Creep Deformation of Polymers Using Nanoindentation

As viscoelastic materials, polymers often undergo a time-dependent deformation under a certain applied load, also known as creep. Creep becomes a critical factor when the polymeric parts are designed to be exposed to continuous stress, such as structural components, joints and fittings, and hydrostatic pressure vessels.

The inherent nature of viscoelasticity plays a vital role in the performance of polymers and directly influences their service reliability. The environmental conditions such as loading and temperature affect the creep behavior of the polymers. Creep failures often occur due to the lack of alertness of the time-dependent creep behavior of the polymer materials used under specific service conditions. As a result, it is important to develop a reliable and quantitative test of the viscoelastic mechanical behaviors of the polymers. The Nano module of the NANOVEA Mechanical Testers applies the load with a high-precision piezo and directly measures the evolution of force and displacement in situ. The combination of accuracy and repeatability makes it an ideal tool for creep measurement.

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Eight different polymer samples were tested by nanoindentation technique using the NANOVEA PB1000 Mechanical Tester. As the load linearly increased from 0 to 40 mN, the depth progressively increased during the loading stage. The creep was then measured by the change of indentation depth at the maximum load of 40 mN for 30 s.

The load vs displacement plot of the nanoindentation tests on different polymer samples is shown in Fig. 1 and the creep curves are compared in Fig. 2. The hardness and Young’s modulus are summarized in Fig. 3, and the creep depth is shown in Fig. 4. As an example in Fig. 1, the AB, BC and CD portions of the load-displacement curve for the nanoindentation measurement represent the loading, creep and unloading processes, respectively.

Delrin and PVC exhibit the highest hardness of 0.23 and 0.22 GPa, respectively, while LDPE possesses the lowest hardness of 0.026 GPa among the tested polymers. In general, the harder polymers show lower creep rates. The softest LDPE has the highest creep depth of 798 nm, compared to ~120 nm for Delrin.

The creep properties of the polymers are critical when they are used in structural parts. By precisely measuring the hardness and creep of the polymers, a better understanding of the time-dependent reliability of the polymers can be obtained. The creep, change of the displacement at a given load, can also be measured at different elevated temperatures and humidity using the NANOVEA PB1000 Mechanical Tester, providing an ideal tool to quantitatively and reliably measure the viscoelastic mechanical behaviors of polymers in the simulated realistic application environment.

In this study, we showcased that the NANOVEA PB1000 Mechanical Tester measures the mechanical properties of different polymers, including hardness, Young’s modulus and creep. Such mechanical properties are essential in selecting the proper polymer material for intended applications. Delrin and PVC exhibit the highest hardness of 0.23 and 0.22 GPa, respectively, while LDPE possesses the lowest hardness of 0.026 GPa among the tested polymers. In general, the harder polymers exhibit lower creep rates. The softest LDPE shows the highest creep depth of 798 nm, compared to ~120 nm for Delrin.

The NANOVEA Mechanical Testers provide unmatched multi-function Nano and Micro modules on a single platform. Both the Nano and Micro modules include scratch tester, hardness tester and wear tester modes, providing the widest and most user-friendly range of testing available on a single system.