Deformation behavior of normal human enamel: A study by nanoindentation

Tooth enamel has an important mechanical function for human dental health, yet characterizing its mechanical properties is not trivial due to its complex nanoporous structures. We examined the distribution of hardness and modulus across the lingual-buccal enamel cross-section by nanoindentation. At the occlusal surface, the hardness and modulus of enamel were found to be 5.00 +/- 0.22 GPa and 97.12 +/- 2.95 GPa, respectively. At the area close to the enamel-dentine-junction (EDJ), the hardness and modulus were 3.72 +/- 0.35 GPa and 76.83 +/- 5.71 GPa, respectively. At the middle region in between the EDJ and the outer enamel layer, the hardness and modulus were found to be 4.23 +/- 0.18 GPa and 87.62 +/- 2.50 GPa, respectively. The surface and area underneath the nanoindent were analyzed using the following microscopy tools: Scanning Electron Microscopy, Focused Ion Beam imaging, and Transmission Electron Microscopy. The deformation mechanisms of enamel were found to be location dependent and influenced by changes in the chemical composition within enamel. The EDJ forms the interface between enamel and dentin. The deformation behavior differed at the EDJ, due to the increased organic phase at the interface. Within the intermediate enamel region, intra-rod cracks were formed at the center of enamel rods and propagated into the neighboring inter-rod region at deviated directions along the orientation of the local crystallites. At the outer enamel layer, crack propagation was constrained by the rigid structure surrounding the indented site. Most of the cracks were formed close to the surface. A significant amount of material was also pushed upwards and delaminated from the enamel surface of the indentation area.