r./. App Ceram. Technol, 7/ 3/263-275(2010) International Journal o Applied p p Ceramic TECHNOLOGY ceramic Product D Single-and Multilayered Interphases in SiC/SiC Composites Exposed to Severe Environmental Conditions An overview Roger R. Naslain, Rene J -F. Pailler, and Jacques L. Lamon Laboratory for ThermoStructural Composites(LCTS), University of Bordeaux, 33600 Pessac, france Pyrocarbon(PyC), the common interphase for SiC/SiC, is not stable under severe environmental conditions. It could be replaced by boron nitride more resistant to oxidation but poorly compatible with nuclear applications. Other materials, such as ternary carbides seem promising but their use in SiC/SiC has not been demonstrated. The most efficient way to improve the behavior of Py C interphase in severe environments is to replace part of PyC by a material displaying a better compatibility, such as SiC itself. Issues related to the design and behavior of layered interphases are reviewed with a view to demonstrate their interest in high-temperature nuclear reactors. Introduction residual thermal stresses. The interphase protects fibers against chemical reactions that could occur during pre The interphase plays a key role in the behavior of cessing and use of CMCs in aggressive environments ceramic matrix composites(CMCs). It prevents the early It has been postulated that the best failure of the fibers, matrix microcracks being arrested terials for SiC/SiC might be those with a layered structure, or/and deflected parallel to fiber axis(the so-called the layers being parallel to fiber surface, weakly bonded to mechanical fuse"function). It also transfers load from one another but strongly adherent to fibers. Pyrocar the fibers to the matrix and eventually releases part of bon(PyC) was thought to be the best interphase material for sic/sic in terms of their mechanical behavior -5 Unfortunately, Py C is oxidation prone even at low This paper was presented in part ar the 33rd Annual International Conference on Advanced temperatures with the result that PyC interphase can be consumed and the FM coupling nce, that C 200) The American Ceramic Society interphase becomes the weak point of SiC/SiC whe
Single- and Multilayered Interphases in SiC/SiC Composites Exposed to Severe Environmental Conditions: An Overview Roger R. Naslain,* Rene´ J.-F. Pailler, and Jacques L. Lamon Laboratory for ThermoStructural Composites (LCTS), University of Bordeaux, 33600 Pessac, France Pyrocarbon (PyC), the common interphase for SiC/SiC, is not stable under severe environmental conditions. It could be replaced by boron nitride more resistant to oxidation but poorly compatible with nuclear applications. Other materials, such as ternary carbides seem promising but their use in SiC/SiC has not been demonstrated. The most efficient way to improve the behavior of PyC interphase in severe environments is to replace part of PyC by a material displaying a better compatibility, such as SiC itself. Issues related to the design and behavior of layered interphases are reviewed with a view to demonstrate their interest in high-temperature nuclear reactors. Introduction The interphase plays a key role in the behavior of ceramic matrix composites (CMCs). It prevents the early failure of the fibers, matrix microcracks being arrested or/and deflected parallel to fiber axis (the so-called ‘‘mechanical fuse’’ function). It also transfers load from the fibers to the matrix and eventually releases part of residual thermal stresses. The interphase protects fibers against chemical reactions that could occur during processing and use of CMCs in aggressive environments. It has been postulated, that the best interphase materials for SiC/SiC might be those with a layered structure, the layers being parallel to fiber surface, weakly bonded to one another but strongly adherent to fibers.1–4 Pyrocarbon (PyC) was thought to be the best interphase material for SiC/SiC in terms of their mechanical behavior.1–5 Unfortunately, PyC is oxidation prone even at low temperatures with the result that PyC interphase can be consumed and the FM coupling degraded. Hence, that interphase becomes the weak point of SiC/SiC when Int. J. Appl. Ceram. Technol., 7 [3] 263–275 (2010) DOI:10.1111/j.1744-7402.2009.02424.x Ceramic Product Development and Commercialization This paper was presented in part at the 33rd Annual International Conference on Advanced Ceramics and Composites, Daytona Beach, FL, January 18–23, 2009 *naslain@lcts.u-bordeaux1.fr r 2009 The American Ceramic Society
