- Email: [email protected]
borosilicate glass matrix
Journal of Materials Processing Technology 169 (2005) 270–280
Processing and characterisation of model optomechanical composites in the system sapphire fibre/borosilicate glass matrix A.R. Boccaccini a,∗ , D. Acevedo a,1 , A.F. Dericioglu b , C. Jana c a
Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba-shi, Ibaraki 305-0047, Japan c Schott-Jenaer Glas GmbH, Otto Schott Straße 13, D-07745 Jena, Germany Received 8 July 2004; accepted 11 March 2005
Abstract Optomechanical composites based on the system sapphire fibre/borosilicate glass matrix were fabricated and characterised. Different techniques of fabrication were used: composites with randomly orientated chopped sapphire fibres were produced by powder technology and pressureless sintering, whilst unidirectionally oriented fibre composites were fabricated by hot-pressing as well as by sandwiching glass slides and arrays of parallel fibres followed by heat-treatment. Pressureless sintered samples were poorly densified and were opaque. Hot-pressed and “sandwich structure” composites were dense and exhibited strong interfaces between fibres and matrix. Only the “sandwich structure” composites were transparent and showed significant light transmittance in the visible wavelength range, only 20% lower than that of the unreinforced matrix. Due to the strong matrix/fibre interface limited fibre pull-out during composite fracture was observed. The fabricated transparent composites represent an improved version of the traditional material wired glass. They are candidate materials for applications in high performance fire and impact resistant windows requiring high impact strength and avoidance of fragmentation upon fracture. © 2005 Elsevier B.V. All rights reserved. Keywords: Composite materials; Glass matrix; Hot-pressing; Sintering; Ceramic fibres; Mechanical properties; Optical properties
1. Introduction Silicate glasses have extremely low fracture toughness values, which leads to the well known low reliability of these materials in load-bearing applications. Hence, strengthening and toughening of silicate glasses are required if these materials are to find wider use in structural applications [1]. Forming composites by incorporation of reinforcing elements in glass and glass-ceramic matrices is a very effective approach to improve the mechanical properties of glasses, including fracture strength, fracture toughness as well as thermal shock and impact resistance [2–5]. Reinforcements most commonly used are in the form of ceramic whiskers, platelets, particulates or fibres [2–5]. Numerous studies have ∗ 1
Corresponding author. Tel.: +44 207 594 6731; fax: +44 207 584 3194. E-mail address: [email protected] (A.R. Boccaccini). Present address: GEMPPM, INSA de Lyon, 69621 Villeurbanne, France.
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.03.011
been carried out to understand the toughening mechanisms induced by the presence of the reinforcements and to investigate the parameters that lead to satisfactory mechanical behaviour of glass and glass-ceramic matrix composites [5–11]. Limited previous research has been carried out focusing on improving simultaneously mechanical and functional properties of glass matrix composites. These include composites for electric and electronic applications (e.g. induction heating equipment, microwave components, electronic package substrates, connectors), high-temperature applications (e.g. thermal insulators, jet engine thermocouples), dimensional stability applications (e.g. mirror supports for telescopes) and optical applications (e.g. impact resistant windows, structures for micro fluidics) [4,12–20]. In particular, composites exhibiting favourable optical and mechanical properties are called «optomechanical composites» [18], which are the focus of the present work.
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
The main difficulty in the development of optomechanical composites is the requirement of being able to improve the mechanical and optical properties simultaneously [18–24]. Indeed, most glass matrix composite materials developed to date are not optically transparent, or even translucent, because of the type of reinforcements used (e.g. SiC or carbon fibres) [2–5], and therefore, they cannot be considered to be suitable materials for optomechanical applications. Hence, means of improving the mechanical properties of glasses without significantly degrading their optical transparency need to be further investigated. The selection of appropriate fibres and matrices for optomechanical composites is a complex matter because numerous factors have to be considered. The main requirement for the fibres is that they should have a higher thermal stability than the glass matrix because of the usually high temperatures needed for matrix densification [18,22]. Furthermore, matching the fibre and matrix thermal expansion coefficients is necessary in order to avoid large residual stresses upon cooling from the fabrication temperature. However, the development of compressive residual stresses in the matrix by having fibres with thermal expansion coefficient higher than that of the matrix may be also favourable [18,22]. Finally, the strength of bonding at the interface between fibres and matrix is an important parameter since it has a large influence on the mechanical behaviour of the final composites, as it is the case in all brittle matrix composites [2–4]. In addition to these requirements, matching of the fibre and matrix refractive indices is also necessary to avoid (or minimise) light scattering, and thus to obtain a transparent or at least a translucent material [15–24]. Another possible option to obtain transparent composites is to include optical windows by a relatively large spacing of the reinforcing fibres in the matrix [18], in a similar way as in the traditional material wired glass. Recently, we have fabricated oxide-fibre reinforced glass matrix composites with high light transmittance (only 30% lower than that of the matrix) using the “optical window” concept, which is based on the presence of relatively large transparent matrix regions surrounded by the reinforcing fibres [22]. Moreover, Dericioglu et al. [18,24] fabricated minicomposite reinforced borosilicate glass matrix optomechanical composites with low volume fraction of reinforcement exhibiting light transmittance higher than 80% of the transmittance of the matrix, showing that even if the fibres incorporated are opaque, it is possible to achieve considerable optical transparency. A few experimental investigations aiming at producing optomechanical composites with technical applications have been carried out, specially in Japan [17–20,24], in Germany [15,16,23] and in the UK [22], yet research in this field remains still rather limited, which has thus motivated the present experimental study. The objective of this work is to explore and optimise different methods to produce optically transparent borosilicate glass matrix composites reinforced by single crystal Al2 O3 (sapphire) fibres. Borosilicate glass was chosen because of
271
its high thermal capability as well as considerable corrosion and thermal shock resistance [25]. In fact, borosilicate glass has been widely used in the past as the matrix for SiC and carbon fibre reinforced composites for structural applications [4,26]. Moreover, borosilicate glass matrices reinforced by ␣-Al2 O3 in the form of particles, platelets and fibres have been the matter of numerous previous investigations due to the favourable thermal expansion mismatch between alumina and borosilicate glass composition [4,6,27]. Additional advantages of borosilicate glass are its optical properties and relatively low dielectric constant [25,28]. Sapphire fibres were selected because they exhibit outstanding high temperature stability, high chemical durability and excellent mechanical properties [29,30]. Single crystal sapphire fibres have been used in previous studies to reinforce ceramic and glass matrices for high-temperature applications [30–33]. In those studies however no special care was placed on the optical property (transparency) of composites, except for some model systems fabricated for academic purposes [21]. Thus, to the authors’ knowledge, this is the first work on the system sapphire fibre/borosilicate glass matrix with the specific aim of producing transparent composites for optomechanical applications.
2. Materials and experimental procedure 2.1. Materials Borosilicate glass was selected as the matrix material and it was used in two different forms: i) powder of mean particle size <40 m (Duran® , Schott Glas, Mainz, Germany) and ii) glass plates of thickness 1.1 mm (Borofloat® 33, Schott Jenaer Glas, Jena, Germany). The properties of the glass are summarized in Table 1 [34]. The chemical composition of Duran® glass is (in wt%) [34]: 81SiO2 , 13B2 O3 , 4(Na2 O + K2 O), 2Al2 O3 , which can be considered to be identical to that of Borofloat® 33. The reinforcement chosen was sapphire fibre of optical quality with nominal diameter 150 m (Saphikon® , Laser Components UK, Ltd.). The fibres were received in length of 1 m and were cut manually to appropriate lengths for composites fabrication by using metallic scissors. For all composites, fibres were used in the as-received condition. Sapphire fibres were selected because they exhibit outstanding thermomechanical properties [29–33]. This fibre is a monocrystal of ␣-Al2 O3 of very high quality exhibiting high strength and hardness. Additionally, because absence of grain boundaries, Table 1 Properties of the borosilicate glass DURAN® [25,34] Density (g cm−3 ) Tensile strength (MPa) Elastic modulus (GPa) Coefficient of thermal expansion (◦ C−1 ) Refractive index
2.23 60 64 3.3 × 10−6 1.473
272
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
Table 2 Properties of the Saphikon® fibres [35] Density (g cm−3 ) Diameter (m) Tensile strength (GPa) Elastic modulus (GPa) Melting point (◦ C) Coefficient of thermal expansion (K−1 ) Uniaxial refractive index
3.99 150 2.1–3.4 386–435 2053 7.9–8.8 × 10−6 1.760–1.768
there is no light scattering within the sapphire fibres. Table 2 summarizes the main properties of the Saphikon® fibres used [35], and Fig. 1 shows a scanning electron microscopy (SEM) image of the fibre. This micrograph confirms that the fibres have a circular cross-section. Moreover, it shows that the fibres have a very smooth surface, which will influence the possible toughening mechanisms acting in the composites: a smooth interface should lead to greater average pull-out lengths, and thus to higher fracture toughness provided there is optimal bonding strength between the fibre and the matrix [36]. 2.2. Preparation of the composites 2.2.1. Pressureless sintering A mixture of borosilicate glass powder and 5% in volume of chopped sapphire fibres with a length of ∼1 mm was prepared. Fibres were cut using scissors to the required length, and matrix powder and fibres were mixed in dry conditions in a rotary mixer for 1 h. The mixture was pressed into cylindrical samples of 8 mm diameter in a die at room temperature by application of a compacting pressure of about 100 MPa for 2 min. No binder was used in this operation. The pellets were then sintered in an electric furnace at 750 ◦ C for 2 h in normal atmosphere. The heating rate used was 5 ◦ C min−1 . Samples were left to cool down in the furnace. The sintering temperature was chosen on the basis of previous investigations on alumina
Fig. 1. Scanning electron microscopy (SEM) image of a Saphikon® fibre used in the present work.
