Journal of the American Ceramic Society-Rebillat et al. Vol 81. No 9 Table L. Description of the Investigated Materials Material the fabrics sequences, I Nature of the C-SiC sequence in the interphase NT 2244 0.1 0.1 B F K NT SiC 0.05 T F/C/ Si 0.05 0.0 0.0 .05 0.15 nformation regarding the investigated materials was taken from Droillard. The abbreviations"NT"and"T"denote nontreated and treated conditions, respectively. 8-F" represent fiber and matrix. respectively. The numbers directly below each phase represent the thickness of the phase(in units of um). received fibers(materials 1, A, and K), the tensile stress-strain um for the composites reinforced with as-received fibers) curves(Fig. 1)exhibit relatively high fracture strains and This feature is in opposition to the concept that significant fiber stresses(0.9% and 300 MPa, respectively ).5,6 The presence of pull-out is necessary to achieve high toughness. As a result, a typical plateaulike behavior during matrix cracking indicates natrix crack spacing at saturation is rather short(Table II)and that the fiber/interphase bond is weak and load transfer is the density of cracks is very high. The residual strains are very small and the hysteresis loops width on unloading-reloading is Similar failure strains and 50%-higher stresses are obtained very narrow, the widest hysteresis loop is approximately one for those composites reinforced with treated fibers(materials J, tenth that observed for composites reinforced with as-receivec and L). The nonlinear stress-strain behavior induced by matrix cracking is observed up to the ultimate strength of the omposite(Fig. 1). This feature demonstrates rather stron ber/matrix interactions. In those composites reinforced with lL. Mechanical Characterization of interfaces treated fibers the deviation of matrix cracks involves extensive and Interphases crack branching within the interphase, as opposed to a single crack in the fiber/carbon interface of composites reinforced () Experimental Conditions with as-received fibers(Fig. 2)5-79 In the former, crack The push-out tests were conducted using the Interfacial Test propagate first along a short distance within the first carbon System developed at Oak Ridge National Laboratory. The sublayer near the matrix, then they cross the sic sublayer and top of the fibers was loaded at a constant displacement rate of continue propagating and branching in the subsequent carbon 0. 1 mm/s, using a diamond indentor mounted on a load cell blayer. Crack arrest is observed as a result of energy release The samples consisted of 500-pum-thick wedges, prepared us- ia the creation of several failure surfaces and the frictional ing standard metallographic techniques sliding of crack surfaces. Push-back tests were also conducted on the The composites that contain treated fibers have high stra inforced with treated fibers(composites J and B) energies (Table Il the fracture surfaces exhibit short es were urned over once fibers fiber pull-out lengths -30 um, in comparison to 1 15-300 completely debonded by pushout and the load was 400 00 曰20 100 Fig. 1. Tensile stress-strain curves for 2D-SiC/SiC composites with multilayered interphases n(C-SiC)and reinforced with as-received(A, L, K) eated(B, J, L) Nicalon fibers(from Droillard5)received fibers (materials I, A, and K), the tensile stress–strain curves (Fig. 1) exhibit relatively high fracture strains and stresses (0.9% and 300 MPa, respectively).5,6 The presence of a typical plateaulike behavior during matrix cracking indicates that the fiber/interphase bond is weak and load transfer is poor.5–11 Similar failure strains and 50%-higher stresses are obtained for those composites reinforced with treated fibers (materials J, B, and L). The nonlinear stress–strain behavior induced by matrix cracking is observed up to the ultimate strength of the composite (Fig. 1). This feature demonstrates rather strong fiber/matrix interactions. In those composites reinforced with treated fibers, the deviation of matrix cracks involves extensive crack branching within the interphase, as opposed to a single crack in the fiber/carbon interface of composites reinforced with as-received fibers (Fig. 2).5–7,9 In the former, cracks propagate first along a short distance within the first carbon sublayer near the matrix, then they cross the SiC sublayer and continue propagating and branching in the subsequent carbon sublayer. Crack arrest is observed as a result of energy release via the creation of several failure surfaces and the frictional sliding of crack surfaces. The composites that contain treated fibers have high strain energies (Table II), although the fracture surfaces exhibit short fiber pull-out lengths (∼20–30 mm, in comparison to 115–300 mm for the composites reinforced with as-received fibers).6 This feature is in opposition to the concept that significant fiber pull-out is necessary to achieve high toughness.1 As a result, matrix crack spacing at saturation is rather short (Table II) and the density of cracks is very high. The residual strains are very small and the hysteresis loops width on unloading–reloading is very narrow; the widest hysteresis loop is approximately one tenth that observed for composites reinforced with as-received fibers.5,6 III. Mechanical Characterization of Interfaces and Interphases (1) Experimental Conditions The push-out tests were conducted using the Interfacial Test System developed at Oak Ridge National Laboratory.12 The top of the fibers was loaded at a constant displacement rate of 0.1 mm/s, using a diamond indentor mounted on a load cell. The samples consisted of 500-mm-thick wedges, prepared us￾ing standard metallographic techniques.13 Push-back tests were also conducted on the composites re￾inforced with treated fibers (composites J and B). For this purpose, the samples were turned over once fibers had been completely debonded by pushout and the load was applied to Table I. Description of the Investigated Materials† Material Nature of the fabric‡ Number of (C–SiC) sequences, n Nature of the C–SiC sequence in the interphase§ I NT 1 F / C / M 0.5 J T 1 F/ C / M 0.5 A NT 2 F / C / SiC / C / M 0.1 0.3 0.1 B T 2 F / C / SiC / C / M 0.1 0.3 0.1 K NT 4 F / C / SiC / C / SiC / C / SiC / C / M 0.05 0.05 0.05 0.1 0.05 0.15 0.05 L T 4 F / C / SiC / C / SiC / C / SiC / C / M 0.05 0.05 0.05 0.1 0.05 0.15 0.05 † Information regarding the investigated materials was taken from Droillard.5 ‡The abbreviations ‘‘NT’’ and ‘‘T’’ denote nontreated and treated conditions, respectively. § ‘‘F’’ and ‘‘M’’ represent fiber and matrix. respectively. The numbers directly below each phase represent the thickness of the phase (in units of mm). Fig. 1. Tensile stress–strain curves for 2D-SiC/SiC composites with multilayered interphases n(C–SiC) and reinforced with as-received (A,I,K) or treated (B,J,L) Nicalon fibers (from Droillard5 ). 2316 Journal of the American Ceramic Society—Rebillat et al. Vol. 81, No. 9