International Journal of Applied Ceramic Technolog-Naslain, Pailler and Lamon Vol. 7, No 3, 2010 used in oxidizing atmospheres(gas turbines). Within the isobaric and P for pressure pulsed) from propane or same conceptual framework, two main alternatives have prop been proposed: boron nitride(BN)and (X-Y)n multi layers (MD). The former displays a structure similar to Fiber/PyC-Interphase Bonding that of graphite while being more oxidation res sistant In the latter, with X= PyC or BN, Y=SiC, and n The second requirement is a strong bonding be- 1-10, Part of the oxidation prone X constituent is tween the fber and the PyC interphase. Achieving replaced by a material Y exhibiting a better oxidation such a strong bonding is first a matter of surface chem- resistance, such as SiC itself. 8- Furthermore, this istry. The surface of desized Si-C-O fibers(Nicalon concept of material multilayering has been extended to the matrix, yielding"self-healing"composites with and free carbon. As a result, there is in SiC/SiC(CVI)a outstanding lifetimes in oxidizing atmospheres. 10,13 thin and irregular dual layer of hous silica and More recently, SiC/SiC composites have beer carbon which introduces a weak link near the fiber sur- envisaged as structural materials in high-temperature face. 4.4 The fibers should be pretreated to clean (HT) nuclear reactors. The interphase appears again as a their surface and achieve a strong fiber/interphase(Fi) possible weak point, PyC being known to undergo bonding. Another example is the SiC+C fibers(Hi- anisotropic volume change when exposed to neutrons, Nicalon or HN, Nippon Carbon, Tokyo, Japan), whose whereas BN undergoes nuclear reactions. 5-7 microstructure is not fully stabilized after processing and is presently pursued following two similar routes: (i)use whose surface may also contain some oxygen. During nd (ii) MLs, to minimize the effect of neutron irradiation. 5-21 CVI processing, fibers undergo a postshrinkage weak ening the FI interface. Again, the fibers should be pre- The aim of the present overview is to recall the basis treated. Finally, stoichiometric SiC fibers(Hi-Nicalon of the layered interphase concept, to discuss its appli type S(HNS)or Tyranno SA (TSA, Ube Industries, cation to SiC/SiC exposed to oxidizing environment Yamaguchi, Japan)), fabricated at higher temperatures and tentatively, to neutron irradiation. are assumed to be dimensionally stable at composite processing temperatures. Further, their surface consists Py C Single-Layer Interphase: The Reference of free carbon(resulting from SiC decomposition) Hence, their bonding with PyC interphase is expected PyC has a structure similar to that of graphite but to be relatively strong Finally, the roughness of fiber, which is low for or less distorted and stacked with rotational disorder. Nicalon and Hi-Nicalon but significant for stoichiomet Our layered interphase concept requires that: (i)the ric fibers, adds a mechanical contribution to the fi bond layers should be oriented parallel to fiber surface and (i ing in a transition zone where the nanometric grapl the bonding between the fber and the PyC interphase lay should be strong enough. Otherwise, debonding/crack deflection would occur at the fiber surface exposing the SiC (Nicalon)/Py C/SiC: A Case History fiber to mechanical damage and to the atmosphere.3.4 Studies on SiC/PyC/SiC (CVI) fabricated with Py c Texture Nicalon or Hi-Nicalon fibers(the as-received or pre- treated), clearly show the positive effect of FM-interfa- PyC displays a variety of microtexture and anisot- cial design on material properties. As shown in Fig. I in polarized light (extinction angle, Ae)or/and trans- PyC interphase(samples posites with a single 500 nm ropy, which can be characterized by optical microscopy tensile curves for the con and D) exhibit extended non- mission electron microscopy(TEM)(L2, N parameters): linear domains related to damaging phenomena, with the larger the Ae, the higher the anisotropy. The pre high failure strains. How vever. co ferred PyC for an interphase is rough laminar (Ae>18) fibers(sample D)is much stronge This RL-PyC has a tendency to grow with graphene The shapes of the curves are different: continuously layers parallel to fiber surface. It is usually deposite convex for the composite with pretreated fibers(sample D) CVI(I-CVI or P-CVI, where I stands for isothe and with a plateau-like feature for that with the as-
