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In instrumented indentation tests for a thin film coated on a substrate (film/substrate composite), it is well known that the substrate-affected contact area estimated through conventional approximations includes significant uncertainties, leading to a crucial difficulty in determining the elastic modulus and the contact hardness. To overcome this difficulty, an instrumented indentation microscope that enables researchers to make an in situ determination of the contact area is applied to an elastoplastic film on substrates having various values of their elastic moduli. Using the indentation microscope, the substrate-affected indentation contact parameters including contact hardness of the film/ substrate composites are determined directly as well as quantitatively without any undesirable assumptions and approximations associated with the contact area estimate. The effect of a stiffer substrate on the contact profile of impression is significant, switching the profile from sinking in to piling up during penetration, and resulting in the substrate-affected contact hardness being highly enhanced at deeper penetrations. Through the present experimental study, it is demonstrated that the instrumented indentation microscopy is highly efficient in determining the substrate-affected elastoplastic contact parameters of film/substrate composite systems. I. INTRODUCTION

Indentation contact mechanics, when used to make a quantitative assessment of the mechanical properties of a thin film coated on a substrate, is not a fully developed science, and a number of problems, issues, and difficulties remain unsolved. The key issue will be the establishment of a quantitative procedure for correctly evaluating the substrate effect, and then extracting the film-only mechanical properties from the contact parameters of film/substrate composites. Theoretical models have been proposed and experimental studies have been conducted to examine the substrate effects in elastic, elastoplastic, as well as in viscoelastic regimes,1–14 although rigorous test techniques/analyses have not yet been established. Prior to an accurate estimate of film-only properties, it is always required to determine in a quantitative manner the substrate-affected contact area Ac of film/substrate composites. Among several empirical techniques and approximations/assumptions for estimating the contact

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Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr_policy DOI: 10.1557/JMR.2009.0244

1950

http://journals.cambridge.org

J. Mater. Res., Vol. 24, No. 6, Jun 2009 Downloaded: 13 Mar 2015

area Ac of homogeneous bulk materials, the Oliver-Pharr and the Field-Swain elastic approximations have been the most utilized.15,16 These elastic assumptions do not take account of the effect of piling up of plastic impression profile. Accordingly, these elasti