Hi-Nicalon/BN/a-Si,N, ceramic-matrix composites 1487 Topography of a virgin fiber [2 um] 1454nm 93953nn 96977nm 969.77nm Topography of a BN coated fiber[2 um] o nm 230.9nm 200881nm 10044nm Fig. 8. Variation of the longitudinal Youngs modulus with the penetration below contact in BN-coated Hi-Nicalon fibers from micro-indentation tests. The maximum applied force is 250 mN force and a contact force of 1 mN. As noticed local the sintering and bn coating processes, it seems a longitudinal Youngs modulus values plotted as a priori that the former parameter has less influence for function of the depth below penetration show a similar the following reasons: (1)the hot pressing was plateau value close to 193 GPa, obtained after a performed in a controled atmosphere(N2),(2)at a penetration of about 100 um(the Poisson's ratio of relatively low pressure (=30 MPa) and (3)at a the matrix being Vm=0-28). Despite the low difterence, temperature (1550"C)where the fibers exhibit the this value is in good agreement with the one measured same longitudinal Youngs modulus as at room during flexure tests. Since this result reflects the mean temperature. Moore" has also shown that, depending values of 55 indentation tests, carried out on various on the BN coating temperature range, the ultimate regions of the specimen, it could be therefore tensile strength and the longitudinal Youngs modulus concluded that the longitudinal Youngs modulus of of SiC Nicalon fibers could be affected by the coating the fiber is Fm=193 GPa. The corresponding hardness process is 19. 8 GPa at a maximum depth of 392 nm For the above reasons, an AFm examination was Figure 8 shows the variations of the longitudinal carried out to check the surface of the virgin Hi- Young's modulus as a function of penetration depth, Nicalon fibers(without coating) and BN-coated fibers as derived from indentation tests performed on high both as-received from Nippon Carbon. An example of Nicalon fiber he composite. The values are mean these two fiber topographies is given in Fig 9 Figure 9 in the case of 2 um and 0.5 um total lengt maximum applied force( the contact force is I mN). scale, respectively. It can be seen that the BN-coated As noticed, a plateau value of 106 GPa in the Young's fiber shows a very rough surface, whereas the virgin modulus is reached aftcr a penetration of about 180 fiber displays much smaller roughness amplitude. The nm. The corresponding hardness is 14 GPa at roughness amplitude for BN-coated fibers is nearly maximum depth of 410 nm, A comparison between double that of uncoated fibers when analyzing the this value and that reported in the literature(250 highest magnification micrograph GPa) leads to the conclusion that the fibers had lost To explain this difference, let us analyze the BN more than 50%of their initial stiffness. With regard to coating process. In fact, boron nitride is deposited onHi-NicalonlBN/cY-Si,N, ceramic-matrix composites Topography of a virgin fiber [2 pm] 1487 Topography of a BN coated fiber [2 pm] Fig. 8. Variation of the longitudinal Young’s modulus with the penetration below contact in BN-coated Hi-Nicalon fibers from micro-indentation tests. The maximum applied force is 2.50 mN. force and a contact force of 1 mN. As noticed, local longitudinal Young’s modulus values plotted as a function of the depth below penetration show a similar plateau value close to 193 GPa, obtained after a penetration of about 100 ,um (the Poisson’s ratio of the matrix being Y, = O-28). Despite the low difference, this value is in good agreement with the one measured during flexure tests. Since this result reflects the mean values of 55 indentation tests, carried out on various regions of the specimen, it could be therefore concluded that the longitudinal Young’s modulus of the fiber is E,= 193 GPa. The corresponding hardness is 198 GPa at a maximum depth of 392 nm. Figure 8 shows the variations of the longitudinal Young’s modulus as a function of penetration depth, as derived from indentation tests performed on high Nicalon fibers in the composite. The values are mean ones obtained by testing 65 fibers up to 250 mN maximum applied force (the contact force is 1 mN). As noticed, a plateau value of 106 GPa in the Young’s modulus is reached after a penetration of about 180 nm. The corresponding hardness is 14 GPa at a maximum depth of 410 nm. A comparison between this value and that reported in the literature3 (250 GPa) leads to the conclusion that the fibers had lost more than 50% of their initial stiffness. With regard to the sintering and BN coating processes, it seems a priori that the former parameter has less influence for the following reasons: (1) the hot pressing was performed in a controled atmosphere (NJ, (2) at a relatively low pressure (= 30 MPa) and (3) at a temperature (1550°C) where the fibers exhibit the same longitudinal Young’s modulus as at room temperature? Moore8 has also shown that, depending on the BN coating temperature range, the ultimate tensile strength and the longitudinal Young’s modulus of Sic Nicalon fibers could be affected by the coating process. For the above reasons, an AFM examination was carried out to check the surface of the virgin Hi￾Nicalon fibers (without coating) and BN-coated fibers, both as-received from Nippon Carbon. An example of these two fiber topographies is given in Fig. 9 Figure 9 in the case of 2 pm and 0.5 pm total length scale, respectively. It can be seen that the BN-coated fiber shows a very rough surface, whereas the virgin fiber displays much smaller roughness amplitude. The roughness amplitude for BN-coated fibers is nearly double that of uncoated fibers when analyzing the highest magnification micrograph. To explain this difference, let us analyze the BN coating process. In fact, boron nitride is deposited on