used in oxidizing atmospheres (gas turbines). Within the same conceptual framework, two main alternatives have been proposed: boron nitride (BN) and (X–Y)n multilayers (ML). The former displays a structure similar to that of graphite while being more oxidation resistant.5–7 In the latter, with X 5 PyC or BN, Y 5 SiC, and n 5 1–10, part of the oxidation prone X constituent is replaced by a material Y exhibiting a better oxidation resistance, such as SiC itself.1–4,8–12 Furthermore, this concept of material multilayering has been extended to the matrix, yielding ‘‘self-healing’’ composites with outstanding lifetimes in oxidizing atmospheres.10,13,14 More recently, SiC/SiC composites have been envisaged as structural materials in high-temperature (HT) nuclear reactors. The interphase appears again as a possible weak point, PyC being known to undergo anisotropic volume change when exposed to neutrons, whereas BN undergoes nuclear reactions.15–17 Research is presently pursued following two similar routes: (i) use of thin single PyC layers and (ii) use of (PyC–SiC)n MLs, to minimize the effect of neutron irradiation.15–21 The aim of the present overview is to recall the basis of the layered interphase concept, to discuss its application to SiC/SiC exposed to oxidizing environment and tentatively, to neutron irradiation. PyC Single-Layer Interphase: The Reference PyC has a structure similar to that of graphite but the elementary graphene layers are of limited size more or less distorted and stacked with rotational disorder. Our layered interphase concept requires that: (i) the layers should be oriented parallel to fiber surface and (ii) the bonding between the fiber and the PyC interphase should be strong enough. Otherwise, debonding/crack deflection would occur at the fiber surface exposing the fiber to mechanical damage and to the atmosphere.3,4 PyC Texture PyC displays a variety of microtexture and anisotropy, which can be characterized by optical microscopy in polarized light (extinction angle, Ae) or/and transmission electron microscopy (TEM) (L2, N parameters): the larger the Ae, the higher the anisotropy.22 The preferred PyC for an interphase is rough laminar (Ae4181) This RL-PyC has a tendency to grow with graphene layers parallel to fiber surface. It is usually deposited by CVI (I-CVI or P-CVI, where I stands for isothermal/ isobaric and P for pressure pulsed) from propane or propylene.1,5,9,23 Fiber/PyC-Interphase Bonding The second requirement is a strong bonding between the fiber and the PyC interphase.3,4 Achieving such a strong bonding is first a matter of surface chemistry. The surface of desized Si–C–O fibers (Nicalon, Nippon Carbon, Tokyo, Japan) is enriched in oxygen and free carbon. As a result, there is in SiC/SiC (CVI) a thin and irregular dual layer of amorphous silica and carbon which introduces a weak link near the fiber surface.1,3–6,24,25 The fibers should be pretreated to clean their surface and achieve a strong fiber/interphase (FI) bonding. Another example is the SiC1C fibers (HiNicalon or HN, Nippon Carbon, Tokyo, Japan), whose microstructure is not fully stabilized after processing and whose surface may also contain some oxygen. During CVI processing, fibers undergo a postshrinkage weakening the FI interface. Again, the fibers should be pretreated. Finally, stoichiometric SiC fibers (Hi-Nicalon type S (HNS) or Tyranno SA (TSA, Ube Industries, Yamaguchi, Japan)), fabricated at higher temperatures are assumed to be dimensionally stable at composite processing temperatures. Further, their surface consists of free carbon (resulting from SiC decomposition).26 Hence, their bonding with PyC interphase is expected to be relatively strong. Finally, the roughness of fiber, which is low for Nicalon and Hi-Nicalon but significant for stoichiometric fibers, adds a mechanical contribution to the FI bonding in a transition zone where the nanometric graphene layers stacks become progressively parallel to fiber axis.1,7 SiC (Nicalon)/PyC/SiC: A Case History Studies on SiC/PyC/SiC (CVI) fabricated with Nicalon or Hi-Nicalon fibers (the as-received or pretreated), clearly show the positive effect of FM-interfacial design on material properties.1–4 As shown in Fig. 1, tensile curves for the composites with a single 500 nm PyC interphase (samples I and J) exhibit extended nonlinear domains related to damaging phenomena, with high failure strains. However, composite with pretreated fibers (sample J) is much stronger. The shapes of the curves are different: continuously convex for the composite with pretreated fibers (sample J) and with a plateau-like feature for that with the as- 264 International Journal of Applied Ceramic Technology—Naslain, Pailler and Lamon Vol. 7, No. 3, 2010
