Journal of the American Ceramic Sociery-Rebillat et al. Vol 81. No 4 Il. Description of the Materials The interfacial parameters were extracted from the experi- mental-stress-versus-fiber-end-displacement curves, using the O Processing and Chemical Analysis The carbon interphase and the Sic matrix were infiltrated placement(u) predicted by the Hsueh model(after subtracting into porous fiber preforms by using the isothermal/isobaric the contribution of the loading-frame deformation) for the chemical vapor infiltration(I-CVI) process. 17 Two series of applied stress, or, is given by previously by h(a++2 Droillard: 13 the first used as-received Nicalon TM fabric(de (1 noted as " I"' in Droillard), and the second used treated Nicalon TM fabric(denoted as"J"and"P"in Droillard where aa is the stress required to initiate debonding and sliding, The free surface of untreated Nicalon TM fibers contains silica h the sliding length, and a, the axial residual stress SiO2 ). Therefore, the fiber/interphase interface is the silica amorphous )anisotropic pyC interface, whereas the surface of The sliding length h and the interfacial shear stress T can be separated into a constant and a variable component treated NicalonTM fibers is rich in free carbon and, as a result, the fiber/interphase interface may be regarded as a carbon/ T=-u(o +on) carbon interface. Such an interface exhibits a stronger bond than the silica/anisotropic pyC interface, which is generall described as weak 8,9 Two carbon thicknesses have been de ted on the fibers: 0. 1(material P)and 0.5 um(materials I and J)(Table 1) where oe is the clamping residual stress, u the coefficient of friction, and o, the average contribution to the clamping stress (2) Stress-Strain Behavior due to Poissons effect. The variable o, is given by Tensile stress-strain curves for the materials were previously ported by Droillard. Two distinct behaviors, depending on l「/ V Em fv the fiber surface, were observed(Fig. 1). A plateaulike behav 2D( E I )+(m) ior was obtained for the as-received fiber-reinforced material ( material 1). However, the mechanical behavior was signifi cantly improved(material J) when fiber-matrix bonding was modified by fiber surface treatment( Fig. 1), as indicated by the following features: (i)a 50% higher failure stress, (ii)a satu- I-f+Vm+ E ration stress similar to that of ultimate failure, (iii)a very small where ve and v are the Poissons ratios for the fiber and crack-spacing distance at saturation(\s 20-30 um, versus matrix, respectively. A uniform Poisson's effect, characterized 115-300 um for the composite reinforced with untreated fi- by an average stress o, is assumed along the sliding interface bers),(iv)small residual deformation, and (v) narrow hyster- Hence, an average interfacial shear stress, T, is determined and esis loops when unloading-reloading was performed(approxi he stress in the fiber, or varies linearly along the sliding mately one-tenth of the width observed for the composites with length, from the applied stress o at the surface to the debond as-received Nicalon TM fibers). Short crack-spacing distances, stress od at the end of the sliding zone: 0 =(0+ gdv as well as small hysteresis loop widths, reflect strong fiber matrix bonding. Examination of interfacial regions via trans- varies with the sliding distance. During crack propagation,the mission electron microscopy(TEM) revealed a complex net axial stress in the fiber at the end of the sliding zone is always work of cracks branching into microcracks within the in equilibrium with o interphase in the treated fiber-reinforced composites(upper in B)Push-Back Tests: By turning over the sample, push- set in Fig. 1), whereas a single crack that propagated along the back tests can be conducted on the pushed-out fibers. Because fiber surface over a long distance was generally observed thes nly frictional for the osites with as-received fibers(lower inset characterized from the stress-versus-displacement curves. This technique is an alternative method to extract the sliding char- acteristics. However, during reverse sliding of the fibers, wear lIL. Determination of Interfacial Characteristics underestimation of interphase/interface properties. Neve of the sliding interphases may occur, which leads to significar Microindentation Tests theless, these tests can provide useful data on the compos- ites reinforced with treated fibers that complement the data (A) Push-Out Tests: A wedge specimen geometry was rmined by the push-out the specimens only needed to be half that commonly used for (2) Nanoindentation Tests oush-in tests, 9(-500 um) to push the fibers out. The tests Another method for fiber push testing uses a nanoindentor (ORNL) Interfacial Test System 20 at a constant displacement (MPM)(Nano Instrument, Knoxville, TN). The MPM that has rate of 0. 