Nanoindentation as a method to interrogate the mechanical properties of polymer coatings

Abstract

Polymer coatings are widely used in many industrial applications such as coatings on car bodies and refrigerators, and as varnishes on floor coverings and wood. As protective coverings polymer coatings are subject to wear and degradation making their mechanical properties a key performance indicator. Mechanical properties of non-polymeric coatings can be successfully determined using nanoindentation. However, the time-dependent nature of the mechanical properties of polymers requires a different approach to that used for time-independent materials. Spherical nanoindentation using a ramp and hold load method and creep analysis was compared to tensile testing and has produced results that fully characterise the timedependent mechanical response. Using this method differences in mechanical properties between different polymers as well as the changes in mechanical properties due to degradation and aging were distinguished. In conventional instrumented indentation tests contact areas are calculated from the measured depth based on Hertzian contact mechanics. Finite element analysis has suggested that spherical indentation is indeed Hertzian for visco-elastic materials. Direct observations of the contact area of visco-elastic materials under load were made simultaneously with depth measurements by indenting transparent polymers at a macro-scale. This novel approach suggested that for some polymers spherical indentation can be non-Hertzian. It appears that the ramp load times as well as surface properties contribute to the non-Hertzian contact. Consequently, moduli obtained from nanoindentation tests may not always be directly comparable to moduli obtained from tensile tests. These results will support the development of standard nanoindentation test procedures for visco-elastic polymers.