S Jacques et al. /Journal of the European Ceramic Society 20(2000)1929-1938 ” SAD SAD SAD SAD fibre 300nm fibre SAD SAD Fig. 12. Bright-field TEM image of the interfacial zone of a batch I minicomposite and corresponding SAD patterns(negatives). Fig. 13 exhibits a deflection of a matrix crack: it occurs close to the matrix within this thin orientated ig. 13. Bright-field TEM image of the interfacial zone of a batch 2 te and corresponding SAd patterns(negatives) sublayer. Because of interface regularity, the crack can propagates along the matrix in mode Il over very long distances. This phenomenon explains SEM observations Table 3 where the whole bn coating was found to remain fixed Average lifetimes, at 700C, under a 9.5 kg load, of the mini- on the fibre with a smooth and regular surface composites Batch Lifetime(h) 3.3. Lifetime at high temperature under tensile loading Dry air Moist air The average values of the lifetimes at 700oC in dry moist air under a 9.5 kg load are reported in Table 3 As a general rule, in these conditions, the mini- composite lifetimes depend on the ability of the inter- >400 phase to protect the fibre from a mechanism of stress corrosion. Indeed. Bertrand et al. 5 have evidenced thanks to similar tests performed on as-received bare For all the minicomposites with BN interphases, the fibre tows, that the Hi-Nicalon fibre lifetime becomes lifetimes are significantly improved which does prove extremely short(below one minute for a tow submitted the advantage of bn with respect to pyrocarbon both in to a load of 4.5 kg! ). Within a cracked matrix of a dry air and moist air. Among the three batches, batch 3 composite, if the interphase is oxidised the fibre is allows to achieve the best results. In dry air the experiment directly exposed to air and breaks, which results in the had to be stop after 400 h of exposure, which evidences failure of the whole composite the very efficient protection.Fig. 13 exhibits a de¯ection of a matrix crack: it occurs close to the matrix within this thin orientated sublayer. Because of interface regularity, the crack can propagates along the matrix in mode II over very long distances. This phenomenon explains SEM observations where the whole BN coating was found to remain ®xed on the ®bre with a smooth and regular surface. 3.3. Lifetime at high temperature under tensile loading The average values of the lifetimes at 700C in dry or moist air under a 9.5 kg load are reported in Table 3. As a general rule, in these conditions, the mini￾composite lifetimes depend on the ability of the inter￾phase to protect the ®bre from a mechanism of stress corrosion. Indeed, Bertrand et al.15 have evidenced, thanks to similar tests performed on as-received bare ®bre tows, that the Hi-Nicalon ®bre lifetime becomes extremely short (below one minute for a tow submitted to a load of 4.5 kg!). Within a cracked matrix of a composite, if the interphase is oxidised the ®bre is directly exposed to air and breaks, which results in the failure of the whole composite. For all the minicomposites with BN interphases, the lifetimes are signi®cantly improved which does prove the advantage of BN with respect to pyrocarbon both in dry air and moist air. Among the three batches, batch 3 allows to achieve the best results. In dry air the experiment had to be stop after 400 h of exposure, which evidences the very ecient protection. Fig. 13. Bright-®eld TEM image of the interfacial zone of a batch 2 minicomposite and corresponding SAD patterns (negatives). Fig. 12. Bright-®eld TEM image of the interfacial zone of a batch 1 minicomposite and corresponding SAD patterns (negatives). Table 3 Average lifetimes, at 700C, under a 9.5 kg load, of the mini￾composites Batch Lifetime (h) Dry air Moist air 0 (PyC) 3 5 1 47 7 2 89 62 3 >400 187 S. Jacques et al. / Journal of the European Ceramic Society 20 (2000) 1929±1938 1935