l154 Journal of the American Ceramic Society--Sauder and Lamon Vol. 90. No. 4 Global cross section .kv×1a.k3白m Core observation Edge observation 75g edge magnification(cross sections were obtained by cutting fibers using a Daue presented in Fig. 7. (a)global cross section, (b)core magnification, (c) Fig 12. Scanning electron microscope micrographs of SA3(2)after the ere g is the tress,Re is the fiber diameter, and e(t) is (6) TEM Analysis of Crept Fibers the thickness of ular carbon layer TEM analysis was performed on SA3 (2)fibers tested at 1600oC In a first step a simple linear time dependence was selected and Hi-Nicalon S fibers tested at 1500.C. Creep tests were in- terrupted when the total deformation of the fiber reached 6% The following features were observed (7 o Carbon was highly preponderant in the superficial region (the thickness was close to 500 nm) The degradation rate was estimated from the thickness of the (i Cavities were not detected at triple junctions. As indi- annular layer determined from micrographs of fibers after in- cated above, it may be thought that accommodation is made ed creep tests. k=2.10-m/s was estimated for the Hi- possible because of carbon anisotropy Stress relation(6)was introduced into Eqs. (3)and (4). Figure 14 shows the creep curve that was predicted for Hi-Nicalon S a good agreement with the experimental results wa IV. Conclusion btained which supports the analysis. Nevertheless, a slight The tensile creep behavior of Sic fibers with a low oxygen con- discrepancy can be noticed, which may result from approxi- tent was investigated up to very high temperatures. Tests of long mations in the annular degradation law and in load sharing duration were carried out on a high-performance testing device. The contribution of the annular carbon layer was negle The three stages of creep were evidenced annular degradation law and contribution of the carbon layer / primary creep was particularly long in the Hi-Nicalon fibers Refinement, if necessary, would introduce a more complex in load sharing ca lasted more than 144 h. Primary creep was attributed to vis- elastic deformation of carbon at grain boundaries. Primarywhere so is the applied stress, Rc is the fiber diameter, and e(t) is the thickness of the annular carbon layer. In a first step, a simple linear time dependence was selected for e(t): eðtÞ ¼ kt (7) The degradation rate was estimated from the thickness of the annular layer determined from micrographs of fibers after in￾terrupted creep tests. k 5 2.10
12 m/s was estimated for the Hi￾Nicalon S fiber. Stress relation (6) was introduced into Eqs. (3) and (4). Figure 14 shows the creep curve that was predicted for Hi-Nicalon S. A good agreement with the experimental results was obtained, which supports the analysis. Nevertheless, a slight discrepancy can be noticed, which may result from approxi￾mations in the annular degradation law and in load sharing. The contribution of the annular carbon layer was neglected. Refinement, if necessary, would introduce a more complex annular degradation law and contribution of the carbon layer in load sharing. (6) TEM Analysis of Crept Fibers TEM analysis was performed on SA3 (2) fibers tested at 16001C and Hi-Nicalon S fibers tested at 15001C. Creep tests were in￾terrupted when the total deformation of the fiber reached 6%. The following features were observed: (i) Carbon was highly preponderant in the superficial region (the thickness was close to 500 nm). (ii) Cavities were not detected at triple junctions. As indi￾cated above, it may be thought that accommodation is made possible because of carbon anisotropy. IV. Conclusion The tensile creep behavior of SiC fibers with a low oxygen con￾tent was investigated up to very high temperatures. Tests of long duration were carried out on a high-performance testing device. The three stages of creep were evidenced. Primary creep was particularly long in the Hi-Nicalon fibers. It lasted more than 144 h. Primary creep was attributed to vis￾coelastic deformation of carbon at grain boundaries. Primary Fig. 12. Scanning electron microscope micrographs of SA3 (2) after the creep test presented in Fig. 7, (a) global cross section, (b) core magnification, (c) edge magnification (cross sections were obtained by cutting fibers using a blade). 1154 Journal of the American Ceramic Society—Sauder and Lamon Vol. 90, No. 4