Independent measurement of Young's modulus and Poisson's ratio of transparent thin films using indentation and surface deformation measurements

Abstract

Instrumented indentation is a common technique for measuring the elastic properties of thin materials, including elastomers, gels, and biological materials. Traditional indentation analysis yields a reduced modulus, which is a function of Young's modulus and Poisson's ratio, thus requiring one of the parameters to be estimated or independently measured to decouple the properties. It is difficult in some cases to know the true deformation of the surface due to substrate deformations, machine compliance, and thermal drift. To address these issues, a new technique is demonstrated in which 3D displacements are measured at discrete points along the surface of a transparent specimen during indentation tests using microscopy and fluorescent micrometer-scale particles embedded in the specimen. The out-of-plane displacements of the particles are measured using a defocused imaging technique, taking advantage of the change in spherical aberration ring radius with distance from the focal plane. A technique for tracking the motion of the particles and calibrating the system is described, and experimental measurements on a silicone elastomer are presented. Two optimization algorithms were developed to extract Young's modulus and Poisson's ratio from the experimental measurements. The first algorithm uses radial and normal displacements measured along the surface of the specimen. The second algorithm uses a combination of traditional indentation analysis and radial surface displacements. The elastic properties of polydimethylsiloxane (PDMS) were calculated from experimental data using both algorithms. The results from both methods were in agreement with each other, as well as with values of Young's modulus reported in the literature.