A.R. Boccaccini et al. Joumal of Materials Processing Technology 169(2005)270-280 matrix Fig 3 SEM micrographs of Sapphire fibre rced 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 mperfect bonding The existence of different"kinds" of porosity in the green lar result was obtained for hot-pressed samples. Indeed it has compact has been also proposed to explain the retardation of been proved in previous studies [27, 28, 43] that cristobalite densification in composites containing rigid inclusions [39]. crystallisation could happen in Al2O3/borosilicate glass com Pores can be embedded in the matrix material only, or they can posites with low volume fractions of Al2O3 (<10%)and even be located at the interface between matrix and inclusions. This when sintering temperature is low (in the range 700-800oC) 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 Counts/s 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 Saphikonfibr reinforced sintered composite. The only crystalline phase detected was corundum, which corresponds to the crystalline of the Saphikon fibres used. No cristobalite has crystallised in the matrix, which is a favourable result from the Fig. 4. XRD patte Saphikon" fibre reinforced sintered composite showing corundum as the only cristalline phase, which corresponds to the point of view of the composite mechanical strength. A simi- structure of the Saphikonfibres274 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 Al2O3/borosilicate glass com￾posites with low volume fractions of Al2O3 (<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