wwceramics. org/ACT Layered Interphases in SiC/SiC Composites be irradiated by much more energetic neutrons Pyc-based interphases have been up to now the in- (14.1 Mev) formed during the deuterium/titrium fu terphases of choice. However, they raise a problem of sion reaction but likely at somewhat lower temperatures. anisotropic dimensional change under neutron irradia- Such severe irradiation conditions are known to tion strongly depending on their degree of crystalliza- their constituents, on the basis of literature da 2a change(in a more or less dramatic manner)the structure tion. Graphite and highly oriented pyrolytic graphite of materials and hence their properties, as briefly undergo a moderate shrinkage along the a-axis, that is discussed in the following sections for SiC/Sic arallel to grap layer, and a significant swelling along the perpendicular c-axis. This anisotropy is strong at low temperature/high irradiation dose but it decreases Irradiation of Silicon Carbide ature is raised, The behavior of turbostratic PyC is more complex. Although similar to Monolithic SiC undergoes a moderate swelling that of graphite parallel to graphene layers, it first when irradiated by neutrons, as the result of am- shrinks in a perpendicular direction at low irradiation orphization or point defect formation at low temper dose and then swells. . Because in a PyC-interphase a minimum(0.2-0.4 vol% for a dose of modify residual thermal stresses(particularly in radial 1-8 dpa)at 1100-1200C, then increases to reach direction)and alter the FM bonding. Hence, the inter 1. vol% at 1600 C 7.73a, b SiC matrix when deposited phase may be again the vulnerable constituent of SiC/ by CVI is pure, well crystallized and assumed to behave Sic when exposed in a prolonged manner to neutron as monolithic SiC irradiation Potential solutions to overcome this diffi- ect neutro fibers strongly depends on their composition and struc- for SiC/SiC Exposed to Oxidizing Atmosphere. 16 17.80 ture. On the one hand, stoichiometric fibers, which are Finally, pseudo-porous SiC interphases might also be well crystallized and with a small impurity content, also an alternative as previously mentioned. o/o However, behave like monolithic SiC. On the other hand, Si-C- their dimensional stability under neutron irradiation is O(Nicalon type)an not well known prepared at lower temperature and poorly crystallized undergo a permanent shrinkage. 74 As a result of this volume change mismatch upon Irradiated sicisic neutron irradiation between SiC(CVD) matrix and 1st/ 2nd SiC fiber generations, debonding at FM interface A compilation of strength data(mostly from Alex- usually occurs with mechanical properties degradation. ural tests)reported by different authors, for a variety of This key feature explains why stoichiometric SiC fibers are SiC/PyC/SiC composites that have been neutron irra- ic to be used in nudlear reactors diated(in a broad temperature range: 200-1000"C) suggests that: (i)there is no loss in strength up to an Q Neutron Irradiation of interphase Materials: Boron irradiation dose of 10 dpa for materials fabricated with itride is poorly compatible with nuclear reactor envi- stoichiometric SiC fibers, but conversely, (ii)the ronment.///Firstly, 5 B isotope(present at a level of strength drops by a 60% almost linearly when irradi- a 20 at.% in natural boron) has an extremely high ation dose increases up to 10 dpa for those with Si-C-O neutron capture cross section. Hence, the use of Bn (Nicalon)or SiC+C(Hi-Nicalon)fibers. ( 6.73. This interphase would suppose that it is deposited from 5B- result is consistent with the dimensional change (per riched gaseous precursor. Secondly, BN when neu- manent shrinkage)reported for lese two latter fibers tron irradiated, undergoes nuclear reactions producing which lowers the FM bonding and load transfer. Hence, gaseous species(helium)and radioactive long-life the analysis of the effects of neutron irradiation and in cies(such as 6C). Further, because in a nuclear reactor terphase design on the mechanical properties of Sic/SiC the atmosphere is, in principle, not oxidizing bn should be pursued for composites with stoichiometric interphases are, for all these reasons, not presently fibers, using exclusively tensile tests and analysis of mi- d in this field of application crostructure-strength relations with appropriate modelsbe irradiated by much more energetic neutrons (14.1 MeV) formed during the deuterium/titrium fu￾sion reaction but likely at somewhat lower temperatures. Such severe irradiation conditions are known to change (in a more or less dramatic