wwceramics. org/ACT Layered Interphases in SiC/SiC Composites I,J: Pycsoo M, N: PyC reated Nicalon as-received Nicalon J- Stronger F\-bonding I- weak FM-bondiag Fig 1. Tensile curves at ambient of 2D-SiC/SiC (CV) fabricated from Nicalon fibers with different interphases: single P)yC layers or (ByC-SiC) ML interphases(adapted from Droillard'y received fibers (sample I). Matrix microcracks are compressive TRS and lower interfacial shear stress deflected near fiber surface (I-inset, right) for the latter, Experimental studies have shown that the mechanical at the level of the weak carbon/ silica interface and within properties. 5.28 Dupel et through an optimum for e(PyC)=few the PyC interphase(in a diffused manner and over short 100 nm. at distance, J-inset, left) for the former. Here, the PyC inter- tensile properties of ID-SiC(NCG)Py C/SiC (P-CVD phase does play its role of mechanical fuse. These two inicomposites were optima for e(PyC)=220 nm.The microcrack deflection modes correspond to very different calculated radial TRS in the interphase was compressive FM coupling(weak in I and stronger in D. Interestingly, (enhancing thus the FM bonding) at low e(PyC)values toughness of the composites fabricated with pretreated and tensile(favoring FM debonding) for e(PyC)> fibers as well as their fatigue resistance in tensile cyclic 400 nm. loading are higher when it is the Py C interphase, which is In sic/sic fabricated with stoichiometric fibers the active mechanical fuse(strong bonding). 22/Similar the situation is different because the CTEs of the main results have been reported for composites fabricated from constituents are now similar but the fiber surface is Hi-Nicalon fibers and Pyc interphas highly crystalline and rough. The mechanical properties of 2D-SiC/PyC/SiC( CVI)are either constant or Influence of Py C-Interphase Thickness slightly dependent on interphase thickness, when increases from 25 to PyC-layer thickness, e(PyC), has an infuence on be expected from fiber roughness, the interfacial fric- the mechanical properties of SiC/Pyc/SiC ce tional stress tf is high composites with Ty through thermal residual stresses(TRS) and fiber surfac fibers than for those with Hi-Nicalon fibers and decreases as e(PyC Increases In SiC/PyC/SiC(CV) fabricated at≈1000°C using Nicalon( Ceramic Grade, NCG)or Hi-Nicalon Crack Deflection Modeling fibers, the coefficient of thermal expansion(CTE)of the fibers is lower than that of the matrix, which results in Crack deflection at an interface in brittle materials compressive radial TRS at the FM interface and has been modeled. ,3 Recently, Pompidou and reinforces FM bonding. Further, the fiber surface is Lamon 3. 34 have proposed a model, derived from the very smooth. Hence, increasing e(PyC) relaxes radial approach of Cook and Gordon, which is applicable to
received fibers (sample I). Matrix microcracks are deflected near fiber surface (I-inset, right) for the latter, at the level of the weak carbon/silica interface and within the PyC interphase (in a diffused manner and over short distance, J-inset, left) for the former. Here, the PyC interphase does play its role of mechanical fuse. These two microcrack deflection modes correspond to very different FM coupling (weak in I and stronger in J). Interestingly, toughness of the composites fabricated with pretreated fibers as well as their fatigue resistance in tensile cyclic loading are higher when it is the PyC interphase, which is the active mechanical fuse (strong bonding).1,2,27 Similar results have been reported for composites fabricated from Hi-Nicalon fibers and PyC interphase. Influence of PyC-Interphase Thickness PyC-layer thickness, e(PyC), has an influence on the mechanical properties of SiC/PyC/SiC composites, through thermal residual stresses (TRS) and fiber surface roughness. In