September 1998 Multilayered Interphases in SiC/SiC CH Composites with"Weak"and" Strong" Interfaces 2323 Table IV. Interface Characteristics, As a Function of the Number of (C-SiC) Sequences(n), for the SiC/SiC Composites Reinforeed with Treated Fibers Embedded Stress drop Shear stress Ratio of cement with extraction Sample (MPa) MPa) (MPa) n (1400) 4370 133 (13 (0.59) (26) L(=4) 3070 4230 (930) (0.38) 6 he value in parentheses given beneath each measurement is the standard deviation, 'From Hsueh. 6 (Table IV), which is almost one order of magnitude higher than grains that correspond to the SiC sublayer (i.e, microcrystal- those determined for untreated fibers. However, they are in line grains on the order of the sublayer thickness ). Con- L from hysteresis loop widths measured using specimens tested showed the presence of thin sections of carbon that appeared under tension(Table II). ,6 The discrepancy between the push- torn and remained partially bonded to the surface. Thus, one out and the hysteresis-loop techniques that is observed for com- the sliding surfaces corresponds to a Sic sublayer and the other posite J has not been elucidated yet. However, it may be related corresponds to a carbon sublayer terface in those specimens tested under tension. Indeed, a par- (B) Push-Back Tests: The critical stress that corresponds to the presence of a much-more-tortuous debonding/sliding in- the limit of linearity during the pushback of strongly bonded ticularly intense crack branching has been detected in compos- fibers is higher than the debonding stress obtained during the te J tensile specimens. 5-7 push-out tests on composites with weak interfaces. This obser Contrary to previous results on composites reinforced with vation confirms that fiber sliding is significantly limited in as-received fibers, the composite that shows the strongest re- composites reinforced with initially strongly bonded fibers sistance to interfacial cracking is that with only a single carbon (Figs. 3 and 7, Tables Ill and V layer interphase, because it exhibits the highest debonding and The interfacial parameters extracted from the push-back maximum stress(Fig. 9). Nevertheless, the displacements at curves show the previously mentioned dependence on the num- maximum stress (Table IV) and the frictional shear stresses ber of n(C-SiC) sequences(n I and 2). The frictional re- ( Fig. 10)that were observed for the SiC/SiC composites with sistance to fiber sliding in a composite with a multilayered multilayered interphases are also indicative of a higher resis- interphase is unambiguously higher than that obtained for a tance to sliding. composite with a single carbon interphase, as indicated by the seM observations of the debond location indicated debonding and the maximum stress and the interfacial shea ences between single-carbon-layer and multiple-layer stress (Table v) he composites with a single carbon The following characteristics of roughness have been de- (0.5 um thick), the debond crack was always detected in the rived from the clamping stresses and the reseating load de- interior of the carbon layer, 4 whereas in composites with mul crease: amplitude of -60 nm and wavelength of -350 nm(note tilayered interphases, it is located at the first carbon/SiC sub- that the wavelength is -500 nm for a single carbon interphase) layer interface near the fiber or near that interface. The crack Figure 13 shows that the interfacial shear stresses are much deviations were similar in tensile specimens. -7 As previously smaller during pushback than those derived from push-out mentioned, TEM and SEM analyses- have shown cracks lo tests. A study on the wear of sliding surfaces during push-back cated within the interphase(single carbon layer)or in the car- tests performed on bon sublayer near the fiber in multilayered interphases, as sum- showed that wear is severe at the onset from reverse sliding. 31 marized by the schematic diagram in Fig. 2. Furthermore, changes in the sliding parameters for a glass- SEM observation of the loaded side of the fibers(Fig. 12(b)) matrix system have been attributed to wear of the interface showed that the sliding surface consists of submicrometer asperities that occur during pushout. 24 In composites with M 0.5 Fig. 12. SEM micrographs of sliding surfaces involved in SiC/SiC composites with treated fibers and multilayered interphases(a) fiber side, composite B, and(b)matrix side, composite L).