibres. Basalt fibres of commercial origin were also used to prepare composites with SLS glass matrix for comparison purposes. The sapphire fibres consist of single-crystal sapphire filaments of 150 µm diameter. They are completely transparent, besides having excellent Young's modulus (435 GPa) and flexural strength (760 MPa), and therefore represent a promising choice for an alltransparent composite system. The Nextel™ fibres consist of a fibre roving with a filament count of 400, while the basalt roving comprises approximately 230 filaments. The former has been developed for use as reinforcement in composites for structural applications, while the latter exhibits excellent thermal insulating properties and has been used in fire blocking textiles. Basalt fibres also exhibit favourable mechanical properties and therefore have been also considered for use as reinforcement in GMCs [17]. For the fabrication of composites, the reinforcing fibres were “sandwiched” between the glass slides, and subsequently submitted to a heat-treatment, during which the viscous flow of the glass between the fibres was exploited for consolidation of the composites. This processing method allowed for the transparency and geometry of the glass slides to be retained, and flat composite sheets were obtained. The parameters employed for composite fabrication with the two different glass matrices are shown in Table 1, this table lists also the softening point of each type of glass which is relevant to select the processing temperature for composite fabrication. The fibres were aligned unidirectionally, with periodical spacings of 2, 3 and 4 mm in the Nextel™ fibre-reinforced composites. In the case of the sapphire fibre-reinforced composites, a 4 mm spacing was employed, since preliminary tests showed that smaller spacings make eliminating
Fig. 2. Schematic drawing of a bend test specimen, showing the fibre plane positioned below the neutral line, in the tensile loading region.
Fig. 3. Schematic drawing of the flat chevron notch test for the measurement of fracture toughness of unidirectional fibre-reinforced composites.
voids during consolidation difficult, resulting in composites with macro-sized pores. Moreover, the relatively large diameter of the sapphire filaments (150 µm) limits the number of fibre filaments that can be introduced in one fibre plane. Higher fibre contents can only be employed with the addition of multiple fibre planes, or in a multilayered composite system, which will be the focus of future investigations. The processing temperatures were selected for each type of glass based on their softening points and after a trial and error approach to determine optimal conditions. A higher temperature was employed to process the borosilicate glasses not only because of their higher
Fig. 1. Schematic diagram of the ZrO2 sol–gel coating process.
D. Desimone et al. / Journal of Non-Crystalline Solids 356 (2010) 2591–2597
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Fig. 4. Soda-lime silicate GMC reinforced with (a) 2 mm spaced Nextel™ fibres (white stripes in the image) and (b) 4 mm spaced sapphire fibre.
Table 2 Fibre spacing before (original) and after (final) thermal treatment for consolidation of Nextel™ and basalt fibre-reinforced composites. Original spacing (mm)
Final spacing (mm)
1 2 3 4
2.5 4.5 5.5 6.8
The measurements were carried out in the visible wavelength range (400–700 nm). Flexural strength of the composites was measured using a 4point bending strength test, in accordance with British Standard BS EN 658-3:2002. Polished test bars of 15 × 70 × 3.5 mm3 were prepared, of both unreinforced glass slides (e.g. processed under the same conditions) and with fibre spacings of 1, 2, 3 and 4 mm. The fibre plane was positioned below the neutral line of the specimen, in the region subjected to tensile loading during the bend test, as shown schematically in Fig. 2. Outer and inner spans of 60 and 20 mm respectively were employed, in compliance with the minimal thickness/span length ratio of 20 for avoiding shear failure, i.e. ensuring that composites fail in tension [21]. At least 5 specimens of each composite system were tested and their fracture surfaces were observed using scanning electron microscopy (SEM) (JEOL). The interfacial bonding was assessed by the push-in test [22]. A Vickers indenter was employed in a load–unload–reload–unload cycle, and the interfacial sliding resistance was calculated from the hysteresis area of the load versus displacement curve, according to [22]:
softening point, but especially to avoid crystallisation and consequent opacification of the glass slides. High crystal growth rates have been observed for the borosilicate glasses employed at temperatures in the range of 700 to 950 °C. A sol–gel dip-coating method was developed for the deposition of a ZrO2 interface on the reinforcing fibres, as shown schematically in Fig. 1. Yttrium–zirconyl oxalate sol was prepared [18–20] with 1 molar solutions of the precursors. The yttrium content was calculated based on a 3 mol% Y2O3 to ZrO2 ratio. The fibres were vacuum infiltrated by the sol, dried at room temperature for 1 day, and subsequently ovendried at 60 °C for 24 h, at 90 °C for 12 h and at 100 °C for 12 h. The asdried dip-coated fibres were calcined according to the thermal treatment shown in Fig. 1, before a second layer was deposited following the same procedure. The second calcination procedure included a sintering step at 1300 °C for 1 h, to densify the interfacial layers. In the case of the Nextel™ fibres, a “minicomposite” was obtained, comprising the fibre filaments embedded in the ZrO2 matrix. Total light transmittance of Nextel™ and basalt fibre composites was measured in the direction perpendicular to the fibre plane using a Shimadzu UV-3600 spectrophotometer. The as-produced composites were polished with diamond suspensions of 6, 3 and 1 µm grain sizes.
