Cold-sprayed AISI 316L stainless steel coatings are treated to form an austenite phase with excessive dissolved nitrogen (known as the S-phase) by plasma nitriding at temperatures below 450 • C. The S-phase is a hard and wear-resistant layer with high corrosion resistance. However, the S-phase layer formed after only nitriding is thin and the hardness abruptly decreases at a certain depth; it lacks mechanical reliability. We examined two types of combined low-temperature plasma treatment to enhance the mechanical reliability of the S-phase layer: (i) sequential and (ii) simultaneous. In the sequential plasma treatment, the carburizing step was followed by nitriding. In the simultaneous treatment, the nitriding and carburizing steps were conducted at the same time. Both combined plasma treatments succeeded in thickening the S-phase layers and changed the hardness depth profiles to decrease smoothly. In addition, anodic polarization measurements indicated that sequential treatment involving carburizing followed by nitriding for 2 h each resulted in high corrosion resistance.However, plasma-sprayed stainless steel coatings include oxide layers, cracks, and pores in their structure, which deteriorate the corrosion resistance considerably.Cold-spraying is not a fusing process but a solid particle stacking process. Hence, cold-sprayed coatings are without any oxides, similar to a bulk steel structure [23]. It was reported that thick cold-sprayed 316L stainless steel coatings show electrochemical polarization behavior similar to bulk stainless steels [24]. Therefore, it is expected that cold-sprayed AISI 316L coatings can be applied as corrosion protective coatings. Additionally, Villa et al. reported that cold-sprayed 316L coatings can be hardened by cold working during the deposition process, which can also result in increased strength, and the abrasive wear resistance of the coating is in the same order of magnitude as that of coatings produced by HVOF [25]. However, the hardness of the 316L coatings was 358 ± 36 HVN, which is insufficient to apply under severe wear conditions.We have previously reported low-temperature plasma nitriding of cold-sprayed AISI 316L coatings to successfully improve the Vickers hardness to 1200 HV [26]. In particular, remelted AISI 316L coatings by a postlaser treatment had excellent corrosion protection for ordinary steel substrates due to a dense microstructure.However, there is a concern that S-phase layers formed by nitriding without carburizing could be delaminated when an external force is applied, because the hardness of the S-phase layers declines drastically at a certain depth. Low-temperature treatments combining nitriding and carburizing have been reported to result in a milder hardness distribution (with hardness not declining drastically), excellent wear resistance, and thickening of the S-phase layers. There are two types of combined treatments-(i) sequential and (ii) simultaneous. In the sequential treatment, carburizing is followed by nitriding [27][28][29]. Meanwhile, in ...

In this study, low-temperature plasma nitriding is applied to austenitic stainless steels at temperatures below 450 • C. This enhances the wear resistance of the steels with maintaining corrosion resistance, by producing expanded austenite (known as the S-phase), which dissolves excessive nitrogen. Austenitic stainless steels contain nickel, which has the potential to play an important role in the formation and properties of the S-phase. In this experiment, austenitic stainless steel layers with different nickel contents were processed using direct laser metal deposition, and subsequently treated using low-temperature plasma nitriding. As a result, the stainless steel layers with high nickel contents formed the S-phase, similar to the AISI 316L stainless steel. The thickness and Vickers hardness of the S-phase layers varied with respect to the nickel contents. Due to lesser chromium atoms binding to nitrogen, the chromium content relatively decreased. Moreover, there was no evident change in the wear and corrosion resistances due to the nickel contents.Coatings 2020, 10, 365 2 of 11 Ordinary austenitic stainless steels contain nickel as an alloy element. Nickel provides the crystal structure of stainless steel in an austenite phase (fcc lattice) and plays an important role in the formation of the S-phase. The effects of nickel on the formation and properties of the S-phase have been reported in several papers. For example, E. Menthe reported that the S phase for nitriding can only be formed if iron, chromium, and nickel are available; however, the fcc structure of the austenite lattice is not necessary to form the S-phase [4]. On the contrary, J. Buhagiar reported that the S-phase can be produced in the surface of Ni-free ASTM F2581 austenitic stainless steel via low-temperature plasma nitriding at 430 • C. Accordingly, nickel is not essential for the formation of an S-phase in austenitic stainless steel [32]. F. Borgioli insisted that low-temperature glow-discharge nitriding can produce the S-phase without the formation of large amounts of nitrides, not only on AISI 316L and AISI 202 austenitic stainless steels but also on the nickel-free P558 alloy [33]. These studies examined the results by using stainless steels, which have a lower nickel content as compared with ordinary austenitic stainless steels. In addition, the results for increased nickel contents have not been reported. Recently, super stainless steels with high nickel contents are being widely used as corrosion-resistant materials for machinery parts. Therefore, the S-phase with high nickel austenitic stainless steels should be considered to further develop the low-temperature nitriding technique.Direct laser metal deposition can synthesize the alloy layers by using several metal powders as the feeding powder. In this study, high-nickel austenitic stainless steel layers were deposited by using AISI 316L powder and nickel powder. Subsequently, these deposited alloy layers were processed by plasma nitriding at the temperature of 450 • C to produce the ...

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Cold-spray techniques have been a significant development for depositing metal coatings in recent years. In cold-spray processes, inexpensive nitrogen gas is widely used as the propellant gas in many industries. However, it is difficult to produce austenitic stainless steel coatings with dense microstructures with cold-spray techniques when using nitrogen propellant gas because of work hardening. In this study, the effects of cold-spray conditions using a nitrogen propellant gas on AISI 316L stainless steel coatings were examined. It was found that a higher nitrogen propellant gas temperature and pressure produce coatings with dense microstructures. The measured AISI 316L coating hardness values suggest that AISI 316L particles sprayed at temperatures of 700 and 800 • C soften due to the heat, allowing uniform deformation on the substrate and consequently forming dense coating microstructures. In addition, AISI 316L powder with particle diameters of 5-20 µm resulted in a denser coating microstructure than powder with particle diameters of 10-45 and 20-53 µm. Finally, the standoff distance between the nozzle and the substrate also affected the AISI 316L coating microstructures; a standoff distance of 40 mm produced the densest microstructure.it is difficult to produce cold-sprayed austenitic stainless steel coatings using nitrogen propellant gas. Austenitic stainless steel exhibits work hardening and resists plastic deformation, but when the sprayed particles are stacked on the substrate, the sprayed particle velocity with nitrogen propellant gas is significantly lower than with helium propellant gas at the same gas temperature [4][5][6][7]. As a result, the stacked stainless steel particles on the substrate do not deform completely, resulting in many voids between the stacked particles.It has also been reported that cold-sprayed 316L stainless steel coatings using nitrogen propellant gas exhibited a porous microstructure as opposed to coatings using helium propellant gas, whereas the use of a heat treatment up to 800 • C enabled the same coating microstructures to be dense [8]. In addition, stainless steel 316L powder mixed with Co-Cr powder has been cold-sprayed using nitrogen propellant gas, and heat treatments were found to cause densification and porosity reduction of the coating microstructures [9]. We have previously investigated low-temperature plasma nitriding for AISI 316L coatings using a cold-spray technique to enhance the wear resistance while maintaining the corrosion resistance [10]. In that work, cold-sprayed AISI 316L coatings using nitrogen propellant gas also exhibited porous microstructures; subsequently, laser annealing was performed. As a result, the pores and cracks in the coatings disappeared completely. Furthermore, by using an optimized rectangular divergent cold-spray nozzle, sprayed stainless steel 316L coatings were fabricated successfully as dense microstructures when using nitrogen propellant gas [11].A porous microstructure significantly deteriorates the corrosion resistance, wear r...