Low pressure plasma arc source ion nitriding compared with glow-discharge plasma nitriding of stainless steel

Surface and Coatings Technology 145 Ž2001. 31᎐37

Low pressure plasma arc source ion nitriding compared with glow-discharge plasma nitriding of stainless steel Wang LiangU , Sun Juncai, Xu Xiaolei Institute of Metals and Technology, Dalian Maritime Uni¨ ersity, Dalian, 116024, PR China Received 26 September 2000; accepted in revised form 27 April 2001

Abstract The nitrided layers produced by low temperature conventional d.c. glow discharge plasma nitriding and low pressure plasma arc source ion nitriding on AISI 304 austenitic stainless steel were studied using X-ray diffraction ŽXRD., transmission electron microscopy ŽTEM. and microhardness testing. The surface nitrogen content was determined by electron probe microanalysis ŽEPMA. and energy dispersive X-ray analysis ŽEDX.. The nitrogen content in the nitrided layer obtained using a low pressure plasma arc source is higher than that in the nitrided layer obtained by d.c. glow discharge plasma nitriding at 420⬚C. Microstructural analyses by XRD show that both treatments at the same temperature of ; 400⬚C lead to predominant formation of the f.c.c. nitrogen wNx solid solution phase ␥ N . However, the concentrations of N and the layer thickness of this phase are clearly different for the various treatments. There are substantial differences in microstructures and phases detected by TEM, which showed that the pure expanded austenite phase with a f.c.c. structure was formed in the nitrided layer for the low pressure plasma arc source, but for glow discharge plasma nitriding, the CrNq ␥ mixture was present in the nitrided layer even when nitriding at temperatures below 450⬚C. The reason for this is not clear. When the temperature was increased to 480᎐500⬚C, there was no evident difference in the microstructures and phases in nitrided layers obtained by both treatments. The nitrided layers all consisted of CrNq ␣-Fe phases. Both nitriding methods are able to harden the surface of austenitic stainless steel by nitrogen diffusion forming nitrided layers. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma nitriding; Austenitic stainless steel; XTEM

1. Introduction Nitriding is an effective surface hardening technique used to improve the wear resistance of steels. Although tribological properties of austenitic stainless steel are improved by nitriding treatment, an adverse effect on corrosion resistance is generally observed for conventional processing temperature Žapprox. 500⬚C. owing to the precipitation of CrN and the induced transforma-

U

Corresponding author. Present address: Thin Film Processing Group, Department of Surface Engineering, Korea Institute of Machinery & Materials, 66, Sangnam-dong, Changwon, Kyungnam, South Korea 641-010. Tel.: q82-55-280-3599; fax: q82-55280-3559. E-mail address: [email protected] ŽW. Liang..

tion of austenite to ferrite. At lower temperatures, a layer of metastable f.c.c. phase with a high concentration of nitrogen in solid solution is produced. This nitrogen supersaturated phase with the f.c.c. structure has been observed in d.c plasma nitriding w1,2,6x, ECR microwave plasma treatment w3x, plasma immersion ion implantation ŽPI 3 . w4x, beam ion implantation ŽBII. w5x and other nitriding treatments w7,8x. The microstructure and nitrogen content of the nitrided layers differ in different nitriding methods. Samandi et al. and Gunzel ¨ et al. w9,10x compared the results of AISI-316 austenitic stainless steel to both PI 3 and glow discharge plasma nitriding at temperatures between 350 and 520⬚C. Wei et al. w11x compared the results of AISI 304 stainless steel nitrided obtained by beam ion implantation,