manner) the structure of materials and hence their properties, as briefly discussed in the following sections for SiC/SiC and their constituents, on the basis of literature data. Irradiation of Silicon Carbide Monolithic SiC undergoes a moderate swelling when irradiated by neutrons, as the result of am￾orphization or point defect formation at low tempera￾ture and cavity and dislocation loops formation at high temperature. It first decreases as temperature is raised, passes through a minimum (0.2–0.4 vol% for a dose of 1–8 dpa) at 1100–12001C, then increases to reach 1.5 vol% at 16001C.17,73a,b SiC matrix when deposited by CVI is pure, well crystallized and assumed to behave as monolithic SiC. The effect of neutron irradiation on SiC-based fibers strongly depends on their composition and struc￾ture. On the one hand, stoichiometric fibers, which are well crystallized and with a small impurity content, also behave like monolithic SiC. On the other hand, Si–C– O (Nicalon type) and SiC1C (Hi-Nicalon) fibers, prepared at lower temperature and poorly crystallized, undergo a permanent shrinkage.74 As a result of this volume change mismatch upon neutron irradiation between SiC (CVI) matrix and 1st/ 2nd SiC fiber generations, debonding at FM interface usually occurs with mechanical properties degradation.73,75 This key feature explains why stoichiometric SiC fibers are preferred for SiC/SiC to be used in nuclear reactors. Neutron Irradiation of Interphase Materials: Boron nitride is poorly compatible with nuclear reactor envi￾ronment.76,77 Firstly, 10 5 B isotope (present at a level of 20 at.% in natural boron) has an extremely high neutron capture cross section. Hence, the use of BN interphase would suppose that it is deposited from 11 5 B￾enriched gaseous precursor. Secondly, BN when neu￾tron irradiated, undergoes nuclear reactions producing gaseous species (helium) and radioactive long-life spe￾cies (such as 14 6 C). Further, because in a nuclear reactor the atmosphere is, in principle, not oxidizing, BN interphases are, for all these reasons, not presently used in this field of application. PyC-based interphases have been up to now the in￾terphases of choice. However, they raise a problem of anisotropic dimensional change under neutron irradia￾tion strongly depending on their degree of crystalliza￾tion. Graphite and highly oriented pyrolytic graphite undergo a moderate shrinkage along the a-axis, that is, parallel to graphene layer, and a significant swelling along the perpendicular c-axis. This anisotropy is strong at low temperature/high irradiation dose but it decreases as irradiation temperature is raised.78 The behavior of turbostratic PyC is more complex. Although similar to that of graphite parallel to graphene layers, it first shrinks in a perpendicular direction at low irradiation dose and then swells.21,79 Because in a PyC-interphase graphene layers are preferably oriented parallel to the fiber surface, this dimensional change anisotropy may modify residual thermal stresses (particularly in radial direction) and alter the FM bonding. Hence, the inter￾phase may be again the vulnerable constituent of SiC/ SiC when exposed in a prolonged manner to neutron irradiation. Potential solutions to overcome this diffi- culty are those already discussed in ‘Layered Interphases for SiC/SiC Exposed to Oxidizing Atmosphere.’16,17,80 Finally, pseudo-porous SiC interphases might also be an alternative as previously mentioned.16,72,80 However, their dimensional stability under neutron irradiation is not well known. Irradiated SiC/SiC A compilation of strength data (mostly from flex￾ural tests) reported by different authors, for a variety of SiC/PyC/SiC composites that have been neutron irra￾diated (in a broad temperature range: 200–10001C) suggests that: (i) there is no loss in strength up to an irradiation dose of 10 dpa for materials fabricated with stoichiometric SiC fibers, but conversely, (ii) the strength drops by 60% almost linearly when irradi￾ation dose increases up to 10 dpa for those with Si–C–O (Nicalon) or SiC1C (Hi-Nicalon) fibers.16,73,81 This result is consistent with the dimensional change (per￾manent shrinkage) reported for these two latter fibers, which lowers the FM bonding and load transfer. Hence, the analysis of the effects of neutron irradiation and in￾terphase design on the mechanical properties of SiC/SiC should be pursued for composites with stoichiometric fibers, using exclusively tensile tests and analysis of mi￾crostructure–strength relations with appropriate models. www.ceramics.org/ACT Layered Interphases in SiC/SiC Composites 271