Nano-Indentation for Characterizing Mechanical Properties of Soft Materials

We have attempted to apply the computer-based finite element analysis (FEA) method to accurately measure the mechanical properties (e.g., hardness and elasticity) of a soft material by an indentation test. First, an axisymmetric model has been developed using commercially available FEA code ANSYS. The FEA model consisted of a thin Al-film resting on Si-substrate. A spherical indenter has been used to indent the Al-film, which traveled a predefined depth during the loading and unloading cycles. First, numerical simulations were conducted to get the force vs. displacement plot, which was later used to determine the modulus of elasticity and hardness of Al-film. The effects of substrate modulus and indentation depth were thoroughly investigated to determine the modulus and hardness of Al-film. The effect of friction, considered at the interface of indenter and Al-film, was found to offer minimum impact for relatively small indentation depth. The induced force on the Al-film by the indenter has been found to be higher with increasing indentation depth when friction was considered. However the contact stiffness, represented by the slope of the unloading curve, has been found almost the same with and without considering friction. The variation of substrate modulus has been found to be ineffective to capture the Al-film modulus for relatively small indentation depth. However for higher indentation depth, the substrate modulus has been found to offer profound effect to capture the film modulus. The hardness of the Al-film has also been found to be relatively unaffected with variation of substrate modulus. However, the hardness of the Al-film has been found to be higher with friction for relatively high indentation depth. Results obtained from this preliminary research are important to continue further investigation and to characterize the mechanical properties of other soft-materials, e.g., biofilms to minimize its detrimental effects and utilize its favorable aspects in industrial and biomedical applications.