Nanoindentation of soft films on hard substrates: the importance of pile-up
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Nanoindentation is a common technique for measuring the mechanical properties of thin films. Here, we address the potential measurement errors caused by pile-up when soft films deposited on hard substrates are tested by nanoindentation methods. Pile-up is exacerbated in soft film/hard substrate systems because of the constraint the substrate exerts on plastic deformation of the film. To experimentally examine pile-up effects, aluminum films with thicknesses of 240 and 1700 nm were deposited on hard glass substrates and tested by standard nanoindentation techniques. The aluminum/glass system is interesting because the film and substrate have similar elastic moduli; thus, any unusual behavior in the nanoindentation results may be attributed to differences in the plastic flow characteristics alone. A detailed scanning electron microscopy examination of nanoindentation hardness impressions in the film revealed that common methods for analyzing nanoindentation data underestimate the true contact areas by as much as 80%, which results in overestimations of the hardness and modulus by as much as 80% and 35%, respectively. The sources of these errors and their influence on the measurement of hardness and elastic modulus are discussed, and a simple model for the composite hardness of the film/substrate system is developed. The model could prove useful in measuring the hardness and elastic modulus of soft-film / hard substrate systems when it is not possible to make indentations shallow enough to avoid the substrate influences.