Yy. Lim et al., Accurate determination of the mechanical properties of thin aluminum filmsdeposited on sapphire flats using nanoindentations, J MATER RES, 14(6), 1999, pp. 2314-2327
Nanoindentations using a Berkovich diamond indenter have been made on 1, 2,
and 5 mu m thick 99.99% purity polycrystalline aluminum films thermally ev
aporated in vacuum on to 2 mm thick R-cut polished sapphire flats. The proj
ected contact areas of the residual indentations were estimated from the un
loading load-displacement curves, and some of the indentations were imaged
with an atomic force microscope (AFM). It was found that a large majority o
f indents showed material pileup, and the projected areas of these indents,
as measured with the AFM, were up to 50% greater than those calculated fro
m the unloading curves. This discrepancy between the calculated and directl
y measured indentation areas has a strong influence on the derived values o
f Young's modulus and hardness of the aluminum films. Using a new analytica
l model, Young's modulus of the aluminum films has been determined to be in
the range of 50-70 GPa, independent of the relative indentation depth. The
composite nanohardness of the 1 and 2 mu m thick films was found to have a
load-independent value of 1 GPa, whereas the composite nanohardness of the
5 mu m film decreased from 1 to 0.7 GPa with increasing indenter penetrati
on. Finally, it has been suggested that in order to improve the accuracy wi
th which the mechanical properties of thin films or bulk specimens can be d
etermined by nanoindentation techniques, the projected contact areas should
be measured by direct methods, such as atomic force microscopy.