SiC/PyC/SiC (CVI) fabricated at 10001C using Nicalon (Ceramic Grade, NCG) or Hi-Nicalon fibers, the coefficient of thermal expansion (CTE) of the fibers is lower than that of the matrix, which results in compressive radial TRS at the FM interface and reinforces FM bonding. Further, the fiber surface is very smooth. Hence, increasing e(PyC) relaxes radial compressive TRS and lower interfacial shear stress. Experimental studies have shown that the mechanical properties go through an optimum for e(PyC) 5 few 100 nm.11,15,28 Dupel et al. 29 have reported that the tensile properties of 1D-SiC (NCG)/PyC/SiC (P-CVI) minicomposites were optima for e(PyC) 5 220 nm. The calculated radial TRS in the interphase was compressive (enhancing thus the FM bonding) at low e(PyC) values and tensile (favoring FM debonding) for e(PyC)4 400 nm. In SiC/SiC fabricated with stoichiometric fibers, the situation is different because the CTEs of the main constituents are now similar but the fiber surface is highly crystalline and rough. The mechanical properties of 2D-SiC/PyC/SiC (CVI) are either constant or slightly dependent on interphase thickness, when e(PyC) increases from 25 to 250 nm.17,30 As it could be expected from fiber roughness, the interfacial frictional stress tf is higher for composites with Tyranno fibers than for those with Hi-Nicalon fibers and decreases as e(PyC) increases.12 Crack Deflection Modeling Crack deflection at an interface in brittle materials has been modeled.31,32 Recently, Pompidou and Lamon33,34 have proposed a model, derived from the approach of Cook and Gordon,32 which is applicable to Fig. 1. Tensile curves at ambient of 2D-SiC/SiC (CVI) fabricated from Nicalon fibers with different interphases: single PyC layers or (PyC–SiC)n ML interphases (adapted from Droillard1 ). www.ceramics.org/ACT Layered Interphases in SiC/SiC Composites 265
266 International Journal of Applied Ceramic Technolog-Naslain, Pailler and Lamon Vol. 7, No 3, 2010 The model has been applied to SiC/PyCSiC to examine crack deflection probability at a given interface or within the interphase, to show the influence of fiber pretreatment and to discuss the effect of graphene-layer SIC/Pyc orientation in the interphase Deflection at first interface 0.8 (PyC /SiCm)is very unlikely because for the related E2/EI ratio (N 0.07), the value of oi should be extremely low(Fig. 2) in accordance with experiments By contrast, deflection at second interface( SiCpPyCi)is PyC/Pyc most likely because for the corresponding E2/E, ratio (10 for Hi-Nicalon) the width of the debonding domain (debonding potential) is very large. This is the most frequently observed case(weak FM bonding). If the fiber has been pretreated to strengthen the FM bonding, the representative point may move above the master curve,with debonding no longer occurring at that interface. However, it may take place within the PyC Fig.2. Values of of of ratio provided by the master curve for for a EyE, value of 1, a still significant deflection p us fber/matrix and fberlinterphase/matrix systems(cracked tential (Fin awep s a f now urf e nene layers are lamn al (material I)cited second)(adapted from Pompidou and treated Nicalon fiber deposited perpendicular to fber surface, crack deflection within the PyC interphase becomes no longer possible composites with single- or multilayered interphases. because the ratio opyc//opyci N 2. 17 is well above When a crack of tip radius p is placed in an elastic the master curve for E2/E1=1 medium and subjected to a uniaxial tension Oz(in a direction z perpendicular to crack plane), it generates a Layered Interphases for SiC/SiC Exposed to multiaxial stress field near crack tip whose orr compo- Oxidizing Atmosphere nent(in radial direction)is maximum at a distance on the order of p(omx=o(r=p)). If an interface is PyC is oxidation prone even at temperature as low as placed perpendicular to primary crack extension direc tion near crack tip, a secondary local crack may nucleat 500C, its oxidation resulting in the formation of gas- eous oxides(active oxidation) and degradation of FM at that interface if om.>o:, where o; is the interface coupling 35.36 Two approaches have been selected to solve debonding stress. Deflection results from coalescence of this problem relying on self-healing (or self-sealing)mech- both cracks. 