Effects of the substrate on the determination of hardness of thin films by the nanoscratch and nanoindentation technique
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Effects of the substrate on the determination of hardness of thin films by the nanoscratch and nanoindentation techniques: A comparative study for the cases of soft film on hard substrate and hard film on soft substrate. Noureddine Tayebi1,2, Andreas A. Polycarpou1 and Thomas F. Conry2 1 Department of Mechanical and Industrial Engineering, the University of Illinois at UrbanaChampaign, Urbana, IL 61801 2 Department of General Engineering, the University of Illinois at Urbana-Champaign, Urbana, IL 61801 ABSTRACT Hardness values of Au/Fused Quartz and SiO2/Al systems, which correspond to the cases of soft film on hard substrate and hard film on soft substrate, were measured using both the nanoindentation and nanoscratch techniques. The effect of substrate interaction on the measurement of hardness using the nanoscratch technique is found to be much less pronounced compared to that of the nanoindentation technique. Such reduction in substrate effect is attributed to the features used in the nanoscratch analysis: (a) direct imaging of residual profile allows for the effect of pile-up/sink-in to be considered, (b) lower normal loads applied as compared to the nanoindentation, (c) effect of elastic recovery of the plastically deformed surfaces is included in the nanoscratch analysis, whereas the nanoindentation analysis is based solely on the load-displacement data. Moreover, experiments with residual scratch depths as shallow as 3 nm are used to estimate hardness of thin films; a promising indication for the use of such technique in the measurements of ultra-thin films. INTRODUCTION Mechanical testing of thin films on a supporting substrate is most often performed directly on the film layer using the nanoindentation technique [1]. However, the interaction of a thin film with a substrate will typically influence the measurement of its intrinsic mechanical properties. As a rule-of-thumb the indentation depth should not exceed 10-20% of the total film thickness in order to minimize or eliminate substrate effects [2]. Accurate estimates of hardness are almost impossible to attain for thin films with a thickness less than 20 nm since extremely shallow indentations, on the order of a few nanometers, produce inconsistent results due to instrument limitations. In this study, the nanoscratch technique [3, 4] is applied to thin films to compare the estimates of hardness with the results obtained by nanoindentation, and to assess the effect of substrate interaction on the estimate of hardness of a thin film. Scratches are generated using a finite-radius diamond tip that traverses along the thin-film surface for a distance of several microns with a prescribed vertical load profile. This technique utilizes measurements of the normal force, the lateral force, and the residual cross-sectional (plowing) area of the resulting scratch. Furthermore, since data are obtained over several to a hundred of micrometers of lateral range, the nanoscratch technique has more of an averaging effect among grains, grain boundaries, and surface
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