974 Journal of the American Ceramic Sociery-Rebillat et al. Vol 8I. No 4 Table v, Interfacial Characteristics Measured by Push-Back Tests for Material j value 3630(590) Maximum stress(MPa) 4345(1570) 2040(650) Decrease in stress at push out(MPa) 935(240 f the fiber (um) 0.7800.31) Interfacial shear stress.T 80(30) 40(8) Fitting(MPa) Coefficient of friction, H 0.034(0.017 Axia S(MP 850(390) Clamping stress(MPa) 030(50) 114 avele 0.4900.11) Ratio of debond st stress decrease 4.2(1.5) 2.5(1.8) Embedded length of -140 um. Values given in parentheses are standard deviations. clearly observed via SEM(Fig. 10). Although the rou V. Discussion mplitude cannot be measured in the micrograph in Fig. 10,it seems comparable to that observed on the pushed-out fibers ()Composites Reinforced with As-Received Fibers (Fg,7 The frictional shear stresses extracted from the push-out tests are similar to those estimated by Droillard 3 for material I from the hysteresis loops of unloading-reloading under tension and (a) by other authors for 2D SiC/SiC composites and even for microcomposites tested under tension. 36,37 A microcomposite consists of a single fiber coated with an interphase and with the matrix. Values of-10 MPa seem to be characteristic of these SiC/SiC composite materials reinforced with as-received Nicalon TM fibers 3, 7, 8, 12, 13,15 Moreover, the approach by Bright et al.23 that was based only on the maximum stresses gave interfacial parameters (u, T) that were similar to those extracted from the nonlinea domain of the push-out curve using the Hsueh model, 18,21 except that the derived clamping residual stress from the Hsueh model was significantly smaller. Furthermore, comparable values of interfacial shear strengths(Ts) were determined using various equations which do and do not account for the presence of residual stresses. 24-/These Ts values are one order of magnitude higher than the interfacial shear stresses(T) As commonly observed in composites with weakly bonded fibers. the fiber/carbon-coating interface is the weakest link in the interfacial sequence(lower inset in Fig. 1). Examination of both sides of samples after push-out tests reveals the smooth surface of the fiber and does not indicate any tearing of the in- terphase, which suggests the presence of a single crack that propa- (b) C gates at the surface of the fiber. Hence, the role of the inter- cau the matrix, only contributing to the relief of residual stress. The analysis shows that, despite the presence of a layer of soft carbon, the effect of surface roughness cannot be This effect is greater than the thermal residual stress; carbon layer is subjected to compression during fib (2) Composites Reinforced with Treated Fibers (A) Debonding: TEM examination of the cracked inter phases after tensile tests has revealed the presence of cracks branching within the interphase(top inset in Fig. 1). 12, 16 The interfacial damage behavior, as well as the interfacial proper ties extracted from the push-out curves, seem to be consistent with these crack patterns The load required for initiation of fiber sliding, as measured by the so-calleddebond stress(oa), is more than a factor of 4 higher for the composites reinforced with treated fibers. which is also reflected by the maximum stress. This stress seems to increase exponentially as the embedded length of the micrographs of (a)protruding fibers and (b)the fiber fiber increases and, thus, is sensitive to very small increases in back tests performed on a Sic/C/SiC com- the thickness of the tested zone(-10 um). Fibers could not be ated fibers(material J) pushed out when the samples were thicker than 180 um, be-is clearly observed via SEM (Fig. 10). Although the roughness amplitude cannot be measured in the micrograph in Fig. 10, it seems comparable to that observed on the pushed-out fibers (Fig. 7). V. Discussion (1) Composites Reinforced with As-Received Fibers The frictional shear stresses extracted from the push-out tests are similar to those estimated by Droillard13 for material I from the hysteresis loops of unloading–reloading under tension and by other authors for 2D SiC/SiC composites19 and even for microcomposites tested under tension.36,37 A microcomposite consists of a single fiber coated with an interphase and with the matrix. Values of ∼10 MPa seem to be characteristic of these SiC/SiC composite materials reinforced with as-received Nicalon™ fibers.3,7,8,12,13,15 Moreover, the approach by Bright et al.23 that was based only on the maximum stresses gave interfacial parameters (m, t) that were similar to those extracted from the nonlinear domain of the push-out curve using the Hsueh model,18,21 except that the derived clamping residual stress from the Hsueh model was significantly smaller. Furthermore, comparable values of interfacial shear strengths (ts) were determined using various equations which do and do not account for the presence of residual stresses.24–27 These ts values are one order of magnitude higher than the interfacial shear stresses (t). As commonly observed in composites with weakly bonded fibers, the fiber/carbon-coating interface is the weakest link in the interfacial sequence (lower inset in Fig. 1). Examination of both sides of samples after push-out tests reveals the smooth surface of the fiber and does not indicate any tearing of the in￾terphase, which suggests the presence of a single crack that propa￾gates at the surface of the fiber. Hence, the role of the inter￾phase may be regarded as limited, because it remains bonded to the matrix, only contributing to the relief of residual stress. The analysis shows that, despite the presence of a layer of soft carbon, the effect of surface roughness cannot be neglected. This effect is greater than the thermal residual stress; thus, the carbon layer is subjected to compression during fiber sliding. (2) Composites Reinforced with Treated Fibers (A) Debonding: TEM examination of the cracked inter￾phases after tensile tests has revealed the presence of cracks branching within the interphase (top inset in Fig. 1).12,16 The interfacial damage behavior, as well as the interfacial proper￾ties extracted from the push-out curves, seem to be consistent with these crack patterns. The load required for initiation of fiber sliding, as measured by the so-called ‘‘debond stress’’ (sd), is more than a factor of 4 higher for the composites reinforced with treated fibers, which is also reflected by the maximum stress. This stress seems to increase exponentially as the embedded length of the fiber increases and, thus, is sensitive to very small increases in the thickness of the tested zone (∼10 mm). Fibers could not be pushed out when the samples were thicker than 180 mm, be￾Table V. Interfacial Characteristics Measured by Push-Back Tests for Material J† Characteristic Value Zero push back (push-out) One push back Debonding stress (MPa) 3630 (590) 970 (350) Maximum stress (MPa) 4345 (1570) 2040 (650) Decrease in stress at push out (MPa) 935 (240) 490 (220) Displacement of the top of the fiber (mm) 0.78 (0.31) 0.57 (0.33) Interfacial shear stress, t Plateau (MPa) 80 (30) 40 (8) Fitting (MPa) 35 (20) Coefficient of friction, m 0.034 (0.017) Axial stress (MPa) −850 (390) Clamping stress (MPa) −1030 (50) Debonded length (mm) 114 (35) Magnitude of roughness (nm) 47 (29) Wavelength of roughness (mm) 0.49 (0.11) Ratio of debond stress to stress decrease 4.2 (1.5) 2.5 (1.8) † Embedded length of ∼140 mm. Values given in parentheses are standard deviations. Fig. 10. SEM micrographs of (a) protruding fibers and (b) the fiber sliding surface after push-back tests performed on a SiC/C/SiC com￾posite reinforced with treated fibers (material J). 974 Journal of the American Ceramic Society—Rebillat et al. Vol. 81, No. 4