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

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.