R. Naslain/ Composites Science and Technology 64(2004)155-170 PI-P sequences with a polymer precursor(to consolidate the Sic particulates deposited from the slurry)and a 3.5 SA FIber deposition of SiC from the gas phase by P-CVI [37, 38 2 ( LorM Tyranno(zMI) 3. Material design In terms of material design, the objective is (1)to Nicalon(NL201) achieve the best mechanical behavior in static and cyclic Hi-Nicalon loading, particularly at high temperatures and (2)to improve oxidation resistance (SiC/Sic being intrinsi 100012001400160018002000 cally oxidation-prone)and durability under load in corrosive environments, such as fuel combustion gas he mechanical behavior of SiC-matrix composites is Fig 4. Variations of the room temperature failure strength of differ mostly controlled by the fibers and the interphase ent SiC-based fibers as a function of their heat treatment temperature whereas the oxidation resistance and durability are Tyranno (Lox M and ZMD)and Nicalon(NL 201)are SiCO(M) depending upon the properties of the fibers, the inter- fibers, Hi-Nicalon is a SiC +C fiber and SA fiber is a quasi-stoichio- phase, the matrix and the external coating. These dif- metric SiC fiber, according to Ishikawa et al.[42] Terent points will be briefly discussed that of pure SiC (Ex400 GPa), their strain at failure 3.. Fibers relatively high (ER=1. 4%)and more importantly, they decompose beyond 1100-1200oC with a strength Generally speaking, two kinds of fibers are used to degradation [40]. Hence, SiC/SiC composites fabri- inforce a SiC-matrix with a view to achieve high cated with these fibers should be processed by low The former are available with a wide range of mechan- limited in temperature (Table l PIP)and their use is ical and thermal properties at high temperatures, and at SiC-based fibers of second generation (such as Hi- a relatively low cost(at least for those also used for the Nicalon) are oxygen-free fibers consisting of a mixture reinforcement of polymer matrices). Further, they are of Sic-nanocrystals(a5 nm in mean size) and free car chemically compatible with Sic almost up to its bon [C/Si (at) ratio=1.39]. They do not undergo decomposition temperature (N2500C). Conversely, decomposition at high temperature since they do not their coefficient of thermal expansion( CTE) is aniso- contain significant amount of SiC.O, phase. They creep tropic (it is very low and even negative along the axis at moderate temperatures (1200C) but their creep but large and positive radially) and different from that resistance is improved (1400C)if they have been of SiC. As a result, C/SiC composites already exhibit a submitted to a heat treatment at 1400-1600C which microcracked matrix in the as-prepared state, these stabilizes the fiber microstructure[40, 41]. The SiC fibers microcracks facilitating the in-depth oxygen diffusion of third generation(such as Hi-Nicalon S, Tyranno $A when exposed to oxidizing atmospheres [39]. More or Sylramic) are oxygen-free and quasi-stoichiometric importantly, carbon fibers undergo active oxidation at [with C/Si (at)=1.00-1. 08][40, 42, 43]. Further, their very low temperature(450C)and could be used only grain size is relatively large(20-200 nm)and their ther in composites where they are perfectly protected again mal stability is excellent, since they are prepared at very oxidation. Finally, the HR carbon fibers(which are the high temperature(1600-2000C) As a result, their use most attractive in terms of cost and availability), have in SiC/SiC composites is compatible with all the fabri- to be properly stabilized through a high temperature cation processes depicted in Section 2, including those treatment(hTT) when used to reinforce SiC at high temperatures. Conversely, the Sic fibers of third The situation is a priori better with SiC fibers since all generation are very stiff (E N400 GPa)and their wea- the problems related to FM-compatibility at high tem- veability is low. Further, their somewhat low strain at peratures are solved and since Sic displays a much bet- failure(0.6-0.8%)limits the extension of the non-linear ter oxidation resistance than carbon. However, all the stress-strain domain partly responsible for the non-brit Sic-based fibers are not pure Sic and most of them are tle character of SiC/Sic composites. Finally, these