Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain and(ii)the(PyC-SiC)n multilayer interphases(with typically I<n<10) used in interphases, the interphase is a layer of a porous material Examples of such interphases are porous alumina(or zirconia layers in alumina fibre/alumina matrix composites..49.One simple way to form such porous oxides is first to deposit a carbon/oxide mixture on the fibre surface then to embed the coated fibres in the matrix and finally, to burn out the carbon of the interphase. Other approaches have been proposed to (a)Type I t b) Type Il weaken the FM bonding in CMCs but have not been applie yet to real composites or have not yielded improved M d F mechanical properties or/and lifetimes n Interesting example, Kriven et al have recently suggested the use of shear induced phase transformations occurring with volume shrink age in some materials, e.g. the proto to clino transformation in enstatite MgSiO3(AV/Vo = -5.5%), to induce a weakening effect (which is thus the opposite of that exploited metastable tetragonal zirconia)and hence to promote debond ing in the(enstatite)interphase 0.51 c】 Type In mposites The very first interphases reported to weaken FM bonding and to yield tough CMCs were probably formed in a fortuitous manner. as the result of an in situ chemical reaction at the FM interfaces during processing, for both CaAl2O19, which crystallizes in the magnetoplumbite glass-ceramic matrix(hot pressing) and SiC-matrix(CVi) structure-type related to B-alumina4-37. Ideally, these reinforced with Si-C-O Nicalon fibres*. These fibres are interphases should be strongly bonded to the fibre surface now known to be metastable at high temperatures, under and the layers perfectly parallel to the fibre surface. Under going decomposition beyond about 1100C and reacting such conditions, the weak fibre surface/interphase interface with most silica based glass-ceramic compositions, with,in is no longer the mechanical fuse and the matrix cracks are either case formation of free carbon, which may act as a deflected in a diffuse manner within the interphase itself. mechanical fuse, as discussed in more detail in the next However, as discussed in the next section, such interphases section. This interphase formation approach, though it has are rarely used in an optimized state. In other words, they limited flexibility, is still successfully used in various are often poorly crystallized and oriented (not to say reactive systems. As an example, rare-earth hexa-aluminate amorphous)and too weakly bonded to the fibre surface, with nterphases, approximating the structural formula the result that they behave as a type I interphase or as a type CeI-1Al12-yO19-(magnetoplumbite alumina) and display I/type II hybrid interphase. Finally, the number of materials ng roughly the proper cleavage plane orientation, have been with a layered crystal structure, displaying an easy cleavage formed within dual coatings, by in situ solid state reaction in parallel to the layers and hence which can be used as a low po2 atmosphere between a ceria-doped zirconia interphases in CMCs, is extremely limited. In type Ill coating deposited on a Saphikonf corundum single crystal interphases, the concept used in type II interphases is fibre and an outer alumina coating(simulating an alumina extended to the scale of the nano-or microstructure. These matrix) interphases consist of a stack of layers of different nature Chemical vapour deposition(CVD)or chemical vapour (say,(X- Yn), strongly bonded to the fibre surface but infiltration(CVI) are the most commor d techniques with weak internal interfaces which can be either the X/Y for the deposition of non-oxide interphases such as PyC. interfaces or even atomic planes if one of the layers, say X, hex-BN or related multilayers, whereas their use is more has a layered crystal structure, as for the type II interphase. difficult(but still possible in most cases) for oxides 8.5 With respect to the latter, type Ill interphases can be widely Their success is caused by several important advantages: (i) tailored, the adjustable parameters being the nature of X and simple volatile precursors are available for the interphase Y, the number of X-Y sequences, n, and the thicknesses of materials of interest, i.e. hydrocarbons such as CH4; C 3 h6 or and y layers in the sequence. Another important C3Hg for pyrocarbon, CH] SiCl3 (MTS)H2 for Sic and advantage of these interphases lies in the fact that the BX3-NH3(with X= F, cl, Br) for BN, (ii) they are low terphase functions can now be decoupled. As an example, temperature and low pressure processes with hence no layer X can act as mechanical fuse and layer Y as diffusion significative degradation of the fibres,(iii) they can be barrier. At least two interphases of this type have been xtensively studied (i)the dual BN-SiC (n= I)interphases From Nippon Carbon, Tokyo. used in silica based glass-ceramic matrix composites 8-4>+From Saphikon Inc,Milford(NH),USA 1148