Mechanistic investigation of plasma-assisted TiC formation and material transfer during electrical discharge coating using low-density additively manufactured electrodes

Electrical discharge coating (EDC) enables the synthesis of metallurgically bonded ceramic–metal composite coatings under ambient conditions through repetitive high-energy electrical discharges. However, the mechanisms governing phase formation, material transfer, and coating growth remain poorly understood. In this study, thick TiC-Fe composite coatings were synthesized on steel using a low-density additively manufactured (LD-AM) Ti6Al4V electrode. Optical emission spectroscopy (OES), debris characterization, and cross-sectional microstructural analyses were performed to investigate the proposed plasma-assisted and melt-pool-assisted TiC formation pathways and coating growth mechanisms. OES detected reactive Ti, Fe, and C species within the plasma channel, while XRD of recovered debris identified TiC among expelled material. Together, these observations support two proposed TiC formation pathways, i.e., plasma-assisted TiC formation within the discharge plasma (pathway 1), and melt-pool-assisted TiC formation within the substrate melt pool (pathway 2). EBSD and FEGSEM revealed a microstructural gradient across the coating thickness, from fine carbide-rich regions near the surface to dendritic and columnar structures near the interface. The LD-AM electrode produced coatings up to 133 μ m thick at an average deposition rate of 15 μ m/min, exceeding the performance of the wrought electrode. The maximum cross-sectional microhardness reached 1455 ± 169 HV0.05, representing an increase of approximately 489% relative to the substrate. Nanoindentation revealed localized hardness values approaching 39 GPa in TiC-rich regions. In addition, the representative polished coated surface exhibited an approximately fourfold reduction in corrosion current in 3.5 wt% NaCl solution compared with the bare substrate. The findings provide mechanistic evidence supporting the proposed TiC formation pathways during EDC and demonstrate the potential of LD-AM electrodes for producing thick, hard, and corrosion-resistant composite coatings under ambient conditions.