270 International Journal of Applied Ceramic Technolog-Naslain, Pailler and Lamon Vol. 7, No 3, 2010 studied also with a view to improve the oxidation resis- ible at high temperatures with oxide fibers and matrices tance. The potential advantage of the former lies in the in alumina-based oxide-oxide CMCs. They have been fact that B atoms are now present in the interphase envisaged recently as potential interphase in SiC/SiC which favors crack healing at intermediate temperature mainly on the basis of their oxidation resistance. 0b-d Unfortunately, experiments with pretreated Hi-Nicalon Firstly, the interlayer bonding in such oxides is not as fibers have shown that the fber/BNi bond is relatively weak as it is in PyC or BN. Secondly, the deposition weak, crack deflection occurring at that interface and of monazite by sol-gel is not as straightforward as the not within the ML interphase. Nevertheless, the lifetime CVi deposition of PyC or Bn (particularly in nD-fiber in air at 700C under load of minicomposites fabricated preforms commonly used in real application at plant with Hi-Nicalon and a(Bn4o-SiC25)10 ML interphase level). Further, monazite has been reported to be stable, was significantly improved. with respect to carbon and SiC, only in relatively narrow (PyC-TiC), ML interphases can also improve the temperature/oxygen partial pressure conditions and oxidation resistance of SiC/SiC, although titanium ox- could be reduced during SiC/SiC Processing ides are not commonly regarded as healing oxides. Such performed at low Po, and long duration (CVD) interphases have been deposited on the as-received Monazite may inhibit to some extent the oxidation of sic Hi-Nicalon, according to a combination of conven- fibers but the inhibition process seems to depend on the tional P-CVI(for PyC) and reactive P-CVI(for occurrence of dopants(e.g, Al or Zr). Finally, other TiC). When the amount of TiC is low, each sublay refractory oxides, such as zirconia, have also been consid er consists of a PyC film reinforced with nanometric ered but they do not have layered crystal structures and TiC particles, which results in a strong FM bondi hence do not fall into our layered interphase concept. The lifetime of such minicomposites, under load in air Finally, porous interphases (also referred to as at 700%C, is much higher(> 300 h)than that(20 h)for " pseudo-porous")consist of a mixture, at the nanometer their counterparts with PyC single-layer interphase. One scale, of a refractory material, such as SiC, with(fugitive) of the reasons, which could explain such unexpected carbon. However, such interphases do not display a behavior might be a strong interfacial bonding marked anisotropic texture nor protect the fibers in an environment and may undergo some sintering Miscellaneous Interp base Materials when exposed for a long time at high temperature A few additional materials have been identified as Interphases in SiC/SiC for HT Nuclear Reactors rential interphase materials for SiC/SiC but with lim- ited success up to now. This is the case for the ternary SiC/SiC are potential structural materials for both carbides(MAX phases), such as Ti3 SiC2 or Ti3AlC2 fission and fusion hT nuclear reactors.5-17.73 This new which display layered crystal structures. However, the and extremely demanding application raises specific deposition by CVD/CVI is difficult. Further, they constraints on the fibers, the matrix and the interphases tend to grow with the layers perpendicular to the sub strate surface and their ability to arrest/deflect a matrix Sic/sic environment in ht Nuclear reactors crack in a CMC has not been formally established. Oxides with layered crystal structures have been The environment that would see SiC/SiC considered as potential interphases in CMCs. 0ad This cooled fast reactors is not so different from that they is the case for the easily cleavable phyllosilicates(and presently experience in advanced gas turbines, in terms related phyllosiloxides), such as mica fluorphlogopite of temperature, gas pressure, and lifetime. However KMg3(AISi3)O1oF2 (and related K(Mg2AlSigO12), they are not expected to see permanently oxidizing whose weak interlayer bonding is somewhat similar to atmosphere importantly, they would be co that in PyC or BN. a However, their deposition uously exposed to intense irradiation by fast neutrons, (according to a multistep I process) is difficult, a-particles, and electromagnetic radiations. In HT their thermal stability limited and their compatibility advanced fission reactors, SiC/SiC will be exposed to with SiC and Sic-CVi can be questioned. Other layered moderately energetic neutrons(N 2 Mev) but at tem oxides such as rare-earth ortho-phosphates(e. g, mona- peratures that could be higher than about 1200%C, te LapO4)are much more refractory and are compat- hereas in Tokamak fusion reactor blankets, they willstudied also with a view to improve the oxidation