Wear and Corrosion Properties of Cold-Sprayed AISI 316L Coatings Treated by Combined Plasma Carburizing and Nitriding at Low Temperature

Abstract

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 ...

Low-Temperature Plasma Nitriding for Austenitic Stainless Steel Layers with Various Nickel Contents Fabricated via Direct Laser Metal Deposition

“…Low-temperature plasma nitriding can be applied to stainless steel plates and machinery parts. Recently, low-temperature plasma nitriding was also applied to the plasma-sprayed and cold-sprayed AISI 316L stainless steel coatings [24][25][26][27]. In addition, HVOF-sprayed stainless steel coatings were treated by gas nitriding and nitrocarburizing at low temperatures [28][29][30].…”

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Phase Stability and Microstructure Evolution of Solution-Hardened 316L Powder Feedstock for Thermal Spraying

“…Most of the hardening techniques for stainless-steel have been developed for wrought materials. However, recent publications prove the possibility of adapting these processes for coatings [7][8][9][10][11][12]. Producing coatings by thermal spraying can be used for various substrate materials, because of their comparatively low heat load.…”

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