1. Davies et al. /Journal of the European Ceramic Society 25(2005)599-604 ↑ 4. Conclusions The distributions of fibre strength, pullout length, and fi- bre/matrix interface shear strength, T, within an individual fibre bundle were investigated for an orthogonal 3-D wo ven SiC/SiC composite tensile tested at 1100C in air. The mean fibre pullout length, (h), varied between 59 um at the centre to zero within an embrittled region(15 um width) 0 at the fibre bundle perimeter. Fibre strength(normalised to a gauge length of 10-m)decreased from 2.25+0.25 GPa Distance from fibre bundle perimeter(om) at the centre to 1.39+0. 15 GPa adjacent to the embrittled Fig. 6. Distribution of fibre/matrix interface shear strength, t, along the region, compared to 3.86+0. 13 GPa for a specimen tensile minor axis within an individual fibre bundle in an orthogonal 3-D wover tested at room temperature. The Sic/SiC composite tested at 1100.C in ai the fibre bundle was 100+ 16 mPa at the centre but this in- creased rapidly to a maximum of 2.25+0. 21 GPa close to indicates the presence of a strong linear relationship between bound for t with respect to suppression of crack deflection the mean fibre pullout length(Fig. 5(a))to the fraction, m, mechanisms at the interface in this composite system.Whilst such a wide variation in t within a single fibre bundle has fm(723±1.2)um not previously been noted, this was attributed to the lack of The value of (h)varied between zero at the perimeter t data available for composites containing short fibre pull- to 59 um at the centre with the lowest non-zero value of out lengths. Overall, the experimental data was consistent (h)being 1.7 um at a distance of 15 um from the bundle with oxygen having surrounded the fibre bundle perimete perimeter; thus providing an estimate for the width of the via matrix cracks in the transverse fibre bundles embrittled region in this specimen, i.e. 15 um. It is clear from Eq (4)that the ratio of (h) between fibre bundle centre and lowest non-zero(h) region would also provide a first Acknowledgements approximation for the ratio of t between these positions (i 9/1.7)and being even higher for the ratio between centre The authors wish to gratefully acknowledge Dr. M and perimeter(where (h)was zero) Shibuya of Ube Industries Ltd, Dr. J. Gotoh of Kawasaki Heavy Industries Ltd, and T. Hirokawa and T. Tanamura of 3.4. Fibre/matrix interface shear strength Shikibo Ltd for the manufacture and supply of all materials used in this study Values of t obtained from Eq (4)have been presented in Fig. 6 with the lowest value of t being 100+ 16 MPa at the fibre bundle centre and approximately 20 times that of References the rT case, illustrating the rapid increase in t which re- sults from even a relatively short exposure time to oxygen I. Curtin, W.A., Theory of mechanical properties of ceramic-matrix at elevated temperature. The value of t gradually increased mposites.J.Am. Ceram. Soc. 1991, 74(11), 2837-2845 2. Davies, 1. J, Ishikawa, T, Shibuya, M. and Hirokawa, T, Fibre away from the bundle centre to reach 149+17 MPa at a dis- strength parameters measured in situ for ceramic-matrix composites tance of 39 um from the bundle perimeter but then increased tested at elevated temperature in vacuum and in air. Compos. Sci. rapidly to a maximum of 2.25+0.21 GPa adjacent to the nbrittled region, with r being necessarily greater than this 3. Davies, L J, Ishikawa, T,Shibuya,M.and Hirokawa,T,Optical within the embrittled region. This value of t at the transition croscopy of a 3-D woven SiC/SiC-based composite. Compos. Sci. echnol.1999,59,429-437 zone between brittle and non-brittle regions thus provides 4. Rebillat, F, Lamon, J, Naslain, R, Lara-Curzio, E, Ferber, M.K. an estimate of the minimum t required for suppression of and Besmann, T. M, Interfacial bond strength in SiC/C/SiC composite crack deflection mechanisms within this composite system materials, as studied by single-fibre push-out tests. J. Am. Ceram. Whilst such high values of t have not previously been re- Soc.1998,81(4),965-978 ported within SiC/SiC composites, the authors attribute this 5. Filipuzzi, L, Camus, G, N: R. and Thebault. J. Oxidation mechanisms and kinetics of lD-SiC/C/SiC composite materials. I.An to the lack of quantitative data available in partially oxidised perimental approach. J. Am. Ceram. Soc. 1994, 77(2), 459-466 CMCs exhibiting h values on the order of several microns 6. Prouhet, S, Camus, G, Labrugere, C Guette, A. and Martin, E, Me However, evidence of high strength fibres exhibiting fracture chanical characterization of Si-C(O) fibre/SiC (CVI) matrix compos mirrors and micron-range pullout lengths, i.e. the require- nterphase. J.Am. Ceram. Soc. 1994, 77(3), 649-656 ments for candidate t values in the gPa range, is available B, Hay, R. S, Marshall, D. B, Morgan, P. E. D. and in the literature, 26, 39 suggesting the data presented in this Influence of interfacial roughness on fibre sliding in oxide omposites with La-monazite interphases J. Am. Ceram. Soc. 2003 work to not be exceptional for CMCs 86(2),305-316.I.J. Davies et al. / Journal of the European Ceramic Society 25 (2005) 599–604 603 Distance from fibre bundle perimeter (∝m) 0 10 20 30 40 50 60 Fibre/matrix interface shear strength (MPa) 0 500 1000 1500 2000 2500 Fig. 6. Distribution of fibre/matrix interface shear strength, τ, along the minor axis within an individual fibre bundle in an orthogonal 3-D woven SiC/SiC composite tested at 1100 ◦C in air. indicates the presence of a strong linear relationship between the mean fibre pullout length (Fig. 5(a)) to the fraction, fm, of fibres exhibiting fracture mirrors (Fig. 5(b)) with h = fm (72.3 ± 1.2)