Fig. 2. Arrangement of Saphikon® fibres in (a) hot-pressed and (b) “sandwich structure” composites, in a volume fraction of approximately 5%.
platelet reinforced glass matrix composites where the same borosilicate glass matrix was used [37]. 2.2.2. Hot-pressing A custom-made vacuum hot-press described in previous works [38] was used. Samples made of pure glass matrix and fibre reinforced composites were fabricated. The average fibre length was about 10 mm. The fibres were arranged parallel to each other on a slightly pressed thin layer of powder in a carbon die. They were separated an average distance of ∼1 mm, as shown in Fig. 2(a). Subsequently, a second layer of powder was added above the fibres and the composites were hot-pressed. The volume fraction of fibres in the rectangular samples, which were cut out of the hot-pressed disc, as shown in Fig. 2(a), was approximately 5%. The parameters used for the fabrication of the samples were: heating rate 100 ◦ C h−1 , holding temperature 750 ◦ C, holding time 1 h, applied pressure 10 MPa and cooling rate 100 ◦ C min−1 . 2.2.3. “Sandwich structure” method This is a simple pressureless method for composite fabrication introduced recently [22]. The method consists of sandwiching the reinforcing fibres between two glass plates, as shown in Fig. 2(b), and then subjecting the “sandwich structure” to a heat treatment to consolidate the composite by exploiting viscous flow of the glass. The as-received plates of borosilicate glass (Borofloat® 33) were cut by means of a diamond tip to the desired dimensions (about 2.5 cm × 1.5 cm). The average length of the fibres was 8 mm. The same disposition as in the hot-pressed samples was used: fibres were arranged parallel to each other, and the average distance between two fibres was about 1 mm (see Fig. 2(b)). The heat-treatment consisted of two main steps. In the first step, the glass plates were heated under a high vacuum to clean the surface by degassing. Subsequently, the sandwich structure was subjected to a second heat-treatment. The heating rate was 100 ◦ C h−1 , the holding temperature was varied between 750 and 775 ◦ C, the holding time was between 2.5
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
and 5 h and the cooling rate was varied between 100 and 260 ◦ C h−1 . The degassing step included two different treatments. Firstly, the sample was under a normal atmosphere and after completion of half of the holding time, a high vacuum was produced in order to clean all the impurities that might have burnt. All samples were produced using the same heating and cooling rates (100 ◦ C h−1 ) in both steps, the holding temperature and holding time for the first step were 500 ◦ C and 4 h, respectively, and they were 750 ◦ C and 5 h for the second step, respectively. 2.3. Characterization techniques The density of sintered and hot-pressed samples was determined by the Archimedes’ method. The relative density was calculated by considering the theoretical density of the composites based on the density data for matrix and fibres, given in Tables 1 and 2, respectively, and the volume fraction of fibres. For microstructural characterisation, samples were cut, then mounted in resin and polished to 1 m finish with diamond paste to obtain flat cross sections for SEM. The microstructures of selected sintered, hot-pressed and sandwich structure samples were examined using conventional SEM (JEOL LV 5610) working in both secondary electron and backscattered electron modes. Sintered samples were analysed using X-ray diffraction (XRD) to detect any crystallisation of the matrix. Samples were crushed to a fine powder, and then analysed with a Philips PW1700 series automated powder diffractometer using Cu K␣ radiation at 40 kV–40 mA with a secondary crystal monochromator. To gain preliminary understanding of the fracture behaviour of the composites and qualitative information about the interaction (bonding) between fibres and matrix during the fracture process, fractures surfaces were also observed by SEM. A preliminary assessment of the optical quality of the samples was obtained by placing selected samples at different heights over a written text on a white surface and assessing the text legibility. This qualitatively demonstrated the light transmitting characteristics of the different composites. A digital camera was used to document this behaviour. Fabricated “sandwich structure” composites were also observed by means of a conventional optical microscope. Light transmittance of the hot-pressed and “sandwich structure” samples (with and without fibres) was measured at room temperature by a UV–visible spectrophotometer (UV1601 Shimadzu, Japan), in the direction perpendicular to the fibres axis. Before light transmittance measurements, samples were cut and polished to obtain the size required for the measurements. The thickness of the samples was 2.3 ± 0.1 mm, and the light transmittance was measured for wavelengths between 350 and 800 nm. The light transmittance of the samples was reported as a percentage of the transmittance of a reference monolithic borosilicate glass slide (Borofloat® 33).
273
3. Results and discussion 3.1. Microstructural characterisation 3.1.1. Sintered composites Successful fabrication of fibre reinforced glass matrix composites relies on knowing the relationship between temperature and viscosity of the glass matrix. As it is well-known for Duran® -type borosilicate glass [34,37], the range of suitable temperatures for sintering glass powder is very narrow (between 720 and 780 ◦ C). A difference of ±20 ◦ C in the sintering temperature can have a large effect on the densification of the composites. Pressureless sintered samples fabricated in this investigation were not translucent and their densification was not completely achieved; the sintered relative densities were in the range 80–83% of theoretical density. Fig. 3(a–c) show SEM micrographs of polished cross-sections of a chopped fibre reinforced sintered composite at different magnifications. Fig. 3(a) shows that the matrix close to the fibres is porous and that larger defects are situated around the fibre, confirming that it has not been possible to obtain a good densification of the matrix in this region by pressureless sintering. The presence of randomly orientated fibres impedes the perfect flow of the viscous glass during sintering and causes a high porosity of the matrix in the region close to the fibres. On the contrary, the matrix far away from the fibres (Fig. 3(b)) exhibits high densification with few isolated pores. Fig. 3(c) is a high magnification image of the fibre/matrix interface showing that the lack of densification impedes the complete bonding of fibre and matrix leading to an imperfect interface. The poor densification of the pressureless sintered samples achieved in the present work is in broad agreement with previous studies that showed that the presence of rigid inclusions makes the densification of a glass matrix composite a difficult task and that the density of the composites decreases with increasing volume fraction of rigid inclusions [39–41]. Different explanations for this phenomenon have been proposed. An inhomogeneous distribution of inclusions in the powder can lead to a poor matrix particle packing and formation of agglomerates, and thus the porosity will increase around the fibres [39]. Moreover, studies conducted using the same materials but different mixing conditions have shown that optimised wet-mixing techniques lead to higher densities as they allow for more homogeneous mixtures [42]. Since the present glass powder/chopped fibres mixture was prepared in dry conditions, the last explanation may be applicable to our results. Another factor affecting the densification of composites containing rigid inclusions is the development of residual stresses as a consequence of different sintering rates of matrix and inclusions. These stresses may cause sintering damage, leading to crack-like voids or isolated pores and consequently to poor mechanical properties of the sintered samples [39–42].
274
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
Fig. 3. SEM micrographs of Sapphire® fibre reinforced pressureless sintered composites at different magnifications: (a) area around a fibre showing porosity and sintering defects, (b) matrix far away from the fibres exhibiting high densification and (c) high magnification image of the fibre/matrix interface showing imperfect bonding.
The existence of different “kinds” of porosity in the green compact has been also proposed to explain the retardation of densification in composites containing rigid inclusions [39]. Pores can be embedded in the matrix material only, or they can be located at the interface between matrix and inclusions. This is confirmed in the present composites by the micrographs analysed above (Fig. 3(a and c)). It is observed that pores are situated both in the matrix and at the interface between matrix and fibres. These different pore types in the initial compact will have different free surface energies, and thus, they will lead to an overall lower driving force for sintering in the composite than in the inclusion-free compact [39] and thus to a less densified composite. However, Fig. 3(b) shows that far away from the fibres, the matrix was very well densified, confirming that the parameters used for sintering (time and temperature) were appropriate for this glass. Fig. 4 shows the XRD pattern for a Saphikon® fibre reinforced sintered composite. The only crystalline phase detected was corundum, which corresponds to the crystalline structure of the Saphikon® fibres used. No cristobalite has crystallised in the matrix, which is a favourable result from the point of view of the composite mechanical strength. A simi-
lar result was obtained for hot-pressed samples. Indeed it has been proved in previous studies [27,28,43] that cristobalite crystallisation could happen in Al2 O3 /borosilicate glass composites with low volume fractions of Al2 O3 (<10%) and even when sintering temperature is low (in the range 700–800 ◦ C).
Fig. 4. XRD pattern of a Saphikon® fibre reinforced sintered composite showing corundum as the only cristalline phase, which corresponds to the structure of the Saphikon® fibres.
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
275
Fig. 5. SEM image of the polished section of a hot-pressed sample containing Saphikon® fibres unidirectionally aligned, showing high densification but the presence crack-like defects.
This should have a negative effect on the mechanical properties of the composites because the different thermal expansion coefficients of cristobalite and borosilicate glass matrix may lead to microcracking, therefore decreasing the tensile strength of the composites. The lack of cristobalite formation in the present composites confirms the high resistance to crystallisation of the Duran® borosilicate composition and supports the choice of this particular glass as the matrix for composites. 3.1.2. Hot-pressed composites Hot-pressed samples without fibres were translucent, while composites containing fibres were opaque, even though the densities achieved were very high in both cases, 99.5 and 98% of theoretical density, respectively. Cristobalite crystallisation was not detected in hot-pressed composites. Fig. 5 shows a polished section of a hot-pressed sample containing 5 vol% unidirectionally aligned Saphikon® fibres. Although the matrix seems to be well densified without the presence of large cavities or pores (compare with Fig. 3(a), pressureless sintered sample), there are still some defects and crack-like voids remaining, mainly close to the fibres, which should be responsible for high light scattering and the loss of transparency of these samples. Thus, despite the relative high density achieved, the parameters used for hot-pressing were not optimal for producing optically transparent or translucent composites. 3.1.3. “Sandwich structure” composites SEM micrographs of polished surfaces of “sandwich structure” composites are shown in Fig. 6(a and b). In some regions the interface between fibres and matrix could only be detected by using SEM in backscattered electrons mode (Fig. 6(b)), indicating an intimate perfect bond. Some defects (voids) were observed in some samples, localised just close to the fibres (Fig. 6(a)). These were probably the result of a too short processing time. When the parameters used were
Fig. 6. SEM micrographs of polished surfaces of “sandwich structure” composites showing (a) some defects (voids), marked by arrows, localised close to the fibres after too short processing time and (b) perfect bonded sample fabricated employing optimised parameters.
