1932 S Jacques et al. /Journal of the European Ceramic Society 20(2000)1929-1938 curve for each kind of interphase. In every case, an fibre/matrix bonding. Here, the interphase remains extended non-linear domain evidencing matrix micro- fixed on the fibre as shown in Fig. 5(a) where the rough cracking and fibre/matrix debonding follows the initial and tortured BN coating surface appears. A highest inear elastic region. Therefore, the four kinds of inter- magnification observation [Fig. 5(b)]shows a debonding phase act as mechanical fuses that occurs within the interphase itself. Minicomposites of batches 0 and 2 exhibit sim laI Concerning batch 3 [Fig. 60 the pull-out lengths tensile behaviours with small residual stresses. The are medium(reaching 100 Hm). If the surface of the identical average failure forces for both batches are the pulled out fibres seems to be smooth as in the case of highest. Therefore, BN interphase 2 processed with batch 1, a meticulous observation shows that they are r=2.5 m/h is as good at room temperature as reference not bare but coated with the whole BN interphase [Fig PyC interphase. Batch 3 obtained with r=3 m/h exhi- 6(b)]. Here, the debonding occurs near the matrix bits weakest mechanical properties with important resi dual strains evidencing that the fibre/matrix load 3. 2. TEM characterization transfer is less good. The characteristics decrease more for batch I interphase prepared with the slowest rate of 3. 2. Batch 2 minicomposite ow travelling. Thus, with these processing conditions, Bright-field observation of interphase 2(Fig. 7)shows the intermediate r of 2.5 m/h appears to result in an that it is made of two sublayers with different textures timum The average proportional limit force for each batch evolves in the same way. These results need not mean debond that for the best composites(batches 0 and 2)the matrix begins to crack later during the composite tension. But as the load transfer is better, the very first crack coming out does not lead to a deflection from linearity detect- able on curves at low load level atrIx 3. 1.2. Interfacial shear stress t Interfacial shear stresses measured by following the different methods(Table 2) bear out the different inter- Fig 4. SEM obsevations of the failure surface of a batch I mini phase characteristics. However, a distinction appears composite: (a)important fibre pull-out; (b)debonding in the fibre/ between batches 0 and 2. If the fibre/matrix bond is interphase interface strong with Pyc interphase, the lowest T values obtained with batch 2 show that the bn coating results in an intermediate bond fibre 3. 1.3. SEM observation of fracture surfaces SEM observation of batch I minicomposite fracture surfaces after tensile tests [Fig. 4(a)] shows an important fibre pull-out. The pull-out lengths can exceed 200 um matrix The surface of the pulled out fibres is smooth and free of any coating. The debonding occurs in the fibre/inter- phase interface as evidenced by highest magnification observation [Fig 4(b). This large debonding corrobo rates the measured low value of t Fig. 5. SEM observations of the failure of a batch 2 minicomposite In the case of batch 2, the fibre pull-out is small and (a)part of BN coating fixed on pull-out fibre; (b)debonding within the generally lower than 50 um evidencing the strongest Average interfacial shear stresses r (%) Is(um) N Eo(GPa) as(MPa) 0.(MPa) o(MPa) I(MPa) 0 (PyC) 73 19.2 17.5 128curve for each kind of interphase. In every case, an extended non-linear domain evidencing matrix micro￾cracking and ®bre/matrix debonding follows the initial linear elastic region. Therefore, the four kinds of inter￾phase act as mechanical fuses. Minicomposites of batches 0 and 2 exhibit similar tensile behaviours with small residual stresses. The identical average failure forces for both batches are the highest. Therefore, BN interphase 2 processed with r=2.5 m/h is as good at room temperature as reference PyC interphase. Batch 3 obtained with r=3 m/h exhi￾bits weakest mechanical properties with important resi￾dual strains evidencing that the ®bre/matrix load transfer is less good. The characteristics decrease more for batch 1 interphase prepared with the slowest rate of tow travelling. Thus, with these processing conditions, the intermediate r of 2.5 m/h appears to result in an optimum. The average proportional limit force for each batch evolves in the same way. These results need not mean that for the best composites (batches 0 and 2) the matrix begins to crack later during the composite tension. But, as the load transfer is better, the very ®rst crack coming out does not lead to a de¯ection from linearity detect￾able on curves at low load level. 3.1.2. Interfacial shear stress  Interfacial shear stresses measured by following the di€erent methods (Table 2) bear out the di€erent inter￾phase characteristics. However, a distinction appears between batches 0 and 2. If the ®bre/matrix bond is strong with PyC interphase, the lowest  values obtained with batch 2 show that the BN coating results in an intermediate bond. 3.1.3. SEM observation of fracture surfaces SEM observation of batch 1 minicomposite fracture surfaces after tensile tests [Fig. 4(a)] shows an important ®bre pull-out. The pull-out lengths can exceed 200 mm. The surface of the pulled out ®bres is smooth and free of any coating. The debonding occurs in the ®bre/inter￾phase interface as evidenced by highest magni®cation observation [Fig. 4(b)]. This large debonding corrobo￾rates the measured low value of . In the case of batch 2, the ®bre pull-out is small and generally lower than 50 mm evidencing the strongest ®bre/matrix bonding. Here, the interphase remains well ®xed on the ®bre as shown in Fig. 5(a) where the rough and tortured BN coating surface appears. A highest magni®cation observation [Fig. 5(b)] shows a debonding that occurs within the interphase itself. Concerning batch 3 [Fig. 6(a)], the pull-out lengths are medium (reaching 100 mm). If the surface of the pulled out ®bres seems to be smooth as in the case of batch 1, a meticulous observation shows that they are not bare but coated with the whole BN interphase [Fig. 6(b)]. Here, the debonding occurs near the matrix. 3.2. TEM characterization 3.2.1. Batch 2 minicomposite Bright-®eld observation of interphase 2 (Fig. 7) shows that it is made of two sublayers with di€erent textures Table 2 Average interfacial shear stresses  batch V0 p…%† ls(mm) N E0(GPa) s(MPa) p(MPa) 0 (MPa) 
(mm)  (MPa) Eq. (1) Eq. (2) 0 (PyC) 22.9 87 462 350 503 455 223 31 117 73 1 18.4 408 98 348 306 311 155 41 15 12 2 19.2 206 194 305 417 409 200 24 75 32 3 17.5 279 143 328 269 255 128 32 29 20 Fig. 4. SEM obsevations of the failure surface of a batch 1 mini￾composite: (a) important ®bre pull-out; (b) debonding in the ®bre/ interphase interface. Fig. 5. SEM observations of the failure of a batch 2 minicomposite: (a) part of BN coating ®xed on pull-out ®bre; (b) debonding within the interphase. 1932 S. Jacques et al. / Journal of the European Ceramic Society 20 (2000) 1929±1938