Determining the viscoelastic parameters of thin elastomer based materials using continuous microindentation

Depth-sensing indentation (DSI) was used to evaluate the viscoelastic properties of three proprietary silicone-based coatings and four butyl-based rubber sheets. The coatings are used as layers in silicone bilayer coatings. The elastic indentation modulus, E∗, irrecoverable deformation energy and elastic energy were determined in an earlier study using the unloading data of DSI tests and applying a creep correction method proposed by Ngan et al. The model used in this study to characterize the coatings was the standard four parameter Maxwell-Voigt viscoelastic model combined with the elastic indentation load-depth model for a Berkovich indenter as proposed by Fischer-Cripps. This model fits the data quite well. The parameters E1∗, E2∗, η1, and η2 were found by curve fitting this model with the indentation depth versus time data while held at a constant maximum load. The parameter E1∗ is equivalent to E∗; values of E1∗ obtained using the Fischer-Cripps technique were 6.8 ± 0.9 MPa, 109 ± 7 MPa, and 4.2 ± 0.5 MPa for bond coat A, bond coat B, and the top coat, respectively. One purpose of this paper is to demonstrate a technique to determine viscoelastic parameters of silicone-based elastomers. It is hoped that these parameters may then be used in the evaluation and development of more durable silicone-based foul release coatings. Another purpose is to apply this technique to other elastomer-based materials such as butyl-based rubbers.