Electrochemical micromachining of high-strength metallic glasses

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High strength bulk metallic glasses are very attractive for applications in micro-devices under high mechanical load or devices with defined functional surface microstructures. For surface structuring of those glasses on the micro- and submicro-scale advanced electrochemical techniques are regarded as very perspective. Aim of the proposed project is to conduct fundamental studies for the evaluation of the potential of micro-electrochemical machining methods using metal tip electrodes for selected Zr- and Fe-based glasses. Fundamental studies of the electrode behaviour of the alloys in aqueous and methanolic working electrolytes enabled the analysis of the complex anodic reaction processes. Zr-based glasses showed characteristic transpassive breakdown behaviour and repassivated gradually. There was no significant effect of the alloy composition and of the structural state (amorphous vs. crystalline) detectable. The possibility of micro-machining with ultra-short transpassive voltage pulses was demonstrated, but so far only irregular hole structures could obtained. By the identification of a more suitable electrolyte a better control of the transpassive local dissolution is aimed to be reached. The electrode behaviour of the softmagnetic Fe-based alloy is principally determined by the single phase amorphous structure and the high P content which both enable a homogeneous strong passivation. It is characterized by a gradual passive-transpassive transition and by a fast repassivation. In particular in methanolic sulphuric acid by micro-machining very homogeneous smooth surface microstructures, hole and line models, could be realized. Thereby, the amorphous structure was revealed to be advantageous for homogeneous dissolution and repassivation. Studies by means of high resolution method are planned to analyze the machined rim areas in more detail regarding possible selective dissolution phenomena and related local crystallization. In the applied last project year the focus will be set on the generation of complex amorphous microstructures/microparts based on the knowledge gained so far. Horizontal undercutting is a unique possibility of the electrochemical machining method and will be used to produce free-standing micro-lamellas and microparts for magnetic actuator applications with very high precision. The complex electrochemical machining processes of multi-component glassy alloys will be described in relation to structural and composition-dependent materials characteristics.