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.