REBILLAT et al: SiC/SIC COMPOSITES mechanical response of composites described by the tress -strain curve Fiber debonding results from the deflection of the cracks that initiate in the matrix(Fig. 1). Then sliding 5 of the fiber debonded in the interface determines the load transfers from the fiber to the matrix and vice 5 200 versa. The fiber sliding is influenced by the misfit E strain, the associated radial component of the ther mally induced residual stress-field, surface roughness Weak interfaces debond easily. A single long debond crack is located at the surface of the fibers in o LONGITUDINAL TE small interface shear stresses, load transfers through Fig. 2: Typical tensile stress-strain behaviors measured on 2D the debonded interfaces are poor. The matrix becomes ricated from untreated or treated Nicalon(ceramic grade)fit subjected to lower stresses and the volume of matrix ers: (a)strong fiber/coating interfaces and (b) that may experience further cracking is reduced the presence of long debonds. The cracks are gener- ally widely opened, whereas the crack spacing dis- tance at saturation as well as the pull out length tend (cohesive failure type, Fig. 1), into short and branched to be rather long (100 um). Toughening results multiple cracks [4, 12]. Short debonds as well as essentially from sliding friction along the debonds. Improved load transfers allow further cracking of the matrix via a scale effect 6, 7] leading to a hig However, due to poor load transfers and long density of matrix cracks(which are slightly opened debonds,the fibers carry most of the load, which Sliding friction within the coating as well as multiple reduces the composite strength. The corresponding cracking of the matrix increase energy absorption, tensile stress-strain curve exhibits a short curved domain limited by a stress at matrix cracking satu- leading to toughening. Limited debonding and fibers, leading to strengthening. The associated tensile In the presence of stronger fiber/coating bonds, the stress strain curve exhibits a wide curved domain and the stress at matrix cracking saturation is close to ulti mate failure(Fig. 2). Table 1 gives various values of the interfacial shear stresses measured using various debond crack methods on SiC/SiC composites with PyC-based fiber coating. It can be noticed that the interfacial shear stresses range between 10 and 20 MPa for the weak interfaces whereas they are larger than 100-300 MPa for the strong interfaces. Additional data can be found in[4,7,8,24,29] 3. SiC/BN/SiC COMPOSITES: TESTING METHODOLOGY AND MICROSTRUCTURAL ANALYSES Fiber 3. 1. Specimen preparation debond crack SIC/BN/SIC microcomposites and woven com posites were manufactured via chemical vapor infil- tration [13]. They were reinforced with either as received or treated(proprietary treatment, SNECMA/SEP, Bordeaux) SiC Nicalon fibers (NL 202 grade). The SiC/BN/SIC mIcrocosmos tes consist of a single fiber (15 um diameter ), coated with a boron nitride layer(0.3-0.9 um thick) and a Sic matrix deposited by CVD. They are tive of Fiber their counterparts in the 2D woven con they are produced using identical deposition conditions [13] Fig 1 Schematic diagram showing crack deflection when the A single or a bilayered BN fiber coating was fiber coating/interface is (a) strong or(b) weak. deposited from a BF3, NH3, Ar gas mixture (Table4610 REBILLAT et al.: SiC/SiC COMPOSITES mechanical response of composites described by the stress–strain curve. Fiber debonding results from the deflection of the cracks that initiate in the matrix (Fig. 1). Then sliding of the fiber debonded in the interface determines the load transfers from the fiber to the matrix and vice versa. The fiber sliding is influenced by the misfit strain, the associated radial component of the ther￾mally induced residual stress-field, surface roughness and debond length. Weak interfaces debond easily. A single long debond crack is located at the surface of the fibers in those composites exhibiting weak interfaces (adhesive failure type, Fig. 1). As a consequence of small interface shear stresses, load transfers through the debonded interfaces are poor. The matrix becomes subjected to lower stresses and the volume of matrix that may experience further cracking is reduced by the presence of long debonds. The cracks are gener￾ally widely opened, whereas the crack spacing dis￾tance at saturation as well as the pull out length tend to be rather long (>100 µm). Toughening results essentially from sliding friction along the debonds. However, due to poor load transfers and long debonds, the fibers carry most of the load, which reduces the composite strength. The corresponding tensile stress–strain curve exhibits a short curved domain limited by a stress at matrix cracking satu￾ration which is significantly smaller than ultimate strength (Fig. 2). In the presence of stronger fiber/coating bonds, the matrix cracks are deflected within the coating Fig. 1. Schematic diagram showing crack deflection when the fiber coating/interface is (a) strong or (b) weak. Fig. 2. Typical tensile stress–strain behaviors measured on 2D SiC/SiC composites possessing PyC based interphases and fab￾ricated from untreated or treated Nicalon (ceramic grade) fib￾ers: (a) strong fiber/coating interfaces and (b) weak fiber/coating interfaces. (cohesive failure type, Fig. 1), into short and branched multiple cracks [4, 12]. Short debonds as well as improved load transfers allow further cracking of the matrix via a scale effect [6, 7] leading to a higher density of matrix cracks (which are slightly opened). Sliding friction within the coating as well as multiple cracking of the matrix increase energy absorption, leading to toughening. Limited debonding and improved load transfers reduce the load carried by the fibers, leading to strengthening. The associated tensile stress strain curve exhibits a wide curved domain and the stress at matrix cracking saturation is close to ulti￾mate failure (Fig. 2). Table 1 gives various values of the interfacial shear stresses measured using various methods on SiC/SiC composites with PyC-based fiber coating. It can be noticed that the interfacial shear stresses range between 10 and 20 MPa for the weak interfaces whereas they are larger than 100–300 MPa for the strong interfaces. Additional data can be found in [4, 7, 8, 24, 29]. 3. SiC/BN/SiC COMPOSITES: TESTING METHODOLOGY AND MICROSTRUCTURAL ANALYSES 3.1. Specimen preparation SiC/BN/SiC microcomposites and woven com￾posites were manufactured via chemical vapor infil￾tration [13]. They were reinforced with either as￾received or treated (proprietary treatment, SNECMA/SEP, Bordeaux) SiC Nicalon fibers (NL 202 grade). The SiC/BN/SiC microcomposites consist of a single fiber (15 µm diameter), coated with a boron nitride layer (0.3–0.9 µm thick) and a SiC matrix deposited by CVD. They are representative of their counterparts in the 2D woven composites, since they are produced using identical chemical vapor deposition conditions [13]. A single or a bilayered BN fiber coating was deposited from a BF3, NH3, Ar gas mixture (Table