where A is the hysteresis area of the graph, Fmax is the maximum applied force before unloading–reloading cycle, Rf is the fibre radius, Ef the fibre's Young's modulus and τ the interfacial sliding resistance. The flat chevron notch (FCN) technique [23] was employed for measurement of the fracture toughness of the composites. This special technique has been developed to capture the effect of the interfacial strength between fibre and matrix in composites, including the effect of the coating (interlayer), on fracture toughness. In addition, the FCN
Fig. 5. Total light transmittance of a soda-lime silicate slide (unreinforced matrix), composites reinforced with Nextel® fibres with various spacings and for sapphire fibre-reinforced composites with a 3 mm spacing.
Fig. 6. Normalized light transmittance of composites as a function of the shadow area measured by transmitted light microscopy, decreasing with increasing shadow area.
3
A=
Fmax ; 24π2 R3f τEf
ð1Þ
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3. Results and discussion 3.1. Composites light transmittance
Fig. 7. Average flexural strength of soda-lime silicate (SS) and borosilicate (BS) composites reinforced with coated and uncoated Nextel™ fibres, showing variation with increasing fibre spacing.
test allows measurement of the strength of sandwiched glass plates and possible matrix changes in the vicinity of the fibres, caused by thermal treatment, can be identified. Fig. 3 shows the schematic drawing of the test-piece, where a chevron notch was introduced at the fibre plane. Samples with the dimension of 10 × 10 mm2 were prepared, with polished surfaces, and loaded as indicated in Fig. 3. The fracture toughness (KIc) was then calculated from the maximum load before failure along the mid-plane (Fmax), a geometrical function obtained from finite element analysis (Y*min) and the specimen dimensions (B and W), according to [23]:
KIC =
Ymin * Fmax 1
BW 2
ð2Þ
The composites produced exhibited excellent optical transparency as shown in Fig. 4. Despite the regular fibre spacings employed, as described previously, these distances were altered during the thermal treatment for consolidation of the composites, as a result of the gravity-induced glass flow and consequent slight deformation of the slides. The actual spacings measured by image analysis of transmitted light microscopy are shown in Table 2. The total light transmittance of the composites was constant throughout the visible wavelength range (Fig. 5). Overall, the composites with the greatest spacing (lowest fibre content) exhibited the highest transmittance values (approximately 77%), as well as the composites reinforced with the transparent sapphire fibres. Dericioglu and Kagawa [3] described the light transmittance of the composites as a function of the matrix total light transmittance, reduced by the shadow area created with the introduction of the reinforcing fibres. A mathematical model was proposed, in which the total light transmittance of the composite (TC) is given as a fraction of the matrix transmittance (TM), considering the transmittance loss caused by the shadow area produced by the opaque fibres, as a function of their radius (Rf) and spacing (dS): TC = TM
2R 1− f dS
ð3Þ
The average composite transmittance values measured were normalized with the matrix transmittance (TC/TM) and plotted against the shadow area (2Rf/dS) measured by image analysis of transmitted light microscopy for the different fibre spacings (Fig. 6). The values followed the trend predicted with the mathematical model,
Fig. 8. Typical brittle failure fracture surface of borosilicate (a) and soda-lime silicate (b) glass reinforced with uncoated Nextel™ fibres, and fibre pull-out occurring within ZrO2 coated Nextel™ fibre bundles ((c) and (d)).
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Fig. 9. Bending test bar showing crack bridging by the reinforcing fibres in a Nextel™ 610 (white stripes in the image) fibre-reinforced soda-lime silicate glass composite with ZrO2 interface.
represented in the graph by the dashed line, although some deviation was observed. The equation was developed based on in-line light transmittance values, hence excluding scattering effects present in the matrix, such as pores and inclusions. The measurements presented are for total light transmittance, which therefore include these scattering effects. 3.2. Flexural strength Fig. 7 shows the flexural strength of Nextel™ fibre-reinforced composites, with and without the ZrO2 interface, as a function of fibre spacing. The fibre spacing employed in the manufacture of the composites, i.e. before thermal treatment, was used in this case, since the real spacing value does not affect the flexural strength values directly, but the fibre content. The comparison between flexural strength values measured for composites with uncoated fibres and the unreinforced glass matrices indicates that no significant improvement was achieved. The introduction of the ZrO2 interface, however, led to increased flexural strength, at approximately 17 and 23% compared with the unreinforced soda-lime silicate and borosilicate matrices, respectively. Considering the flexural strength of the composites reinforced with uncoated Nextel™ fibres, increases of 27 and 94% were observed for the borosilicate and soda-lime silicate matrix composites, respectively. Observation of the fracture surfaces of the composites after bend testing indicated that some fibre pull-out occurs within the ZrO2 coated fibre tow (Fig. 8(c) and (d)), whereas limited pull-out occurs in the composites reinforced with the uncoated fibres, which revealed a fracture surface characteristic of brittle failure.
