Formation of Alpha-Al2O3 Coatings on Tungsten Substrate by Plasma Electrolytic Oxidation

Oxide coatings formed by plasma electrolytic oxidation (PEO) of tungsten substrate for 10 min in a phosphate alkaline electrolyte (PAE, 2 g/L Na3PO4·12H2O) with an addition of 2 g/L, 3 g/L, and 4 g/L NaAlO2 were investigated by SEM/EDS and XRD. In PAE + 2 g/L NaAlO2, a weakly crystalline coating is formed, consisting of amorphous Al2O3, the triclinic phase of WO3, the cristobalite phase of AlPO4 and the gamma and alpha phases of Al2O3. Strong micro-discharges during PEO in PAE with the addition of 3 g/L and 4 g/L NaAlO2 lead to the crystallization of amorphous Al2O3 into gamma-Al2O3 and alpha-Al2O3 phases. The coating formed in PAE + 4 g/L NaAlO2 is well crystallized and rich in alpha-Al2O3, which makes it suitable for high-temperature applications. To explain the composition of the formed coatings and the transformation of the amorphous Al2O3 into gamma and alpha phases, we followed the change in morphology, thickness, chemical and phase composition of the coatings during PEO in PAE + 4 g/L NaAlO2.