Nanosurface Mechanical Properties of Polymers Based on Continuous Stiffness Indentation

A systematic experimental study of the large strain surface mechanical properties of a number of polymers {polymethylmethacrylate (PMMA), polyetheretherketone (PEEK), polystyrene (PS), polycarbonate (PC), polypropylene (PP), and ultra-high molecular weight polyethylene (UHMWPE)} at the nanometer scale is described. The polymeric surfaces were indented and the data were analyzed using a contact compliance method in conjunction with a nano-indenter system. The indentation experiments were performed using a Berkovich Tip indenter with a continuous contact compliance indentation mode. The indentations were performed using a constant loading rate (300 μN/sec) to a maximum penetration depth of 5 μm. The experimental results showed a considerable strain-rate hardening effect for the polymers and a peculiarly harder response of these surfaces at the near-to-surface (submicron) layers. PMMA was the hardest polymer of the selection, whereas UHMWPE and PP were observed to be the softest polymers. The paper includes practical consideration of a creeping effect and appropriateness of tip calibration using harder surfaces for nanoindentation experimentation of polymers.