optimised (775 ◦ C, 3.5 h), the two plates of glass were seen to be well bonded together and the interface between them was “invisible” under SEM (Fig. 6(b)). “Sandwich structure” samples with and without fibres were transparent. There was some contamination on the surfaces after heat-treatment, but this disappeared after polishing. The two slides of glass were in all cases very well bonded. Fig. 7 shows the matrix region between two fibres (“optical window”) in a sandwich structure composite observed with an optical microscope; fibres are also visible. Heat-treatment at 775 ◦ C for 3.5 h was found to be the best thermal process for the fabrication of “sandwich structure” composites. 3.2. Fracture behaviour Previous research has proved that ␣-Al2 O3 /borosilicate glass systems containing ␣-Al2 O3 platelets as reinforcement lead to composites with higher fracture strength, Young’s modulus, hardness and fracture toughness than the matrix without reinforcement [6,37]. In the present investigation, fracture behaviour of the composites was anal-
276
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
Fig. 7. Optical microscopy micrograph showing the matrix region of about 1mm width between two sapphire fibres (“optical window”) in a (nonpolished) transparent sandwich structure composite.
ysed qualitatively by examining fracture surfaces of different samples. 3.2.1. Sintered composites Fig. 8(a and b) are SEM images of fracture surfaces of sintered samples. Fig. 8(a) shows that there is pull-out of the fibres during the fracture of the composite. Since there are many defects at the interface between fibres and matrix (see also Fig. 3(c)), the bonding at the interface is weak and it is easy to have fibre/matrix debonding and fibre pull out. These mechanisms (debonding and pull-out) dissipate energy, and thus they will contribute to increase the toughness of the composite. It is also seen that there are cracks in the matrix in regions around the fibres and at the interface, which are most probably a consequence of pores and crack-like flaws produced during sintering. However, Fig. 8(b) indicates that in regions where the glass matrix is in close contact with the fibres, bonding between matrix and fibres can be free of defects. In this case a strong fibre/matrix bonding may be assumed. Furthermore, the presence of layers of glass on the pulled-out fibres (arrows in Fig. 8(a)) confirms that the bonding between borosilicate glass and sapphire fibres was very strong after sintering at 750 ◦ C if there was close contact between them. Thus, it may be anticipated that fracture toughness of the samples will be only partially improved by debonding and pull-out mechanisms. Moreover, the samples will show low fracture strength because of the relatively high porosity. 3.2.2. Hot-pressed composites The fracture behaviour of the hot-pressed composites was very similar, in qualitative terms, to that of pressureless sintered samples, as inferred from fracture surface analyses.
Fig. 8. SEM images of fracture surfaces of pressureless sintered samples showing (a) evidence of pull-out of the Saphikon® fibres during fracture of the composite and (b) defect-free matrix-fibre interface in an area of good matrix-fibre bonding. The arrows in (a) indicate rest of glass from the matrix adhered to the surface of the fibre.
Typical fracture surfaces showing evidence of fibre debonding and pull-out are presented in Fig. 9(a and b). Due to the lower porosity of these composites compared to pressureless sintered materials, they are expected to exhibit higher fracture strength. Moreover, due to the unidirectional alignment of the Saphikon® fibres in these composites, the materials will exhibit anisotropic mechanical behaviour with highest strength and stiffness in the fibre direction, exploiting a loadtransfer mechanism [7]. 3.2.3. “Sandwich structure” composites The “sandwich method” led to defect-free composites without porosity. These composites therefore should have higher fracture strength than the sintered samples. SEM micrographs of fracture surfaces of “sandwich structure” composites, shown in Fig. 10(a–c), confirm that when the matrix is in close contact with the fibres a strong bonding may develop between them. Fig. 10(a) shows an impression left in the matrix by the fibre when the sample was fractured. The matrix was broken and some glass matrix stayed adhered to
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
277
strength and elastic modulus than the matrix. Moreover, the higher thermal expansion coefficient of the fibre than that of the matrix implies that, upon cooling from the fabrication temperature, the matrix will be in tangential compression [7]. This should increase the tensile fracture strength of the composite. It has been also shown that compressive residual thermal stresses in the matrix caused by thermal expansion coefficient mismatch have important effects on the toughening mechanisms acting in brittle matrix composites [5,8], and that a compressive residual stress field in the matrix should increase the matrix microcracking stress [7]. 3.3. Optical properties
Fig. 9. Typical fracture surfaces of a hot-pressed sample showing evidence of Saphikon® fibre (a) debonding and (b) pull-out.
the fibre surface after fracture. This result confirms that there exists a strong bonding between ␣-Al2 O3 and borosilicate glass. As proved in previous investigations [31,36,37], chemical compatibility between Al2 O3 inclusions and borosilicate glass produces a very strong chemical bonding and leads to strong, but brittle composites. This was confirmed by the very short “pull-out” length in these composites, as shown in Fig. 10(b), and the relatively sharp interface between sapphire fibre and borosilicate glass matrix, as shown in Fig. 10(c). The results indicate the need to coat the fibres with a suitable material to produce the weak interface required for fully exploiting toughening mechanisms such as fibre pull-out. A typical material suggested for composites with silicate matrices and alumina-containing fibres is SnO2 [31,36]. Chawla et al. have studied the role of SnO2 coating, which has no diffusion in alumina and very little diffusion in silicate glass [31]. Other works have been conducted using TiO2 coatings on aluminosilicate fibres, which were embedded in silicate glass matrices [22]. In principle, sapphire fibres have suitable properties to reinforce borosilicate glass matrices, i.e. much higher tensile
The matching of refractive index of fibre and matrix in the present composites is not perfect; meaning that only low volume fraction of fibres may be used to achieve a transparent material, exploiting the optical window concept [18,24] similar to wired glass. Due to the lack of densification of the pressureless sintered composites, only hot-pressed and sandwich structure composites are discussed in this section. The macroscopic appearance of polished “sandwich structure” composites is shown in Fig. 11(a and b), which qualitatively demonstrates the transparency of the samples. The images show that it is possible to see through the composites, and thus light scattering effects are minimised. The underlaying text remains clearly legible even if the composite is not directly above the text (Fig. 11(b)). This behaviour was expected because of the disposition of the fibres in the matrix leaving optical windows (Fig. 7), as in the conventional material wired glass. The matrix regions between the fibres should have the same light transmittance as the monolithic borosilicate glass matrix. Measurements of the light transmittance of the composites in the UV and visible wavelength ranges were carried out to quantify the transparency of the samples. The results are shown in Fig. 12. The figure shows the light transmittance of three samples: a hot- pressed sample without fibres, a “sandwich structure” sample without fibres (reference matrix) and a “sandwich structure” sample with fibres, before and after polishing. It is seen that the hot-pressed sample has lost almost all of its optical transparency; it has a transmittance of 18% relative to the borosilicate glass plate. It is thus difficult to envisage that it will be feasible to obtain a transparent fibre reinforced composite by this technique of fabrication, even if high densities (as high as 98% of theoretical density) could be achieved, as discussed in Section 3.1.2. The “sandwich structure” sample without fibres has a 100% transmittance in the wavelength range 350–800 nm. This means that there was no loss of transparency by joining two plates of glass together in a sandwich structure. This also confirms that the processing parameters used for fabrication were optimised for this system. The transmittance of a “sandwich structure” composite before and after polishing (to 3 m using a diamond grid) was also measured. As expected, Fig. 12 shows that the pol-
278
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
Fig. 10. SEM micrographs of fracture surfaces of “sandwich structure” composites showing (a) impression left in the matrix by a fibre debonded during fracture, (b) very short “pull-out” length in these composites and (c) the relatively sharp interface between sapphire fibre and borosilicate glass, indicating strong fibre/matrix bonding.
ished sample has better transmittance properties than the nonpolished sample; polishing the sample increases its optical transmittance by 20%. The “sandwich structure” composites exhibit an almost constant transmittance of about 60% before polishing and of about 80% after polishing. These results are in agreement with the images in Fig. 11, showing the high transparency of the composites, and they confirm that the “optical window” concept is a convenient way to fabricate optomechanical composites, as proposed in the literature [18,24]. The present results are better in terms of transparency than those obtained in our previous work [22], where only 60% transparency in Nextel® fibre reinforced soda-lime glass composites fabricated by the “sandwich structure” method was achieved. The results are similar to those obtained recently by Dericioglu and Kagawa [44], who achieved 80% relative transparency in SiC fibre reinforced MgAl2 O4 matrix optomechanical composites, in which the fibre diameter and fibre to fibre spacing were identical to those of the present “sandwich structure” composites. The light transmittance of the polished “sandwich structure” composite, which is given relative to that of the monolithic borosilicate glass matrix in
Fig. 12, can be analysed quantitatively by the following relation, which was developed for optomechanical composites with regularly aligned fibres in a transparent matrix [24]: Df Tc = Tm 1 − Lf
(1)
where Tc and Tm are the light transmittance of the composite and the monolithic matrix, respectively, and Df and Lf are the fibre diameter and fibre to fibre spacing, respectively. Incorporating the diameter of the sapphire fibre (∼150 m) and the fibre to fibre spacing (∼1 mm) in Eq. (1), the light transmittance of the polished “sandwich structure” composite relative to that of the monolithic matrix (Tc /Tm ) is found to be ∼85%. This value is in good agreement with the experimentally determined relative light transmittance value given in Fig. 12 for the polished composite that reads ∼80%. This result demonstrates the effectiveness of the optical window concept in the present sapphire fibre/borosilicate glass composites and its efficiency in providing optical transparency to the resulting optomechanical composite.
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
279
4. Conclusions
Fig. 11. Macroscopic appearance of polished “sandwich structure” composites qualitatively demonstrating the transparency of the samples. The images show that it is possible to see through the composites. The under-laying text remains clearly legible if the sample is placed (a) in direct contact with the text and (b) even if the composite is not directly above the text.
