The creep viscosity of chemical-precursor-derived silicon carbonitride (SiCN), which is known to remain predominantly amorphous at temperatures below 1400°C, was measured in the temperature range 1090-1280°C. Experiments were done in uniaxial compression at constant loads in pure nitrogen atmosphere. The creep behavior exhibited three stages. In stage I the strain rate decreased rapidly with time and deformation was accompanied by densification. In stage II the samples exhibited a steady-state creep rate. In stage III, which commenced after long-term deformation, creep gradually declined to rates that were below the sensitivity of our apparatus. The relative density of the specimens during stage II and stage III remained constant at ≈2.3 g/cm 3 . The shear viscosity in stage II was nearly Newtonian and was measured to be 1.3 × 10 13 -5.0 × 10 13 Paؒs at 1280°C, which is approximately 10 3 times the value for fused silica. The creep-hardened as well as uncrept specimens contained silicon nitride crystallites. The volume fraction of these crystals was variable but always less than 5%. Such a small volume fraction of crystals does not explain the dramatic creep-hardening behavior in stage III, even if it is assumed that the crystals formed during creep deformation in stage II.
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