A method for making substrate-independent hardness measurements of soft metallic films on hard substrates by nanoindenta
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C.A. Ross Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Mass Avenue, 13-4005 Cambridge, Massachusetts 02139
G.M. Pharr Department of Materials Science & Engineering, The University of Tennessee and Oak Ridge National Laboratory, 434 Dougherty Engineering Building, Knoxville, Tennessee 37996 (Received 30 October 2002; accepted 11 March 2003)
A new method for making substrate-independent hardness measurements by nanoindentation techniques that applies to soft metallic films on very hard substrates is presented. The primary issue to be addressed is substrate-induced enhancement of indentation pileup and the ways it influences the indentation contact area. On the basis of experimental observations of soft aluminum films deposited on silicon, glass, and sapphire substrates, an empirical relationship was derived that relates the amount of pileup to the contact depth. From this relationship and the associated experimental observations, a method was developed that allows the intrinsic hardness of the film to be estimated, even when the indenter penetrates through the film into the substrate. The method should prove useful for very thin films ( 1), the actual hardnesses are generally less than those predicted by the nanoindentation measurements because the nanoindentation analysis ignores the influences of pileup on the contact area.25,26 For depths less than the film thickness (hmax/tf < 1), the values of Hactual are essentially independent of depth in the range 0.8–1.0 GPa, in contrast to the slowly rising hardness observed in the nanoindentation measurements. This difference, which is consistent with previous observations,15 indicates that the substrate properties do not pose a significant influence on the film hardness when the actual area is used in the calculations. In this context, it is interesting to note
that the slight rise in the nanoindentation hardnesses is not real but an artifact due to the depth dependent pileup. It is also notable that the actual film hardnesses are close to the nanoindentation hardnesses at penetration depths of about 10% of the film thickness, implying that the 10% rule appears to work well in these systems. IV. A NEW METHOD OF ANALYSIS
On the basis of the observations presented in Sec. III, it is possible to develop a new method for measuring the hardness of soft films on hard substrates that accounts for the influences of the substrate on pileup. The method may be used to estimate the intrinsic hardness of the film even when the indenter penetrates through the film into the substrate. Since parts of the method are empirically based on the data in Fig. 3 and the fit to this data given by Eq. (1), it formally applies only to soft aluminum films on hard substrates. However, it may also prove useful for other soft film materials such as gold and copper on similar hard substrates. The method is based on a simple model for the composite hardness of a soft film on a hard substrate proposed by Tsui, Oliver, and Pharr.27 The model, which is shown
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