H Mei/Composites Science and Technology 68(2008)3285-3292 2. Experimental procedures tooling and further coated with Sic by I-CVi under the same condi tions(final coating thickness 50 um). The specimens had a gage- 2.1. Material preparation section volume of approximately 25 8x 3 mm. To avoid surface damage of the specimen during the tensile tests, the specimen The C/Sic composites used in this investigation were processed edges were protected by bonding two pairs of Aluminum tab by isothermal chemical vapor infiltration( CVl)of SiC into woven (see Fig. 3) 0°90° fiber performs at~1000° The classical CVI SiC processing methodology has been described elsewhere [12]. The preforms 2.2. Mechanical tests were made from 1KT-300 carbon fibers and fiber volume contents of the as-processed composites was about 40 voL %. Fiber architec- Both monotonic tensile and periodic unloading-reloading tests tures of the as-received composite panels are shown in Fig. la and were conducted at room temperature on a servohydraulic load- (microstructures in this study were observed with a scanning frame( Model 8801, Instron Ltd, High Wycombe, England) with electron microscope, SEM, Hitachi S-2700, Tokyo, Japan). The mag- a loading rate of 0.06 mm/min. The monotonic tensile tests were ified surface observation of the crossover fiber bundles indicate eal-time monitored by the acoustic emission technique(Model the deposition morphology of the Cvi-Sic matrix. As is typical of MICRo-80D, Physical Acoustic Corp, NJ, USA). The cyclic unload lultiple-ply composites, not all plies(X-Y planes)were perfectly ing-reloading tests were preformed wit an Incremen igned in the Z-direction of the panels( Fig. 1a). SEM views of loading of 20 MPa per cycle up to final rupture. To the polished composites exhibit porosity present in the as-pro- the effect of Trs on the macroscopic mechanical respons cessed CVI C/SiC, including the inter-bundle pores in Fig. 2a and ferent CMC composites, a 2D Hi-Nicalon Sic composite with the the inter-filament pores in Fig. 2b. The TEM observation of the same 0 90 Sic fiber(Nippon Carbon Co., Tokyo, Japan)architec- upper-left picture in Fig. 2b shows that in the as-processed com- tures was used to conduct the same unloading-reloa posite, the carbon fibers were uniformly coated with the PyC inter references. The used 2D SiC/SiC composite proce hase(200 nm)and then the column nanocrystals of Sic matrix rently with the previou opposites vertically grew on the soft PyC interphase I-CVI technology. SEM micrographs in Fig. 4 present the eu. The bulk density and porosity of the infiltrated composite pan- fiber architectures and the constituent microstructures is 2.2 g/cm and 13% in average, respectively As shown in Fig 3. fiber, Pyc interphase(200 nm)and CVI-SiC matrix in the 2D tensile specimens were machined from the panels using diamond Sic/Sic composite Fig 1. SEM micrographs showing the fiber architectures of the 2D C/ sic composite prepared by Cvl (a)3D view and (b) top view. The magnified observation indicating the morphology of the CVI-SiC matrix. b Fig. 2. Micrographs showing porosity present in the virgin 2D C/SiC. (a)inter-bundle pores and(b) inter- filament pores. The TEM observation indicating the constituen and CVI-SiC matrix2. Experimental procedures 2.1. Material preparation The C/SiC composites used in this investigation were processed by isothermal chemical vapor infiltration (CVI) of SiC into woven 0/90 fiber performs at 1000 C. The classical CVI SiC processing methodology has been described elsewhere [12]. The preforms were made from 1 K T-300 carbon fibers and fiber volume contents of the as-processed composites was about 40 vol.%. Fiber architec￾tures of the as-received composite panels are shown in Fig. 1a and b (microstructures in this study were observed with a scanning electron microscope, SEM, Hitachi S-2700, Tokyo, Japan). The mag￾nified surface observation of the crossover fiber bundles indicates the deposition morphology of the CVI-SiC matrix. As is typical of multiple-ply composites, not all plies (X–Y planes) were perfectly aligned in the Z-direction of the panels (Fig. 1a). SEM views of the polished composites exhibit porosity present in the as-pro￾cessed CVI C/SiC, including the inter-bundle pores in Fig. 2a and the inter-filament pores in Fig. 2b. The TEM observation of the upper-left picture in Fig. 2b shows that in the as-processed com￾posite, the carbon fibers were uniformly coated with the PyC inter￾phase (200 nm) and then the column nanocrystals of SiC matrix vertically grew on the soft PyC interphase. The bulk density and porosity of the infiltrated composite pan￾els is 2.2 g/cm3 and 13% in average, respectively. As shown in Fig. 3, tensile specimens were machined from the panels using diamond tooling and further coated with SiC by I-CVI under the same condi￾tions (final coating thickness 50 lm). The specimens had a gage￾section volume of approximately 25  8  3 mm3 . To avoid surface damage of the specimen during the tensile tests, the specimen edges were protected by bonding two pairs of Aluminum tabs (see Fig. 3). 2.2. Mechanical tests Both monotonic tensile and periodic unloading-reloading tests were conducted at room temperature on a servohydraulic load￾frame (Model 8801, Instron Ltd., High Wycombe, England) with a loading rate of 0.06 mm/min. The monotonic tensile tests were real-time monitored by the acoustic emission technique (Model MICRO-80D, Physical Acoustic Corp., NJ, USA). The cyclic unload￾ing-reloading tests were preformed with an incremental step loading of 20 MPa per cycle up to final rupture. To compare the effect of TRS on the macroscopic mechanical response of dif￾ferent CMC composites, a 2D Hi-Nicalon/SiC composite with the same 0/90 SiC fiber (Nippon Carbon Co., Tokyo, Japan) architec￾tures was used to conduct the same unloading–reloading tests as references. The used 2D SiC/SiC composite processed concur￾rently with the previous C/SiC composites under the same I-CVI technology. SEM micrographs in Fig. 4 present the woven fiber architectures and the constituent microstructures of SiC fiber, PyC interphase (200 nm) and CVI-SiC matrix in the 2D SiC/SiC composite. Fig. 1. SEM micrographs showing the fiber architectures of the 2D C/SiC composite prepared by CVI, (a) 3D view and (b) top view. The magnified observation indicating the morphology of the CVI-SiC matrix. Fig. 2. Micrographs showing porosity present in the virgin 2D C/SiC, (a) inter-bundle pores and (b) inter-filament pores. The TEM observation indicating the constituent microstructures of carbon fiber, PyC interphase and CVI-SiC matrix. 3286 H. Mei / Composites Science and Technology 68 (2008) 3285–3292