Chemical and mechanical characterization of TiO ₂/Al ₂O ₃ atomic layer depositions on AISI 316 L stainless steel

In this work, four different nanometric mono and multi Al ₂O ₃ and TiO ₂ layers have been applied on polished AISI 316 L Stainless Steel substrates by Atomic Layer Depositions (ALD) in order to improve their intrinsic corrosion resistance. The coating morphology has been investigated using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Thickness analysis has been performed using AFM and Glow Discharge Optical Emission Spectrometry (GDOES). Compositional analysis has been performed using GDOES and EDXS. The corrosion protection offered to the substrate by the different coatings has been investigated using polarization curves. All four different coatings appeared to be perfectly adherent to the substrate with a thickness close to the nominal value of 100nm. Compositional analysis confirmed the presence of mono/multi layers of TiO ₂ and Al ₂O ₃ on the surface of the samples, even if only qualitative data have been obtained by GDOES. Vickers indentations used to evaluate coating adhesion showed that TiO ₂ mono layer seems to have the lowest adherence between the different ALD coatings, while the four-layer configuration showed complete adherence at low indentation loads. Polarization curves showed that all ALD coatings improve the natural corrosion resistance of AISI 316 L Stainless Steel. The three configurations containing an Al ₂O ₃ layer showed the higher corrosion protection, with corrosion current densities lower than 10 ^-10A/cm ² and wide passive regions, in the order of 1V, while uncoated AISI 316 L had a passive region in the range of 0.4V.