Surface integrity studies on ZrB2 and graphene reinforced ZrB2 ceramic matrix composite in EDM process

Abstract

Zirconium diboride is a potential ceramic for high temperature applications because of its combination of properties, such as very high melting point, high hardness, elevated temperature strength, good thermal and electrical conductivity. Although the near net shape components are made from this material, machining is required to create additional features for the application requirements. This ceramic's high hardness and low fracture toughness make it very difficult to use the conventional processes for machining. The good electrical conductivity of this ceramic makes electrical discharge machining (EDM) the preferred machining process. This paper presents the surface and sub-surface modifications during Wire cut EDM (WEDM) and EDM of ZrB2 an ultra high temperature ceramic (UHTC) and ZrB2-GNP (ceramic matrix composite with 7 vol% Graphene Nano Platelets) along with the surface characterization of the copper electrode (EDM tool). Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopy (EDS) and Raman spectroscopy (for GNPs) were used to characterize the melt-solidified layer and the recrystallized region. The presence of micro cracks and pores was observed in the melt-solidified layer. Growing dendrites of ZrB2 were observed in the high magnification SEM images of the melt-solidified layer surface. Under the melt-solidified layer, a recrystallized fine grain structure was observed with reduced porosity compared to the rest of the bulk. After the WEDM of ZrB2-GNP (7 vol%), the stacks of cut GNP were retained, whereas GNPs were damaged after EDM The end surface of the copper electrode showed the partial deposition of ZrB2 and the formation of a micro scale pattern on it. The effect of peak current and pulse ON time on surface roughness (Sa), material removal rate (MRR) and relative tool wear (%) were studied for ZrB2-GNP (7 vol%) The results indicated that the surface roughness (Sa), MRR and the relative tool wear (%) increase with increasing peak current. The rise in pulse ON time also increases surface roughness (Sa) and MRR but reduces the relative tool wear rate (%)