A.R. Boccaccini et al. Joumal of Materials Processing Technology 169(2005)270-280 The main difficulty in the development of optomechanical its high thermal capability as well as considerable corrosion composites is the requirement of being able to improve the and thermal shock resistance [25]. In fact, borosilicate glass mechanical and optical properties simultaneously [18-24]. has been widely used in the past as the matrix for SiC and Indeed, most glass matrix composite materials developed to carbon fibre reinforced composites for structural application date are not optically transparent, or even translucent, because [4, 26]. Moreover, borosilicate glass matrices reinforced by of the type of reinforcements used(e.g. SiC or carbon fibres) a-Al2O3 in the form of particles, platelets and fibres have [2-5], and therefore, they cannot be considered to be suitable been the matter of numerous previous investigations due to materials for optomechanical applications. Hence, means of the favourable thermal expansion mismatch between alu- improving the mechanical properties of glasses without sig- mina and borosilicate glass composition [4, 6, 27]. Additional nificantly degrading their optical transparency need to be advantages of borosilicate glass are its optical propertie further investigated and relatively low dielectric constant [25, 28]. Sapphire The selection of appropriate fibres and matrices for fibres were selected because they exhibit outstanding high ptomechanical composites is a complex matter because temperature stability, high chemical durability and excellent numerous factors have to be considered. The main require- mechanical properties[29, 30]. Single crystal sapphire fibres ment for the fibres is that they should have a higher ther- have been used in previous studies to reinforce ceramic and mal stability than the glass matrix because of the usually glass matrices for high-temperature applications [30-331 high temperatures needed for matrix densification [18, 22]. In those studies however no special care was placed on Furthermore, matching the fibre and matrix thermal expan- the optical property(transparency)of composites, except sion coefficients is necessary in order to avoid large residual for some model systems fabricated for academic purposes stresses upon cooling from the fabrication temperature. How- [21]. Thus, to the authors'knowledge, this is the first work ever, the development of compressive residual stresses in the on the system sapphire fibre/borosilicate glass matrix with matrix by having fibres with thermal expansion coefficient the specific aim of producing transparent composites for higher than that of the matrix may be also favourable [ 18, 22]. optomechanical applications Finally, the strength of bonding at the interface between fibres and matrix is an important parameter since it has a large influ- ence on the mechanical behaviour of the final composites, as 2. Materials and experimental procedure it is the case in all brittle matrix composites [2-4]. In addi- tion to these requirements, matching of the fibre and matrix 2.1. Materials refractive indices is also necessary to avoid (or minimise) light scattering, and thus to obtain a transparent or at least Borosilicate glass was selected as the matrix material and 4]. Another possible option to it was used in two different forms: i) powder of mean particle obtain transparent composites is to include optical windows size <40 um(Duran", Schott Glas, Mainz, Germany)and ii) by a relatively large spacing of the reinforcing fibres in the glass plates of thickness 1. 1 mm(Borofloat33, Schott Jenaer matrix[18], in a similar way as in the traditional material Glas, Jena, Germany ) The properties of the glass are summa- wired glass. Recently, we have fabricated oxide-fibre rein- rized in Table 1[34]. The chemical composition of Duran forced glass matrix composites with high light transmittance glass is(in wt%)[34]: 81SiO2, 13B203, 4(Na20+K2O) (only 30% lower than that of the matrix)using the"optical 2Al2O3, which can be considered to be identical to that of windowconcept, which is based on the presence of relatively Borofloat33 large transparent matrix regions surrounded by the reinforc The reinforcement chosen was sapphire fibre of optical ing fibres [22]. Moreover, Dericiogluet al [18, 24] fabricated quality with nominal diameter 150 um(Saphikon",Laser minicomposite reinforced borosilicate glass matrix optome- Components UK, Ltd ) The fibres were received in length of chanical composites with low volume fraction of reinforce- I m and were cut manually to appropriate lengths for compos. ment exhibiting light transmittance higher than 80% of the ites fabrication by using metallic scissors. For all composites transmittance of the matrix, showing that even if the fibres fibres were used in the as-received condition. Sapphire fibres incorporated are opaque, it is possible to achieve considerable were selected because they exhibit outstanding thermome- optical transparency chanical properties [29-33]. This fibre is a monocrystal of A few experimental investigations aiming at producing a-Al2O3 of very high quality exhibiting high strength and optomechanical composites with technical applications have hardness. Additionally, because absence of grain boundaries 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 Properties of the borosilicate glass durAN[25, 34] present experimental study Density(gcm-3) The objective of this work is to explore and optimise dif- Tensile strength(MPa) 60 ferent methods to produce optically transparent borosilicate Elastic modulus(GPa) 64 glass matrix composites reinforced by single crystal Al2O Coefficient of thermal expansion(C) Refractive index 1.473 (sapphire)fibres. Borosilicate glass was chosen because ofA.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280 271 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 sig￾nificantly 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 require￾ment for the fibres is that they should have a higher ther￾mal stability than the glass matrix because of the usually high temperatures needed for matrix densification [18,22]. Furthermore, matching the fibre and matrix thermal expan￾sion coefficients is necessary in order to avoid large residual stresses upon cooling from the fabrication temperature. How￾ever, 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 influ￾ence on the mechanical behaviour of the final composites, as it is the case in all brittle matrix composites [2–4]. In addi￾tion 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 rein￾forced 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 reinforc￾ing fibres [22]. Moreover, Dericioglu et al. [18,24] fabricated minicomposite reinforced borosilicate glass matrix optome￾chanical composites with low volume fraction of reinforce￾ment 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 dif￾ferent methods to produce optically transparent borosilicate glass matrix composites reinforced by single crystal Al2O3 (sapphire) fibres. Borosilicate glass was chosen because of 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
-Al2O3 in the form of particles, platelets and fibres have been the matter of numerous previous investigations due to the favourable thermal expansion mismatch between alu￾mina 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 <40m (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 summa￾rized in Table 1 [34]. The chemical composition of Duran® glass is (in wt%) [34]: 81SiO2, 13B2O3, 4(Na2O+K2O), 2Al2O3, 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 compos￾ites fabrication by using metallic scissors. For all composites, fibres were used in the as-received condition. Sapphire fibres were selected because they exhibit outstanding thermome￾chanical properties [29–33]. This fibre is a monocrystal of
-Al2O3 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) 2.23 Tensile strength (MPa) 60 Elastic modulus (GPa) 64 Coefficient of thermal expansion (◦C−1) 3.3 × 10−6 Refractive index 1.473