January 2004 Effect of a BN Interphase That Debonds between the interphase and the Matrix in SiC/Sic Composites was estimated from the measured final crack density of failed composites multiplied by the normalized cumulative AE energy (Fig. 2), assuming the latter represented the stress-dependent Inside Debonair distribution of matrix cracks, which has been demonstrated for inside Debonding" similar systems'.8 Therefore, the only variable not known was T which was adjusted to best fit the predicted stress-strai 3008吵 the experimental stress-strain curve. For the case where SYL-IBN lengths overlap Ahn and Curtin"showed that if the ° Outside Debonding still equally spaced, the composite strain could then be modeled by o/(fE+ao/ -(o+o4ES(o)p] 00.10.20.304050607 E E品 <28 Strain stress-strain curves for ide-debonding Fig. 3. Room-temperature tensile stress-strain curves for 8.7 epcm ecimens are shown in Figs. 2(a) and SYL- iBN SiC/SiC composites and 7, I epcm HNS SiC/SiC composites (b), respectively. steresis loops removed). Note the HNS composites are displaced by The interfacial shear stress was also measured directly from the 0. 2% in strain for clarity. fiber push-in technique. Results of the two techniques are listed fo individual specimens in Table I for systems that displayed global outside debonding, mixed outside/inside debonding, and global the bn interphase and the matrix was often observed to occur inside debonding. Both techniques confirmed that the interfacial around groups of fibers that were linked to one another by the thin shear strength of global outside-debonding composites(-10 MPa Bn that was deposited on two closely spaced fibers. Usually, these was significantly less than that of inside-debonding composites fiber groups were made up of a few fibers that formed a row of (-70 MPa). Mixed outside/inside debonding had intermediate fibers as shown in Fig 4(b ). Debonding at the BN interphase/Sic values of interfacial shear strength. It is important to note that even matrix was observed for individual fibers that were well separated though the interfacial shear strength of outside debonding is lower from other fibers. For some composites, regions of outside than that of inside-debonding composites, there was no loss in debonding and inside debonding were observed in different re- ultimate strengths for outside-debonding composites and often u gions or bundles of the fracture surface, i.e., mixed debonding ultimate strength increased(e.g, compare the f= 0.2 composites In addition to a low elastic modulus, outside-debonding com- in Fig 3(a). posites often displayed a secondary modulus before significant matrix cracking. Figure 5 shows a family of stress-strain curves for a number of different outside-debonding composites with (2) Intermediate-Temperature Mechanical Behavior ifferent volume fractions, The initial elastic moduli were very Stress rupture at 815C was performed on SYL and SYL- iBN consistent(218 GPa)and all of the curves showed an inflection composites with inside and outside debonding(fig. 6).The x70 MPa that resulted in stress-rupture data for SYL SiC/SiC composites displaying inside MPa). This inflection was not associated with any AE activity; i.e debonding have been reported in refs 20 and 21. since the par it appears that this inflection was not due to matrix crack varied in fiber volume fraction, the rupture stress data are plotted ormation as the stress on the fibers i e. the load in a matrix crack that was Finally, the interfacial shear strength of several different inside- carried by the fibers. For comparison, the rupture stress corre- debonding and outside-debonding composites was determined sponding to a composite with f=0.2 in the loading direction is using two techniques. 4 First, the interfacial shear strength was shown on the right axis. Each set of data for the different types of estimated by modeling the stress-strain curve based on the composites had at least one panel with f =0.2. stress-dependent crack density(from AE) Composite strain was First, note that there is a difference in rupture behavior between determined in the same fashion as Pryce and Smith. Using the inside-debonding SYL-ibN fiber composites and SYL nomenclature of Curtin et al, composite strain can be modeled posites. Inside-debonding SYL-iBn composites outperform (i.e assuming equally spaced cracks fail after a longer time at a given stress) inside-debonding SYL composites because the fibers in SYL-iBn composites E=a/Ee+a6()pE0+σt) rally spread apart from one another with the formation of the 100 the fiber surface. 