The role of Argon in the AISI 420 Stainless-Steel Low-Temperature Plasma Nitriding

Abstract

AISI 420 steel samples were subjected to three heat treatment conditions: annealed, hardened, and tempered at 400°C, followed by low-temperature nitriding using N2, H2, and varying Ar proportions in a pulsed DC glow discharge. The study aimed to investigate the impact of varying Ar content (10–50 vol.%) on glow discharge characteristics and surface properties of nitrided samples, using an 80% N2 + 20% H2 base gas mixture. The samples underwent characterization including optical microscopy, X-ray diffractometry, microhardness, and roughness measurements. Plasma characterization was conducted using optical emission spectroscopy. The results indicated that higher Ar concentrations led to increased thickness of the nitrided layer (up to 67%), as well as hardness (up to 14%) and surface roughness (up to 50%). These improvements stemmed from increased Ar-based species bombardment on the surface, enhancing the cleaning effect of surface oxides. This facilitated nitrogen adsorption onto the steel surface, increasing the atomic nitrogen concentration in the outermost layer of the steel. The increased nitrogen concentration facilitated diffusion, resulting in significant physical-chemical reactions at the surface-plasma interface. These reactions, including sputtering, molecule dissociation, and recombination, led to enhanced high-diffusivity pathways within the martensitic microstructures of both the as-hardened and 400°C-tempered samples.

Keywords:
AISI 420 stainless steel; low-temperature pulsed DC plasma nitriding; argon in nitriding gas mixtures; optical emission spectroscopy; glow discharge (plasma) diagnostics

1. Introduction

Plasma nitriding represents a widely adopted technology for surface modification of engineering materials¹-³. This technique enables the enhancement of mechanical, tribological, and chemical surface properties of metallic alloys while preserving its bulk properties of the substrate⁴,⁵. Since the 1980s, significant attention has been directed towards advancing plasma nitriding techniques for stainless steels. This heightened focus stems from the inherent advantages offered by plasma-assisted methods over conventional solid and gas nitriding treatments⁶. Presently, plasma-assisted treatments conducted at low temperatures have undergone extensive scrutiny and have experienced a burgeoning application within industrial settings⁷-⁹.

The low temperatures employed in stainless steels treatments decrease the nitriding efficiency due to the reduction of the active nitriding species density¹⁰. One way to overcome this problem is the proper selection and adjustment of the nitriding gas mixture¹¹, since the ionization rate is low in pure N₂ plasma due to the highly stable binding strength of N₂ molecule¹². As such, the gas mixture composition plays important role in order to optimize the surface properties of the nitrided materials. This is based on the fact that the gas dilution, which is a variable directly related to the utilized gas mixture, usually determines the atoms saturation rate available to react with the material surface¹³.

It is very well established that the H₂ gas is usually used in addition to N₂ in plasma nitriding gas mixtures, since the hydrogen enhances the rates of N₂ dissociation and the plasma ionization¹⁴. These effects are due to the fact that hydrogen has a minor collisional cross-section on nitrogen and, additionally, for acting in the removal of the oxide protective layer existing on the stainless steels surface¹⁵,¹⁶, which acts as a barrier to nitrogen diffusion. The oxides removal also enhances the emissions of secondary electron and, consequently, the discharge ionization increasing the plasma assisted treatments efficiency⁵,¹⁷,¹⁸. It is worth mentioning, as shown by some authors⁵,¹⁹, the ideal proportion of H₂ added to the nitriding gas mixture is about 20%.

In view of evaluation for the metallurgical features of the nitrided material, similarly there are many studies devoted to the analysis of the influence of H₂ ratio on nitriding gas mixture⁵,¹⁵,²⁰-³². As can be evidenced in the referred works, there is no agreement on the role of the H₂, since ref¹⁵,²⁵,²⁹,³⁰. show that by increasing the H₂ content, both the hardness and thickness of the treated layer tend to be also increased. Differently, ref⁵,²⁴,²⁶,²⁸,³². show opposite results, id. est, tendency of growth for the nitrided layer hardness and thickness, by decreasing the H₂ content in the gas mixture. In the first case, the greatest nitriding efficiency is credited to the role of hydrogen on the surface oxide layer reduction, and to the increasing active nitriding species density in the plasma. For the second case, the authors claim that the increasing nitriding efficiency is due to the increased nitrogen potential, owing to the smaller H₂ content. Among the referred works, all studies comprising the employment of a pure N₂ atmosphere²⁴-³¹, agree well that the use of a N₂ (single gas) plasma is less efficient than those based on H₂-N₂ gas mixtures.