Fig. 10. 4-point bend strengths of sapphire fibre-reinforced soda-lime silicate and borosilicate glass matrices showing the increased strength of the borosilicate GMC.
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Fig. 11. Interfacial sliding resistance of sapphire fibre-reinforced soda-lime silicate (SS/Sap) and borosilicate GMCs (BS/Sap), compared to values reported in the literature.
In addition, a fail-safe behaviour, e.g. non-catastrophic failure, was observed for the composites with the highest fibre content (2 mm fibre spacing), especially for those containing ZrO2-coated fibres. In these composites, crack bridging occurs and the glass matrix fragments are held together by the fibres, preventing the composites from failing into pieces (Fig. 9). Introducing sapphire fibres into the glass matrices did not induce the same effect, and the composites failed in a completely brittle manner. An increase in strength was observed, however for the borosilicate GMCs containing sapphire fibres with a 3 mm spacing. Composites with higher fibre content could not be fabricated due to the difficulty in eliminating residual porosity when fibre spacings smaller than 3 mm are employed. Fig. 10 shows the measured bend strength values of the sapphire fibre-reinforced composites, revealing a 49% increase in strength for the borosilicate GMCs. Measurement of the interfacial sliding resistance of both composites (Fig. 11) showed that a greater stress is generated at the borosilicate glass/sapphire fibre interface than that at the interface of the same fibre with the soda-lime silicate glass, although in both cases the values obtained were comparable to values reported in the literature for a sapphire fibre-reinforced epoxy matrix composite [24], a Nextel™ 720 fibre-reinforced mullite matrix composite [25] and SiC reinforced LAS glass–ceramic [26] and borosilicate glass [22] matrix composites. The higher interfacial sliding resistance present in the borosilicate GMC can lead to increased energy dissipation, resulting in higher flexural strength. In the case of the Nextel™ fibre-reinforced composites, measurement of the interfacial sliding resistance was
Fig. 12. Flat chevron notch fracture toughness of soda-lime silicate and borosilicate matrix composites reinforced with Nextel™ 610 and sapphire fibres.
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fracture toughness. The same effect was observed in the sapphire fibre-reinforced composites (Fig. 14(b)), while in the case of composites containing a ZrO2 interface, crack propagation occurred at the fibre/matrix interface as shown in Fig. 14(c) and (d). Fractured pieces of the coating can be seen at the interface, indicating that crack deflection and fibre debonding occurred, which explains the increase in bend strength for composites with ZrO2 interface. The introduction of a ZrO2 layer, on the other hand, seems to provide improved fibre/matrix interfacial bonding, as the crack is observed to propagate along the fibre/matrix interface, and most importantly along the fibre/coating interface as well. This indicates the potential of the interface to act efficiently in toughening the composite, as fibre pull-out was also observed during fracture, as shown previously in Fig. 8(c) and (d). Fig. 13. Fracture surface of a Nextel™ 610 reinforced borosilicate composites showing the crack initiation and the notch tip, propagating along the fibre plane.
not possible due to cracking of the fibre prior to sliding, suggesting that a strong bond has formed. Measurement of the fracture toughness showed that there is little contribution of the reinforcements to fracture toughness of the composites, as the values obtained are close to those of the unreinforced matrix processed under the same conditions (Fig. 12). It was noted that crack propagation occurred, as expected, in the plane of the chevron notch and fibre tows (see Fig. 13) so that no fibre pullout or similar toughening mechanisms were operative. The results suggest that at the fibre contents investigated, the fibre/matrix interfacial area present in the composites does not provide sufficient energy dissipation for improved fracture toughness. Fig. 14(a) shows typical fracture surfaces of uncoated Nextel™ fibre-reinforced composites after the FCN test. The crack is initiated at the notch tip as expected, and propagates along the fibre plane, although not at the fibre/matrix interface as desired for improved
4. Conclusions Highly transparent GMCs were produced in the systems comprising the combination of soda-lime silicate and borosilicate glass matrices with Nextel™ or sapphire fibres. Analysis of the composites light transmittance with different fibre content showed that optimum transparency can be achieved without compromising the fibre content up to a maximum at which minimal transparency losses occur with the introduction of opaque fibres. Nearly complete light transmittance (93%) was obtained when transparent (but costly) sapphire fibres were employed as reinforcement. Mechanical properties, however, indicate that there is little contribution of the reinforcements to the composites' mechanical integrity at the fibre contents investigated. Due to the relatively high bonding between fibres and matrices fracture toughness is barely affected, although evidence of energy dissipating mechanisms was confirmed by analysing the morphology of fractured specimens. It is worthwhile emphasizing that the present work was carried out as a preliminary assessment of the potential of these composite
Fig. 14. Fracture surfaces of borosilicate glass composites reinforced with (a) uncoated Nextel™ fibres, (b) uncoated sapphire fibres and (c) ZrO2 coated Nextel™ fibres, and (d) detail of the ZrO2coating/matrix interface, showing the crack propagation at the coated fibre/matrix interface and crack deflection at the coating layer.
