Accurate determination of the mechanical properties of thin aluminum films deposited on sapphire flats using nanoindentations

Nanoindentations using a Berkovich diamond indenter have been made on 1, 2, and 5 mu m thick 99.99% purity polycrystalline aluminum films thermally evaporated in vacuum on to 2 mm thick R-cut polished sapphire flats. The projected contact areas of the residual indentations were estimated from the unloading load-displacement curves, and some of the indentations were imaged with an atomic force microscope (AFM). It was found that a large majority of indents showed material pileup, and the projected areas of these indents, as measured with the AFM, were up to 50% greater than those calculated from the unloading curves. This discrepancy between the calculated and directly measured indentation areas has a strong influence on the derived values of Young's modulus and hardness of the aluminum films. Using a new analytical 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 penetration. Finally, it has been suggested that in order to improve the accuracy with which the mechanical properties of thin films or bulk specimens can be determined by nanoindentation techniques, the projected contact areas should be measured by direct methods, such as atomic force microscopy.