AFM nanomechanical properties and durability of new hybrid fluorinated sol-gel coatings

Microscopically homogeneous and thin fluorinated hybrid coatings showing significantly high mechanical and photochemical durability are prepared through the sol-gel chemistry and studied in this work. Their physico-chemical and nanomechanical properties are investigated. In this study, a special focus is given to the effects of organic/inorganic ratio and inorganic precursor type on surface and mechanical properties at the nanoscale. Different coatings containing low and high contents of silica as well as a mixed oxide composed of high amount of titania and low amount of silica are investigated. Nanoscratch and nanoindentation tests are performed by atomic force microscopy (AFM) and employed to assess the nanomechanical behavior of the hybrid coatings at a proper scale. Dynamic hardness results obtained from AFM are found to be in good agreement with technological pencil scratch tests performed at a macroscopic scale. Owing to the high inorganic content and good cross-linking degree achieved, coatings with a high amount of silica show considerably high dynamic hardness (nearly 2 GPa), which remains unchanged even after a long-term UV-vis light exposure. Notwithstanding the comparable inorganic amount, the hardness of mixed oxide coatings is markedly lower than high silica coatings and is subjected to modifications when exposed to UV-vis light in the long term, as also corroborated by x-ray photoelectron spectroscopy (XPS). Young's modulus results obtained by AFM nanoindentations appear to be dependent mostly on the inorganic/organic ratio. Higher moduli are determined for samples with higher inorganic/organic ratio, and no particular effect of the nature of inorganic precursor upon such property is observed.