To investigate alternatives to the H₂-N₂ gas mixtures, several studies were performed encompassing other mixtures such as N₂-Ar¹³,¹⁶,³³-⁴⁴, N₂-H₂-Ar¹⁶,³⁷, N₂-Ne⁴⁴, and N₂-O₂-H₂-²H (deuterium)⁴. Particularly, the Ar addition to form a binary mixture with N₂ substantially increases the N active species production through Penning ionization and dissociation processes¹⁶, promoting an increased nitriding efficiency, as verified on different plasma-assisted nitriding systems. In conventional³⁶, and active screen³⁷ dc plasma configurations, Ar content additions up to 70 vol.% increase the plasma (electrons or ions) density, its temperature, and the atomic N-based (neutrals, excited and ions) species density, although meaningly reduce the density of molecular ionized species ($N_2^{+}$) beyond 30%³⁸, as it was also observed in the optical emission spectroscopy results obtained here. In rf plasma configuration, it was found that the N₂ dissociation and active species concentration were appreciably enhanced, by rising the Ar content up to 50-60 vol.%, while the concentration of $N_2^{+}$ decreased³⁹. In the case of inductively coupled discharges⁴⁰ and electron cyclotron resonance plasmas⁴¹, it has been observed that the highest rates of N₂ dissociation, and consequently, the peak production of atomic N species, were achieved at approximately 30% and 80% Ar contents, respectively. Such enhancement on atomic N species density due to the Ar addition promoted increased nitrided layers thickness and hardness, as shown in refs¹⁶,³³,⁴²,⁴³. Lastly, regarding N₂-H₂-Ar ternary gas mixtures, it was demonstrated by Naeem et al.¹⁶,³⁷ that Ar contents up to 30% benefit the generation of atomic N- and molecular N₂-based species, promoting the growth of the hardness and wear resistance of the nitrided material.

Investigating the influence of argon concentration in the gas mixture employed for nitriding, particularly concerning AISI 420 martensitic stainless steel (MSS), is paramount due to its widespread usage, especially in environments prone to rapid corrosion and wear⁴⁵-⁴⁸. The imperative of utilizing surface treatments, such as thermochemical treatments, to enhance the properties and performance of AISI 420 MSS in such environments is crucial for ensuring the durability and reliability of components derived from it⁴⁹,⁵⁰. Nitriding is a surface treatment process that facilitates the formation of nitrogen-expanded martensitic phase, resulting in substantial enhancements in corrosion and wear resistance of martensitic steels. During nitriding, nitrogen diffuses into the material's surface, forming a hardened layer known as the ‘nitrided case’. This layer primarily comprises iron nitrides and nitrogen dissolved in the martensitic matrix⁸.

The presence of the nitrogen-expanded martensitic phase, induced by nitriding, imparts superior surface properties to the material, including increased hardness, wear, and corrosion resistance⁹. Thus, nitriding stands as a pivotal process for enhancing performance and extending the lifespan of components manufactured from martensitic steels, significantly contributing to various industrial applications. As previously mentioned, the role of argon in the nitriding atmosphere is significant, impacting the kinetic and thermodynamic reactions of the process. However, there exists a limited comprehension of how argon concentration influences the nitriding behavior of AISI 420 MSS. Therefore, investigating this relationship is essential to optimize process parameters, enhance energy efficiency, and reduce operational costs. Moreover, advancing scientific understanding of gas-solid interactions in metallurgy contributes to the development of more efficient and sustainable surface modification techniques.

