S Jacques et al. /Journal of the European Ceramic Society 20(2000)1929-1938 to the pivot point of 1300C)limits the crystallisation to the fibre surface. This local inhomogeneity involves a 4.1. Relation between structures of fibre/matrix interfaces local embrittlement instead of global reinforcement and id minicomposite mechanical behaviour so an important fibre pull-out. The fibre/matrix bonding becomes very weak (low t values)and the load transfer From the above results, the processing parameter from the matrix to the fibre is no more ensured appears to play a very important role in the structure of A schematic representation, which summarises the the resulting BN interphase and so in the control of the different structures and the fracture behaviour of the BN fibre/matrix bonding. Indeed, the resident time of the interphases, is given in Fig. 14 tow inside the hot areas and the gap between the actual fibre temperature and the susceptor temperature depends 4. 2. Lifetime on r. Thus, with a high rate(r>2 m/h), the fibre tem- perature does not have time to reach the susceptor tem- As expected, the lifetime of the reference batch 0 with perature and does not follow exactly the temperature a pyrocarbon interphase is not very dependent on the profile presented in Fig. 2 moisture content and is the shortest despite an applied Concerning batch 2, the bn coating located close to the matrix is made of very well organised BN domains a slow enough rate of displacement tow (r=2.5 m/h) allows the large extension of the domains. The result is a rough interphase surface and an irregular BN/matrix interface and consequently a good bonding of the matrix During deflections of matrix cracks from mode I to mode Il occurring in the course of unloading-reloading cycles, intense frictions(cf. t values) between two rough sur- faces restrict the sliding. These frictions and tearing result in a SEM observation of a tortured interphase surface fixed on pulled out fibres. These features explain that, among the three bn interphases, interphase 2 confers the best mechanical properties. When r increases up to 3 m/h(batch 3), the wel matrix organised sublayer does not have time to thicken remains thin and regular. Hence, debonding and sliding can occur over long distances without any obstacles. As a matter of fact, the surface coating of the pulled out fibres appears smooth on SEM examinations. The interfacial shear stress is weak and so are the mechanical properties B Previous studies on CMCs with a PyC interphase1617 ive already reported the importance of tailoring the interphase thickness and orientation. But, whereas a thick well-organised pyrocarbon results in a too weak Interphase 2 fibre/matrix bonding, here a sufficient highly crystallised Bn thickness allows, on the contrary, to limit the sliding For slowest rates of displacement tow (r=2 m/h) batch 1), the second bn sublayer located near the matrix becomes isotropic because of a more important homogeneous nucleation; but above all, the fibre surface starts to crystallise by thermal treatment. This feature is rising for the Hi-Nicalon fibre is ex →, thermodynamically stable owing to its poor content of oxygen 18-20 Nevertheless, a recent study 2 performed on heat-treated Hi-Nicalon fibres has shown that a growth of the B-SiC crystallite size occurs in the bulk fibre crystallised 450 nm from and above 1300 c resulting in a mechanical interphase 1 enhancement of the fibre. In the present case, the part Fig. 14. Schematic representation of the BN interphases structures cular treatment (rather short time at a temperature close and the matrix cracks deflection4. Discussion 4.1. Relation between structures of ®bre/matrix interfaces and minicomposite mechanical behaviour From the above results, the processing parameter r appears to play a very important role in the structure of the resulting BN interphase and so in the control of the ®bre/matrix bonding. Indeed, the resident time of the tow inside the hot areas and the gap between the actual ®bre temperature and the susceptor temperature depends on r. Thus, with a high rate (r>2 m/h), the ®bre tem￾perature does not have time to reach the susceptor tem￾perature and does not follow exactly the temperature pro®le presented in Fig. 2. Concerning batch 2, the BN coating located close to the matrix is made of very well organised BN domains. A slow enough rate of displacement tow (r=2.5 m/h) allows the large extension of the domains. The result is a rough interphase surface and an irregular BN/matrix interface and consequently a good bonding of the matrix. During de¯ections of matrix cracks from mode I to mode II occurring in the course of unloading-reloading cycles, intense frictions (cf.  values) between two rough sur￾faces restrict the sliding. These frictions and tearing result in a SEM observation of a tortured interphase surface ®xed on pulled out ®bres. These features explain that, among the three BN interphases, interphase 2 confers the best mechanical properties. When r increases up to 3 m/h (batch 3), the well￾organised sublayer does not have time to thicken; it remains thin and regular. Hence, debonding and sliding can occur over long distances without any obstacles. As a matter of fact, the surface coating of the pulled out ®bres appears smooth on SEM examinations. The interfacial shear stress is weak and so are the mechanical properties. Previous studies on CMCs with a PyC interphase16,17 have already reported the importance of tailoring the interphase thickness and orientation. But, whereas a thick well-organised pyrocarbon results in a too weak ®bre/matrix bonding, here a sucient highly crystallised BN thickness allows, on the contrary, to limit the sliding. For slowest rates of displacement tow (r=2 m/h) (batch 1), the second BN sublayer located near the matrix becomes isotropic because of a more important homogeneous nucleation; but above all, the ®bre surface starts to crystallise by thermal treatment. This feature is surprising for the Hi-Nicalon ®bre is expected to be thermodynamically stable owing to its poor content of oxygen.18ÿ20 Nevertheless, a recent study12 performed on heat-treated Hi-Nicalon ®bres has shown that a growth of the b-SiC crystallite size occurs in the bulk from and above 1300C resulting in a mechanical enhancement of the ®bre. In the present case, the parti￾cular treatment (rather short time at a temperature close to the pivot point of 1300C) limits the crystallisation to the ®bre surface. This local inhomogeneity involves a local embrittlement instead of global reinforcement and so an important ®bre pull-out. The ®bre/matrix bonding becomes very weak (low  values) and the load transfer from the matrix to the ®bre is no more ensured. A schematic representation, which summarises the di€erent structures and the fracture behaviour of the BN interphases, is given in Fig. 14. 4.2. Lifetime As expected, the lifetime of the reference batch 0 with a pyrocarbon interphase is not very dependent on the moisture content and is the shortest despite an applied Fig. 14. Schematic representation of the BN interphases structures and the matrix cracks de¯ection. 1936 S. Jacques et al. / Journal of the European Ceramic Society 20 (2000) 1929±1938