(Table IV), which is almost one order of magnitude higher than those determined for untreated fibers. However, they are in excellent agreement with those extracted for composites B and L from hysteresis loop widths measured using specimens tested under tension (Table II).5,6 The discrepancy between the push￾out and the hysteresis-loop techniques that is observed for com￾posite J has not been elucidated yet. However, it may be related to the presence of a much-more-tortuous debonding/sliding in￾terface in those specimens tested under tension. Indeed, a par￾ticularly intense crack branching has been detected in compos￾ite J tensile specimens.5–7 Contrary to previous results on composites reinforced with as-received fibers, the composite that shows the strongest re￾sistance to interfacial cracking is that with only a single carbon layer interphase, because it exhibits the highest debonding and maximum stress (Fig. 9). Nevertheless, the displacements at maximum stress (Table IV) and the frictional shear stresses (Fig. 10) that were observed for the SiC/SiC composites with multilayered interphases are also indicative of a higher resis￾tance to sliding. SEM observations of the debond location indicated differ￾ences between single-carbon-layer and multiple-layer inter￾faces (Fig. 12). In the composites with a single carbon layer (0.5 mm thick), the debond crack was always detected in the interior of the carbon layer,14 whereas in composites with mul￾tilayered interphases, it is located at the first carbon/SiC sub￾layer interface near the fiber or near that interface. The crack deviations were similar in tensile specimens.5–7 As previously mentioned, TEM and SEM analyses5–7 have shown cracks lo￾cated within the interphase (single carbon layer) or in the car￾bon sublayer near the fiber in multilayered interphases, as sum￾marized by the schematic diagram in Fig. 2. SEM observation of the loaded side of the fibers (Fig. 12(b)) showed that the sliding surface consists of submicrometer grains that correspond to the SiC sublayer (i.e., microcrystal￾line grains on the order of the sublayer thickness.5 ). Con￾versely, SEM observation of the surface of protruding fibers showed the presence of thin sections of carbon that appeared torn and remained partially bonded to the surface. Thus, one of the sliding surfaces corresponds to a SiC sublayer and the other corresponds to a carbon sublayer. (B) Push-Back Tests: The critical stress that corresponds to the limit of linearity during the pushback of strongly bonded fibers is higher than the debonding stress obtained during the push-out tests on composites with weak interfaces. This obser￾vation confirms that fiber sliding is significantly limited in composites reinforced with initially strongly bonded fibers (Figs. 3 and 7, Tables III and V). The interfacial parameters extracted from the push-back curves show the previously mentioned dependence on the num￾ber of n(C–SiC) sequences (n 4 1 and 2). The frictional re￾sistance to fiber sliding in a composite with a multilayered interphase is unambiguously higher than that obtained for a composite with a single carbon interphase, as indicated by the debonding and the maximum stress and the interfacial shear stress (Table V). The following characteristics of roughness have been de￾rived from the clamping stresses and the reseating load de￾crease: amplitude of ∼60 nm and wavelength of ∼350 nm (note that the wavelength is ∼500 nm for a single carbon interphase). Figure 13 shows that the interfacial shear stresses are much smaller during pushback than those derived from push-out tests. A study on the wear of sliding surfaces during push-back tests performed on composites with weakly bonded fibers showed that wear is severe at the onset from reverse sliding.31 Furthermore, changes in the sliding parameters for a glass￾matrix system have been attributed to wear of the interface asperities that occur during pushout.24 In composites with Fig. 12. SEM micrographs of sliding surfaces involved in SiC/SiC composites with treated fibers and multilayered interphases ((a) fiber side, composite B, and (b) matrix side, composite L). Table IV. Interface Characteristics, As a Function of the Number of (C–SiC) Sequences (n), for the SiC/SiC Composites Reinforced with Treated Fibers† Sample Embedded length (mm) Debonding stress (MPa) Maximum stress (MPa) Imposed displacement (mm) Stress drop with extraction (MPa) Shear stress from plateau (MPa) Ratio of debonding/drop stresses J ‡ (n 4 1) 140 3730 4950 1.01 890 100 5.2 (20) (400) (1400) (0.43) (390) (38) (2.6) B (n 4 2) 110 2650 4370 1.22 960 133 3.7 (40) (600) (1390) (0.59) (400) (51) (2.6) L (n 4 4) 160 3070 4230 1.13 1120 90 3.2 (80) (630) (930) (0.38) (420) (26) (1.6) † The value in parentheses given beneath each measurement is the standard deviation. ‡ From Hsueh.16 September 1998 Multilayered Interphases in SiC/SiC CVI Composites with ‘‘Weak’’ and ‘‘Strong’’ Interfaces 2323