2I The where o is the applied stress. the residual (thermal)stress in more time with increasing separation distance. In addition, the the matrix(compression is negative), E is the elastic modulus, debonding interface for inside-debonding SYL-ibN actually oc- subscripts m, f, and c refer to matrix, fiber, and composite, curs between the in situ BN and the CVI-deposited BN. 0In other respectively, and p. is the matrix crack density. The fir rst part of the words, for inside-debonding SYL-iBN composites, the debonding equation corresponds to the elastic strain response of an uncracked and sliding interface was some distance(-100 nm)away from the composite and the second part of the equation corresponds to the fiber surface, which contained Sic in(displacement) of the fibers at and away from a or both fiber composite systems possessing an outside- through-thickness matrix crack dictated by the sliding lengt debonding interface, further improvements in intermediate mperature stress-rupture life were observed(Fig. 6). For SYL (2) composites with outside debonding compared with SYL inside bonding co ed by over 25 a=(1-∫)Em/E outside-debonding SYL-ibn composites in comparison to inside- debonding SYL-BN composites, the and r is the fiber radius, f is the fiber volume fraction in the magnitude in time improvement at resses and -200 MPa direction, and T is the interfacial shear strength. E and o improvement in fiber stress(40 determined from the stress-strain curves. Er is 380 GPa and at lower stresses near the run-out c It should be noted that was determined from the rule-of-mixtures. The stress-dependent p. these high-stress conditions for stress-rupture are significantlyJanuary 2004 Effect of a BN Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites 107 6oo 1 500 - 'Outside Debonding' "Inside Debonding" 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Strain, % Fig. 3. Room-temperature tensile stress-strain curves for 8.7 epcm SYL-iBN SiC/SiC composites and 7. I epcm HNS SiC/SiC composites (hysteresis loops removed). Note the HNS composites are displaced by 0.2% in strain for clarity. the BN interphase and the matrix was often observed to occur around groups of fibers that were linked to one another by the thin BN that was deposited on two closely spaced fibers. Usually, these fiber groups were made up of a few fibers that formed a row of fibers as shown in Fig. 4(b). Debonding at the BN interphaselSiC matrix was observed for individual fibers that were well separated from other fibers. For some composites, regions of outside debonding and inside debonding were observed in different re￾gions or bundles of the fracture surface, i.e., mixed debonding. In addition to a low elastic modulus, outside-debonding com￾posites often displayed a secondary modulus before significant matrix cracking. Figure 5 shows a family of stress-strain curves for a number of different outside-debonding composites with different volume fractions. The initial elastic moduli were very consistent (-218 GPa) and all of the curves showed an inflection at -70 MPa that resulted in a lower secondary modulus (-177 MPa). This inflection was not associated with any AE activity; i.e., it appears that this inflection was not due to matrix crack formation. Finally, the interfacial shear strength of several different inside￾debonding and outside-debonding composites was determined using two technique^.'^ First, the interfacial shear strength was estimated by modeling the stress-strain curve based on the stress-dependent crack density (from AE). Composite strain was determined in the same fashion as Pryce and Smith.I6 Using the nomenclature of Curtin et aZ.,I7 composite strain can be modeled assuming equally spaced cracks: E zz U/E, + CX~(U)~,JE~(U + uth) (1) (for pi' > 26) where u is the applied stress, uIh is the residual (thermal) stress in the matrix (compression is negative), E is the elastic modulus, subscripts m, f, and c refer to matrix, fiber, and composite, respectively, and p, is the matrix crack density. The first part of the equation corresponds to the elastic strain response of an uncracked composite and the second part of the equation corresponds to the extra strain (displacement) of the fibers at and away from a through-thickness matrix crack dictated by the sliding length: 6 = cir(u + u,h)/2T (2) where a = (1 -f)E,,/fE, (3) and r is the fiber radius, f is the fiber volume fraction in the loading direction, and T is the interfacial shear strength. Ec and ulh were determined from the stress-strain curves. Ef is 380 GPa and Em was determined from the rule-of-mixtures. The stress-dependent pc was estimated from the measured final crack density of failed composites multiplied by the normalized cumulative AE energy (Fig. 2), assuming the latter represented the stress-dependent distribution of matrix cracks, which has been demonstrated for similar systems.''