m. The value of h varied between zero at the perimeter to 59
m at the centre with the lowest non-zero value of h being 1.7
m at a distance of 15
m from the bundle perimeter; thus providing an estimate for the width of the embrittled region in this specimen, i.e. 15
m. It is clear from Eq. (4) that the ratio of h between fibre bundle centre and lowest non-zero h region would also provide a first approximation for the ratio of τ between these positions (i.e. 59/1.7) and being even higher for the ratio between centre and perimeter (where h was zero). 3.4. Fibre/matrix interface shear strength Values of τ obtained from Eq. (4) have been presented in Fig. 6 with the lowest value of τ being 100 ± 16 MPa at the fibre bundle centre and approximately 20 times that of the RT case,2 illustrating the rapid increase in τ which re￾sults from even a relatively short exposure time to oxygen at elevated temperature. The value of τ gradually increased away from the bundle centre to reach 149±17 MPa at a dis￾tance of 39
m from the bundle perimeter but then increased rapidly to a maximum of 2.25 ± 0.21 GPa adjacent to the embrittled region, with τ being necessarily greater than this within the embrittled region. This value of τ at the transition zone between brittle and non-brittle regions thus provides an estimate of the minimum τ required for suppression of crack deflection mechanisms within this composite system. Whilst such high values of τ have not previously been re￾ported within SiC/SiC composites, the authors attribute this to the lack of quantitative data available in partially oxidised CMCs exhibiting h values on the order of several microns. However, evidence of high strength fibres exhibiting fracture mirrors and micron-range pullout lengths, i.e. the require￾ments for candidate τ values in the GPa range, is available in the literature,26,39 suggesting the data presented in this work to not be exceptional for CMCs. 4. Conclusions The distributions of fibre strength, pullout length, and fi- bre/matrix interface shear strength, τ, within an individual fibre bundle were investigated for an orthogonal 3-D wo￾ven SiC/SiC composite tensile tested at 1100 ◦C in air. The mean fibre pullout length, h, varied between 59
m at the centre to zero within an embrittled region (∼15
m width) at the fibre bundle perimeter. Fibre strength (normalised to a gauge length of 10−3 m) decreased from 2.25 ± 0.25 GPa at the centre to 1.39 ± 0.15 GPa adjacent to the embrittled region, compared to 3.86 ± 0.13 GPa for a specimen tensile tested at room temperature. The lowest value of τ within the fibre bundle was 100 ± 16 MPa at the centre but this in￾creased rapidly to a maximum of 2.25 ± 0.21 GPa close to the embrittled region, suggesting this value to be a lower bound for τ with respect to suppression of crack deflection mechanisms at the interface in this composite system. Whilst such a wide variation in τ within a single fibre bundle has not previously been noted, this was attributed to the lack of τ data available for composites containing short fibre pull￾out lengths. Overall, the experimental data was consistent with oxygen having surrounded the fibre bundle perimeter via matrix cracks in the transverse fibre bundles. Acknowledgements The authors wish to gratefully acknowledge Dr. M. Shibuya of Ube Industries Ltd., Dr. J. Gotoh of Kawasaki Heavy Industries Ltd., and T. Hirokawa and T. Tanamura of Shikibo Ltd. for the manufacture and supply of all materials used in this study. References 1. Curtin, W. A., Theory of mechanical properties of ceramic-matrix composites. J. Am. Ceram. Soc. 1991, 74(11), 2837–2845. 2. Davies, I. J., Ishikawa, T., Shibuya, M. and Hirokawa, T., Fibre strength parameters measured in situ for ceramic-matrix composites tested at elevated temperature in vacuum and in air. Compos. Sci. Technol. 1999, 59, 801–811. 3. Davies, I. J., Ishikawa, T., Shibuya, M. and Hirokawa, T., Optical microscopy of a 3-D woven SiC/SiC-based composite. Compos. Sci. Technol. 1999, 59, 429–437. 4. Rebillat, F., Lamon, J., Naslain, R., Lara-Curzio, E., Ferber, M. K. and Besmann, T. M., Interfacial bond strength in SiC/C/SiC composite materials, as studied by single-fibre push-out tests. J. Am. Ceram. Soc. 1998, 81(4), 965–978. 5. Filipuzzi, L., Camus, G., Naslain, R. and Thebault, J., Oxidation mechanisms and kinetics of 1D-SiC/C/SiC composite materials. I. An experimental approach. J. Am. Ceram. Soc. 1994, 77(2), 459–466. 6. Prouhet, S., Camus, G., Labrugere, C., Guette, A. and Martin, E., Me￾chanical characterization of Si–C(O) fibre/SiC (CVI) matrix compos￾ites with a BN-interphase. J. Am. Ceram. Soc. 1994, 77(3), 649–656. 7. Davis, J. B., Hay, R. S., Marshall, D. B., Morgan, P. E. D. and Sayir, A., Influence of interfacial roughness on fibre sliding in oxide composites with La-monazite interphases. J. Am. Ceram. Soc. 2003, 86(2), 305–316