International Journal of Applied Ceramic Technolog-Naslain, Pailler and Lamon Vol. 7, No 3, 2010 used in oxidizing atmospheres(gas turbines). Within the isobaric and P for pressure pulsed) from propane or same conceptual framework, two main alternatives have prop been proposed: boron nitride(BN)and (X-Y)n multi layers (MD). The former displays a structure similar to Fiber/PyC-Interphase Bonding that of graphite while being more oxidation res sistant In the latter, with X= PyC or BN, Y=SiC, and n The second requirement is a strong bonding be- 1-10, Part of the oxidation prone X constituent is tween the fber and the PyC interphase. Achieving replaced by a material Y exhibiting a better oxidation such a strong bonding is first a matter of surface chem- resistance, such as SiC itself. 8- Furthermore, this istry. The surface of desized Si-C-O fibers(Nicalon concept of material multilayering has been extended to the matrix, yielding"self-healing"composites with and free carbon. As a result, there is in SiC/SiC(CVI)a outstanding lifetimes in oxidizing atmospheres. 10,13 thin and irregular dual layer of hous silica and More recently, SiC/SiC composites have beer carbon which introduces a weak link near the fiber sur- envisaged as structural materials in high-temperature face. 4.4 The fibers should be pretreated to clean (HT) nuclear reactors. The interphase appears again as a their surface and achieve a strong fiber/interphase(Fi) possible weak point, PyC being known to undergo bonding. Another example is the SiC+C fibers(Hi- anisotropic volume change when exposed to neutrons, Nicalon or HN, Nippon Carbon, Tokyo, Japan), whose whereas BN undergoes nuclear reactions. 5-7 microstructure is not fully stabilized after processing and is presently pursued following two similar routes: (i)use whose surface may also contain some oxygen. During nd (ii) MLs, to minimize the effect of neutron irradiation. 5-21 CVI processing, fibers undergo a postshrinkage weak ening the FI interface. Again, the fibers should be pre- The aim of the present overview is to recall the basis treated. Finally, stoichiometric SiC fibers(Hi-Nicalon of the layered interphase concept, to discuss its appli type S(HNS)or Tyranno SA (TSA, Ube Industries, cation to SiC/SiC exposed to oxidizing environment Yamaguchi, Japan)), fabricated at higher temperatures and tentatively, to neutron irradiation. are assumed to be dimensionally stable at composite processing temperatures. Further, their surface consists Py C Single-Layer Interphase: The Reference of free carbon(resulting from SiC decomposition) Hence, their bonding with PyC interphase is expected PyC has a structure similar to that of graphite but to be relatively strong Finally, the roughness of fiber, which is low for or less distorted and stacked with rotational disorder. Nicalon and Hi-Nicalon but significant for stoichiomet Our layered interphase concept requires that: (i)the ric fibers, adds a mechanical contribution to the fi bond layers should be oriented parallel to fiber surface and (i ing in a transition zone where the nanometric grapl the bonding between the fber and the PyC interphase lay should be strong enough. Otherwise, debonding/crack deflection would occur at the fiber surface exposing the SiC (Nicalon)/Py C/SiC: A Case History fiber to mechanical damage and to the atmosphere.3.4 Studies on SiC/PyC/SiC (CVI) fabricated with Py c Texture Nicalon or Hi-Nicalon fibers(the as-received or pre- treated), clearly show the positive effect of FM-interfa- PyC displays a variety of microtexture and anisot- cial design on material properties. As shown in Fig. I in polarized light (extinction angle, Ae)or/and trans- PyC interphase(samples posites with a single 500 nm ropy, which can be characterized by optical microscopy tensile curves for the con and D) exhibit extended non- mission electron microscopy(TEM)(L2, N parameters): linear domains related to damaging phenomena, with the larger the Ae, the higher the anisotropy. The pre high failure strains. How vever. co ferred PyC for an interphase is rough laminar (Ae>18) fibers(sample D)is much stronge This RL-PyC has a tendency to grow with graphene The shapes of the curves are different: continuously layers parallel to fiber surface. It is usually deposite convex for the composite with pretreated fibers(sample D) CVI(I-CVI or P-CVI, where I stands for isothe and with a plateau-like feature for that with the as-used in oxidizing atmospheres (gas turbines). Within the same conceptual framework, two main alternatives have been proposed: boron nitride (BN) and (X–Y)n multi￾layers (ML). The former displays a structure similar to that of graphite while being more oxidation resistant.5–7 In the latter, with X 5 PyC or BN, Y 5 SiC, and n 5 1–10, part of the oxidation prone X constituent is replaced by a material Y exhibiting a better oxidation resistance, such as SiC itself.1–4,8–12 Furthermore, this concept of material multilayering has been extended to the matrix, yielding ‘‘self-healing’’ composites with outstanding lifetimes in oxidizing atmospheres.10,13,14 More recently, SiC/SiC composites have been envisaged as structural materials in high-temperature (HT) nuclear reactors. The interphase appears again as a possible weak point, PyC being known to undergo anisotropic volume change when exposed to neutrons, whereas BN undergoes nuclear reactions.15–17 Research is presently pursued following two similar routes: (i) use of thin single PyC layers and (ii) use of (PyC–SiC)n MLs, to minimize the effect of neutron irradiation.15–21 The aim of the present overview is to recall the basis of the layered interphase concept, to discuss its appli￾cation to SiC/SiC exposed to oxidizing environment and tentatively, to neutron irradiation. PyC Single-Layer Interphase: The Reference PyC has a structure similar to that of graphite but the elementary graphene layers are of limited size more or less distorted and stacked with rotational disorder. Our layered interphase concept requires that: (i) the layers should be oriented parallel to fiber surface and (ii) the bonding between the fiber and the PyC interphase should be strong enough. Otherwise, debonding/crack deflection would occur at the fiber surface exposing the fiber to mechanical damage and to the atmosphere.3,4 PyC Texture PyC displays a variety of microtexture and anisot￾ropy, which can be characterized by optical microscopy in polarized light (extinction angle, Ae) or/and trans￾mission electron microscopy (TEM) (L2, N parameters): the larger the Ae, the higher the anisotropy.22 The pre￾ferred PyC for an interphase is rough laminar (Ae4181) This RL-PyC has a tendency to grow with graphene layers parallel to fiber surface. It is usually deposited by CVI (I-CVI or P-CVI, where I stands for isothermal/ isobaric and P for pressure pulsed) from propane or propylene.1,5,9,23 Fiber/PyC-Interphase Bonding The second requirement is a strong bonding be￾tween the fiber and the PyC interphase.3,4 Achieving such a strong bonding is first a matter of surface chem￾istry. The surface of desized Si–C–O fibers (Nicalon, Nippon Carbon, Tokyo, Japan) is enriched in oxygen and free carbon. As a result, there is in SiC/SiC (CVI) a thin and irregular dual layer of amorphous silica and carbon which introduces a weak link near the fiber sur￾face.1,3–6,24,25 The fibers should be pretreated to clean their surface and achieve a strong fiber/interphase (FI) bonding. Another example is the SiC1C fibers (Hi￾Nicalon or HN, Nippon Carbon, Tokyo, Japan), whose microstructure is not fully stabilized after processing and whose surface may also contain some oxygen. During CVI processing, fibers undergo a postshrinkage weak￾ening the FI interface. Again, the fibers should be pre￾treated. Finally, stoichiometric SiC fibers (Hi-Nicalon type S (HNS) or Tyranno SA (TSA, Ube Industries, Yamaguchi, Japan)), fabricated at higher temperatures are assumed to be dimensionally stable at composite processing temperatures. Further, their surface consists of free carbon (resulting from SiC decomposition).26 Hence, their bonding with PyC interphase is expected to be relatively strong. Finally, the roughness of fiber, which is low for Nicalon and Hi-Nicalon but significant for stoichiomet￾ric fibers, adds a mechanical contribution to the FI bond￾ing in a transition zone where the nanometric graphene layers stacks become progressively parallel to fiber axis.1,7 SiC (Nicalon)/PyC/SiC: A Case History Studies on SiC/PyC/SiC (CVI) fabricated with Nicalon or Hi-Nicalon fibers (the as-received or pre￾treated), clearly show the positive effect of FM-interfa￾cial design on material properties.1–4 As shown in Fig. 1, tensile curves for the composites with a single 500 nm PyC interphase (samples I and J) exhibit extended non￾linear domains related to damaging phenomena, with high failure strains. However, composite with pretreated fibers (sample J) is much stronger. The shapes of the curves are different: continuously convex for the composite with pretreated fibers (sample J) and with a plateau-like feature for that with the as- 264 International Journal of Applied Ceramic Technology—Naslain, Pailler and Lamon Vol. 7, No. 3, 2010