Graded viscoelastic behavior of human enamel by nanoindentation

•Graded viscoelastic behavior of enamel is studied by nanoindentation.•Δh graded increases from the outer to the inner enamel layers.•Energy dissipation graded increases gradually across the enamel layers.

Graded viscoelastic behavior of enamel, including displacement deviation (Δh) between single- and multi-cycling nanoindentations and energy dissipation, is studied using nanoindentation. The imposed load of emerging Δh is about 160 μN, 90 μN and 40 μN, and the maximum value of Δh is 1.86±1.50 nm, 2.21±0.55 nm and 4.15±2.85 nm from the outer to the inner layers of enamel, respectively. In all three enamel sections, the hysteresis loop areas emerge from the third loading cycle, but perform different appearance. Due to the graded structure of enamel, a graded increasing viscoelasticity was found, which is essential to dissipating energy, and preventing cracks from propagating in enamel, therefore retaining the structural integrity of the tooth. The possible mechanism of enamel graded viscoelastic behavior might be the reversible unfolding and refolding of the protein layer.