S. Mall,J.L Ryba / Composites Science and Technology 68(2008)274-282 mediate temperature range, which in turn caused a rela- Acknowledgements ively more reduction in stress rupture performance relative to that at below or above the intermediate temperature test The support of Ruth Sikorski, Propulsion Directorate, condition Air Force Research Laboratory, Wright-Patterson AFB Similar characteristics were observed in the fracture sur- as well as Drs. Jim DiCarlo and Greg Morscher, Ceramics faces of the residual strength tests(Fig. 11). The fracture Branch, NASA-Glenn Research Center is highly surfaces are relatively rougher in tests at 400C and appreciated 950C i.e outside the intermediate range(Fig. lla, c, d and f) than the more planar fracture in tests at 750"C References within the intermediate range(Fig. llb and e). Finally, 1g. 12 shows the effect of the exposure time on the damage []DiCarlo JA, Dutta S. Continuous ceramics fibers for progression. This is shown for one test condition, at 950C composites. In: Lehman R, El-Rahaiby S, Wachtman J in the steam environment, for the sake of brevity. This Handbook on continuous fiber reinforced ceramic matrix composites. clearly shows that damage of BN interphase increased with CIAC, Purdue University: 1995. p. 137-83 test exposure duration, and it is already prevalent at about 22] Ichikawa H, Ishikawa. Slicon carbide fibers (organometa 4h, or in-between l and 4 h. In summary, the micro- opic analysis suggests that the tested CMC composite [3] Yun HM, DiCarlo JA. Comparison of tensile, creep and rupture experienced relatively more embrittlement of the fiber/ trength properties of stoichiometric sic fibers. In: Proceedings of the matrix interphase in the presence of moisture and at the 3rd annual conference on composites, materials and structures, voL. intermediate temperature range. This resulted in relatively 20, Cocoa Beach;1999,p.259-72. more reduction in stress rupture performance in these test [4] Brennan JJ. Interfacial characterization of glass and matrix/nicalon SiC fiber composites. Mater Sci Res I 5] Cooper RF, Chyung K. Structure and chemistry of ceramics. J Mater Sci 1987- 22-3148-60. 4. Conclusions 6] Cao H, Bischoff E, Sbaizero O, Ruhle M, Evans AG, Marshall DB, et al. Effect of interfaces on the properties of fiber-reinforced The stress rupture behavior of silicon carbide fiber(Syl) ceramics. J Am Ceram Soc 1990: 73: 1691-9 [7 Morscher GN, Hurst J, Brewer D. intermediate-temperature stress reinforced in silicon carbide(SiC)matrix with a boron rupture of a woven Hi-nicalon, BN-interphase, SiC matrix in air. J nitride(BN) interphase( Syl-iBN/BN/SiC)was invest Am Ceram Soc 2000: 83: 1441-9. at three temperatures,400°C,750°Cand950°%C, [8] Morscher GN. Tensile stress rupture of SiC/SiCm minicomposites laboratory air and 100% steam test environments with carbon and boron nitride interphases at elevated temperatures in air. J Am Ceram Soc 1997: 80: 2029-42 rupture strength versus time to failure relationships were 9]Lara-Curzio E, Ferber MK, Tortorelli PE. Interface established under these six elevated temperature and envi- stress-rupture of Nicalon SiC CVCCs at Intermed ronment conditions. Time to failure in the steam test envi- tures. Key engineerin publica. ronment was shorter than that in the laboratory air test tions;1997.10691082. environment at a given stress level and at an elevated tem- [Io] Lara-Curzio E. stress-rupture of n/Sic continuous fiber °C. J Am Ceram soc perature. In addition time to failure decreased as the test an enhanced temperatures. J Am Ceram BN interphase between the fiber and the matrix. The pres- Soc1998;81:2140 ence of more moisture in the steam environment test condi- [12] Heredia FE, McNulty JC, Zok FW, Evans AG. Oxidation embrit- tion accelerated this process, and