Fabrication of ultra-low porosity plasma electrolytic oxidation coating on Ta-12W alloys and its formation mechanism

The sealing of pores in plasma electrolytic oxidation (PEO) coatings is critical to further enhance the protection of the underlying substrate and maintain the mechanical performance of coatings. This study focuses on the modification of the surface microstructure of PEO coatings on Ta-12W alloy by introducing NH4F additive into a mixed silicate-phosphate electrolyte. As a result, ultra-low porosity coating with a surface porosity of less than 1 % is effectively prepared. Detailed investigations have been conducted to study the effects of NH4F content and treatment duration on various coating characteristics, including microscopic morphology, elemental distribution, thickness, and phase composition. It is found that optimizing the concentration of NH4F eliminates the large pore structures and promotes the development of Ta-rich structures resembling pancakes on the coating surface. Furthermore, the intensities of diffraction peaks from different crystal faces of Ta2O5 in coatings exhibit variations, and selective growth is observed specifically on its (110) crystal plane. After a 20-minute treatment of Ta12W alloys in the electrolyte containing 4 g/L NH4F, the surface of the coating displays predominantly pancakelike structures with sealed pore/non-pore features. The formation process of these specific structures is discussed based on the evolution of coating morphology with oxidation time. It is proposed that the increase in anodic voltage and gas production in the late stage favors the breakdown of gas bubbles, leading to the creation of nonporous pancake-like structures on the surface. In addition, the breakdown of the dielectric layer at the periphery of gas bubbles results in the influx of film-forming materials into pores, which causes the formation of pancakelike structures with sealed pores.