DYNAMIC FAILURE PROCESSES UNDER CONFINING STRESS IN AlON, A TRANSPARENT POLYCRYSTALLINE CERAMIC

An experimental technique is developed to impose a planar lateral confinement in a prismatic specimen (with rectangular cross-section); the setup enabled a controlled and homogeneous stress state with high lateral compressive stresses. A transparent polycrystalline aluminum oxynitride (AlON) specimen was used for the study. The statically pre-compressed specimen was then subjected to axial dynamic compressive loading using a modified compression Kolsky bar setup. Experimental design was performed using 3D computational modeling. Initial exploratory experiments were conducted on AlON at average planar confinement of 400 – 410 MPa; the results suggested a higher compressive strength and a nonlinear stress evolution in AlON due to the confinement. A high-speed camera was used to observe the damage evolution in the specimen during the course of loading. The photographs and stress evolution are suggestive of an additional inelastic deformation mechanism, evolution of which is slower than the typical brittle-cracking type of damage apparent in the unconfined case. The TEM and HREM analysis indicated dislocation plasticity in some fragments; dislocations in the slip bands were characterized to be dissociated <110> dislocations on {111} planes. The width between two partial dislocations was about 15 nm, suggesting low stacking fault energy of AlON. Microscopic characterization also reveals that the eventual fragmentation of AlON is by cleavage mainly along low-index {111} planes.