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systems to exhibit enhanced mechanical properties as well as optical transparency. Therefore, only one fibre plane was introduced at this stage, despite the consequently resultant low fibre content. The use of higher fibre contents is currently being investigated, e.g. by including additional fibre layers, or in a multilayered system, which is challenging since the composite light transmittance should not be impaired by addition of fibres. Moreover, an increase in bend strength was observed in composites with ZrO2 interface. Samples containing the ZrO2 interface showed a bend strength improvement of 17 and 23%, for the soda-lime silicate and borosilicate matrix composites, respectively. These composites are thus promising materials for optomechanical applications, where high optical transparency and mechanical resistance/integrity are concurrently required. Future work will therefore focus in the optimisation of coating conditions, where further characterisation of interfacial properties (e.g. measurement of interfacial sliding resistance) will also be carried out to achieve a greater improvement in mechanical properties. Acknowledgments We thank Prof. J. R. Taylor and Dr J. C. Travers for kindly allowing us to carry out the light transmittance experiments in their facilities (Physics Department of Imperial College London). The authors acknowledge the financial support of the EU Commission, Marie Curie Fellowship (MEST-CT-2004-514667) “MatEnv-Trans and PowGen”. Financial support provided by the Czech Science Foundation under project GA101/09/1821 is gratefully acknowledged.
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Optically-transparent oxide fibre-reinforced glass matrix composites Deborah Desimone a, Ivo Dlouhy b, William E. Lee a, Dietmar Koch c, Jürgen Horvath c, Aldo R. Boccaccini a,d,⁎ a
Imperial College London, Department of Materials, Prince Consort Road, London SW7 2BP, UK Institute of the Physics of Materials, Zizkova 22, 61662 Brno, Czech Republic c Advanced Ceramics Group, University of Bremen, Germany d Department of Materials Science and Engineering, University of Erlangen-Nuremberg, 91058 Erlangen, Germany b
a r t i c l e
i n f o
Article history: Received 15 October 2009 Received in revised form 2 April 2010 Available online 9 June 2010 Keywords: Fracture toughness; Glass matrix composites; Light transmittance; Optomechanical
a b s t r a c t The development of ‘optomechanical composites’, based on glass matrices reinforced with continuous oxide fibre bundles (Nextel™) or filaments (sapphire) is described. The fibres were arranged unidirectionally between soda-lime silicate or borosilicate glass slides, and heat-treated to achieve densification by viscous flow of the glass matrix between the fibres. Small decreases in light transmittance (up to approximately 20%) upon introduction of fibres indicated that the composites are promising materials for use in optomechanical applications, where high transparency or translucency is required, in addition to improved fracture resistance. Fibre pull-out and crack deflection were identified at fibre/matrix interfaces, and a crack bridging effect was observed, with fibres holding the glass pieces together, thus preventing catastrophic failure. A relatively low fibre/matrix interfacial sliding resistance was measured by the fibre push-in test, although flexural strength measurements indicated the need to improve interfacial bonding. For this purpose, a tetragonal zirconia interfacial layer was introduced by sol–gel dip-coating of the fibres, to modify the fibre/ matrix bonding, providing weaker interfaces that facilitate the occurrence of toughening mechanisms. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Incorporation of stiff and strong ceramic fibres into brittle glass matrices is an effective way to increase the mechanical strength, fracture toughness, impact strength and thermal shock resistance of glass [1,2]. Moreover, when this improvement can be achieved without significant transparency loss (optomechanical composites [3]), the area of application is expected to expand enormously, especially in the building (architecture) and optics industries, as well as for armour, high temperature, electro-electronics and aerospace applications [4]. The selection of appropriate fibres and matrices for optomechanical composites is, however, extremely complex, as both mechanical and optical properties must be considered. The fibres must exhibit higher thermal stability than that of the matrix, besides adequate (e.g. matching) coefficient of thermal expansion and refractive index to that of the matrix. The bonding between matrix and fibres determines to a great extent the mechanical behaviour of the composites. A relatively weak interfacial bonding is required to induce crack deflection at the interface and fibre pull-out during the fracture process, mechanisms which lead to increased fracture toughness [5]. Different types of fibres, with different arrangements and processing routes for the fabrication of
