The working principle of dry micro-electro-discharge machining of vertically aligned carbon-nanotube forests is investigated by evaluating the effect of oxygen on the process. The machining experiments with controlled oxygen/nitrogen ratios indicate a correlation between the peak current of discharge pulses and the oxygen concentration, suggesting not only a vital role for oxygen in the process, but also a removal mechanism fundamentally different from that in typical electro-discharge machining based on direct melting and evaporation of the sample material. The highest surface quality and uniformity in the machined forest microstructures as well as smooth machining without short circuiting are achieved at an approximate oxygen concentration of 20% under the discharge condition of 30 V and 10 pF, revealing that air is an optimal medium for the removal process. Elemental and molecular analyses show no evidence of significant crystalline deterioration or contamination in the nanotubes processed with the technique.

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