JOURNAL OF MATERIALS SCIENCE 38 (2 0 0 3 ) 2935 – 2944 Hot shock compaction of nanocrystalline alumina G. J. VENZ, P. D. KILLEN Centre for Medical, Health and Environmental Physics, Queensland University of Technology, Brisbane, QLD 4001, Australia E-mail: garyvenz@hotmail.com N. W. PAGE Department of Mechanical Engineering, University of Newcastle, Newcastle, NSW 2308, Australia An experimental investigation of hot shock compaction of a nanocrystalline alumina powder was performed. The effects of variations in shock pressure and compaction temperature on the properties of the compacted materials were studied. It was found that the bulk density and hardness of the compacted material increased with shock pressure. Increasing compaction temperature resulted in increases in compact hardness and bonding, and reductions in cracking within the compacted specimens. The results suggest that dense, well bonded, crack free nanocrystalline ceramics may be fabricated more effectively using hot shock compaction, than by room temperature shock compaction followed by sintering or room temperature static compaction followed by sintering. C  2003 Kluwer Academic Publishers 1. Introduction The properties of nanocrystalline ceramics have been shown to be superior to those of conventional micro- crystalline ceramics in a number of ways. These include increased ductility and plastic deformation, improved sintering behaviour, and increased strength and hard- ness [1–4]. It is difficult to produce bulk nanocrystalline ceramic materials using traditional powder processing techniques such as sintering, as these techniques often require the material to be exposed to elevated tempera- tures for extended periods of time. This leads to crystal growth within the material, thus destroying the desired nanocrystalline microstructure. One method which has been proposed for the production of nanocrystalline ce- ramics is shock compaction. Shock compaction is essentially a rigid die pressing technique in which consolidation is achieved by the propagation of a shock wave through the material. The shock wave may be initiated either by the detonation of an explosive charge in contact with or near the target material, or the impact of a high-speed projectile or flyer plate onto the target material. The projectile or flyer plate may be accelerated to the desired impact velocity by either the rapid expansion of a compressed gas, or detonation of an explosive. In the case of powdered materials, propagation of the shock wave through the material results in collaps- ing of the voids between particles, thus increasing the density of the material. As the powder is compressed, friction between the powder particles results in local- ized heating at the particle surfaces. After passage of the shock wave, heat is conducted from the surface to the interior of the particles resulting in an increase in temperature of the bulk material. The material then eventually returns to ambient temperature and pressure conditions. High energy deposition in the powders during com- paction leads to the possibility of obtaining very high densities (i.e., >90%) [5, 6]. This is highly desirable if post-compaction sintering is to be performed, as lin- ear shrinkage (and therefore warping and cracking) decrease with increasing green density, enabling in- creased accuracy in dimensional tolerances. Addition- ally, the sintering rate increases with increasing green density, reducing the sintering times and/or tempera- tures required [7]. The high energy deposition also al- lows for the compaction of powders which are normally very difficult to consolidate by conventional methods [6, 8]. The high rates of heating and subsequent cool- ing (10 8 –10 11 K · s −1 for metals) [9] which occur dur- ing the compaction process (i.e., densification, particle surface melting, resolidification, surface cooling) mean that grain growth may be minimised, whilst desirable metastable properties may be retained during and after compaction [5]. When consolidating metal powders, the tempera- tures and pressures established at the particle surfaces are usually sufficient to induce surface deformation or melting, which assist in densification and bond- ing of the powder particles. Ceramic materials, how- ever, usually have much higher hardness and melting points than metals. The pressure and temperature con- ditions produced during compaction are not normally high enough to induce surface deformation and/or melt- ing, and hence sufficient densification and interparticle bonding do not occur. (Meyers et al. [10] predicted the pressure required for room temperature shock consoli- dation of Al 2 O 3 powder to be approximately 23 GPa.) 0022–2461 C  2003 Kluwer Academic Publishers 2935