Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain composites,the goal being to achieve a non-brittle mechan- under load(at a stress level higher than the proportional ical behaviour in the first step, then to improve the oxidation limit, i. e in a microcracked state)significantly improved, 8.In resistance and lifetime, in the second. However, since a second route followed by many investigators,the Nicalon(or Hi-Nicalon)/SiC composites display basically pyrocarbon interphase was replaced by a boron nitride non-reactive FM interfaces, the interphase design strategy interphase. As a matter of fact, hex-BN has a layered crystal was based exclusively on the use of CVD/CVI fibre structure close to that of graphite. Hence, it is expected to act as a mechanical fuse, the interphase as yet, in principle Pyrocarbon has been and is still the most commonly used type Il(Figure 3). Further BN can be deposited by CVD/ phase in Nicalon/SiC composites inasmuch as: (i) it is CVI at low temperatures from a variety of precursors an efficient mechanical fuse, (ii)it can be deposited from a including BX3-NH3 mixtures(with X= F, Cl, Br), borane variety of hydrocarbons, e.g. CH4, C Hs or C3H, and (ii)it (such as B2H6), borazine B, N3 H6 and related species4.86-9 is compatible with both the fibres and the matrix at high The main advantage of Bn is its higher oxidation resistance, temperatures. Although it is relatively easy to produce non- the oxidation of bn starting at about 800 C(vs 450Cfor brittle Nicalon/PyC/SiC composites at the laboratory scale, pyrocarbon) and remaining passive, i.e. with formation of a chieving high mechanical properties at room temperature protective condensed oxide B203, up to about 1100oC (in terms of stress and strain at failure and fatigue However, the use of bn interphases in Nicalon/Sic resistance) is not possible without some interphase composites raises several points of concern, mainly related design". It has been shown that the best mechanical to its crystallization state and reactivity. First, there is often behaviour is observed with a highly anisotropic pyrocarbon at the Nicalon/BN interface, a dual carbon/silica thin layer d to the fibre surface (which displaying a shear strength which is lower than that of the supposes some fibre surface pretreatment)and in which the BN interphase itself and actually acts as a mechanical fuse carbon atomic layers are parallel to the fibre surface(type II As a result the FM load transfer is low when the Sic-matrix interphase in Figure 3). Such pyrocarbon interphases can be becomes microcracked, the tensile curve being type a with deposited in a controlled manner by I-CVI or P-cVT a plateau-like feature and a somewhat low failure stress Under such conditions, the matrix microcracks are deflected (Figure 1). Second, Bn interphases are usually amor in a diffuse mode within the interphase(and not at the fibre phous or poorly crystalline. However, hex- BN interphases, surface) between the carbon atomic layers themselves, each strongly anisotropic and with the bn atomic layers parallel matrix crack giving rise to an infinity of nanocracks in to the fibre surface have been deposited from BF3-NH3 pyrocarbon with the result that the interphase keeps a high mixtures but under CvD/CVI conditions which are load transfer capability up to failure. Hence, the composites chemically aggressive for the fibres. Hence as far as we exhibit a type B tensile behaviour curve with a high failure know, diffuse crack deflection in a highly anisotropic BN stress(Figure 1)3-.As far as we know, such a diffuse crack interphase strongly bonded to the fibre surface and yielding deflection mode is unique and was never reported for other a type b tensile curve similar to that observed for Nicalon/ interphase materials (it corresponds to relatively high T, and PyC/SiC(Figure 1), has not been reported yet. It is not even Ti values with respect to the Nicalon/glass-ceramic formally established whether the shear failure strength of hex -BN parallel to the basal plane is low enough to permit In a second step, the interphase design was oriented such diffuse crack deflection mode. A second point of towards improving the oxidation resistance of the compo concern is the sensitivity of Bn to moisture, which has been sites. The following discussion will be strictly limited to the reported to be extremely low for well crystallized hex-BN, issues directly related to the FM interphases, though ther but high when BN is turbostratic. Since most BN exists complementary approaches to improve the oxidation interphases are deposited at low temperatures in an resistance of Nicalon/SiC composites, such as the use of amorphous or poorly crystalline state, they are expected to coatings deposited on the external surface of the be moisture sensitive. It has been shown that there is indeed composites,28, 81 or that of chemically modified Sic. a direct correlation between the BN deposition temperature matrices,.. A first route was through the addition of and the interphase durability in H20 containing environ boron to pyrocarbon (by adding a gaseous precursor of ments, on the one hand, and that Si doping increases the boron, e.g. BClyH2, to the hydrocarbon). This boron moisture resistance of BN on the other hand. Thus, though ddition has two positive consequences: (1) it improves BN is a promising interphase material, its use in Nicalon the anisotropy of pyrocarbon, the effect being maximum for SiC composites is far from being fully optimized, with 8 at B and the interphase remaining of type II respect to pyrocarbon Figure 3), and (ii) it increases the oxidation resistance of The third and last route for improving the oxidation the pyrocarbon interphase(the oxidation of boron yielding a resistance of Nicalon/Sic composites, which will be low melting glass healing the microcracks, as already discussed, is based on the use of multilayer type Il mentioned). In order to take into account these two effects, interphases(Figure 3). The oxidation of Nicalon/PyC/SiC C(B)interphases with a graded composition were designe composites involves mainly two phenomena: (i) chemical Figure 7)and successfully used in model microcomposites. reactions between oxygen and both carbon and Sic and (ii Crack deflection occurred in (or near)the C(B)sublayer mass transfer of gaseous species (oxygen and carbon with the highest anisotropy, and the lifetime of micro- oxides). The former are responsible for the formation of composites exposed to an oxidizing atmosphere at 600 c an annular pore around each fibre, whose length, increases