0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 2 8 3 - X

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W. Liang et al. r Surface and Coatings Technology 145 (2001) 31᎐37

plasma ion implantation and plasma nitriding. Compared with PI 3 and beam ion implantation methods, ion nitriding results in lower contents of nitrogen at approximately 400⬚C. It has been shown that various techniques Žbeam implantation, plasma ion implantation and ion nitriding. can lead to the same surface phase Žexpanded austenite . at approximately 400⬚C but this conclusion is mainly based on XRD experimental results. Although Li et al. w13x reported differences between XRD and TEM, but that was due to preparation problems. Recently, Menthe et al. w14x gave the TEM results of a nitrided layer on AISI 304L austenitic stainless steel obtained by pulsed-glow discharge plasma nitriding at temperatures of 450⬚C for 5 h and Blawert et al. w15x carried out TEM observations on near surface of the austenitic stainless steel X6CrNiTi1810 after a PI 3 treatment at 400⬚C. The results show that TEM observations can give more details about the microstructure of the nitrided layer and can provide some different results from that obtained by XRD. Among different variants of the plasma nitriding technique, low pressure plasma arc source ion nitriding has been shown to be more effective in comparison to PIII and conventional glow discharge plasma nitriding in hardening austenitic stainless steels at low temperature, thus avoiding degradation of the corrosion resistance w12x. The similarities and differences of the nitrided layer obtained by conventional glow discharge plasma nitriding and low pressure plasma arc source ion nitriding have not been studied previously. In this work, the structure and properties of conventional glow discharge glow discharge plasma and low pressure plasma arc source ion nitrided AISI 304 austenitic stainless steel were characterized in order to establish the similarities and differences between these two nitriding processes.

2. Experimental details The material used in this work is AISI 304 austenitic stainless steel with the following chemical compositions Žwt.%.: C 0.03, Si 0.70, Ti 0.14, Mn 2.0, Cr 18.9, Ni 9.2, and Fe in balance. Samples 15 mm in diameter and 5 mm in thickness were cut from a bar. Before nitriding, the specimens were surface ground, followed by mechanical polishing. The original structures of the samples were austenite with a trace of ferrite created by mechanical polishing process in preparation. In the case of conventional glow discharge glow discharge ion-nitriding the specimens were mounted on the cathode in a commercial 50 kVA ion-nitriding furnace made by Dalian Maritime University. The chamber was evacuated to approximately 10 Pa and then filled with the process gas ŽNH 3 . to the operating pressure of approximately 500 Pa. A d.c. glow discharge

was generated. The specimens were heated by the ion-bombardment reaching the different nitriding temperatures. A glow discharge voltage of 750᎐800 V was applied on the cathode. The substrate current density was 2.2 mArcm2 . Low pressure plasma arc source ion nitriding was carried out using a 15-kW ion nitriding and ion plating unit made by Lanzhou vacuum equipment factory in China. The chamber was evacuated to a pressure approximately 5 = 10y2 Pa and the specimens were heated in vacuum to the intended treatment temperatures. The plasma was generated by a low pressure plasma arc source with a power of 10 kW. In this system a hollow cathode generates an arc discharge plasma which contains a high proportion of directed electrons with an enhanced mean energy, the so-called low voltage electron beam, resulting in a very effective ionization of the gas. Consequently, very high plasma densities Ž; 10 12 to 10 13rcm3 . can be achieved. The chamber was then filled with ammonia ŽNH 3 . to the intended working pressure of 0.4 Pa. A glow discharge negative bias voltage with amplitude of 0.8 kV was applied to the substrate. The arc current was 40 A with the voltage of 50 V. The ion current density was approximately 0.4᎐0.6 mArcm2 . The detail of this system was described previously w12,16x. The nitriding process started immediately as soon as the temperature was reached without pre-cleaning by ion sputtering. The processing time of 2 h was kept constant and the temperature was varied for the two processes. After the treatments, the samples were cooled in vacuum to room temperature. For nitriding of stainless steel, low pressure plasma arc source ion nitriding and glow discharge ion nitriding were performed for the same treatment time of 2 h, using the same treatment voltage of 800 V and the same working gas ŽNH 3 .. The differences in the experimental conditions were the plasma generation and the working pressure, during treatment, of 0.4 Pa in the case of low pressure plasma arc source and 500 Pa in the case of glow discharge glow discharge ion nitriding. The microstructure of the samples was examined by optical microscopy, X-ray diffraction ŽXRD. and transmission electron microscopy ŽTEM.. The samples were electrochemically thinned and ion thinned from the unnitrided side for TEM examination. The investigations were carried out on a HITACH H-800 electron microscope at an acceleration voltage of 175 kV. The phases were determined by evaluating the electron diffraction patterns as well as by X-ray diffraction in ␪᎐2␪ geometry using Cu K ␣ radiation. The surface hardness was measured with a MH-6 Vikers microhardness tester with loads of 1 N. The nitrogen concentration of nitrided layers was measured using EDX and EPMA. The corrosion resistance was examined utilizing the anodic polarization curves tested

W. Liang et al. r Surface and Coatings Technology 145 (2001) 31᎐37

in a 3.5% NaCl solution. The reference electrode was a standard calomel electrode ŽSCE..

