Fatigue resistant silicon nitride ceramics due to anelastic deformation and energy dissipation

To investigate the link between internal friction and fatigue resistance of sintered silicon nitride at elevated temperatures, impulse excitation, torsion pendulum and uniaxial tension-compression tests were performed. At low stress-amplitudes, an internal friction peak is observed and associated with the glass transition of amorphous intergranular phases (IGPs). This peak constitutes a reliable means of energy dissipation for otherwise brittle ceramics. The high amplitude uniaxial tests have revealed a new and much larger internal friction effect than previously could be expected from low amplitude test results. Rheological analysis indicates that the underlying deformation is truly anelastic. Within the investigated stress-amplitude range (30-150 MPa), the damping is shown to increase linearly with stress-amplitude. As a consequence, energy dissipation in a cyclically loaded component will increase locally at stress concentrations such as crack tips. This results in an increased crack propagation resistance, and explains earlier observations of a positive fatigue effect in sintered silicon nitride at high temperature.