Given the limited number of studies, particularly concerning stainless steel substrates, investigating the impact of Ar additions to N₂-H₂-based gas mixtures, the precise role of Ar in enhancing nitriding efficiency remains incompletely understood. To contribute to this area, this study explores the influence of Ar on the plasma nitriding of AISI 420 MSS, examining its effects on both glow discharge characteristics and metallurgical properties of the treated material. Optical emission spectroscopy is employed as a diagnostic tool to gain insights into the formation of reactive species responsible for nitriding. Microstructural features of the nitrided layers are analyzed through optical microscopy, X-ray diffractometry, microhardness, and roughness measurements. The role of Ar in the plasma gas mixture on nitrided layer formation is investigated by integrating results from plasma diagnostics with metallurgical characterization.

2. Experimental Procedures

Cylindrical samples of AISI 420 MSS with 10 mm in height were cut from an 9.5 mm diameter commercial annealed rod (for a composition of 0.17% C, 0.70% Mn, 0.50% Si, 12.2% Cr, 0.23% P, 0.03% S and Fe balance, in wt.%). Part of these samples were austenitized at 1050 °C for 0.5 h and air-cooled to obtain full-martensite (bct) structure. Half of these air-hardened samples were tempered at 400°C for 1 h to acquire tempered-martensite (bcc) structure. These three groups of samples were subsequently ground using SiC sandpaper ranging from 180 to 1200 grade and mirror polished using 1 μm Al₂O₃ abrasive suspension. Posteriorly, samples were cleaned with alcohol in ultrasound bath, dried in a heated airflow, and finally introduced into the discharge chamber. The measured hardness of as-hardened, as-400°C-tempered, and as-annealed samples was 488±12, 421±24, and 327±13 HV_0.3, respectively.

A pulsed dc power supply operating at 4.2 kHz was used to perform the nitriding treatments. The glow discharge was generated using a 600 V pulse (peak) voltage, and the duty cycle was adjusted to obtain the desired temperature (the heating of samples acting as the cathode was provided by plasma species bombardment only). In order to promote ultimate cleaning and aiming to remove the (Cr₂O₃) native oxide layer from the samples surface, prior to the nitriding step, treating samples were sputter-cleaned at 300 °C for 0.5 h in a glow discharge generated using a gas mixture of 80% H₂+ 20% Ar. All plasma nitriding treatments were carried out at 350 °C for 6 h times, employing a gas flow rate of 3.34 × 10⁻⁶Nm³s⁻¹ (200 sccm), under a pressure of 400 Pa (3 Torr). These parameters were selected based on results obtained in previous studies⁵¹-⁵³. A scheme and more detailed description of the nitriding plant and procedure can be obtained in ref⁵¹,⁵⁴.

The choice of a treatment temperature of 350°C was primarily aimed at avoiding the precipitation of chromium nitride phases, which are detrimental to the material's corrosion resistance. Additionally, this temperature was selected because it is sufficiently low to induce significant changes in the material's structure, which could influence the kinetics of the nitriding process. Finally, this temperature is recommended for the tempering of the material in its as-quenched state, allowing for the simultaneous execution of two operations (tempering + nitriding) in a single cycle. In the case of samples that have already been tempered, this temperature is lower than that employed in the previous tempering process and therefore does not exert any additional effect.

To study the effect of Ar on features of the glow discharge and surface properties of the nitrided samples, six-gas mixture compositions were evaluated, which are indicated in Table 1, being also indicated the adopted samples codification. The choice of argon values, ranging proportionally in volume from 0% to 50%, was based on research findings documented in the literature¹⁶,³⁶-⁴³. These studies indicate that higher argon content improves nitriding efficiency by facilitating the removal of surface oxides from stainless steels and promoting the formation of reactive nitrogen radicals that react with the steel surface. However, these sources also note that argon levels exceeding 50% lead to a decrease in the density of reactive nitrogen species.

Table 1
Plasma nitriding treatment conditions.