The system sapphire fibre reinforced borosilicate glass matrix composite was studied aiming at developing “optomechanical composites”. Different techniques of fabrication were used: randomly orientated chopped fibre reinforced composites were fabricated by pressureless sintering, unidirectionally oriented fibre reinforced composites were fabricated by hot-pressing and by sandwiching two slides of glass and an array of parallel fibres (sandwich structure composites). Pressureless sintered samples were porous, specially near the fibre/matrix interfaces and there was poor contact between the fibres and the matrix. Hot-pressed and “sandwich structure” composites were dense and showed a strong interface between fibres and matrix. Interface engineering should be introduced by coating the fibres with a suitable material (e.g. SnO2 ) in order to obtain a weaker interface and improve toughness by inducing significant fibre pull-out effect. Pressureless sintered and hot-pressed samples were opaque due to residual porosity and sintering defects. However, the hot-pressed unreinforced matrix was translucent. On the other hand, “sandwich structure” composites were transparent and showed significant light transmittance in the visible wavelength range, only 20% lower than that of the unreinforced matrix (borosilicate glass slides). These results indicate that this technique of fabrication is viable for production of “optomechanical composites” with borosilicate glass matrix. The samples fabricated, which exhibit strong fibre/matrix interface bonding, represent an improved (but less cost-effective) version of the traditional fire and impact resistant material wired glass. The composites should therefore be interesting materials for high performance fire resistant windows, requiring high impact strength and avoidance of fragmentation upon fracture, obviously in cases where stringent requirements may justify the higher costs of the material.
Acknowledgments Experimental assistance of Mr. Norbert Galy is appreciated. ARB acknowledges financial support of the Royal Society, London, UK (Grant nr. 574006.G503).
References
Fig. 12. Results of the measurements of light transmittance of selected samples in the UV and visible wavelength ranges: hot-pressed sample without fibres (a), “sandwich structure” sample without fibres (b) and “sandwich structure” sample with fibres, before and after polishing (c and d, respectively). For the composites, light transmittance was measured perpendicularly to the fibre axes.
[1] I.W. Donald, Review. Methods for improving the mechanical properties of oxide glasses, J. Mater. Sci. 24 (1989) 4177–4208. [2] K.M. Prewo, J.J. Brennan, G.K. Layden, Fiber reinforced glasses and glass-ceramics for high performance applications, Ceram. Bull. 65 (1986) 305–322. [3] R.D. Rawlings, Glass-ceramic matrix composites, Composites 25 (1994) 372–379. [4] A.R. Boccaccini, Glass and glass-ceramic matrix composite materials: a review, J. Ceram. Soc.Jpn. 109 (7) (2001) S99–0S109.
280
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
[5] I.W. Donald, Preparation and properties applications of glass and glass-ceramic matrix composites, Key Eng. Mater. 108–110 (1995) 123–144. [6] V. Cannillo, C. Leonelli, A.R. Boccaccini, Numerical models for thermal residual stresses in Al2 O3 platelets/borosilicate glass matrix composites, Mater. Sci. Eng. A323 (2002) 246–250. [7] K.K. Chawla, Ceramic Matrix Composites, Chapman and Hall, first ed., 1993. [8] A.G. Evans, F.W. Zok, Review. The physics and mechanics of fibre-reinforced brittle matrix composites, J. Mater. Sci. 29 (1994) 3857–3896. [9] M.D. Thouless, O. Sbaizero, L.S. Sigl, A.G. Evans, Effect of interface mechanical properties on pull-out in a SiC fibre-reinforced lithium aluminum silicate glass-ceramic, J. Am. Ceram. Soc. 72 (1989) 525–532. [10] H. Hegeler, R. Br¨uckner, Fibre-reinforced glasses: influence of thermal expansion of the glass matrix on strength and fracture toughness of the composites, J. Mater. Sci. 25 (1990) 4836–4846. [11] M.J. Blisset, P.A. Smith, J.A. Yeomans, Flexural mechanical properties of thermally treated unidirectional and cross-ply nicalonreinforced calcium aluminosilicate composites, J. Mater. Sci. 33 (1998) 4181–4190. [12] D.D.L. Chung, Composite Materials: Science and Applications, Springer, London/Berlin/Heidelberg, 2003. [13] P.N. Kumta, Processing aspects of glass-nicalon fibre and interconnected porous aluminium nitride ceramic and glass composites, J. Mater. Sci. 31 (1996) 6229–6240. [14] J.A. Rice, C.S. Hazelton, M.J. Haun, Optimisation of the electrical and mechanical properties of a low dielectric loss, continuous fibre ceramic composite, Ceram. Trans. 74 (1996) 497–508. [15] T. Leutbecher, D. Hulsenberg, Oxide fibre reinforced glass: a challenge to new composites, Adv. Eng. Mater. 2 (2000) 93– 99. [16] B. Fankhanel, E. Muller, K. Weise, G. Marx, Translucent fibre reinforced glass, Key Eng. Mater. 206–213 (2002) 1109–1112. [17] Y. Kagawa, T. Yamada, Tensile properties of optical fibre-glass matrix sensor-based composite, J. Mater. Sci. Lett. 13 (1994) 1403–1405. [18] A.F. Dericioglu, S. Zhu, Y. Kagawa, Improvement of fracture resistance in a glass matrix optomechanical composite reinforced by Al2 O3 –ZrO2 minicomposite, Ceram. Eng. Sci. Proc. 23 (2002) 485–492. [19] H. Iba, T. Chang, Y. Kagawa, H. Minakuchi, K. Kanamaru, Fabrication of optically transparent short fibre-reinforced glass matrix composites, J. Am. Ceram. Soc. 79 (1996) 881–884. [20] P. Kangutkar, T. Chang, Y. Kagawa, M.J. Koczak, H. Minakuchi, K. Kanamaru, Fabrication of optically transparent SiCaON fiber reinforced glass matrix composites, Ceram. Eng. Sci. Proc. 14 (9–10) (1993) 963–970. [21] D. Banerjee, H. Rho, H.E. Jackson, R.N. Singh, Characterization of residual stresses in a sapphire-fibre-reinforced glass-matrix composite by micro-fluorescence spectroscopy, Compos. Sci. Technol. 61 (2001) 1639–1647. [22] A.R. Boccaccini, S. Atiq, G. Helsh, Optomechanical glass matrix composites, Compos. Sci. Technol. 63 (2003) 779–783. [23] D. Huelsenberg, P. Feehling, T. Mache, V. Winkler, D. Raab, A.R. Boccaccini, Herstellung und charakterisierung neuer faserverstaerkter glasmatrix-verbundwerkstoffe, in: B. Wielage, G. Leonhardt (Eds.),
[24]
[25] [26]
[27]
[28]
[29]
[30] [31]
[32]
[33]
[34] [35] [36]
[37] [38] [39]
[40] [41]
[42]
[43] [44]
Verbundwerkstoffe und Werkstoffverbunde, Wiley-VCH, Weinheim, 2001, pp. 365–369. A.F. Dericioglu, Y. Kagawa, Mechanical behaviour of Al2 O3 –ZrO2 minicomposite reinforced glass matrix optomechanical composite, Mater. Sci. Technol. 19 (2003) 1119–1124. R.H. Doremus, Glass Science, Second ed., John Wiley & Sons, New York, 1994. A.R. Boccaccini, A.J. Strutt, K.S. Vecchio, D. Mendoza, K.K. Chawla, C.B. Ponton, D.H. Pearce, Behavior of nicalon-fiber reinforced glass matrix composites under thermal cycling conditions, Composites Part A 29 (11) (1998) 1343–1352. R.C.C. Monteiro, M.M.R.A. Lima, Effect of compaction on the sintering of borosilicate glass/alumina composites, J. Eur. Ceram. Soc. 23 (2003) 1813–1818. M.F. Zawrah, E.M.A. Hamzawy, Effect of cristobalite formation on sinterability, microstructure and properties of glass/ceramic composites, Ceram. Int. 28 (2002) 123–130. V.N. Kurlov, V.M. Kiiko, A.A. Kolchin, S.T. Mileiko, Sapphire fibres grown by a modified internal crystallisation method, J. Cryst. Growth 204 (1999) 499–504. J. Wendorff, R. Janssen, N. Claussen, Model experiments on pure oxide composites, Mater. Sci. Eng. A250 (2) (1998) 186–193. K.K. Chawla, M.K. Ferber, Z.R. Xu, R. Venkatesh, Interface engineering in alumina/glass composites, Mater. Sci. Eng. A162 (1993) 35–44. R.W. Goettler, K.T. Faber, Interfacial shear stresses in SiC and Al2 O3 fibre reinforced glasses, Ceram. Eng. Sci. Proc. 9 (7–8) (1988) 861–870. D.H. Pearce, A.J. Jickells, C.B. Ponton, Fabrication of sapphire fibre reinforced ceramic matrix composite, Br. Ceram. Trans. 95 (4) (1996) 141–145. Schott® Technical Glasses, Company information bulletin, 1982. Properties and benefits of sapphire for the semiconductor processing industry, www.saphikon.com, accessed on 21th May 2003. R. Venkatesh, K.K. Chawla, Effect of interfacial roughness on fibre pull-out in alumina/SnO2 /glass composites, J. Mater. Sci. Lett. 11 (1992) 650–652. A.R. Boccaccini, P.A. Trusty, Toughening and strengthening of glass by Al2 O3 platelets, J. Mater. Sci. Lett. 15 (1996) 60–63. H.S. Kim, Ph.D. Thesis, Imperial College, University of London, 1989, p. 85. A.R. Boccaccini, E.A. Olevsky, Processing of platelet-reinforced glass matrix composites: effect of inclusions on sintering anisotropy, J. Mater. Process. Technol. 96 (1999) 92–101. G.W. Scherer, Sintering with rigid inclusions, J. Am. Ceram. Soc. 70 (1987) 719–725. M.N. Rahaman, L.C. De Jonghe, Effect of rigid inclusions on the sintering of glass powder compacts, J. Am. Ceram. Soc. 70 (1987) C348–C351. A.R. Boccaccini, D.H. Pearce, P.A. Trusty, Pressureless sintering and characterization of Al2 O3 -platelet reinforced barium– magnesium–aluminosilicate glass–ceramic composites, Composites Part A 28A (1997) 505–510. M.M. Lima, R. Monteiro, Characterisation and thermal behaviour of a borosilicate glass, Thermochim. Acta 373 (2001) 69–74. A.F. Dericioglu, Y. Kagawa, Fail-safe light transmitting SiC fiberreinforced spinel matrix optomechanical composite, J. Mater. Sci. 37 (2002) 523–530.
