A.R. Boccaccini et al. Joumal of Materials Processing Technology 169(2005)270-280 273 and 5h and the cooling rate was varied between 100 and 3. Results and discussion 260Ch-. The degassing step included two different treat ments. Firstly, the sample was under a normal atmosphere and 3.1. Microstructural characterisation after completion of half of the holding time, a high vacuum vas produced in order to clean all the impurities that might 3.1.1. Sintered composit have burnt. All samples were produced using the same heat Successful fabrication of fibre reinforced glass matrix ing and cooling rates(100Ch-l)in both steps, the holding composites relies on knowing the relationship between tem- temperature and holding time for the first step were 500oc perature and viscosity of the glass matrix. As it is well-known and 4 h, respectively, and they were 750C and 5 h for the for Duran-type borosilicate glass [34,371, the range of suit second step, respectively able temperatures for sintering glass powder is very narrow between720and780°C). A difference of±20° C in the sin- 2.3. Characterization techniques tering temperature can have a large effect on the densification The density of sintered and hot-pressed samples was deter- mined by the Archimedes'method. The relative density was gation were not translucent and their densification was not calculated by considering the theoretical density of the com- completely achieved; the sintered relative densities were in posites based on the density data for matrix and fibres, given the range 80-83% of theoretical density. Fig. 3(a-c)show in Tables 1 and 2, respectively, and the volume fraction of SEM micrographs of polished cross-sections of a chopped fibres. For microstructural characterisation, samples were fibre reinforced sintered composite at different magnifica cut,then mounted in resin and polished to 1 um finish with tions. Fig. 3(a)shows that the matrix close to the fibres is diamond paste to obtain flat cross sections for SEM. The porous and that larger defects are situated around the fibre, microstructures of selected sintered, hot-pressed and sand- confirming that it has not been possible to obtain a good wich structure samples were examined using conventional densification of the matrix in this region by pressureless sin- SEM(JEOL LV 5610)working in both secondary electron tering. The presence of randomly orientated fibres impedes and backscattered electron modes the perfect flow of the viscous glass during sintering and Sintered samples were analysed using X-ray diffraction causes a high porosity of the matrix in the region close to the (XRD) to detect any crystallisation of the matrix. Sam- fibres. On the contrary, the matrix far away from the fibres ples were crushed to a fine powder, and then analysed with (Fig 3(b ))exhibits high densification with few isolated pores a Philips PW1700 series automated powder diffractometer Fig. 3(c)is a high magnification image of the fibre/matrix using Cu Ka radiation at 40 k V-40 mA with a seconda interface showing that the lack of densification impedes the ono To gain preliminary understanding of the fracture interface behaviour of the composites and qualitative information The poor densification of the pressureless sintered sam about the interaction(bonding) between fibres and matrix ples achieved in the present work is in broad agreement with during the fracture process, fractures surfaces were also previous studies that showed that the presence of rigid inclu- observed by SEM. ions makes the densification of a glass matrix composite a A preliminary assessment of the optical quality of the sam- difficult task and that the density of the composites decreases ples was obtained by placing selected samples at different with increasing volume fraction of rigid inclusions [39-411 heights over a written text on a white surface and assessing Different explanations for this phenomenon have been pro- the text legibility. This qualitatively demonstrated the light posed. An inhomogeneous distribution of inclusions in the transmitting characteristics of the different composites. a powder can lead to a poor matrix particle packing and for digital camera was used to document this behaviour. Fab- mation of agglomerates, and thus the porosity will increase ricated"sandwich structure" composites were also observed around the fibres [39]. Moreover, studies conducted using the by means of a conventional optical microscope same materials but different mixing conditions have shown Light transmittance of the hot-pressed and"sandwich that optimised wet-mixing techniques lead to higher densities ructure"samples(with and without fibres) was measured at as they allow for more homogeneous mixtures[42]. Since the room temperature by a UV-visible spectrophotometer (UV- present glass powder/chopped fibres mixture was prepared in 1601 Shimadzu, Japan ), in the direction perpendicular dry conditions, the last explanation may be applicable to our the fibres axis. Before light transmittance measurements, results samples were cut and polished to obtain the size required Another factor affecting the densification of composites r the measurements. The thickness of the samples was containing rigid inclusions is the development of residual 2.3+0.1 mm, and the light transmittance was measured for stresses as a consequence of different sintering rates of matrix wavelengths between 350 and 800 nm. The light transmit- and inclusions. These stresses may cause sintering damage, ance of the samples was reported as a percentage of the leading to crack-like voids or isolated pores and conse- transmittance of a reference monolithic borosilicate gla quently to poor mechanical properties of the sintered samples slide(Borofloat 33) [39-42]A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280 273 and 5 h and the cooling rate was varied between 100 and 260 ◦C h−1. The degassing step included two different treat￾ments. 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 heat￾ing 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 deter￾mined by the Archimedes’ method. The relative density was calculated by considering the theoretical density of the com￾posites 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 sand￾wich 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. Sam￾ples 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 sam￾ples 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. Fab￾ricated “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 (UV- 1601 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 transmit￾tance of the samples was reported as a percentage of the transmittance of a reference monolithic borosilicate glass slide (Borofloat® 33). 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 tem￾perature and viscosity of the glass matrix. As it is well-known for Duran®-type borosilicate glass [34,37], the range of suit￾able temperatures for sintering glass powder is very narrow (between 720 and 780 ◦C). A difference of ±20 ◦C in the sin￾tering temperature can have a large effect on the densification of the composites. Pressureless sintered samples fabricated in this investi￾gation 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 magnifica￾tions. 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 sin￾tering. 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 sam￾ples achieved in the present work is in broad agreement with previous studies that showed that the presence of rigid inclu￾sions 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 pro￾posed. An inhomogeneous distribution of inclusions in the powder can lead to a poor matrix particle packing and for￾mation 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 conse￾quently to poor mechanical properties of the sintered samples [39–42].