*2.5. Mechanical Properties of the Coatings*

The external surfaces of the specimens were investigated with a nano-indentation apparatus (model ENT-1100a; Elionix, Tokyo, Japan). The specimens were fixed to the specimen stage with adhesive resin (Superbond Orthomite; Sun Medical, Shiga, Japan). Nano-indentation testing was carried out at 28 ◦C using a Berkovich indenter with a 10-mN peak load for ca. 1000 nm depth analysis (*n* = 10). Linear extrapolation methods (ISO Standard 14577) were used for the unloading curve between 95% and 70% of the maximum test force to calculate the elastic modulus [30–33]. The hardness and elastic modulus of the buccal enamel surfaces were calculated using the software bundled with the nano-indentation apparatus.
