Investigation of consolidation mechanisms induced by applied electric/electromagnetic fields during the early stages of spark plasma sintering

Abstract

This work aims at investigating the role of the electric/electromagnetic fields generated by the pulsed current applied during Spark Plasma Sintering (SPS) process on the sintering mechanisms. The selected model material was a metallic granular medium composed of microsized particles of pre-oxidized copper. Its electrical behavior and correlated microstructural modifications were studied in two complementary enclosures. First, a demonstrator equipped with a modulable pulsed electric current generator has allowed analyzing, at low temperature and without mechanical loading, the effects related to the application of an electric wave, by controlling and modulating its characteristics (i.e. shape, frequency, amplitude). An abrupt electrical transition, named “Branly effect” from an insulating to a conductive state, is observed in the granular copper medium. The increase of pulses frequency is shown to strongly reduce the critical time to generate the electrical transition. Moreover, a commercial SPS device, with specific electrical and thermal instrumentation, was implemented by varying applied stress and using conductive and insulating dies. The analysis of the electrical behavior coupled with post-mortem microstructural observations allowed to highlight that the coupling between pulsed current and mechanical stress promotes specific mechanisms in SPS. Under high stress, interparticle contact area increase and the insulating oxide layer is damaged by microcracking. The coupling with the flow of the pulsed current involves local overheating and dielectric breakdown, favoring ignition of Branly effect. Micro-welds are formed between particles, creating privileged paths of the pulsed current and initiating densification at low temperature (250 °C).