Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain x▲v BMAS TEST TEMPERATURE rC) ST TEMPERATURE(c】 h(3p) vs temperature in air for0°9o°LAS composites with and without CVD Nicalon: O: BN coated Nicalon: o: SiC/BN DEPTH NTO MATRIX(nm) DEPTH INTO FIBRE ( to Ref. 6 Figure 5 Scanning Auger electron spectroscopy depth profiles for a Si-c-O Nicalon fibre/LAS-IIl matrix composite, showing that interface debond has occurred within in situ formed carbon layer, according to Ref. 65 composites with Hi-Nicalon fibre. The bn coatings (100-300 nm thick) were either amorphous or amorphous unstable in the presence of SiC, such as alkali metal oxides to partly turbostratic, as is, usually the case for low temperature BN-CVD. First, the BN/LAs interface was often present in glass compositions, fluxing or refining observed to be reactive with: () Si and Al diffusing from the agents(ZnO, AS2O, Sb205)or nucleating agents used to matrix into the coating and inducing the crystallization of favour the glass-ceramic conversion, such as TiO2 or the boron nitride and (i) in the opposite direction, B Nb2O5, may act as a source of oxygen and favour the for diffusing into the matrix and impeding its glass-ceramic mation of carbon at the FM interface ,/ As an example, in conversion. Second, in order to prevent these interdiffusion Nicalon/LAS-IlI composites(the atomic composition of the phenomena, a Sic diffusion barrier( 100-250 nm thick)was matrix being close to 5% Li, 2% Mg, 7% Al, 24% Si and deposited by CVD on the BN coating. Nicalon/LAS and 62%O, neglecting the 5 wt. Nb20s and 2 wt. ZrO2, Nicalon/BMAS composites fabricated with such BN/SiC additives), the interfacial zone comprised a carbon-rich dual interphases exhibited high strength and thermal/ layer (Figure 5) with a thickness of 350 nm, with on the oxidative stability to 1100 and 1200.C, respectively, as long as the Bn and SiC layers were tightly bonded for a LAS-I-matrix exhibiting a composition close to that of (Figure 6. Tensile fatigue tests as well as tensile stress- LAS-Ill but with no Nb,Os addition, the thickness of the rupture tests performed in air on Nicalon/BMAS composites arbon layer was reported to be lower(about 10 nm of pure showed that these composites could withstand stress levels arbon plus another 20 nm of carbon-rich layer grading into higher than the proportional limit for long periods at high the usual fibre composition) with, as expected, no NbC temperatures(1100-1200C). These results support the layer.7.6. The composition of the fibre may also play a usefulness of the dual interphase concept, one layer(BN role, possibly through kinetic considerations. As ar acting as an oxidation resistant mechanical fuse and the example, the chemical reaction occurring between vapo other (SiC)as an oxidation resistant diffusion barrier. grown stoichiometric Sic whiskers and the LAS-Il matrix However, in bn deposited at low temperature in amorphous previously mentioned resulted in the formation of NbC crys- or poorly crystalline states which are highly reactive, an tals, but with no carbon layer. For the carbon mechanic important subject for future research might be the effect of fuse formed by in situ reactions sensitive to oxidation, moisture attempts have been made to improve its oxidation resistance through the alteration of the matrix composition and hence of the FM interfacial zone. As an example, br has In Nicalon/SiC composites reported that replacing part of the Nb Os additive in LAS Sic-matrix composites are fabricated according to a Ill by 3.5 wt. B2O3(UTRC-200 LAS composition) liquid or a gas phase route. In the commonly used I-CV significantly improved the oxidation resistance of the com process (I standing for isothermal-isobaric), a porous posite within the 450-850.C temperature range(which multidirectional fibre preform is slowly densified at critical for carbon interphases), because of the formation in 1100 C with the Sic-matrix deposited in situ on the fibres oxidizing atmospheres of a low melting glass healing the surface as the result of a chemical reaction involving a microcracks and protecting the carbon interphase. Hoy ever. such a B,O, addition to the LAS-matrix reduced the gaseous precursor, such as 8-10 practical use of the composite to 1000"C under stress, by CH3 SiCl3(g)SiC()+ 3HCI(& enhancing the creep of the matrix in its ceramed state In a second step, a different approach centred on bn Nicalon/Sic composites are brittle when fabricated wit based CVD fibre coatings, and which will be discussed in interphase addition, suggesting that a strong FM bonding is detail in the next subsection, was used to improve the formed during processing. The two-step approach which oxidation resistance of Nicalon/LAS and Nicalon/BMAs has been used to design the interphase has followed the osites under load nd extended to same logic as that depicted for glass-ceramic matrix