Material removal by spalling during EDM of ceramics

Abstract This paper introduces the concept of a rapid rough-machining regime for die-sink electrical discharge machining (EDM) of advanced ceramics, offering potentially high rates of material removal and improved process efficiency. Whereas the conventional material removal mechanisms of spark erosion involve melting, dissociation and evaporation, the authors have established that prolonged high-energy discharges (in this paper as arc-related discharges) promote areas of spalling in which surface layers of ceramic material are released as large flakes by thermal-shock induced fracture. This paper describes tests on sialon–TiN and SiC–TiB2 composites. No catastrophic fracture of these materials was observed under arcing conditions, clearly demonstrating their robustness. Flakes of material typically up to several hundred microns across and one hundred microns thickness were isolated from captured debris, often segmented by vertical cracking. Sectioning of the eroded work-piece revealed shallow sub-surface cracking which ran parallel to the machined surface, typically following its profile at near-constant depth and which usually limited the extent of vertical cracking to the layer of material above. Subject to optimising electrode pulsation and delivering controlled, randomly distributed high energy discharges, this regime offers the potential for a reliable and fast “fissile planing” technique in which material is removed in shallow layers.