Investigation of cyclic and frequency nanoindentation effects in polydimethylsiloxane

The nanoindentation response of polydimethylsiloxane (PDMS) is examined using single nanoindentation loading and small-scale fatigue. It is well known that viscoelastic material response is inherently related to the local loading and environmental conditions. First, quasistatic nanoindentation experiments were performed at various depths through the specimen to benchmark our nanoindentation results with literature data. The PDMS cyclic and frequency dependence to quasi-static and dynamic nanoindentation loading was studied and a ‘load/partial-unload’ technique was employed to investigate nanoindentation modulus variation through the thickness of the specimen. The frequencies of the small-scale fatigue tests were varied to study periodic response. The average indentation modulus for PDMS at 2mN load-controlled tests was 4.37 ± 0.1 MPa. The PDMS sample had an average indentation modulus value of 3.94 ± 0.06 MPa for 3mN load-controlled tests. The indentation moduli decreased as the maximum depth increased because the stiffness reduced when indentations were performed further from the surface. The single nanoindentation data was confirmed with literature values and validated the precision of nanoindentation testing. Small-scale fatigue tests were implemented at 50 cycles with frequencies of 1, 0.5, and 0.033 Hz. The lower frequencies displayed an increase in maximum depth at a given controlled load due to relaxation and creep effects. As with the single nanoindentations, the small-scale fatigue tests confirmed the decreasing trend of indentation moduli as the maximum depth increased. Overall, the two nanoindentation methods corroborated similar trends in changes of the PDMS mechanical response.