w. Yang et al Materials Science and Engineering 4345(2003)28-35 Table I Several selected properties of the Tyranno-SA fiber C/Si Diameter(um) Structure Tensile strength (MPa) Tensile modulus( Gpa) Elongation(%) Thermal conductivity (W m-IK-) 1.087.5 Crystal 2510 646(RT) The fiber and properties were provided by the Ube Industry Ltd. Japan. with much improved thermal conductivity and thermal be found elsewhere [18]. The densification process stability. Good radiation resistance is also expected due generally required x 15 h to its stoichiometric chemistry and high-crystalline structure [16]. Furthermore, the fabrication cost is 2.2. Mechanical tests only about one third of that for the Hi-Nicalon Type- S fiber [1]. The excellent performance and low fabrica Three-point bending tests(with a support span of 18 tion cost make the Tyranno-SA fiber very attractive. nm) were performed at room temperature. Bending particularly for nuclear fusion application. However, 1 specimens were cut parallel to one of the fiber bundle remains unclear about the mechanical performances of directions of the fabric cloth using a diamond cutter and SiC/SiC composites reinforced with the Tyranno-sa both the tensile and compression surfaces were carefully fiber. The Tyranno-sa fiber possesses quite different ground using diamond slurry to eliminate the effects of surface characteristics such as near stoichiometric sic surface CVD-SiC layers which were formed at the end of surface chemistry with rough surface compared with the the CVI process [18]. Three tests were conducted for Nicalon-CG and Hi-Nicalon fibers [17]. Pure Sic each composite. The dimension of the specimen is 30X surface chemistry and rough fiber surface may have 4.0Wx1.5 mm. The crosshead speed was 0.0083 significant affections on the interfacial bonding and mm s-I. The load-displacement data was recorded fiber sliding, and therefore, on the mechanical properties of the composites. This is also an important issue to be 2.3 Microstructural characterization understood In this study, several CVI-SiC/SiC composites rein forced with the Tyranno-SA fiber were fabricated with The morphologies, thickness, and uniformities of the interlayers were examined with scanning electron mict varIous PyC and SiC/PyC interlayers. The flexural scope(SEM) using JOEL JIM-6700F. The interlayer properties and fracture behaviors were studied using thickness was measured with an estimated resolution of three-point bending tests. The main objectives are to get 10 nm understanding of the mechanical properties of The fracture surfaces were observed with seM various composites. The simple bending tests were used interfacial debonding and fiber pullout to study the effects of the rough fiber surface character istics and the various interlayers 3.1. Compo 2. 1. Composite processing Totally six composites were fabricated. The interlayer structures and thickness and composite densities/poros- Fibrous preforms were fabricated by stacking ities are listed in table 2. the fiber volume fractions are layers of 2D plain-woven Tyranno-SA fiber cloths in 42-44%. No intentional interlayer was applied in 0/90. The preforms were compressed to keep a fiber composite T-NL. Single PyC interlayer was deposited volume fraction of x43% using a set of graphite in composites T-C50, T-C100, and T-C200. In order fixtures. The normal size of the preforms was 40 mm investigate the effect of Sic layer in Tyranno-SA/SIC in diameter and 2.0 mm in thickness. The preforms were composites, two composites, T-SiC/C80 and T-SiC/ pre-coated with single PyC layers of different thickness C150, were deposited with SiC/Pyc bi-interlayers. The or SiC/Pyc bi-layers using an isothermal chemical vapor interlayer thickness and space homogeneity are as in infiltration(CVI) process through the thermal decom- Table 2. Fig. I shows the SEM images of the interlayers position of methane and CH3Sicl3(MTS), respectively. of composites T-C200 and T-SiC/C150. A uniform 200 MTS was carried by hydrogen. nIm- In compo The pre-coated preforms were finally densified with C200 while in composite T-SiC/C150, a SiC layer of 150 Sic matrix by an isothermal- forced flow CVi proces nm thickness was deposited on the fibers prior to the 1273 K and 14.7 kPa. Detailed fabrication process can deposition of 150 nm-thick PyC layer. SEM interlayerwith much improved thermal conductivity and thermal stability. Good radiation