Processing and characterisation of model optomechanical composites in the system sapphire fibre/borosilicate glass matrix A.R. Boccaccini a,∗ , D. Acevedo a,1 , A.F. Dericioglu b , C. Jana c a
Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba-shi, Ibaraki 305-0047, Japan c Schott-Jenaer Glas GmbH, Otto Schott Straße 13, D-07745 Jena, Germany Received 8 July 2004; accepted 11 March 2005
Abstract Optomechanical composites based on the system sapphire fibre/borosilicate glass matrix were fabricated and characterised. Different techniques of fabrication were used: composites with randomly orientated chopped sapphire fibres were produced by powder technology and pressureless sintering, whilst unidirectionally oriented fibre composites were fabricated by hot-pressing as well as by sandwiching glass slides and arrays of parallel fibres followed by heat-treatment. Pressureless sintered samples were poorly densified and were opaque. Hot-pressed and “sandwich structure” composites were dense and exhibited strong interfaces between fibres and matrix. Only the “sandwich structure” composites were transparent and showed significant light transmittance in the visible wavelength range, only 20% lower than that of the unreinforced matrix. Due to the strong matrix/fibre interface limited fibre pull-out during composite fracture was observed. The fabricated transparent composites represent an improved version of the traditional material wired glass. They are candidate materials for applications in high performance fire and impact resistant windows requiring high impact strength and avoidance of fragmentation upon fracture. © 2005 Elsevier B.V. All rights reserved. Keywords: Composite materials; Glass matrix; Hot-pressing; Sintering; Ceramic fibres; Mechanical properties; Optical properties
1. Introduction Silicate glasses have extremely low fracture toughness values, which leads to the well known low reliability of these materials in load-bearing applications. Hence, strengthening and toughening of silicate glasses are required if these materials are to find wider use in structural applications [1]. Forming composites by incorporation of reinforcing elements in glass and glass-ceramic matrices is a very effective approach to improve the mechanical properties of glasses, including fracture strength, fracture toughness as well as thermal shock and impact resistance [2–5]. Reinforcements most commonly used are in the form of ceramic whiskers, platelets, particulates or fibres [2–5]. Numerous studies have ∗ 1
Corresponding author. Tel.: +44 207 594 6731; fax: +44 207 584 3194. E-mail address: [email protected] (A.R. Boccaccini). Present address: GEMPPM, INSA de Lyon, 69621 Villeurbanne, France.
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.03.011
been carried out to understand the toughening mechanisms induced by the presence of the reinforcements and to investigate the parameters that lead to satisfactory mechanical behaviour of glass and glass-ceramic matrix composites [5–11]. Limited previous research has been carried out focusing on improving simultaneously mechanical and functional properties of glass matrix composites. These include composites for electric and electronic applications (e.g. induction heating equipment, microwave components, electronic package substrates, connectors), high-temperature applications (e.g. thermal insulators, jet engine thermocouples), dimensional stability applications (e.g. mirror supports for telescopes) and optical applications (e.g. impact resistant windows, structures for micro fluidics) [4,12–20]. In particular, composites exhibiting favourable optical and mechanical properties are called «optomechanical composites» [18], which are the focus of the present work.
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
The main difficulty in the development of optomechanical composites is the requirement of being able to improve the mechanical and optical properties simultaneously [18–24]. Indeed, most glass matrix composite materials developed to date are not optically transparent, or even translucent, because of the type of reinforcements used (e.g. SiC or carbon fibres) [2–5], and therefore, they cannot be considered to be suitable materials for optomechanical applications. Hence, means of improving the mechanical properties of glasses without significantly degrading their optical transparency need to be further investigated. The selection of appropriate fibres and matrices for optomechanical composites is a complex matter because numerous factors have to be considered. The main requirement for the fibres is that they should have a higher thermal stability than the glass matrix because of the usually high temperatures needed for matrix densification [18,22]. Furthermore, matching the fibre and matrix thermal expansion coefficients is necessary in order to avoid large residual stresses upon cooling from the fabrication temperature. However, the development of compressive residual stresses in the matrix by having fibres with thermal expansion coefficient higher than that of the matrix may be also favourable [18,22]. Finally, the strength of bonding at the interface between fibres and matrix is an important parameter since it has a large influence on the mechanical behaviour of the final composites, as it is the case in all brittle matrix composites [2–4]. In addition to these requirements, matching of the fibre and matrix refractive indices is also necessary to avoid (or minimise) light scattering, and thus to obtain a transparent or at least a translucent material [15–24]. Another possible option to obtain transparent composites is to include optical windows by a relatively large spacing of the reinforcing fibres in the matrix [18], in a similar way as in the traditional material wired glass. Recently, we have fabricated oxide-fibre reinforced glass matrix composites with high light transmittance (only 30% lower than that of the matrix) using the “optical window” concept, which is based on the presence of relatively large transparent matrix regions surrounded by the reinforcing fibres [22]. Moreover, Dericioglu et al. [18,24] fabricated minicomposite reinforced borosilicate glass matrix optomechanical composites with low volume fraction of reinforcement exhibiting light transmittance higher than 80% of the transmittance of the matrix, showing that even if the fibres incorporated are opaque, it is possible to achieve considerable optical transparency. A few experimental investigations aiming at producing optomechanical composites with technical applications have been carried out, specially in Japan [17–20,24], in Germany [15,16,23] and in the UK [22], yet research in this field remains still rather limited, which has thus motivated the present experimental study. The objective of this work is to explore and optimise different methods to produce optically transparent borosilicate glass matrix composites reinforced by single crystal Al2 O3 (sapphire) fibres. Borosilicate glass was chosen because of
271
its high thermal capability as well as considerable corrosion and thermal shock resistance [25]. In fact, borosilicate glass has been widely used in the past as the matrix for SiC and carbon fibre reinforced composites for structural applications [4,26]. Moreover, borosilicate glass matrices reinforced by ␣-Al2 O3 in the form of particles, platelets and fibres have been the matter of numerous previous investigations due to the favourable thermal expansion mismatch between alumina and borosilicate glass composition [4,6,27]. Additional advantages of borosilicate glass are its optical properties and relatively low dielectric constant [25,28]. Sapphire fibres were selected because they exhibit outstanding high temperature stability, high chemical durability and excellent mechanical properties [29,30]. Single crystal sapphire fibres have been used in previous studies to reinforce ceramic and glass matrices for high-temperature applications [30–33]. In those studies however no special care was placed on the optical property (transparency) of composites, except for some model systems fabricated for academic purposes [21]. Thus, to the authors’ knowledge, this is the first work on the system sapphire fibre/borosilicate glass matrix with the specific aim of producing transparent composites for optomechanical applications.
2. Materials and experimental procedure 2.1. Materials Borosilicate glass was selected as the matrix material and it was used in two different forms: i) powder of mean particle size <40 m (Duran® , Schott Glas, Mainz, Germany) and ii) glass plates of thickness 1.1 mm (Borofloat® 33, Schott Jenaer Glas, Jena, Germany). The properties of the glass are summarized in Table 1 [34]. The chemical composition of Duran® glass is (in wt%) [34]: 81SiO2 , 13B2 O3 , 4(Na2 O + K2 O), 2Al2 O3 , which can be considered to be identical to that of Borofloat® 33. The reinforcement chosen was sapphire fibre of optical quality with nominal diameter 150 m (Saphikon® , Laser Components UK, Ltd.). The fibres were received in length of 1 m and were cut manually to appropriate lengths for composites fabrication by using metallic scissors. For all composites, fibres were used in the as-received condition. Sapphire fibres were selected because they exhibit outstanding thermomechanical properties [29–33]. This fibre is a monocrystal of ␣-Al2 O3 of very high quality exhibiting high strength and hardness. Additionally, because absence of grain boundaries, Table 1 Properties of the borosilicate glass DURAN® [25,34] Density (g cm−3 ) Tensile strength (MPa) Elastic modulus (GPa) Coefficient of thermal expansion (◦ C−1 ) Refractive index
2.23 60 64 3.3 × 10−6 1.473
272
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
Table 2 Properties of the Saphikon® fibres [35] Density (g cm−3 ) Diameter (m) Tensile strength (GPa) Elastic modulus (GPa) Melting point (◦ C) Coefficient of thermal expansion (K−1 ) Uniaxial refractive index
3.99 150 2.1–3.4 386–435 2053 7.9–8.8 × 10−6 1.760–1.768
there is no light scattering within the sapphire fibres. Table 2 summarizes the main properties of the Saphikon® fibres used [35], and Fig. 1 shows a scanning electron microscopy (SEM) image of the fibre. This micrograph confirms that the fibres have a circular cross-section. Moreover, it shows that the fibres have a very smooth surface, which will influence the possible toughening mechanisms acting in the composites: a smooth interface should lead to greater average pull-out lengths, and thus to higher fracture toughness provided there is optimal bonding strength between the fibre and the matrix [36]. 2.2. Preparation of the composites 2.2.1. Pressureless sintering A mixture of borosilicate glass powder and 5% in volume of chopped sapphire fibres with a length of ∼1 mm was prepared. Fibres were cut using scissors to the required length, and matrix powder and fibres were mixed in dry conditions in a rotary mixer for 1 h. The mixture was pressed into cylindrical samples of 8 mm diameter in a die at room temperature by application of a compacting pressure of about 100 MPa for 2 min. No binder was used in this operation. The pellets were then sintered in an electric furnace at 750 ◦ C for 2 h in normal atmosphere. The heating rate used was 5 ◦ C min−1 . Samples were left to cool down in the furnace. The sintering temperature was chosen on the basis of previous investigations on alumina
Fig. 1. Scanning electron microscopy (SEM) image of a Saphikon® fibre used in the present work.
