International y ournal of Applied Ceramic TechnologyNaslain Vol.2,No.2,2005 Fiber Matrix 500 Fig 1. Interphases for SiC/SiC composites with layered crystal structure or microstructure: (a)anisotropic pyrocarbon single-layer ase and (b)(pyC-SiCIo multilayered interphase. oxidizing atmospheres is improved by self-healing phe- of damaging phenomena, mainly including multiple nomena(silica or SiO2-B2O3 scales formed by oxidation matrix microcracking and FM-debonding. As a result, healing the narrow annular pore created around each fiber their stiffness progressively decreases as the applied load by oxidation)(Fig. 1b). Another interphase concept is raised beyond the proportional limit(SiC/SiC com- that has been less explored is the use of a porous SiC layer, posites), with little permanent deformation upon un- a porous solid displaying a lower failure energy than its loading (at least for well-processed materials). Hence, dense counterpart. However, such a porous interface they are often referred to as damageable elastic materials would favor the oxidation of the fibers as mentioned pre- TI he extent of the nonlinear domain in which the ma- ously for porous matrices terials are damage-tolerant is related to the ultimate fail- Finally, a seal-coating is usually deposited on the ex- ure strain of the fibers (e. g, the latter becoming low, ternal surface of C/SiC and SiC/SiC composites, mainly typically 0.6-0.7% for stoichiometric SiC fibers). Fur- to seal the residual open porosity(composites fabricated ther, the damage features are strongly related to the in- by the Pip or CVI processes)or/and to improve their tensity of the FM-bonding, a point that is often resistance to corrosive environments. Dense single layer underestimated. When the FM-bonding is too weak ceramic coatings(such as SiC or Si3N4) displaying a the matrix microcrack density is low, the microcracks tendency to microcracking(as a result of CTE-mismatch are widely open under load, and debonding occurs over or mechanical loading) multilayered coatings are prefer a long distance(and sometimes over the whole fiber able, as it will be discussed in the next section. Such ngth, exposing the oxidation-prone fibers to the am- coatings are deposited by PVd or P-CVD bient environment). By contrast, when the FM-bonding is stronger and the interphase is strongly adherent to the Selected Properties fiber. it is the reverse situation that is observed. the composite displaying a higher failure stress(Fig. 2)and Mechanical Bebavior a better oxidation resistance. SiC-matrix ce tough when properly designed and fabricated SiC-matrix composites display a nonlinear stress- toughness, expressed in terms of critical energy release ain behavior when tensile loaded in one of the fiber rate of the order of 10 kJ/m2, whereas that of monolithic ections. This nonlinearity is related to the occurrence SiC-ceramics is of the order of a few 100oxidizing atmospheres is improved by self-healing phe￾nomena (silica or SiO2–B2O3 scales formed by oxidation healing the narrow annular pore created around each fiber by oxidation) (Fig. 1b).4,18 Another interphase concept that has been less explored is the use of a porous SiC layer, a porous solid displaying a lower failure energy than its dense counterpart. However, such a porous interface would favor the oxidation of the fibers as mentioned pre￾viously for porous matrices. Finally, a seal-coating is usually deposited on the ex￾ternal surface of C/SiC and SiC/SiC composites, mainly to seal the residual open porosity (composites fabricated by the PIP or CVI processes) or/and to improve their resistance to corrosive environments. Dense single layer ceramic coatings (such as SiC or Si3N4) displaying a tendency to microcracking (as a result of CTE-mismatch or mechanical loading) multilayered coatings are prefer￾able, as it will be discussed in the next section.19 Such coatings are deposited by PVD or P-CVD. Selected Properties Mechanical Behavior SiC-matrix composites display a nonlinear stress– strain behavior when tensile loaded in one of the fiber directions. This nonlinearity is related to the occurrence of damaging phenomena, mainly including multiple matrix microcracking and FM-debonding. As a result, their stiffness progressively decreases as the applied load is raised beyond the proportional limit (SiC/SiC com￾posites), with little permanent deformation upon un￾loading (at least for well-processed materials). Hence, they are often referred to as damageable elastic materials. The extent of the nonlinear domain in which the ma￾terials are damage-tolerant is related to the ultimate fail￾ure strain of the fibers (e.g., the latter becoming low, typically 0.6–0.7% for stoichiometric SiC fibers). Fur￾ther, the damage features are strongly related to the in￾tensity of the FM-bonding, a point that is often underestimated.15 When the FM-bonding is too weak, the matrix microcrack density is low, the microcracks are widely open under load, and debonding occurs over a long distance (and sometimes over the whole fiber length, exposing the oxidation-prone fibers to the am￾bient environment). By contrast, when the FM-bonding is stronger and the interphase is strongly adherent to the fiber, it is the reverse situation that is observed, the composite displaying a higher failure stress (Fig. 2) and a better oxidation resistance. SiC-matrix composites are tough when properly designed and fabricated with toughness, expressed in terms of critical energy release rate of the order of 10 kJ/m2 , whereas that of monolithic SiC-ceramics is of the order of a few 100 J/m2 . 15 Fig. 1. Interphases for SiC/SiC composites with layered crystal structure or microstructure: (a) anisotropic pyrocarbon single-layer interphase15 and (b) (PyC–SiC)10 multilayered interphase.18 78 International Journal of Applied Ceramic Technology—Naslain Vol. 2, No. 2, 2005