3. Results and discussion A typical optical micrograph showing the cross-section of the nitrided layer produced at various temperatures for 2 h is shown in Fig. 1. The treated specimens are characterized by white layers of different thicknesses and display a nitrided layer with an abrupt interface towards the substrate when nitrided below 450⬚C. The thickness of the nitrided layer depends on treatment temperature ranging from 2 ␮m for glow discharge plasma nitriding and 10 ␮m for low pressure plasma arc source ion nitriding at 400⬚C, 13 ␮m for glow discharge plasma nitriding and 30 ␮m for low pressure plasma arc source ion nitriding at 460⬚C. No other microstructural features were revealed in the cross-sections of layers produced at temperature below 450⬚C by the optical metallographic technique. However, after nitriding at temperatures above 450⬚C, some dark phases were observed in the nitrided layer. Obviously, the formation of CrN phase significantly deteriorates the corrosion resistance of the nitrided layer.

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Fig. 2 shows XRD ␪᎐2␪ spectra in a selected angular range for samples that were treated with different processes at different temperature for 2 h. The influence of nitriding temperature on the microstructure of the modified layer can be seen from the X-ray diffraction patterns. The austenitic stainless steel in untreated state has the typical austenite peaks except that there is an additional small peak which can be attributed to a martensitic transformation of the very near surface that occurs during polishing of the sample. The sample that was d.c. glow discharge plasma nitrided at 350⬚C shows broader ␥ N Ž111. and ␥ N Ž200. peaks from the modified layer besides the ␥ Ž111. and ␥ Ž200. peaks from the substrate, indicating that ␥ N phase was formed. Strong austenite peaks from the substrate were observed indicating a shallow nitrided layer. The 350⬚C low pressure plasma arc source nitriding sample displays a series of distinct ␥ N peaks that are shifted to lower angles than those for the plasma nitrided samples, signifying a greater degree of nitrogen supersaturation and, hence, austenite expansion. An increase in the nitriding temperature results in an increase of the intensity of ␥ N peaks and a reduction of the ␥ peaks. At a temperature of 400⬚C, the substrate diffraction peaks disappear completely, and the only visible peaks

Fig. 1. Micrographs showing the morphology of nitrided layers produced by at different temperatures for 2 h Ža. glow discharge plasma nitriding Žb. low pressure plasma arc source nitriding.

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W. Liang et al. r Surface and Coatings Technology 145 (2001) 31᎐37

are the ␥ N Ž111. and ␥ N Ž200.. The full widths at half maximum of the ␥ N peaks are generally broader than those of the ␥ peaks, indicating that the nitrided layers formed are rich in defects and are highly stressed. The shift of ␥ N peaks indicates that the incorporated nitrogen concentration increases with nitriding temperature under the same processing time. The CrN phases could not be detected for both treatments at the temperature of approximately 400⬚C. In contrast, at 450᎐480⬚C, the main peaks were from ferrite and CrN. These results suggest that relatively high temperature treatments facilitate the precipitation of CrN and formation of ferrite in agreement with metallographic observations ŽFig. 1.. Contrary to the work of Samandi et al. w9x, no ␥⬘-Fe 4 N phase was detected in the modified layers. XRD reveals the formation of an expanded austenite phase for both nitriding techniques. However, the phase formed with low pressure plasma arc source ion nitriding is more expanded than that with glow discharge plasma nitriding, indicating the presence of a larger amount of nitrogen, which is consistent with the results of EDX measurement. After 400⬚C treatment, the N concentrations in the nitrided layers were 12 wt.% and 5.8 wt.% for low pressure plasma arc source ion nitriding and glow discharge plasma nitriding, respectively. From the measurements by EDX, the glow discharge plasma nitriding 304 SS exhibits lower nitrogen concentration in the nitrided layer obtained at 400⬚C, while

