Availableonlineatwww.sciencedirect.com °" Science Direct c000e5 Part B: engineering ELSEVIER Composites: Part B 39(2008)362-373 www.elsevier.com/locate/compositesb Damage tolerant, translucent oxide fiber/glass matrix composites Dagmar Hulsenberg", Peer Fehling, Thomas Leutbecher TU Ilmenau, Department of Glass and Ceramics Technology, P.O. Box 100565, 98684 Ilmenau, German Received 21 July 2006: accepted 16 January 2007 Available online 26 January 2007 Abstract Starting out from damage-tolerant oxidic composites with carbon (C)-coated fibers, the main part of our investigations was concentrated on Nextel 440-fibers/glass matrix composites. The matrix glasses were chosen in consideration of suitable thermal expan- sion coefficients and optical refractive indices. The fiber pull-out is enabled by a CVd coating with boron nitride(BN). In comparison with a single BN layer, a BN /TiO -double layer yields better mechanical and optical properties. All composites are made by means of a traditional lab method including hot pressing. Varying differences of the thermal expansion coefficients of fibers and matrix glasses(minus, zero, plus) allow the preparation of composites presenting various mechanical properties and also different damage tolerances. A composite, which is both damage-tolerant and translucent at one and the same time requires an identical thermal expansion coefficient and, in addition, an identical optical efractive index of the fibers and the matrix. Bubbles must be avoided in any case. e 2007 Elsevier Ltd. All rights reserved Keywords: A. Glass fibres; B. Damage tolerance; B. Optical properties/techniques: E. Heat treatment; Glass-matrix composites 1. Introduction the load transported by the fiber will be. By applying a suit ible intermediate layer between the fibers and the glass lass is characterized by some very positive properties matrix (i.e an appropriate coating of the fibers), the crack such as its transparency in the visible wavelength range, propagation as well as the fracture toughness can advanta- its high electrical resistance and adaptable thermal expan- geously be influenced. As soon as a crack develops in the sion coefficient. However, its application is limited in par- matrix, the coated fiber will either divert or bridge it. From ticular because of its low crack resistance, the sudden a certain limit load on, the glass matrix will present multi- crack propagation and the resulting catastrophic fracture. crack growth. If the load increases further, the fibers will be Different attempts were made to overcome these problems. extended even more, thus entailing a relative movement The mechanical strength and the fracture toughness of between the fibers and the matrix. This, in turn, will cause act glass products can be improved by its reinforcing the fiber debonding and a fiber pull-out. When the loading rs. The material will not become ductile, but que has reached the ultimate strength of the fibers, the compos- ite will definitely br If a fiber-reinforced glass matrix is exposed to stress, the If the bond between the fibers and the matrix is so load will be transferred to the fibers so that the brittle glass strong that the fibers cannot be pulled out, the composite matrix is relieved. The degree of load transfer depends on will remain brittle. This depends- among other factors both E-moduluses and on the fiber volume content of the also on the properties of the fiber coating, whether chemi composite. The higher the ratio Efiber/Ematrix is, the higher cal reactions take place between the constituents, whether surface roughness brought about by nanocrystals prevents Corresponding adua or. TeL: +493677692801: fax: +49 3677691436. the fibers from sliding in the glass matrix, or whether the de(D.Holsenberg). desired pulling-out of the fibers really takes place. The 1359-8368/S.see front matter e 2007 Elsevier Ltd. All rights reserved doi:10.1016/j.compositesb.2007.01.00
Damage tolerant, translucent oxide fiber/glass matrix composites Dagmar Hu¨lsenberg *, Peer Fehling, Thomas Leutbecher TU Ilmenau, Department of Glass and Ceramics Technology, P.O. Box 100565, 98684 Ilmenau, Germany Received 21 July 2006; accepted 16 January 2007 Available online 26 January 2007 Abstract Starting out from damage-tolerant oxidic composites with carbon (C)-coated fibers, the main part of our investigations was concentrated on Nextel 440-fibers/glass matrix composites. The matrix glasses were chosen in consideration of suitable thermal expansion coefficients and optical refractive indices. The fiber pull-out is enabled by a CVD coating with boron nitride (BN). In comparison with a single BN layer, a BN/TiO2-double layer yields better mechanical and optical properties. All composites are made by means of a traditional lab method including hot pressing. Varying differences of the thermal expansion coefficients of fibers and matrix glasses (minus, zero, plus) allow the preparation of composites presenting various mechanical properties and also different damage tolerances. A composite, which is both damage-tolerant and translucent at one and the same time requires an identical thermal expansion coefficient and, in addition, an identical optical refractive index of the fibers and the matrix. Bubbles must be avoided in any case. