Effect of Viscous Behavior on Young’s Modulus Estimation of Thermoplastic Glassy Polymers via Nanoindentation

We have determined Young’s modulus ( \(E_{N}\) $E_N$ ) value via nanoindentation experiments for thermoplastic glassy polymers; polycarbonate (PC) and poly(methyl methacrylate) (PMMA). Four types of load functions are followed to examine the viscous behavior effect on unloaded data which affects \(E_{N}\) $E_N$ estimation. It is seen that the viscous deformation during unloading is ~ 10% for PMMA and ~ 3.5% for PC with a no-hold + slow-unloading rate and by following a fast unloading rate it reduces to < 1%. The viscoelastic recovery is less for a fast unloading rate. We have estimated the elastic stiffness ( \(S\) $S$ ) value via hold + fast-unloading rate data to eliminate the viscous deformation and minimize the viscoelastic recovery. We show that the fitting range of unloaded data from which \(S\) $S$ value is estimated, is not universal to avail \(E_{N}\) $E_N$ equal to tensile modulus ( \(E\) $E$ ). Our results show that the fitting range depends on the applied load, choice of the load function, contact area estimation, phenomenological correction factor, and viscous behavior of the material.