Enabling Jet-Electrochemical Discharge Machining on Niobium-Like Passivating Metal and the Single Step Fabrication of Coated Microstructures

Abstract

Analogous to difficult-to-cut materials in mechanical machining, Niobium (Nb) is a difficult-to-dissolve metal by electrochemical machining (ECM) due to its prone-to-passivation nature. Jet-electrochemical discharge machining (Jet-ECDM), which uses anodic discharge rather than the cathodic discharge as in traditional ECDM, has been previously demonstrated as an efficient method for machining inert semiconductors like Si, 4H-SiC. However, applying Jet-ECDM on Nb-like passivating metal is very challenging because plasma electrolytic oxidation (PEO) is very prone to occur in the machining area due to anodic discharges, forming an insoluble, highly stable oxide coating that prevents material removal. This study reveals the principles of overcoming PEO to enable Jet-ECDM on metals. The findings show that the discharge intensity, affected by the electric field and pulse frequency, is decisive in achieving Nb material removal. The electrolyte chemistry significantly influences material removal fashions, depending on the electrolyte's chemical activity. NaOH is the preferred choice for higher removal efficiency (maximum 1.92 mm/min) and machining localization, while NaCl ensures better surface quality (Sa 99.4 nm). Moreover, the Jet-ECDM generated microstructure surface presents a thin layer of PEO coatings (<1 μm). This demonstrates the high potential of Jet-ECDM technology for efficient fabrication of coated microstructures in a single-step process.