April 1998 Interfacial Bond Strength in SiC/C/SiC Composite Materials Table IV. Interfacial Characteristics Measured by anoindentation Tests Performed on Composite j value Displacement Fiber radius(um) 7.9(0.5) Debonding stress(MPa) 1024(128) 1013(129 Clamping stress(MPa) 1426(40 Coefficient of friction, u 0.15(0.03) 0.1700.02) Interfacial shear stress, T(MP 247(138) 51(100) 0033(0.055) 0061(0.043) Maximum displacement(nm) 171(73) 67(56 Maximum stress(MPa) 2233(266) 2355(98 24(8) Roughness, A(from Eq (3))(um 0.08(0.04) 008(0.03) TFor a sample thickness of-200 um. Values given in parentheses are standard deviations tions and sliding is less significant than during loading. 21 The rectly related to the wavelength of the roughness along the high friction coefficient calculated for the loading phase is sliding surface similar to that which has been measured for two sliding carbon The values measured in push-back tests are naturally smaller surfaces(0.11-0.13).3 than those measured by push-out tests(Table V). The frictional sliding resistance is still larger than the shear stresses obtaine (7 Interfacial Parameters from from push-out tests on composites reinforced with untreated Single- Fiber Push-Back Tests on Treated fibers fibers(Fig. 5). Furthermore, all the interfacial characteristics. The curves recorded during the push-back tests are typical ncluding the debonding and the maximum stresses, the re- and include a linear domain followed by a nonlinear domain sidual clamping stress, and the magnitude of roughness(Table vith a downward curvature(Fig. 9). An initial linear domain of V), confirm the higher resistance to fiber sliding in composites elastic deformation is still present at applied stresses that are reinforced with treated fibers. Thus, comparison with the re smaller than the critical stress required for sliding. The non- sults of push-out tests performed on material I indicates that the inear domain(b'c" in Fig 9)is attributed to the progres stress at the end of the initial linear-elastic domain and the sive increase in fiber sliding length, The curves exhibit a load maximum stress of the push-back curve of material J are much drop before the plateau, as observed during push out, whic higher--1000 MPa vs 500 MPa and 2100 MPa vs 1400 MPa. reflects the catastrophic sliding of the rest of the fiber. The respectively. Yet, the displacement at the load drop to obtain debonding stress now represents the resistance to sliding. Fi- sliding of the entire fiber is smaller: 0.57 um vs 0.8 um nally, the reseating peak load in the plateau indicates that the The surface roughness of the fibers is estimated from the fiber recovers its initial position before protruding clamping stresses and the reseating load: amplitude of -50 nm The reseating peak load has been often described as evidence (+30 nm)and wavelength of -500 nm(*100 nm). The mag of the contribution of surface roughness to fiber sliding 34, 35 nitude of roughness(A)is larger than that estimated for mate Therefore, the wavelength of the reseating peak should be di- rial I from push-out tests. The roughness of the sliding surface 2500+ Thickness= 150 HI 1500 Fig 9. Stress-displacement from a push-back test performed on composite Jtions and sliding is less significant than during loading.21 The high friction coefficient calculated for the loading phase is similar to that which has been measured for two sliding carbon surfaces (0.11–0.13).33 (7) Interfacial Parameters from Single-Fiber Push-Back Tests on Treated Fibers The curves recorded during the push-back tests are typical and include a linear domain followed by a nonlinear domain with a downward curvature (Fig. 9). An initial linear domain of elastic deformation is still present at applied stresses that are smaller than the critical stress required for sliding. The non￾linear domain (‘‘b’’–‘‘c’’ in Fig. 9) is attributed to the progres￾sive increase in fiber sliding length. The curves exhibit a load drop before the plateau, as observed during push out, which reflects the catastrophic sliding of the rest of the fiber. The debonding stress now represents the resistance to sliding. Fi￾nally, the reseating peak load in the plateau indicates that the fiber recovers its initial position before protruding. The reseating peak load has been often described as evidence of the contribution of surface roughness to fiber sliding.34,35 Therefore, the wavelength of the reseating peak should be di￾rectly related to the wavelength of the roughness along the sliding surface.35 The values measured in push-back tests are naturally smaller than those measured by push-out tests (Table V). The frictional sliding resistance is still larger than the shear stresses obtained from push-out tests on composites reinforced with untreated fibers (Fig. 5). Furthermore, all the interfacial characteristics, including the debonding and the maximum stresses, the re￾sidual clamping stress, and the magnitude of roughness (Table V), confirm the higher resistance to fiber sliding in composites reinforced with treated fibers. Thus, comparison with the re￾sults of push-out tests performed on material I indicates that the stress at the end of the initial linear-elastic domain and the maximum stress of the push-back curve of material J are much higher—1000 MPa vs 500 MPa and 2100 MPa vs 1400 MPa, respectively. Yet, the displacement at the load drop to obtain sliding of the entire fiber is smaller: 0.57 mm vs 0.8 mm. The surface roughness of the fibers is estimated from the clamping stresses and the reseating load: amplitude of ∼50 nm (±30 nm) and wavelength of ∼500 nm (±100 nm). The mag￾nitude of roughness (A) is larger than that estimated for mate￾rial I from push-out tests. The roughness of the sliding surface Table IV. Interfacial Characteristics Measured by Nanoindentation Tests Performed on Composite J† Characteristic Value Displacement control Load control Fiber radius (mm) 7.9 (0.5) 8.2 (0.2) Debonding stress (MPa) 1024 (128) 1013 (129) Clamping stress (MPa) −1525 (664) −1426 (404) Axial stress (MPa) −831 (219) −950 (268) Loading Coefficient of friction, m 0.15 (0.03) 0.17 (0.02) Interfacial shear stress, t (MPa) 247 (138) 251 (100) Unloading mn 0.033 (0.055) 0.061 (0.043) tn (MPa) 36 (49) 93 (71) Maximum displacement (nm) 171 (73) 167 (56) Maximum stress (MPa) 2233 (266) 2355 (98) Debonded length (mm) 22 (9) 24 (8) Roughness, A (from Eq. (3)) (mm) 0.08 (0.04) 0.08 (0.03) † For a sample thickness of ∼200 mm. Values given in parentheses are standard deviations. Fig. 9. Stress–displacement curve from a push-back test performed on composite J. April 1998 Interfacial Bond Strength in SiC/C/SiC Composite Materials 973