Comparison of tensile and compressive creep behavior in silicon nitride

The creep behavior of a commercial grade of Si3N4 was studied at 1350 degre es and 1400 degrees C. Stresses ranged from 10 to 200 MPa in tension and fr om 30 to 300 MPa in compression. In tension, the creep rate increased linea rly with stress at low stresses and exponentially at high stresses. By cont rast, the creep rate in compression increased linearly with stress over the entire stress range, Although compressive and tensile data exhibited an Ar rhenius dependence on temperature, the activation energies for creep In ten sion, 715.3 +/- 22.9 kJ/mol, and compression, 489.2 +/- 62.0 kJ/mol, were t rot the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is cont rolled by the formation of cavities, The cavity volume fraction increased l inearly with increased tensile creep strain with a slope of unity. A cavita tion 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 simi lar to 18% of that in tension at 1.8% axial strain and approached zero at s trains <1%, The linear dependence of creep rate on applied stress is consis tent with a model for compressive creep involving solution-precipitation of Si3N4. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution-p recipitation creep of Si3N4 did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.