Samples code	Gas mixture (vol.%)
#0	0% Ar + 100% (80% N2 + 20% H2)	80% N2 + 20% H2
#10	10% Ar + 90% (80% N2 + 20% H2)	10% Ar + 72% N2 + 18% H2
#20	20% Ar + 80% (80% N2 + 20% H2)	20% Ar + 64% N2 + 16% H2
#30	30% Ar + 70% (80% N2 + 20% H2)	30% Ar + 56% N2 + 14% H2
#40	40% Ar + 60% (80% N2 + 20% H2)	40% Ar + 48% N2 + 12% H2
#50	50% Ar + 50% (80% N2 + 20% H2)	50% Ar + 40% N2 + 10% H2

In order to evaluate the active species as a function of the Ar content added on the base gas mixture, an experiment was specially specified for optical emission spectroscopy measurements, following a procedure similar to that adopted in ref⁵². The experimental apparatus was equipped with a 2048-element linear silicon CCD array and a 600 lines/mm grating, set to operate on the range of 200-850 nm, presenting a wavelength resolution of 1.5 nm (full width at half maximum). In the related experiments, the Ar content was changed, starting from de gas mixture of 80% N₂+ 20% H₂ (condition #0, in Table 1), and being added for amounts of 10 vol.% up to 50 vol.% Ar to each 60 minutes (see in Table 1, conditions from #10 to #50, respectively). The spectrum was achieved by recording the data to each 5 minutes. For this purpose, the t_ON was adjusted as a function of the applied gas mixture, in order to maintain the temperature at 350 °C (623 K). It is to be noted that the experiment was designed this way aiming to ensure the same solid angle, so the emission spectra for each gas mixture condition was acquired without any changes in the experimental setup. As the position between sample and optical fiber was kept unaltered, intensity of the obtained emission lines could be successfully confronted. Five spectra were obtained for each gas mixture condition. The average values indicated for the optical emission spectroscopy data were estimate by the spectrometer software from these five spectra. The respective measurement errors (indicated by error bars on the points of the graphic of optical emission spectroscopy results) were estimated from the standard deviation determined from the two spectra showing the larger measurement data differences for each gas mixture condition.

Nitrided samples were cross-sectioned and prepared for microstructural analysis following the conventional metallographic procedure. After polishing, samples were etched using Vilella’s reagent (1 g of picric acid + 4 mL of HCl + 96 mL of ethanol). The etched samples cross-section was analyzed using an Olympus BX51M optical microscope. To identify the phases present in the nitrided layers, Xray diffractometry (XRD) technique was performed using a Shimadzu XDR7000 Xray diffractometer with a Cu K_α Xray tube in the Bragg-Brentano configuration. The samples roughness was evaluated using the profilometry module of an Olympus LEXT OLS 3000 confocal laser scanning microscope (CLSM), same equipment used to analyze the nitrided surfaces morphology. Microhardness measurement was performed on the nitrided surface (sample top), on the non-nitrided surface (sample bottom) and on the samples cross-section with a Shimadzu Micro Hardness Tester HMV‐2T, applying a load of 10 gf to determine the microhardness profile (in the sample cross-section), and a load of 300 gf to determine the top and bottom hardness, for a peak-load contact of 15 s.

3. Results and Discussion

3.1. Surface and microstructural characterization

Figure 1 presents typical cross-section micrographs of the as-hardened, as-400°C-tempered and as-annealed samples nitrided for the different gas mixtures used here. As expected for the 350 °C nitriding temperature⁵¹, the N (nitrogen)-chemically altered surface is not attacked or etched by the Villella’s etchant while the N-unalloyed substrate bulk is etched. This result empirically indicates the greater resistance of the nitrided layer to acid corrosion than the original steel. The white-aspect layer occurrence suggests the nitrogen diffusion and nitrogen enrichment in interstitial solid solution on the body-centered cubic (bcc) or body-centered tetragonal (bct) structures, promoting the formation of the so-called expanded ferrite⁵⁵ and/or expanded martensite⁵⁶ phase, respectively. Another aspect worth highlighting is that no significant visual changes are observed among the nitrided layers obtained in the samples subjected to different prior heat treatments.

Figure 1
Cross-section micrographs of as-hardened, as-400 °C–tempered and as-annealed plasma nitrided AISI 420 steel samples treated under different argon content in gas mixture.

