Plasma Nitriding of Austenitic Stainless Steel using Ti–Mo Combination Screen

Abstract

Austenitic stainless steels such as SUS304 are widely used in various applications due to their excellent corrosion resistance; however, their low hardness and wear resistance limit their structural use. Plasma nitriding was applied using a Ti–Mo combination screen to simultaneously form a Ti–Mo composite nitride layer and a nitrogen diffusion layer in SUS304 stainless steel. The microstructure and mechanical properties of the plasma-nitrided SUS304 were evaluated. XRD analysis revealed peaks of δ-(Ti,Mo)N, which originated from the constituent elements of the screen, indicating the successful formation of a Ti–Mo–N composite nitride layer on the sample surface. The thickness of the Ti–Mo–N layer increased with higher gas pressure and nitrogen content in the gas mixture. The formation of the Ti–Mo–N layer on the nitrogen diffusion layer significantly enhanced the hardness and reduced the kinetic friction coefficient. The results suggest that this surface treatment method could be beneficial for improving the durability and lifespan of stainless steel components in demanding applications.

1. Introduction

Austenitic stainless steels are used in a wide variety of applications, including machine parts and household products. However, their low hardness and wear resistance limit their use as structural materials. To improve these properties, surface modification techniques such as plasma nitriding can be employed. Plasma nitriding involves using nitrogen in an active plasma state by generating a glow discharge in a reduced-pressure nitrogen–hydrogen mixed gas atmosphere. This method is noted for its low environmental impact and effectiveness in treating stainless steel, a challenging material for nitriding. Plasma nitriding using a metal screen has been studied,^{1,2,3,4,5,6,7,8,9,10,11,12,13)} and the authors investigated the conventional DC plasma nitriding (DCPN) method with the addition of a metal screen as the cathode material (hereinafter referred to as ASDCPN). This approach not only forms a nitrogen diffusion layer through nitrogen diffusion into the sample but also deposits a nitride layer formed by the sputtered cathode material on the sample surface. The formation of the nitrogen diffusion layer has been studied by applying ASDCPN treatment to SUS304, austenitic stainless steel, using a screen made of SUS304.¹⁴⁾ In this study, the authors developed a “combination screen” fabricated from two types of metal wire to enhance functionality. By using a combination screen consisting of metal wires of multiple nitride-forming elements in the ASDCPN treatment, it is believed that hardness and wear resistance can be increased by forming various compound nitride layers on top of the nitrogen diffusion layer produced in the DCPN treatment. In contrast to the combined treatment^15,16) of nitriding followed by coating, the method developed by the authors allows simultaneous nitriding and coating. To date, and to the best of our knowledge, no studies have investigated the simultaneous formation of a composite nitride layer on top of a nitrogen diffusion layer via plasma nitriding. Commercially available expanded metal materials that can be used as screens are limited, and those that can be processed into screens have restricted workability. Therefore, the authors have devised a plasma nitriding method using a metal combination screen, which can be easily fabricated from difficult-to-work materials using wire-shaped metal components and is expected to form various types of composite nitrides. Ti–Mo composite nitride, a nonferrous nitride with high hardness,^17,18) was used in this study. ASDCPN treatment with a Ti–Mo combination screen was applied to simultaneously form a Ti–Mo composite nitride layer and a nitrogen diffusion layer in SUS304. The microstructure and mechanical properties of the ASDCPN-treated SUS304 were then evaluated.

