Time Dependent Nanomechanical Response of Nacre

ABSTRACT Nacre, the shiny inner layer of seashells is a model biomimetic system composed of 95% of inorganic (aragonite) phase and 5% of organic phase (mainly proteins and polysaccharides). Nacre exhibits an interlocked layered “brick and mortar” structure where the bricks are made up of aragonite and mortar is the organic phase. We have performed nanoindentation and dynamic nanoindentation tests to study the nanomechanical and dynamic nanomechanical response of nacre. The indentation experiments performed at low loads indicate an elastic modulus of about 15 GPa for the organic phase. The low load, low penetration experiments appear to be better indicators of nanomechanical behavior. Dynamic nanomechanical response of nacre was studied using dynamic nanoindentation (nano-DMA). Significant increase in the values of tan δ was observed with increase in frequency. Also the dynamic nanoindentation experiments indicate that nacre exhibits viscoelastic behavior. Further, fourier transform spectroscopy experiments of nacre in innate and undisturbed state indicate the presence of water in nacre. The nanograin structure of nacre platelets, as well as the entrapped and adsorbed water, is two important contributors to the viscoelastic response of nacre. Atomic force microscopy experiments also indicate a very high force to remove organic from the aragonite in nacre. These experiments provide important insight into nanomechanical response of nacre, its constituents and also interfaces.