H. Mei, L. Cheng Materials Letters 59(2005)3246-3251 2.3. Thermal cycling tests experiences thermal stress. The thermal stress created due to the temperature gradient is given by Ref. [9 Thermal cycling tests were conducted with a specific system including a high frequency induction heating fumace xE△T and a servo-hydraulic machine(Model INSTRON 8801 (3) from INSTRON Ltd, in England). The temperature was measured by an infrared pyrometer through a small window where o, is the thermal stress, a the coefficient of thermal in the wall of the furnace and the wall was internally cut out expansion, E the Youngs modulus, ATc the critical to enable the circulating cold water to reach all over the temperature gradient, and v the Poisson's ratio surfaces of it. Thermal cycling was carried out between two In the present work, according to the Table l, the mean selected temperatures by a programmable microprocessor tensile strength of the as-received 2D-C/SiC composites is and the period was 120 s: holding for 30 s at the lower about 248 MPa, the value of v approximates to 0.32, E temperature(less than 700C), heating to 1200C in 60 s about 70 GPa and o is 5.3x 10 /C in average. Therefore, and holding for 30 S, and then cooling back to the lower from formula(3), we obtain: ATc455C, which is slightly temperature immediately. The temperature difference AT lower than the temperature difference in testing(AT=500 was about 500C. Only the middle parts of specimens(about C). More and more cracks, thus, were produced on the 40 mm long, 3 mm wide and 3 mm thick as shown in Fig. 1) ceramic coating surfaces, and then propagated inwards were kept in the hot zone and wet oxygen atmosphere when thermal stress exceeded the strength of the matrix including dry oxygen 8000 Pa and water-vapor 15000 Pa atera(△7>△Tc) (about 54C). The flux of gases was accurately controlled by oth environmental atmospheres and thermal cycling a mass flow controller(5850 i series of BROOKS in Japan) should be responsible for mechanical degradation of the and its precision could reach 0. 1 SCCM composites in testing. The typical micrographs of the fracture sections of the 2D C/SiC composites after 50 2. 4. Measurements and observations thermal cycles in the wet oxygen atmosphere are presented in Fig. 2. It can be seen that the fibers were oxidized and/ Residual strengths of the specimens after the given or pulled out(Fig 2a) and that matrix cracked and thermal cycling numbers in the wet oxygen were measured delaminated as regular spacing(Fig. 2b). The wet oxygen on an INSTRON-8801 device and changes in resistance atmosphere and cyclic thermal stress must be taken into were in situ monitored by SDIA. The microstructural consideration to explain the phenomena. These fissures observations were conducted on a scanning electron micro- opened by cyclic stress provided paths through which scope(SEM, HITACHI S-4700) oxygen could migrate toward the carbon reinforcement and reacted with it. Therefore, thermal cycling resulted in a physical damage while environmental atmospheres (i.e, 3. Results and discussion wet oxygen) caused a chemical degradation Damage and degradation of the fibers is very severe in 3.1. Effect of thermal cycling in the wet oxygen atmosphere the wet oxygen atmosphere under the cyclic temperatures on mechanical properti Fig. 3 shows two types of major fibers failure pattern: (1) physical fracture under thermal cycling and (ii) chemical The sudden change in the surrounding temper rature recession in oxidizing atmosphere. In testing, matrix crack generates temperature gradient, thereby, the ceramic body were opened transversely as regular spacing and superficial b AAsN Fig. 2. Typical micrographs of the fracture sections of the 2D C/SiC composites after 50 thermal cycles in the wet oxygen atmosphere. (a)Superficial and2.3. Thermal cycling tests Thermal cycling tests were conducted with a specific system including a high frequency induction heating furnace and a servo-hydraulic machine (Model INSTRON 8801 from INSTRON Ltd., in England). 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 of it. Thermal cycling was carried out between two selected temperatures by a programmable microprocessor and the period was 120 s: holding for 30 s at the lower temperature (less than 700 -C), heating to 1200 -C in 60 s and holding for 30 s, and then cooling back to the lower temperature immediately. The temperature difference DT was about 500 -C. Only the middle parts of specimens (about 40 mm long, 3 mm wide and 3 mm thick as shown in Fig. 1) were kept in the hot zone and wet oxygen atmosphere including dry oxygen 8000 Pa and water-vapor 15 000 Pa (about 54 -C). The flux of gases was accurately controlled by a mass flow controller (5850 i series of BROOKS in Japan) and its precision could reach 0.1 SCCM. 2.4. Measurements and observations Residual strengths of the specimens after the given thermal cycling numbers in the wet oxygen were measured on an INSTRON-8801 device and changes in resistance were in situ monitored by SDIA. The microstructural observations were conducted on a scanning electron micro￾scope (SEM, HITACHI S-4700). 3. Results and discussion 3.1. Effect of thermal cycling in the wet oxygen atmosphere on mechanical properties The sudden change in the surrounding temperature generates temperature gradient, thereby, the ceramic body experiences thermal stress. The thermal stress created due to the temperature gradient is given by Ref. [9], rt ¼ aEDTc 1  m ð3Þ where rt is the thermal stress, a the coefficient of thermal expansion, E the Young’s modulus, DTc the critical temperature gradient, and v the Poisson’s ratio. In the present work, according to the Table 1, the mean tensile strength of the as-received 2D-C/SiC composites is about 248 MPa, the value of v approximates to 0.32, E is about 70 GPa and a is 5.3106 /-C in average. Therefore, from formula (3), we obtain: DTc
455 -C, which is slightly lower than the temperature difference in testing (DT
500 -C). More and more cracks, thus, were produced on the ceramic coating surfaces, and then propagated inwards when thermal stress exceeded the strength of the matrix material (DT >DTc). Both environmental atmospheres and thermal cycling should be responsible for mechanical degradation of the composites in testing. The typical micrographs of the fracture sections of the 2D C/SiC composites after 50 thermal cycles in the wet oxygen atmosphere are presented in Fig. 2. It can be seen that the fibers were oxidized and/ or pulled out (Fig 2a) and that matrix cracked and delaminated as regular spacing (Fig. 2b). The wet oxygen atmosphere and cyclic thermal stress must be taken into consideration to explain the phenomena. These fissures opened by cyclic stress provided paths through which oxygen could migrate toward the carbon reinforcement and reacted with it. Therefore, thermal cycling resulted in a physical damage while environmental atmospheres (i.e., wet oxygen) caused a chemical degradation. Damage and degradation of the fibers is very severe in the wet oxygen atmosphere under the cyclic temperatures. Fig. 3 shows two types of major fibers failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. In testing, matrix cracks were opened transversely as regular spacing and superficial a b 50 µm 25 µm Fig. 2. Typical micrographs of the fracture sections of the 2D C/SiC composites after 50 thermal cycles in the wet oxygen atmosphere. (a) Superficial and (b) central. 3248 H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251