Adu: Composite Mate. VoL 8, No I, Pp. 3-16(1999) Materials design and processing of high temperature ceramic matrix composites: state of the art and future trends R. NASLAIN Laboratory for Thermostructural Composites. UMR-580/(CNRS-SEP/SNECMA-UB1) University of Bordeaux- 1. 3 Allee de La Boetie, 3 3600 Pessac, france Abstract-The material design of ceramic matrix composites(CMCs)is discussed in terms of fiber nature and architecture, interphases(pyrocarbon, BN, multilayers), matrices and seal-coating taking into account lifetime considerations. CMCs are processed by liquid or gas phase routes Besides the well established processing routes, such as polymer impregnation and pyrolysis (PIP) slurry impregnation/hot pressing(SIHP)and isothermal/isobaric chemical vapor infiltration(I-CvI) techniques, emerging processes allowing densification times of the order of a day, are discussed Keywords: CMC: C/C: SiC/SiC: design: interphase: processing L INTRODUCTION The ceramic matrix composites(CMCs) considered here consist of continuous (or semi-continuous ceramic fibers embedded in a ceramic matrix, such as carbon, silicon carbide, silicon nitride or a refractory oxide. They are usually designed for applications in severe environments. It is now well established that CMCs are tough materials when the fiber-matrix(FM)bonding is properly controlled during processing, i. e. through the use of a thin layer of a compliant material with a low shear strength, referred to as the interphase. It has been recently suggested that the best interphase materials might be those with a layered crystal structure, e.g pyrocarbon, or with a layered microstructure, such as(PyC-SiC)n multilayers [lI The design of a CMC, in terms of materials, is dictated by the nature of the application. It usually includes: (i) the choice of a proper fiber and fiber architecture: (ii) the selection of suitable interphase and matrix materials and, when necessary: (iii) the use of an external seal coating. As the composition of the CMC is fixed, a processing technique has to be selected on the basis of everal considerations, including the size and shape of the part as well as the
R. narain number of parts to be produced. Because ceramic remains brittle within a broad range of temperature, CMCs should be fabricated according to processes that do not damage the fibers, which generally means pressureless techniques and use of matrix precursors. Further, in order to avoid strong chemical reactions between the constituents or detrimental change in the fiber microstructure, low temperature processing techniques are often preferred. The starting material can be a simple continuous fiber tow (or tape), fiber fabrics or, more often, a multidirectional nD fiber preform. The interphase material is classically deposited on the fiber surface by chemical vapor deposition(CVD)or infiltration(CVD). Then the fiber archite densified according to liquid or gas phase routes, or a combination of both(h processes)[21 2. MATERIALS DESIGN 2. Fiber architecture In the field of CMCs, carbon fibers are by far the most attractive reinforcements Carbon is one of the most refractory materials and one of the very few materials whose mechanical properties increase as temperature is raised. Further, a variety of carbon fibers are available, some of them being relatively cheap. Unfortunately carbon reacts with oxygen at temperatures as low as 400-500oC, with formation f gaseous oxides(active oxidation). As a result, the use of carbon fibers in CMCs that will be exposed to oxidizing atmospheres, demands internal/external oxidation protection. Hence, non-oxide fibers, such as silicon carbide or related fibers, known to undergo passive oxidation with formation of a protective silica scale are more appropriate. During many years, the SiC-based fibers available on the market such as the Si-C-O Nicalon fibers(from Nippon Carbon) or the Si-C-O(Ti) Tyranno fibers from Ube Industries, were limited in thermal stability, undergoing decomposition at 1100-1200C 13-111. As a result, their use was limited to low temperature processing techniques and applications. Modern oxygen-free fibers with a stoichiometry close to C/Si (at)= I and displaying a much higher thermal stability (Hi-Nicalon S from Nippon Carbon. Tyranno(SA)from Ube Industries or Sylramic from Dow Corning )are now available [12-17. Such fibers might become attractive for CMCs if their cost is reduced significantly. Finally, refractory oride fibers would be the best reinforcements for CMCs used in a prolonged manner at high temperature in oxidizing atmospheres. These fibers actually exist on the market, as pure alumina(Nextel 610 from 3M)or as mixed alumina-mullite(Nextel 720 also from 3M) fibers, but their high temperature mechanical properties are relatively poor beyond 1000-1 100C due to grain growth and creep[18-231 Ceramic yarn fibers are sometimes employed as continuous single tows. After im pregnation with a liquid precursor of the matrix and winding, they yield a prepreg type or semi-product material, which is used to produce laminates according to techniques similar to those developed for their polymer matrix counterparts. How ever, ceramic fibers are more generally employed as self-standing multidirectional
