Petch Relation in Single‐Phase Oxide Ceramics

Detailed examination of recent strength-grain size data for MgO, Al2O3, and BeO ceramics showed that for large grain sizes behavior was of the form of the Knudsen equation: . For small grain sizes, behavior fitted the form of the Petch equation, or appeared similar but with negative slope. Three characteristic fracture-initiation modes are inferred, all based on the principles of the Griffith criterion and on a critical crack length c≃D: (1) true brittle or elastic failure, (2) initiation by plastic yield, and (3) initiation by viscous sliding of grain boundaries. Fracture initiation is viewed as the restart of crack growth which was previously interrupted at a grain boundary.