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Glass fibres; B. Damage tolerance; B. Optical properties/techniques; E. Heat treatment; Glass-matrix composites 1. Introduction Glass is characterized by some very positive properties such as its transparency in the visible wavelength range, its high electrical resistance and adaptable thermal expansion coefficient. However, its application is limited in particular because of its low crack resistance, the sudden crack propagation and the resulting catastrophic fracture. Different attempts were made to overcome these problems. The mechanical strength and the fracture toughness of compact glass products can be improved by its reinforcing by fibers. The material will not become ductile, but quasiductile [1–3]. If a fiber-reinforced glass matrix is exposed to stress, the load will be transferred to the fibers so that the brittle glass matrix is relieved. The degree of load transfer depends on both E-moduluses and on the fiber volume content of the composite. The higher the ratio Efiber/Ematrix is, the higher the load transported by the fiber will be. By applying a suitable intermediate layer between the fibers and the glass matrix (i.e., an appropriate coating of the fibers), the crack propagation as well as the fracture toughness can advantageously be influenced. As soon as a crack develops in the matrix, the coated fiber will either divert or bridge it. From a certain limit load on, the glass matrix will present multicrack growth. If the load increases further, the fibers will be extended even more, thus entailing a relative movement between the fibers and the matrix. This, in turn, will cause the fiber debonding and a fiber pull-out. When the loading has reached the ultimate strength of the fibers, the composite will definitely break. If the bond between the fibers and the matrix is so strong that the fibers cannot be pulled out, the composite will remain brittle. This depends – among other factors – also on the properties of the fiber coating, whether chemical reactions take place between the constituents, whether surface roughness brought about by nanocrystals prevents the fibers from sliding in the glass matrix, or whether the desired pulling-out of the fibers really takes place. The 1359-8368/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2007.01.002 * Corresponding author. Tel.: +49 3677 692801; fax: +49 3677 691436. E-mail address: dagmar.huelsenberg@tu-ilmenau.de (D. Hu¨lsenberg). www.elsevier.com/locate/compositesb Available online at www.sciencedirect.com Composites: Part B 39 (2008) 362–373
D. Hiilsenberg et al/ Composites: Part B 39(2008)362-373 latter will lead to a controlled, slow breaking, i.e., damage between the fibers and the matrix. All sorts of chemical tolerance. Consequently, it is necessary to evaluate the interaction, force- or form-fit connection must be properties of the interface, in particular the shear strength prevented so as to be able to optimize the bond between the fibers and In contrast to this, the optical transparency requires to the matrix have as few disturbing interfaces as possible, so no inter- Earlier studies and publications concentrated only on nal cracks. flaws or bubbles this aspect of damage tolerance [4-6. Carbon or Sic-fibers with C-coating were used. The coating similar to graphite In order to achieve these contradicting aims, the follow allowed the fibers to slide in the glass matrix. The compos- ing prerequisites must be provided ites were characterized by excellent mechanical properties at room temperature and a good thermal stability. How Both the fibers and the matrix must be oxidic and opti- ever, they were black. The transparency of the matrix glass cally transpar or the nanocrystalline glass ceramic matrix was not The covering layers must also be optically transparent adopted by the composite Both the fibers and the matrix must present absolutely Japanese authors investigated the possibility of produc- identical optical refractive index. ing transparent glass matrix composites [7, 8]. The trans-- The thermal expansion coefficients of both the fibers and mission of the inspected oxynitride fibers/glass matrix the matrix must be adapted composites amounted to 40% in the visible wavelength The fibers shall soften at considerably higher tempera range, with the fiber volume portion being only 7%. It tures than the matrix glass was not possible to explain the negative influence exerted The E-modulus of the fibers must be clearly