very expensive( Fig. 4). The early Si-C-O fibers [such as advanced Sic fibers are costly. Obviously, there is still a the Nicalon(from Nippon Carbon, Japan) fiber), refer- need for low cost Sic-based fibers exhibiting moderate red to as Sic-based fibers of first generation, consist of stiffness and relatively high failure strength as well as Sic-nanocrystals(1-2 nm in size) and free carbon high thermal stability and creep resistance(Table 1) embedded in an amorphous SiCrOy matrix. As a nterestingly, a new ceramic fiber, corresponding to result, their stiffness(E-220 GPa)is much lower than the overall formula SiBN3 C, has been recently proposed
PI-P sequences with a polymer precursor (to consolidate the SiC particulates deposited from the slurry) and a deposition of SiC from the gas phase by P-CVI [37,38]. 3. Material design In terms of material design, the objective is (1) to achieve the best mechanical behavior in static and cyclic loading, particularly at high temperatures and (2) to improve oxidation resistance (SiC/SiC being intrinsically oxidation-prone) and durability under load in corrosive environments, such as fuel combustion gas. The mechanical behavior of SiC-matrix composites is mostly controlled by the fibers and the interphase whereas the oxidation resistance and durability are depending upon the properties of the fibers, the interphase, the matrix and the external coating. These different points will be briefly discussed. 3.1. Fibers Generally speaking, two kinds of fibers are used to reinforce a SiC-matrix with a view to achieve high toughness and reliabilty: carbon and SiC-based fibers. The former are available with a wide range of mechanical and thermal properties at high temperatures, and at a relatively low cost (at least for those also used for the reinforcement of polymer matrices). Further, they are chemically compatible with SiC almost up to its decomposition temperature ( 2500 C). Conversely, their coefficient of thermal expansion (CTE) is anisotropic (it is very low and even negative along the axis but large and positive radially) and different from that of SiC. As a result, C/SiC composites already exhibit a microcracked matrix in the as-prepared state, these microcracks facilitating the in-depth oxygen diffusion when exposed to oxidizing atmospheres [39]. More importantly, carbon fibers undergo active oxidation at very low temperature ( 450 C) and could be used only in composites where they are perfectly protected again oxidation. Finally, the HR carbon fibers (which are the most attractive in terms of cost and availability), have to be properly stabilized through a high temperature treatment (HTT) when used to reinforce SiC. The situation is a priori better with SiC fibers since all the problems related to FM-compatibility at high temperatures are solved and since SiC displays a much better oxidation resistance than carbon. However, all the SiC-based fibers are not pure SiC and most of them are very expensive (Fig. 4). The early Si–C–O fibers [such as the Nicalon (from Nippon Carbon, Japan) fiber], referred to as SiC-based fibers of first generation, consist of SiC-nanocrystals (1–2nm in size) and free carbon embedded in an amorphous SiCxOy matrix. As a result, their stiffness (E=220 GPa) is much lower than that of pure SiC (E 400 GPa), their strain at failure relatively high ("R=1.4%) and more importantly, they decompose beyond 1100–1200 C with a strength degradation [40]. Hence, SiC/SiC composites fabricated with these fibers should be processed by low temperature techniques (CVI or PIP) and their use is limited in temperature (Table 1). SiC-based fibers of second generation (such as HiNicalon) are oxygen-free fibers consisting of a mixture of SiC-nanocrystals ( 5 nm in mean size) and free carbon [C/Si (at) ratio=1.39]. They do not undergo decomposition at high temperature since they do not contain significant amount of SiCxOy phase. They creep at moderate temperatures ( 1200 C) but their creep resistance is improved ( 1400 C) if they have been submitted to a heat treatment at 1400–1600 C which stabilizes the fiber microstructure [40,41]. The SiC fibers of third generation (such as Hi-Nicalon S, Tyranno SA or Sylramic) are oxygen-free and quasi-stoichiometric [with C/Si (at)=1.00–1.08] [40,42,43]. Further, their