D. Raab er al./ Materials Science and Engineering A 417(2006)341-347 Fig8. SEM micrographs of fracture surfaces in biaxial flexure test: borosilicate glass/Zen Tron M continuous fiber composites containing desized fibers(left)and 70 nm tin oxide-coated fibers(night) increase of fracture toughness or flexure strength in both contint ous and short fiber reinforced composites. No crack deflection at the matrix-tin dioxide-fi ber interfaces. neither indication ofother toughening mechanisms, such as fiber debonding or pull-out were found. The decrease of fiber strength during CVD coating and the possible diffusion of Sn-ions during hot pressing could 30μm be responsible for the relatively poor mechanical performance of these composite Fig9. Laser scanning micrograph of a Vickers'microindentation on a borosil- Acknowledgements ate glass/ZenTron M continuous fiber composite with 70 nm tin oxide-coated The authors thank Mrs. J.A. Roether(Imperial College Lon- don) for revising the manuscript. The work was supported by Investigating fracture surfaces by SEM, no feature could the German Research Foundation(FG). The authors are grate- be discerned which could be linked with a mechanism acting ful to TELUX WeiBwasser(Germany) for providing the matrix to enhance the fracture toughness in the present composites glass. Thanks are also due to AGY Europe for the Zen TronM (Fig 8). Besides the clearly visible rings around fibers represent- glass fiber samples ing the tin oxide coating in Fig. 8 (right ), there are no apparent differences between the two fracture surfaces. Moreover, neither References significant crack deflection was detected at fiber-matrix inter faces, nor indication of fiber debonding or pull-out could be [1]AR. Boccaccini, R D. Rawlings, Glass Technol. 43C(2002)191-201 observed 2]K M. Prewo, J.J. Brennan, G K. Layden, Am. Ceram. Soc. Bull. 6: These sEM observations were confirmed by Vickers (1986)305-313. microindentation tests. As shown in Fig9 cracks starting from [4]W. Beier, Faserverstarkte Glaser, 44th Intemational Scientific Collo- the corners of the Vickers'impression were not deflected at matrix-tin dioxide-fiber interfaces. It was found that cracks [5].R. Boccaccini, in: N.P. Bansal(Ed ) Handbook of Ceramic Compos- propagate unimpeded through matrix, interfaces and fibers ites, Kluwer Academic Publishers, New York, 2005, p. 461(Chapter demonstrating that the composites would fail in a brittle manner, [6]IW. Donald, Key Eng Mater. 108-110(1995)123-144 with little or no crack deflection or fiber pull-out [ D. Hulsenberg, P. Fehling, H. Kem, D. Raab, T. Mache, G. Marx, K. Weise, A.R. Boccaccini, Mater. Congr. (2002) 4. Conclusions []D. Hulsenberg, P. Fehling, T. Leutbecher, D Raab, Machine Dyn. Prob- Turbostratic boron nitride CVD-coated Nextel TM 440 fibers are suitable reinforcements for borosilicate glass matrix compos- [10] H. Iba, T. Chang, Y. Kagawa, H. Minakuchi, K. Kanamaru, J. Am ites. Thermal degradation of the fiber and changing of surfac roughness due to the C VD coating process were found to be neg. [II]B. Fankanel, E. Muller, K. Weise, G. Marx, Key Eng. Mater. 206-213 (2002)11091112. igible. The composites exhibited enhanced fracture toughness [12] A.R. Boccaccini,S Atiq, G. Helsch, Comp. Sci. Technol. 63(2003) in comparison to the unreinforced matrix, especially in the case 779-783. of continuous fiber reinforcement Enhanced fracture strength [13] H. Iba, T. Naganuma, K. Matsumura, Y. Kagawa, J. Mater. Sci for continuous and short fiber reinforcement was confirmed (1999)5701-5705 in three-point bending strength tests. However, no significant [14R.J Kerans, R.S. Hay, T.A. Parthasarathy, M. Cinibulk, J. Am. Cerar Soe.85(1l)(2002)2599-2632. increase of fracture strength in both types of composites was [15]AMaheshwari, K KChawla, TA.Michalske, Mater. Sci.Eng.A observed in biaxial flexure strength tests (1989)269276.346 D. Raab et al. / Materials Science and Engineering A 417 (2006) 341–347 Fig. 8. SEM micrographs of fracture surfaces in biaxial flexure test: borosilicate glass/ZenTronTM continuous fiber composites containing desized fibers (left) and 70 nm tin oxide-coated fibers (right). Fig. 9. Laser scanning micrograph of a Vickers’ microindentation on a borosil￾icate glass/ZenTronTM continuous fiber composite with 70 nm tin oxide-coated fibers. Investigating fracture surfaces by SEM, no feature could be discerned which could be linked with a mechanism acting to enhance the fracture toughness in the present composites (Fig. 8). Besides the clearly visible rings around fibers represent￾ing the tin oxide coating in Fig. 8 (right), there are no apparent differences between the two fracture surfaces. Moreover, neither significant crack deflection was detected at fiber–matrix inter￾faces, nor indication of fiber debonding or pull-out could be observed. These SEM observations were confirmed by Vickers’ microindentation tests. As shown in Fig. 9 cracks starting from the corners of the Vickers’ impression were not deflected at matrix-tin dioxide–fiber interfaces. It was found that cracks propagate unimpeded through matrix, interfaces and fibers, demonstrating that the composites would fail in a brittle manner, with little or no crack deflection or fiber pull-out. 4. Conclusions Turbostratic boron nitride CVD-coated NextelTM 440 fibers are suitable reinforcements for borosilicate glass matrix compos￾ites. Thermal degradation of the fiber and changing of surface roughness due to the CVD coating process were found to be neg￾ligible. The composites exhibited enhanced fracture toughness in comparison to the unreinforced matrix, especially in the case of continuous fiber reinforcement. Enhanced fracture strength for continuous and short fiber reinforcement was confirmed in three-point bending strength tests. However, no significant increase of fracture strength in both types of composites was observed in biaxial flexure strength tests. Tin oxide coating on ZenTronTM glass fibers does not bring an increase of fracture toughness or flexure strength in both continu￾ous and short fiber reinforced composites. No crack deflection at the matrix-tin dioxide-fiber interfaces, neither indication of other toughening mechanisms, such as fiber debonding or pull-out were found. The decrease of fiber strength during CVD coating and the possible diffusion of Sn-ions during hot pressing could be responsible for the relatively poor mechanical performance of these composites. Acknowledgements The authors thank Mrs. J.A. Roether (Imperial College Lon￾don) for revising the manuscript. The work was supported by the German Research Foundation (DFG). The authors are grate￾ful to TELUX Weißwasser (Germany) for providing the matrix glass. Thanks are also due to AGY Europe for the ZenTronTM glass fiber samples. 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