Use of nanoindentation for the determination of visco-elastic properties of polymers

The aim of this application report is to demonstrate the application of spherical indentation on polymethylmethacrylate (PMMA). Several types of indentation experiments were performed on the PMMA in order to determine its visco-elastic behavior. Corresponding analytical model was applied to calculate its time-dependent mechanical properties.

For several decades, spherical indentation has been used for the determination of materials characteristics such as elastic modulus, hardness and elastic- plastic properties. One of the main reasons for the popularity of these measurements was that with a spherical indenter (see Fig. 1 for an SEM image) and sufficiently low loads essentially elastic deformations could be obtained. The contact problem can be solved relatively simply by solving well-known Hertz equations and the elastic constants of the material are easily calculated.

Such idea was very advantageous also for determination of visco-elastic properties of polymers, where low deformations would lead to suppression of irreversible flow of the material and the visco-elastic properties of the polymer could therefore be determined. This would be a great advantage over commonly performed indentation with Berkovich indenter, where immediately the limits of plastic flow are reached and the deformation is inevitably irreversible. Also, the stresses in many applications (e.g. damping components in civil and automotive engineering) susceptible to creep are relatively low and the viscoelastic deformations disappear sometime after unloading. Therefore, the stresses in the indentation tests for the determination of the viscoelastic properties should also be low.

This Application report deals with several aspects of low load spherical indentation including long hold periods and demonstrates a way to calculated viscoelastic properties from these measurements.

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