T. Taguchi et al. I Journal of Nuclear Materi The intermediate C layers are used only to separate each Microstructure was observed by scanning electro SiC layers. It was also reported that thin C layer reduces microscopy (SEM) and transmission electron micros- the sensitivity of oxidation [13] and irradiation [14].a copy (TEM) Composite fracture surface was evaluated thin C layer(50 nm)was therefore chosen in this study. by sEM. Electron probe microanalysis (EPMA)was The characterization of SiC/C multi-layer and the effect carried out to evaluate the chemical composition of the of Sic/C multi-layer on the mechanical properties were fracture surface within the Sic/C multi-layer investigated in this study 3. Results and discussion 2. Experimental procedure The porosity of SiC/SiC composite fabricated in this Tyranno SA fiber SiC fabrics(Ube Industries, Ube, tudy was approximately 20%o with fiber volume fraction Japan)were used as reinforcement for the SiC/Sic of approximately 39%. omposite in this study. The precursor for C deposition A typical cross-sectional SEM microphotograph of was propylene( C3H6). Technical grade methyltrichlo- SiC/SiC composite is shown in Fig. 1. The average rosilane (MTS, CH3,, Gelest Inc, Tullytown, PA, thickness of each layer in SiC/C multi-layer interphase USA)was also used for SiC infiltration SiC fabric layers is given in Table 1. Although the deposition time of with fabric layer orientation of (0%/90%)were restrained each C layer was same, the thickness of outer C layer in a graphite fixture. The Sic and c interphase coating was thicker than that of inner C layer. The reasor on the fiber were sequentially deposited at 1100C and that the deposition area decreased at the outer C layer at 5 Pa. The C deposition condition of flow rate is 50 compared to inner C layer since the space among cm/min C3H6 and 1000 cm/min Ar. The SiC deposition vicinal fibers was filled with the inner SiC and C lay- condition of flow rate is 0. 15 g/min MTS carried by 250 ers m/min of H2. Table I shows the condition parameters Fig 2 shows the cross-sectional TEM micrographs of of Sic/c multi-layer interphase deposition. After the Sic/Sic composite. The Sic/c multi-layer consisted of interphase layer was deposited, the preform was infil- six(SiC/C)layers. Electron diffraction patterns for the trated at 1200C under atmospheric pressure with a corresponding Sic layers and Sic fiber are also shown in MTS flow rate of 0.3 g/min carried by 750 cm/min of Fig. 2. The electron diffraction pattern of second Sic H,. A composite with single C layer as an interphase. layer was similar to that of sic fiber with fine grain size was also fabricated (40 nm)[4]. The TEM observation reveals that the Mechanical properties of the composite were evalu- inner SiC layers (first to fourth SiC layers) consisted of ated by 3-point bending and tensile testing. Three-point finer Sic grains compared to the outer Sic layers(fifth bending was carried out at ambient temperature with and sixth SiC layers)and Sic matrix. The grain sizes of cross-head speed of 0. 1 mm/min and a support span fifth and sixth SiC layers were almost the same as that of length of 20 mm. The test specimen was 2x5x25 mm Sic matrix. A very thin C layer formed between SiC Tensile testing was carried out at ambient temperature fiber and first Sic layer. The Sic fibers were coated by with cross-head speed of 0.5 mm/min. Further details of poly vinyl alcohol(PVA)as sizing material [4]. A very tensile test and specimen is described elsewhere [10, 11]. thin C layer formed by carbonizing the PVa since the The each number of specimens measured by bending fibers were heated at 1100C prior to the initial SiC and tensile testing is three interphase deposition Table I Condition parameters of Sic/C multi-layer interphase deposition Material Designed thickness(nm) Measured thickness(nm) Deposition time(min) (SiC/C×6 000000 03050 00000The intermediate C layers are used only to separate each SiC layers. It was also reported that thin C layer reduces the sensitivity of oxidation [13] and irradiation [14]. A thin C layer (
