Nano-indentation of polymeric surfaces

This paper presents results of normal hardness, plasticity index and elastic modulus for a selection of organic polymers (a poly(methylmethacrylate), PMMA, a poly(styrene), PS, a poly(carbonate), PC, and an ultra-high molecular weight poly(ethylene), UHMWPE) obtained using the contact compliance method. The paper describes in detail the dependence of the imposed penetration depth, the maximum load and the deformation rate upon the hardness and elastic modulus values for these polymeric surfaces; typical penetration depths range from about 10 nm to 10 mu m where the imposed loads are less than 300 mN. The results show a considerable strain-rate hardening effect for the present systems and possibly a peculiarly harder response of these materials at the near-to-surface (submicron) layers. The paper includes considerations of a practical nature which are drawn in order to overcome some intrinsic limitations of this technique when it is used for polymeric surfaces, especially for a creeping phenomenon which may be observed at the incipient unloading experimental segments. The appropriateness of using a tip calibration constructed upon hard substrates when indenting polymers is reviewed at the conclusion of the paper.