Figure 2a shows the XRD data of as-hardened samples for the untreated and nitrided condition as a function of the Ar content in the gas mixture. The untreated (as-hardened) sample presents on the adopted scan range (35-60°) the main (relative to 100% intensity) martensite phase (α′-Fe) peak. All post-nitrided as-hardened samples (which were in fact also simultaneously plasma tempered at 350 °C, as the nitriding treatment was carried out) show the formation of the nitrogen-expanded martensite (α′_N-Fe) and epsilon iron nitride (ε-Fe_2-3N) phase. As known, α'_N-Fe peak is usually identified on XRD data as broad peaks displaced to smaller 2θ angles, with greater interplanar distances when compared to the corresponding α'-Fe peak⁵⁷. The α_N-Fe phase formation occurs as a result of deformation and consequent stress generation in the crystal lattice, due to nitrogen incorporation in solid solution⁵⁶. As can be also seen from the Figure 2a analysis, the broader peaks also comprise contributions of ε-Fe_2-3N precipitates, which exists for a large range of stoichiometry, comprised on ~25 to 33 at.% N, leading to overlapping peaks on the diffractogram⁵⁸.

Figure 2
XRD patterns for: (a) as-hardened samples nitrided under different argon content in gas mixtures, (b) different samples pretreatment conditions and plasma nitrided under 50% argon content in the gas mixture, and (c) evolution of the lattice parameters on nitrided samples as a function of argon content in the gas mixture.

Figure 2b shows a comparison between the XRD patterns of as-annealed, as-hardened and as-400°C-tempered, for untreated and treated samples at the 50% Ar + 50% (80% N₂+ 20% H₂) gas mixture (#50, in Table 1). From the comparison among nitrided as-hardened and as-400°C-tempered samples, it is noted that the former shows a higher intensity α'_N-Fe phase peak compared to the latter. This result can be attributed to the higher N solubility/supersaturation in the martensite lattice for the case of the as-hardened samples⁵³, which would be directly associated to the higher chromium content dissolved in solid solution and, theoretically, homogenously distributed all over the steel matrix. The nitrided as-annealed sample XRD pattern, in turn, shows the occurrence of the N-expanded ferrite phase (α_N-Fe), characterized by the broadened original ferrite (α-Fe) peak shifted to smaller angles⁵⁹-⁶¹, which confirms that the phase lattice parameter is increased by the N. The broadened peak feature is an indicative of the existence of N gradients occurring from the surface top into the substrate bulk that usually results in compressive residual stresses in the sample surface. In addition, no peaks related to the chromium nitride phases were identified, despite that second-phase particles in the form of chromium carbides are present in the as-annealed steel microstructure, as evidenced in the sample substrate bulk, in Figure 1. It is worth emphasizing that the absence of chromium nitrides and such second-phases rich in chromium (in the carbide form) at the sample surface could indicate that the treated material corrosion resistance was not affected. XRD data for the remaining evaluated conditions were not presented in this section, as they exhibit a similar pattern to that illustrated herein.

The lattice parameter (a) of the as-annealed, and as-400°C-tempered samples (both conditions showing bcc structure), and as-tempered samples (with bct structure), before and after nitriding, was determined using the obtained XRD data taken the plan (110) as reference. From this estimation, it was possible to obtain a comparative evaluation of the lattice expansion caused by the N diffusion under the indicated evaluated conditions. A deconvolution analysis of the XRD data of the nitrided samples for 2θ angles ranging from 41° to 46° was performed in order to more accurately determine the position of the α’_N-Fe (110) peak (see Table 2). Results indicated that the a lattice parameter increment, expressed in percentage values, for the bct structure of the as-hardened samples and those estimated for the as-400°C-tempered samples showing cubic martensite (bcc) phase post nitriding are similar, and agree well to those reported in the literature⁵¹,⁵⁶. In contrast, the estimated values for the bcc structure of annealed samples post nitriding are far below the values reported in the literature for ferritic stainless steels (~ 5.24 for (110) plane)⁶⁰ and higher than those estimated for the ferrite phase present in nitrided super duplex stainless steel samples (~ 1.04% for (110) plane)⁵⁹. It is believed that these differences are linked to variations in chemical composition between the compared alloys. Finally, as shown in Figure 2c, the lattice parameters (a_nitrided) of bcc and bct structures of nitrided samples grow linearly with the addition of Ar content in the gas mixture.