therefore it caused more tlement probe for ceramic-matrix composites. J Am Ceram Soc 199578:2097-10 reduction in stress rupture performance of the tested CMC [13] Brewer D. HSR/EPM combustor materials development program system in the steam test environment than in the laboratory Mater Sci Eng A 1999: 261: 284-94 air test environment at a given elevated temperature. Fur- [14] Yun HM, Gyenkenyesi JZ, Chen YL, Wheeler DR, DiCarlo JA ther, the tests conducted at 750C experienced relatively Tensile behavior of SiC/SiC composites reinforced by more damage than expected which resulted in more reduc sylramic fibers. In: Proceedings of the 25th annual confer composites, materials and structures, voL 20, Cocoa Beach; tion in stress rupture performance than the interpolated performance between 400C and 950C. Damage analysis [15] LaRochelle KJ, Mall S Temperature and moisture effects upon stress showed the degradation of BN interphase between fiber upture life of Syl-iBN/BN/SiC composites. Ceram Eng Sci Proc and matrix; however it was of different type at 400C or 2003:24:45964 950C versus 750C. It was in the form of fracture, reces- [16] Mall S Effects of moisture on fatigue behavior of SiC/SiC composite at elevated temperature. Mater Sci Eng: A 2005: 412: 165- sion and removal of Bn interphase at 400C or 950C. On [17] Mall S, LaRochelle KJ. Fatigue and stress-rupture behaviors of Sic/ the other hand. there was formation of borosilicate which Sic composite under humid environment at elevated temperature. caused pesting of fibers at 750C Compos Sci Technol 2006: 66: 2925-34mediate temperature range, which in turn caused a rela￾tively more reduction in stress rupture performance relative to that at below or above the intermediate temperature test condition. Similar characteristics were observed in the fracture sur￾faces of the residual strength tests (Fig. 11). The fracture surfaces are relatively rougher in tests at 400 C and 950 C i.e., outside the intermediate range (Fig. 11a, c, d and f) than the more planar fracture in tests at 750 C within the intermediate range (Fig. 11b and e). Finally, Fig. 12 shows the effect of the exposure time on the damage progression. This is shown for one test condition, at 950 C in the steam environment, for the sake of brevity. This clearly shows that damage of BN interphase increased with test exposure duration, and it is already prevalent at about 4 h, or in-between 1 and 4 h. In summary, the micro￾scopic analysis suggests that the tested CMC composite experienced relatively more embrittlement of the fiber/ matrix interphase in the presence of moisture and at the intermediate temperature range. This resulted in relatively more reduction in stress rupture performance in these test conditions. 4. Conclusions The stress rupture behavior of silicon carbide fiber (Syl) reinforced in silicon carbide (SiC) matrix with a boron nitride (BN) interphase (Syl-iBN/BN/SiC) was investigated at three temperatures, 400 C, 750 C and 950 C, under laboratory air and 100% steam test environments. Stress rupture strength versus time to failure relationships were established under these six elevated temperature and envi￾ronment conditions. Time to failure in the steam test envi￾ronment was shorter than that in the laboratory air test environment at a given stress level and at an elevated tem￾perature. In addition time to failure decreased as the test temperature increased in a given test environment. The pri￾mary damage mechanism involved the degradation of the BN interphase between the fiber and the matrix. The pres￾ence of more moisture in the steam environment test condi￾tion accelerated this process, and therefore it caused more reduction in stress rupture performance of the tested CMC system in the steam test environment than in the laboratory air