Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride
K J. Yoon, Sheldon M. Wiederhorn, William E. Luecke
The creep behavior of most commercial grade of Si3N4 was studied at 1350 and 1400 C. Stresses ranged from 10 to 200 Mpa in tension and from 30 to 300 Mpa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 22.9 kJ/mol, and compression, 489.2 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was 18% of of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution-precipitation of Si3N4. Although the tensile and compressive creep rates overlapped at lowest stresses, cavity volumme fraction measurements showed that solution-precipitation creep of Si3N4 did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.