for low pressure plasma arc source ion nitriding 304SS results in a much higher N concentrations in the nitrided layers obtained at the same temperature. Fig. 3 shows the hardness as a function of temperature. The microhardness tests Žwith an applied load of 1 N. performed on the samples treated by both kinds of nitriding methods showed a clear hardness increase compared to the untreated samples. But the nitrided layers obtained by low pressure plasma arc source ion nitriding are much harder than those obtained by conventional glow discharge glow discharge ion nitriding especially for nitriding temperatures below 450⬚C. It is clear that larger improvements are associated with the samples which have the thicker modified layer and higher average N contents in solid solution. With nitriding temperatures above 450⬚C, the nitrided layers obtained by these two treatments have almost the same hardness values and consist of similar phases. Significant increases in surface hardness were observed owing to the formation of continuous nitride layers on the nitrided surface. The hardness reaches a maximum value at a temperature of 480᎐500⬚C. The bright and dark field images of nitrided layers obtained at 400⬚C by both methods near the surface are shown in Fig. 4. The difference in the microstructure of the modified layer is obvious for both nitriding processes. It is clear that the microstructure of the low pressure plasma arc source ion nitrided layer in this

Fig. 2. XRD spectra for glow discharge plasma nitrided samples and low pressure plasma arc source ion nitrided samples.

W. Liang et al. r Surface and Coatings Technology 145 (2001) 31᎐37

Fig. 3. Microhardness profiles of glow discharge plasma nitriding Ža. and low pressure plasma arc source nitriding Žb. as a function of treatment temperature.

region consists of a single phase, while a mixture of phases is seen for glow discharge plasma nitriding sample. The electron diffraction pattern confirms that only the ␥ N phase was present for low pressure plasma arc source ion nitriding carried out at 400⬚C. However, the sample that was nitrided at 400⬚C by glow discharge plasma nitriding shows, besides the ␥ reflections, also reflections that can be attributed to CrN. This phase

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has not been observed by XRD; evidently, the concentration of the CrN phase in the modified layer is very low, beyond the sensitivity of the X-ray diffractometer. After being nitrided at a temperature of approximately 460᎐480⬚C the modified layers display a lamellar structure showing a pearlite-type appearance by both methods. The typical bright and dark field images with selected area electron diffraction for this sample are shown in Fig. 5. The samples nitrided at 460⬚C are found to consist of a polycrystalline structure of CrN and ␣-Fe phase in accordance with the XRD results. The results presented above demonstrate unequivocally that distinct differences exist between the microstructure of austenitic stainless steel resulting from low pressure plasma arc source and glow discharge plasma nitriding at similar temperatures, especially at lower temperature. At temperatures above 450⬚C low pressure plasma arc source ion nitriding produces a nitrided layer with a microstructure and phase composition very similar to that produced by glow discharge plasma nitriding at the same temperature. In fact, high temperature nitriding by low pressure plasma arc source ion nitriding results in a distribution of nitrided layers similar to that observed in conventional plasma nitriding processing except the different thickness obtained for same treatment time.

Fig. 4. TEM of stainless steel nitrided at 400⬚C by glow discharge plasma nitriding Ža. micrograph of nitrided layer Žb. electron diffraction of nitrided layer showing mixture of CrN and ␥ phase Žc. micrograph of nitrided layer nitrided at 400⬚C by low pressure plasma arc source Žd. electron diffraction of the nitrided layer showing ␥ N single phase.

W. Liang et al. r Surface and Coatings Technology 145 (2001) 31᎐37

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Fig. 5. TEM of stainless steel nitrided at 460᎐480⬚C Ža. by glow discharge plasma nitriding and Žb. by low pressure plasma arc source nitriding.

