Defining the limits to long-term nano-indentation creep measurement of viscoelastic materials

An ultra-stable instrumented nano-indentation tester (UNHT, Anton Paar) was used to study extremely long (30,000 s) indentation creep of polymers. Total drift rate, measured on fused silica and sapphire samples, was less than 0.2 pm/s for up to 10 h, enabling collection of valid, low-uncertainty, long-term creep data - orders of magnitude longer than previously possible. Fits of the popular N-element Kelvin model to indentation creep data gave values of instant elastic modulus E0∗ and infinite modulus E∞∗ strongly dependent on the time-span of data fitted. Comparison with the long-term experimental data showed that the model was unable to use short term data to predict creep at the much longer times required by industry. A new analysis method is proposed to obtain a better estimate of the true value of infinite modulus, E∞∗, which is the simplest indicator of maximum dimensional change of polymeric material components subject to long-term stress.