optomechanical composites have been considered [2,6–9]. Oxide fibres show high tensile strength and modulus, as well as high temperature resistance. They are therefore candidates for production of tough glass matrix composites (GMC) [7]. Alumina fibres have been investigated previously as reinforcement in silicate matrix composites, and control of the interface was shown to be critical to improving fracture toughness [2,6–16]. Dericioglu and Kagawa have demonstrated enhanced fracture toughness on incorporation of a ZrO2 interface in a glass matrix/alumina fibre composite, due to the absence of strong chemical bonding between the ZrO2 layer and the alumina fibres. In this case, pull-out is observed at the alumina/zirconia interface, and a load-bearing capability provided by fibre bridging is achieved [8,9]. In the present work, novel GMCs were produced in four different systems: borosilicate glass/sapphire fibre, borosilicate glass/Nextel™ fibre, soda-lime-silica (SLS) glass/basalt fibre and SLS glass/Nextel™ fibre. A novel fabrication technique was investigated, that comprises “sandwiching” the reinforcements (fibres) between glass slides and subsequent thermal treatment to densify the composite. A sol–gel process was investigated for coating the fibres with a ZrO2 layer, to optimize the fibre/matrix interface. This should promote toughening mechanisms such as fibre pull-out and debonding. 2. Experimental
⁎ Corresponding author. Imperial College London, Department of Materials, Prince Consort Road, London SW7 2BP, UK. E-mail addresses: [email protected], [email protected] (A.R. Boccaccini). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.015
Schott Nexterion® glass B (borosilicate) and soda-lime-silica (SLS) microscope glass (Agar Scientific Ltd.) slides were selected for the manufacture of composites reinforced with sapphire (Saphikon®,
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D. Desimone et al. / Journal of Non-Crystalline Solids 356 (2010) 2591–2597
Table 1 Heat-treatment parameters for the consolidation of the GMCs produced. Glass matrix
Ts (°C)
Heating rate (°C/min)
Tmax (°C)
Holding time at Tmax (h)
Cooling rate (°C/min)
Borosilicate Soda-lime silicate
820 720
20 20
1000 850
0.5 0.5
5 5
Laser Components, UK) and Nextel™ (Ceramic Textiles & Composites Europe, 3 M Deutschland GmbH, Neuss) fibres. Basalt fibres of commercial origin were also used to prepare composites with SLS glass matrix for comparison purposes. The sapphire fibres consist of single-crystal sapphire filaments of 150 µm diameter. They are completely transparent, besides having excellent Young's modulus (435 GPa) and flexural strength (760 MPa), and therefore represent a promising choice for an alltransparent composite system. The Nextel™ fibres consist of a fibre roving with a filament count of 400, while the basalt roving comprises approximately 230 filaments. The former has been developed for use as reinforcement in composites for structural applications, while the latter exhibits excellent thermal insulating properties and has been used in fire blocking textiles. Basalt fibres also exhibit favourable mechanical properties and therefore have been also considered for use as reinforcement in GMCs [17]. For the fabrication of composites, the reinforcing fibres were “sandwiched” between the glass slides, and subsequently submitted to a heat-treatment, during which the viscous flow of the glass between the fibres was exploited for consolidation of the composites. This processing method allowed for the transparency and geometry of the glass slides to be retained, and flat composite sheets were obtained. The parameters employed for composite fabrication with the two different glass matrices are shown in Table 1, this table lists also the softening point of each type of glass which is relevant to select the processing temperature for composite fabrication. The fibres were aligned unidirectionally, with periodical spacings of 2, 3 and 4 mm in the Nextel™ fibre-reinforced composites. In the case of the sapphire fibre-reinforced composites, a 4 mm spacing was employed, since preliminary tests showed that smaller spacings make eliminating
Fig. 2. Schematic drawing of a bend test specimen, showing the fibre plane positioned below the neutral line, in the tensile loading region.
Fig. 3. Schematic drawing of the flat chevron notch test for the measurement of fracture toughness of unidirectional fibre-reinforced composites.
voids during consolidation difficult, resulting in composites with macro-sized pores. Moreover, the relatively large diameter of the sapphire filaments (150 µm) limits the number of fibre filaments that can be introduced in one fibre plane. Higher fibre contents can only be employed with the addition of multiple fibre planes, or in a multilayered composite system, which will be the focus of future investigations. The processing temperatures were selected for each type of glass based on their softening points and after a trial and error approach to determine optimal conditions. A higher temperature was employed to process the borosilicate glasses not only because of their higher
Fig. 1. Schematic diagram of the ZrO2 sol–gel coating process.