larger than that of the ma chemical bond between the fibers and the matrix. the com osites showed catastrophic fracture. No comment was The fibers may be amorphous(like S-glass) or nanocrys- nade by the authors concerning the interface between the talline(without influence on the transparency like Nextel fibers and the matrix Other authors [9] investigated the influence exerted by The fiber coating should fulfill the following functions different transparent fiber coatings on the mechanical prop- erties of oxidic composites. They tested uncoated, SnOx-or 1 It should make sure that no chemical interactions BN-coated Nextel 480-fibers. Compared with non-rein between the fibers and the matrix due to diffusion pro- forced matrix glass, only the BN-coated Nextel 480-fiber cesses or other reactions may take place was able to increase the strength of the material. Neither 2. It should form a smooth layer without any disturbing in the composite with the uncoated fibers nor in the one crystallizations so as to allow the fibers to slide with the SnOz-coated fibers, the fibers pulled out. The 3. The transparency in the visible wavelength range should pull-out was only observed in the case of BN coating. be guaranteed The SnO2 layer caused a strong bond with the matrix. 4. It should be thermally resistant and resistant to environ- Although SnO2 does not react with the Al_O3 of the Nextel mental influences even in an oxidizing atmosphere 480-fibers, it reacts very intensely with the Sio of the glass matrix instead. The fact that the BN-coated fibers slide As the thickness of the covering layers is far below the within the glass matrix may be due to the plate-like nano- wavelength of the visible light, the optical refractive index crystalline structure of the BN layers. Surprisingly, the does not play a crucial role cited authors did not test other coatings like, for example, TiO2 or ZrOz. One could possibly expect that they react 3 Experiments less intensely with the glass matrix, compared with Sno The aim of our own research work was to investigate com- 3. 1. Overview of the constituents tested posites presenting three properties: they should be damage tolerant, stronger than compact glass, and translucent [1 Two different types of fibers were chosen, the above mentioned S-glass fiber and the Nextel 440 fiber. Their 2. Preconditions composition and properties are shown in Table 1 In order to study some certain combinations of fibers The following opposing parameters have to be and matrix glasses with adapted and non-adapted thermal optimize expansion coefficients and refractive indices, four matrix glasses were used. Table 2 represents their composition The improvement of fracture toughness by crack deflec- and properties. Glass type 8650 contains a big quantity tion, the debonding and pulling out of the fibers from of lead oxide, therefore requiring oxidizing conditions dur the matrix can only be achieved through a soft interface ing the preparation of the composite
latter will lead to a controlled, slow breaking, i.e., damage tolerance. Consequently, it is necessary to evaluate the properties of the interface, in particular the shear strength, so as to be able to optimize the bond between the fibers and the matrix. Earlier studies and publications concentrated only on this aspect of damage tolerance [4–6]. Carbon or SiC-fibers with C-coating were used. The coating similar to graphite allowed the fibers to slide in the glass matrix. The composites were characterized by excellent mechanical properties at room temperature and a good thermal stability. However, they were black. The transparency of the matrix glass or the nanocrystalline glass ceramic matrix was not adopted by the composite. Japanese authors investigated the possibility of producing transparent glass matrix composites [7,8]. The transmission of the inspected oxynitride fibers/glass matrix composites amounted to 40% in the visible wavelength range, with the fiber volume portion being only 7%. It was not possible to explain the negative influence exerted by the constituents used on the transparency of the composites although the thermal expansion coefficients and the refractive indices were adapted. Due to the strong chemical bond between the fibers and the matrix, the composites showed catastrophic fracture. No comment was made by the authors concerning the interface between the fibers and the matrix. Other authors [9] investigated the influence exerted by different transparent fiber coatings on the mechanical properties of oxidic composites. They tested uncoated, SnO2- or BN-coated Nextel 480-fibers. Compared with non-reinforced matrix glass, only the BN-coated Nextel 480-fiber was able to increase the strength of the material. Neither in the composite with the uncoated fibers nor in the one with the SnO2-coated fibers, the fibers pulled out. The pull-out was only observed in the case of BN coating. The SnO2 layer caused a strong bond with the matrix. Although SnO2 does not react with the Al2O3 of the Nextel 480-fibers, it reacts very intensely with the SiO2 of the glass matrix instead. The fact that the BN-coated fibers slide within the glass matrix may be due to the plate-like nanocrystalline structure of the BN layers. Surprisingly, the cited authors did not test other coatings like, for example, TiO2 or ZrO2. One could possibly expect that they react less intensely with the glass matrix, compared with SnO2. The aim of our own research work was to investigate composites presenting three properties: they should be damagetolerant, stronger than compact glass, and translucent [10]. 