The Tribological Behavior of Nanocrystalline TiO2 Coating Produced by Plasma Electrolytic Oxidation

Nanocrystalline TiO2 coatings were produced on titanium substrates using the plasma electrolytic oxidation technique. The effects of frequency, duty cycle, and type of applied current (pulse and direct) were evaluated on the microstructure of the coatings and the tribological behavior of the samples. Morphological evaluations demonstrated that the pancake structure was developed from coatings created with a unipolar pulsed current. However, a volcano-like surface morphology resulted from a constant current. The XRD analysis results showed that the coatings were composed mainly of the rutile phase by 77.80-96.34 wt. %. In comparison, 22.20 wt. % of the anatase phase was identified in samples produced with direct current. These phases were determined to be nanocrystalline (29.5-48.3 nm), which led to significant improvements in the tribological properties. The sample produced with direct current had larger pores, greater roughness, and a four-times higher thickness than samples created with unipolar current. Furthermore, the tribological study results showed that wear resistance was significantly higher in the unipolar pulsed current coatings than in those obtained with direct current. Moreover, samples made at a higher frequency and lower duty cycle showed better tribological behavior.