Apparent depth‐dependent modulus and hardness of polymers by nanoindentation: Investigation of surface detection error and pressure effects

Abstract

Nanoindentation is a widely used technique to characterize the mechanical properties of polymeric materials at the nanoscale. Extreme surface stiffening has been reported for soft polymers such as poly(dimethylsiloxane) (PDMS) rubber. Our recent work [J. Polym. Sci. Part B Polym. Phys. 2017, 55, 30-38] provided a quantitative model which demonstrates such extreme stiffening can be associated with experimental artifacts, for example, error in surface detection. In this work, we have further investigated the effect of surface detection error on the determination of mechanical properties by varying the sample modulus, instrument surface detection criterion, and probe geometry. We have examined materials having Young's moduli from 2 MPa (PDMS) to 3 GPa (polystyrene) using two different nanoindentation instruments (G200 and TI 950) which implement different surface detection methods. The results show that surface detection error can lead to apparent large stiffening. The errors are lower for the stiffer materials, but can still be significant if care is not taken to establish the range of the surface detection error in a particular experimental situation. We have also examined the effect of pressure beneath the probe on the nanoindentation-determined modulus of polystyrene with different probe geometries.

Glass Transition Phenomenon for Conjugated Polymers

“…For instance, it has been reported that for a variety of polymers, the cooling rate dependence of T g exhibits the following relationship: dT g / d log q ≈ 3 K . Also, the T g shows a pressure dependence, which increases with the pressure by 0.3 °C per MPa …”

Effect of bias voltage on the tribological and sealing properties of rubber seals modified by DLC films

“…According to Qian et al [22], the smaller error in modulus and hardness calculated by Oliver and Pharr method is strongly associated with larger nanoindentation depth (> 800 nm) in polymer, because of the incomplete recovery after unloading to zero. It can be seen that the maximum depth of indentation on every sample was obviously different.…”

Water‐assisted mechanical testing of polymeric thin‐films

“…One of the main inaccuracies of nanoindentation is the quantification of the initial contact event and the contact area, which is penetration-depth dependent, thus obfuscating the measured modulus and hardness. 58 Meanwhile, it is challenging to detect the surface especially for soft systems. 59 Next, the effect of the substrate starts to play a role when the penetration depth exceeds 10%-25% of the film thickness.…”

“…There are still multiple limitations for the nanoindentation method to measure thin‐film samples. One of the main inaccuracies of nanoindentation is the quantification of the initial contact event and the contact area, which is penetration‐depth dependent, thus obfuscating the measured modulus and hardness 58 . Meanwhile, it is challenging to detect the surface especially for soft systems 59 .…”