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datapile-up do not in account for theon extra contact area produced by the pile-up [18-21],for thefilm enhancement of soft films hard substrates has important consequences property measurement accuracy. In particular, the hardness and the elastic modulus are determined from nanoindentation load-displacement data using the relations 12]: H-

Pmax

(1)

A and •-•

•

Ei

(2)

where, Pmax is the peak indentation load, A is the projected indentation contact area, • is a constant depending on the indenter geometry, S is the experimentally measured contact stiffness, V is Poisson's ratio of the specimen, and vi and Ei are Poisson's ratio and the elastic modulus of the indenter. Thus, the accuracy with which the hardness and modulus can be measured depends on how well the contact area is known, since the area appears explicitly in both equations. In most nanoindentation methods [2,3], the contact area is measured by evaluating an empiricallydetermined indenter shape function at the contact depth, hc, where the contact depth has been estimated from the indentation load-displacement data using a procedure based on a solution for the indentation of an elastic half-space by a rigid punch [22]. However, since the solution is only elastic, contributions due to plasticity and pile-up are inherently ignored, and this leads to an underestimation of the contact area and thus an overestimation of the hardness and modulus in materials which pile-up [20,21 ]. In order to gain a better understanding of the importance of pile-up in nanoindentation mechanical property measurement of soft films on hard substrates, an experimental investigation was undertaken to explore the pile-up behavior of a model material system. Finite element simulations were also performed to gain physical insight into the pile-up behavior as well as to explore the importance of the work-hardening characteristics of the film. In this paper, important observations and their implications for nanoindentation mechanical property measurement accuracy are presented and discussed. EXPERIMENTAL PROCEDURE The model system used in the investigation was high purity aluminum deposited on glass. In addition to a large difference in hardness (the hardness of the film is in the range 0.5-1.0 GPa while that for the substrate is 7.0 GPa), an equally important consideration in the choice of the system was the similarity of the elastic moduli of the two components. The modulus of aluminum is 70 GPa, while that for the glass used in the study is 57 GPa, as measured by nanoindentation methods. The fact that the moduli are similar minimizes the role that a filndsubstrate modulus difference would play in determining the indentation load-displacement behavior, thus simplifying the interpretation of results. The films were sputter-deposited to three different thicknesses, tf- 240 nm, 650 nm, and 1700 nm, and indented to various penetration depths, h, with a sharp Berkovich diamond indenter. The load-displacement data obtained at each depth were analyzed by the method of Oliver and Pharr [2