International Journal of Applied Ceramic Technolog-Naslain, Pailler and Lamon Vol. 7, No 3, 2010 interphase matrIx matrix deflected crack fiber 300nm Fig 4. ID-SiC(HN)BN/SiC (CVI) minicomposite with BN interphase deposited from tris(dimethylamino)borane: matrix crack deflected within the BN interphase, as seen by transmission electron microscopy(BFmode) at low(a)and higb()magnifications(adapted from acqu Another efficient way to improve the oxidation resis- reported that such a treatment increased the interfacial of BN is to dope the bn precursor with a silane shear stress, without achieving the high values required (such as The resulting posits for crack deflection within the interphase. Finally, with a Si content ranging from 15 to 40 wt%, display crack deflection according to these different scenarios oxidation rates(at 1200C in oxygen or N 1500.C in can be modeled, as already discussed for the composites air)two to three orders of magnitude lower than those for with PyC interphases(see Fig. 2)33.34 undoped BN. Further, their use in SiC/SiC signifi cantly improves the resistance of the composite to oxy- BN Interphases as Deposited by CvI from On lics: Another way to reduce corrosion during bn dep temperatures.48.5ob However, such Si-doped BN is amor- sition is through the use of halogen-free organometallic precursors. S. Jacques and colleagues have deposited phous and hence at variance with the requirements of our bN interphases from tris(dimethylamino) borane, layered interphase concept. Interestingly, it has been s gested to use dual BN-based interphases containing one BIN(CH3)2]3 in H2-NH3 Aow, on Hi-Nicalon fibers layer of textured pure BN (acting as mechanical fuse)and (NH, being used to avoid the occurrence of free carbon in phase for SiC/Sic in gas turbine application. ar intra one layer of BN(Si) for improving oxidation the coating). The tensile curves of their minicomposite characteristic of SiC/SiC with relatively strong FM bond tance ing, high matrix crack density at failure and high inter- weak, owing to the occurrence of addi g is relatively acial shear stress(here, t=230 MPa). The latter is,to our knowledge, the highest value reported for SiC/BN/ of silica orland carbon at the fber bn coating orland at Sic composites. Finally, crack deflection did occur within the SiC matrix/bn coating interfaces. #-47.51.52 Matrix crack deflection often occurs at these weak interfaces quirement of our layered interphase concept. (mostly at the former)and not within the bn inter- ed on Sic fibers with In situ phase. Carbon is also present as single layer in compos Generated BN Surface: The preceeding sections suggest tes fabricated with pretreated fibers(Nicalon and that SiC fibers with a carbon surface may not be the Nicalon) or stoichiometric fibers. It can be chemi- nost appropriate for BN deposition. An altern cally removed before BN deposition. LeGallet et al have would be to use SiC fibers with a BN surface 54-56Another efficient way to improve the oxidation resis￾tance of BN is to dope the BN precursor with a silane (such as HSiCl3).48,50a–c The resulting BN(Si) deposits with a Si content ranging from 15 to 40 wt%, display oxidation rates (at 12001C in oxygen or 15001C in air) two to three orders of magnitude lower than those for undoped BN.50a Further, their use in SiC/SiC signifi- cantly improves the resistance of the composite to oxy￾gen- and water-containing atmospheres at intermediate temperatures.48,50b However, such Si-doped BN is amor￾phous and hence at variance with the requirements of our layered interphase concept. Interestingly, it has been sug￾gested to use dual BN-based interphases containing one layer of textured pure BN (acting as mechanical fuse) and one layer of BN(Si) for improving oxidation resis￾tance.50c Finally, BN(Si) has been envisaged as inter￾phase for SiC/SiC in gas turbine application.50d Generally speaking, the FM bonding is relatively weak, owing to the occurrence of additional thin layers of silica or/and carbon at the fiber/BN coating or/and at the SiC matrix/BN coating interfaces.44–47,51,52 Matrix crack deflection often occurs at these weak interfaces (mostly at the former) and not within the BN inter￾phase. Carbon is also present as single layer in compos￾ites fabricated with pretreated fibers (Nicalon and Hi-Nicalon) or stoichiometric fibers. It can be chemi￾cally removed before BN deposition. LeGallet et al. have reported that such a treatment increased the interfacial shear stress, without achieving the high values required for crack deflection within the interphase.52 Finally, crack deflection according to these different scenarios can be modeled, as already discussed for the composites with PyC interphases (see Fig. 2).33,34 BN Interphases as Deposited by CVI from Organometal￾lics: Another way to reduce corrosion during BN depo￾sition is through the use of halogen-free organometallic precursors.53 S. Jacques and colleagues have deposited BN interphases from tris(dimethylamino) borane, B[N(CH3)2]3 in H2–NH3 flow, on Hi-Nicalon fibers (NH3 being used to avoid the occurrence of free carbon in the coating). The tensile curves of their minicomposites is characteristic of SiC/SiC with relatively strong FM bond￾ing, high matrix crack density at failure and high inter￾facial shear stress (here, t 5 230 MPa). The latter is, to our knowledge, the highest value reported for SiC/BN/ SiC composites. Finally, crack deflection did occur within the BN interphase (Fig. 4), in accordance with the requirement of our layered interphase concept. BN Interphases Deposited on SiC Fibers with In Situ Generated BN Surface: The preceeding sections suggest that SiC fibers with a carbon surface may not be the most appropriate for BN deposition. An alternative would be to use SiC fibers with a BN surface.54–56 Fig. 4. 1D-SiC (HN)/BN/SiC (CVI) minicomposite with BN interphase deposited from tris(dimethylamino)borane: matrix crack deflected within the BN interphase, as seen by transmission electron microscopy (BF mode) at low (a) and high (b) magnifications (adapted from Jacques et al.53). 268 International Journal of Applied Ceramic Technology—Naslain, Pailler and Lamon Vol. 7, No. 3, 2010