Effects of substrate on determination of hardness of thin films by nanoscratch and nanoindentation techniques
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Andreas A. Polycarpoua) Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
Thomas F. Conry Department of General Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 19 December 2003; accepted 16 March 2004)
A comparative study on the effects of the substrate on the determination of hardness of thin films by the use of the nanoscratch and nanoindentation techniques was conducted. Gold films deposited on fused quartz substrates and silicon dioxide films deposited on aluminum substrates with variant film thicknesses were investigated. These two systems correspond to a soft film on a hard substrate and a hard film on a soft substrate, respectively. The effect of substrate interaction on the measurement of hardness using the nanoscratch technique was found to be less pronounced compared to that of the nanoindentation technique due to: (i) the lower normal loads applied to achieve the penetration depths that occur at higher loads when using the nanoindentation method; (ii) the direct imaging of the residual deformation profile that is used in the nanoscratch technique, which allows for the effects of pileup or sink-in to be taken into account, whereas in the nanoindentation technique the contact area is estimated from the load-displacement data, which does not include such effects; and (iii) the account of elastic recovery of the plastically deformed surfaces from scratch tests. The film thickness did not appear to have any effect on the hardness of Au and SiO2 films obtained from nanoscratch data. This observation allowed, for the case of SiO2 films, the determination of the “free substrate effect region” and the derivation of an empirical relationship that relates the composite hardness of the film/substrate system to the contact-depth-to-film-thickness ratio, even when the indenter penetrates into the substrate. Such findings can allow for the determination of the intrinsic hardness of ultrathin hard films (∼1–5 nm thick), where the substrate effect is unavoidable. I. INTRODUCTION
Mechanical properties of thin films deposited on a supporting substrate are most often characterized by direct testing on the film layer using the load-depth sensing nanoindentation technique.1–3 However, such a technique has primarily been developed for monolithic materials3 and therefore does not take into consideration the interaction of a thin film with its substrate, which typically influences the measurement of the film intrinsic
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0233 J. Mater. Res., Vol. 19, No. 6, Jun 2004
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mechanical properties.4–13 As a rule-of-thumb, the indentation depth should not exceed 10–20% of the total film thickness to confine the plastic flow region (caused by the indenter penetration) within the film, and thus minimize or eliminate the effect of the substrate on the measured hardness.6–8 Therefore
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