266 International Journal of Applied Ceramic Technolog-Naslain, Pailler and Lamon Vol. 7, No 3, 2010 The model has been applied to SiC/PyCSiC to examine crack deflection probability at a given interface or within the interphase, to show the influence of fiber pretreatment and to discuss the effect of graphene-layer SIC/Pyc orientation in the interphase Deflection at first interface 0.8 (PyC /SiCm)is very unlikely because for the related E2/EI ratio (N 0.07), the value of oi should be extremely low(Fig. 2) in accordance with experiments By contrast, deflection at second interface( SiCpPyCi)is PyC/Pyc most likely because for the corresponding E2/E, ratio (10 for Hi-Nicalon) the width of the debonding domain (debonding potential) is very large. This is the most frequently observed case(weak FM bonding). If the fiber has been pretreated to strengthen the FM bonding, the representative point may move above the master curve,with debonding no longer occurring at that interface. However, it may take place within the PyC Fig.2. Values of of of ratio provided by the master curve for for a EyE, value of 1, a still significant deflection p us fber/matrix and fberlinterphase/matrix systems(cracked tential (Fin awep s a f now urf e nene layers are lamn al (material I)cited second)(adapted from Pompidou and treated Nicalon fiber deposited perpendicular to fber surface, crack deflection within the PyC interphase becomes no longer possible composites with single- or multilayered interphases. because the ratio opyc//opyci N 2. 17 is well above When a crack of tip radius p is placed in an elastic the master curve for E2/E1=1 medium and subjected to a uniaxial tension Oz(in a direction z perpendicular to crack plane), it generates a Layered Interphases for SiC/SiC Exposed to multiaxial stress field near crack tip whose orr compo- Oxidizing Atmosphere nent(in radial direction)is maximum at a distance on the order of p(omx=o(r=p)). If an interface is PyC is oxidation prone even at temperature as low as placed perpendicular to primary crack extension direc tion near crack tip, a secondary local crack may nucleat 500C, its oxidation resulting in the formation of gas- eous oxides(active oxidation) and degradation of FM at that interface if om.>o:, where o; is the interface coupling 35.36 Two approaches have been selected to solve debonding stress. Deflection results from coalescence of this problem relying on self-healing (or self-sealing)mech- both cracks. 2 When applied to a microcomposite anisms by condensed oxides(passive oxidation). The first loaded in tension along fiber axis, debonding would occur when of/of s omax / oma(with r>D, where of whereas in the second, part of PyC is replaced, in the the failure stress of iber and, l the distance so-called Ml interphases, by Sic or tic to reduce the between crack tip and interface (or ligament), thickness of each elementary PyC sublayer to a few 10 nm om and omax were computed and their ratio plotted and to favor self-healing phenomena? versus Youngs moduli ratio E2/E1, as shown in Fig. 2 The domain under this master curve corresponds to the Boron-Doped PyCInterpbase debonding situation and that above to conditions where debonding cannot occur. The curve exhibits a The addition of boron to Py C increases its graphitic mum corresponding to the highest debonding potential. character at low B concentration and improves its Conversely, when E2/En decreases and tends to zero, oxidation resistance at high B levels by blocking the debonding becomes quite impossible. But, the crack can so-called active sites and forming a Auid oxide(B2O3)in be arrested. Failure of the reinforcing material depends a temperature range(500-900oC), where the growth on its strength versus stress operating: of versus omx. kinetics of silica is still too slow.composites with single- or multilayered interphases. When a crack of tip radius r is placed in an elastic medium and subjected to a uniaxial tension szz (in a direction z perpendicular to crack plane), it generates a multiaxial stress field near crack tip whose srr compo￾nent (in radial direction) is maximum at a distance on the order of r (smax rr ¼ srrðr ¼ rÞ). If an interface is placed perpendicular to primary crack extension direc￾tion near crack tip, a secondary local crack may nucleate at that interface if smax rr > sc i , where si c is the interface debonding stress. Deflection results from coalescence of both cracks.32 When applied to a microcomposite loaded in tension along fiber axis, debonding would occur when sc i =sc f smax rr =smax zz (with r4l), where sf c is the failure stress of the fiber and, l, the distance between crack tip and interface (or ligament),33,34 smax rr and smax zz were computed and their ratio plotted versus Young’s moduli ratio E2/E1, as shown in Fig. 2. The domain under this master curve corresponds to the debonding situation and that above to conditions where debonding cannot occur. The curve exhibits a maxi￾mum corresponding to the highest debonding potential. Conversely, when E2/E1 decreases and tends to zero, debonding becomes quite impossible. But, the crack can be arrested. Failure of the reinforcing material depends on its strength versus stress operating: sf c versus smax zz . The model has been applied to SiC/PyC/SiC to examine crack deflection probability at a given interface or within the interphase, to show the influence of fiber pretreatment and to discuss the effect of graphene-layer orientation in the interphase. Deflection at first interface (PyCi/SiCm) is very unlikely because for the related E2/E1 ratio ( 0.07), the value of si c should be extremely low (Fig. 2) in accordance with experiments. By contrast, deflection at second interface (SiCf/PyCi) is most likely because for the corresponding E2/E1 ratio (10 for Hi-Nicalon) the width of the debonding domain (debonding potential) is very large. This is the most frequently observed case (weak FM bonding). If the fiber has been pretreated to strengthen the FM bonding, the representative point may move above the master curve, with debonding no longer occurring at that interface. However, it may take place within the PyC interphase, that is, at a PyCi/PyCi interface that shows for a E2/E1 value of 1, a still significant deflection po￾tential (Fig. 2). This is the situation observed, for pre￾treated Nicalon fiber.1–4 If now the graphene layers are deposited perpendicular to fiber surface, crack deflection within the PyC interphase becomes no longer possible because the ratio sc PyC===sc PyC? 2:17 is well above the master curve for E2/E1 5 1. Layered Interphases for SiC/SiC Exposed to Oxidizing Atmosphere PyC is oxidation prone even at temperature as low as 5001C, its oxidation resulting in the formation of gas￾eous oxides (active oxidation) and degradation of FM coupling.35,36 Two approaches have been selected to solve this problem relying on self-healing (or self-sealing) mech￾anisms by condensed oxides (passive oxidation). The first one is based on single-layer interphases containing boron whereas in the second, part of PyC is replaced, in the so-called ML interphases, by SiC or TiC to reduce the thickness of each elementary PyC sublayer to a few 10 nm and to favor self-healing phenomena.9,10 Boron-Doped PyC Interphase The addition of boron to PyC increases its graphitic character at low B concentration and improves its oxidation resistance at high B levels by blocking the so-called active sites and forming a fluid oxide (B2O3) in a temperature range (500–9001C), where the growth kinetics of silica is still too slow.37 Fig. 2. Values of sc i =sc 2 ratio provided by the master curve for various fiber/matrix and fiber/interphase/matrix systems (cracked material (material 1) cited second) (adapted from Pompidou and Lamon34). 266 International Journal of Applied Ceramic Technology—Naslain, Pailler and Lamon Vol. 7, No. 3, 2010