2. Experimental Procedure

A SUS304 plate specimen (15 mm in diameter and 5 mm thick) was used as the sample. A Ti–Mo combination screen with an aperture ratio of 5% was fabricated using 0.50 mm-diameter Ti (purity, 99.5%) and Mo (purity, 99.95%) wires at a length ratio of 5:1. For comparison, an SUS304 screen was also fabricated using commercially available expanded metal. Figure 1 illustrates a schematic of the ASDCPN equipment. ASDCPN treatment was conducted using a DC plasma nitriding apparatus (JIN-1S, NDK, Japan). SUS304 bases were placed at equal intervals on the furnace stage, and samples were positioned on these bases. A Ti–Mo combination screen, with a diameter of 170 mm and a height of 200 mm, was installed to cover the samples. The distance between the screen and the center of the sample was 30 mm. The ASDCPN treatment conditions were as follows: treatment temperature of 773 K, treatment time of 16 h, treatment pressure of 100 or 200 Pa, N₂:H₂ gas ratio of 1:1 or 3:1, and voltage of 230–290 V. Plasma nitriding was performed with the furnace wall as the anode and the sample and screen as the cathodes. The plasma-nitrided samples were analyzed using X-ray diffraction (XRD) (RINT-2500, Rigaku, Japan) with CuKα radiation, cross-sectional scanning electron microscopy (SEM) (JSM-6060LV, JEOL, Japan), electron probe microanalysis (EPMA) (JXA-820, JEOL, Japan), and glow-discharge optical emission spectroscopy (GD-OES) analysis (GD-Profiler2, Horiba, Japan). Micro-Vickers hardness tests were performed at 10 points with a load of 0.25 N. Friction and wear tests (Tribogear CAT, Shinto Scientific, Japan) were conducted with a load of 10.6 N, 10-mm-diameter WC ball as a mating material, sliding distance of 5 mm, sliding speed of 600 mm/min, and reciprocating sliding frequency of 1000 times. Additionally, an adhesion evaluation test was performed to measure the adhesion of the nitride layer to the sample surface. An indentation was made with a Rockwell hardness tester (MRK-M, Matsuzawa, Japan) and examined using field-emission SEM (JSM-6330FII, JEOL, Japan); adhesion was assessed based on the cracks and delamination observed around the indentation.¹⁹⁾

Fig. 1. Schematic of ASDCPN equipment.

3. Results and Discussions

3.1. Surface Microstructure

Figure 2 displays the XRD patterns of the whole surface of the plasma-nitrided samples. Diffraction peaks of the S-phase,²⁰⁾ a supersaturated solid solution of nitrogen in austenite, were observed for SUS304 100 Pa1:1. Similar diffraction peaks of the S-phase were confirmed for Ti–Mo 100 Pa1:1, Ti–Mo 200 Pa1:1, Ti–Mo 100 Pa3:1, Ti–Mo 200 Pa3:1, and SUS304 100 Pa1:1; however, the intensities of the S-phase peaks from Ti–Mo 200 Pa1:1, Ti–Mo 100 Pa3:1, and Ti–Mo 200 Pa3:1 were lower than those of SUS304 100 Pa1:1, due to the formation of a nitride layer on top of the S-phase. In addition to the S-phase peaks, the diffraction peaks of CrN were observed for all the nitrided samples, and the peaks of γ’-Fe₄N were also observed for SUS304 100 Pa1:1 and Ti–Mo 100 Pa1:1, indicating the decomposition of a part of the S-phase at the high treatment temperature of 773 K. Furthermore, a part of CrN formed by sputtering on the surface of the sample and the base was included in the nitride layer during nitriding. Peaks of δ-(Ti,Mo)N^17,21) were observed for Ti–Mo 200 Pa1:1, Ti–Mo 100 Pa3:1, and Ti–Mo 200 Pa3:1, suggesting the formation of a Ti–Mo–N composite nitride layer on the sample surface. The gas pressure and nitrogen content were found to influence the formation of the S-phase and Ti–Mo–N layers. During plasma nitriding with a metal screen, nitrogen ions sputter the screen, forming nitrides with nitrogen in the plasma, which are subsequently deposited on the sample surface.^1,3,9) Previous studies detected single nitrides, such as TiN and CrN, in steel plasma-nitrided with Ti²²⁾ or Cr²³⁾ screens. This study detected δ-(Ti,Mo)N, a compound nitride, using a Ti–Mo combination screen with two nitride-forming elements. Figure 3 presents cross-sectional SEM images of the plasma-nitrided samples. An S-phase was observed in SUS304 100 Pa1:1, consistent with the XRD results. In Ti–Mo 100 Pa1:1, the S-phase was present with a similar thickness to that in SUS304 100 Pa1:1. Ti–Mo 100 Pa3:1 and Ti–Mo 200 Pa1:1 had a thinner S-phase compared to SUS304 100 Pa1:1 and Ti–Mo 100 Pa1:1, while no S-phase was observed in Ti–Mo 200 Pa3:1. Figure 4 illustrates elemental mapping images of the plasma-nitrided samples obtained using EPMA. In Ti–Mo 100 Pa1:1, Ti–Mo 200 Pa1:1, and Ti–Mo 100 Pa3:1, N was confirmed to be in solid solution within the substrate. No significant enrichment of Ti or Mo was observed in Ti–Mo 100 Pa1:1. However, Ti and Mo were enriched on the top surfaces of Ti–Mo 200 Pa1:1, Ti–Mo 100 Pa3:1, and Ti–Mo 200 Pa3:1, with N, Ti, and Mo being co-located on these surfaces. Plasma nitriding with a Ti–Mo combination screen produced a composite nitride, the Ti–Mo–N layer, on the sample surface. This layer was thicker when the gas pressure or nitrogen content was higher, resulting in more deposits from the screen. Figure 5 illustrates the GD-OES analysis results for Ti and Mo in the plasma-nitrided samples. Ti–Mo 200 Pa1:1 and Ti–Mo 100 Pa3:1 had higher concentrations of Ti and Mo than Ti–Mo 100 Pa1:1, and the enriched zone was thicker. The area with high concentrations of Ti and Mo is the Ti–Mo–N layer. Among the samples, Ti–Mo 200 Pa3:1 exhibited the thickest Ti and Mo enriched areas. In a previous study, a TiN layer approximately 1 μm thick was formed using a Ti screen under similar conditions; however, at a higher treatment temperature of 823 K, treatment time of 10 h, treatment pressure of 500 Pa, and an N₂:H₂ gas ratio of 3:1.²²⁾ Figure 6 presents the GD-OES analysis results for N in the plasma-nitrided samples. The thickness of the N-enriched area was consistent with the observations from cross-sectional SEM images. The nitrogen diffusion layers in Ti–Mo 200 Pa1:1, Ti–Mo 100 Pa3:1, and Ti–Mo 200 Pa3:1 were thinner than those of Ti–Mo 100 Pa1:1.

