April 2007 SiC-Based Fibers and Low Oxygen Conten 1153 16 14 12 E0.8 Test st SEM observation 04 02 0 0 12 Temps(h) x35.0 Fig. 11. Creep test at 1450C and under a stress of 500 MPa for Hi-Nicalon S fiber, and scanning electron microscope micrographs of cross section after test interruption(cross sections were obtained by cutting fibers using a blade) that the annular the fb was made of pure carbon. Furthermore, more serious effect. Under such vacuu oxidation the diameter of the fiber was unchanged. These results suggest products (Sio (g)and co (g)) would be hat silicon volatilized and that this phenomenon advanced from ed, so that the fiber would be completely le authors the surface toward the core. Data in the literature support this who observed degradation of SiC fibers aperature assumption. Thus, Fig. 13 shows that Sic decomposes at tem- under argon" attributed this phenomenon to active oxidation. peratures above 1400C when the pressure is identical to that in But they did not go into an in-depth investigation to ascertain the chamber(10 Pa). Furthermore, the authors have shown that gaseous Si (g) is produced preponderantly under these con- The creep rate acceleration would result from a change in the ditions.4- The experimental conditions were favorable to sil- stress state in the SiC fiber, caused by the annular degradation of icon volatilization. Thermodynamic equilibrium could not be fiber. During Si volatilization, stiff SiC is replaced by a porous reached and the pressure of gaseous si (g) remained above the carbon material that is much more compliant. As a conse- uilibrium value in the chamber. quence, the load is carried preponderantly by the Sic core. The pressure in the chamber (10 Pa) is smaller than As annular degradation proceeds, there is an increase in stress, that of si (g) at temperatures above 1400C, according to according to the following equation: Fig. 13. As a consequence, gaseous species can be eliminated; (i The chamber wall was covered with a deposit after the gaseous products condense The above phenomenon cannot be attributed to active oxi- a(1) FoRc dation from residual oxygen. Active oxidation would have athat the annular region was made of pure carbon. Furthermore, the diameter of the fiber was unchanged. These results suggest that silicon volatilized and that this phenomenon advanced from the surface toward the core. Data in the literature support this assumption. Thus, Fig. 13 shows that SiC decomposes at tem￾peratures above 14001C when the pressure is identical to that in the chamber (10
4 Pa). Furthermore, the authors have shown that gaseous Si (g) is produced preponderantly under these con￾ditions.41–48 The experimental conditions were favorable to sil￾icon volatilization. Thermodynamic equilibrium could not be reached and the pressure of gaseous Si (g) remained above the equilibrium value in the chamber: (i) The pressure in the chamber (10
4 Pa) is smaller than that of Si (g) at temperatures above 14001C, according to Fig. 13. As a consequence, gaseous species can be eliminated; (ii) The chamber wall was covered with a deposit after the tests, indicating that gaseous products condensed. The above phenomenon cannot be attributed to active oxi￾dation from residual oxygen. Active oxidation would have a more serious effect. Under such vacuum conditions, oxidation products (SiO (g) and CO (g)) would be continuously eliminat￾ed, so that the fiber would be completely destroyed. The authors who observed degradation of SiC fibers at a high temperature under argon49 attributed this phenomenon to active oxidation. But they did not go into an in-depth investigation to ascertain their interpretation. The creep rate acceleration would result from a change in the stress state in the SiC fiber, caused by the annular degradation of fiber. During Si volatilization, stiff SiC is replaced by a porous carbon material that is much more compliant. As a conse￾quence, the load is carried preponderantly by the SiC core. As annular degradation proceeds, there is an increase in stress, according to the following equation: sðtÞ ¼ soR2 c ðRc
eðtÞÞ2 (6) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 4 8 12 16 20 24 Temps (h) Strain (%) Test stop for SEM observation Fig. 11. Creep test at 14501C and under a stress of 500 MPa for Hi-Nicalon S fiber, and scanning electron microscope micrographs of cross section after test interruption (cross sections were obtained by cutting fibers using a blade). April 2007 SiC-Based Fibers and Low Oxygen Content 1153