Microstructural Alterations in Alumina Ceramics Associated with Ballistic Impact Events

Surface topography and defect structures associated with ballistic impact in a commercial 94 percent alumina ceramic armor are described. Fracture surfaces were examined by scanning electron microscopy, augmenting previous replication electron fractography studies. Direct transmission electron microscopy was employed in characterizing defect structures in extracted fracture fragments on replicas and in thin foils taken from bulk polycrystalline alumina in undamaged and damaged states. A variety of dislocation interactions and tangled masses of 11 -2 dislocations with densities as high as 2 * 10 cm have been observed near intersecting surfaces in thin fracture fragments. Similar deformation textures within the bulk have been observed in impact-deunaged alumina for a distance of at least 20 ym beneath a fracture surface. Localized plastic flow is most prominently evidenced in the case of cracks which are intersecting at acute angles to create fracture fragments, and in worked zones abutting propagating cracks in more massive material. These observations stro igly support the concept that portions of the high stresses concentrated at the tips of propagating cracks during an impact event are relaxed by plastic deformation processes, thereby absorbing excess energy and/or delaying the fracture process to some degree.