esults CVD-Carbon(45nm, t=25MPa) 4.1. Experiments with C-coated fibers The tests and results obtained are described in greater detail in the doctoral thesis submitted by the co-author Leutbecher [13]. The composites are, of course, black and opaque. The aim of these experiments was to gain experi 55nm, t=34MPa ence with the generation of fiber pull-out and with the damage-tolerant behavior. Here, two composites are --- CVD-Carbon (25nm,= S-glass fibers/C-layer/756 glass matrix. Nextel 440 fibers/C-layer/ Duran glass matrix. Fig. 6. Stress-strain-curves of differently carbon coated Nextel 440-fibers in a Duran glass matrix In the case of the black composite, the refractive indices and their adaptation do not play any role at all. In both also show the brittle fracture behavior of pure(non-rein- cases. th Terence of the thermal expansion coefficients forced )hot-pressed matrix glass powder. One can clearly is A%Xp-M#0. One could expect that during cooling after recognize the typical damage-tolerant behavior when C- hot pressing, the 756 glass clamps the S-glass fibers, thus coated fibers are employed. The composites contain a preventing them from pulling out. The expansion difference portion of fibers ranging from 30 to 35 vol%.The more between the Nextel 440 fibers and Duran glass is just the homogeneous and dense the coatings on the fibers are, opposite, which could support the fiber pull-out. Surpris- the better their mechanical properties will be. Therefore, abi, we found fiber pull-out in both cases, which is prob- CVD-carbon layers bring about better results of the com- due to the high elasticity and the good sliding posite compared with C-layers made from Novolak. The behavior of the nanocrystalline graphite layers thickness of the carbon layer influences the stress-strain The mechanical properties of the composites were tested behavior in the way as can be seen from Fig. 6. Despite this through three-point bending in our laboratory. For this, fact. however, the absolute bending strength mainly ( this machine had been used as hot press before). The test ence of the thermal expansion coefficients and particularly The lowering speed of the punch was I mm/min, the force matrix) seem to cause this effect was measured by means of a 100 N-dial gauge. The sam Fig. 7 illustrates the fiber pull-out for the combination ples presented the following dimensions: length: 50 mm, of S-glass fiber/C-layer/756 glass matrix, whereas it width: 6mm, thickness: 2 mm, distance between the shown for the combination of Nextel 440 fiber/C-layer supports: 40 mm. For one measuring value, a total of 20 duran glass matrix in Fig 8 The different thermal expansion coefficients of the fibers were determined and matrix glasses exert an influence on the shear stress Fig. 5 shows the stress-strain curves for the S-glass/756 glass matrix composites, Fig. 6 illustrates this curve for the Nextel 440/Duran glass matrix composites. Both figures CVD-Carbon (25nm, t=78MPa) hot-pressed strain [% Fig. 5. Stress-strain-curves of differently carbon coated Sglass-fibers in a Fig. 7. SEM-picture of fiber pull-out, composite S-glass-fiber/CVD.4. Results 4.1. Experiments with C-coated fibers The tests and results obtained are described in greater detail in the doctoral thesis submitted by the co-author Leutbecher [13]. The composites are, of course, black and opaque. The aim of these experiments was to gain experi￾ence with the generation of fiber pull-out and with the damage-tolerant behavior. Here, two composites are discussed: – S-glass fibers/C-layer/756 glass matrix. – Nextel 440 fibers/C-layer/Duran glass matrix. In the case of the black composite, the refractive indices and their adaptation do not play any role at all. In both cases, the difference of the thermal expansion coefficients is DaF–M 6¼ 0. One could expect that during cooling after hot pressing, the 756 glass clamps the S-glass fibers, thus preventing them from pulling out. The expansion difference between the Nextel 440 fibers and Duran glass is just the opposite, which could support the fiber pull-out. Surpris￾ingly, we found fiber pull-out in both cases, which is prob￾ably due to the high elasticity and the good sliding behavior of the nanocrystalline graphite layers. The mechanical properties of the composites were tested through three-point bending in our laboratory. For this, the testing machine type INSTRON 4467 was used again (this machine had been used as hot press before). The test procedure is based on the DIN ENV 658-3 standard [16]. The lowering speed of the punch was 1 mm/min, the force was measured by means of a 100 N-dial gauge. The sam￾ples presented the following dimensions: length: 50 mm, width: 6 mm, thickness: 2 mm, distance between the supports: 40 mm. For one measuring value, a total of 20 samples were tested. Then, the average and the variability were determined. Fig. 5 shows the stress–strain curves for the S-glass/756 glass matrix composites, Fig. 6 illustrates this curve for the Nextel 440/Duran glass matrix composites. Both figures also show the brittle fracture behavior of pure (non-rein￾forced) hot-pressed matrix glass powder. One can clearly recognize the typical damage-tolerant behavior when C￾coated fibers are employed. The composites contain a portion of fibers ranging from 30 to 35 vol%. The more homogeneous and dense the coatings on the fibers are, the better their mechanical properties will be. Therefore, CVD-carbon layers bring about better results of the com￾posite compared with C-layers made from Novolak. The thickness of the carbon layer influences the stress–strain behavior in the way as can be seen from Fig. 6. Despite this fact, however, the absolute bending strength mainly depends on the fibers and matrix glasses used. The differ￾ence of the thermal expansion coefficients and particularly their signs (clamping of the S-glass fiber by the 756 glass matrix) seem to cause this effect. Fig. 7 illustrates the fiber pull-out for the combination of S-glass fiber/C-layer/756 glass matrix, whereas it is shown for the combination of Nextel 440 fiber/C-layer/ Duran glass matrix in Fig. 8. The different thermal expansion coefficients of the fibers and matrix glasses exert an influence on the shear stress Fig. 5. Stress–strain-curves of differently carbon coated S-glass-fibers in a 756 glass matrix. Fig. 6. Stress–strain-curves of differently carbon coated Nextel 440-fibers in a Duran glass matrix. Fig. 7. SEM-picture of fiber pull-out, composite S-glass-fiber/CVD￾Carbon/756 glass matrix. D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373 367