D. Hilsenberg et al/Composites: Part B 39(2008)362-373 Table I could possibly be avoided by applying a microwave Composition and properties of used fibers(specification by the producers) assisted CVD at reaction temperatures of around 500oC Dimension Nextel 440 This could be a task to be solved in the future The Sno, layers prepared presented relatively big cassit erite crystals which -together with certain reactions taking place with the glass matrix prevented the coated fibers %%K from sliding along within the matrix when stress was exerted. The composites turned brittle and broke, as other 2.8 authors had already stated earlier [9]. Therefore, research work with SnO -coated fibers was stopped Refractive index n 1.523 Transformation temperature, Tg 3.2. Single fiber tensile strength Tensile strength MPa 4580 2070 E-modulus The tensile strength of the single fibers (as received Fiber diameter desized and coated) was measured at room temperature Before the measurement was made. some of the fibers were exposed to different temperatures in order to simulate the Table 2 and properties of the used matrix glasses(specifications by conditions prevailing during CVD coating and hot pressing of the composites Fig. I shows the results of the delivered S-glass and Ne Dimension Duran 756 N-SK 4 tel 440 fibers after a 5 h pre-treatment at different temper- Producer Schott Telux SchottSchott atures in air. Although the tensile strength of the S-glass 63.035.2 35.4 fibers in their original state is very high, it decreases so mass% 3.1 8.0 4.6 3.8 strongly, already after a pre-treatment at 500C, that it 134 falls below that of the Nextel 440 fibers. In the tensile strength of the Nextel 440 fibers remains nearly unchanged in the interesting temperature range of the hot g of the composites(up to 850C)and the previous K,O CVD coating(at 900C). Therefore, this type of fiber is 43.8 well suitable for the experiments Cs,O Fig. 2 shows an overview of the tensile strengths of the Thermal 10-K differently coated Nextel 440 fibers measured. The thin Refractive index n 143 1491.613 1.6 black lines illustrate how the measuring values vary around 500658 475 the averages. From the results, the conclusion can be drawn that the coating process alone does not remarkably influ- MPa 805257 ence the tensile strength(cf. Table 1). Only after exposing the composite to high temperatures for another 5 h, the ten- sile strength will decrease. However, some differences can be seen here as well. A statistically solid fact is that heating to 6 The intermediate layers investigated were pyrolytic car- 750 C over 5 h reduces the tensile strength only in the case on(for comparison purposes), SnO,(cassiterite crystals), of the TiO2-and BN-single layers Double BN/TiO2 coated TiO2(anatase nanocrystals), BN(turbostratic) and BN/ fibers will not lose the tensile strength considerably TiO2 double layers. Fig. 3 shows the surface of a coated Nextel 440 fiber The carbon coating was effected either by the conversion without any heating at all and after reheating. Here, atten- of novolak into amorphous carbon or by the chemical tion has to be drawn to the fact that Nextel 440 is a nano- vapour deposition(CVD). All other layers were prepared crystalline fiber [13]. The thin(30 nm BN+ 30 nm TiO2 by CVD. The authors wish to thank Prof. Marx and his fel- surface layer(Fig 3, left-hand side)allows the crystals of low workers from the TU Chemnitz, Institute of Physical the fibers to be imaged almost directly. They have a spher- Chemistry, for preparing the CVD layers [11, 12]. Before ical shape, thus generating a smooth surface, which allows coating, the fibers were thermally desized(over 3 h at a the fiber to slide. During the thermal sourcing out( Fig. 3, temperature of 500C). As will be shown later, the strength right-hand side), the crystals grow slightly. Nevertheless of the S-glass fibers is strongly decreasing with increasing their topographical structure will not be destroyed temperature. However, coating with BN requires tempera- tures of about 900C. As such temperatures are too high 3.3. Preparation of the test samples for S-glass fibers, the following investigations are mainly concentrated on composites containing Nextel 440-fibers. The method applied was taken from [15]and is sch The problem of the temperature stability of S-glass fibers ically illustrated in Fig. 4. The description begins in the topThe intermediate layers investigated were pyrolytic car￾bon (for comparison purposes), SnO2 (cassiterite crystals), TiO2 (anatase nanocrystals), BN (turbostratic) and BN/ TiO2 double layers. The carbon coating was effected either by the conversion of novolak into amorphous carbon or by the chemical