Fig. 2. Arrangement of Saphikon® fibres in (a) hot-pressed and (b) “sandwich structure” composites, in a volume fraction of approximately 5%.
platelet reinforced glass matrix composites where the same borosilicate glass matrix was used [37]. 2.2.2. Hot-pressing A custom-made vacuum hot-press described in previous works [38] was used. Samples made of pure glass matrix and fibre reinforced composites were fabricated. The average fibre length was about 10 mm. The fibres were arranged parallel to each other on a slightly pressed thin layer of powder in a carbon die. They were separated an average distance of ∼1 mm, as shown in Fig. 2(a). Subsequently, a second layer of powder was added above the fibres and the composites were hot-pressed. The volume fraction of fibres in the rectangular samples, which were cut out of the hot-pressed disc, as shown in Fig. 2(a), was approximately 5%. The parameters used for the fabrication of the samples were: heating rate 100 ◦ C h−1 , holding temperature 750 ◦ C, holding time 1 h, applied pressure 10 MPa and cooling rate 100 ◦ C min−1 . 2.2.3. “Sandwich structure” method This is a simple pressureless method for composite fabrication introduced recently [22]. The method consists of sandwiching the reinforcing fibres between two glass plates, as shown in Fig. 2(b), and then subjecting the “sandwich structure” to a heat treatment to consolidate the composite by exploiting viscous flow of the glass. The as-received plates of borosilicate glass (Borofloat® 33) were cut by means of a diamond tip to the desired dimensions (about 2.5 cm × 1.5 cm). The average length of the fibres was 8 mm. The same disposition as in the hot-pressed samples was used: fibres were arranged parallel to each other, and the average distance between two fibres was about 1 mm (see Fig. 2(b)). The heat-treatment consisted of two main steps. In the first step, the glass plates were heated under a high vacuum to clean the surface by degassing. Subsequently, the sandwich structure was subjected to a second heat-treatment. The heating rate was 100 ◦ C h−1 , the holding temperature was varied between 750 and 775 ◦ C, the holding time was between 2.5
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
and 5 h and the cooling rate was varied between 100 and 260 ◦ C h−1 . The degassing step included two different treatments. Firstly, the sample was under a normal atmosphere and after completion of half of the holding time, a high vacuum was produced in order to clean all the impurities that might have burnt. All samples were produced using the same heating and cooling rates (100 ◦ C h−1 ) in both steps, the holding temperature and holding time for the first step were 500 ◦ C and 4 h, respectively, and they were 750 ◦ C and 5 h for the second step, respectively. 2.3. Characterization techniques The density of sintered and hot-pressed samples was determined by the Archimedes’ method. The relative density was calculated by considering the theoretical density of the composites based on the density data for matrix and fibres, given in Tables 1 and 2, respectively, and the volume fraction of fibres. For microstructural characterisation, samples were cut, then mounted in resin and polished to 1 m finish with diamond paste to obtain flat cross sections for SEM. The microstructures of selected sintered, hot-pressed and sandwich structure samples were examined using conventional SEM (JEOL LV 5610) working in both secondary electron and backscattered electron modes. Sintered samples were analysed using X-ray diffraction (XRD) to detect any crystallisation of the matrix. Samples were crushed to a fine powder, and then analysed with a Philips PW1700 series automated powder diffractometer using Cu K␣ radiation at 40 kV–40 mA with a secondary crystal monochromator. To gain preliminary understanding of the fracture behaviour of the composites and qualitative information about the interaction (bonding) between fibres and matrix during the fracture process, fractures surfaces were also observed by SEM. A preliminary assessment of the optical quality of the samples was obtained by placing selected samples at different heights over a written text on a white surface and assessing the text legibility. This qualitatively demonstrated the light transmitting characteristics of the different composites. A digital camera was used to document this behaviour. Fabricated “sandwich structure” composites were also observed by means of a conventional optical microscope. Light transmittance of the hot-pressed and “sandwich structure” samples (with and without fibres) was measured at room temperature by a UV–visible spectrophotometer (UV1601 Shimadzu, Japan), in the direction perpendicular to the fibres axis. Before light transmittance measurements, samples were cut and polished to obtain the size required for the measurements. The thickness of the samples was 2.3 ± 0.1 mm, and the light transmittance was measured for wavelengths between 350 and 800 nm. The light transmittance of the samples was reported as a percentage of the transmittance of a reference monolithic borosilicate glass slide (Borofloat® 33).
273
3. Results and discussion 3.1. Microstructural characterisation 3.1.1. Sintered composites Successful fabrication of fibre reinforced glass matrix composites relies on knowing the relationship between temperature and viscosity of the glass matrix. As it is well-known for Duran® -type borosilicate glass [34,37], the range of suitable temperatures for sintering glass powder is very narrow (between 720 and 780 ◦ C). A difference of ±20 ◦ C in the sintering temperature can have a large effect on the densification of the composites. Pressureless sintered samples fabricated in this investigation were not translucent and their densification was not completely achieved; the sintered relative densities were in the range 80–83% of theoretical density. Fig. 3(a–c) show SEM micrographs of polished cross-sections of a chopped fibre reinforced sintered composite at different magnifications. Fig. 3(a) shows that the matrix close to the fibres is porous and that larger defects are situated around the fibre, confirming that it has not been possible to obtain a good densification of the matrix in this region by pressureless sintering. The presence of randomly orientated fibres impedes the perfect flow of the viscous glass during sintering and causes a high porosity of the matrix in the region close to the fibres. On the contrary, the matrix far away from the fibres (Fig. 3(b)) exhibits high densification with few isolated pores. Fig. 3(c) is a high magnification image of the fibre/matrix interface showing that the lack of densification impedes the complete bonding of fibre and matrix leading to an imperfect interface. The poor densification of the pressureless sintered samples achieved in the present work is in broad agreement with previous studies that showed that the presence of rigid inclusions makes the densification of a glass matrix composite a difficult task and that the density of the composites decreases with increasing volume fraction of rigid inclusions [39–41]. Different explanations for this phenomenon have been proposed. An inhomogeneous distribution of inclusions in the powder can lead to a poor matrix particle packing and formation of agglomerates, and thus the porosity will increase around the fibres [39]. Moreover, studies conducted using the same materials but different mixing conditions have shown that optimised wet-mixing techniques lead to higher densities as they allow for more homogeneous mixtures [42]. Since the present glass powder/chopped fibres mixture was prepared in dry conditions, the last explanation may be applicable to our results. Another factor affecting the densification of composites containing rigid inclusions is the development of residual stresses as a consequence of different sintering rates of matrix and inclusions. These stresses may cause sintering damage, leading to crack-like voids or isolated pores and consequently to poor mechanical properties of the sintered samples [39–42].
274
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
Fig. 3. SEM micrographs of Sapphire® fibre reinforced pressureless sintered composites at different magnifications: (a) area around a fibre showing porosity and sintering defects, (b) matrix far away from the fibres exhibiting high densification and (c) high magnification image of the fibre/matrix interface showing imperfect bonding.
The existence of different “kinds” of porosity in the green compact has been also proposed to explain the retardation of densification in composites containing rigid inclusions [39]. Pores can be embedded in the matrix material only, or they can be located at the interface between matrix and inclusions. This is confirmed in the present composites by the micrographs analysed above (Fig. 3(a and c)). It is observed that pores are situated both in the matrix and at the interface between matrix and fibres. These different pore types in the initial compact will have different free surface energies, and thus, they will lead to an overall lower driving force for sintering in the composite than in the inclusion-free compact [39] and thus to a less densified composite. However, Fig. 3(b) shows that far away from the fibres, the matrix was very well densified, confirming that the parameters used for sintering (time and temperature) were appropriate for this glass. Fig. 4 shows the XRD pattern for a Saphikon® fibre reinforced sintered composite. The only crystalline phase detected was corundum, which corresponds to the crystalline structure of the Saphikon® fibres used. No cristobalite has crystallised in the matrix, which is a favourable result from the point of view of the composite mechanical strength. A simi-
lar result was obtained for hot-pressed samples. Indeed it has been proved in previous studies [27,28,43] that cristobalite crystallisation could happen in Al2 O3 /borosilicate glass composites with low volume fractions of Al2 O3 (<10%) and even when sintering temperature is low (in the range 700–800 ◦ C).
Fig. 4. XRD pattern of a Saphikon® fibre reinforced sintered composite showing corundum as the only cristalline phase, which corresponds to the structure of the Saphikon® fibres.
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
275
Fig. 5. SEM image of the polished section of a hot-pressed sample containing Saphikon® fibres unidirectionally aligned, showing high densification but the presence crack-like defects.
This should have a negative effect on the mechanical properties of the composites because the different thermal expansion coefficients of cristobalite and borosilicate glass matrix may lead to microcracking, therefore decreasing the tensile strength of the composites. The lack of cristobalite formation in the present composites confirms the high resistance to crystallisation of the Duran® borosilicate composition and supports the choice of this particular glass as the matrix for composites. 3.1.2. Hot-pressed composites Hot-pressed samples without fibres were translucent, while composites containing fibres were opaque, even though the densities achieved were very high in both cases, 99.5 and 98% of theoretical density, respectively. Cristobalite crystallisation was not detected in hot-pressed composites. Fig. 5 shows a polished section of a hot-pressed sample containing 5 vol% unidirectionally aligned Saphikon® fibres. Although the matrix seems to be well densified without the presence of large cavities or pores (compare with Fig. 3(a), pressureless sintered sample), there are still some defects and crack-like voids remaining, mainly close to the fibres, which should be responsible for high light scattering and the loss of transparency of these samples. Thus, despite the relative high density achieved, the parameters used for hot-pressing were not optimal for producing optically transparent or translucent composites. 3.1.3. “Sandwich structure” composites SEM micrographs of polished surfaces of “sandwich structure” composites are shown in Fig. 6(a and b). In some regions the interface between fibres and matrix could only be detected by using SEM in backscattered electrons mode (Fig. 6(b)), indicating an intimate perfect bond. Some defects (voids) were observed in some samples, localised just close to the fibres (Fig. 6(a)). These were probably the result of a too short processing time. When the parameters used were
Fig. 6. SEM micrographs of polished surfaces of “sandwich structure” composites showing (a) some defects (voids), marked by arrows, localised close to the fibres after too short processing time and (b) perfect bonded sample fabricated employing optimised parameters.
