Phenomena-based optimization of load function during nanoindentation experiments for estimating elastic aspect of viscous materials

Abstract

We propose an optimized load function designed to acquire the elastic aspect during nanoindentation for two viscous polymers, i.e., polycarbonate (PC) and poly (methyl methacrylate) (PMMA). To do so, various load functions are followed where constant strain rate (P˙/P) varies from 0.05 /s to 50.0 /s, maximum load (Pm) from 1000 μN to 9000 μN and unloading rate (P˙UL) from 9.0 × 102 μN/s to 107 μN/s. Depending upon the P˙/P, extent of viscous deformation varies during loading, e.g., slow P˙/P shows more viscous deformation due to sufficient loading time and vice-versa. However, for fast P˙/P, a large amount of viscous deformation is observed only when the tip decelerates at the end of loading since the deceleration zone is the favor of viscous deformation. During holding, it is seen that a large amount of viscous deformation takes place for fast P˙/P at any Pm. Because viscous deformation could not take place during loading due to insufficient time that is why it takes place during holding. In addition, at high Pm large amount of viscous deformation takes place than low Pm. Since it is observed that different time is required for various P˙/P and Pm to saturate the viscous deformation during holding; hence, a novel method is suggested to estimate the required time for any P˙/P and Pm indentation. During unloading, it is noticed that for fast P˙UL less amount of viscoelastic recovery takes place towards Pm, hence, it is recommended that 95%–40% and 95%–60% unloading range should be considered to evaluate the elastic aspect for PC and PMMA, respectively.