Surface Modification of Stainless Steel by Electro-Spark Deposition

Abstract

Electrospark Deposition (ESD) is a pulsed micro-welding process that is used to apply surface coatings for repair of damaged high value precision products or modify their surfaces for specific properties. The low heat input, small heat affected zone and the ability to form metallurgical bonding of coating to substrate are some of the major advantages of ESD process. Many applications require the components to have excellent surface performance, such as wear resistance and corrosion resistance. To meet these requirements, some components are built with specific materials, compromising other properties and cost. ESD technique provides an approach to modify the component surface without compromising the bulk properties. Stainless steel is an ideal material for many applications such as industrial equipment, surgical instruments, household hardware etc., due to its resistance to corrosion. Surface modification of stainless steel may improve its performance and may open new applications.

In this study, surface modification of 304 stainless steel by ESD was investigated. TiC, WC and Molybdenum (Mo) were employed as coating materials. The ESD processing windows for these coatings were investigated. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis was conducted to characterize the microstructure and composition of coated stainless steel. Micro-hardness and wear resistance tests were carried out to evaluate the mechanical properties of coated stainless steel. TiC and WC coatings dramatically increase the micro-hardness of 304 stainless steel. WC coating improves the abrasion wear resistance of stainless steel by more than 5 times, while TiC and Mo coatings also improve it by 2.5 times.

Electrochemical tests were conducted to investigate the corrosion resistance of coated stainless steel. Mo coating exhibits significant improvement on corrosion resistance in 5% NaCl solutions, which corrodes 350 times slower than stainless steel. TiC coating also increases the corrosion resistance with 10 times slower corrosion rate. WC coating does not show improvement on the corrosion resistance. Electrochemical impedance spectroscopy (EIS) was employed to further investigate the electrochemical behavior of coated stainless steel. The results showed the polarization resistance of Mo coated sample is much larger than that of base metal stainless steel. XRD analysis indicate the phase transformation from austenite to ferrite after ESD of Mo.

Comprehensive metallurgical analysis of Mo coated 304 stainless steel is performed after heat treatment at 400ºC, 650ºC and 900ºC. The effects of heat treatment atmosphere are investigated by comparing the sample treated in air and Ar gas. SEM and EDX results show the coating thickness decreases with the increase of heat treatment temperature. Localized Mo rich area is found in heat-treated samples. More cracks, porosities and rougher surface conditions are observed in heat-treated samples. XRD analysis display phase transformation from austenite to ferrite at 400ºC. Mo rich intermetallic is detected at 650ºC under Ar gas. Mo and Cr oxides are found in heat-treated samples above 650ºC in air. XPS results show metallic state Mo disappears after heat treatment in air, while metallic state Mo only disappears at 650ºC in Ar gas. It is suggested that Mo rich intermetallic is formed at specific temperature range around 650ºC.

Electrochemical test indicated heat-treated samples, either in Ar or in air atmosphere, have lower corrosion resistance than as-deposited sample. Metallic state Mo and a certain ratio of austenite and ferrite can contribute to better corrosion resistance. EIS analysis with modified equivalent circuit is conducted to further investigate the electrochemical behavior. The results indicate that heat-treated samples introduce more nonuniform coating layers because of oxidation and diffusion of alloy elements. Mo rich intermetallic phase decreases the corrosion potential of the heat-treated sample at 650ºC in Ar, and also decreases the corrosion rate of the sample.