Fig. 2. XRD patterns of plasma-nitrided samples. (Online version in color.)

Fig. 3. Cross-sectional SEM images of plasma-nitrided samples.

Fig. 4. EPMA elemental mapping images of plasma-nitrided samples.

Fig. 5. GD-OES analysis results for Ti and Mo in plasma-nitrided samples. (Online version in color.)

Fig. 6. GD-OES analysis results for N in plasma-nitrided samples. (Online version in color.)

R.R.M. de Sousa et al. reported that using a SUS316 screen with uniformly perforated holes during plasma nitriding resulted in a larger mean free path of ion particles reaching the screen at lower gas pressures, which led to increased sputtering on the screen.²⁴⁾ Additionally, plasma nitriding with an expanded metal SUS304 screen at a treatment temperature of 723 K, a treatment time of 5 h, a gas ratio of N₂:H₂ = 1:3, and gas pressures of 100, 600, and 1200 Pa, revealed that the sample nitrided at 100 Pa exhibited the thickest nitrogen diffusion layer.²⁵⁾ For iron-based screens, nitrogen diffuses into the sample as the screen-derived iron nitrides decompose into stable nitrides, forming only a nitrogen diffusion layer.^{5,10,12,13,14)} Consequently, with an iron-based screen, the nitrogen diffusion layer was thicker at lower gas pressures due to increased deposits at lower pressures. Conversely, in this study, both the number of deposits and the nitride layer thickness increased with higher gas pressure. The Ti–Mo combination screen used in this study, consisting of metal wires with a low aperture ratio of 5%, has a large screen surface area for sputtering. Therefore, a higher nitrogen content in the furnace increases the number of sputtered particles as nitrogen impinges on the screen. Additionally, since the sample in this study is not insulated and serves as a cathode, screen-derived nitrides are readily deposited on the sample surface.²⁶⁾ It has been reported that in plasma nitriding with a metal screen, when the sample is not insulated and serves as a cathode or is insulated and not a cathode, the growth rate of the nitrogen diffusion layer is less than that of the deposited layer if the sample is not a cathode, with the deposited layer suppressing nitrogen diffusion layer formation.²⁶⁾ For the Ti–Mo combination screen, the thicker nitride layer on the sample surface results in a thinner nitrogen diffusion layer, indicating that the composite nitride is minimally decomposed and does not significantly contribute nitrogen. Due to the high affinity of Ti and Mo with nitrogen, the composite nitride formed using the Ti–Mo combination screen remains stable, forming a nitride layer atop the nitrogen diffusion layer. Consequently, the formation of a Ti–Mo–N layer on the sample surface suppresses nitrogen diffusion.