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Fig. 4. Soda-lime silicate GMC reinforced with (a) 2 mm spaced Nextel™ fibres (white stripes in the image) and (b) 4 mm spaced sapphire fibre.
Table 2 Fibre spacing before (original) and after (final) thermal treatment for consolidation of Nextel™ and basalt fibre-reinforced composites. Original spacing (mm)
Final spacing (mm)
1 2 3 4
2.5 4.5 5.5 6.8
The measurements were carried out in the visible wavelength range (400–700 nm). Flexural strength of the composites was measured using a 4point bending strength test, in accordance with British Standard BS EN 658-3:2002. Polished test bars of 15 × 70 × 3.5 mm3 were prepared, of both unreinforced glass slides (e.g. processed under the same conditions) and with fibre spacings of 1, 2, 3 and 4 mm. The fibre plane was positioned below the neutral line of the specimen, in the region subjected to tensile loading during the bend test, as shown schematically in Fig. 2. Outer and inner spans of 60 and 20 mm respectively were employed, in compliance with the minimal thickness/span length ratio of 20 for avoiding shear failure, i.e. ensuring that composites fail in tension [21]. At least 5 specimens of each composite system were tested and their fracture surfaces were observed using scanning electron microscopy (SEM) (JEOL). The interfacial bonding was assessed by the push-in test [22]. A Vickers indenter was employed in a load–unload–reload–unload cycle, and the interfacial sliding resistance was calculated from the hysteresis area of the load versus displacement curve, according to [22]:
softening point, but especially to avoid crystallisation and consequent opacification of the glass slides. High crystal growth rates have been observed for the borosilicate glasses employed at temperatures in the range of 700 to 950 °C. A sol–gel dip-coating method was developed for the deposition of a ZrO2 interface on the reinforcing fibres, as shown schematically in Fig. 1. Yttrium–zirconyl oxalate sol was prepared [18–20] with 1 molar solutions of the precursors. The yttrium content was calculated based on a 3 mol% Y2O3 to ZrO2 ratio. The fibres were vacuum infiltrated by the sol, dried at room temperature for 1 day, and subsequently ovendried at 60 °C for 24 h, at 90 °C for 12 h and at 100 °C for 12 h. The asdried dip-coated fibres were calcined according to the thermal treatment shown in Fig. 1, before a second layer was deposited following the same procedure. The second calcination procedure included a sintering step at 1300 °C for 1 h, to densify the interfacial layers. In the case of the Nextel™ fibres, a “minicomposite” was obtained, comprising the fibre filaments embedded in the ZrO2 matrix. Total light transmittance of Nextel™ and basalt fibre composites was measured in the direction perpendicular to the fibre plane using a Shimadzu UV-3600 spectrophotometer. The as-produced composites were polished with diamond suspensions of 6, 3 and 1 µm grain sizes.
where A is the hysteresis area of the graph, Fmax is the maximum applied force before unloading–reloading cycle, Rf is the fibre radius, Ef the fibre's Young's modulus and τ the interfacial sliding resistance. The flat chevron notch (FCN) technique [23] was employed for measurement of the fracture toughness of the composites. This special technique has been developed to capture the effect of the interfacial strength between fibre and matrix in composites, including the effect of the coating (interlayer), on fracture toughness. In addition, the FCN
Fig. 5. Total light transmittance of a soda-lime silicate slide (unreinforced matrix), composites reinforced with Nextel® fibres with various spacings and for sapphire fibre-reinforced composites with a 3 mm spacing.
Fig. 6. Normalized light transmittance of composites as a function of the shadow area measured by transmitted light microscopy, decreasing with increasing shadow area.
3
A=
Fmax ; 24π2 R3f τEf
ð1Þ
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3. Results and discussion 3.1. Composites light transmittance
Fig. 7. Average flexural strength of soda-lime silicate (SS) and borosilicate (BS) composites reinforced with coated and uncoated Nextel™ fibres, showing variation with increasing fibre spacing.
