Morphology and Wear Behaviour of Single and Multi-layer Electrical Discharge Coatings

Electrical discharge coating (EDC) is a version of electrical discharge machining (EDM) which can produce a coating of material from a tool or from a powder suspended in the oil, onto a workpiece. Since spark temperatures are high, a large range of materials, such as hard ceramics, can be melted and deposited in the coating. These material properties, combined with the rapid quenching of the EDC layer, result in coatings which are typically very hard. Since EDC is an adaptation of EDM, coatings can be produced on components with conformal surfaces. In this study, hard ceramic materials tungsten carbide and powder-sintered titanium carbide are used as negative polarity tool electrodes, from which material is donated to produce hard coatings. The workpiece material is 304 stainless steel. Mechanical properties of the single and multi-layer coatings are assessed by dry sliding wear testing using the ball-on-flat method. Morphology and composition both on the surface and in cross-section are also analysed using SEM and EDX. The TiC single layer coating produced the best wear results for the conditions of the experiment, with the lowest and most constant coefficient of friction of approximately 0.2-0.3 despite cracking remaining in this layer. The TiC + Si double coating, despite better morphology including eliminated surface cracks, produced worse friction characteristics compared to TiC.