As observed in the cross-sectional SEM image (Fig. 3), the elemental mapping by EPMA (Fig. 4), and the GD-OES analysis results (Figs. 5 and 6), the Ti–Mo–N layer is thicker and the S-phase is thinner when either the gas pressure or nitrogen ratio is increased. In this study, the plasma nitriding process incorporates a metal screen as a cathode material into the conventional DCPN method. Consequently, the nitride layer formed from the screen develops concurrently with the nitrogen diffusion layer produced by the DCPN process, thereby influencing the thickness of the nitrogen diffusion layer.

3.2. Characterization of Surface Mechanical Properties

Figure 7 illustrates the adhesion evaluation test results of the plasma-nitrided samples. Ti–Mo 200 Pa1:1 and Ti–Mo 100 Pa3:1 exhibited HF1 and HF2, respectively, indicating strong adhesion. In contrast, Ti–Mo 200 Pa3:1, which had a thin nitrogen diffusion layer, showed HF5, indicating weak adhesion. The adhesion of a hard layer in surface modification is influenced by the consistency of the physical properties between the substrate and the hard layer. Adhesion is typically stronger when a hard intermediate layer is present between the substrate and the hard layer, compared to a hard layer on a substrate with low hardness.^27,28,29,30) Cross-sectional SEM images confirmed the presence of a hard S-phase and a nitrogen diffusion layer beneath the nitride layer in Ti–Mo 200 Pa1:1 and Ti–Mo 100 Pa3:1. Therefore, the hardness difference between the substrate and the Ti–Mo–N layer was minimal, and the deformation of the substrate during the Rockwell hardness test followed that of the Ti–Mo–N layer, making delamination difficult. This suggests that the formation of a nitrogen diffusion layer between the substrate and the Ti–Mo–N layer suppresses the exfoliation of the nitride layer. Figure 8 presents the Vickers microhardness test results of the plasma-nitrided samples. The hardness of the SUS304 100 Pa1:1 sample was higher than that of the non-treated sample due to the formation of a nitrogen diffusion layer on the surface. Ti–Mo 100 Pa1:1 exhibited greater hardness than SUS304 100 Pa1:1 because the Ti–Mo combination screen was deposited on the nitrogen diffusion layer. Ti–Mo 200 Pa1:1 and Ti–Mo 100 Pa3:1 demonstrated higher hardness than Ti–Mo 100 Pa1:1, attributed to the formation of a Ti–Mo–N layer on the nitrogen diffusion layer. However, the hardness of Ti–Mo 200 Pa3:1 was lower than that of SUS304 100 Pa1:1 because the nitride layer in Ti–Mo 200 Pa3:1 was thick, while the nitrogen diffusion layer was relatively thin. Figure 9 presents the friction and wear test results of the plasma-nitrided samples. Ti–Mo at 100 Pa1:1 exhibited a lower kinetic friction coefficient than SUS304 at 100 Pa1:1, due to its higher hardness, as confirmed by the Vickers microhardness test results. Ti–Mo 200 Pa1:1 and Ti–Mo 100 Pa3:1 exhibited lower kinetic friction coefficients than both SUS304 100 Pa1:1 and Ti–Mo 100 Pa1:1. This can be attributed to the formation of a Ti–Mo–N layer on top of the nitrided layer. Conversely, Ti–Mo 200 Pa3:1 exhibited a higher coefficient of kinetic friction than SUS304 100 Pa1:1, likely because Ti–Mo 200 Pa3:1 had lower hardness and weaker adhesion between the nitride layer and the substrate (Fig. 7). Ti–Mo 200 Pa3:1 exhibited a higher kinetic friction coefficient than the non-treated sample, suggesting that the nitride layer was exfoliated. Ti–Mo 200 Pa1:1 and Ti–Mo 100 Pa3:1 exhibited lower friction coefficients compared to a TiN coating deposited via the arc-PVD method.³¹⁾

Fig. 7. Adhesion evaluation test results of plasma-nitrided samples.

Fig. 8. Vickers microhardness test results of plasma-nitrided samples. (Online version in color.)

