Extracting the mechanical properties of a viscoelastic polymeric film on a hard elastic substrate
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K.Y. Zeng Institute of Materials Research and Engineering, Singapore 117302, Singapore (Received 20 May 2004; accepted 9 July 2004)
A semi-analytical approach was developed to study the creep response of a viscoelastic polymeric thin film on a hard elastic substrate under flat-ended punch indentation. This approach allows one to separate mechanical properties of the film from its substrate. To verify the approach, flat-ended punch creep indentation tests were performed to obtain the long-time viscoelastic behaviors of a polymethyl-methacrylide film overlying on an aluminum substrate. Three viscoelastic models describing the constitutive equations of the polymer film are compared in this paper. “Film-only” viscoelastic parameters were extracted by fitting with the experimental creep curves of flat-ended punch indentation. It is shown that the present approach provides an effective way to characterize the mechanical properties of a polymeric film on an elastic substrate.
I. INTRODUCTION
There are two major challenging issues in determining mechanical properties of a viscoelastic polymeric thin film on a substrate by indentation. The first issue is due to the influence of the substrate on the mechanical behaviors of the thin film, causing difficulties to extract “film-only” parameters. The second issue is due to the influence of the viscoelasticity of the polymeric film itself, causing the load–displacement curves to depend not only on the loading time but also on the loading or unloading rate. A possible way to deal with the first issue is to limit the indentation depth to less than 10% of the film thickness.1 However, this can only alleviate the substrate effect but cannot completely eliminate it. In addition, when the thickness of the film is less than the radius of indenter, in particular, when the film thickness approaches nanometer level,2 its implementation becomes difficult. Several methods have been proposed to extract viscoelastic parameters of bulk polymeric materials from the load-depth curves obtained by indentation.3–11 However, to extract the thin film properties of a polymeric thin film/substrate system from the load–depth curves is still a challenging issue. Flat-ended punch indentation has been used to study
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0382 J. Mater. Res., Vol. 19, No. 10, Oct 2004
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the viscoelasticity of polymers.3,6,8,12–14 From the viewpoint of contact mechanics, the flat-ended punch indentation is the simplest compared with those by other indenters, such as conical, spherical and Berkovich indenters since the contact area is given directly by the geometry of the punch end and remains constant during indentation. The elastic solution of an elastic layer overlying a semi-infinite elastic substrate under flat-ended punch indentation was obtained by Yu et al.13 The solutions of flat-punch indentation on bulk viscoelastic solids in the two- and three-dimensions were given in R
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