Critical issues in measuring the mechanical properties of hard films on soft substrates by nanoindentation techniques
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This study explores the difficulties encountered when using conventional nanoindentation techniques to measure the Young's modulus and hardness of hard films on soft substrates. In general, the indentation measurement of film/substrate systems is affected by four material properties: the Young's modulus and hardness of the film, and the Young's modulus and hardness of the substrate. For the particular case of a hard film on a soft substrate, there is a tendency for the material around the hardness impression to sink-in which results from the large difference in yielding of the two materials. In this study, a `model' system consisting of NiP on annealed Cu was used to explore the behavior. This system is interesting because the film and substrate have similar Young's moduli, minimizing the elastic behavior as a variable. In contrast, the hardness of NiP is approximately 7-8 GPa, and that of the annealed copper is less than 1 GPa, providing a factor of 10 difference in the plastic flow characteristics. Experimental results indicate that standard analytical methods for determining the contact depth, hardness and Young's modulus do not work well for the case of a hard film on a soft substrate. At shallow contact depths, the measured indentation modulus is close to that of the film, but at larger depths sink-in phenomena result in an overestimation of the contact area, and an indentation modulus which is less than the Young's modulus of both the film and substrate. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) provide critical details of the physical processes involved, and illustrate how the standard data analyses overestimate the true contact area.