The precipitation of CrN depletes the austenite of chromium, favoring the formation of ferrite and CrN in a lamellar structure. ␥ N phase is unstable due to supersaturation of N in the interstitial sites. Consequently, thermal decomposition into a more stable CrN is likely to occur with the release of nitrogen. From the results shown above, it is obvious that the temperature of the nitriding treatment plays a decisive role in the evolution of the surface microstructure. A distinct difference between low pressure plasma arc source ion nitriding and glow discharge plasma nitriding has been identified with respect to the degree of nitrogen supersaturation of the austenite and microstructure that can be achieved at low temperature. At higher temperature, these differences are not evident. It is proposed w9x that the extent of supersaturation and the resultant microstructure be primarily controlled by the nitrogen mass transfer mechanism. The layer thickness, phases and hardness of each treatment are compared in Table 1. For comparison, the polarization curves of the untreated substrate AISI 304 stainless steel, glow discharge plasma nitriding sample and low pressure plasma arc source ion nitriding sample nitrided at 400⬚C are displayed together in Fig. 6 with the measured corrosion potential and corrosion current densities given in Table 2. In 3.5% NaCl solution, the unnitrided samples did not passivate, but both the glow discharge plasma nitrided and low pressure plasma arc source nitrided passivate.

For the untreated substrate, the rapid increase in corrosion current density with applied potential is typical for non-nitrided substrate that does not form a protective passive oxide film. This rapid increase in corrosion current associated with the initiation and propagation of localized corrosion indicating that pitting corrosion occurred from the initial stage. Very clear evidence of pitting could be seen on the untreated substrate surface after testing. The glow discharge plasma nitrided stainless steel has a low corrosion potential of y265.5 mV SCE in the test environment and very high corrosion current density being of 7.15 ␮A cmy2 , indicating high dissolution rates. The polarization plot for low pressure plasma arc source nitrided stainless steel indicates a corrosion potential of 61.35 mV SCE and the presence of a pitting potential, at approximately 1000 mV. Below this pitting potential, the corrosion current remains low at less than 1 ␮A cmy2 . The low pressure plasma arc source nitrided sample possesses the most positive value of the corrosion potential and the lowest corrosion current density. After testing, the surface retains the original appearance just in the as received condition. On the other hand, the low pressure plasma arc source nitrided sample also shows a substantially wider passivation region compared to the glow discharge plasma nitrided sample. The low pressure plasma arc source nitrided sample treated at 400⬚C showed the lowest current density and the most positive breakdown potential and a 70-fold decrease in the passive current was achieved

Table 1 The layer thickness, phases and hardness of each treatment Process method

Treatment temp. Ž⬚C.

Nitrided layer thickness Ž␮m.

Phase ŽXRD.

Surface hardness ŽHV0.1 .

Glow discharge plasma nitriding

350 400 460 500 350 400 460 500

1.5 3 15 35 3 9 30 50

␥N ␥N ␥N q ␥ q CrNq ␣ CrNq ␣ ␥N ␥N ␥ N q CrNq ␣ CrNq ␣

380 460 580 1200 500 1120 1200 1200

Low pressure plasma arc source ion nitriding

W. Liang et al. r Surface and Coatings Technology 145 (2001) 31᎐37 Table 2 The corrosion test results of substrates and nitrided samples Sample

Ecorr ŽmV.

Icorr Ž␮Arcm2 .

Substrate Glow discharge plasma nitriding Low pressure plasma arc source ion nitriding

y178 y265.46

2.58 7.15

8.41 3.03

y164.5

0.29

75.41

R

Fig. 6. Potentiodynamic polarization behavior of modified layer obtained at 400⬚C Ža. glow discharge plasma nitriding, Žb. low pressure plasma arc source nitriding and Žc. 304 stainless steel substrate.

as compared with the glow discharge plasma nitrided sample. The corrosion resistance of the low pressure plasma arc source nitrided stainless steel is better than that of the glow discharge plasma nitrided one, implying that there are no or less CrN precipitates in the layer and this kind of nitrided layer may exhibit stronger passivation capacity than the glow discharge plasma nitrided layer. In contrast, the sample treated at a higher temperature exhibited very poor corrosion resistance. Clearly, the processing method and temperature play important roles in determining the corrosion resistance of the modified layers. The results show that for both the low pressure plasma arc source ion nitriding and glow discharge plasma nitriding treated specimens, a significant improvement in corrosion resistance was obtained when they were treated at a temperature of approximately 400⬚C. For these specimens, the improved corrosion resistance is attributed to the nitrogen diffusing into the modified layer.

