| Shock-conditioning of ceramic particulates--first introduced in 1966 by Bergmann and Barrington--is known to introduce excess internal energy (plastic deformation), as evidenced by residual strain and crystallite size reduction, and subsequently to enhance sinterability. These effects have been reconfirmed in this study of a fine, pure alumina oxide powder. However, the main sequence experiment undertaken in this work, spanning the nominal shock pressure range 5.1-10.7 GPa, clearly shows, apparently for the first time, the adverse consequences of bimodal morphology resulting from overcompaction during shocking. In overcompacted powders, uncontrolled grain growth occurs during the late intermediate and early final stages of sintering, resulting in very coarse-grained microstructure. This research has been directed toward identification and optimization of the parameters affecting the process of (1) shock conditioning, and (2) intermediate (post-shock) processing of the powders. In a broader sense, it has also been directed toward identifying the mechanisms responsible for shock-induced enhancement of sinterability of ceramic materials.;In this paper, the experimental findings are reviewed; the adverse effects of excessive shock compaction on the densification kinetics and microstructural development are considered; shock-induced morphological alteration and/or aggregation of the powders is examined; competing thermally-activated, rate-sensitive, morphology-dependent processes (recrystallization, grain growth) which could impede sintering and alter final microstructures are identified; and based on controlled atmosphere precision-digital dilatometry, some examples of enhanced sinterability, due to shocking conditioning per se (reasonably free of confounding morphological effects) are presented.;A well-known and sinterable grade of alumina (CR-10 Al(,2)O(,3), from Baikowski International) was chosen as a high-purity research grade material ((TURN) 99.98% Al(,2)O(,3), (TURN) 10 m('2)/g, 0.13 (mu)m). It was subjected to nominal plane strain shock conditioning in precompacted, steel encapsulated disk form by an explosively-driven flyer plate. After removal from the recovery fixture, the shocked powders were comminuted and reconstituted as small (9.5 mm diameter) cylindrical compacts (D(,0) (TURN) 0.62). In supporting studies of statically-compacted powder, pressure was applied hydrostatically (to 5.5 GPa, static) using a modified belt type high pressure apparatus (courtesy of NIRIM, Japan). |
