wwceramics. org/ACT Layered Interphases in SiC/SiC Composites A straightforward approach is to start with a stoic- st Pyc-1 layer biometric SiC fiber containing some boron(acting narily as sintering aid), as shown by Sacks and Brennan. When such a fiber is treated at high tem perature in an N-containing atmosphere, B atoms diffuse radially from fiber core to react with nitrogen yielding a strongly adherent BN layer at fiber surface (typically, 100-200 nm thick). Further, diffusion in BN being anisotropic, the bn coating tends to grow with Bn layers perpendicular to fiber surface and hence strongly bonded to the fiber. If now a BN interphase is deposited on such a substrate by CVI, the BN layers would have a tendency to be oriented, after some gime, parallel to the fiber o-called Sylramic-iBN and Super Sylramic-iBN fibers may have been developed on the basis of some related In the composites, crack deflection Fig. 5. ID-SiC (HN/SiC (CVI) with(lyC-SiCIo ML would occur either within the BN interphase or at the interphase: transmission electron microscopic image of a matrix BN-SiCm interface(outside debonding). These features microcrack deflected within the ML interphase(adapted from these composites at high temperatures dizing ranging from 3 to 100 nm for PyC and 10 to 500 nm for environmen Sic while the number of PyC-SiC sequences is in the ML C-D Interp bases funge of 3-10. The first material deposited on fiber sur- face is usually pyc but it could also be SiC in an attempt to strengthen the FI bonding. 1> Both MLs with con- ML interphases, (X-Y)m extend the concept of stant sublayer thickness or graded sublayer thickness(on layered interphase from the atomic to the nanometer PyC or/and SiC) have been used. scales, the interphase being now a stack of films of Replacing a Pyc single interphase by a(Pyc-sic)m different materials X and Y, and the X-Y elementary ML interphase does not change markedly tensile properties, sequence repeated n times. Their main advantage is that as shown for composites fabricated with Nicalon fibers they can be highly tailored. 42. 78 As an example, the (Fig. 1). Tensile curves fall into two groups oxidation resistance of Sic/SiC could be improved by depending on whether the fibers are pretreated(strong replacing PyC or BN single-layer interphases(100- FM bonding) or not(weak FM bonding). Similar con- 200 nm thick) by(PyC-SiC), or( BN-SiC), ML inter- clusion can be drawn for SiC/SiC with Hi-Nicalon phases in which the thickness of the oxidation-prone fibers" or TSA stoichiometric fibers. Crack deflect- PyC or BN mechanical fuse is reduced to a few nan tion occurs at the FI (or/and interphase/matrix) interfa meters. This design criterion is based on oxygen gas when the FM bonding is weak (e-g, for the phase diffusion consideration"> and formation of heal- as-received Nicalon or Hi-Nicalon fibers) and withi ng condensed oxides(silica or/and boria). s ML inter- the ML interphase when it is strong enough(treated are deposited by CVI (switching from X to Y fibers). In this latter case, a matrix microcrack exhibits rs). A key requirement a dual propagation mode across the ml interphase (Fig. 5)with an overall propagation path significantly (PyC-SiC)n ML Interphases: Since the pioneering work The lifetime of SiC/SiC with ML interphase under of Droillard and colleagues, (PyC-SiC), ML interphas- load, at high temperature in air is improved with respect es have been used in a variety of SiC/SiC. -6>Depending to their counterparts with single PyC interphas on CVI conditions, SiC sublayers are either microcrystal lized(with rough SiC/PyC interfaces)or nanocrystallized Other(X-Yn ML Interphases: At least two other M h smooth SiC/PyC interfaces. Sublayer thickness interphases,(BN-SiC)n and(PyC-TiC)m have beA straightforward approach is to start with a stoic￾hiometric SiC fiber containing some boron (acting primarily as sintering aid),26 as shown by Sacks and Brennan.54 When such a fiber is treated at high tem￾perature in an N-containing atmosphere, B atoms diffuse radially from fiber core to react with nitrogen, yielding a strongly adherent BN layer at fiber surface (typically, 100–200 nm thick). Further, diffusion in BN being anisotropic, the BN coating tends to grow with BN layers perpendicular to fiber surface and hence strongly bonded to the fiber. If now a BN interphase is deposited on such a substrate by CVI, the BN layers would have a tendency to be oriented, after some transition regime, parallel to the fiber surface.25,27 The so-called Sylramic-iBN and Super Sylramic-iBN fibers may have been developed on the basis of some related mechanism.55 In the composites, crack deflection would occur either within the BN interphase or at the BN–SiCm interface (outside debonding). These features could explain the good mechanical properties of these composites at high temperatures in oxidizing environment.43,55,57 ML (X–Y)n Interphases ML interphases, (X–Y)n, extend the concept of layered interphase from the atomic to the nanometer scales, the interphase being now a stack of films of different materials X and Y, and the X–Y elementary sequence repeated n times. Their main advantage is that they can be highly tailored.1,4,5,58 As an example, the oxidation resistance of SiC/SiC could be improved by replacing PyC or BN single-layer interphases (100– 200 nm thick) by (PyC–SiC)n or (BN–SiC)n ML inter￾phases in which the thickness of the oxidation-prone PyC or BN mechanical fuse is reduced to a few nano￾meters. This design criterion is based on oxygen gas phase diffusion consideration35 and formation of heal￾ing condensed oxides (silica or/and boria).13 ML inter￾phases are deposited by CVI (switching from X to Y gaseous precursors).58 A key requirement is again a strong bonding between fiber surface and interphase. (PyC–SiC)n ML Interphases: Since the pioneering work of Droillard and colleagues,1,2 (PyC–SiC)nML interphas￾es have been used in a variety of SiC/SiC.59–65 Depending on CVI conditions, SiC sublayers are either microcrystal￾lized (with rough SiC/PyC interfaces) or nanocrystallized with smooth SiC/PyC interfaces. Sublayer thickness is ranging from 3 to 100 nm for PyC and 10 to 500 nm for SiC while the number of PyC–SiC sequences is in the range of 3–10. The first material deposited on fiber sur￾face is usually PyC but it could also be SiC in an attempt to strengthen the FI bonding.64,65 Both MLs with con￾stant sublayer thickness or graded sublayer thickness (on PyC or/and SiC) have been used.1,65 Replacing a PyC single interphase by a (PyC–SiC)n ML interphase does not change markedly tensile properties, as shown for composites fabricated with Nicalon fibers (Fig. 1).1 Tensile curves fall into two groups depending on whether the fibers are pretreated (strong FM bonding) or not (weak FM bonding). Similar con￾clusion can be drawn for SiC/SiC with Hi-Nicalon fibers59,60 or TSA stoichiometric fibers.65 Crack deflec￾tion occurs at the FI (or/and interphase/matrix) interface when the FM bonding is weak (e.g., for the as-received Nicalon or Hi-Nicalon fibers) and within the ML interphase when it is strong enough (treated fibers). In this latter case, a matrix microcrack exhibits a dual propagation mode across the ML interphase (Fig. 5) with an overall propagation path significantly lengthened.59,60 The lifetime of SiC/SiC with ML interphase under load, at high temperature in air is improved with respect to their counterparts with single PyC interphase.60,66 Other (X–Y)n ML Interphases: At least two other ML interphases, (BN–SiC)n and (PyC–TiC)n, have been Fig. 5. 1D-SiC (HN)/SiC (CVI) with (PyC–SiC)10 ML interphase: transmission electron microscopic image of a matrix microcrack deflected within the ML interphase (adapted from Bertrand et al.60). www.ceramics.org/ACT Layered Interphases in SiC/SiC Composites 269