2. Preconditions The following opposing parameters have to be optimized: – The improvement of fracture toughness by crack deflection, the debonding and pulling out of the fibers from the matrix can only be achieved through a soft interface between the fibers and the matrix. All sorts of chemical interaction, force- or form-fit connection must be prevented. – In contrast to this, the optical transparency requires to have as few disturbing interfaces as possible, so no internal cracks, flaws or bubbles. In order to achieve these contradicting aims, the following prerequisites must be provided: – Both the fibers and the matrix must be oxidic and optically transparent. – The covering layers must also be optically transparent. – Both the fibers and the matrix must present absolutely identical optical refractive index. – The thermal expansion coefficients of both the fibers and the matrix must be adapted. – The fibers shall soften at considerably higher temperatures than the matrix glass. – The E-modulus of the fibers must be clearly larger than that of the matrix. – While joining the fibers and the matrix, no bubbles, grain boundaries or cracks may be left. The fibers may be amorphous (like S-glass) or nanocrystalline (without influence on the transparency like Nextel 440). The fiber coating should fulfill the following functions: 1. It should make sure that no chemical interactions between the fibers and the matrix due to diffusion processes or other reactions may take place. 2. It should form a smooth layer without any disturbing crystallizations so as to allow the fibers to slide. 3. The transparency in the visible wavelength range should be guaranteed. 4. It should be thermally resistant and resistant to environmental influences even in an oxidizing atmosphere. As the thickness of the covering layers is far below the wavelength of the visible light, the optical refractive index does not play a crucial role. 3. Experiments 3.1. Overview of the constituents tested Two different types of fibers were chosen, the abovementioned S-glass fiber and the Nextel 440 fiber. Their composition and properties are shown in Table 1. In order to study some certain combinations of fibers and matrix glasses with adapted and non-adapted thermal expansion coefficients and refractive indices, four matrix glasses were used. Table 2 represents their composition and properties. Glass type 8650 contains a big quantity of lead oxide, therefore requiring oxidizing conditions during the preparation of the composite. D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373 363
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 top
The intermediate layers investigated were pyrolytic carbon (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 fellow 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 temperatures 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 cassiterite 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 Nextel 440 fibers after a 5 h pre-treatment at different temperatures 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 influence the tensile strength (cf. Table 1). Only after exposing the composite to high temperatures for another 5 h, the tensile 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, attention has to be drawn to the fact that Nextel 440 is a nanocrystalline 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 spherical 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 schematically 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
4000 Nextel 440 fiber 0100200300400500600700800900100011001200130014001500 Fig. 1. Single fiber tensile strength at room temperature of differently pre-treated(5 h in air, different temperatures)S-glass-and Nextel 440-fibers. 40nm BN+110nmTIo2 40nm BN+40nmTIO2 0onm BN+40 nmT:o2 desized 40nm BN+110nmT:O2 desized 1500 2000 2500 Fig. 2. Single fiber tensile strength of differently coated Nextel 440-fibers, not pre-treated and 5 h heated up to 750C[14] left-hand corner and is continued clockwise. The starting blowing air and then pulled by rolls through a slurry bath. materials(at the top, left-hand side) are, on the one hand, The slurry consists only of the glass powder and distilled differently prepared rovings(bundles of differently coated water. During this transport process, the powder starts fibers) wound on coils. On the other hand, pulverized sticking to the surface of the fibers. In this way, the rovings matrix glass is employed. Then, the rovings are infiltrated transport the matrix glass powder out of the slurry by by the glass powder(at the top, in the middle). For this, adhesion. Then, they are reeled up in parallel on a capstan the fibers of the rovings are separated from each other by The resulting layers are now cut and removed from the