50 nm) was therefore chosen in this study. The characterization of SiC/C multi-layer and the effect of SiC/C multi-layer on the mechanical properties were investigated in this study. 2. Experimental procedure Tyranno SA fiber SiC fabrics (Ube Industries, Ube, Japan) were used as reinforcement for the SiC/SiC composite in this study. The precursor for C deposition was propylene (C3H6). Technical grade methyltrichlo￾rosilane (MTS, CH3SiCl3, Gelest Inc., Tullytown, PA, USA) was also used for SiC infiltration. SiC fabric layers with fabric layer orientation of (0/90) were restrained in a graphite fixture. The SiC and C interphase coating on the fiber were sequentially deposited at 1100 C and at 5 Pa. The C deposition condition of flow rate is 50 cm3/min C3H6 and 1000 cm3/min Ar. The SiC deposition condition of flow rate is 0.15 g/min MTS carried by 250 cm3/min of H2. Table 1 shows the condition parameters of SiC/C multi-layer interphase deposition. After the interphase layer was deposited, the preform was infil￾trated at 1200 C under atmospheric pressure with a MTS flow rate of 0.3 g/min carried by 750 cm3/min of H2. A composite with single C layer as an interphase was also fabricated. Mechanical properties of the composite were evalu￾ated by 3-point bendingand tensile testing. Three-point bendingwas carried out at ambient temperature with cross-head speed of 0.1 mm/min and a support span length of 20 mm. The test specimen was 2 · 5 · 25 mm3. Tensile testingwas carried out at ambient temperature with cross-head speed of 0.5 mm/min. Further details of tensile test and specimen is described elsewhere [10,11]. The each number of specimens measured by bending and tensile testingis three. Microstructure was observed by scanningelectron microscopy (SEM) and transmission electron micros￾copy (TEM). Composite fracture surface was evaluated by SEM. Electron probe microanalysis (EPMA) was carried out to evaluate the chemical composition of the fracture surface within the SiC/C multi-layer. 3. Results and discussion The porosity of SiC/SiC composite fabricated in this study was approximately 20% with fiber volume fraction of approximately 39%. A typical cross-sectional SEM microphotograph of SiC/SiC composite is shown in Fig. 1. The average thickness of each layer in SiC/C multi-layer interphase is given in Table 1. Although the deposition time of each C layer was same, the thickness of outer C layer was thicker than that of inner C layer. The reason is that the deposition area decreased at the outer C layer compared to inner C layer since the space among vicinal fibers was filled with the inner SiC and C lay￾ers. Fig. 2 shows the cross-sectional TEM micrographs of SiC/SiC composite. The SiC/C multi-layer consisted of six (SiC/C) layers. Electron diffraction patterns for the correspondingSiC layers and SiC fiber are also shown in Fig. 2. The electron diffraction pattern of second SiC layer was similar to that of SiC fiber with fine grain size (
40 nm) [4]. The TEM observation reveals that the inner SiC layers (first to fourth SiC layers) consisted of finer SiC grains compared to the outer SiC layers (fifth and sixth SiC layers) and SiC matrix. The grain sizes of fifth and sixth SiC layers were almost the same as that of SiC matrix. A very thin C layer formed between SiC fiber and first SiC layer. The SiC fibers were coated by poly vinyl alcohol (PVA) as sizingmaterial [4]. A very thin C layer formed by carbonizingthe PVA since the fibers were heated at 1100 C prior to the initial SiC interphase deposition. Table 1 Condition parameters of SiC/C multi-layer interphase deposition Material Designed thickness (nm) Measured thickness (nm) Deposition time (min) (SiC/C)·6 SiC 50 56 4 C 50 37 30 SiC 200 119 15 C 50 35 30 SiC 200 132 15 C 50 40 30 SiC 200 153 15 C 50 41 30 SiC 500 432 38 C 50 62 30 SiC 500 572 38 C 50 83 30 T. Taguchi et al. / Journal of Nuclear Materials 329–333 (2004) 572–576 573