H. Mei et al. I Scripta Materialia 54(2006)163-168 cycling under a constant load in oxidizing atmosphere have Figs. I and 2. The properties of the composite are listed not been reported in a detailed manner, despite recent ad- in Table I vances in the architecture design and processing of these materials 2. 2. Thermal cycling test When evaluating C/SiC composites for potential use in structural applications where periodically changing tem- Thermal cycling experiments under load constraints peratures occur, the basic characterization of the materials were conducted with a newly-developed integrated system btained from mechanical and environmental testing is including an induction heating furnace(with a controlled very important in understanding the fundamental proper- atmosphere chamber providing various kinds and concen ties of the materials. In this paper, thermal cycling testing trations of oxidizing gas) monitored by a programmable results of 3D braided C/SiC composites under a constant microprocessor and a servo-hydraulic machine(Model In load of 60 MPa in a wet oxygen atmosphere are presented. stron 8801, Instron Ltd, England). Many experimental Corresponding thermal stress or thermal strain during test- conditions/ parameters must be taken into consideration ing will be measured, calculated and analyzed by theoreti- especially regarding(1)the load alignment, (2)the config- cal and experimental methods. Effects of thermal cycling uration of the specimen, (3)the heater, (4)the cooling on mechanical properties of composites will be discussed water, (5) the measurement for temperature, (6)the induc and the morphologies of fracture sections and coating tion coil for cyclic temperature, (7)the grip holder and( 8) surfaces will be observed the pressure and flow of the controlled atmosphere (as shown in Fig. 3). The temperature was measured by an 2. Experimental infrared pyrometer through a small window in the wall of the furnace and the wall was internally cut out to enable 2. 1. Preparation of C/SiC composite the circulating cold water to reach all over the surfaces Thermal cycling was carried out between two selected tem- T-300TM carbon fiber from Toray (Japan) was employe peratures and the period was 120 s: holding for 30 s at The fiber preform was prepared using a 3D braid method. 900C, heating to 1200C in 60 s and holding for 30 s, The volume fraction of fibers was about 40% and the braid- then cooling back to 900C immediately (temperature dif- ing angle was about 20. Low pressure CVi was employed ference ATA300C). Only the middle parts of the spec to deposit a pyrolytic carbon layer and the silicon carbide mens(40 mm long, 3 mm wide and 3 mm thick)were atrix. A thin pyrolytic carbon layer was deposited on kept in the hot zone and oxidizing atmosphere. In testing, the surface of the carbon fiber as the interfacial layer with a constant load of 60 MPa was applied to both the longitu C3Hs at 800C. Methyltrichlorosilane(MTS, CH3 SiCl3) was used for the deposition of the Sic-matrix MTS vapor was carried by bubbling hydrogen. Typical conditions for 4-R85 deposition were 1000C, a hydrogen: MTS ratio of 10: 1, and a pressure of 5 kPa. Argon was employed as the dilu- ent gas to slow down the chemical reaction rate of deposi- tion. Finally, the test specimens were machined from the fabricated composites and further coated with Sic by iso- thermal CvI under the same conditions. The morphology and dimensions of the as-received specimens are shown in Fig. 2. Drawing of as-prepared C/SiC specimen (all dimensions in mm) Fig 1.(a) Substrate of the 3D-C/SiC composite and(b) its top surface.cycling under a constant load in oxidizing atmosphere have not been reported in a detailed manner, despite recent ad￾vances in the architecture design and processing of these materials. When evaluating C/SiC composites for potential use in structural applications where periodically changing tem￾peratures occur, the basic characterization of the materials obtained from mechanical and environmental testing is very important in understanding the fundamental proper￾ties of the materials. In this paper, thermal cycling testing results of 3D braided C/SiC composites under a constant load of 60 MPa in a wet oxygen atmosphere are presented. Corresponding thermal stress or thermal strain during test￾ing will be measured, calculated and analyzed by theoreti￾cal and experimental methods. Effects of thermal cycling on mechanical properties of composites will be discussed and the morphologies of fracture sections and coating surfaces will be observed. 2. Experimental 2.1. Preparation of C/SiC composite T-300TM carbon fiber from Toray (Japan) was employed. The fiber preform was prepared using a 3D braid method. The volume fraction of fibers was about 40% and the braid￾ing angle was about 20. Low pressure CVI was employed to deposit a pyrolytic carbon layer and the silicon carbide matrix. A thin pyrolytic carbon layer was deposited on the surface of the carbon fiber as the interfacial layer with C3H8 at 800 C. Methyltrichlorosilane (MTS, CH3 SiCl3) was used for the deposition of the SiC-matrix. MTS vapor was carried by bubbling hydrogen. Typical conditions for deposition were 1000 C, a hydrogen:MTS ratio of 10:1, and a pressure of 5 kPa. Argon was employed as the dilu￾ent gas to slow down the chemical reaction rate of deposi￾tion. Finally, the test specimens were machined from the fabricated composites and further coated with SiC by iso￾thermal CVI under the same conditions. The morphology and dimensions of the as-received specimens are shown in Figs. 1 and 2. The properties of the composite are listed in Table 1. 2.2. Thermal cycling test Thermal cycling experiments under load constraints were conducted with a newly-developed integrated system including an induction heating furnace (with a controlled atmosphere chamber providing various kinds and concen￾trations of oxidizing gas) monitored by a programmable microprocessor and a servo-hydraulic machine (Model In￾stron 8801, Instron Ltd., England). Many experimental conditions/parameters must be taken into consideration, especially regarding (1) the load alignment, (2) the config￾uration of the specimen, (3) the heater, (4) the cooling water, (5) the measurement for temperature, (6) the induc￾tion coil for cyclic temperature, (7) the grip holder and (8) the pressure and flow of the controlled atmosphere (as shown in Fig. 3). The temperature was measured by an infrared pyrometer through a small window in the wall of the furnace and the wall was internally cut out to enable the circulating cold water to reach all over the surfaces. Thermal cycling was carried out between two selected tem￾peratures and the period was 120 s: holding for 30 s at 900 C, heating to 1200 C in 60 s and holding for 30 s, then cooling back to 900 C immediately (temperature dif￾ference DT
300 C). Only the middle parts of the speci￾mens (40 mm long, 3 mm wide and 3 mm thick) were kept in the hot zone and oxidizing atmosphere. In testing, a constant load of 60 MPa was applied to both the longitu￾Fig. 1. (a) Substrate of the 3D-C/SiC composite and (b) its top surface. Fig. 2. Drawing of as-prepared C/SiC specimen (all dimensions in mm). 164 H. Mei et al. / Scripta Materialia 54 (2006) 163–168