wwceramics. org/ACT Layered Interphases in SiC/SiC Composites I,J: Pycsoo M, N: PyC reated Nicalon as-received Nicalon J- Stronger F\-bonding I- weak FM-bondiag Fig 1. Tensile curves at ambient of 2D-SiC/SiC (CV) fabricated from Nicalon fibers with different interphases: single P)yC layers or (ByC-SiC) ML interphases(adapted from Droillard'y received fibers (sample I). Matrix microcracks are compressive TRS and lower interfacial shear stress deflected near fiber surface (I-inset, right) for the latter, Experimental studies have shown that the mechanical at the level of the weak carbon/ silica interface and within properties. 5.28 Dupel et through an optimum for e(PyC)=few the PyC interphase(in a diffused manner and over short 100 nm. at distance, J-inset, left) for the former. Here, the PyC inter- tensile properties of ID-SiC(NCG)Py C/SiC (P-CVD phase does play its role of mechanical fuse. These two inicomposites were optima for e(PyC)=220 nm.The microcrack deflection modes correspond to very different calculated radial TRS in the interphase was compressive FM coupling(weak in I and stronger in D. Interestingly, (enhancing thus the FM bonding) at low e(PyC)values toughness of the composites fabricated with pretreated and tensile(favoring FM debonding) for e(PyC)> fibers as well as their fatigue resistance in tensile cyclic 400 nm. loading are higher when it is the Py C interphase, which is In sic/sic fabricated with stoichiometric fibers the active mechanical fuse(strong bonding). 22/Similar the situation is different because the CTEs of the main results have been reported for composites fabricated from constituents are now similar but the fiber surface is Hi-Nicalon fibers and Pyc interphas highly crystalline and rough. The mechanical properties of 2D-SiC/PyC/SiC( CVI)are either constant or Influence of Py C-Interphase Thickness slightly dependent on interphase thickness, when increases from 25 to PyC-layer thickness, e(PyC), has an infuence on be expected from fiber roughness, the interfacial fric- the mechanical properties of SiC/Pyc/SiC ce tional stress tf is high composites with Ty through thermal residual stresses(TRS) and fiber surfac fibers than for those with Hi-Nicalon fibers and decreases as e(PyC Increases In SiC/PyC/SiC(CV) fabricated at≈1000°C using Nicalon( Ceramic Grade, NCG)or Hi-Nicalon Crack Deflection Modeling fibers, the coefficient of thermal expansion(CTE)of the fibers is lower than that of the matrix, which results in Crack deflection at an interface in brittle materials compressive radial TRS at the FM interface and has been modeled. ,3 Recently, Pompidou and reinforces FM bonding. Further, the fiber surface is Lamon 3. 34 have proposed a model, derived from the very smooth. Hence, increasing e(PyC) relaxes radial approach of Cook and Gordon, which is applicable toreceived fibers (sample I). Matrix microcracks are deflected near fiber surface (I-inset, right) for the latter, at the level of the weak carbon/silica interface and within the PyC interphase (in a diffused manner and over short distance, J-inset, left) for the former. Here, the PyC inter￾phase does play its role of mechanical fuse. These two microcrack deflection modes correspond to very different FM coupling (weak in I and stronger in J). Interestingly, toughness of the composites fabricated with pretreated fibers as well as their fatigue resistance in tensile cyclic loading are higher when it is the PyC interphase, which is the active mechanical fuse (strong bonding).1,2,27 Similar results have been reported for composites fabricated from Hi-Nicalon fibers and PyC interphase. Influence of PyC-Interphase Thickness PyC-layer thickness, e(PyC), has an influence on the mechanical properties of SiC/PyC/SiC composites, through thermal residual stresses (TRS) and fiber surface roughness. In SiC/PyC/SiC (CVI) fabricated at 10001C using Nicalon (Ceramic Grade, NCG) or Hi-Nicalon fibers, the coefficient of thermal expansion (CTE) of the fibers is lower than that of the matrix, which results in compressive radial TRS at the FM interface and reinforces FM bonding. Further, the fiber surface is very smooth. Hence, increasing e(PyC) relaxes radial compressive TRS and lower interfacial shear stress. Experimental studies have shown that the mechanical properties go through an optimum for e(PyC) 5 few 100 nm.11,15,28 Dupel et al. 29 have reported that the tensile properties of 1D-SiC (NCG)/PyC/SiC (P-CVI) minicomposites were optima for e(PyC) 5 220 nm. The calculated radial TRS in the interphase was compressive (enhancing thus the FM bonding) at low e(PyC) values and tensile (favoring FM debonding) for e(PyC)4 400 nm. In SiC/SiC fabricated with stoichiometric fibers, the situation is different because the CTEs of the main constituents are now similar but the fiber surface is highly crystalline and rough. The mechanical properties of 2D-SiC/PyC/SiC (CVI) are either constant or slightly dependent on interphase thickness, when e(PyC) increases from 25 to 250 nm.17,30 As it could be expected from fiber roughness, the interfacial fric￾tional stress tf is higher for composites with Tyranno fibers than for those with Hi-Nicalon fibers and decreases as e(PyC) increases.12 Crack Deflection Modeling Crack deflection at an interface in brittle materials has been modeled.31,32 Recently, Pompidou and Lamon33,34 have proposed a model, derived from the approach of Cook and Gordon,32 which is applicable to Fig. 1. Tensile curves at ambient of 2D-SiC/SiC (CVI) fabricated from Nicalon fibers with different interphases: single PyC layers or (PyC–SiC)n ML interphases (adapted from Droillard1 ). www.ceramics.org/ACT Layered Interphases in SiC/SiC Composites 265