test allows measurement of the strength of sandwiched glass plates and possible matrix changes in the vicinity of the fibres, caused by thermal treatment, can be identified. Fig. 3 shows the schematic drawing of the test-piece, where a chevron notch was introduced at the fibre plane. Samples with the dimension of 10 × 10 mm2 were prepared, with polished surfaces, and loaded as indicated in Fig. 3. The fracture toughness (KIc) was then calculated from the maximum load before failure along the mid-plane (Fmax), a geometrical function obtained from finite element analysis (Y*min) and the specimen dimensions (B and W), according to [23]:
KIC =
Ymin * Fmax 1
BW 2
ð2Þ
The composites produced exhibited excellent optical transparency as shown in Fig. 4. Despite the regular fibre spacings employed, as described previously, these distances were altered during the thermal treatment for consolidation of the composites, as a result of the gravity-induced glass flow and consequent slight deformation of the slides. The actual spacings measured by image analysis of transmitted light microscopy are shown in Table 2. The total light transmittance of the composites was constant throughout the visible wavelength range (Fig. 5). Overall, the composites with the greatest spacing (lowest fibre content) exhibited the highest transmittance values (approximately 77%), as well as the composites reinforced with the transparent sapphire fibres. Dericioglu and Kagawa [3] described the light transmittance of the composites as a function of the matrix total light transmittance, reduced by the shadow area created with the introduction of the reinforcing fibres. A mathematical model was proposed, in which the total light transmittance of the composite (TC) is given as a fraction of the matrix transmittance (TM), considering the transmittance loss caused by the shadow area produced by the opaque fibres, as a function of their radius (Rf) and spacing (dS): TC = TM
2R 1− f dS
ð3Þ
The average composite transmittance values measured were normalized with the matrix transmittance (TC/TM) and plotted against the shadow area (2Rf/dS) measured by image analysis of transmitted light microscopy for the different fibre spacings (Fig. 6). The values followed the trend predicted with the mathematical model,
Fig. 8. Typical brittle failure fracture surface of borosilicate (a) and soda-lime silicate (b) glass reinforced with uncoated Nextel™ fibres, and fibre pull-out occurring within ZrO2 coated Nextel™ fibre bundles ((c) and (d)).
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Fig. 9. Bending test bar showing crack bridging by the reinforcing fibres in a Nextel™ 610 (white stripes in the image) fibre-reinforced soda-lime silicate glass composite with ZrO2 interface.
represented in the graph by the dashed line, although some deviation was observed. The equation was developed based on in-line light transmittance values, hence excluding scattering effects present in the matrix, such as pores and inclusions. The measurements presented are for total light transmittance, which therefore include these scattering effects. 3.2. Flexural strength Fig. 7 shows the flexural strength of Nextel™ fibre-reinforced composites, with and without the ZrO2 interface, as a function of fibre spacing. The fibre spacing employed in the manufacture of the composites, i.e. before thermal treatment, was used in this case, since the real spacing value does not affect the flexural strength values directly, but the fibre content. The comparison between flexural strength values measured for composites with uncoated fibres and the unreinforced glass matrices indicates that no significant improvement was achieved. The introduction of the ZrO2 interface, however, led to increased flexural strength, at approximately 17 and 23% compared with the unreinforced soda-lime silicate and borosilicate matrices, respectively. Considering the flexural strength of the composites reinforced with uncoated Nextel™ fibres, increases of 27 and 94% were observed for the borosilicate and soda-lime silicate matrix composites, respectively. Observation of the fracture surfaces of the composites after bend testing indicated that some fibre pull-out occurs within the ZrO2 coated fibre tow (Fig. 8(c) and (d)), whereas limited pull-out occurs in the composites reinforced with the uncoated fibres, which revealed a fracture surface characteristic of brittle failure.
Fig. 10. 4-point bend strengths of sapphire fibre-reinforced soda-lime silicate and borosilicate glass matrices showing the increased strength of the borosilicate GMC.
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Fig. 11. Interfacial sliding resistance of sapphire fibre-reinforced soda-lime silicate (SS/Sap) and borosilicate GMCs (BS/Sap), compared to values reported in the literature.
In addition, a fail-safe behaviour, e.g. non-catastrophic failure, was observed for the composites with the highest fibre content (2 mm fibre spacing), especially for those containing ZrO2-coated fibres. In these composites, crack bridging occurs and the glass matrix fragments are held together by the fibres, preventing the composites from failing into pieces (Fig. 9). Introducing sapphire fibres into the glass matrices did not induce the same effect, and the composites failed in a completely brittle manner. An increase in strength was observed, however for the borosilicate GMCs containing sapphire fibres with a 3 mm spacing. Composites with higher fibre content could not be fabricated due to the difficulty in eliminating residual porosity when fibre spacings smaller than 3 mm are employed. Fig. 10 shows the measured bend strength values of the sapphire fibre-reinforced composites, revealing a 49% increase in strength for the borosilicate GMCs. Measurement of the interfacial sliding resistance of both composites (Fig. 11) showed that a greater stress is generated at the borosilicate glass/sapphire fibre interface than that at the interface of the same fibre with the soda-lime silicate glass, although in both cases the values obtained were comparable to values reported in the literature for a sapphire fibre-reinforced epoxy matrix composite [24], a Nextel™ 720 fibre-reinforced mullite matrix composite [25] and SiC reinforced LAS glass–ceramic [26] and borosilicate glass [22] matrix composites. The higher interfacial sliding resistance present in the borosilicate GMC can lead to increased energy dissipation, resulting in higher flexural strength. In the case of the Nextel™ fibre-reinforced composites, measurement of the interfacial sliding resistance was
Fig. 12. Flat chevron notch fracture toughness of soda-lime silicate and borosilicate matrix composites reinforced with Nextel™ 610 and sapphire fibres.