optimised (775 ◦ C, 3.5 h), the two plates of glass were seen to be well bonded together and the interface between them was “invisible” under SEM (Fig. 6(b)). “Sandwich structure” samples with and without fibres were transparent. There was some contamination on the surfaces after heat-treatment, but this disappeared after polishing. The two slides of glass were in all cases very well bonded. Fig. 7 shows the matrix region between two fibres (“optical window”) in a sandwich structure composite observed with an optical microscope; fibres are also visible. Heat-treatment at 775 ◦ C for 3.5 h was found to be the best thermal process for the fabrication of “sandwich structure” composites. 3.2. Fracture behaviour Previous research has proved that ␣-Al2 O3 /borosilicate glass systems containing ␣-Al2 O3 platelets as reinforcement lead to composites with higher fracture strength, Young’s modulus, hardness and fracture toughness than the matrix without reinforcement [6,37]. In the present investigation, fracture behaviour of the composites was anal-
276
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
Fig. 7. Optical microscopy micrograph showing the matrix region of about 1mm width between two sapphire fibres (“optical window”) in a (nonpolished) transparent sandwich structure composite.
ysed qualitatively by examining fracture surfaces of different samples. 3.2.1. Sintered composites Fig. 8(a and b) are SEM images of fracture surfaces of sintered samples. Fig. 8(a) shows that there is pull-out of the fibres during the fracture of the composite. Since there are many defects at the interface between fibres and matrix (see also Fig. 3(c)), the bonding at the interface is weak and it is easy to have fibre/matrix debonding and fibre pull out. These mechanisms (debonding and pull-out) dissipate energy, and thus they will contribute to increase the toughness of the composite. It is also seen that there are cracks in the matrix in regions around the fibres and at the interface, which are most probably a consequence of pores and crack-like flaws produced during sintering. However, Fig. 8(b) indicates that in regions where the glass matrix is in close contact with the fibres, bonding between matrix and fibres can be free of defects. In this case a strong fibre/matrix bonding may be assumed. Furthermore, the presence of layers of glass on the pulled-out fibres (arrows in Fig. 8(a)) confirms that the bonding between borosilicate glass and sapphire fibres was very strong after sintering at 750 ◦ C if there was close contact between them. Thus, it may be anticipated that fracture toughness of the samples will be only partially improved by debonding and pull-out mechanisms. Moreover, the samples will show low fracture strength because of the relatively high porosity. 3.2.2. Hot-pressed composites The fracture behaviour of the hot-pressed composites was very similar, in qualitative terms, to that of pressureless sintered samples, as inferred from fracture surface analyses.
Fig. 8. SEM images of fracture surfaces of pressureless sintered samples showing (a) evidence of pull-out of the Saphikon® fibres during fracture of the composite and (b) defect-free matrix-fibre interface in an area of good matrix-fibre bonding. The arrows in (a) indicate rest of glass from the matrix adhered to the surface of the fibre.
Typical fracture surfaces showing evidence of fibre debonding and pull-out are presented in Fig. 9(a and b). Due to the lower porosity of these composites compared to pressureless sintered materials, they are expected to exhibit higher fracture strength. Moreover, due to the unidirectional alignment of the Saphikon® fibres in these composites, the materials will exhibit anisotropic mechanical behaviour with highest strength and stiffness in the fibre direction, exploiting a loadtransfer mechanism [7]. 3.2.3. “Sandwich structure” composites The “sandwich method” led to defect-free composites without porosity. These composites therefore should have higher fracture strength than the sintered samples. SEM micrographs of fracture surfaces of “sandwich structure” composites, shown in Fig. 10(a–c), confirm that when the matrix is in close contact with the fibres a strong bonding may develop between them. Fig. 10(a) shows an impression left in the matrix by the fibre when the sample was fractured. The matrix was broken and some glass matrix stayed adhered to
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
277
strength and elastic modulus than the matrix. Moreover, the higher thermal expansion coefficient of the fibre than that of the matrix implies that, upon cooling from the fabrication temperature, the matrix will be in tangential compression [7]. This should increase the tensile fracture strength of the composite. It has been also shown that compressive residual thermal stresses in the matrix caused by thermal expansion coefficient mismatch have important effects on the toughening mechanisms acting in brittle matrix composites [5,8], and that a compressive residual stress field in the matrix should increase the matrix microcracking stress [7]. 3.3. Optical properties
Fig. 9. Typical fracture surfaces of a hot-pressed sample showing evidence of Saphikon® fibre (a) debonding and (b) pull-out.
the fibre surface after fracture. This result confirms that there exists a strong bonding between ␣-Al2 O3 and borosilicate glass. As proved in previous investigations [31,36,37], chemical compatibility between Al2 O3 inclusions and borosilicate glass produces a very strong chemical bonding and leads to strong, but brittle composites. This was confirmed by the very short “pull-out” length in these composites, as shown in Fig. 10(b), and the relatively sharp interface between sapphire fibre and borosilicate glass matrix, as shown in Fig. 10(c). The results indicate the need to coat the fibres with a suitable material to produce the weak interface required for fully exploiting toughening mechanisms such as fibre pull-out. A typical material suggested for composites with silicate matrices and alumina-containing fibres is SnO2 [31,36]. Chawla et al. have studied the role of SnO2 coating, which has no diffusion in alumina and very little diffusion in silicate glass [31]. Other works have been conducted using TiO2 coatings on aluminosilicate fibres, which were embedded in silicate glass matrices [22]. In principle, sapphire fibres have suitable properties to reinforce borosilicate glass matrices, i.e. much higher tensile
The matching of refractive index of fibre and matrix in the present composites is not perfect; meaning that only low volume fraction of fibres may be used to achieve a transparent material, exploiting the optical window concept [18,24] similar to wired glass. Due to the lack of densification of the pressureless sintered composites, only hot-pressed and sandwich structure composites are discussed in this section. The macroscopic appearance of polished “sandwich structure” composites is shown in Fig. 11(a and b), which qualitatively demonstrates the transparency of the samples. The images show that it is possible to see through the composites, and thus light scattering effects are minimised. The underlaying text remains clearly legible even if the composite is not directly above the text (Fig. 11(b)). This behaviour was expected because of the disposition of the fibres in the matrix leaving optical windows (Fig. 7), as in the conventional material wired glass. The matrix regions between the fibres should have the same light transmittance as the monolithic borosilicate glass matrix. Measurements of the light transmittance of the composites in the UV and visible wavelength ranges were carried out to quantify the transparency of the samples. The results are shown in Fig. 12. The figure shows the light transmittance of three samples: a hot- pressed sample without fibres, a “sandwich structure” sample without fibres (reference matrix) and a “sandwich structure” sample with fibres, before and after polishing. It is seen that the hot-pressed sample has lost almost all of its optical transparency; it has a transmittance of 18% relative to the borosilicate glass plate. It is thus difficult to envisage that it will be feasible to obtain a transparent fibre reinforced composite by this technique of fabrication, even if high densities (as high as 98% of theoretical density) could be achieved, as discussed in Section 3.1.2. The “sandwich structure” sample without fibres has a 100% transmittance in the wavelength range 350–800 nm. This means that there was no loss of transparency by joining two plates of glass together in a sandwich structure. This also confirms that the processing parameters used for fabrication were optimised for this system. The transmittance of a “sandwich structure” composite before and after polishing (to 3 m using a diamond grid) was also measured. As expected, Fig. 12 shows that the pol-
278
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
Fig. 10. SEM micrographs of fracture surfaces of “sandwich structure” composites showing (a) impression left in the matrix by a fibre debonded during fracture, (b) very short “pull-out” length in these composites and (c) the relatively sharp interface between sapphire fibre and borosilicate glass, indicating strong fibre/matrix bonding.
ished sample has better transmittance properties than the nonpolished sample; polishing the sample increases its optical transmittance by 20%. The “sandwich structure” composites exhibit an almost constant transmittance of about 60% before polishing and of about 80% after polishing. These results are in agreement with the images in Fig. 11, showing the high transparency of the composites, and they confirm that the “optical window” concept is a convenient way to fabricate optomechanical composites, as proposed in the literature [18,24]. The present results are better in terms of transparency than those obtained in our previous work [22], where only 60% transparency in Nextel® fibre reinforced soda-lime glass composites fabricated by the “sandwich structure” method was achieved. The results are similar to those obtained recently by Dericioglu and Kagawa [44], who achieved 80% relative transparency in SiC fibre reinforced MgAl2 O4 matrix optomechanical composites, in which the fibre diameter and fibre to fibre spacing were identical to those of the present “sandwich structure” composites. The light transmittance of the polished “sandwich structure” composite, which is given relative to that of the monolithic borosilicate glass matrix in
Fig. 12, can be analysed quantitatively by the following relation, which was developed for optomechanical composites with regularly aligned fibres in a transparent matrix [24]: Df Tc = Tm 1 − Lf
(1)
where Tc and Tm are the light transmittance of the composite and the monolithic matrix, respectively, and Df and Lf are the fibre diameter and fibre to fibre spacing, respectively. Incorporating the diameter of the sapphire fibre (∼150 m) and the fibre to fibre spacing (∼1 mm) in Eq. (1), the light transmittance of the polished “sandwich structure” composite relative to that of the monolithic matrix (Tc /Tm ) is found to be ∼85%. This value is in good agreement with the experimentally determined relative light transmittance value given in Fig. 12 for the polished composite that reads ∼80%. This result demonstrates the effectiveness of the optical window concept in the present sapphire fibre/borosilicate glass composites and its efficiency in providing optical transparency to the resulting optomechanical composite.
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
279
4. Conclusions
Fig. 11. Macroscopic appearance of polished “sandwich structure” composites qualitatively demonstrating the transparency of the samples. The images show that it is possible to see through the composites. The under-laying text remains clearly legible if the sample is placed (a) in direct contact with the text and (b) even if the composite is not directly above the text.
