Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain EXAMPLES OF TAILORED INTERFACIAL ZONE MIS+H posites posites with a glass-ceramic matrix rei Nicalon# fibres, namely the Si-C-O ceramic grade fibres (NLM-202)and more recently Hi-Nicalon"fibres 60, hav been developed during the last two decades for applications at medium or high temperatures. The most commonly used H2 matrix compositions belong to the LAS (Li,o-A12 03-SiO2) MAS(MgO-Al2O3-SiO2), CAS(CaO-Al2OSiO2)and BMAS(Bao-MgO-Al203-SiO2) systems and also contain small amounts of various additives. The composites are processed according to a prepreg route comprising slurry Figure 4 Deposition of (Pyc-Sic), multilayer interphases by PCVDI impregnation and hot pressing steps. The design of their FM interfaces has been performed in two steps, first by taking advantage of in situ reactions occurring at the FM interfaces during hot pressing and which result in tough materials and performed with the same equipment as that used for the CVI second, through fibre CVD precoating, in order to of non-oxide matrices, and(iv) they yield well-controlled improve the composite lifetime under stress in oxidizing interphase deposits, in terms of thickness, composition and environments 6 7.41 structure. Furthermore, in one of their last versions, namely Nicalon/glass-ceramic composites are reactive systems pressure pulsed CVD or CVI (P-CVD/CVi)the interphase in the temperature range corresponding to the hot pressing can be deposited layer by layer, with a layer thickness which step typically 1200-1400C. Chemical and structural can be as low as I nm if necessary, giving an extremely analyses at the nm scale have shown that a complex large flexibility to these processes. As an example, if the multilayer FM interfacial zone is formed in situ as the result nature of the gaseous precursor is periodically changed, of an oxidation of the fibre surface(which will be assumed multilayer interphases such as(PyC-SiC)m, are obtained, the to consist of SiC, for simplicity) by oxygen from the thicknesses of the PyC and Sic layers beir matrix. 7.41.60-69. The nature of the interfacial zone, its controlled by the number of hydrocarbon and kinetics of growth and thus its thickness, depend mainly on pulses, respectively(Figure 4) the matrix composition and hot pressing conditions. Thus, Solution/gel or sol/gel processes are particularly appro the FM bonding and hence the mechanical properties of the priate for the deposition of simple, binary or ternary oxide composites, can be tailored. The key point is the presence in hase is the FM interfacial zone, of a thin layer of carbon, often formed on the fibre surface through the repetition of dip strongly textured, which acts as mechanical fuse(with low oating/gelification/drying/firing sequences. Metal alkoxides in and sometimes extremely low r; values, and low to medium water-alcohol solution are often chosen as precursors T, values depending on the state of residual stresses, i. e. CTE inasmuch as different alkoxides can be mixed together at mismatch. 5. 63. 66.67.70-72). The mechanism responsible for the molecular scale with a view to form ultimately complex the formation of the FM interfacial zone is still a matter of oxide interphases, yielding after hydrolysis/polycondensa- controversy. The most commonly accepted is a passive tion, homogeneous singie phase gels. Conversely, the use of oxidation of SiC (from the fibre) by oxygen (or carbon mixtures of sols or mixtures of sols and organometallic monoxide)from the matrix, according to one of the species yields diphasic gels. As an example, the fluorophlo- following overall equations gopite mica interphase, KMg3 (AlSi ,)O oF2 and the related phyllosiloxide interphase KMg 12, were both SiC+O2→C+SiO2 prepared according to an all-alkoxide route. 303233.For SiC+2CO→3C+SiO the latter, the precursor was a mixture of KOCH Ag(OC2H5)2; Al(OC,H9)3 and Si(oc?Hs)4 in 2-methoxy the source of oxygen in eqn (1)being either oxygen dis- ethanol. The oxide equivalents of alkoxide solutions being lved in the slurry glass particles or oxygen generated by low, several sequences are necessary to achieve a gel specific oxides used as additives.73-76 However. more deposit of significant thickness, e.g. 5 for a phyllosiloxide complex mechanisms involving e.g. the active oxidation of Sic were also proposed". The processing parameters gel thickness of I um. Further, the gel-oxide conversion, which can be adjusted are the matrix composition (and par- including solvent vapourization, removal of volatiles and sintering/crystallization of the porous amorphous deposit, ticularly the nature and concentration of the specific oxides occurs with an important shrinkage and needs to be sed as additives )and the hot pressing conditions( tempera- conducted with care in order to avoid crack formation in ture, duration and atmosphere). Based on thermodynamic the deposit. The solution/gel or sol/gel processes could be considerations, it has been shown that oxides which are extended to multilayer interphases, (X-Y)m, where X and Y are now oxide layers of different compositions f Nicalon and Hi-Nicalon fibres from Nippon Carbon, Tokyo 1149