368 D. Hiilsenberg et al Composites: Part B 39(2008) 4.2. Mechanical properties of the Nextel 440 fibers/756 glass matrix composites If Nextel 440 fibers and 756 glass as matrix are used, the sign of the difference of the thermal expansion coefficients becomes inverted compared with the S-glass fibers. Theo- retically, a fiber pull-out can already be expected here Now, no graphite-like carbon is being used any more as sliding layer, but boron nitride(BN) instead. Will it react with the fiber and/or the matrix? We did not find any reac- tions between Nextel 440 fibers and the Bn layers. How- ever,any chemical reactions do take place between the Bn layers and the matrix glasses. Tiny bubbles of a size of a few nanometers were found in the interface between BN and the matrix(see Section 4.5, Fig. 18). They cause an internal light scattering. In combination with the Fig8. SEM-picture of fiber pull-out, composite Nextel 440-fiber/CVD. absorption of light by atomic boron(which resulted from Carbon/Duran glass matrix(35% fiber volume content) the chemical reactions), both effects will cause a reduction which can be transmitted between the fiber and the matrix the bN layer and t r er to prevent the reactions between when being under load conditions. These shear stresses with a second fiber coating, in our case TiO2, to protect it. may either support(when they are very low) or prevent Although not all transmission problems can be solved in (when they are high) the fiber pull-out. The method of this way, this measure will not only bring about a consid- measuring them in the interface between fiber and matrix erable improvement in the optical properties, but will also is described in detail by Kuntz [17]. The measurements influence the mechanical properties positively. Therefore, were made by Leutbecher together with Kuntz at the Uni- most of the composites were prepared using double-coated versity of Bremen. The schematic layout for the"push-in"(BN/TiO2) Nextel 440 fibers test can be seen from Fig 9. The shear stresses t measured Fig. 10 shows the stress-strain curves of composites between the matrix and the fibers are also represented in made of differently coated Nextel 440 fibers and 756 glass Figs. 5 and 6. As expected, t is generally higher for the matrix. In principle, these curves are similar to those repre composite S-glass fibers/756 glass matrix compared with sented by Fig. 5. However, instead of the C-coating, there the composites Nextel 440 fibers/Duran glass matrix. is a BN/TiO2 intermediate layer, which leads to a white Therefore, the fiber pull-out is more distinct in the latter composite(see also Fig. 4, down on the left(2). In com- parison with Fig. 5, the ultimate fracture strain is much higher,which is due to AF_M=+0.5x 10-6K-.The more the fibers contract during cooling after hot pressing compared with the matrix glass, the more clearly a gap will develop between them, which facilitates the sliding of the fibers in the matrix. The maximum stress sustained will not differ that greatly if C-coated S-glass fibers(Fig. 5)or BN/TiO2-coated Nextel 440 fibers are used( Fig. 10) Indenter As expected, the non-reinforced hot-pressed 756 matrix class(Fig. 10, curve 1)presents again-also in this new test-nearly the same brittle behavior as in Fig. 5. Its rein- forcement with desized Nextel 440 fibers(curve 2)results in a higher E-modulus and only a slight improvement strength. Due to the fact that there is no sliding lay Matrix and that a direct reaction takes place between the fib and the matrix, also this composite will become brittle If a fiber pull-out takes place, the exact position of the Fiber photograph of a broken composite. The fiber pull-out can clearly be recognized. Fig. 12 illustrates the transition between the fiber and the matrix. The fiber bN, TIO and also the matrix can clearly be distinguished, with ther flaws nor other defects being detected. The EDX anal- Fig.9. Principle of the push-in test [17] ysis carried out in well-chosen points on the surface of thewhich can be transmitted between the fiber and the matrix when being under load conditions. These shear stresses may either support (when they are very low) or prevent (when they are high) the fiber pull-out. The method of measuring them in the interface between fiber and matrix is described in detail by Kuntz [17]. The measurements were made by Leutbecher together with Kuntz at the Uni￾versity of Bremen. The schematic layout for the ‘‘push-in’’ test can be seen from Fig. 9. The shear stresses s measured between the matrix and the fibers are also represented in Figs. 5 and 6. As expected, s is generally higher for the composite S-glass fibers/756 glass matrix compared with the composites Nextel 440 fibers/Duran glass matrix. Therefore, the fiber pull-out is more distinct in the latter composite. 4.2. Mechanical properties of the Nextel 440 fibers/756 glass matrix composites If Nextel 440 fibers and 756 glass as matrix are used, the sign of the difference of the thermal expansion coefficients becomes inverted compared with the S-glass fibers. Theo￾retically, a fiber pull-out can already be expected here. Now, no graphite-like carbon is being used any more as sliding layer, but boron nitride (BN) instead. Will it react with the fiber and/or the matrix? We did not find any reac￾tions between Nextel 440 fibers and the BN layers. How￾ever, any chemical reactions do take place between the BN layers and the matrix glasses. Tiny bubbles of a size of a few nanometers were found in the interface between BN and the matrix (see Section 4.5, Fig. 18). They cause an internal light scattering. In combination with the absorption of light by atomic boron (which resulted from the chemical reactions), both effects will cause a reduction in transmission. In order to prevent the reactions between the BN layer and the matrix glasses, the BN was provided with a second fiber coating, in our case TiO2, to protect it. Although not all transmission problems can be solved in this way, this measure will not only bring about a consid￾erable improvement in the optical properties, but will also influence the mechanical properties positively. Therefore, most of the composites were prepared using double-coated (BN/TiO2) Nextel 440 fibers. Fig. 10 shows the stress–strain curves of composites made of differently coated Nextel 440 fibers and 756 glass matrix. In principle, these curves are similar to those repre￾sented by Fig. 5. However, instead of the C-coating, there is a BN/TiO2 intermediate layer, which leads to a white composite (see also Fig. 4, down on the left (2)). In com￾parison with Fig. 5, the ultimate fracture strain is much higher, which is due to DaF–M = +0.5 · 106 K1 . The more the fibers contract during cooling after hot pressing compared with the matrix glass, the more clearly a gap will develop between them, which facilitates the sliding of the fibers in the matrix. The maximum stress sustained will not differ that greatly if C-coated S-glass fibers (Fig. 5) or BN/TiO2-coated Nextel 440 fibers are used (Fig. 10). As expected, the non-reinforced hot-pressed 756 matrix glass (Fig. 10, curve 1) presents again – also in this new test – nearly the same brittle behavior as in Fig. 5. Its rein￾forcement with desized Nextel 440 fibers (curve 2) results in a higher E-modulus and only a slight improvement in strength. Due to the fact that there is no sliding layer and that a direct reaction takes place between the fiber and the matrix, also this composite will become brittle. If a fiber pull-out takes place, the exact position of the sliding plane will be important. Fig. 11 shows the REM photograph of a broken composite. The fiber pull-out can clearly be recognized. Fig. 12 illustrates the transition between the fiber and the matrix. The fiber, BN, TiO2 and also the matrix can clearly be distinguished, with nei￾ther flaws nor other defects being detected. The EDX anal￾ysis carried out in well-chosen points on the surface of the Fig. 8. SEM-picture of fiber pull-out, composite Nextel 440-fiber/CVD￾Carbon/Duran glass matrix (35% fiber volume content). Fig. 9. Principle of the push-in test [17]. 368 D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373