1934 S Jacques et al. Journal of the European Ceramic Society 20(2000)1929-1938 fibre Fig 10. HR-TEM image of the middle of the interphase of a batch 2 The observation of the second sublayer located near the matrix(Fig. 10, right) reveals a very high structural organisation even if some 002 fringe distortions still remain. The fringes can exceed 50 nm in length. These large coherent domain sizes explain the sublayer thick debonding ness inhomogeneity and consequently the interphase/ matrix interface irregularity. 800nm 3.2.2. Batch I minicomposite As in the case of batch 2, batch I minicomposite study expected because of the slower rate of toy ayers. As Fig. 11. Bright-field TEM image of the interphase of a batch I mini- Fig. 11)evidences the occurrence of two su (r=2 m/h for batch I against 2.5 m/h for batch 2)and (located near the matrix) is made of an isotropic BN, hence a higher reactor resident time, the total interphase 002 diffraction arcs meet together and form continuous thickness is higher: about 950 nm. But here, the second rings; there is no more preferential orientation. Here sublayer located near the matrix is homogeneous in again, this difference with batch 2 can be explained by thickness. Another important difference: in addition, two the slower rate of tow displacement. The gaseous phase dark"edges"appear at the fibre/interphase interface infiltrated in the tow porosity is drag off more slowly to with a large debonding located between the both; this the hottest areas, which allows it to be raised to a high debonding extends over long distances est temperature. Homogeneous nucleation can be then From a comparison between a SAd performed on favoured to the detriment of heterogeneous chemical these dark edges and a Sad performed more inside the surface reactions. This phenomenon associated with a fibre(Fig. 12, bottom), it is clear that the SiC fibre sur- high yield of homogeneous reaction, results in an face has been crystallised. Both SAD patterns are typical important production of HF and intermediate species of Sic but, whereas diffraction rings are quasi-con- and consequently leads to a material disorganisation tinuous within the fibre (Sic nanometric grains size) these are spotted at the fibre surface(more extended Sic 3. 2.3. Batch 3 minicomposite grains size). This crystallisation has not been observed Batch 3 interphase is also made of two sublayers(Fig. in batch 2. For batch l, it can be due to a slower rate of 13): a first preponderant one located near the fibre tow displacement, which gives the fibre enough time to identical with those of the other batches, about 250 nm be raised to a highest temperature while going through in thickness, and a very thin one located near the matrix he reactor hottest area(1250C). This thermal treat- (about 20 nm in thickness)where BN organisation is ment is responsible for the fibre crystallisation that very pronounced. The faster tow rate (3 m/h)can starts from its surface, causes local embrittlement and explain this disproportion. The fibre resident time in the results in a large debonding hottest area is reduced; the second organised sublayer SAD observations of interphase(Fig. 12, top) show has no more time to grow as in the case of batch 2. But, that the first bn sublayer (located near the fibre) is contrary to batch 2, this restricted growth allows this identical with that of batch 2: it is an anisotropic tur- sublayer to keep a constant thickness and therefore to bostratic BN. On the contrary, the second sublayer form a regular and linear interface with the matrixThe observation of the second sublayer located near the matrix (Fig. 10, right) reveals a very high structural organisation even if some 002 fringe distortions still remain. The fringes can exceed 50 nm in length. These large coherent domain sizes explain the sublayer thick￾ness inhomogeneity and consequently the interphase/ matrix interface irregularity. 3.2.2. Batch 1 minicomposite As in the case of batch 2, batch 1 minicomposite study (Fig. 11) evidences the occurrence of two sublayers. As expected because of the slower rate of tow travelling (r=2 m/h for batch 1 against 2.5 m/h for batch 2) and hence a higher reactor resident time, the total interphase thickness is higher: about 950 nm. But here, the second sublayer located near the matrix is homogeneous in thickness. Another important di€erence: in addition, two dark ``edges'' appear at the ®bre/interphase interface with a large debonding located between the both; this debonding extends over long distances. From a comparison between a SAD performed on these dark edges and a SAD performed more inside the ®bre (Fig. 12, bottom), it is clear that the SiC ®bre sur￾face has been crystallised. Both SAD patterns are typical of SiC but, whereas di€raction rings are quasi-con￾tinuous within the ®bre (SiC nanometric grains size), these are spotted at the ®bre surface (more extended SiC grains size). This crystallisation has not been observed in batch 2. For batch 1, it can be due to a slower rate of tow displacement, which gives the ®bre enough time to be raised to a highest temperature while going through the reactor hottest area (1250C). This thermal treat￾ment is responsible for the ®bre crystallisation that starts from its surface, causes local embrittlement and results in a large debonding. SAD observations of interphase (Fig. 12, top) show that the ®rst BN sublayer (located near the ®bre) is identical with that of batch 2: it is an anisotropic tur￾bostratic BN. On the contrary, the second sublayer (located near the matrix) is made of an isotropic BN, 002 di€raction arcs meet together and form continuous rings; there is no more preferential orientation. Here again, this di€erence with batch 2 can be explained by the slower rate of tow displacement. The gaseous phase in®ltrated in the tow porosity is drag o€ more slowly to the hottest areas, which allows it to be raised to a high￾est temperature. Homogeneous nucleation can be then favoured to the detriment of heterogeneous chemical surface reactions. This phenomenon associated with a high yield of homogeneous reaction, results in an important production of HF and intermediate species and consequently leads to a material disorganisation. 3.2.3. Batch 3 minicomposite Batch 3 interphase is also made of two sublayers (Fig. 13): a ®rst preponderant one located near the ®bre identical with those of the other batches, about 250 nm in thickness, and a very thin one located near the matrix (about 20 nm in thickness) where BN organisation is very pronounced. The faster tow rate (3 m/h) can explain this disproportion. The ®bre resident time in the hottest area is reduced; the second organised sublayer has no more time to grow as in the case of batch 2. But, contrary to batch 2, this restricted growth allows this sublayer to keep a constant thickness and therefore to form a regular and linear interface with the matrix. Fig. 10. HR-TEM image of the middle of the interphase of a batch 2 minicomposite. Fig. 11. Bright-®eld TEM image of the interphase of a batch 1 mini￾composite. 1934 S. Jacques et al. / Journal of the European Ceramic Society 20 (2000) 1929±1938