International Journal of Applied Ceramic Technolog-Naslain, Pailler and Lamon Vol. 7, No 3, 2010 mg TSA (where the fm bonding seems to be higher and not to markedly depend on Py C thickness) Unirrad Replacing the PyC single-layer interphase by a (PyC-SiC)n ML interphase (with e(PyC)=20 sic)=100 nm and n= 5) yields relatively brittle composites, with extremely limited nonlinear domain Dsc件 HNS)/Pyc/sic and very low strain at failure( a.1%). Their tensile curves after neutron irradiation(N 8 dpa; 800C)ar similar to those of their unirradiated counterparts. Fi Strain, E(%) nally, composites with"pseudo-porous"SiC interphase- es have also been irradiated and mechanically tested. 6. 83 Fig. 6. Tensile curves of Sic/pyC/SiC composites with Hi-Nicalon type S SiC fibers and thick py C interphase, before and after neutron irradiation, (adapted from Ozawa et al) Concluding Remarks 1. The interphases in SiC/SiC are ideally materials with a layered structure, in which the layers are parallel to Figure 6 shows tensile curve for SiC(HNS)/PyC/ the fiber surface, weakly bonded to one another bu SiC(CVI)composites recorded at ambient after neu- strongly adherent to the fiber. Matrix crack deflection tron irradiation at 1000@C 9-21,82For such a composite occurs within the interphase in a diffused manner and with a thick PyC single-layer interphase, the featu res of over short distances the tensile curve after irradiation show a weakening of 2. Anisotropic PyC could be regarded as the inter- the FM bonding(with a plateau-like shape, broad hys- phase of choice. It is deposited by CVI with graphene teresis loops, and some residual strain after unloading) layers parallel to fber surface. Achieving a strong bond Further, the strain to failure is high(and close to that of ing between the interphase and the fiber is not straight ry tow under tension) as opposed to that of the unir- forward, and it may require a fiber pretreatment. The radiated material(which seems to exhibit a premature optimal Py C thickness depends on both residual thermal failure). Irradiation at higher dose(up to a 8 dpa at stresses and fiber roughness. SiC/SiC with optimized 800C)of composites with still thicker PyC interphase PyC interphase displays high load transfer and good (a 700 nm)did not change markedly the tensile behav- mechanical properties under static or cyclic loadings ior after irradiation. Because the thickness of the Pyc in a broad temperature range. Unfortunately, PyC is interphase is here extremely large(vs 100-200 nm in oxidation prone even at low temperatures most SiC/SiC), the change in tensile behavior could be 3. BN-layered interphase is more resistant to oxida- tentatively attributed to an evolution of the PyC nano- tion. However, its deposition by CVI with optimal texture during irradiation. structure and bonding to the fiber is not straight 2D-SiC(HNS)/PyC/SiC(CVn)cor th forward. BFs-NH3 ereas much thinner PyC interphase(50-60 nm), display after BCl3-NH3-H2 requires high temperature to achieve neutron irradiation (750C with dose up to 12 dpa),a high crystallinity and corrosion resistance. Corrosion very different behavior. Their tensile curves(not shown problem could be solved through the use of TG-CVI or in Fig. 6), before and after irradiation are very similar, halogen-free organometallic precursor. Finally, achieving with convex curvature up to failure, relatively narrow strong fiber/BN bonding remains a key issue. One way to hysteresis loops and limited residual strain after unload- solve this difficulty might be to use pretreated ng. These features suggest a relatively strong FM bonding (stoichiometric) fibers with a BN surface and little evolution of the interfacial zone during irradi- 4. Another way to improve oxidation resistance of ation. However, in both cases, the strain to failure is low SiC/SiC is to reduce the thickness of PyC interphase and comparatively to that of the fiber. Hence, reducing the to play with self-healing phenomena. SiC/SiC with thickness of the PyC interphase in composites fabricated (X-Y)n interphases (with X= PyC or BN and Y= SiC with Hi-Nicalon S fibers seems to enhance the stability of or TiC) displays improved lifetime under load in oxidizing the FM-interphase bonding. It is noteworthy that such a atmospheres. The concept of Ml material associated conclusion cannot be drawn for those produced from self-healing phenomenaFigure 6 shows tensile curve for SiC (HNS)/PyC/ SiC (CVI) composites recorded at ambient after neu￾tron irradiation at 10001C.19–21,82 For such a composite with a thick PyC single-layer interphase, the features of the tensile curve after irradiation show a weakening of the FM bonding (with a plateau-like shape, broad hys￾teresis loops, and some residual strain after unloading). Further, the strain to failure is high (and close to that of dry tow under tension) as opposed to that of the unir￾radiated material (which seems to exhibit a premature failure).82 Irradiation at higher dose (up to 8 dpa at 8001C) of composites with still thicker PyC interphase ( 700 nm) did not change markedly the tensile behav￾ior after irradiation.20 Because the thickness of the PyC interphase is here extremely large (vs 100–200 nm in most SiC/SiC), the change in tensile behavior could be tentatively attributed to an evolution of the PyC nano￾texture during irradiation. 2D-SiC (HNS)/PyC/SiC (CVI) composites, with much thinner PyC interphase (50–60 nm), display after neutron irradiation (7501C with dose up to 12 dpa), a very different behavior.19 Their tensile curves (not shown in Fig. 6), before and after irradiation are very similar, with convex curvature up to failure, relatively narrow hysteresis loops and limited residual strain after unload￾ing. These features suggest a relatively strong FM bonding and little evolution of the interfacial zone during irradi￾ation. However, in both cases, the strain to failure is low comparatively to that of the fiber. Hence, reducing the thickness of the PyC interphase in composites fabricated with Hi-Nicalon S fibers seems to enhance the stability of the FM-interphase bonding. It is noteworthy that such a conclusion cannot be drawn for those produced from TSA (where the FM bonding seems to be higher and not to markedly depend on PyC thickness).17 Replacing the PyC single-layer interphase by a (PyC–SiC)n ML interphase (with e(PyC) 5 20 nm, e(SiC) 5 100 nm and n 5 5) yields relatively brittle composites, with extremely limited nonlinear domain and very low strain at failure ( 0.1%). Their tensile curves after neutron irradiation ( 8 dpa; 8001C) are similar to those of their unirradiated counterparts.20 Fi￾nally, composites with ‘‘pseudo-porous’’ SiC interphas￾es have also been irradiated and mechanically tested.16,83 Concluding Remarks 1. The interphases in SiC/SiC are ideally materials with a layered structure, in which the layers are parallel to the fiber surface, weakly bonded to one another but strongly adherent to the fiber. Matrix crack deflection occurs within the interphase in a diffused manner and over short distances. 2. Anisotropic PyC could be regarded as the inter￾phase of choice. It is deposited by CVI with graphene layers parallel to fiber surface. Achieving a strong bond￾ing between the interphase and the fiber is not straight￾forward, and it may require a fiber pretreatment. The optimal PyC thickness depends on both residual thermal stresses and fiber roughness. SiC/SiC with optimized PyC interphase displays high load transfer and good mechanical properties under static or cyclic loadings in a broad temperature range. Unfortunately, PyC is oxidation prone even at low temperatures. 3. BN-layered interphase is more resistant to oxida￾tion. However, its deposition by CVI with optimal structure and bonding to the fiber is not straight￾forward. BF3–NH3 is a corrosive precursor whereas BCl3–NH3–H2 requires high temperature to achieve high crystallinity and corrosion resistance. Corrosion problem could be solved through the use of TG-CVI or halogen-free organometallic precursor. Finally, achieving a strong fiber/BN bonding remains a key issue. One way to solve this difficulty might be to use pretreated (stoichiometric) fibers with a BN surface. 4. Another way to improve oxidation resistance of SiC/SiC is to reduce the thickness of PyC interphase and to play with self-healing phenomena. SiC/SiC with (X–Y)n interphases (with X 5 PyC or BN and Y 5 SiC or TiC) displays improved lifetime under load in oxidizing atmospheres. The concept of ML material associated with self-healing phenomena is still more efficient when Fig. 6. Tensile curves of SiC/PyC/SiC composites with Hi-Nicalon type S SiC fibers and thick PyC interphase, before and after neutron irradiation, (adapted from Ozawa et al.82). 272 International Journal of Applied Ceramic Technology—Naslain, Pailler and Lamon Vol. 7, No. 3, 2010