resistance is also expected due to its stoichiometric chemistry and high-crystalline structure [16]. Furthermore, the fabrication cost is only about one third of that for the Hi-Nicalon Type￾S fiber [1]. The excellent performance and low fabrica￾tion cost make the Tyranno-SA fiber very attractive, particularly for nuclear fusion application. However, it remains unclear about the mechanical performances of SiC/SiC composites reinforced with the Tyranno-SA fiber. The Tyranno-SA fiber possesses quite different surface characteristics such as near stoichiometric SiC surface chemistry with rough surface compared with the Nicalon-CG and Hi-NicalonTM fibers [17]. Pure SiC surface chemistry and rough fiber surface may have significant affections on the interfacial bonding and fiber sliding, and therefore, on the mechanical properties of the composites. This is also an important issue to be understood. In this study, several CVI-SiC/SiC composites rein￾forced with the Tyranno-SA fiber were fabricated with various PyC and SiC/PyC interlayers. The flexural properties and fracture behaviors were studied using three-point bending tests. The main objectives are to get an understanding of the mechanical properties of the various composites. The simple bending tests were used to study the effects of the rough fiber surface character￾istics and the various interlayers. 2. Experimental 2.1. Composite processing Fibrous preforms were fabricated by stacking 11 layers of 2D plain-woven Tyranno-SA fiber cloths in 0/908. The preforms were compressed to keep a fiber volume fraction of
/43% using a set of graphite fixtures. The normal size of the preforms was 40 mm in diameter and 2.0 mm in thickness. The preforms were pre-coated with single PyC layers of different thickness or SiC/PyC bi-layers using an isothermal chemical vapor infiltration (CVI) process through the thermal decom￾position of methane and CH3SiCl3 (MTS), respectively. MTS was carried by hydrogen. The pre-coated preforms were finally densified with SiC matrix by an isothermal-forced flow CVI process at 1273 K and 14.7 kPa. Detailed fabrication process can be found elsewhere [18]. The densification process generally required
/15 h. 2.2. Mechanical tests Three-point bending tests (with a support span of 18 mm) were performed at room temperature. Bending specimens were cut parallel to one of the fiber bundle directions of the fabric cloth using a diamond cutter and both the tensile and compression surfaces were carefully ground using diamond slurry to eliminate the effects of surface CVD-SiC layers which were formed at the end of the CVI process [18]. Three tests were conducted for each composite. The dimension of the specimen is 30L/ 4.0W/
/1.5T mm3 . The crosshead speed was 0.0083 mm s1 . The load/displacement data was recorded. 2.3. Microstructural characterization The morphologies, thickness, and uniformities of the interlayers were examined with scanning electron micro￾scope (SEM) using JOEL JIM-6700F. The interlayer thickness was measured with an estimated resolution of
/10 nm. The fracture surfaces were observed with SEM interfacial debonding and fiber pullouts. 3. Results 3.1. Composites and interlayer structures Totally six composites were fabricated. The interlayer structures and thickness and composite densities/poros￾ities are listed in Table 2. The fiber volume fractions are 42/44%. No intentional interlayer was applied in composite T-NL. Single PyC interlayer was deposited in composites T-C50, T-C100, and T-C200. In order to investigate the effect of SiC layer in Tyranno-SA/SiC composites, two composites, T-SiC/C80 and T-SiC/ C150, were deposited with SiC/PyC bi-interlayers. The interlayer thickness and space homogeneity are as in Table 2. Fig. 1 shows the SEM images of the interlayers of composites T-C200 and T-SiC/C150. A uniform 200 nm-thick PyC interlayer was deposited in composite T￾C200 while in composite T-SiC/C150, a SiC layer of 150 nm thickness was deposited on the fibers prior to the deposition of 150 nm-thick PyC layer. SEM interlayer Table 1 Several selected properties of the Tyranno-SA fiber C/Si Diameter (mm) Structure Tensile strength (MPa) Tensile modulus (Gpa) Elongation (%) Thermal conductivity (W m1 K1 ) 1.08 7.5 Crystal 2510 409 0.7 64.6 (RT) The fiber and properties were provided by the Ube Industry Ltd. Japan. W. Yang et al. / Materials Science and Engineering A345 (2003) 28 /35 29