*2.3. Mechanical Properties of the Coatings*

The mechanical durability of the coatings is a very important criterion for industrial applications. The mechanical stability of the prepared SiO2-TiO2 coatings is depicted in Figure 9. The bare PC substrates get easily scratched at the applied force of less than 4.2 mN. For the coating prepared from 7 vol% of SiO2 in TiO2, the coating material was removed at the applied force of ~16.1 mN (Figure 9a), whereas this force was decreased to 12.2, 10.9 and 9.8 mN for the coating prepared from 10, 15 and 20 vol% of SiO2 in TiO2 (Figure 9b–d), respectively. The smooth, crack-free morphology and lower surface roughness on the coating resulted in the stable and adhesive coating on the PC substrate. Even rubbed by the fingers, the coating material was not easily removed. This is ascribed to the formation of an intermediate layer connecting the TiO2 nanoparticles and the silica network. However, in the case of the coating prepared from higher concentration of SiO2 in TiO2, due to significant cracks and increased surface roughness, the coating material could be removed at relatively lower applied force.

**Figure 9.** Adhesion test performed on the coatings prepared from (**a**) 7; (**b**) 10; (**c**) 15; and (**d**) 20 vol% of SiO2 in TiO2.