resis￾tance. The potential advantage of the former lies in the fact that B atoms are now present in the interphase, which favors crack healing at intermediate temperatures. Unfortunately, experiments with pretreated Hi-Nicalon fibers have shown that the fiber/BN1 bond is relatively weak, crack deflection occurring at that interface and not within the ML interphase. Nevertheless, the lifetime in air at 7001C under load of minicomposites fabricated with Hi-Nicalon and a (BN40–SiC25)10 ML interphase was significantly improved.9 (PyC–TiC)n ML interphases can also improve the oxidation resistance of SiC/SiC, although titanium ox￾ides are not commonly regarded as healing oxides. Such interphases have been deposited on the as-received Hi-Nicalon, according to a combination of conven￾tional P-CVI (for PyC) and reactive P-CVI (for TiC).67 When the amount of TiC is low, each sublay￾er consists of a PyC film reinforced with nanometric TiC particles, which results in a strong FM bonding. The lifetime of such minicomposites, under load in air at 7001C, is much higher (4300 h) than that (20 h) for their counterparts with PyC single-layer interphase. One of the reasons, which could explain such unexpected behavior might be a strong interfacial bonding. Miscellaneous Interphase Materials A few additional materials have been identified as potential interphase materials for SiC/SiC but with lim￾ited success up to now. This is the case for the ternary carbides (MAX phases), such as Ti3SiC2 or Ti3AlC2, which display layered crystal structures. However, their deposition by CVD/CVI is difficult.68 Further, they tend to grow with the layers perpendicular to the sub￾strate surface and their ability to arrest/deflect a matrix crack in a CMC has not been formally established.69 Oxides with layered crystal structures have been considered as potential interphases in CMCs.70a–d This is the case for the easily cleavable phyllosilicates (and related phyllosiloxides), such as mica fluorphlogopite KMg3(AlSi3)O10F2 (and related K(Mg2Al)Si4O12), whose weak interlayer bonding is somewhat similar to that in PyC or BN.70a However, their deposition (according to a multistep sol–gel process) is difficult, their thermal stability limited and their compatibility with SiC and SiC-CVI can be questioned. Other layered oxides such as rare-earth ortho-phosphates (e.g., mona￾zite LaPO4) are much more refractory and are compat￾ible at high temperatures with oxide fibers and matrices in alumina-based oxide–oxide CMCs. They have been envisaged recently as potential interphase in SiC/SiC mainly on the basis of their oxidation resistance.70b–d Firstly, the interlayer bonding in such oxides is not as weak as it is in PyC or BN.70b Secondly, the deposition of monazite by sol–gel is not as straightforward as the CVI deposition of PyC or BN (particularly in nD-fiber preforms commonly used in real application at plant level). Further, monazite has been reported to be stable, with respect to carbon and SiC, only in relatively narrow temperature/oxygen partial pressure conditions and could be reduced during SiC/SiC processing (usually performed at low PO2 and long duration (CVI)).70c Monazite may inhibit to some extent the oxidation of SiC fibers but the inhibition process seems to depend on the occurrence of dopants (e.g., Al or Zr).70d Finally, other refractory oxides, such as zirconia, have also been consid￾ered but they do not have layered crystal structures and hence do not fall into our layered interphase concept.71 Finally, porous interphases (also referred to as ‘‘pseudo-porous’’) consist of a mixture, at the nanometer scale, of a refractory material, such as SiC, with (fugitive) carbon.72 However, such interphases do not display a marked anisotropic texture nor protect the fibers in an oxidizing environment and may undergo some sintering when exposed for a long time at high temperature. Interphases in SiC/SiC for HT Nuclear Reactors SiC/SiC are potential structural materials for both fission and fusion HT nuclear reactors.15–17,73 This new and extremely demanding application raises specific constraints on the fibers, the matrix and the interphases. SiC/SiC Environment in HT Nuclear Reactors The environment that would see SiC/SiC in e.g. gas cooled fast reactors is not so different from that they presently experience in advanced gas turbines, in terms of temperature, gas pressure, and lifetime. However, they are not expected to see permanently oxidizing atmospheres. More importantly, they would be contin￾uously exposed to intense irradiation by fast neutrons, a-particles, and electromagnetic radiations. In HT advanced fission reactors, SiC/SiC will be exposed to moderately energetic neutrons ( 2 MeV) but at tem￾peratures that could be higher than about 12001C, whereas in Tokamak fusion reactor blankets, they will 270 International Journal of Applied Ceramic Technology—Naslain, Pailler and Lamon Vol. 7, No. 3, 2010