Nanoindentation creep behavior of enamel biological nanocomposites

Creep behaviour and mechanical properties of developing and mature porcine molar enamels in natural, lyophilized and burnt states were investigated by an AFM-attached nanoindentation system. The natural premature enamel has nearly 3 to 15 times more creep than mature enamel because of abundant organics and water contents. The natural mature enamel has the balanced mechanical properties, i.e., relatively high modulus and hardness with viscoelastic/viscoplastic deformation ability. After lyophilization, the mature enamel and those at late maturation stage had a significant increase in creep, while those at the transition stage with higher organics content showed slight but not statistically significant increase. A heat treatment to 600 degrees C destroyed most organics and led to the crystal growth of HAp crystallites in enamel mineral. As a result, the modulus and hardness of the burnt samples were significantly increased, while the ability of viscous deformation was compromised. A small amount of soft organic matrix between the nanoscale building units of hard mineral is thus one of the important factors for the stress transfer, as well as the viscous deformation. More significantly, water seems to play a vital role for the organics to fulfil its function and is essential in the design of the natural biological nanocomposite to resist external force. This study can provide a deeper understanding of the relationship between enamel's composition, structure and mechanical performance.