Abstract

Soft-spark micro-arc oxidation (MAO) is considered a potential solution for eliminating the porosity on the surface coating of titanium alloys during the MAO process. However, the mechanisms behind soft-sparking and the growth of coatings on Ti alloys are not yet fully understood. This study aimed to investigate the growth pattern of soft-spark MAO coating on titanium alloys by using sodium silicate as the electrolyte and manipulating the soft-sparking duration. The research focused on analyzing the positive and negative pulse voltage-time response curves, surface micromorphology, surface composition, cross-sectional structure, phase composition, dynamic potential polarization curve, microhardness, and thickness growth curve during the soft-spark process. The research findings revealed that soft spark discharges occurred in the plasma atmosphere near the surface of the external porous layer. The sparks penetrated the weak points of the amorphous interface layer (existing between the MAO coating and the substrate), forming filamentary discharges within the coating. This phenomenon led to the formation of a significant amount of rutile phases and bubble pores within the soft-spark coating, promoting the expansion of soft-sparks into the coating and facilitating the growth of the soft-spark coating. While the inner hardness of the soft-spark MAO film reached 1146 HV, the plasma atmosphere discharge also led to the formation of numerous filamentary channels and gas pores within the coating. Consequently, the coating's corrosion resistance did not show any improvement.

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