Composites Part A 29A(1998)1145-115 1359-835X/98/S- see front mat c 1998 Elsevier Science Ltd. All rights reserved ELSEVIER PII:S1359835X(9700128-0 The design of the fibre-matrix interfacial zone in ceramic matrix composites Roger R. Naslain Laboratory for Thermostructural Composites, UMR-47(CNRS-SEP-UB1) University of Bordeaux, 3 Allee de La boetie, 33600 Pessac, france atrix interactions or deposited on the fibre surface prior to composite fabrication. It has several key functions, including crack defection, load transfer, diffusion barrier and residual stress relaxation. Four types of interphase are depicted involving weak interfaces, materials with a layered crystal structure(pyrocarbon, BN, micas and phyllosiloxides r materials with the B-alumina/magnetoplumbite structures), multilayers such as(PyC-SiC)n or(BN-SiC), or finally, porous materials. Achieving high mechanical properties and long lifetimes in severe environments require a subtle design of the fibre-matrix interfacial zone, which is depicted for Nicalon/glass-ceramic and Nicalon/SiC matrix composites. C 1998 Elsevier Science Ltd. All rights reserved eywords: A. ceram composites ( CMCs); B interface/interphase; pyrocarbon; boron nitride; multilayers INTRODUCTION FM interfacial zone in CMCs from a processing standpoint The first approach is through the in situ formation of a weak Ceramic matrix composites(CMCs)display, with respect to interphase resulting from some chemical reaction at the FM many other polymer or metal matrix composites, the key interfaces during the high temperature step of composite oroperty of being inverse composites, which is to say that processing. It has been extensively used during the early the strain at failure of the matrix, Em, is much lower than that development of glass-ceramic matrix composites,. The of the fibres, Ef. As a result in such composites the matrix second approach, which offers much more design flexibility undergoes microcracking, Since in CMCs the two inasmuch as it no longer relies on an in situ reaction, is constituents, namely the fibres and the matrix, are brittle, based on the use of precoated fibres, the weak interphase the matrix microcracks should not induce the early failure of being deposited on the fibre surface prior to the deposition he fibres by a notch effect, which means that the matrix of the matrix itself. This second approach was extensively microcracks should be defected at the fibre-matrix(FM) used for non-oxide composites, such as C/SiC and SiC/SiC interfaces. This key function is actually achieved when, composites (the fibre being designated first, as usual) after processing, the FM bonding is weak enough. Further, produced according to the cvi process. A variety of in CMCs the FM interfaces have another key function, i.e interphase materials was suggested, the most studied being that of load transfer as in any fibre-reinforced composite, pyrocarbon, h xagona which supposes conversely a strong enough FM bonding such as micas, B-aluminas and rare-earth phosphates Finally, CMCs are fabricated at relatively high temperatures The aim of the present contribution is, first, to present the (typically, -1000C in the so-called chemical vapour basis of the interphase concept in CMCs(in terms of infiltration (Cvn) process and even higher in the slurry terphase functions and interphase ma aterial nature)and impregnation/hot pressing technique) and they are used in second, to show how such a concept has been used to design severe conditions(high temperatures and oxidizing atmos the FM interfacial zone, focusing on some important pheres), a feature which introduces a constraint of presently used composites thermochemical and thermomechanical compatibilities The design of the FM interfacial zone, or interphase should take into account all these requirements, which are INTERPHASE CONCEPT IN CMCs often contradictory. It becomes an extremely difficult materials science challenge, it the lifetime of the composites Mechanical behaviour of CMCs required in advanced jet engines CMCs are inverse and damageable composites displaying There are two classical approaches to the design of the a non-linear stress-strain behaviour under tensile loading
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 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
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
Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain and(ii)the(PyC-SiC)n multilayer interphases(with typically I+From Saphikon Inc,Milford(NH),USA 1148
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
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
Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain composites,the goal being to achieve a non-brittle mechan- under load(at a stress level higher than the proportional ical behaviour in the first step, then to improve the oxidation limit, i. e in a microcracked state)significantly improved, 8.In resistance and lifetime, in the second. However, since a second route followed by many investigators,the Nicalon(or Hi-Nicalon)/SiC composites display basically pyrocarbon interphase was replaced by a boron nitride non-reactive FM interfaces, the interphase design strategy interphase. As a matter of fact, hex-BN has a layered crystal was based exclusively on the use of CVD/CVI fibre structure close to that of graphite. Hence, it is expected to act as a mechanical fuse, the interphase as yet, in principle Pyrocarbon has been and is still