*'8 Therefore, the only variable not known was T, which was adjusted to best fit the predicted stress-strain curve to the experimental stress-strain curve. For the case where the sliding lengths overlap, Ahn and Curtin'' showed that if the cracks are still equally spaced, the composite strain could then be modeled by E = a/(fEJ + CKU~/E~ - CK(U + U~~)/[~E&(U)P,] (4) (for p;' < 26) Therefore, for higher applied stress conditions, if pC-' < 26 was predicted, Eq. (4) was used. Examples of best-fit stress-strain curves for inside-debonding (T - 73 MPa) and outside-debonding (T - 18 MPa) composite specimens are shown in Figs. 2(a) and (b), respectively. The interfacial shear stress was also measured directly from the fiber push-in technique. Results of the two techniques are listed for individual specimens in Table I for systems that displayed global outside debonding, mixed outsidelinside debonding, and global inside debonding. Both techniques confirmed that the interfacial shear strength of global outside-debonding composites (- 10 MPa) was significantly less than that of inside-debonding composites (-70 MPa). Mixed outsidehide debonding had intermediate values of interfacial shear strength. It is important to note that even though the interfacial shear strength of outside debonding is lower than that of inside-debonding composites, there was no loss in ultimate strengths for outside-debonding composites and often the ultimate strength increased (e.g., compare thef = 0.2 composites in Fig. 3(a)). (2) Intermediate-Temperature Mechanical Behavior Stress rupture at 815°C was performed on SYL and SYL-iBN composites with inside and outside debonding (Fig. 6). The stress-rupture data for SYL SiC/SiC composites displaying inside debonding have been reported in Refs. 20 and 21. Since the panels varied in fiber volume fraction, the rupture stress data are plotted as the stress on the fibers, i.e., the load in a matrix crack that was carried by the fibers. For comparison, the rupture stress corre￾sponding to a composite withf = 0.2 in the loading direction is shown on the right axis. Each set of data for the different types of composites had at least one panel with f = 0.2. First, note that there is a difference in rupture behavior between inside-debonding SYL-iBN fiber composites and SYL fiber com￾posites. Inside-debonding SYL-iBN composites outperform (i.e., fail after a longer time at a given stress) inside-debonding SYL composites because the fibers in SYL-iBN composites are natu￾rally spread apart from one another with the formation of the - 100 nm BN layer on the fiber surface.*' The rupture life depends on the time it takes to bond nearest-neighbor fibers together, which takes more time with increasing separation distance. In addition, the debonding interface for inside-debonding SYL-iBN actually oc￾curs between the in situ BN and the CVI-deposited BN." In other words, for inside-debonding SYL-iBN composites, the debonding and sliding interface was some distance (-100 nm) away from the fiber surface, which contained Sic. For both fiber composite systems possessing an outside￾debonding interface, further improvements in intermediate￾temperature stress-rupture life were observed (Fig. 6). For SYL composites with outside debonding compared with SYL inside￾debonding composites, stress-rupture improved by over 250 MPa in fiber stress (-50 MPa for an f = 0.2 composite). For outside-debonding SYL-iBN composites in comparison to inside￾debonding SYL-BN composites, there was over an order of magnitude in time improvement at high stresses and -200 MPa improvement in fiber stress (-40 MPa for an f = 0.2 composite) at lower stresses near the run-out condition. It should be noted that these high-stress conditions for stress-rupture are significantly