Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain 400 300 200 microcrack 浦排时 0.6 0.8 .ONGITUDINAL TENSILE STRAIN ( % Figure 1 Tensile curves and crack deflection mechanisms for 2D-Nicalor/Py c/SiC composites corresponding to different fiber-matrix bonding(specimen I is a composite of type A and specimen J a composite of type B), according to Refs 4, 36,94 and a non-brittle failure. 2. However, these features, which crack density is low and the debonding length large, (ii)the are quite uncommon for ceramics, depend strongly on the stress-strain tensile curve displays a typical plateau-like FM bonding. Since in CMCS, R<Ef, the matrix fails first, feature, and(iv) at failure, the matrix microcracks are e it undergoes microcracking at low strain. When the load widely open exposing the fibre surface to the atmosphere, pplied along the fibre direction(1D-composites)or along the composite failing at a moderate stress with extensive one of the fibre directions (nD-composites ), the matrix fibre pullout(Figure 1). By contrast, in composites with microcracks usually propagate perpendicular to that fibre relatively strong bonding, which is the preferred situation direction(mode I). Even when the fibre volume fraction is from a mechanical standpoint and from that of the high(namely, for V,> V, where V( is the critical fibre environment, exactly opposite features are observed volume fraction), the formation and propagation of the first At saturation, the microcrack density is very high but the matrix microcrack(s)may induce the failure of the brittle crack opening and debond length small, rendering more fibres by a notch effect, if the FM bonding is too strong difficult the in-depth diffusion of the atmosphere constitu Therefore, weakening the FM bonding through appropriate ents(particularly, oxygen). Furthermore, since load transfer rocessing is a well-established requirement in CMCs. remains good up to failure, the tensile curve no longer When this requirement is fulfilled, the matrix microcracks exhibits the plateau-like feature, failure occurring at a propagate all over the composite cross-section without higher stress level and with limited fibre pullout. Hence, breaking the fibres, which are thus protected by the weak failure with extensive fibre pullout is not necessary, as often interfaces acting as mechanical fuses, the microcracks being stated as a valuable criterion for appreciating the quality of a deflected in the interfaces(mode il)over a distance, l which CMC. depends on the FM bonding(it can be of the order of 10- From the above discussion, it appears that the FM 20 um for relatively strong or of the order of 100 um or bonding should be neither too strong nor too weak. Hence, more, for very weak FM bonding). Matrix microcracking an important issue is the characteriz of the fm bonding and interface debonding occur within a given strain range up from a quantitative(or at least semiquantitative)standpoint, to a saturation state, the density of cracks, their spacing and which could be used to optimize processing conditions. The opening at saturation depending once again upon the FM FM bonding is usually depicted with two parameters bonding. Beyond this point, the applied load is mainly associated with fracture and slip, respectively. The arried by the fibres alone and the composite stiffness former is considered to involve an interfacial debond reduced as the result of damage energy, T, and the latter is expected to occur with The FM interfaces have also a load transfer function interfacial shear resistance, Ti, which is written as ti= which is fulfilled in an efficient manner when the FM T。μσr, where To constant'term associated with bonding is relatively strong, whereas a too weak FM roughness(most ceramic fibres exhibit some roughness ng yields poor characteristics at failure. Hence, the the nm scale), u is a Coulomb friction coefficient and ar is shape of the stress-strain tensile curve depends both on the the clamping stress normal to the interface For debonding damage extension and FM load transfer In composites with and sliding to occur, rather than brittle matrix crack a too weak FM bonding: (i)matrix microcracking occurs propagation through the fibres, the debond energy r;must within a relatively narrow strain range, (ii)at saturation the not exceed an upper limit, relative to the fibre fracture 1146