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fracture toughness. The same effect was observed in the sapphire fibre-reinforced composites (Fig. 14(b)), while in the case of composites containing a ZrO2 interface, crack propagation occurred at the fibre/matrix interface as shown in Fig. 14(c) and (d). Fractured pieces of the coating can be seen at the interface, indicating that crack deflection and fibre debonding occurred, which explains the increase in bend strength for composites with ZrO2 interface. The introduction of a ZrO2 layer, on the other hand, seems to provide improved fibre/matrix interfacial bonding, as the crack is observed to propagate along the fibre/matrix interface, and most importantly along the fibre/coating interface as well. This indicates the potential of the interface to act efficiently in toughening the composite, as fibre pull-out was also observed during fracture, as shown previously in Fig. 8(c) and (d). Fig. 13. Fracture surface of a Nextel™ 610 reinforced borosilicate composites showing the crack initiation and the notch tip, propagating along the fibre plane.
not possible due to cracking of the fibre prior to sliding, suggesting that a strong bond has formed. Measurement of the fracture toughness showed that there is little contribution of the reinforcements to fracture toughness of the composites, as the values obtained are close to those of the unreinforced matrix processed under the same conditions (Fig. 12). It was noted that crack propagation occurred, as expected, in the plane of the chevron notch and fibre tows (see Fig. 13) so that no fibre pullout or similar toughening mechanisms were operative. The results suggest that at the fibre contents investigated, the fibre/matrix interfacial area present in the composites does not provide sufficient energy dissipation for improved fracture toughness. Fig. 14(a) shows typical fracture surfaces of uncoated Nextel™ fibre-reinforced composites after the FCN test. The crack is initiated at the notch tip as expected, and propagates along the fibre plane, although not at the fibre/matrix interface as desired for improved
4. Conclusions Highly transparent GMCs were produced in the systems comprising the combination of soda-lime silicate and borosilicate glass matrices with Nextel™ or sapphire fibres. Analysis of the composites light transmittance with different fibre content showed that optimum transparency can be achieved without compromising the fibre content up to a maximum at which minimal transparency losses occur with the introduction of opaque fibres. Nearly complete light transmittance (93%) was obtained when transparent (but costly) sapphire fibres were employed as reinforcement. Mechanical properties, however, indicate that there is little contribution of the reinforcements to the composites' mechanical integrity at the fibre contents investigated. Due to the relatively high bonding between fibres and matrices fracture toughness is barely affected, although evidence of energy dissipating mechanisms was confirmed by analysing the morphology of fractured specimens. It is worthwhile emphasizing that the present work was carried out as a preliminary assessment of the potential of these composite
Fig. 14. Fracture surfaces of borosilicate glass composites reinforced with (a) uncoated Nextel™ fibres, (b) uncoated sapphire fibres and (c) ZrO2 coated Nextel™ fibres, and (d) detail of the ZrO2coating/matrix interface, showing the crack propagation at the coated fibre/matrix interface and crack deflection at the coating layer.
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systems to exhibit enhanced mechanical properties as well as optical transparency. Therefore, only one fibre plane was introduced at this stage, despite the consequently resultant low fibre content. The use of higher fibre contents is currently being investigated, e.g. by including additional fibre layers, or in a multilayered system, which is challenging since the composite light transmittance should not be impaired by addition of fibres. Moreover, an increase in bend strength was observed in composites with ZrO2 interface. Samples containing the ZrO2 interface showed a bend strength improvement of 17 and 23%, for the soda-lime silicate and borosilicate matrix composites, respectively. These composites are thus promising materials for optomechanical applications, where high optical transparency and mechanical resistance/integrity are concurrently required. Future work will therefore focus in the optimisation of coating conditions, where further characterisation of interfacial properties (e.g. measurement of interfacial sliding resistance) will also be carried out to achieve a greater improvement in mechanical properties. Acknowledgments We thank Prof. J. R. Taylor and Dr J. C. Travers for kindly allowing us to carry out the light transmittance experiments in their facilities (Physics Department of Imperial College London). The authors acknowledge the financial support of the EU Commission, Marie Curie Fellowship (MEST-CT-2004-514667) “MatEnv-Trans and PowGen”. Financial support provided by the Czech Science Foundation under project GA101/09/1821 is gratefully acknowledged.
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