J Mater Sci(2007)42:5046-50 5055 O HNL ranging from 1.300 1.900C in Ar for 1 h. After ●HNLs annealing, the microstructural characteristics and fracture properties were investigated, respectively As a result. excellent microstructure and mechanical stabilities were observed for sic fibers with near-stoi- chiometric composition and high-crystalline structure Also, the correlation between the mechanical proper ties and the microstructure of sic-based fibers was clarified. Combining the microstructure examination with mechanical test indicates that the thermal and mechanical stabilities of Sic fibers at high temperature 040.50.6 were mainly controlled by their crystallization and composition as well as other factors. The crystallization of amorphous phase and impurities could Fig8 The fiber tensile strength versus the square root of the grain coarsening, decomposition and oxidation of Sic fracture mirror size; the slops of fitting lines yield the mirror Based on the present result, the near stoichiometric constant Am=3.93 MPam" for as-received HNL fiber, and high crystallite SiC fibers showed a high potenti to be applied at very high temperatures. This work is existence of other degradation mechanisms such as useful to the optimization of fabrication and applica- contaminants during annealing and metallic impurities tion condition of high pertormance CMCS induced during process 34-36]. The existence of metallic impurities within the fibers is possible, because all these fibers are polymer derived. The metallic References impurities can easily enter the fibers during the various steps of polymer handling and can cause rapid or 1. Kohyama a (2004) Ceramics 39: 838(in Japanese) abnormal grain growth in local areas. There are at least 3. Ohnabe H. Masaki S. Onozuka M. Miyahara K. Sasa T two indirect observations supporting above mentioned 999) Compos: Part A 30: 489 mechanism: (i)Observation of fracture surface in Fig. 5 4. Ichikawa H(2000) Ann Chim Sci Mat 25: 523 for the HNL and HNLS fiber showed that the strength- 5. Ishikawa T, Kohoku Y, Kumagawa K, Yamamura T limiting flaws after annealing are larger than the average 6. Dong S Katoh Y, Kohyama A(2003)J Am Ceram Soc 86: 26 grain size, indicating rapid defect growth in selected 7. Lee SP, Katoh Y, Park JS, Dong S Kohyama A, Suyama S areas of the fiber and thus suggesting the possible exis- Yoon HK (2001)J Nucl Mater 289: 30 tence of metallic impurites; (i) the UF fiber showed high 8. Dong SM, cholon g, Labrugere C Lahaye M, Guette A strength retention than HNL fiber 23]. This suggests sci36:2371 that the UF fiber during processing did not introduce 9. Havel M, Colomban Ph(2003)J Raman Spectr 34:786 metallic impurities to the degree that those employed for 10. Havel M, Colomban Ph(2004)Compos: Part B 35: 139 the hnl fiber 11. Bunsell AR, Berger MH (2000)J Euro Ceram Soc 20: 2249 12. Sha JJ, Nozawa T, Park JS, Katoh Y, Kohyama A(2004) J Nucl Mater 329-333. 592 13. Cullity BD(1978) Elements of X-ray diffraction, 2nd edn Summary Addison Wesley, Reading. MA, p 284-285 14. ASTM D3379-75(reapproved 1989)Standard test method Sic-based fibers, Hi-Nicalon, Hi-Nicalon type S for tensile strength and Youngs modulus for high-modul terials and TyrannoM-SA, were annealed at temperatures 15. Youngblood GE, Lewinsohn C, Jones RH, Kohyama A (2001)J Nucl Mater 289: 1 Table 1 Fracture toughness and critical fracture energy for 16. Shimoo T, Tsukada I, Narisawa M, Seguchi T, Okamura K annealed fibers (1997)J Ceram Soc Jpn 105: 559 17. Hollon G. Pailler R. Naslain R. Laanami F. monthioux M Condition Olry P(1997)J Mater Sci 32: 327 As-received 1300C"C 1600C 19. Ichikawa H, Ishikawa T(2000) In: Kelly A, Zweben C, INL, Kle(MPa m") 1.56 Chou T(eds) Silicon carbide fibers (organometallic Pyroly INL, 7e(J/m 4.51 s), Comprehensive composite Materials, vol 1. Elsevier HNLS, KI(MPa m)1.74 1611.45134 cience Ltd, Oxford, England, pp 107-145 HNLS,,e(J/m 3.09 2.14 20. Takeda M, Saeki A, Sakamoto J, Imai Y, Ichikawa H (1999 Compos Sci Technol 59: 787existence of other degradation mechanisms such as contaminants during annealing and metallic impurities induced during process [34–36]. The existence of metallic impurities within the fibers is possible, because all these fibers are