Design and processing of high temperature CMCs preforms, fabricated by weaving processes. Such n D-fiber architectures are well suited to rapid processing techniques and complex loading schemes [24, 25 2.2. Interphase materials In most CMCs, an interphase is used to control the FM-bonding during processing It is a key constituent with several functions, including FM load transfer, matrix microcrack deflection (the so-called fuse function) and diffusion barrier (in reactive FM systems). The design of the FM-interfacial zone in a given CMC for a given application is somewhat difficult, and it is rarely performed in an optimized manner. The best interphase materials might be those with a layered structure or microstructure, the layers being parallel to the fiber surface and weakly bonded to one another. and the whole interphase strongly adherent to the fiber [, 26]. The most commonly employed interphase consists of a thin layer(less than I um)of anisotropic pyrocarbon deposited on the fiber surface by CVD/CVI from a suitable hydrocarbon(Fig. la Pyrocarbon is an excellent interphase material from a mechanical standpoint However, it is oxidation prone. The oxidation of carbon starts at about 450C. It occurs with formation of gaseous species exclusively (CO or/and CO2)and hence with a weight loss, the material being progressively consumed (active oxidation ). It is also anisotropic and strongly depending on the occurrence of specific impuritie (such as alkali or alkaline earth cations) often present in carbonaceous materials. When used in a SiC/SiC composite exposed to an oxidizing atmosphere under cyclic loading, a pyrocarbon interphase can be consumed with a dramatic change in the FM interfacial bonding and hence in the overall mechanical behavior of the com- Figure 1. Interphases in SiC/SiC composites: (a) anisotropic pyrocarbon; (b) hex-BN: and (c)(PyC-SiC)1o multilayers according to [73].131I. and [33 respectively
6 R. Masai posite. Under such conditions, interphases with an improved oxidation resistance should be employed, A first approach is to select a material displaying intrinsi cally a high oxidation resistance, ideally a layered refractory oxide compatible with both the fiber and the matrix. Indeed, the number of such oxide interphases is ex- tremely limited. Known examples are magnetoplumbite-type oxides, e.g. hibonite CaAl12O19), or mica-type oxides such as fluorophlogopite KMg3 (SiAlOo F2 or related phyllosiloxides(KMg2 Al(Si4 )012 [26-29]. Moreover, the deposition of a layered oxide thin film with a controlled microtexture(the layers being parallel to the fiber surface) by sol-gel or CVD/CVI techniques, is not straightforward. The thermal stability of layered oxides at high temperatures is often limited, which is typically the case for micas and related materials. Another potential interphase material is hexagonal BN which has a layered crystal structure similar to that of raphite but whose oxidation starts at about 850.C and is passive(formation of condensed B2O3)[30]. Hex-BN can be deposited by CVD/CVI from a variety of gaseous precursors, the most commonly used being BX3-NH3 mixtures(with X=F, CD. It is often turbostratic(as its pyrocarbon counterpart)and even amor phous, depending on the nature of the precursor and the T-P conditions. Highly ordered hex-BN has been deposited at low temperatures from BF3-NH3 but under conditions which are aggressive for SiC-based fibers(Fig 1b)131. However, hex BN is sensitive to moisture when poorly crystallized [32]. A second approach is to design self-healing multilayered(X-Y)n interphases, combining at a nm-scale a compliant X material such as pyrocarbon with a stiff glass-former Y, such as SiC (Fig. Ic), the formation of the glass at a high enough temperature, protecting the mechanical fuse against oxidation [26, 33]. The concept can even be extended to X-Y sequences in which the mechanical fuse is a glass-former itself, an example being the recently proposed (BN-SiC), multilayered interphases [34]. Finally, in a few CMCs, such as the C/C composites, an interphase is not used, the mechanical fuse being the FM interface itself which has been weakened enough by, for example, thermal treatment at very high temperature 2.3. Matrix and seal-coating Ceramic matrices also fall into two categories, namely the non-oxide and the oxide matrices. Among the first family, the carbon matrix (associated with carbon fibers) is the most commonly used material on the basis of cost, processing and properties considerations, C/C are by far the most developed CMCs and the only materials used in volume production. The carbon matrix can be formed either from iquid or gaseous precursors, with a variety of microtextures and hence a variety of properties [35, 36]. Isotropic carbon displays low mechanical characteristics Highly anisotropic carbons, such as those deposited by CVD/CvI with the so-cal rough laminar(RL) microtexture, exhibit after a graphitization treatment beyond 2000C. a high thermal conductivity The advantage of SiC and Si3N4 matrices lies in their oxidation resistance(they are protected by a silica scale up to 1500-1600 C). However, most CMCs ar
Design and processing of high temperature CMCs Figure 2. Model composites with self-healing multilayered matrix, reinforced with carbon fibers. according to [391 inverse composites(ER and the matrix undergoes microcracking when loaded at a high enoug level, e.g. in cyclic loading. As a result, oxygen can diffuse via the matrix microcracks and reach the oxidation-prone interphas and fibers [37 To impede or slow down the in-depth diffusion of oxygen, the oncept of multilayered ceramics has been extended to the matrix [38]. An example of a composite with a self-healing matrix reinforced with carbon fibers, has recentl been reported by Lamouroux et al.(Fig. 2)[39]. The multilayered matrix comprises mechanical fuse layers arresting the cracks and glass former layers entrapping oxygen in a low viscosity glass A variety of oxide matrices, such as alumina, mullite, stabilized zirconia or silica-based glass-ceramics can also be considered. Glass-ceramics, such as those from the Li,O-AlO3-SiO,(LAS). CaO-AlO3-SiO,(CAS)or Bao-Mgo AlO3-SiO,(BMAS)systems, have been extensively studied. One of the advan- tages of such glass-ceramic matrices lies in the fact that they are used in their low viscosity state(beyond T,) to embed the fibers and then ceramed to increase their refractoriness [401 Finally, non-oxide CMCs often receive a seal-coating to seal the open residual porosity and hence to impede the in-depth diffusion of oxygen. A variety of seal- coatings has been suggested including a single layer of a glass-former such as SiC or multilayered smart coatings [41 In the specific case of C/C composites, the oxidation protection usually comprises both inhibiting particles dispersed within the matrix and a complex external seal-coating [42, 431 3. PROCESSING CMCs are fabricated via liquid or gas phase routes, from fiber tows, fabrics or nD- reforms. Low temperature, pressureless, near net shape processes are preferred