Surface Modification of 304 Stainless Steel by Electro-Spark Deposition

Electro-spark deposition (ESD) is a pulsed microwelding process that is used to apply surface coatings for the repair of damaged high value and precision products or modify their surfaces for specific properties. The low heat input, minimal heat-affected zone and the ability to form metallurgical bonding of coating to substrate are major advantages of the ESD process. Many applications require the components to have excellent surface performance, such as wear and corrosion resistance. ESD technique provides an approach to modify the component surface without compromising the bulk properties. In this study, surface modifications of 304 stainless steel by ESD were investigated. Titanium carbide (TiC), tungsten carbide (WC) and molybdenum (Mo) were employed as coating materials. Scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX) analysis were conducted to characterize the microstructure and composition of the coatings. The coatings thicknesses were all around 40 µm. The results showed that TiC and WC coatings showed a dramatic increase in the microhardness, up to 5 times. WC coating improved the wear resistance by more than 5 times, while TiC and Mo coatings also improved it by approximately 2.5 times. Electro-chemical tests were conducted to investigate the corrosion resistance of the coatings. Mo coating exhibited a significant improvement in the corrosion resistance in 5% NaCl solutions, corroding 350 times slower than stainless steel. Synchrotron x-ray diffraction was performed to investigate the microstructure changes of the Mo-coated sample. Heat treatment was also carried out to investigate the corrosion behavior of Mo-coated 304 stainless steel at elevated service temperature in air or argon.