grain size is relatively large (20–200 nm) and their thermal stability is excellent, since they are prepared at very high temperature (1600–2000 C). As a result, their use in SiC/SiC composites is compatible with all the fabrication processes depicted in Section 2, including those at high temperatures. Conversely, the SiC fibers of third generation are very stiff (E 400 GPa) and their weaveability is low. Further, their somewhat low strain at failure (0.6–0.8%) limits the extension of the non-linear stress-strain domain partly responsible for the non-brittle character of SiC/SiC composites. Finally, these advanced SiC fibers are costly. Obviously, there is still a need for low cost SiC-based fibers exhibiting moderate stiffness and relatively high failure strength as well as high thermal stability and creep resistance (Table 1). Interestingly, a new ceramic fiber, corresponding to the overall formula SiBN3C, has been recently proposed Fig. 4. Variations of the room temperature failure strength of different SiC-based fibers as a function of their heat treatment temperature: Tyranno (Lox M and ZMI) and Nicalon (NL 201) are Si–C–O (M) fibers, Hi-Nicalon is a SiC+C fiber and SA fiber is a quasi-stoichiometric SiC fiber, according to Ishikawa et al. [42]. 160 R. Naslain / Composites Science and Technology 64 (2004) 155–170
R. Naslain/ Composites Science and Technology 64 (2004)155-170 Table I Properties of Sic-based and related fibers from literature sources(properties may vary from lot to lot and some of the fibers are still experimental) Properties Nicalon Hi-Nical Hi- Nical Sylramic Carborundum SHC-O) on si-c SA SIC(AI) SIC(B. T1) SIB-C-N Y-SiC Diameter (um) 8-10 8-14 Density(g/cm) 2.55 3.0-3.1 18-2.0 Tensile strength(GPa) Young s modulus(GPa) 386-40 0.4 CTE(10-6K- 3.1-3.2 3.0-3. Thermal conduct(W/mK) 1.4-3.0 40-45 Resistivity(S2 cm 0.1 ≈1470 ≈1500 Depending on processing conditions. bAt25°C c Temperature at which m=0.5 in BSR-test. [44]. It is prepared from a polymer precursor and it dis-(interphases with a layered crystal structure)or/and plays surprising properties. Firstly, it remains amor along the interfaces between the elementary layers phous up to extremely high temperatures(1700-1900oC, (interphases with a layered microstructure)and in both depending on the atmosphere) and hence it exhibits a case roughly parallel to the fiber surface (mode high failure strength(3-4 GPa). Its weaveability is good I-mode II deflection mode). The last condition, i.e owing to its small diameter(8-14 um)and relatively low that requiring a strong bonding between the interphase stifness (E-180-350 GPa). Surprisingly, its creep and the fiber is often underestimated although it is resistance is similar to that of the best Sic polycrystal essential. If it is not satisfied, FM-debonding will pre- line fibers, although it is actually amorphous(Table 1). ferentially occurred at the fiber surface, sometimes over Finally, it may have a good compatibility with boron very long distances, with the result that the load trans nitride(one of the best interphase materials for SiC/Sic fer function is largely lost and the fibers exposed to the composites) and it displays an oxidation resistance bet- ter than those of Si, N4 and SiC. Unfortunately, fibers from the Si-B-C-N quarternary system are still experi mental and their cost. relative to that of other advanced Sic fibers is not known 3.2. Interp The interphase is a thin film of a compliant material with a low shear strength(typically, 0.I-l um in thick- ness), which is deposited on the fiber surface prior to the infiltration of the matrix and whose main function (b) is to arrest or/and deflect the matrix microcracks hence protecting the fibers from an early failure by notch effect(mechanical fuse function). In addition, the interphase has a load transfer function(as in any composite) and may act as diffusion barrier during composite processing, when necessary. Although different kinds of interphase concepts have been suggested 145, it has been postulated that the best interphase materials might be those with a layered crys- tal structure(Pyrocarbon, hexagonal-BN)or a layered microstructure [(PyC-SiCn or(BN-SiC)nl, the layers being deposited parallel to the fiber surface and weakly Fig. 5. D bonded to one another, and the interphase being composites: (a) single layer pyrocarbon or hexagonal BN strongly bonded to the fiber surface( Fig. 5)[46]. When (b) porous SiC single layer interphase, (c)multilayered(X- phase, with X=PyC or BN and y=SiC (schematic), (d) these conditions are properly satisfied, crack deflection tion in a multilayered(Pyc-SiC)o interphase, according to Naslain occurs within the interphase either along atomic planes (a-c)[46] and Bertrand et al. [50]
