H. Mei, L. Cheng / Materials Letters 59(2005)3246-3251 accumulated resistance curve reveals clearly three sequent the newly-developed SDIa has an agreement with the damage stages: (i)physical destruction unde experimental statistical result. cycling within the first 15 cycles, (ii) interaction According to the subtle electrical resistance measure destruction and chemical recession through ment, three sequent damage stages of the C/SiC composites derived from the stage (i) between about 15 and 35 cycles, during thermal cycling in the wet oxygen are obtained and (ini slow chemical oxidation after cracks reaches physical destruction under thermal cycling within the first saturations at 35 cycles. Consequently, the damage revealed 15 cycles, (ii) interaction of physical destruction and y in situ monitoring of electrical resistance is subtle related chemical recession through the cracks derived from the to mechanical testing stage (i) between about 15 and 35 cycles, and (iii) slow chemical oxidation after cracks reaches saturations at 35 4. Conclusions he C/SiC composites subjected to thermal cycling are Acknowledgements rery sensitive to the wet oxygen atmosphere. Under cyclic temperatures, fibers are susceptible to oxidation in the wet The authors acknowledge the financial support of Natural oxygen and the role playing by SiC as an oxidation barrier Science Foundation of China(Contract No. 90405015) H20. Microscopic observations indicate two types of major 5042se al Young Elitists Foundation(Contract No seems to be limited although its oxidation is noticeable in and Nation fiber failure patten: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere They were considered to be responsible for mechanical References Damage in 2D C/SiC composites has been characterized []N Chawla, J.W. Holmes, R.A. Lowden, Scr. Mater. 35(1996)1411 y both mechanical properties testing and electrical resist- 22]F. Lamouroux, X. Bourrat, J. Sevely, R. Naslain, Carbon 31(1993) nce measurement. Mechanical testing results show that 3]O. Ceysson, M. Salvia, L. Vincent, E.C. Lyon, Scr. Mater. 34( 1996) thermal cycling damage to C/SiC composites is limited and 1273. there exists a critical cycling number (38 times). The 44S. Wang, D D.L. Chung, Carbon 35(1997)621 damage revealed by real time electrical resistance measure- 5] xJ.Wang, D D.L. Chung, Comp. 29B(1998)B63 ment is subtle related to mechanical testing. Resistance 6 N. Angelidis, C.Y. Wei, P.E. Irving, Comp. 35(2004)A1135 decreases upon heating and increases with cooling in each [7 D.C. Seo, J.J. Lee, Comp. Struct. 47(1999)525 [8]BK Jang, H. Matsubara, Mater. Lett. 59(2005)266 cycle. As cycling progresses, the baseline resistance [9]RN Singh, H. Wang, Comp. Eng. 5(1995)1 decreases continuously and then levels off after the critical [10]R H Doremus, J Phys. Chem.80(1976)1773 cycling number(35 times). The critical number predicted by [I G.H. Schiroky, Adv Ceram. Mater. 2(1987)137accumulated resistance curve reveals clearly three sequent damage stages: (i) physical destruction under thermal cycling within the first 15 cycles, (ii) interaction of physical destruction and chemical recession through the cracks derived from the stage (i) between about 15 and 35 cycles, and (iii) slow chemical oxidation after cracks reaches saturations at 35 cycles. Consequently, the damage revealed by in situ monitoring of electrical resistance is subtle related to mechanical testing. 4. Conclusions The C/SiC composites subjected to thermal cycling are very sensitive to the wet oxygen atmosphere. Under cyclic temperatures, fibers are susceptible to oxidation in the wet oxygen and the role playing by SiC as an oxidation barrier seems to be limited although its oxidation is noticeable in H2O. Microscopic observations indicate two types of major fiber failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. They were considered to be responsible for mechanical degradation. Damage in 2D C/SiC composites has been characterized by both mechanical properties testing and electrical resist￾ance measurement. Mechanical testing results show that thermal cycling damage to C/SiC composites is limited and there exists a critical cycling number (38 times). The damage revealed by real time electrical resistance measure￾ment is subtle related to mechanical testing. Resistance decreases upon heating and increases with cooling in each cycle. As cycling progresses, the baseline resistance decreases continuously and then levels off after the critical cycling number (35 times). The critical number predicted by the newly-developed SDIA has an agreement with the experimental statistical result. According to the subtle electrical resistance measure￾ment, three sequent damage stages of the C/SiC composites during thermal cycling in the wet oxygen are obtained: (i) physical destruction under thermal cycling within the first 15 cycles, (ii) interaction of physical destruction and chemical recession through the cracks derived from the stage (i) between about 15 and 35 cycles, and (iii) slow chemical oxidation after cracks reaches saturations at 35 cycles. Acknowledgements The authors acknowledge the financial support of Natural Science Foundation of China (Contract No. 90405015) and National Young Elitists Foundation (Contract No. 50425208). References [1] N. Chawla, J.W. Holmes, R.A. Lowden, Scr. Mater. 35 (1996) 1411. [2] F. Lamouroux, X. Bourrat, J. Sevely, R. Naslain, Carbon 31 (1993) 1273. [3] O. Ceysson, M. Salvia, L. Vincent, E.C. Lyon, Scr. Mater. 34 (1996) 1273. [4] S. Wang, D.D.L. Chung, Carbon 35 (1997) 621. [5] X.J. Wang, D.D.L. Chung, Comp. 29B (1998) B63. [6] N. Angelidis, C.Y. Wei, P.E. Irving, Comp. 35 (2004) A1135. [7] D.C. Seo, J.J. Lee, Comp. Struct. 47 (1999) 525. [8] B.K. Jang, H. Matsubara, Mater. Lett. 59 (2005) 266. [9] R.N. Singh, H. Wang, Comp. Eng. 5 (1995) 1287. [10] R.H. Doremus, J. Phys. Chem. 80 (1976) 1773. [11] G.H. Schiroky, Adv. Ceram. Mater. 2 (1987) 137. H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251 3251