CYCLIC FATIGUE BEHAVIOR AND FRACTURE-TOUGHNESS OF SILICON-NITRIDE CERAMICS SINTERED WITH RARE-EARTH-OXIDES

The cyclic fatigue behavior and fracture toughness of silicon nitride ceramics sintered with selected rare-earth oxides were investigated to determine whether crystallization,of grain boundaries affected room temperature crack-growth behavior under cyclic and monotonic loads. Stable crack propagation under cyclic loads was observed in all compositions, and shown to be a distinct cyclic fatigue phenomenon rather than a manifestation of stress corrosion cracking in ambient air. Extensive wear debris on the fatigue fracture surfaces of the rare-earth oxide specimens provided evidence of a frictional wear crack-growth mechanism. Compact-tension fatigue tests yielded crack-growth rates which were power law dependent upon the applied stress-intensity range, with exponents between 15 and 31. Subsequent fracture toughness testing of fatigue-cracked specimens yielded values of the critical stress intensity K(c) of approximately 9 MPa square-root m. Comparison of fatigue crack-growth rates and fracture toughness values obtained in this study with those for a reference commercial silicon nitride (with amorphous grain-boundary phases) indicated that crystallization of grain boundary phases does not appear to degrade cyclic fatigue resistance or fracture toughness. This observation may be explained by considering that impurity-stabilized amorphous regions tend to be preserved at interfaces between adjacent silicon nitride grains and between silicon nitride grains and crystalline grain boundary pockets. Since these interfaces form the preferred crack propagation paths in both stable and fast fracture, the immediate crack tip environment may not be affected by crystallization of the bulk grain boundary phases.