Discharge-induced graphitization of monocrystalline diamond: A spark erosion approach for BMD and non-conductive MCD

Abstract

Monocrystalline diamond (MCD), a superhard material widely used in various industrial applications due to its exceptional mechanical, physical, and chemical properties, is considered a key material for next-generation technologies. To enable cost-effective machining of MCD, a discharge-induced graphitization method using electrical discharge machining (EDM) is proposed. A resistance-capacitance pulse-width modulation (RC-PWM) discharge power source is designed to support this process. By adjusting the duty ratio of the discharge pulse train, the RC-PWM power source delivers the required working energy, allowing MCD to be machined at its breakdown voltage while minimizing excessive thermal damage. An instantaneous voltage detection method is introduced to autonomously regulate the wire-electrode feed-rate, effectively preventing short-circuits and electrode breakage. The electrical discharge processability (EDP) of MCD is evaluated through the spark erosion rate (SER) achieved with the RC-PWM discharge power source. During the machining of boron-doped monocrystalline diamond (BMD), the formation of a graphite deposition layer and discharge debris enables continuous EDM of otherwise non-conductive MCD. Experimental results show that BMD and non-conductive MCD can be machined at a discharge frequency of 300 kHz, with optimal pulse-on-time ratios of 30 % and 40 %, respectively. Four material removal mechanisms: erosion, vaporization, vaporization with erosion, and ablation are identified in BMD. In contrast, only vaporization and ablation occur in non-conductive MCD due to the absence of conductive charge carriers. This study presents a cost-effective and technically promising approach for the EDM processing of non-conductive diamond materials, with potential for technological autonomy and future commercialization.