the most commonly used type Il(Figure 3). Further BN can be deposited by CVD/ phase in Nicalon/SiC composites inasmuch as: (i) it is CVI at low temperatures from a variety of precursors an efficient mechanical fuse, (ii)it can be deposited from a including BX3-NH3 mixtures(with X= F, Cl, Br), borane variety of hydrocarbons, e.g. CH4, C Hs or C3H, and (ii)it (such as B2H6), borazine B, N3 H6 and related species4.86-9 is compatible with both the fibres and the matrix at high The main advantage of Bn is its higher oxidation resistance, temperatures. Although it is relatively easy to produce non- the oxidation of bn starting at about 800 C(vs 450Cfor brittle Nicalon/PyC/SiC composites at the laboratory scale, pyrocarbon) and remaining passive, i.e. with formation of a chieving high mechanical properties at room temperature protective condensed oxide B203, up to about 1100oC (in terms of stress and strain at failure and fatigue However, the use of bn interphases in Nicalon/Sic resistance) is not possible without some interphase composites raises several points of concern, mainly related design". It has been shown that the best mechanical to its crystallization state and reactivity. First, there is often behaviour is observed with a highly anisotropic pyrocarbon at the Nicalon/BN interface, a dual carbon/silica thin layer d to the fibre surface (which displaying a shear strength which is lower than that of the supposes some fibre surface pretreatment)and in which the BN interphase itself and actually acts as a mechanical fuse carbon atomic layers are parallel to the fibre surface(type II As a result the FM load transfer is low when the Sic-matrix interphase in Figure 3). Such pyrocarbon interphases can be becomes microcracked, the tensile curve being type a with deposited in a controlled manner by I-CVI or P-cVT a plateau-like feature and a somewhat low failure stress Under such conditions, the matrix microcracks are deflected (Figure 1). Second, Bn interphases are usually amor in a diffuse mode within the interphase(and not at the fibre phous or poorly crystalline. However, hex- BN interphases, surface) between the carbon atomic layers themselves, each strongly anisotropic and with the bn atomic layers parallel matrix crack giving rise to an infinity of nanocracks in to the fibre surface have been deposited from BF3-NH3 pyrocarbon with the result that the interphase keeps a high mixtures but under CvD/CVI conditions which are load transfer capability up to failure. Hence, the composites chemically aggressive for the fibres. Hence as far as we exhibit a type B tensile behaviour curve with a high failure know, diffuse crack deflection in a highly anisotropic BN stress(Figure 1)3-.As far as we know, such a diffuse crack interphase strongly bonded to the fibre surface and yielding deflection mode is unique and was never reported for other a type b tensile curve similar to that observed for Nicalon/ interphase materials (it corresponds to relatively high T, and PyC/SiC(Figure 1), has not been reported yet. It is not even Ti values with respect to the Nicalon/glass-ceramic formally established whether the shear failure strength of hex -BN parallel to the basal plane is low enough to permit In a second step, the interphase design was oriented such diffuse crack deflection mode. A second point of towards improving the oxidation resistance of the compo concern is the sensitivity of Bn to moisture, which has been sites. The following discussion will be strictly limited to the reported to be extremely low for well crystallized hex-BN, issues directly related to the FM interphases, though ther but high when BN is turbostratic. Since most BN exists complementary approaches to improve the oxidation interphases are deposited at low temperatures in an resistance of Nicalon/SiC composites, such as the use of amorphous or poorly crystalline state, they are expected to coatings deposited on the external surface of the be moisture sensitive. It has been shown that there is indeed composites,28, 81 or that of chemically modified Sic. a direct correlation between the BN deposition temperature matrices,.. A first route was through the addition of and the interphase durability in H20 containing environ boron to pyrocarbon (by adding a gaseous precursor of ments, on the one hand, and that Si doping increases the boron, e.g. BClyH2, to the hydrocarbon). This boron moisture resistance of BN on the other hand. Thus, though ddition has two positive consequences: (1) it improves BN is a promising interphase material, its use in Nicalon the anisotropy of pyrocarbon, the effect being maximum for SiC composites is far from being fully optimized, with 8 at B and the interphase remaining of type II respect to pyrocarbon Figure 3), and (ii) it increases the oxidation resistance of The third and last route for improving the oxidation the pyrocarbon interphase(the oxidation of boron yielding a resistance of Nicalon/Sic composites, which will be low melting glass healing the microcracks, as already discussed, is based on the use of multilayer type Il mentioned). In order to take into account these two effects, interphases(Figure 3). The oxidation of Nicalon/PyC/SiC C(B)interphases with a graded composition were designe composites involves mainly two phenomena: (i) chemical Figure 7)and successfully used in model microcomposites. reactions between oxygen and both carbon and Sic and (ii Crack deflection occurred in (or near)the C(B)sublayer mass transfer of gaseous species (oxygen and carbon with the highest anisotropy, and the lifetime of micro- oxides). The former are responsible for the formation of composites exposed to an oxidizing atmosphere at 600 c an annular pore around each fibre, whose length, increases
