G N. Morscher, J D. Cawley/ Journal of the European Ceramic Society 22(2002)2777-2787 4. Improvements in intermediate temperature stress- All three of the earlier approaches have been demon rupture of SiC/BN SiC composites strated for SYL reinforced composites. Fig 8 shows the dramatic improvements in intermediate stress rupture Based on the understanding of the mechanistic pro- properties for the three approaches described above in cess leading to intermediate temperature stress-rupture, comparison to the data from the material modeled in in part derived from the development and verification of Fig. 7. 14,15 500-h rupture stresses in excess of 200 MPa the earlier model, a few approaches have been employed were common for all three of the approaches, the"out to improve intermediate temperature stress-rupture. The side debonding"and Si-doped bn interphase compo- most desirable would to use a more durable interphase sites performed the best. Precrack experiments were than BN. However, to date, no real interphase has pre- performed for fiber-spread composites and for"outside sented itself, but the bn can be improved. For example, debonding "composites in Ref. 31( Fig. 9). The fiber composites with higher bn processing temperature or spread composites did show a decrease in stress-rupture bn doped with Si have shown greater resistance to properties with increased crack density whereas the oxygen and water containing environments at inter- outside debonding composites did not, in fact"out mediate temperatures. Si-doped BN was especially side debonding "composites showed improvement. In asistant to these environments; however, it requires other words, stress-rupture of fiber-spread composites processing temperatures on the order of 1400C, too still possess agage-length'"dependence whereas for high for processing of preforms. Some progress has outside debonding "this does not appear to be the been made with coating large individual pieces of woven case. The kinetic limit as determined for the composites loth with the higher processing temperature Si-doped modeled in Fig. Sb no longer applies to either of these BN interphase and then stacking the woven cloth to fabricate MI composites. 28 Since fiber separation is one of the controlling factors de Debonding-350 in fiber-to-fiber fusion, one logical technique to slow e-S-doped BN this process down would be to spread fibers further apart. By increasing the time for fiber fusion, more 14001Fiber-Spreading fibers would fail in a matrix crack under global load Spread SYL or SYL旧B L1200 sharing conditions, thereby reducing the available fibers that could fail after fibers are strongly bonded to one 5 1000 another or to the matrix. Two techniques have enabled fiber spreading that results in greater degree of fiber-to- fiber separation:(1)a proprietary approach to spread fibers in woven fabric and(2) heat-treating woven fabric to produce a-100 nm thick in-situ BN layer on the SYL fibers. 30 The former mechanically separates Time to fail, hr fibers resulting in fewer near fiber-to-fiber contacts and Fig. 8. Stress-rupture properties at 815oC in air of cor greater coverage of all the fibers with BN; the latter woven BN interphase composites and composites with produces a high-temperature BN layer that separates improvements fibers by an additional 200 nm. The in-situ BN Sylra mic R fibers are referred to as syl-iBN Another improvement has been to alter the interface outside debonding BN' where interface debonding and sliding occur within the interphase region from the fiber/BN interface to the N/matrix interface, i.e. "outside debonding. 3I This 350 MPa Precrack would be similar in concept to multi-layer coatings 32 where interphase oxidation is engineered to occur as far away from the fiber surface as possible. The multi-layer C/SiC interphase coating of NIC/SiC composites in Ref 37 does show slight improvement over other C-inter phase NC/SiC composites at intermediate tempera- tures. For"outside debonding"BN interphases, oxygen and water vapor do not have direct access to the fibers 30 In order to fuse fibers to the matrix or to one another Time. Hours oxidation must occur through the thickness of the bn, which is relatively slow because the boria reacts with the Fig. 9. Stress-rupture of fiber-spread composite and outside-debond- ing BN composite. Both composites had a fiber volume fraction in the Sic in the matrix crack to effectively seal the matrix crack loading direction of 0.2.4. Improvements in intermediate temperature stress￾rupture of SiC/BN/SiC composites Based on the understanding of the mechanistic pro￾cess leading to intermediate temperature stress-rupture, in part derived from the development and verification of the earlier model, a few approaches have been employed to improve intermediate temperature stress-rupture. The most desirable would to use a more durable interphase than BN. However, to date, no real interphase has pre￾sented itself, but the BN can be improved. For example, composites with higher BN processing temperature or BN doped with Si have shown greater resistance to oxygen and water containing environments at inter￾mediate temperatures.27 Si-doped BN was especially resistant to these environments; however, it requires processing temperatures on the order of 1400
C, too high for processing of preforms. Some progress has been made with coating large individual pieces of woven cloth with the higher processing temperature Si-doped BN interphase and then stacking the woven cloth to fabricate MI composites.28 Since fiber separation is one of the controlling factors in fiber-to-fiber fusion, one logical technique to slow this process down would be to spread fibers further apart. By increasing the time for fiber fusion, more fibers would fail in a matrix crack under global load sharing conditions, thereby reducing the available fibers that could fail after fibers are strongly bonded to one another or to the matrix. Two techniques have enabled fiber spreading that results in greater degree of fiber-to- fiber separation: (1) a proprietary approach to spread fibers in woven fabric 29 and (2) heat-treating woven fabric to produce a100 nm thick in-situ BN layer on the SYL fibers.30 The former mechanically separates fibers resulting in fewer near fiber-to-fiber contacts and greater coverage of all the fibers with BN; the latter produces a high-temperature BN layer that separates fibers by an additional 200 nm. The in-situ BN Sylra￾mic1 fibers are referred to as SYL-iBN. Another improvement has been to alter the interface where interface debonding and sliding occur within the interphase region from the fiber/BN interface to the BN/matrix interface, i.e. ‘‘outside debonding’’. 31 This would be similar in concept to multi-layer coatings 32 where interphase oxidation is engineered to occur as far away from the fiber surface as possible. The multi-layer C/SiC interphase coating of NIC/SiC composites in Ref. 37 does show slight improvement over other C-inter￾phase NIC/SiC composites at intermediate tempera￾tures. For ‘‘outside debonding’’ BN interphases, oxygen and water vapor do not have direct access to the fibers. In order to fuse fibers to the matrix or to one another, oxidation must occur through the thickness of the BN, which is relatively slow because the boria reacts with the SiC in the matrix crack to effectively seal the matrix crack. All three of the earlier approaches have been demon￾strated for SYL reinforced composites. Fig. 8 shows the dramatic improvements in intermediate stress rupture properties for the three approaches described above in comparison to the data from the material modeled in Fig. 7.14,15 500-h rupture stresses in excess of 200 MPa were common for all three of the approaches, the ‘‘out￾side debonding’’ and Si-doped BN interphase compo￾sites performed the best. Precrack experiments were performed for fiber-spread composites and for ‘‘outside debonding’’ composites in Ref. 31 (Fig. 9). The fiber￾spread composites did show a decrease in stress-rupture properties with increased crack density whereas the ‘‘outside debonding’’ composites did not, in fact ‘‘out￾side debonding’’ composites showed improvement. In other words, stress-rupture of fiber-spread composites still possess a ‘‘gage-length’’ dependence whereas for ‘‘outside debonding’’ this does not appear to be the case. The kinetic limit as determined for the composites modeled in Fig. 5b no longer applies to either of these Fig. 8. Stress-rupture properties at 815
C in air of conventional woven BN interphase composites and composites with interphase improvements. Fig. 9. Stress-rupture of fiber-spread composite and outside-debond￾ing BN composite. Both composites had a fiber volume fraction in the loading direction of 0.2. G.N. Morscher, J.D. Cawley / Journal of the European Ceramic Society 22 (2002) 2777–2787 2785