An unexpected plasticization phenomenon and a constant of the change rate of viscoelastic properties for polymers during nanoindentation test

Abstract

The effect of loading force, loading rate and unloading rate on the viscoelastic behavior of three representative polymers: poly(methyl methacrylate) (PMMA, amorphous polymer), polyvinylidene fluoride (PVDF, semicrystalline polymer), and epoxy (crosslinked polymer) have been investigated using nanoindentation. The results showed that the maximum indentation depth increased with the increase of loading force, and the relationship between loading force and depth became linear when the loading force is beyond 3000 lN. At the beginning, the plasticity index changed substantially with the increase of loading force, and after reaching a critical loading force, the plasticity index almost remained constant. The maximum indentation depth decreased with the increase of loading rate, which followed a power law curve. With the increase of loading rate, a plasticization phenomenon happened, and a possible reason is that the heat may accumulate and raise the local temperature. The plasticity index initially followed the power law with the increase of unloading rate and then almost remained constant. A constant, the change rate of viscoelastic properties with the unloading rate, for the three representative polymers studied in this research, around À0.033, has been obtained, which may be another manifestation of the phenomenon that many polymers have similar time/temperature shifts and that their WLF equation constants are approximately the same.

Characterization of indentation size effects in epoxy

“…7 also depicts the effect of loading time on the hardness of epoxy. Consistent with the results reported in [32], the hardness decreases at large h with increasing loading time from 5 to 80 s. However, time dependence of the hardness markedly decreases with decreasing h (see [33e34] and references therein for time dependent behavior of polymeric materials during nanoindentation).…”

Indentation size effect of multiple orders of magnitude in polydimethylsiloxane

“…For the Berkovich indenter tip, this A S ( h ) is given as follows: where h is the indentation depth under the applied test force. The H U for each indentation test was calculated according to ISO 14557‐1,18 that is where F is the maximum applied force.…”

“…Indentation is a widely applied testing approach for polymers,16–18 and both nanoindentation and microindentation tests have been conducted on PDMS samples. The nanoindentations were performed on MTS NanoXP indentation systems in the XP mode (MTS Systems, Eden Prairie, MN), whereas larger depths were tested with a Fischerscope HM2000S microindenter (MI; Fischer Technologies, Windsor, CT).…”

Characterization of Properties of Poly(vinylidene Fluoride) Using Instrumented Microindentation Test

“…The instrumented indentation technique (IIT) allows the testing of micro and nano scale structures and the mapping of the mechanical properties of those regions, which are associated with the processing conditions and polymer morphology. [1] This characterization technique is commonly used to measure material hardness and can also be used to obtain information on mechanical properties related to hardness, usually the modulus of DOI: 10.1002/masy.202000136 elasticity. [2] In this test, an indenter is pressed onto the surface of a material to a predetermined depth or force.…”

“…[3] Since microindentation deals with small scale deformations, there are some factors that affect the measurements and would be negligible or constant in large deformation tests. The micromechanisms of plastic deformation depend on the specific local morphology of the material, [1] which allows the identification of microstructural variations.…”