Fig. 9. Friction and wear test results of plasma-nitrided samples. (Online version in color.)

As noted, incorporating the metal combination screen into the DCPN treatment successfully formed a hard composite nitride layer on the nitrogen diffusion layer. Plasma nitriding with a metal combination screen represents a novel surface-treatment technology capable of easily creating a composite nitride layer.

4. Conclusions

In this study, ASDCPN treatment was applied using a Ti–Mo combination screen to simultaneously form a Ti–Mo composite nitride layer and a nitrogen diffusion layer in SUS304. The microstructure and mechanical properties of the ASDCPN-treated SUS304 were evaluated. The primary findings of this study are as follows:

(1) XRD peaks of δ-(Ti,Mo)N, derived from the screen’s constituent elements, were observed in samples plasma-nitrided using a Ti–Mo combination screen.

(2) A Ti–Mo–N composite nitride layer was successfully formed on the sample surface via plasma nitriding with a Ti–Mo combination screen.

(3) The Ti–Mo–N layer thickness increased with higher gas pressure and nitrogen content in the gas.

(4) Higher gas pressure and nitrogen content in the gas corresponded to a thinner S-phase.

(5) The formation of the Ti–Mo–N layer on the nitrogen diffusion layer increased the hardness and reduced the kinetic friction coefficient.

Statement for Conflict of Interest

The authors have no conflicts of interest related to the conduct of this research.

Acknowledgment(s)

This work was supported by JSPS KAKENHI Grant Number JP23K04403.

