276 A.R. Boccaccini et al. Joumal of Materials Processing Technology 169(2005)270-280 N 1 mm Fig. 7. Optical microscopy microgra ng the matrix region of about Imm width between two sapphire fibres ("optical window)in a polished)transparent sandwich structure composite ysed qualitatively by examining fracture surfaces of different 3.2. 1. Sintered composites Fig. 8(a and b)are SEM images of fracture surfaces of sintered samples. Fig 8(a) shows that there is pull-out of the 19sE1 fibres during the fracture of the composite. Since there are many defects at the interface between fibres and matrix(see Fig. 8. SEM images of fracture surfaces of pressureless sintered samples also Fig. 3(c)), the bonding at the interface is weak and it is showing(a)evidence of pull-out of the Saphikonfibres during fracture of easy to have fibre/matrix debonding and fibre pull out. These the composite and(b) defect-free matrix-fibre interface thus they will contribute to increase the toughness of the adhered to the surface of the fibre (a) indicate rest of glass from the matrix mechanisms(debonding and pull-out )dissipate energy, and matrix-fibre bonding. The arrow composite. It is also seen that there are cracks in the matrix in regions around the fibres and at the interface, which are Typical fracture surfaces showing evidence of fibre debond- most probably a consequence of pores and crack-like flaws ing and pull-out are presented in Fig 9(a and b). Due to the es compared to press in regions where the glass matrix is in close contact with sintered materials, they are expected to exhibit higher frac- the fibres, bonding between matrix and fibres can be free ture strength. Moreover, due to the unidirectional alignment of defects. In this case a strong fibre/matrix bonding may will exhibit anisotropic mechanical behaviour with highest the pulled-out fibres(arrows in Fig. 8(a)confirms that the strength and stiffness in the fibre direction, exploiting a load bonding between borosilicate glass and sapphire fibres w transfer mechanism [71 very strong after sintering at 750C if there was close con- tact between them. Thus, it may be anticipated that fracture 3.2.3. "Sandwich structure"composites ughness of the samples will be only partially improved by sandwich method" led to defect-free composites debonding and pull-out mechanisms. Moreover, the samples without These composites therefore should have will show low fracture strength because of the relatively high higher fracture strength than the sintered samples. SEM micrographs of fracture surfaces of"sandwich structure composites, shown in Fig. 10(a-c), confirm that when the 3. 2.2. Hot-pressed composites matrix is in close contact with the fibres a strong bonding may very similar, in qualitative terms, to that of pressureless was develop between them. Fig. 10(a)shows an impression left The fracture behaviour of the hot-pressed composite in the matrix by the fibre when the sample was fractured. The tered samples, as inferred from fracture surface analyses matrix was broken and some glass matrix stayed adhered to276 A.R. Boccaccini et al. / Journal of Materials Processing Technology 169 (2005) 270–280 Fig. 7. Optical microscopy micrograph showing the matrix region of about 1mm width between two sapphire fibres (“optical window”) in a (non￾polished) transparent sandwich structure composite. ysed qualitatively by examining fracture surfaces of different samples. 3.2.1. Sintered composites Fig. 8(a and b) are SEM images of fracture surfaces of sintered samples. Fig. 8(a) shows that there is pull-out of the fibres during the fracture of the composite. Since there are many defects at the interface between fibres and matrix (see also Fig. 3(c)), the bonding at the interface is weak and it is easy to have fibre/matrix debonding and fibre pull out. These mechanisms (debonding and pull-out) dissipate energy, and thus they will contribute to increase the toughness of the composite. It is also seen that there are cracks in the matrix in regions around the fibres and at the interface, which are most probably a consequence of pores and crack-like flaws produced during sintering. However, Fig. 8(b) indicates that in regions where the glass matrix is in close contact with the fibres, bonding between matrix and fibres can be free of defects. In this case a strong fibre/matrix bonding may be assumed. Furthermore, the presence of layers of glass on the pulled-out fibres (arrows in Fig. 8(a)) confirms that the bonding between borosilicate glass and sapphire fibres was very strong after sintering at 750 ◦C if there was close con￾tact between them. Thus, it may be anticipated that fracture toughness of the samples will be only partially improved by debonding and pull-out mechanisms. Moreover, the samples will show low fracture strength because of the relatively high porosity. 3.2.2. Hot-pressed composites The fracture behaviour of the hot-pressed composites was very similar, in qualitative terms, to that of pressureless sin￾tered samples, as inferred from fracture surface analyses. Fig. 8. SEM images of fracture surfaces of pressureless sintered samples showing (a) evidence of pull-out of the Saphikon® fibres during fracture of the composite and (b) defect-free matrix-fibre interface in an area of good matrix-fibre bonding. The arrows in (a) indicate rest of glass from the matrix adhered to the surface of the fibre. Typical fracture surfaces showing evidence of fibre debond￾ing and pull-out are presented in Fig. 9(a and b). Due to the lower porosity of these composites compared to pressureless sintered materials, they are expected to exhibit higher frac￾ture strength. Moreover, due to the unidirectional alignment of the Saphikon® fibres in these composites, the materials will exhibit anisotropic mechanical behaviour with highest strength and stiffness in the fibre direction, exploiting a load￾transfer mechanism [7]. 3.2.3. “Sandwich structure” composites The “sandwich method” led to defect-free composites without porosity. These composites therefore should have higher fracture strength than the sintered samples. SEM micrographs of fracture surfaces of “sandwich structure” composites, shown in Fig. 10(a–c), confirm that when the matrix is in close contact with the fibres a strong bonding may develop between them. Fig. 10(a) shows an impression left in the matrix by the fibre when the sample was fractured. The matrix was broken and some glass matrix stayed adhered to