A.R. Boccaccini et al. Joumal of Materials Processing Technology 169(2005)270-280 20kV X2591gg从m 20kU 18 2 SE I Fig. 5. SEM image of the polished section of a hot-pressed sample contain- ing Saphikon" fibres unidirectionally aligned, showing high densification but the presence crack-like defects This should have a negative effect on the mechanical prop- erties of the composites because the different thermal expan- sion coefficients of cristobalite and borosilicate glass matrix may lead to microcracking, therefore decreasing the tensile gth of the composites. The lack of cristobalite forma- tion 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 1936 BES 3.1.2. Hot-pressed composites Hot-pressed samples without fibres were translucent, while composites containing fibres were opaque, even though Fig. 6. SEM micrographs of polished surfaces of"sandwich structure"com- posites showing(a)some defects(voids), marked by arrows, localised close the densities achieved were very high in both cases, 99.5 and to the fibres after too short processing time and(b) perfect bonded sample 98% of theoretical density, respectively. Cristobalite crys fabricated employing optimised parameters tallisation was not detected in hot-pressed composites. Fig. 5 shows a polished section of a hot-pressed sample containing optimised(775C,3.5h), the two plates of glass were seen 5 vol% unidirectionally aligned Saphikonfibres. Although to be well bonded together and the interface between them the matrix seems to be well densified without the presence of was"invisible" under SEM(Fig. 6(b)) large cavities or pores(compare with Fig. 3(a), pressureless Sandwich structure" samples with and without fibres sintered sample, there are still some defects and crack-like were transparent. There was some contamination on the sur- voids remaining, mainly close to the fibres, which should be faces after heat-treatment, but this disappeared after polish responsible for high light scattering and the loss of trans- ing. The two slides of glass were in all cases very well bonded parency of these samples. Thus, despite the relative high Fig. 7 shows the matrix region between two fibres ("optical density achieved, the parameters used for hot-pressing were window )in a sandwich structure composite observed with not optimal for producing optically transparent or translucent an optical microscope; fibres are also visible Heat-treatment composites at 775C for 3.5 h was found to be the best thermal process for the fabrication of sandwich structure c 3.1.3. Sandwich structure"composites SEM micrographs of polished surfaces of""sandwich 3. 2. Fracture behaviour structure"composites are shown in Fig. 6(a and b). In some regions the interface between fibres and matrix could only Previous research has proved that a-Al2O3/borosilicate be detected by using SEM in backscattered electrons mode glass systems containing a-Al2O3 platelets as reinforce (Fig. 6(b)), indicating an intimate perfect bond. Some defects ment lead to composites with higher fracture strength (voids) were observed in some samples, localised just cl Young's modulus. hardness and fracture toughness than the to the fibres(Fig. 6(a)). These were probably the result of a matrix without reinforcement [6,371. In the present inves- too short processing time. When the parameters used were tigation, fracture behaviour of the composites was analA.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 contain￾ing Saphikon® fibres unidirectionally aligned, showing high densification but the presence crack-like defects. This should have a negative effect on the mechanical prop￾erties of the composites because the different thermal expan￾sion coefficients of cristobalite and borosilicate glass matrix may lead to microcracking, therefore decreasing the tensile strength of the composites. The lack of cristobalite forma￾tion 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 crys￾tallisation 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 trans￾parency 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” com￾posites 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 sur￾faces after heat-treatment, but this disappeared after polish￾ing. 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
-Al2O3/borosilicate glass systems containing
-Al2O3 platelets as reinforce￾ment lead to composites with higher fracture strength, Young’s modulus, hardness and fracture toughness than the matrix without reinforcement [6,37]. In the present inves￾tigation, fracture behaviour of the composites was anal-