vapour deposition (CVD). All other layers were prepared by CVD. The authors wish to thank Prof. Marx and his fel￾low workers from the TU Chemnitz, Institute of Physical Chemistry, for preparing the CVD layers [11,12]. Before coating, the fibers were thermally desized (over 3 h at a temperature of 500 C). As will be shown later, the strength of the S-glass fibers is strongly decreasing with increasing temperature. However, coating with BN requires tempera￾tures of about 900 C. As such temperatures are too high for S-glass fibers, the following investigations are mainly concentrated on composites containing Nextel 440-fibers. The problem of the temperature stability of S-glass fibers could possibly be avoided by applying a microwave assisted CVD at reaction temperatures of around 500 C. This could be a task to be solved in the future. The SnO2 layers prepared presented relatively big cassit￾erite crystals which – together with certain reactions taking place with the glass matrix – prevented the coated fibers from sliding along within the matrix when stress was exerted. The composites turned brittle and broke, as other authors had already stated earlier [9]. Therefore, research work with SnO2-coated fibers was stopped. 3.2. Single fiber tensile strength The tensile strength of the single fibers (as received, desized and coated) was measured at room temperature. Before the measurement was made, some of the fibers were exposed to different temperatures in order to simulate the conditions prevailing during CVD coating and hot pressing of the composites. Fig. 1 shows the results of the delivered S-glass and Nex￾tel 440 fibers after a 5 h pre-treatment at different temper￾atures in air. Although the tensile strength of the S-glass fibers in their original state is very high, it decreases so strongly, already after a pre-treatment at 500 C, that it falls below that of the Nextel 440 fibers. In comparison, the tensile strength of the Nextel 440 fibers remains nearly unchanged in the interesting temperature range of the hot pressing of the composites (up to 850 C) and the previous CVD coating (at 900 C). Therefore, this type of fiber is very well suitable for the experiments. Fig. 2 shows an overview of the tensile strengths of the differently coated Nextel 440 fibers measured. The thin black lines illustrate how the measuring values vary around the averages. From the results, the conclusion can be drawn that the coating process alone does not remarkably influ￾ence the tensile strength (cf. Table 1). Only after exposing the composite to high temperatures for another 5 h, the ten￾sile strength will decrease. However, some differences can be seen here as well. A statistically solid fact is that heating to 750 C over 5 h reduces the tensile strength only in the case of the TiO2- and BN-single layers. Double BN/TiO2 coated fibers will not lose the tensile strength considerably. Fig. 3 shows the surface of a coated Nextel 440 fiber without any heating at all and after reheating. Here, atten￾tion has to be drawn to the fact that Nextel 440 is a nano￾crystalline fiber [13]. The thin (30 nm BN + 30 nm TiO2) surface layer (Fig. 3, left-hand side) allows the crystals of the fibers to be imaged almost directly. They have a spher￾ical shape, thus generating a smooth surface, which allows the fiber to slide. During the thermal sourcing out (Fig. 3, right-hand side), the crystals grow slightly. Nevertheless, their topographical structure will not be destroyed. 3.3. Preparation of the test samples The method applied was taken from [15] and is schemat￾ically illustrated in Fig. 4. The description begins in the top Table 1 Composition and properties of used fibers (specification by the producers) Dimension S-glass Nextel 440 Producer – Owens-Corning 3W SiO2 mass% 65 28 Al2O3 mass% 25 70 B2O3 mass% – 2 MgO mass% 10 – Thermal expansion coefficient, a 106 K1 2.8 5.3 Refractive index, n – 1.523 1.616 Transformation temperature, Tg C 816 – Tensile strength MPa 4580 2070 E-modulus GPa 87 186 Fiber diameter lm 10 11.6 Table 2 Compositions and properties of the used matrix glasses (specifications by the producers) Dimension Duran 756 N-SK 4 8650 Producer – Schott Telux Schott Schott SiO2 mass% 79.7 63.0 35.2 35.4 Al2O3 mass% 3.1 8.0 4.6 3.8 B2O3 mass% 10.3 20.0 12.2 13.4 MgO mass% 0.9 – – – CaO mass% 0.8 – – – BaO mass% 3.7 2.5 46.9 – Na2O mass% 5.2 3.5 – – K2O mass% – 3.0 – – PbO mass% – – – 43.8 Cs2O mass% – – – 3.55 Thermal expansion coefficient, a 106 K1 3.3 4.8 7.4 5.1 Refractive index, n – 1.473 1.49 1.613 1.61 Transformation temperature, Tg C 530 500 658 475 Bending strength MPa 80 52 57 50 E-modulus GPa 63 45 84 58 364 D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373