Exceptional contact elasticity of human enamel in nanoindentation test

Tooth enamel has unsurpassed hardness and stiffness among mammalian tissue structures. Such stiff materials are usually brittle, yet mature enamel can survive for a lifetime. Understanding the nanoscale origin of enamel durability is important for developing advanced load-bearing biomaterials. Here, nanoscale exceptional contact elasticity of the human tooth enamel, based on nanoindentation tests, is reported.

Spherical indenter tips with radii of 243 and 1041nm were used to determine stress-strain curves of enamel. Force-displacement curves were recorded using quasi-static loading strain rates of 0.031, 0.041, and 0.061s^-1. The storage moduli from a superimposed signal amplitude (dynamic strain at 220Hz) embedded during primary quasi-static loading and from quasi-static elastic theory were simultaneously measured. Modulus mapping was considered to be an extremely low quasi-static loading strain rate indentation test.

The elastic limits were 7-9GPa and 5-6GPa for the small and large indenters, respectively. The elastic-plastic transition point and elastic modulus value increased with substantially increased quasi-static loading strain rate. The results suggested that the increase of the elastic limit during high-loading strain was associated with exceptional contact elasticity at the nanoscale of the enamel structure and the consequent extension of the contact area (i.e., a temporary pile-up response, dependent on the enamel nanocrystals and protein glue).

Structural modification at this scale effectively prevents the initiation of cracking from localized strain, thus reinforcing the bulk structure. These results may provide valuable insight for conceptualizing bio-inspired nanocomposites.