left-hand corner and is continued clockwise. The starting materials (at the top, left-hand side) are, on the one hand, differently prepared rovings (bundles of differently coated fibers) wound on coils. On the other hand, pulverized matrix glass is employed. Then, the rovings are infiltrated by the glass powder (at the top, in the middle). For this, the fibers of the rovings are separated from each other by blowing air and then pulled by rolls through a slurry bath. The slurry consists only of the glass powder and distilled water. During this transport process, the powder starts sticking to the surface of the fibers. In this way, the rovings transport the matrix glass powder out of the slurry by adhesion. Then, they are reeled up in parallel on a capstan. The resulting layers are now cut and removed from the Fig. 1. Single fiber tensile strength at room temperature of differently pre-treated (5 h in air, different temperatures) S-glass- and Nextel 440-fibers. Fig. 2. Single fiber tensile strength of differently coated Nextel 440-fibers, not pre-treated and 5 h heated up to 750 C [14]. D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373 365
Composites: Part B 39(2008)362-373 170nm 2,3Hm 207nm 2,3μm Coated fiber After thermal annealing. Fig 3. Topography of the Nextel 440-fiber, coated with 30 nm BN and 30 nm TiO2 [14(AFM picture). Fig. 4. Principle of composites preparation surface of the capstan. Next, the disks are stacked and Table 3 middle)shows a vacuum furnace to provide reducing or Fiber dirction bonlcar' es preparation dried(Fig. 4, on the right). Hot pressing is carried out in an INSTRON installation. Fig. 4(at the bottom, in the doo of the matrix glass <70um 8-35% also neutral atmosphere during hot pressing. For the Slurry Deionized water without additives matrix glass 8650, which requires oxidizing atmospherePressure during hot pressing, an additional furnace(not shown here) Temperature is available which can be inserted in the hot press. In Fig4 N-SK4-matrix 850°C (down on the left), several composite disks are shown: In Duran-matrix 850°C the middle, a black one(1) with C-coated fibers, then two white ones(2) whose optical and thermal properties of 8650-matrix the fibers and the matrix glass are not adapted, and four Atmosphere translucent disks(3)with differently -in any case better N-SK4-matrix roperties Table 3 shows an overview of the manufacturing param 8650matrix eters employed. The hot pressing temperature is mainly influenced by the transformation temperature Tg of results presented in this paper were all obtained in tests car matrix glass. The pressure varied from 5 to 15 MPa ried a pressure of 5 MPa
surface of the capstan. Next, the disks are stacked and dried (Fig. 4, on the right). Hot pressing is carried out in an INSTRON installation. Fig. 4 (at the bottom, in the middle) shows a vacuum furnace to provide reducing or also neutral atmosphere during hot pressing. For the matrix glass 8650, which requires oxidizing atmosphere during hot pressing, an additional furnace (not shown here) is available which can be inserted in the hot press. In Fig. 4 (down on the left), several composite disks are shown: In the middle, a black one (1) with C-coated fibers, then two white ones (2) whose optical and thermal properties of the fibers and the matrix glass are not adapted, and four translucent disks (3) with differently – in any case better adapted – properties. Table 3 shows an overview of the manufacturing parameters employed. The hot pressing temperature is mainly influenced by the transformation temperature Tg of the matrix glass. The pressure varied from 5 to 15 MPa. The results presented in this paper were all obtained in tests carried out at a pressure of 5 MPa. Fig. 3. Topography of the Nextel 440-fiber, coated with 30 nm BN and 30 nm TiO2 [14] (AFM picture). Fig. 4. Principle of composites preparation. Table 3 Parameters of the composites preparation d90 of the matrix glass <70 lm Fiber volume content 8–35% Fiber direction Unidirectional Slurry Deionized water without additives Pressure 5 Mpa Temperature N-SK 4-matrix 850 C Duran-matrix 850 C 756-matrix 700 C 8650-matrix 600 C Atmosphere N-SK 4-matrix Vacuum Duran-matrix Vacuum 756-matrix Vacuum 8650-matrix Oxidizing 366 D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373