Fibre-matrix interfacial zone in ceramic matrix composites: R. R. Naslain 0.1 um 0.1pm 15%t,B 01m 0% at B(Py FIBRE 100 lifetime at 600%C air 800MIPa time (h) Figure7 Nicalon/C(BySiC microcomposites with a graded composition interphase: (a) structure of the interphase, (b)deflection mode of a matrix k(TEM image), and(c)lifetime curves of microcomposites with a single PyC interphase or C(B)interphases. in air at 600C(interphase A is similar to a' but without layer V), according to Ref. 85 with time as the pyrocarbon interphase is consumed (the so shown in Figure 8a, the sublayer thicknesses were in the called pipeline oxidation), and for that of silica on the pore range 50-100 nm and their morphology relatively rough, as wall(assuming passive oxidation ). Oxygen has thus to a result of the preferred growth of B-Sic in the [lll diffuse along the pore for continued reaction with direction. The main advantage of replacing part of the carbon.8. Since this pipeline oxidation is easier when compliant pyrocarbon by stiff Sic was to improve the FM the PyC interphase is thick, a design strategy which has been load transfer and the tensile behaviour whereas the effect of cpajordx is to reduce the PyC thickness by oxidizing environments is discussed elsewhere. Further, lacing, in a sequencial manner, part of the pyrocarbon by lowering the sublayer thicknesses to a few nm and by a glass former such as SiC, the glass being expected to replacing stoichiometric SiC by smooth nanocrystalline SiC plug the annular pores(which now display a much lower +C deposits, more regular(PyC-SiC), interphases witl cross-section) and hence to slow down the oxidation typically 10 n 30 were deposited, utilizing P-CVI process. Multilayer(PyC-SiC), interphases, with typically (Figure &b)47. Preliminary experiments on model micro 1 n<4, have been deposited by conventional I-CVI in composites have shown that such highly engineered 2D-Nicalon fibre preforms, utilizing alternately a carbon interphases, which have common feature with the structure and a Sic precursor,97 Under such conditions, as of seashells, increased both the tensile failure strength and 1152
Fibre-matrix interfacial zone in ceramic matrix composites: R. R.Naslain M M sic 3 500nm 100 (a) Figure 8(PyC-SiC)n multilayer interphases (TE ages), fabricated by: (a)conventional I-CVI(n=4; e(PyC)= 50 nm: e(SiC)=100 nm), and(b)P-CVI (n= 10; e(PyC)=20 nm: e(SiC)= 30 nm). according to Refs 46, 47 respectively the lifetime in oxidizing atmospheres of the materials, with REFERENCES respect to their counterparts with a simple PyC interphase has still to be transferred to real composite. cerphase but The concept has been extended to(BN-SiC 1. Aveston, J, Cooper, G. A and Kell re. In Proc. Conf on the properties m 2 F. w. and Mackin, T.J., The structural pc CONCLUSION rmance of ceramic matrix composites. In High Temperature Ceramic matrix composites are tough materials when the K. Jakus. Butterworth-Heinemann, Boston, 1995, pp 3-84 3. Naslain, R, Fiber-matrix interphases and interfaces in ceramic fibre-matrix bonding has been properly controlled during matrix composites processed by CVl. Composites Interface. rocessing, via the use of an interphase exhibiting a low enough failure shear strength. The interphase can be formed lain, R, The concept of layered interphases in SiC/SiC Ceram he fibre-matrix interfaces by in reactions or 5. Naslain, R.R., Interphases in ceramic matrix composites Ceram. deposited on the fibre surface prior to composite fabrication 1996,79,37 Four main types of interphase matrix composites, In High Temperature Ceramic Matrix Com promising in terms of design flexibility and mechanical osites, eds R. Naslain, J. Lamon and D. Doumeingts. Woodhead 7. Brennnan, J J. Interfacial chemistry and bonding in fiber reinforced structure materials or multilayers. Although it is relatively posites. Mater, Sci, Res, 1987. asy to fabricate non-brittle ceramic matrix composites achieving high mechanical properties and long lifetimes 8. Naslain, R and langlais, f, CVD composite materials. Mater. Sci. Research,, 1986, 20, 145. under load in oxidizing atmospheres at high temperatures Naslain, R. R. Ceramic matrix composites. Philosophical Trans requires a subtle design of the fibre-matrix interfacial zone. Encouraging results have been already obtained for glass- 10. Lowden, R. A, Fiber coatings and the mechanical properties of a ceramic and SiC-matrix composites with SiC based fibres, continuous fiber-reinforced SiC-matrix composite. In Designing Ceramic interfaces ll, ed S. Peteves CEC, Luxembourg, 1993 whereas the case of all-oxide composites remains an open field of research amAC5以m131 n, M. G. Devel ACKNOWLEDGEMENT history. In Designing Ceramic Interfaces I,ed, s, D. Peteves. CEC 14. DanieL. A. M. and Lewis. M. H. measurement of interfacial micro- The author is indebted to his colleagues from LCTS and x composites. British Ceram. Tr SEP for their contribution and valuable discussion, as well s J. Forget and M. Saux for their assistance in the 15. Lewis. M. H. Daniel. A. M. and Cain. M. G. Interface character- manuscript preparation Pmoc,l995,365,269 1153
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