A novel method to fabricate composite coatings via ultrasonic-assisted electro-spark powder deposition

To improve traditional electro-spark deposition (ESD) technology and meet the requirements for special applications, a novel ultrasonic-assisted electro-spark powder deposition (UEPD) process is developed in this study. The unique electrode tool in this new method consists of a NiCrBSi alloy powder as a deposition material and a Cu electrode core as a support component, which are bonded to each other by means of a silver-containing conductive adhesive. By using this UEPD process, a metal matrix composite (MMC) coating is successfully fabricated on ASTM 1045 steel. The coating has an average thickness of approximately 52.3 μm and exhibits efficient metallurgical bonding with the substrate as well as good continuity. The microstructure in the coating mainly presents submicron-columnar dendrites that epitaxially grow perpendicular to the substrate surface, which mainly contain a Ni-rich γ-(Ni, Fe) solid solution, intermetallic FeNi3 and Cr-rich carbide and boride precipitates. These refined grains and hard reinforcements significantly increase the hardness and tribological properties of the coating. Combining the discharge morphology, microstructure and chemical composition of the UEPD electrode and coating, the formation process of the UEPD coating is analysed and discussed in detail. The adhesive NiCrSiB powder layer was melted and transferred to the surface of the substrate during the UEPD process. The separation of the powder tends to start from the interface of the powder layer and electrode core due to weak connectivity. This behaviour is considered beneficial to the quantitative control of powder deposition. In addition, the introduction of ultrasonication can promote dispersion of the metal droplets to obtain a better quality coating.