Analysis of nanoindentation curves in the case of bulk amorphous polymers

Abstract

Abstract Load versus displacement curves obtained from nanoindentation experiments on polymers are particularly difficult to analyze due to their complex mechanical and especially time-dependent behavior. The indentation curves obtained on two amorphous polymers (polycarbonate (PC) and polymethyl methacrylate (PMMA)) show an increase in the indentation depth during load hold time. A non-linear time-dependent model shows that this phenomenon could be decomposed in a first part due to a viscoelastic response and a second part assigned both to a viscoelastic and a viscoplastic response. If the first part is not achieved, the values of the elastic modulus computed using the Oliver and Pharr method are abnormally high. If it is achieved, the beginning of the unloading curve presents an abnormality that is linked to the increase in indentation depth during hold load time. Nevertheless, if the load hold time is high enough and if the contact stiffness is calculated from the initial slope of the power fit of the unloading curve, the values of the elastic modulus computed using the Oliver and Pharr method are close to those obtained by tensile tests.