Effects of soft sparking on micro/nano structure and bioactive components of microarc oxidation coatings on selective laser melted Ti6Al4V alloy

We used the soft sparking mode to fabricate microarc oxidation coatings containing micro/nano-scale pores and bioactive Ca/P components on Ti6Al4V alloy samples formed by selective laser melting. The effect of soft sparking on the microstructure and corrosion resistance of the coatings and the codeposition mechanism of the electrolyte anions in the soft sparking layer were investigated using different negative/positive charge ratios. The results showed that the negative charge significantly decreased the intensity of microarc discharge, resuling in the soft sparking phenomenon, which promotes the formation of a thin amorphous TiO2 layer at the barrier layer/substrate interface. In turn, this interface facilitated the electron tunneling and aggregation of the electrolyte anions, which leads to soft sparking at voltages significantly lower than the breakdown voltage of the anodic oxide film on the selective laser melted Ti alloys. Soft sparking promoted the codeposition of the electrolyte anions, including PO43− and Ca2+ ions, in the form of bulk amorphous phases in the coating formed by soft sparking. The soft sparking layer contained amorphous phases, which were corroded more readily than that the outer ceramic layer. However, the submicro/nano-scale pores in the soft sparking layer improved its corrosion resistance. With an increase in negative/positive charge ratio, the effect of soft sparking on the surface morphology of the MAO coating weakened, and the amounts of Ca and P codeposited in the soft sparking layer increased. Finally, the corrosion resistance of the coating was found to first increase and then decrease, reaching the maximum at the 1.1 ratio.