2 When applied to a microcomposite anisms by condensed oxides(passive oxidation). The first loaded in tension along fiber axis, debonding would occur when of/of s omax / oma(with r>D, where of whereas in the second, part of PyC is replaced, in the the failure stress of iber and, l the distance so-called Ml interphases, by Sic or tic to reduce the between crack tip and interface (or ligament), thickness of each elementary PyC sublayer to a few 10 nm om and omax were computed and their ratio plotted and to favor self-healing phenomena? versus Youngs moduli ratio E2/E1, as shown in Fig. 2 The domain under this master curve corresponds to the Boron-Doped PyCInterpbase debonding situation and that above to conditions where debonding cannot occur. The curve exhibits a The addition of boron to Py C increases its graphitic mum corresponding to the highest debonding potential. character at low B concentration and improves its Conversely, when E2/En decreases and tends to zero, oxidation resistance at high B levels by blocking the debonding becomes quite impossible. But, the crack can so-called active sites and forming a Auid oxide(B2O3)in be arrested. Failure of the reinforcing material depends a temperature range(500-900oC), where the growth on its strength versus stress operating: of versus omx. kinetics of silica is still too slow
composites with single- or multilayered interphases. When a crack of tip radius r is placed in an elastic medium and subjected to a uniaxial tension szz (in a direction z perpendicular to crack plane), it generates a multiaxial stress field near crack tip whose srr component (in radial direction) is maximum at a distance on the order of r (smax rr ¼ srrðr ¼ rÞ). If an interface is placed perpendicular to primary crack extension direction near crack tip, a secondary local crack may nucleate at that interface if smax rr > sc i , where si c is the interface debonding stress. Deflection results from coalescence of both cracks.32 When applied to a microcomposite loaded in tension along fiber axis, debonding would occur when sc i =sc f smax rr =smax zz (with r4l), where sf c is the failure stress of the fiber and, l, the distance between crack tip and interface (or ligament),33,34 smax rr and smax zz were computed and their ratio plotted versus Young’s moduli ratio E2/E1, as shown in Fig. 2. The domain under this master curve corresponds to the debonding situation and that above to conditions where debonding cannot occur. The curve exhibits a maximum corresponding to the highest debonding potential. Conversely, when E2/E1 decreases and tends to zero, debonding becomes quite impossible. But, the crack can be arrested. Failure of the reinforcing material depends on its strength versus stress operating: sf c versus smax zz . The model has been applied to SiC/PyC/SiC to examine crack deflection probability at a given interface or within the interphase, to show the influence of fiber pretreatment and to discuss the effect of graphene-layer orientation in the interphase. Deflection at first interface (PyCi/SiCm) is very unlikely because for the related E2/E1 ratio ( 0.07), the value of si c should be extremely low (Fig. 2) in accordance with experiments. By contrast, deflection at second interface (SiCf/PyCi) is most likely because for the corresponding E2/E1 ratio (10 for Hi-Nicalon) the width of the debonding domain (debonding potential) is very large. This is the most frequently observed case (weak FM bonding). If the fiber has been pretreated to strengthen the FM bonding, the representative point may move above the master curve, with debonding no longer occurring at that interface. However, it may take place within the PyC interphase, that is, at a PyCi/PyCi interface that shows for a E2/E1 value of 1, a still significant deflection potential (Fig. 2). This is the situation observed, for pretreated Nicalon fiber.1–4 If now the graphene layers are deposited perpendicular to fiber surface, crack deflection within the PyC interphase becomes no longer possible because the ratio sc PyC===sc PyC? 2:17 is well above the master curve for E2/E1 5 1. Layered Interphases for SiC/SiC Exposed to Oxidizing Atmosphere PyC is oxidation prone even at temperature as low as 5001C, its oxidation resulting in the formation of gaseous oxides (active oxidation) and degradation of FM coupling.35,36 Two approaches have been selected to solve this problem relying on self-healing (or