Effects of ultrasonic vibration on the silver deposition in powder mixed EDM for antibacterial surface modification

Previous studies have demonstrated that silver nano-powder mixed electrical discharge machining (PMEDM) is a potential technique for concurrent ablative shaping medical implants and generating antibacterial layers on the surfaces. PMEDM modified surfaces with deposited silver offer excellent antibacterial properties along with very good biocompatibility. However, for medical applications, PMEDM is still facing challenges providing sufficiently uniform distribution of deposited silver and preventing spattered layers that might lead to delamination from the modified surface. It was indicated that ultrasonic vibration (UV) has remarkable effects on the fluid dynamics in the spark gap, which positively affect the silver distribution and reduce the generation of spattered layers. Consequently, in this study, a Ti6Al4V workpiece is vibrated with an ultrasonic frequency of 22.3 kHz and an amplitude of 2.5 μm during the PMEDM process. The effect of vibration direction in X and Z axis on the silver deposition is evaluated while discharge energy is varied up to 125 μJ. For comparative analyses also PMEDM without vibration assistance is investigated. Ti6Al4V hollow tool electrodes with an outer diameter of 3 mm and an inner diameter of 2.5 mm are utilised for internal flushing with a pressure of 4 bar. A hydrocarbon-based dielectric fluid is mixed with silver nanopowder at a concentration of 15 g/l. Results show that UV assisted PMEDM increases the content of deposited silver up to 1.7 times compared to PMEDM without vibration. UV with vibration in Z direction and pulse energies of 55 μJ and 125 μJ results in uniform silver distribution and significantly decreases spattered layer formation, whereas the use of vibration in X-direction leads to negative effects. By carefully selecting machining conditions such as vibration direction and amplitude, wall thickness of hollow tool electrode and powder concentration, PMEDM provides surfaces with promising characteristics for medical applications regarding silver deposition and antibacterial properties.