Study of protective hard coatings of SiO2-TiO2 on aluminum substrates

Aluminum alloys are frequently employed in the aeronautics industry due to the remarkable mechanical properties and lightweight nature of these materials. Moreover, thin film coatings are commonly applied in order to improve the corrosion resistance under harsh environments. In this work, Al 7075-T6 substrates were coated with nanostructured SiO2-TiO2 films using a sol-gel method. The experimental approach initially consisted in the preparation of a precursor agent using tetraethyl orthosilicate (TEOS) and triethoxy(octyl)silane (ETOS). Subsequently, nanoparticles of SiO2-TiO2 were mixed in order to develop thin films using a one-step dip coating method. The roughness, nanoindentation and corrosion properties were evaluated for the coated substrates. A finite element model was created for the nanoindentation test, which determined the mechanical response between the film -contact interface during loading conditions. The average hardness, elastic modulus and critical loads leading to fracture were verified. The nanoindentation test presented a significant increase in hardness for the coated Al 7075-T6 alloy, reaching a value of 4.6 GPa. The SiO2-TiO2 thin films presented uniform and compact surface coatings with high mechanical properties. Furthermore, the performed corrosion tests indicated moderate protection by the SiO2-TiO2 thin films. The SiO2-TiO2 thin films displayed a generalized corrosion throughout the surface, presenting oxides and fractured crystals in localized regions.