References

• 1)   C.  Zhao,  C.X.  Li,  H.  Dong and  T.  Bell: Surf. Coat. Technol., 201 (2006), 2320. https://doi.org/10.1016/j.surfcoat.2006.03.045
• 2)   C.X.  Li and  T.  Bell: Wear, 256 (2004), 1144. https://doi.org/10.1016/j.wear.2003.07.006
• 3)   S.C.  Gallo and  H.  Dong: Vacuum, 84 (2009), 321. https://doi.org/10.1016/j.vacuum.2009.07.002
• 4)   Sh.  Ahangarani,  F.  Mahboubi and  A.R.  Sabour: Vacuum, 80 (2006), 1032. https://doi.org/10.1016/j.vacuum.2006.01.013
• 5)   A.  Nishimoto,  K.  Nagatsuka,  R.  Narita and  H.  Nii: Surf. Coat. Technol., 205 (2010), S365. https://doi.org/10.1016/j.surfcoat.2010.08.034
• 6)   C.X.  Li,  J.  Georges and  X.Y.  Li: Surf. Eng., 18 (2002), 453. https://doi.org/10.1179/026708402225006240
• 7)   C.X.  Li,  T.  Bell and  H.  Dong: Surf. Eng., 18 (2002), 174. https://doi.org/10.1179/026708401225005250
• 8)   B.C.E.S.  Kurelo,  G.B.  de Souza,  S.L.  Rutz da Silva,  N. de F.  Daudt,  C.  Alves, Jr.,  R.D.  Torres and  F.C.  Serbena: Surf. Coat. Technol., 275 (2015), 41. https://doi.org/10.1016/j.surfcoat.2015.03.052
• 9)   A.  Saeed,  A.W.  Khan,  F.  Jan,  M.  Abrar,  M.  Khalid and  M.  Zakaullah: Appl. Surf. Sci., 273 (2013), 173. https://doi.org/10.1016/j.apsusc.2013.02.008
• 10)   C.X.  Li: Surf. Eng., 26 (2010), 135. https://doi.org/10.1179/174329409X439032
• 11)   S.C.  Galloa and  H.  Dong: Scr. Mater., 67 (2012), 89. https://doi.org/10.1016/j.scriptamat.2012.03.028
• 12)   Y.  Hoshiyama,  R.  Mizobata and  H.  Miyake: Surf. Coat. Technol., 307 (2016), 1041. https://doi.org/10.1016/j.surfcoat.2016.07.032
• 13)   Y.  Hoshiyama,  K.  Chiba and  T.  Maruoka: Metals, 11 (2021), 412. https://doi.org/10.3390/met11030412
• 14)   Y.  Hoshiyama,  R.  Takatera and  T.  Maruoka: Mater. Trans., 60 (2019), 1638. https://doi.org/10.2320/matertrans.M2019066
• 15)   J.C.A.  Batistab,  M.C.  Josepha,  C.  Godoy and  A.  Matthews: Wear, 249 (2002), 971. https://doi.org/10.1016/S0043-1648(01)00833-X
• 16)   H.A.  Raza,  M.  Shafiq,  M.  Naeem,  M.Y.  Naz,  J.C.  Díaz-Guillén and  C.M.  Lopez-Badillo: J. Mater. Eng. Perform., 28 (2019), 20. https://doi.org/10.1007/s11665-018-3780-1
• 17)   S.  Komiyama,  Y.  Sutou and  J.  Koike: Metall. Mater. Trans. A, 42 (2011), 3310. https://doi.org/10.1007/s11661-010-0522-x
• 18)   T.  Toyoda,  Y.  Sutou,  S.  Komiyama,  D.  Ando,  J.  Koike and  M.  Wang: J. Japan Inst. Met. Mater., 79 (2015), 220 (in Japanese). https://doi.org/10.2320/jinstmet.JBW201401
• 19)   J.  Gerth and  U.  Wiklund: Wear, 264 (2008), 885. https://doi.org/10.1016/j.wear.2006.11.053
• 20)   Z.L.  Zhang and  T.  Bell: Surf. Eng., 1 (1985), 131. https://doi.org/10.1179/sur.1985.1.2.131
• 21)   S.  Komiyama,  Y.  Sutou and  J.  Koike: J. Japan Inst. Metals, 74 (2010), 135 (in Japanese). https://doi.org/10.2320/jinstmet.74.135
• 22)   A.  Yazdani,  M.  Soltanieh,  H.  Aghajani and  S.  Rastegari: Vacuum, 86 (2011), 131. https://doi.org/10.1016/j.vacuum.2011.04.020
• 23)   N.  Toshioka and  A.  Nishimoto: Mater. Trans., 61 (2020), 1115. https://doi.org/10.2320/matertrans.H-M2020815
• 24)   R.R.M.  de Sousa,  F.O.  de Araújo,  J.A.P.  da Costa,  T.  Dumelow,  R.S.  de Oliveira and  C.  Alves, Jr.: Vacuum, 83 (2009), 1402. https://doi.org/10.1016/j.vacuum.2009.04.054
• 25)   A.  Nishimoto,  K.  Nagatsuka,  R.  Narita,  H.  Nii and  K.  Akamatsu: J. ASTM Int., 8 (2011), 1. https://doi.org/10.1520/JAI103286
• 26)   A.  Nishimoto,  T.  Fukube and  T.  Tanaka: Mater. Trans., 57 (2016), 1811. https://doi.org/10.2320/matertrans.M2016209
• 27)   E.  De Las Heras,  D.A.  Egidi,  P.  Corengia,  D.  Gonzalez-Santamaria,  A.  Garcia-Luis,  M.  Brizuela,  G.A.  Lopez and  M.F.  Martinez: Surf. Coat. Technol., 202 (2008), 2945. https://doi.org/10.1016/j.surfcoat.2007.10.037
• 28)   M.  Raoufi,  Sh.  Mirdamadi,  F.  Mahboubi,  Sh.  Ahangarani,  M.S.  Mahdipoor and  H.  Elmkhah: Appl. Surf. Sci., 258 (2012), 7820. https://doi.org/10.1016/j.apsusc.2012.04.041
• 29)   F.  Cemin,  L.T.  Bim,  C.M.  Menezes,  M.E.H.M.  Costa,  I.J.R.  Baumvol,  F.  Alvarez and  C.A.  Figueroa: Surf. Coat. Technol., 283 (2015), 115. https://doi.org/10.1016/j.surfcoat.2015.10.031
• 30)   M.  Nöthe,  A.  Buuron,  F.  Koch,  H.J.  Penkalla,  W.P.  Rehbach and  H.  Bolt: J. Anal. Chem., 365 (1999), 249. https://doi.org/10.1007/s002160051482
• 31)   S.S.  Malvajerdi,  A.S.  Malvajerdi and  M.  Ghanaatshoar: Ceram. Int., 45 (2019), 3816. https://doi.org/10.1016/j.ceramint.2018.11.051