Table 2
Experimental XRD data for 2θ peaks, the phase lattice parameter (a, being a = d = interplanar distance) of the untreated substrate bulk (2θ_matrix, a_matrix), and the nitrided surface (2θ_nitrided, a_nitrided), and estimated variation (Δ) of the lattice parameter before (a_matrix) and after nitriding (a_nitrided), for the three distinct prior heat treatment (and the respective phase) conditions.

Condition		2θmatrix	2θnitrided	amatrix (Å)	anitrided (Å)	Δ = a nitrided − a matrix (%)
#0	as-annealed (α-Fe)	44.68	43.97	2.86	2.909	1.745
(0% Ar)	as-hardened (α'bct-Fe)	44.18	42.96	2.87-	2.974-	3.373-
	as-400°C-tempered (α'bcc-Fe)	44.52	42.99	2.87	2.973	3.304
#10	as-annealed (α-Fe)	44.68	43.92	2.86	2.913	1.855
	as-hardened (α'bct-Fe)	44.18	42.88	2.87-	2.980-	3.557-
(10% Ar)
	as-400°C-tempered (α'bcc-Fe)	44.52	42.93	2.87	2.976	3.442
#20	as-annealed (α-Fe)	44.68	43.87	2.86	2.916	1.965
	as-hardened (α'bct-Fe)	44.18	42.79	2.87-	2.986-	3.764-
(20% Ar)
	as-400°C-tempered (α'bcc-Fe)	44.52	42.81	2.87	2.984	3.718
#30	as-annealed (α-Fe)	44.68	43.84	2.86	2.918	2.032
	as-hardened (α'bct-Fe)	44.18	42.65	2.87-	2.995-	4.089-
(30% Ar)
	as-400°C-tempered (α'bcc-Fe)	44.52	42.75	2.87	2.988	3.857
#40	as-annealed (α-Fe)	44.68	43.79	2.86	2.921	2.142
(40% Ar)	as-hardened (α'bct-Fe)	44.18	42.63	2.87-	2.996-	4.135-
	as-400°C-tempered (α'bcc-Fe)	44.52	42.72	2.87	2.990	3.926
#50	as-annealed (α-Fe)	44.68	43.77	2.86	2.922	2.187
	as-hardened (α'bct-Fe)	44.18	42.55	2.87-	3.002-	4.322-
(50% Ar)
	as-400°C-tempered (α'bcc-Fe)	44.52	42.69	2.87	2.992	3.996

Figure 3 shows nitrided layers thickness values as a function of the Ar content of the studied gas mixtures for the as-hardened, as-400°C-tempered, and as-annealed samples. It appears that the higher the Ar content the higher is the nitrided layer thickness. Likewise, it is possible to assert that a small variation in the thickness values was observed among the as-hardened, and as-400°C-tempered samples. In contrast, significantly smaller values were determined for the previously annealed samples. This behavior can be credited to the lower crystalline defect density present in the ferritic (α-Fe) structure, or its lower chromium content in solid solution, as it is expected due to the relatively intense chromium carbide precipitation dispersed all over the iron matrix, for the as-annealed condition. The previous heat treatment of hardening (applied for the other two heat treatment conditions studied here) leads to a higher crystalline defect density in the steel matrix structure, which constitutes high diffusivity paths favoring the nitrogen diffusion to greater depths⁵³. Similarly, the higher chromium content dissolved in solid solution in both the martensitic matrixes (namely, the bct as well as the bcc one), due to the chromium carbide dissolution during the austenitization step of the hardening treatment, increases the nitrogen solubility⁶². Furthermore, the activation energy for the nitrogen diffusion in bcc structure for the annealed martensitic stainless steel (Q_α = 164 kJ mol^-1)⁶² is greater than in bct structure of as-hardened steel (Q_α’ = 136 kJ mol^{− 1})⁶³, leading to a lower nitrogen diffusivity and minor layer thickness.

