The Importance of Type of Ti-based Additives on the PEO Process and Properties of Al2O3-TiO2 Coating

Al2O3-TiO2 coatings were obtained from a silicate-based electrolyte using pulsed bipolar current by PEO process. Nano-particle titania (NP-TiO2) and potassium titanyl oxalate (PTO) were used as Ti-based additive sources in the PEO electrolytic solution, separately. The coatings were characterized using scanning electron microscope, energy dispersive spectroscopy, and an X-ray diffractometer. The mechanical properties of the coatings were investigated using nanoindentation and ball-on-disk tests. SEM results showed that the PTO developed a more compact inner layer besides the increase of coating thickness. However, NP-TiO2 created an inner layer with less thickness but with higher compactness, without any effect on the outer layer morphology. XRD and Raman spectroscopy analyses showed that the NP-TiO2 had inert incorporation into the alumina. However, TiO2 produced by PTO had reactive incorporation into alumina and made a polymeric titanium oxide structure on the coating with doped rutile and anatase phases. Nanoindentation and tribology analyses approved that the Ti incorporation through PTO provides appropriate mechanical properties owing to the more compact and thicker inner layer. The mechanism of PTO performance in the PEO process was discussed regarding its effect on coating characteristics.