test environment at a given elevated temperature. Fur￾ther, the tests conducted at 750 C experienced relatively more damage than expected which resulted in more reduc￾tion in stress rupture performance than the interpolated performance between 400 C and 950 C. Damage analysis showed the degradation of BN interphase between fiber and matrix; however it was of different type at 400 C or 950 C versus 750 C. It was in the form of fracture, reces￾sion and removal of BN interphase at 400 C or 950 C. On the other hand, there was formation of borosilicate which caused pesting of fibers at 750 C. Acknowledgements The support of Ruth Sikorski, Propulsion Directorate, Air Force Research Laboratory, Wright-Patterson AFB as well as Drs. Jim DiCarlo and Greg Morscher, Ceramics Branch, NASA-Glenn Research Center is highly appreciated. References [1] DiCarlo JA, Dutta S. Continuous ceramics fibers for ceramic composites. In: Lehman R, El-Rahaiby S, Wachtman J, editors. Handbook on continuous fiber reinforced ceramic matrix composites. CIAC, Purdue University; 1995. p. 137–83. [2] Ichikawa H, Ishikawa T. Silicon carbide fibers (organometallic pyrolysis). In: Kelly A, Zweben C, Chou T, editors. Comprehensive composite materials, vol. I. Elsevier Science; 2000. p. 107–45. [3] Yun HM, DiCarlo JA. Comparison of tensile, creep and rupture strength properties of stoichiometric sic fibers. In: Proceedings of the 23rd annual conference on composites, materials and structures, vol. 20, Cocoa Beach; 1999, p. 259–72. [4] Brennan JJ. Interfacial characterization of glass and glass-ceramic matrix/nicalon SiC fiber composites. Mater Sci Res 1986;20:546–60. [5] Cooper RF, Chyung K. Structure and chemistry of fiber-reinforced ceramics. J Mater Sci 1987;22:3148–60. [6] Cao H, Bischoff E, Sbaizero O, Ruhle M, Evans AG, Marshall DB, et al. Effect of interfaces on the properties of fiber-reinforced ceramics. J Am Ceram Soc 1990;73:1691–9. [7] Morscher GN, Hurst J, Brewer D. intermediate-temperature stress rupture of a woven Hi-nicalon, BN-interphase, SiC matrix in air. J Am Ceram Soc 2000;83:1441–9. [8] Morscher GN. Tensile stress rupture of SiCf/SiCm minicomposites with carbon and boron nitride interphases at elevated temperatures in air. J Am Ceram Soc 1997;80:2029–42. [9] Lara-Curzio E, Ferber MK, Tortorelli PF. Interface oxidation and stress-rupture of NicalonTM/SiC CVCCs at Intermediate Tempera￾tures. Key engineering materials. Switzerland: Trans Tech Publica￾tions; 1997. 1069–1082. [10] Lara-Curzio E. stress-rupture of Nicalon/SiC continuous fiber ceramic matrix composites in air at 950 C. J Am Ceram Soc 1997;80:3268–72. [11] Steyer TE, Zok FW, Walls DP. Stress rupture of an enhanced NicalonTM/SiC composite at intermediate temperatures. J Am Ceram Soc 1998;81:2140–6. [12] Heredia FE, McNulty JC, Zok FW, Evans AG. Oxidation embrit￾tlement probe for ceramic-matrix composites. J Am Ceram Soc 1995;78:2097–100. [13] Brewer D. HSR/EPM combustor materials development program. Mater Sci Eng A 1999;261:284–94. [14] Yun HM, Gyenkenyesi JZ, Chen YL, Wheeler DR, DiCarlo JA. Tensile behavior of SiC/SiC composites reinforced by treated sylramic fibers. In: Proceedings of the 25th annual conference on composites, materials and structures, vol. 20, Cocoa Beach; 2001, p. 521–31. [15] LaRochelle KJ, Mall S. Temperature and moisture effects upon stress rupture life of Syl-iBN/BN/SiC composites. Ceram Eng Sci Proc 2003;24:459–64. [16] Mall S. Effects of moisture on fatigue behavior of SiC/SiC composite at elevated temperature. Mater Sci Eng: A 2005;412:165–70. [17] Mall S, LaRochelle KJ. Fatigue and stress-rupture behaviors of SiC/ SiC composite under humid environment at elevated temperature. Compos Sci Technol 2006;66:2925–34. 282 S. Mall, J.L. Ryba / Composites Science and Technology 68 (2008) 274–282