The system sapphire fibre reinforced borosilicate glass matrix composite was studied aiming at developing “optomechanical composites”. Different techniques of fabrication were used: randomly orientated chopped fibre reinforced composites were fabricated by pressureless sintering, unidirectionally oriented fibre reinforced composites were fabricated by hot-pressing and by sandwiching two slides of glass and an array of parallel fibres (sandwich structure composites). Pressureless sintered samples were porous, specially near the fibre/matrix interfaces and there was poor contact between the fibres and the matrix. Hot-pressed and “sandwich structure” composites were dense and showed a strong interface between fibres and matrix. Interface engineering should be introduced by coating the fibres with a suitable material (e.g. SnO2 ) in order to obtain a weaker interface and improve toughness by inducing significant fibre pull-out effect. Pressureless sintered and hot-pressed samples were opaque due to residual porosity and sintering defects. However, the hot-pressed unreinforced matrix was translucent. On the other hand, “sandwich structure” composites were transparent and showed significant light transmittance in the visible wavelength range, only 20% lower than that of the unreinforced matrix (borosilicate glass slides). These results indicate that this technique of fabrication is viable for production of “optomechanical composites” with borosilicate glass matrix. The samples fabricated, which exhibit strong fibre/matrix interface bonding, represent an improved (but less cost-effective) version of the traditional fire and impact resistant material wired glass. The composites should therefore be interesting materials for high performance fire resistant windows, requiring high impact strength and avoidance of fragmentation upon fracture, obviously in cases where stringent requirements may justify the higher costs of the material.
Acknowledgments Experimental assistance of Mr. Norbert Galy is appreciated. ARB acknowledges financial support of the Royal Society, London, UK (Grant nr. 574006.G503).
References
Fig. 12. Results of the measurements of light transmittance of selected samples in the UV and visible wavelength ranges: hot-pressed sample without fibres (a), “sandwich structure” sample without fibres (b) and “sandwich structure” sample with fibres, before and after polishing (c and d, respectively). For the composites, light transmittance was measured perpendicularly to the fibre axes.
[1] I.W. Donald, Review. Methods for improving the mechanical properties of oxide glasses, J. Mater. Sci. 24 (1989) 4177–4208. [2] K.M. Prewo, J.J. Brennan, G.K. Layden, Fiber reinforced glasses and glass-ceramics for high performance applications, Ceram. Bull. 65 (1986) 305–322. [3] R.D. Rawlings, Glass-ceramic matrix composites, Composites 25 (1994) 372–379. [4] A.R. Boccaccini, Glass and glass-ceramic matrix composite materials: a review, J. Ceram. Soc.Jpn. 109 (7) (2001) S99–0S109.
280
A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280
[5] I.W. Donald, Preparation and properties applications of glass and glass-ceramic matrix composites, Key Eng. Mater. 108–110 (1995) 123–144. [6] V. Cannillo, C. Leonelli, A.R. Boccaccini, Numerical models for thermal residual stresses in Al2 O3 platelets/borosilicate glass matrix composites, Mater. Sci. Eng. A323 (2002) 246–250. [7] K.K. Chawla, Ceramic Matrix Composites, Chapman and Hall, first ed., 1993. [8] A.G. Evans, F.W. Zok, Review. The physics and mechanics of fibre-reinforced brittle matrix composites, J. Mater. Sci. 29 (1994) 3857–3896. [9] M.D. Thouless, O. Sbaizero, L.S. Sigl, A.G. Evans, Effect of interface mechanical properties on pull-out in a SiC fibre-reinforced lithium aluminum silicate glass-ceramic, J. Am. Ceram. Soc. 72 (1989) 525–532. [10] H. Hegeler, R. Br¨uckner, Fibre-reinforced glasses: influence of thermal expansion of the glass matrix on strength and fracture toughness of the composites, J. Mater. Sci. 25 (1990) 4836–4846. [11] M.J. Blisset, P.A. Smith, J.A. Yeomans, Flexural mechanical properties of thermally treated unidirectional and cross-ply nicalonreinforced calcium aluminosilicate composites, J. Mater. Sci. 33 (1998) 4181–4190. [12] D.D.L. Chung, Composite Materials: Science and Applications, Springer, London/Berlin/Heidelberg, 2003. [13] P.N. Kumta, Processing aspects of glass-nicalon fibre and interconnected porous aluminium nitride ceramic and glass composites, J. Mater. Sci. 31 (1996) 6229–6240. [14] J.A. Rice, C.S. Hazelton, M.J. Haun, Optimisation of the electrical and mechanical properties of a low dielectric loss, continuous fibre ceramic composite, Ceram. Trans. 74 (1996) 497–508. [15] T. Leutbecher, D. Hulsenberg, Oxide fibre reinforced glass: a challenge to new composites, Adv. Eng. Mater. 2 (2000) 93– 99. [16] B. Fankhanel, E. Muller, K. Weise, G. Marx, Translucent fibre reinforced glass, Key Eng. Mater. 206–213 (2002) 1109–1112. [17] Y. Kagawa, T. Yamada, Tensile properties of optical fibre-glass matrix sensor-based composite, J. Mater. Sci. Lett. 13 (1994) 1403–1405. [18] A.F. Dericioglu, S. Zhu, Y. Kagawa, Improvement of fracture resistance in a glass matrix optomechanical composite reinforced by Al2 O3 –ZrO2 minicomposite, Ceram. Eng. Sci. Proc. 23 (2002) 485–492. [19] H. Iba, T. Chang, Y. Kagawa, H. Minakuchi, K. Kanamaru, Fabrication of optically transparent short fibre-reinforced glass matrix composites, J. Am. Ceram. Soc. 79 (1996) 881–884. [20] P. Kangutkar, T. Chang, Y. Kagawa, M.J. Koczak, H. Minakuchi, K. Kanamaru, Fabrication of optically transparent SiCaON fiber reinforced glass matrix composites, Ceram. Eng. Sci. Proc. 14 (9–10) (1993) 963–970. [21] D. Banerjee, H. Rho, H.E. Jackson, R.N. Singh, Characterization of residual stresses in a sapphire-fibre-reinforced glass-matrix composite by micro-fluorescence spectroscopy, Compos. Sci. Technol. 61 (2001) 1639–1647. [22] A.R. Boccaccini, S. Atiq, G. Helsh, Optomechanical glass matrix composites, Compos. Sci. Technol. 63 (2003) 779–783. [23] D. Huelsenberg, P. Feehling, T. Mache, V. Winkler, D. Raab, A.R. Boccaccini, Herstellung und charakterisierung neuer faserverstaerkter glasmatrix-verbundwerkstoffe, in: B. Wielage, G. Leonhardt (Eds.),
[24]
[25] [26]
[27]
[28]
[29]
[30] [31]
[32]
[33]
[34] [35] [36]
[37] [38] [39]
[40] [41]
[42]
[43] [44]
Verbundwerkstoffe und Werkstoffverbunde, Wiley-VCH, Weinheim, 2001, pp. 365–369. A.F. Dericioglu, Y. Kagawa, Mechanical behaviour of Al2 O3 –ZrO2 minicomposite reinforced glass matrix optomechanical composite, Mater. Sci. Technol. 19 (2003) 1119–1124. R.H. Doremus, Glass Science, Second ed., John Wiley & Sons, New York, 1994. A.R. Boccaccini, A.J. Strutt, K.S. Vecchio, D. Mendoza, K.K. Chawla, C.B. Ponton, D.H. Pearce, Behavior of nicalon-fiber reinforced glass matrix composites under thermal cycling conditions, Composites Part A 29 (11) (1998) 1343–1352. R.C.C. Monteiro, M.M.R.A. Lima, Effect of compaction on the sintering of borosilicate glass/alumina composites, J. Eur. Ceram. Soc. 23 (2003) 1813–1818. M.F. Zawrah, E.M.A. Hamzawy, Effect of cristobalite formation on sinterability, microstructure and properties of glass/ceramic composites, Ceram. Int. 28 (2002) 123–130. V.N. Kurlov, V.M. Kiiko, A.A. Kolchin, S.T. Mileiko, Sapphire fibres grown by a modified internal crystallisation method, J. Cryst. Growth 204 (1999) 499–504. J. Wendorff, R. Janssen, N. Claussen, Model experiments on pure oxide composites, Mater. Sci. Eng. A250 (2) (1998) 186–193. K.K. Chawla, M.K. Ferber, Z.R. Xu, R. Venkatesh, Interface engineering in alumina/glass composites, Mater. Sci. Eng. A162 (1993) 35–44. R.W. Goettler, K.T. Faber, Interfacial shear stresses in SiC and Al2 O3 fibre reinforced glasses, Ceram. Eng. Sci. Proc. 9 (7–8) (1988) 861–870. D.H. Pearce, A.J. Jickells, C.B. Ponton, Fabrication of sapphire fibre reinforced ceramic matrix composite, Br. Ceram. Trans. 95 (4) (1996) 141–145. Schott® Technical Glasses, Company information bulletin, 1982. Properties and benefits of sapphire for the semiconductor processing industry, www.saphikon.com, accessed on 21th May 2003. R. Venkatesh, K.K. Chawla, Effect of interfacial roughness on fibre pull-out in alumina/SnO2 /glass composites, J. Mater. Sci. Lett. 11 (1992) 650–652. A.R. Boccaccini, P.A. Trusty, Toughening and strengthening of glass by Al2 O3 platelets, J. Mater. Sci. Lett. 15 (1996) 60–63. H.S. Kim, Ph.D. Thesis, Imperial College, University of London, 1989, p. 85. A.R. Boccaccini, E.A. Olevsky, Processing of platelet-reinforced glass matrix composites: effect of inclusions on sintering anisotropy, J. Mater. Process. Technol. 96 (1999) 92–101. G.W. Scherer, Sintering with rigid inclusions, J. Am. Ceram. Soc. 70 (1987) 719–725. M.N. Rahaman, L.C. De Jonghe, Effect of rigid inclusions on the sintering of glass powder compacts, J. Am. Ceram. Soc. 70 (1987) C348–C351. A.R. Boccaccini, D.H. Pearce, P.A. Trusty, Pressureless sintering and characterization of Al2 O3 -platelet reinforced barium– magnesium–aluminosilicate glass–ceramic composites, Composites Part A 28A (1997) 505–510. M.M. Lima, R. Monteiro, Characterisation and thermal behaviour of a borosilicate glass, Thermochim. Acta 373 (2001) 69–74. A.F. Dericioglu, Y. Kagawa, Fail-safe light transmitting SiC fiberreinforced spinel matrix optomechanical composite, J. Mater. Sci. 37 (2002) 523–530.