self-sealing) mechanisms by condensed oxides (passive oxidation). The first one is based on single-layer interphases containing boron whereas in the second, part of PyC is replaced, in the so-called ML interphases, by SiC or TiC to reduce the thickness of each elementary PyC sublayer to a few 10 nm and to favor self-healing phenomena.9,10 Boron-Doped PyC Interphase The addition of boron to PyC increases its graphitic character at low B concentration and improves its oxidation resistance at high B levels by blocking the so-called active sites and forming a fluid oxide (B2O3) in a temperature range (500–9001C), where the growth kinetics of silica is still too slow.37 Fig. 2. Values of sc i =sc 2 ratio provided by the master curve for various fiber/matrix and fiber/interphase/matrix systems (cracked material (material 1) cited second) (adapted from Pompidou and Lamon34). 266 International Journal of Applied Ceramic Technology—Naslain, Pailler and Lamon Vol. 7, No. 3, 2010
Layered Interphases in SiC/SiC Composites Jacques et al. have studied the influence of B-doped X> PyC interphase on the oxidation resistance of lD-SiC/SiC 必 matrix s fiber (CVD)microcomposites with pretreated Nicalon fibers Their interphases contain up to 30 at. %B. They showed as expected, that the microtexture of the PyC interphase was significantly improved at low B addition, 8 at. %B, amorphous). More importantly, lifetime in tensile static fatigue(beyond proportional limit PL) in air at 700C was dramatically improved as the B content was raised, the best results being observed for graded composition terphase. Crack deflection and failure occur with che interphase at a location where the interphase micro- texture and graphene-layer orientation were optimal (at 112 ≈8a%B) BN-inte BN Interphases Hi-Nicalon fiber tow moving in a temperature grader The use of BN interphase in SiC/SiC raises sev Ts1100C: T2=1150C 13=1250C) at medium residence time(v=2.5 mb)(adapted from Jacques et al. 12) problems, which still remain imperfectly solved. They include the occurrence of corrosion by precursor and the chemical reactivity of BN with oxygen and moisture hen prepared at low temperature. SiCm/BN interface, these two scenarios corresponding to the"inside"and"outside"debonding reported by Mo F scher et al, in related experiments. In the case of NH3: BF3-NH3 precursor has the advantage of outside" debonding, both the interfacial shear stress yielding crystallized Bn deposits at relatively low tem- and tensile failure stress were lower but the lifetime in rature.' Unfortunately, it involves gaseous species tensile static fatigue at 700C in dry or wet air was dra- (BFs and HF), which are corrosive for Sic-based fibers matically improved(crack deflection occurring far from and alter their strength (as received Nicalon and fiber surface) Nicalon). Conversely, this precursor is compatible with carbon substrates and it could be used to deposit BN Interphases as Deposited by CVI from BClg-NHs- Bn on fibers with a carbon layer surface (pretreated or H2: BCl3-NH3-H2 precursor is usually preferred be- stoichiometric fibers). However, an extra carbon layer cause it is much less corrosive. -/ In principle,BN often remains between the SiC fiber and the BN coating, could be deposited at temperature as low as 700%C. which could be the weakest link in the interfacial zone. However, under such mild conditions, it is nanoporous, ne way to solve the corrosion problem and to play poorly organized and highl ly reactive. Hence, the pro- with the mechanical fuse location could be to deposit cessing temperature should be increased. In the BN in a temperature gradient (TG-CVI). Jacques case of complex fber architectures(nD-preforms), BN et al. have fabricated ID-SiC/BN/SiC minicomposites can be deposited at the highest temperature compatible with a radial crystallinity gradient by simply passing a with the ICVI process(N 1100%C)and further an Hi-Nicalon tow through a three-temperature zones fur- nealed at a temperature corresponding to the up nace Under optimized conditions, in terms of fiber pro- limit of the thermal stability dor main of the fibers.a gression speed, the FI bonding was strong (crack alternative is to deposit Bn on fber tows, which can be deflection occurring within BN interphase(Fig. 3) done at higher temperature(1400-1600%),partic and both interfacial shear stress and tensile stress were larly for stoichiometric SiC fibers. As an example, BN high. At lower fiber speed, crack deflection occurred at deposited on a TSA tow at 1580%C was reported to be fiber surface (as a result of some surface crystallization) nearly stoichiometric, with an impurity content hereas for higher fiber speed it was observed at th <5 at. %, highly crystallized and textured. 49
