The effects of hydrostatic pressure and indentation strain on nanoindentation elastic modulus ( \({E}_{\text{s}}\) ) and hardness ( \(H\) ) were investigated in the five polymers poly(methyl methacrylate) (PMMA), polycarbonate (PC), polystyrene (PS), poly(ether ether ketone) (PEEK), and low density polyethylene (LDPE) using four different pyramidal probes with equivalent cone angle ( \(\theta \) ) = 47.7° (cube corner), 35.0°, 19.7° (Berkovich), or 10.0°. Both \({E}_{\text{s}}\) and \(H\) generally increased with \(\theta \) , with the strongest effects in the glassy polymers. Consistent with a hydrostatic pressure effect, the measured \({E}_{\text{s}}\) were 10–50% higher than conventional elastic moduli measured using bulk specimens. To remove this effect, \({E}_{\text{s}}\) versus \(H\) was extrapolated to \(H=0\) assuming a linear relationship. These extrapolated values were within 15% of the conventional values for all the materials except for LDPE, which showed anomalous behavior likely associated with its rubbery state. Hardness-derived flow stress \(({\sigma}_{\text{H}}\) )–representative strain ( \({\epsilon}_{\text{r}}\) ) curves were also calculated and compared to literature uniaxial compression stress–strain curves. \({\sigma}_{\text{H}}\) compared best to the literature for data for a Berkovich probe at an \({\epsilon}_{\text{r}}\) = 0.07. The empirical Tabor–Marsh–Johnson correlation used to calculate \({\sigma}_{\text{H}}\) does not work as well for these polymers at either higher or lower \({\epsilon}_{\text{r}}\) .
