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 compositelatter 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 compos￾ites were characterized by excellent mechanical properties at room temperature and a good thermal stability. How￾ever, 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 produc￾ing transparent glass matrix composites [7,8]. The trans￾mission 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 com￾posites although the thermal expansion coefficients and the refractive indices were adapted. Due to the strong chemical bond between the fibers and the matrix, the com￾posites 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 prop￾erties of oxidic composites. They tested uncoated, SnO2- or BN-coated Nextel 480-fibers. Compared with non-rein￾forced 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 nano￾crystalline 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 com￾posites presenting three properties: they should be damage￾tolerant, stronger than compact glass, and translucent [10]. 2. Preconditions The following opposing parameters have to be optimized: – The improvement of fracture toughness by crack deflec￾tion, 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 inter￾nal cracks, flaws or bubbles. In order to achieve these contradicting aims, the follow￾ing prerequisites must be provided: – Both the fibers and the matrix must be oxidic and opti￾cally 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 tempera￾tures 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 nanocrys￾talline (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 pro￾cesses 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 environ￾mental 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 above￾mentioned 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 dur￾ing the preparation of the composite. D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373 363