Figure 3
Evolution of the nitrided layer thickness as a function of argon content in the treated gas mixture.

The evolution of surface hardness as a function of Ar content in the gas mixture for as-annealed, as-hardened, and as-400°C-tempered samples is presented in Figure 4a. The hardness values clearly grow with the Ar content increase on the gas mixture. This behavior is linked both to the rise of nitrided layer thickness with the Ar increment, and the raise of N amount interstitially dissolved in α′_N-Fe phase allied to the presence of ε-Fe_2-3N nitride phase in treated layers. The highest average hardness observed for the as-hardened condition is attributed to its higher crystalline defect density favoring N diffusion. The impact of increasing crystal defects density on nitrogen diffusion kinetics and the efficiency of nitriding treatment has been demonstrated by several authors in the literature⁶⁴-⁶⁸.

Figure 4
(a) Surface hardness and hardness profile for previous (b) as-hardened samples plasma nitrided under different argon content in gas mixtures, and (c) different samples pretreatment conditions and plasma nitrided under 10% and 50% argon content in the gas mixture.

Figure 4b confirms the important role of Ar regarding the surface hardening. Note that the hardening depth is increased for nitrided as-hardened samples as the greater is the Ar content in the used gas mixture, within the studied values range. In addition, the smallest hardening depth occurs justly for the nitriding condition without the use of Ar in the gas mixture. In turn, Figure 4c, confronting the hardness profiles obtained for gas mixtures with 10 and 50 vol.% Ar, confirm the validity of this finding also for the nitrided as-400°C-tempered and as-annealed samples. It is worth highlighting that the increases observed in the thickness and hardness of the nitrided layer, as demonstrated in Figure 3 and Figure 4, are directly related to the formation of the phases identified in the XRD patterns presented in Figure 2.

Figure 5a shows the Rz and Ra roughness of the nitrided surfaces as a function of the Ar content used in the nitriding mixture. Results indicate roughness increase for all studied conditions as the Ar content is incremented. This result is related to the sputtering phenomenon caused by Ar-based species, in higher density in the plasma, as its content in the gas mixture is increased¹⁵. The obtained surface morphology after nitriding, as shown in Figure 5b (for i) as-hardened, ii) as-400°C-tempered; and iii) as-annealed samples), reveals the occurrence of a slight network, which seems to outline grain boundaries, an effect more evidenced for the as-annealed condition (Figure 5b(iii)). This morphology may be the consequence of the preferential nucleation and growth of iron nitrides along grains boundaries⁶⁹. It is worth mentioning that here, it was not identified the transformation from lath- to plate-type martensite due to the enhancement of the martensite N content on the as-hardened and 400 °C–tempered nitrided samples, in disagreement to the evidenced for plasma carburized AISI 420 martensitic stainless steel samples, in ref⁷⁰.

Figure 5
(a) Ra and Rz roughness and (b) surface morphology of 1) as-hardened, 2) as-400 °C–tempered and 3) as-annealed samples plasma nitrided under 50% argon content in the gas mixture.

3.2. Optical emission spectroscopy characterization

Figure 6 shows the brief of the optical emission spectroscopy results obtained for the analyzed 200-850 nm wavelength range. On the analyzed wavelength range, it was observed no significant signal for the ArH⁺ (at 767.4 nm), H₂, H₂⁺, H⁺, and N⁺ species in the abnormal glow discharge region. Results show the observed evolutions for the N₂, N₂⁺, N, H, Ar⁺ species, the later one occurring for the 763.51 nm wavelength. As expected, the Ar⁺ density is increased for higher Ar contents, as expense of the decreased N- and H-based species densities in the gas mixture, which is supposedly related to the higher cross-section for the reaction (1), as follows⁷¹:

$$e^{-}+Ar\to e^{-}+e^{-}+A{r}^{+}$$ (1)

Figure 6
Optical emission spectroscopy results obtained for the analyzed 200-850 nm wavelength range.

