Nanoindentation of Polymers with a Sharp Indenter

A five-step indentation scheme is proposed to extract the elastic and viscoelastic properties of polymeric materials using a sharp indenter. In the formulation, analytical solutions to the elastic-viscoelastic deformation based on the concept of “effective indenters” proposed by both Pharr and Bolshakov [Understanding nanoindentation unloading curves. J. Mater. Res.17, 2660 (2002)] and Sakai [Elastic recovery in the unloading process of pyramidal microindentation. J. Mater. Res.18, 1631 (2003)] were derived. Indentation experiments on polymethylmethacrylate following the five-step scheme were performed. The elastic-viscoelastic parameters were extracted by fitting the solution based on Sakai’s effective indenter to the experimental results using a genetic algorithm. It was found that the solution based on Sakai's effective indenter was able to correctly extract the elastic properties. Based on this prediction and the experimental results, Pharr and Bolshakov's effective indenter profile could be determined. The extracted elastic-viscoelastic parameters using the solution based on Pharr and Bolshakov's effective indenter were independent of the reloading levels.