Nanoindentation method for determining the initial contact and adhesion characteristics of soft polydimethylsiloxane
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Dehua Yang Hysitron Inc., Minneapolis, Minnesota 55344
Wole Soboyejoya Princeton Institute for the Science and Technology of Materials (PRISM) and Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544 (Received 1 December 2004; accepted 14 April 2005)
In this paper, we present a method for determining the initial contact point and nanoindentation load–indentation depth characteristics for soft materials. The method is applied to the prediction of the load–indentation depth characteristics of polydimethylsiloxane. It involves the combined use of Johnson–Kendall–Roberts and Maugis–Dugdale adhesion theories and nonlinear least squares fitting in the determination of the initial contact point, the transition parameter, and the contact radius at zero contact load. The elastic modulus and the work of adhesion are also extracted from the load–indentation depth curves.
I. INTRODUCTION
In recent years, there has been significant interest in the measurement of the mechanical properties of soft materials relevant to micro-electronics1 and biomedical systems.2,3 In most cases, depth-sensing indentation (DSI) and atomic force microscopy (AFM) techniques have been used to extract the basic mechanical properties and adhesion characteristics.3–6 However, the interpretation of the measured load–displacement profiles has been complex, due to the surface interactions that occur between the tips and the surfaces, and the nonlinear deformation characteristics of soft materials. It is important to determine the initial contact so that we can transform the translated displacement into actual separation or indentation depth.7,8 In the literature, there are some methods to determine the zero separation for AFM contact mode tests.8 However, most of them are for hard material systems or have some other restrictions such as the need for extra measurements.8 Fitting approaches based on non-adhesive contact mechanics models with a zero offset parameter were also used in the determination of zero contact in DSI nanoindentation systems.5,9 However, adhesive interactions have to be taken account when they become
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0256 2004
J. Mater. Res., Vol. 20, No. 8, Aug 2005
significant. Although fitting adhesive contact mechanics models, such as Johnson–Kendall–Roberts (JKR),10 Maugis–Dugdale (MD),11 and extended viscoelastic JKR12 models, to contact data is well recorded in the literature,13–15 determination of the zero contact using those adhesive contact models is not well described. In this paper, the initial contact in a nanoindentation test will be determined using a fitting method that establishes the initial contact position as a point corresponding to zero indentation depth in adhesive contact mechanics models. Here, we use such a distinction between “displacement” and “indentation depth.” This latter term should be used only for the displacement starting from the initial contact position, rather t
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