A nanoscale investigation on the influence of anodization parameters during plasma electrolytic oxidation of titanium by high-resolution electron energy loss spectroscopy

• Published in: Applied surface science 2021-12, Vol.570, p.151133, Article 151133
• Main Authors: Casanova, Luca, Arosio, Mattia, Taghi Hashemi, Mohammad, Pedeferri, MariaPia, Botton, Gianluigi A., Ormellese, Marco
• Format: Article
• Language: English
• Subjects: Barrier layer; Discharge; EELS; PEO; Structural evolution
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2021.151133

[Display omitted] •EELS combined with advanced data analysis enhances process understanding.•Use of DC regime favors growth of TiO and Ti2O3 in contact with the substrate.•Pulsed PEO homogenizes the oxide stoichiometry favoring Ti+4 oxide bearing phases.•Cathodic polarization and high frequency promoted development of type-B plasma.•Plasma events generally cause oxide vaporization in the form of TiO molecules. High-resolution electron energy loss spectroscopy (EELS) in a transmission electron microscope is performed on titanium oxide coatings obtained by plasma electrolytic oxidation (PEO) with application of different electrical parameters. Core loss spectra were used to evaluate the structural evolution occurring at the two main regions characterizing a PEO coating, i.e. barrier and porous layers. Local crystalline information, extracted from EELS, was correlated to macroscopic technological parameters such as duty cycle and frequency, based on advanced data analysis. Using the spectral differences, structural maps are, for the first time, provided for titanium oxide grown anodically. Cathodic current was found to favour the growth of a mainly crystalline barrier layer, responsible for abundant O2 evolution during the treatment. A detailed mechanism regarding the stimulation of type-B discharges, when using cathodic polarization at high frequency, is given comparing outcomes from optical emission spectroscopy and structural information. As the result of the intense plasma interaction with the growing layer, the structure evolved towards the formation of 18 nm of titanium oxide characterized by a strong Ti+3 fingerprint, followed by 85 nm of Ti3O5 formed according to high temperatures and the de-oxidative condition encountered.