[44]. It is prepared from a polymer precursor and it displays surprising properties. Firstly, it remains amorphous up to extremely high temperatures (1700–1900 C, depending on the atmosphere) and hence it exhibits a high failure strength (3–4 GPa). Its weaveability is good owing to its small diameter (8–14 mm) and relatively low stiffness (E=180–350 GPa). Surprisingly, its creep resistance is similar to that of the best SiC polycrystalline fibers, although it is actually amorphous (Table 1). Finally, it may have a good compatibility with boron nitride (one of the best interphase materials for SiC/SiC composites) and it displays an oxidation resistance better than those of Si3N4 and SiC. Unfortunately, fibers from the Si–B–C–N quarternary system are still experimental and their cost, relative to that of other advanced SiC fibers, is not known. 3.2. Interphases The interphase is a thin film of a compliant material with a low shear strength (typically, 0.1–1 mm in thickness), which is deposited on the fiber surface prior to the infiltration of the matrix and whose main function is to arrest or/and deflect the matrix microcracks, hence protecting the fibers from an early failure by notch effect (mechanical fuse function). In addition, the interphase has a load transfer function (as in any fiber composite) and may act as diffusion barrier during composite processing, when necessary. Although different kinds of interphase concepts have been suggested [45], it has been postulated that the best interphase materials might be those with a layered crystal structure (Pyrocarbon, hexagonal-BN) or a layered microstructure [(PyC–SiC)n or (BN–SiC)n], the layers being deposited parallel to the fiber surface and weakly bonded to one another, and the interphase being strongly bonded to the fiber surface (Fig. 5) [46]. When these conditions are properly satisfied, crack deflection occurs within the interphase either along atomic planes (interphases with a layered crystal structure) or/and along the interfaces between the elementary layers (interphases with a layered microstructure) and in both case roughly parallel to the fiber surface (mode I!mode II deflection mode). The last condition, i.e. that requiring a strong bonding between the interphase and the fiber is often underestimated although it is essential. If it is not satisfied, FM-debonding will preferentially occurred at the fiber surface, sometimes over very long distances, with the result that the load transfer function is largely lost and the fibers exposed to the Table 1 Properties of SiC-based and related fibers from literature sources (properties may vary from lot to lot and some of the fibers are still experimental) Properties Nicalon (Si–C–O) Hi-Nical on Si–C Hi-Nical on S SiC Tyranno SA SiC (Al) Sylramic SiC (B, Ti) Bayer Si–B–C–N Carborundum a-SiC Diameter (mm) 14 14 128–10 10 8–14 28 Density (g/cm3 ) 2.55 2.74 3.10 3.0–3.1 3.0–3.1 1.8–2.0 3.1 Tensile strength (GPa) 3.0 2.8 2.6 2.8–3.0 3.0–3.2 3–4a 1.4 Young’s modulus (GPa) 220 270 390–420 390–420 386–400 180–350a 420 Failure strain (%) 1.4 1.0 0.6 0.7 0.6–0.8 0.7–1.5a 0.4 CTE (106 K1 ) 3.1–3.23.3–3.5 – – 4.0–5.4 3.0–3.5a 4.5 Thermal conduct (W/mK)b 1.4–3.0 5.0–7.8 18 65 40–45 – – Resistivity ( cm) 103 –104 1.4 0.1 – – – – Creep parameter (T0.5) c 1110 1240 1470 1500 1400 1500 1550 a Depending on processing conditions. b At 25 C. c Temperature at which m=0.5 in BSR-test. Fig. 5. Different simple or engineered interphases used in SiC-matrix composites: (a) single layer pyrocarbon or hexagonal BN interphases, (b) porous SiC single layer interphase, (c) multilayered (XY)n interphase, with X=PyC or BN and Y=SiC (schematic), (d) crack deflection in a multilayered (PyC–SiC)10 interphase, according to Naslain (a–c) [46] and Bertrand et al. [50]. R. Naslain / Composites Science and Technology 64 (2004) 155–170 161