polymer derived. The metallic impurities can easily enter the fibers during the various steps of polymer handling and can cause rapid or abnormal grain growth in local areas. There are at least two indirect observations supporting above mentioned mechanism: (i) Observation of fracture surface in Fig. 5 for the HNL and HNLS fiber showed that the strength￾limiting flaws after annealing are larger than the average grain size, indicating rapid defect growth in selected areas of the fiber and thus suggesting the possible exis￾tence of metallic impurites; (ii) the UF fiber showed high strength retention than HNL fiber [23]. This suggests that the UF fiber during processing did not introduce metallic impurities to the degree that those employed for the HNL fiber. Summary SiC-based fibers, Hi-NicalonTM, Hi-NicalonTM type S and TyrannoTM-SA, were annealed at temperatures ranging from 1,300 to 1,900 C in Ar for 1 h. After annealing, the microstructural characteristics and fracture properties were investigated, respectively. As a result, excellent microstructure and mechanical stabilities were observed for SiC fibers with near-stoi￾chiometric composition and high-crystalline structure. Also, the correlation between the mechanical proper￾ties and the microstructure of SiC-based fibers was clarified. Combining the microstructure examination with mechanical test indicates that the thermal and mechanical stabilities of SiC fibers at high temperature were mainly controlled by their crystallization and composition as well as other factors. The crystallization of amorphous phase and impurities could cause the grain coarsening, decomposition and oxidation of SiC. Based on the present result, the near stoichiometric and high crystallite SiC fibers showed a high potential to be applied at very high temperatures. This work is useful to the optimization of fabrication and applica￾tion condition of high performance CMCs. References 1. Kohyama A (2004) Ceramics 39:838 (in Japanese) 2. Naslain R (2004) Compos Sci Technol 64:155 3. Ohnabe H, Masaki S, Onozuka M, Miyahara K, Sasa T (1999) Compos: Part A 30:489 4. Ichikawa H (2000) Ann Chim Sci Mat 25:523 5. Ishikawa T, Kohtoku Y, Kumagawa K, Yamamura T, Nagasawa T (1998) Nature 391:773 6. Dong S, Katoh Y, Kohyama A (2003) J Am Ceram Soc 86:26 7. Lee SP, Katoh Y, Park JS, Dong S, Kohyama A, Suyama S, Yoon HK (2001) J Nucl Mater 289:30 8. Dong SM, Chollon G, Labrugere C, Lahaye M, Guette A, Bruneel JL, Couzi M, Naslain R, Jiang DL (2001) J Mater Sci 36:2371 9. Havel M, Colomban Ph (2003) J Raman Spectr 34:786 10. Havel M, Colomban Ph (2004) Compos: Part B 35:139 11. Bunsell AR, Berger MH (2000) J Euro Ceram Soc 20:2249 12. Sha JJ, Nozawa T, Park JS, Katoh Y, Kohyama A (2004) J Nucl Mater 329–333:592 13. Cullity BD (1978) Elements of X-ray diffraction, 2nd edn. Addison Wesley, Reading, MA, p 284–285 14. ASTM D3379-75 (reapproved 1989) Standard test method for tensile strength and Young’s modulus for high-modulus single-filament materials 15. Youngblood GE, Lewinsohn C, Jones RH, Kohyama A (2001) J Nucl Mater 289:1 16. Shimoo T, Tsukada I, Narisawa M, Seguchi T, Okamura K (1997) J Ceram Soc Jpn 105:559 17. Chollon G, Pailler R, Naslain R, Laanami F, Monthioux M, Olry P (1997) J Mater Sci 32:327 18. Ichikawa H (2000) Ann Chim Sci Mat 25:523 19. Ichikawa H, Ishikawa T (2000) In: Kelly A, Zweben C, Chou T (eds) Silicon carbide fibers (organometallic Pyroly￾sis), Comprehensive composite Materials, vol 1. Elsevier Science Ltd, Oxford, England, pp 107–145 20. Takeda M, Saeki A, Sakamoto J, Imai Y, Ichikawa H (1999) Compos Sci Technol 59:787 0 1 2 3 4 5 6 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 HNL HNLS (rm) -0.5 Tensile strength, σ /GPa Fig. 8 The fiber tensile strength versus the square root of the fracture mirror size; the slops of fitting lines yield the mirror constant Am = 3.93 MPam-1/2 for as-received HNL fiber, Am = 4.33 Mpam–1/2 for as-received HNLS fiber Table 1 Fracture toughness and critical fracture energy for annealed fibers Condition As-received 1300 C 1400 C 1600 C HNL, K1c(MPa m1/2) 1.56 1.46 1.40 1.27 HNL, cc (J/m2 ) 4.51 3.95 3.63 2.99 HNLS, K1c(MPa m1/2) 1.74 1.61 1.45 1.34 HNLS, cc (J/m2 ) 3.60 3.09 2.50 2.14 J Mater Sci (2007) 42:5046–5056 5055 123