1 um/s. A flat-end diamond tip was used to push the been used for the present study allows an accurate application fl of loads in the millinewton range via a Berkovich pyramidal Table L. Investigated Materials and Main Tensile Mechanical Properties Failure Maximum Material cMi Nontreated 241 1.07 183 185 15.2-21.3 Treate 356 24-293 Treated 353 20-30II. Description of the Materials (1) Processing and Chemical Analysis The carbon interphase and the SiC matrix were infiltrated into porous fiber preforms by using the isothermal/isobaric chemical vapor infiltration (I-CVI) process.17 Two series of 2D-SiC/C/SiC composites have been prepared previously by Droillard:13 the first used as-received Nicalon™ fabric (de￾noted as ‘‘I’’ in Droillard13), and the second used treated Nicalon™ fabric14 (denoted as ‘‘J’’ and ‘‘P’’ in Droillard13). The free surface of untreated Nicalon™ fibers contains silica (SiO2). Therefore, the fiber/interphase interface is the silica (amorphous)/anisotropic pyC interface, whereas the surface of treated Nicalon™ fibers is rich in free carbon and, as a result, the fiber/interphase interface may be regarded as a carbon/ carbon interface.13 Such an interface exhibits a stronger bond￾ing than the silica/anisotropic pyC interface, which is generally described as weak.8,9 Two carbon thicknesses have been de￾posited on the fibers: 0.1 (material P) and 0.5 mm (materials I and J) (Table I). (2) Stress–Strain Behavior Tensile stress–strain curves for the materials were previously reported by Droillard.13 Two distinct behaviors, depending on the fiber surface, were observed (Fig. 1). A plateaulike behav￾ior was obtained for the as-received fiber-reinforced material (material I). However, the mechanical behavior was signifi￾cantly improved (material J) when fiber–matrix bonding was modified by fiber surface treatment (Fig. 1), as indicated by the following features: (i) a 50% higher failure stress, (ii) a satu￾ration stress similar to that of ultimate failure, (iii) a very small crack-spacing distance at saturation (ls 4 20–30 mm, versus 115–300 mm for the composite reinforced with untreated fi￾bers), (iv) small residual deformation, and (v) narrow hyster￾esis loops when unloading–reloading was performed (approxi￾mately one-tenth of the width observed for the composites with as-received Nicalon™ fibers). Short crack-spacing distances, as well as small hysteresis loop widths, reflect strong fiber– matrix bonding. Examination of interfacial regions via trans￾mission electron microscopy (TEM) revealed a complex net￾work of cracks branching into microcracks within the interphase in the treated fiber-reinforced composites (upper in￾set in Fig. 1), whereas a single crack that propagated along the fiber surface over a long distance was generally observed for the composites with as-received fibers (lower inset in Fig. 1).16,17 III. Determination of Interfacial Characteristics (1) Microindentation Tests (A) Push-Out Tests: A wedge specimen geometry was used for the indentation tests.18 However, the thickest part of the specimens only needed to be half that commonly used for push-in tests18,19 (∼500 mm) to push the fibers out. The tests were conducted using the Oak Ridge National Laboratory (ORNL) Interfacial Test System20 at a constant displacement rate of 0.1 mm/s. A flat-end diamond tip was used to push the fibers. The interfacial parameters were extracted from the experi￾mental-stress-versus-fiber-end-displacement curves, using the push-out model proposed by Hsueh.21,22 The fiber-end dis￾placement (u) predicted by the Hsueh model (after subtracting the contribution of the loading-frame deformation) for the applied stress, s, is given by u = h~s + sd + 2sz! 2Ef (1) where sd is the stress required to initiate debonding and sliding, h the sliding length, and sz the axial residual stress. The sliding length h and the interfacial shear stress t can be separated into a constant and a variable component: t = −m~sc + sp! (2) h = r~sd − s! 2t (3) where sc is the clamping residual stress, m the coefficient of friction, and sp the average contribution to the clamping stress due to Poisson’s effect. The variable sp is given by sp = 1 2D FS nf Em Ef − f nm 1 − f Ds + S nf Em Ef − fnm 1 − f DsdG (4) with D = 1 + f 1 − f + nm + ~1 − nf!Em Ef (5) where nf and nm are the Poisson’s ratios for the fiber and matrix, respectively. A uniform Poisson’s effect, characterized by an average stress sp, is assumed along the sliding interface. Hence, an average interfacial shear stress, t, is determined and the stress in the fiber, sf , varies linearly along the sliding length, from the applied stress s at the surface to the debond stress sd at the end of the sliding zone: s 4 (s + sd)/2. However, it is noted that the average interfacial shear stress varies with the sliding distance. During crack propagation, the axial stress in the fiber at the end of the sliding zone is always in equilibrium with sd. (B) Push-Back Tests: By turning over the sample, push￾back tests can be conducted on the pushed-out fibers. Because these fibers are already debonded, only frictional sliding is characterized from the stress-versus-displacement curves. This technique is an alternative method to extract the sliding char￾acteristics. However, during reverse sliding of the fibers, wear of the sliding interphases may occur, which leads to significant underestimation of interphase/interface properties. Never￾theless, these tests can provide useful data on the compos￾ites reinforced with treated fibers that complement the data determined by the push-out test. (2) Nanoindentation Tests Another method for fiber push testing uses a nanoindentor that is also called the Mechanical Properties Microprobe (MPM) (Nano Instrument, Knoxville, TN). The MPM that has been used for the present study allows an accurate application of loads in the millinewton range via a Berkovich pyramidal Table I. Investigated Materials and Main Tensile Mechanical Properties† Material Nature of the fabric Interphase thickness (mm) Failure stress (MPa) Failure strain (%) Young’s modulus (GPa) Matrix crack spacing at saturation (mm) Interfacial shear stress (MPa) Maximum strain energy (kJ/m2 ) I Nontreated 0.5 241 1.07 183 185 4 15.2–21.3 J Treated 0.5 356 1.00 170 20 370 24–29.3 P Treated 0.1 P Treated 0.1 353 0.9 250 20 210 20–30 † From Droillard.13 966 Journal of the American Ceramic Society—Rebillat et al. Vol. 81, No. 4