4. Conclusions Compared to the glow discharge plasma nitriding treatment in this paper, low pressure plasma arc source

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ion nitriding results in both higher concentration and deeper penetration of nitrogen at the same temperature and bias voltage for the same treatment time. It has been proved that the low pressure plasma arc source ion nitriding treated sample possesses outstanding corrosion resistance when measured in a 3.5% NaCl solution. Nitriding AISI 304 stainless steel by both methods mentioned above at 400⬚C is successful to significantly improve pitting corrosion in 3.5.% NaCl solution. When conventional plasma nitriding and low pressure plasma arc source ion nitriding were compared using Bragg᎐Brentano X-ray diffraction, a ␥ N-like phase was detected on both nitriding processes. However, electron diffraction analysis of the nitrided layer show CrN precipitates in the conventionally nitrided sample but not in the low pressure plasma arc source ion nitriding, indicating that the concentration of CrN is below the resolution limit of X-ray diffraction methods. When nitriding at above 460⬚C, no substantial differences were found in the crystalline structure and phase constitution of the nitrided layers. The microstructure of the nitrided layer depends on the treatment temperature and technique. We observed in both cases, that an increase of the treatment temperature increases the tendency of CrN precipitating. This suggests that temperature should play an important role during the nitriding process. References w1x S.P. Hannula, P. Nenonen, J.P. Hirvonen, Thin Solid Films 181 Ž1989. 343. w2x Y. Sun, X.Y. Li, T. Bell, J. Mater. Sci. 34 Ž1999. 4793. w3x M.K. Lei, Z.L. Zhang, J. Vac. Sci. Technol. A15 Ž1997. 421. w4x M. Samandi, B.A. Shedden, D.I. Smith, G.A. Collins, R. Hutchings, J. Tendys, Surf. Coat. Technol. 59 Ž1993. 261. w5x D.L. Williamson, O. Ozturk, R. Wei, P.J. Wilbur, Surf. Coat. Technol. 65 Ž1994. 15. w6x E. Menthe, K.-T. Rie, J.W. Schultze, S. Simson, Surf. Coat. Technol. 74-75 Ž1995. 412. w7x M.J. Baldwin, M.P. Fewell, S.C. Haydon, S. Kumar, G.A. Collins, K.T. Short et al., Surf. Coat. Technol. 98 Ž1998. 1187. w8x N. Renevier, P. Collignon, H. Michel, T. Czerwiec, Surf. Coat. Technol. 111 Ž1999. 128. w9x M. Samandi, B.A. Shedden, T. Bell, G.A. Collins, R. Hutchings, J. Tendys, J. Vac. Sci. Technol. B12 Ž1994. 935. w10x R. Gunzel, M. Betzl, I. Alphonsa, B. Ganguly, P.I. John, S. ¨ Mukherjee, Surf. Coat. Technol. 112 Ž1999. 307. w11x R. Wei, J.J. Vajo, J.N. Matossian, P.J. Wilbur, J.A. Davis, D.L. Williamson, Surf. Coat. Technol. 83 Ž1996. 235. w12x L. Wang, X. Xu, Z. Yu, Z. Hei, Surf. Coat. Technol. 124 Ž2000. 93. w13x X. Li, M. Samandi, D. Dunne, G.A. Collins, J. Tendys, K.T. Short et al., Surf. Coat. Technol. 85 Ž1996. 28. w14x E. Menthe, K.-T. Rie, Surf. Coat. Technol. 116-119 Ž1999. 199. w15x C. Blawert, B.L. Mordike, Y. Jiraskova, ´ ´ O. Schneeweiss, Surf. Coat. Technol. 116-119 Ž1999. 189. w16x W. Liang, X. Bin, Y. Zhiwei, S. Yaqin, Surf. Coat. Technol. 130 Ž2000. 304᎐308.