FABRICATION OF TRANSPARENT GAMMA-AL2O3 FROM NANOSIZE PARTICLES

The compaction and heat-treatment behavior of nanosize gamma-Al2O3 pow der (average diameter = 20 nm) was studied. A diamond anvil high-press ure cell was used to compact the powder at pressures up to 3 GPa, both in air at room temperature and under liquid nitrogen, followed by pre ssureless heat treatment at 800-degrees-C. For all conditions studied, the fabricated compacts were optically transparent. X-ray diffraction confirmed retention of the gamma-phase. The compacts were also charac terized before and after heat treatment by microhardness measurements and by transmission electron microscopy. For both ambient and cryogeni c compaction, sample hardness increased with pressure, and heat treatm ent resulted in about a 50% increase in hardness independent of the in itial green-state value. Samples compacted in LN2 were significantly h arder (up to 9.6 GPa) than those compacted in air. TEM examination rev ealed a random-dense-packed particle structure and interconnected poro sity; interstitial void dimensions, however, were always less than the average particle diameter (20 nm). Observed effects on the increase i n hardness could not be explained by microstructural changes normally attributed to increased compaction pressure or heat treatment, most no tably densification. Alternative explanations are proposed.