Formation of zirconia coatings on ZrN-coated substrates by plasma electrolytic oxidation

Zirconia coatings exhibiting widely technological applications were produced by plasma electrolytic oxidation (PEO) on ZrN-coated Si substrates in alkaline electrolytes. Firstly, zirconium nitride films were deposited on the substrates by an air-based reactive sputtering technique. After that, the specimens were electrochemically oxidized in 2 M NaOH by applying voltages ranging from 42 to 55 V for 30 s. Spark discharge was observed over the specimens at the reaction voltages above 46 V. X-ray diffraction patterns show resultant coatings comprised mainly tetragonal ZrO2 with slightly monoclinic ZrO2. The existence of fully spark discharge, highly crystalline phases, porous morphology, and high growth rate characterized the PEO coatings. The highly conductive ZrN seedling layer could facilitate significantly the growth of the coatings. Three types of reaction regimes were identified, including electrochemical oxidation, transition of electrochemical oxidation to PEO, and PEO. Biocorrosion tests revealed that the obtained PEO coatings could improve greatly the biocorrosion resistance. The formation mechanisms of the PEO-produced ZrO2 coatings are also discussed.