1488 M. Drissi-Habti, K. Nakano Topography of a virgin fiber [0.5 uml 48487nm 48487nm 24244m Topography of a BN coated fiber (0. 5 umI 24244n nm l nm Fig. 9. AFM image(with a total length scale of 2 um), showing the topography of the virgin and coated fibers. As noticed, the coated fiber shows greater roughness amplitude the fibers at a temperature of 1500 C via the following the mechanical properties of Hi-Nicalon fibers can reaction only be achieved by means of uniaxial tensile tests on BCl、+NH3→BN+3HCl (11) both virgin and BN-coated fibers. Such a study is now In progress. Therefore, two explanations could be involved. The Moreover, in addition to the bn coating process, it first one is related to the atmosphere in which the as not been excluded that the combined effects of hot reaction is conducted. Indeed, if the atmosphere is not pressing and the high content of sintering additives controlled sufficiently, the excess fiber roughness could (5% alumina+5% yttria) can have a detrimental originate simply from an increase in the mean grain effect on the mechanical properties of the fibers Such size of the Hi-Nicalon fibers. Such explanation is in a case may involve a diffusion mechanism of accordance with previous results. The second explana- aluminum(coming from the sintering additives)into tion could be that an undesirable reaction between the the fibers. This point is also of major importance and fibers and the reactants occurs at 1500C which leads must be verified. to the evaporation of a gaseous product. In this case the mechanical properties of the fiber bulk could altered. This could explain the drastic decrease in the 4 CONCLUSIONS longitudinal Young's modulus of the fibers. In any The microstructure and mechanical propcrties case, on the basis of these preliminary results and Nicalon/ BN/silicon-nitride composites have observations, the BN coating conditions seem to have investigated. Results show that the longitudinal an effect in the increasing the roughness amplitude of Youngs modulus of the fibers in the composite the Hi-Nicalon fibers. It is worth noting, however. that decreases drastically. Preliminary results and observa final clarification of the effects of the bn coating on tions tend to incriminate the bn coating stage.1488 Act. Drissi-Habti, K. Nakano virgin fiber IO.5 pm] I Fig. 9. AFM image (with a total length scale of 2 pm), showing the topography of the virgin and coated fibers. As noticed, coated fiber shows greater roughness amplitude. the the fibers at a temperature of 1500°C via the following reaction: BC13 + NH3 + BN + 3HCl (11) Therefore, two explanations could be involved. The first one is related to the atmosphere in which the reaction is conducted. Indeed, if the atmosphere is not controlled sufficiently, the excess fiber roughness could originate simply from an increase in the mean grain size of the Hi-Nicalon fibers. Such explanation is in accordance with previous results.’ The second explana￾tion could be that an undesirable reaction between the fibers and the reactants occurs at 1500°C which leads to the evaporation of a gaseous product. In this case, the mechanical properties of the fiber bulk could altered. This could explain the drastic decrease in the longitudinal Young’s modulus of the fibers. In any case, on the basis of these preliminary results and observations, the BN coating conditions seem to have an effect in the increasing the roughness amplitude of the Hi-Nicalon fibers. It is worth noting, however, that final clarification of the effects of the BN coating on the mechanical properties of Hi-Nicalon fibers can only be achieved by means of uniaxial tensile tests on both virgin and BN-coated fibers. Such a study is now in progress. Moreover, in addition to the BN coating process, it has not been excluded that the combined effects of hot pressing and the high content of sintering additives (5% alumina +5% yttria) can have a detrimental effect on the mechanical properties of the fibers. Such a case may involve a diffusion mechanism of aluminum (coming from the sintering additives) into the fibers. This point is also of major importance and must be verified. 4 CONCLUSIONS The microstructure and mechanical properties of Hi￾Nicalon/BN/silicon-nitride composites have been investigated. Results show that the longitudinal Young’s modulus of the fibers in the composite decreases drastically. Preliminary results and observa￾tions tend to incriminate the BN coating stage