272 A.R. Boccaccini er al. Joumal of Materials Processing Technology 169(2005)270-280 Table 2 erties of the Saphikon"fibres 35 Diameter(um) ~10mm Tensile strength(GPa) 2.1-3.4 Elastic modulus(GPa) 386-435 Melting point(°C) Coefficient of thermal expansion( K-) 79-8.8×10-6 Uniaxial refractive index 1.760-1.768 there is no light scattering within the sapphire fibres. Table 2 summarizes the main properties of the Saphikon"fibres used 351, and Fig. I shows a scanning electron microscopy (SEM) image of the fibre. This micrograph confirms that the fibres ive a circular cross-section. moreover. it shows that Fig. 2. Arrangement of Saphikon fibres in(a) hot-pressed and(b)"sand- fibres have a very smooth surface, which will influence the wich structure"composites, in a volume fraction of approximately 5% possible toughening mechanisms acting in the composites a smooth interface should lead to greater average pull-out platelet reinforced glass matrix composites where the same lengths, and thus to higher fracture toughness provided there borosilicate glass matrix was used [37] is optimal bonding strength between the fibre and the matrix [36] 2.2.2. Hot-pressing A custom-made vacuum hot-press described in previous 2.2. Preparation of the composites works [38 was used. Samples made of pure glass matrix and fibre reinforced composites were fabricated. The average 2.2 Pressureless sintering fibre length was about 10 mm. The fibres were arranged par A mixture of borosilicate glass powder and 5% in volume allel to each other on a slightly pressed thin layer of powder of chopped sapphire fibres with a length of - l mm was pre- in a carbon die. They were separated an average distance of pared. Fibres were cut using scissors to the required length, I mm, as shown in Fig. 2(a). Subsequently, a second layer and matrix powder and fibres were mixed in dry conditions of powder was added above the fibres and the composites in a rotary mixer for Ih were hot-pressed The mixture was pressed into cylindrical samples of 8mm The volume fraction of fibres in the rectangular samples diameter in a die at room temperature by application of a which were cut out of the hot-pressed disc, as shown in ompacting pressure of about 100 MPa for 2 min No binder Fig. 2(a), was approximately 5%. The parameters used for was used in this operation. The pellets were then sintered in the fabrication of the samples were heating rate 100.Ch-I an electric furnace at 750"C for 2h in normal atmosphere. holding temperature 750 C, holding time I h, applied pres- to cool down in the furnace. The sintering temperature was chosen on the basis of previous investigations on alumina 2.2.3. "Sandwich structure"method This is a simple pressureless method for composite fab- rication 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 ofa dia- mond tip to the desired dimensions(about 2.5 cm x 1. 5 cm) The average length of the fibres was 8 mm. The same dispo- sition as in the hot-pressed samples was used: fibres wer arranged parallel to each other, and the average distance between two fibres was about I mm(see Fig. 2(b)) he heat-treatment consisted of two main steps. In the ⊙AQ1991sEI 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 heat- Fig. 1. Scanning electron microscopy (SEM) image of a Saphikon" fibre ing rate was 100oCh-, the holding temperature was varied used in the present work. between 750 and 775 C the holding time was between 2.5272 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) 3.99 Diameter (m) 150 Tensile strength (GPa) 2.1–3.4 Elastic modulus (GPa) 386–435 Melting point (◦C) 2053 Coefficient of thermal expansion (K−1) 7.9–8.8 × 10−6 Uniaxial refractive index 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 pre￾pared. 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) “sand￾wich 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 par￾allel 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 pres￾sure 10 MPa and cooling rate 100 ◦C min−1. 2.2.3. “Sandwich structure” method This is a simple pressureless method for composite fab￾rication 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 dia￾mond tip to the desired dimensions (about 2.5 cm × 1.5 cm). The average length of the fibres was 8 mm. The same dispo￾sition 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 heat￾ing rate was 100 ◦C h−1, the holding temperature was varied between 750 and 775 ◦C, the holding time was between 2.5