Regarding the atomic N-based species, which naturally are the main responsible by the successful of the samples nitriding, our optical emission spectroscopy results when confronted with our nitrided samples (surface and microstructural characterization) results, clearly indicate that the formation of atomic N species in the glow region of the obtained electrical discharges, due to the increased Ar contents, is not significant, since its density is decreased for higher Ar contents (strongly disagreeing with Figure 2 and Figure 4a results). This finding gives support for the assumption that atomic N-based species would preferentially be formed in profusion on the samples surface, from N₂-based species condensed at the treating surface, by thermal dissociation as well as by a dissociation intensified by the Ar-based fast species bombarding the treating surface. In other words, our successfully achieved nitrided surface features are due to the increment of the momentum transfer from Ar-based species at the samples surface as well as the increased atomic N chemical potential supplied for the steel treating surface, caused by the higher density of Ar-based fast (neutrals, excited and ions) plasma species bombarding the surface. As it is very well established that plasma nitriding is a diffusion-controlled treatment⁵¹, despite the so expected diffusional limitation, since the treatment temperature and time was the same for all studied conditions, it can be also affirmed that the increased layer thickness and hardness observed in our Figure 2 and Figure 4a results are a consequence of the surface bombardment by a higher Ar-based species density, increasing the atomic N concentration in the outer superficial layer of the steel and the driving force for its diffusion, leading to a significant increment on the physical-chemical reactions (mainly sputtering, molecules dissociation and recombination) in surface-plasma interface, associated with the incremented higher-diffusivity paths present in the martensitic microstructures of the as-hardened and as-400°C-tempered samples.

4. Conclusion

Low-temperature plasma nitriding treatments were conducted using various N₂-H₂-Ar gas mixtures to assess the impact of Ar on glow discharge characteristics and surface properties of nitrided AISI 420 MSS samples. These samples underwent prior heat treatments including annealing, hardening, and hardening & 400°C tempering. The main conclusions of the study are summarized as follows:

• Low-temperature DC plasma nitriding in N₂-H₂-Ar gas mixtures effectively enhances the surface hardness of AISI 420 steel samples. This hardness improvement results from the formation of a nitrided layer comprising ε-Fe_2-3N iron nitride and nitrogen-expanded phases.

• The previous heat treatment has proven to be a crucial parameter in the plasma nitriding treatment of steels exhibiting metastable phases. From the same base material, the mechanical properties of the resulting modified surface can only be altered through careful selection of the preceding heat treatment.

• For the conditions examined, the rise in Ar content within the assessed range led to an increase in nitride layer thickness. Surface hardness and roughness likewise increased accordingly.

• Hardness profile measurements reveal a smoothed interface between the nitrided layer and the bulk, indicating a hardening depth of approximately 20 μm for both the nitrided as-hardened and as-hardened & 400°C-tempered samples, and approximately 10 μm for the as-annealed samples.

5. Acknowledgments

This work was supported by CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior–Brasil–Finance Code 001), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), CNPq-Universal Grant N. 482380/2012-8, MCTI/CNPq/CT-Aquaviário Grant N. 456347/2013-5 and Fundação Araucária do Estado do Paraná (PRONEX - Grant N. 46744). Dr. M. Mafra thanks to CNPq by the financial support through Process n. 311716/2018-0. Dr. C.J. Scheuer thanks to FAPERGS (Research Support Foundation of the Rio Grande do Sul State) by the financial support through Grant N. 19/2551-0001749-9, and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) through Grant No. 309675/2022-7. The authors also wish to express thanks to the Laboratory of X-ray Optics and Instrumentation – LORXI, by the use of the X-ray diffraction equipment, and Additive Manufacturing and Surface Engineering Laboratory – LAMSE, by the use of the confocal laser scanning microscope equipment, both laboratories from the Universidade Federal do Paraná (UFPR).

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Publication Dates

• Publication in this collection
  20 May 2024
• Date of issue
  2024

History

• Received
  24 Dec 2023
• Reviewed
  06 Apr 2024
• Accepted
  24 Apr 2024