Anisotropic wear behavior of human enamel at the rod level in terms of nanoscratching

In-situ nanoscratch experiments were carried out on occlusal surface and axial section (on which axis is perpendicular to rods and axis parallels to rods, respectively) of human enamel using a sphero-conical indenter (∼1 μm tip radius) at different normal loads. The elastic recovery and inelastic deformation during scratching process was evaluated in terms of three curves along the track: surface roughness, scratch depth, and post depth. Coefficient of friction (COF) and wear rate was addressed for the single rod. Results reveal that the amount of elastic recovery and inelastic deformation of a single rod on the axial section is more than that on the occlusal surface. At the enamel rod scale, COF depends on loads and locations. The wear resistance of a single rod on the occlusal surface is greater than that at direction of axis perpendicular to rods and axis parallel to rods. The load-dependence is attributed to the ploughing and pile-up. The location-dependence is ascribed to the angle change of normal loading direction, hydroxyapatite crystals orientation and the scratching direction. In addition, viscoelastic protein layers between the apatite crystals play an important role in the anisotropic wear behavior at microscale.