Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain bonding strengthened by clamping stress, depending on the sign of the Cte difference Aa=am-af. In modem CMCs, an interphase, i.e. a thin layer of a material with a low shear G strength, is systematically deposited on the fibre surface before composite processing or formed in situ at the FM interfaces, to control the FM bonding. The thickness of the to abou for fibres displaying a diameter of 7-20 um 10.24.25 However, the use of much thinner interphases, e.g. as low Debonding as 4-14 nm, was also reported The two main functions of the interphase are: (i)to act as 1.0 a mechanical fuse, i.e. to deflect the matrix microcracks and (ii)to maintain a good load transfer between the fibres and Figure 2 A debond diagram for CMCs, according to Ref. 2. The elastic the matrix as previously discussed. In addition, the interphase may act as a buffer, absorbing at least partially from materil li of meaterial ateriahere E I and E: are the plain strain the residual stresses at the FMinterfaces resulting from CTE compliant and thick enough. Further, in very reactive Cle tgy r. The upper limit of the r T ratio depends on the systems, such as non-oxide fibres embedded in a silica based stic mismatch a, as shown in Figure 2. Thus, the glass-ceramic matrix, the interphase may also act as a interface will act as mechanical fuse when the following diffusion barrier, which supposes that it is thermodynamic- nequality, T/ s 1/4, is satisfied for composites in which cally compatible with both the fibres and the matrix, on the the elastic mismatch is small (a=O). Since most ceramic one hand, and thick enough, on the other hand. Finally, most fibres have a fracture energy of the order of rr=20 J/m, an CMCs being used at high temperatures and in oxidizing upper limit for Ti is s 5J/m, which is broadly consistent atmospheres, the interphase should be preferably resistant to ith most experimental data although some higher values oxidation. This last requirement is especially important were mentio d-. Different tests were suggested to one remembers that CMCs are often microcracked. the measure the interface parameters, the most commonly used microcrack network facilitating the in-depth diffusion of being:()the push-through test performed with a flattened oxygen towards the interphases and the fibres.Unfortu- diamond tip which is applied under an increasing load to the nately, as will be apparent in the following sections, the best fibre end in a composite thin foil cut perpendicular to the interphase materials in terms of mechanical fuse function fibre axis4-2I and (ii tensile tests with unloading- are non-oxides, e.g. pyrocarbon or hex- BN. Hence, the loading hysteresis loops, performed on ID model effect of the environment on the interphases and the fibres. opposites in the non-linear stress-strain domain 22 both being usually non-oxides, is the major issue in the The former can be used on real composites whereas the design of modern CMCs latter, performed on model composites, yields data which may not be always representative of real composites. All of them require some skill when performed on small diameter Interphase types fibres. further, the treatment of the data relies on models As shown schematically in Figure 3, different kinds of which may not always be adequate. As an example, most t interphase have been suggested and tested in a variety of models assume that the FM interface has no thickness, CMCs, the main objective being to introduce a weak link in whereas in most composites, the interfacial zone is not a strongly bonded FM system. In type I interphases, a simple homogeneous and has a thickness ranging from 0. 1 to 1 um. weak interface, usually between the fibre and the interphase There is thus usually some discrepancy in the interface is introduced in the fm interfacial zone acting as parameter values derived from different tests, on the one mechanical fuse. Examples of such weak interfaces are hand, and these values should be regarded as an estimate, (i)the silica glass/anisotropic pyrocarbon interface which is useful for processing optimization, when assessed according often present in Nicalon/PyC/Sic or Nicalon/BN/SiC to a given experimental procedure and data treatment. on the composites fabricated by CVI from as-received Nicalon Si-C-O fibres'or (ii) the lanthanum phosphate LaPo4/ alumina interface In type II interphases, which are by far the most commonly used, the interphase is a lay Interphase functions material exhibiting a layered crystal structure, the layers CMCs being fabricated at high temperatures, strong being parallel to the fibre surface and weakly bonded to each interactions governed by solid state diffusion, may occur at the Fm interfaces during processing, which usually result in turbostratic pyrocarbon and hex-boron nitride, in the non strong FM bonding, fibre weakening and brittle behaviour. oxides family, as well as phyllosilicates (such as the Further, in specific FM systems displaying significant CTE fluorophlogopite mica, KMg3(AISi3)O1oF2)30.3 the struc- interfaces may become debonded when turally related synthetic phyllosiloxides(such as KMg2Al cooling the material to room temperature, or the FM Si4O12)34 and cleavable hexaluminate, such as hibonite 1147