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. 49Jacques 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 micro￾texture 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 tem￾perature.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 Hi￾Nicalon).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 fur￾nace. Under optimized conditions, in terms of fiber pro￾gression 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 Mo￾rscher 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 dra￾matically 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 be￾cause 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 pro￾cessing 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 an￾nealed 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), particu￾larly 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