Dynamic nanoindentation as a tool for the examination of polymeric materials
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The determination of the mechanical properties of polymers is more complex than that of many other structural materials, because they display time-dependence in their response to load. Generally, dynamic analysis techniques, such as dynamic mechanical thermal analysis (DMTA), are used to characterize the viscoelastic properties of bulk polymers. However, polymers are increasingly being used as thin films, the properties of which are not readily determined using conventional techniques. Nanoindentation offers the possibility of determining the properties of thin films but has generally only been used to measure static properties. Dynamic nanoindentation equipment has recently become available, but its accuracy with soft polymers is unproven. This paper presents results of a comparison between dynamic nanoindentation, DMTA, and differential scanning calorimetry (DSC) for the determination of the thermal response of four different polymers. A favorable comparison is shown, indicating that dynamic nanoindentation is capable of measuring the time-dependent properties of small samples of polymers.
I. INTRODUCTION
Depth-sensing nanoindentation is a technique by which the mechanical properties of small volumes of material can be determined. In this technique, an indenter probe is placed in contact with the sample surface and then pushed into it. The resistance to indentation and the indent depth are continuously monitored throughout the experiment, and from this data a load:indentation depth trace can be obtained. The trace can then be analyzed using various fitting procedures to determine the Young’s modulus and the hardness of the sample. The most common fitting procedure is that described by Oliver and Pharr.1 Typically, indentation depths would range from less than 10 nm to greater than 100 nm, so the technique is capable of determining the properties over very small deformed volumes of material. Most nanoindentation studies have concentrated on relatively hard materials, including the characterization of diamond-like coatings and wear-resistant layers.2–5 It has also been used to probe differences in grain properties in metallic materials.5–6, More recently, nanoindentation has proven a valuable tool in the study of glassy polymers,5,7–12 although studies on rubbery polymers
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0437 3298
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 11, Nov 2004 Downloaded: 17 Mar 2015
are, to date, limited in number.10,11,13 This is because the study of soft, rubbery materials presents some difficulties, including the determination of the location of the surface, because of high specimen compliance, and also their time-dependent characteristics, as outlined below. Even those authors who have examined viscoelastic properties of polymers explicitly have often done so using glassy or high-modulus materials where viscoelasticity is limited. The high compliance of soft materials means that a considerable indent can be imp
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