T Nozawa et aL /Joumal of Nuclear Materials 384(2009)195-211 Hinoki et al. [11] closely investigated the irradiation effect on While there is a growing body of work evaluating the F/M inter- le interfacial shear properties by the push-in technique and com- phase fundamental knowledge about the irradiation effect on the pared the influence on varied interphases: a PyC monolayer, a PyC F/M interphase is still insufficient. Specifically, a fundamental Sic multilayer, and a pseudo 'porous'Sic interphase. By neutron question about the irradiation tolerance of the multilayer inter irradiation up to 0.5 dpa at 300-500C, slight deformation for phase is unanswered. The goal of the following study is to provide any interphase studies was observed. a decrease of interfacial data and analysis of the effect of neutron irradiation on the F/M shear properties was also identified by the double notch shear test interphase to help bridge this knowledge gap method for irradiation up to 1 dpa at 800-1000C for the same se ies of composites [12]. In both Sic/Sic composites with do-porous Sic interphase severely deteriorated their interfacial properties. In recent work by the authors [13], interfacial shear 21. materials properties for Hi-Nicalon Type-S/CVI-SiC composites with either yer or a PyC/SiC multilayer interphase were investi- Materials evaluated were unidirectional SiC/Sic composites gated for the limited irradiation condition (o-7.7 dpa, (Hyper-Therm High-Temperature Composites Inc, Huntington Tirr=800C). The results indicate(1)a very minor irradiation effect Beach, CA)with varied F/M interphases(Table 1). Reinforcing Sic on interfacial shear properties for the monolayer composites and fibers were Hi-Nicalon"M Type-S(3. 1 g/cm Si/C=1.05).A high- (2)a slight irradiation-induced decrease of shear stresses for the purity and high-crystallinity Sic matrix was chemical-vapor-infiI multilayer composites. This difference is somewhat surprising trated. A PyC monolayer or a Py/Sic multilayer interphase was since it has been believed that the thinly-layered carbon interpha- chemically deposited on the fiber surface in advance of matrix den ses would be more radiation-stable sification. Thickness of the Pyc monolayer was 520-720 nm. In contrast, the multilayer interphase d of a sequel Table 1 of w20 nm-thick PyC and 100 nm-thick Sic sub-layers( Fig. 1). Key characteristics of unidirectional Hi-Nicalon Type-S(CVI-SiC composites The fiber volume fraction and porosity were 30-40% and 15-20% Interphase Fiber volume fraction Porosity Density (glcm respectively. The high-porosity of these CVI-SiC/Sic composites ~0.29 ~257 was due primarily to the presence of columnar pores along the Multilayer ~038 ~0.14 2.66 fiber bundles. Key characteristics of these composites have been reported elsewhere [12-14 a b 》 5 1um d 4 um 500nm Fig 1. Typical microstructural images of (a).(b)Pyc monolayer and(c). (d) Pyc/Sic multilayer interphases of as-received SiC/SiC composites[13]Hinoki et al. [11] closely investigated the irradiation effect on the interfacial shear properties by the push-in technique and com￾pared the influence on varied interphases: a PyC monolayer, a PyC/ SiC multilayer, and a pseudo ‘porous’ SiC interphase. By neutron irradiation up to 0.5 dpa at 300–500 C, slight deformation for any interphase studies was observed. A decrease of interfacial shear properties was also identified by the double notch shear test method for irradiation up to 1 dpa at 800–1000 C for the same ser￾ies of composites [12]. In both cases, SiC/SiC composites with pseu￾do-porous SiC interphase severely deteriorated their interfacial properties. In recent work by the authors [13], interfacial shear properties for Hi-NicalonTM Type-S/CVI-SiC composites with either a PyC monolayer or a PyC/SiC multilayer interphase were investi￾gated for the limited irradiation condition (/ = 7.7 dpa, Tirr = 800 C). The results indicate (1) a very minor irradiation effect on interfacial shear properties for the monolayer composites and (2) a slight irradiation-induced decrease of shear stresses for the multilayer composites. This difference is somewhat surprising since it has been believed that the thinly-layered carbon interpha￾ses would be more radiation-stable. While there is a growing body of work evaluating the F/M inter￾phase, fundamental knowledge about the irradiation effect on the F/M interphase is still insufficient. Specifically, a fundamental question about the irradiation tolerance of the multilayer inter￾phase is unanswered. The goal of the following study is to provide data and analysis of the effect of neutron irradiation on the F/M interphase to help bridge this knowledge gap. 2. Experimental 2.1. Materials Materials evaluated were unidirectional SiC/SiC composites (Hyper-Therm High-Temperature Composites Inc., Huntington Beach, CA) with varied F/M interphases (Table 1). Reinforcing SiC fibers were Hi-NicalonTM Type-S (3.1 g/cm3 , Si/C = 1.05). A high￾purity and high-crystallinity SiC matrix was chemical-vapor-infil￾trated. A PyC monolayer or a PyC/SiC multilayer interphase was chemically deposited on the fiber surface in advance of matrix den￾sification. Thickness of the PyC monolayer was 520–720 nm. In contrast, the multilayer interphase was composed of a sequence of 20 nm-thick PyC and 100 nm-thick SiC sub-layers (Fig. 1). The fiber volume fraction and porosity were 30–40% and 15–20%, respectively. The high-porosity of these CVI-SiC/SiC composites was due primarily to the presence of columnar pores along the fiber bundles. Key characteristics of these composites have been reported elsewhere [12–14]. Table 1 Key characteristics of unidirectional Hi-NicalonTM Type-S/CVI-SiC composites. Interphase Fiber volume fraction Porosity Density (g/cm3 ) PyC 0.29 0.15 2.57 Multilayer 0.38 0.14 2.66 Fig. 1. Typical microstructural images of (a), (b) PyC monolayer and (c), (d) PyC/SiC multilayer interphases of as-received SiC/SiC composites [13]. 196 T. Nozawa et al. / Journal of Nuclear Materials 384 (2009) 195–211