l150 Journal of the American Ceramic Society--Sauder and Lamon Vol. 90. No. 4 sA3(1) (2) Fig. 5. Scanning electron microscope micrographs of the cross sections of (a) Hi-Nicalon. (b)Hi-Nicalon S(c) Tyranno SA3(1), and(d) Tyranno SA3 produces substantial basal plane shear. Furthermore, the mag- It is worth mentioning that primary creep was more signifi nitude of the viscoelastic response of carbon fibers under tension cant in those fibers that contained a large amount of carbon(Hi- depends on the orientation of the graphitic planes. It increases Nicalon). This supports the above carbon deformation-driven with the fraction of graphitic planes with a large angle to loading mechanism direction(isotropic carbon). By contrast, it is limited in the ani Although the Hi-Nicalon and SA3 (1) fibers possessed the sotropic fibers, in which most of the graphitic planes are orient same fraction of carbon, Hi-Nicalon fiber showed a larger sen- ed parallel to the loading direction. In SiC fibers, the orientation sitivity to creep. This discrepancy can be attributed to grain size of graphitic planes is influenced by grain-boundary distribution. which is much smaller in the Hi- Nicalon fiber. It could also Thus, graphitic planes can take on all orientations. Further- be related to the structure of carbon, which displayed a better more, it was indicated earlier that the carbon present in these rganization in the SA3(1) fiber (smaller distance between iC fibers consists of stacks of a few graphitic planes. As a con- two successive layers: doo), as a result of a higher-processing sequence, primary creep may involve deformation of carbon at temperature. A low doog implies a larger stiffness and smaller grain boundaries and grain sliding due to basal plane shear deformationsproduces substantial basal plane shear.20 Furthermore, the mag￾nitude of the viscoelastic response of carbon fibers under tension depends on the orientation of the graphitic planes.21 It increases with the fraction of graphitic planes with a large angle to loading direction (isotropic carbon). By contrast, it is limited in the ani￾sotropic fibers, in which most of the graphitic planes are orient￾ed parallel to the loading direction. In SiC fibers, the orientation of graphitic planes is influenced by grain-boundary distribution. Thus, graphitic planes can take on all orientations. Further￾more, it was indicated earlier that the carbon present in these SiC fibers consists of stacks of a few graphitic planes. As a con￾sequence, primary creep may involve deformation of carbon at grain boundaries and grain sliding due to basal plane shear. It is worth mentioning that primary creep was more signifi- cant in those fibers that contained a large amount of carbon (Hi￾Nicalon). This supports the above carbon deformation-driven mechanism. Although the Hi-Nicalon and SA3 (1) fibers possessed the same fraction of carbon, Hi-Nicalon fiber showed a larger sen￾sitivity to creep. This discrepancy can be attributed to grain size, which is much smaller in the Hi-Nicalon fiber. It could also be related to the structure of carbon, which displayed a better organization in the SA3 (1) fiber (smaller distance between two successive layers: d002), as a result of a higher-processing temperature. A low d002 implies a larger stiffness and smaller deformations. (a) Hi-Nicalon (b) Hi-Nicalon S SA3 (1) SA3 (2) (c) (d) Zoom Fig. 5. Scanning electron microscope micrographs of the cross sections of (a) Hi-Nicalon, (b) Hi-Nicalon S, (c) Tyranno SA3 (1), and (d) Tyranno SA3 (2) fibers. 1150 Journal of the American Ceramic Society—Sauder and Lamon Vol. 90, No. 4