Jacques et al. 38 have studied the influence of B-doped PyC interphase on the oxidation resistance of 1D-SiC/SiC (CVI) microcomposites with pretreated Nicalon fibers. Their interphases contain up to 30 at.% B. They showed, as expected, that the microtexture of the PyC interphase was significantly improved at low B addition, 8 at.% B, and degraded beyond this value (the interphase becoming amorphous). More importantly, lifetime in tensile static fatigue (beyond proportional limit PL) in air at 7001C was dramatically improved as the B content was raised, the best results being observed for graded composition interphase. Crack deflection and failure occur within the interphase at a location where the interphase microtexture and graphene-layer orientation were optimal (at 8 at.% B). BN Interphases The use of BN interphase in SiC/SiC raises several problems, which still remain imperfectly solved. They include the occurrence of corrosion by precursor and the chemical reactivity of BN with oxygen and moisture when prepared at low temperature. BN Interphases as Deposited by CVI from BF3– NH3: BF3–NH3 precursor has the advantage of yielding crystallized BN deposits at relatively low temperature.6,39 Unfortunately, it involves gaseous species (BF3 and HF), which are corrosive for SiC-based fibers and alter their strength (as received Nicalon and HiNicalon).40 Conversely, this precursor is compatible with carbon substrates and it could be used to deposit BN on fibers with a carbon layer surface (pretreated or stoichiometric fibers).41 However, an extra carbon layer often remains between the SiC fiber and the BN coating, which could be the weakest link in the interfacial zone. One way to solve the corrosion problem and to play with the mechanical fuse location could be to deposit BN in a temperature gradient (TG-CVI). Jacques et al. 42 have fabricated 1D-SiC/BN/SiC minicomposites with a radial crystallinity gradient by simply passing a Hi-Nicalon tow through a three-temperature zones furnace. Under optimized conditions, in terms of fiber progression speed, the FI bonding was strong (crack deflection occurring within BN interphase (Fig. 3)) and both interfacial shear stress and tensile stress were high. At lower fiber speed, crack deflection occurred at fiber surface (as a result of some surface crystallization) whereas for higher fiber speed it was observed at the SiCm/BN interface, these two scenarios corresponding to the ‘‘inside’’ and ‘‘outside’’ debonding reported by Morscher et al.,43 in related experiments. In the case of ‘‘outside’’ debonding, both the interfacial shear stress and tensile failure stress were lower but the lifetime in tensile static fatigue at 7001C in dry or wet air was dramatically improved (crack deflection occurring far from fiber surface).42 BN Interphases as Deposited by CVI from BCl3–NH3– H2: BCl3–NH3–H2 precursor is usually preferred because it is much less corrosive.44–47 In principle, BN could be deposited at temperature as low as 7001C. However, under such mild conditions, it is nanoporous, poorly organized and highly reactive. Hence, the processing temperature should be increased.5,44–47 In the case of complex fiber architectures (nD-preforms), BN can be deposited at the highest temperature compatible with the ICVI process ( 11001C) and further annealed at a temperature corresponding to the upper limit of the thermal stability domain of the fibers. An alternative is to deposit BN on fiber tows, which can be done at higher temperature (1400–16001C), particularly for stoichiometric SiC fibers.48 As an example, BN deposited on a TSA tow at 15801C was reported to be nearly stoichiometric, with an impurity content o5 at.%, highly crystallized and textured.49 Fig. 3. BN interphase deposited from BCl3–NH3–H2 on Hi-Nicalon fiber tow moving in a temperature gradient (T1r11001C; T2 5 11501C; T3 5 12501C) at medium residence time (v 5 2.5 m/h) (adapted from Jacques et al.42). www.ceramics.org/ACT Layered Interphases in SiC/SiC Composites 267