Effect of plasma immersion ion implantation of nitrogen on the wear and corrosion behavior of 316LVM stainless steel

Effect of plasma immersion ion implantation of nitrogen on the wear and corrosion behavior of 316LVM stainless steel

Surface & Coatings Technology 201 (2007) 8131 – 8135 www.elsevier.com/locate/surfcoat Effect of plasma immersion ion implantation of nitrogen on the ...

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Surface & Coatings Technology 201 (2007) 8131 – 8135 www.elsevier.com/locate/surfcoat

Effect of plasma immersion ion implantation of nitrogen on the wear and corrosion behavior of 316LVM stainless steel P. Saravanan a , V.S. Raja a,⁎, S. Mukherjee b a

Corrosion Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai-400 076, India b FCIPT, Institute for Plasma Research, Gandhinagar, 382044, Gujarat, India Available online 12 March 2007

Abstract Low energy plasma immersion ion implantation (PIII) of nitrogen ions on vacuum arc melted 316L (316LVM) austenitic stainless steel has been carried out at three different temperatures namely, 250 °C, 380 °C and 500 °C. X-ray diffraction analysis indicated that PIII results in the formation mixed iron-nitrides along with expanded austenite phase at all temperatures. Microhardness measurements revealed a significant increase in hardness after PIII treatment. Corrosion resistance in 3.5% NaCl increases when implanted for 3 h. The passive current density seems to increase with treatment temperature. Wear measurements carried out using a pin-on-disc machine show an increase in wear resistance with rise in treatment temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: 316LVM; Plasma immersion ion implantation; Corrosion; Wear

1. Introduction Most of the research and development in stainless steel continues to generate new ideas for improving the mechanical and corrosion properties of this important class of engineering materials. Development of austenitic stainless steels with improved properties had become wide spread in the 1980s. Then the addition of nitrogen to these steels was made to improve corrosion resistance and wear properties [1]. Since corrosion and wear behavior of an alloy primarily concern its surface, surface modification of austenitic stainless steels will be a viable route to improve their corrosion and tribological properties [2]. Plasma immersion ion implantation (PIII) developed in 1988 [3] has been proved to be one of the most promising surface modification techniques that can be exploited to modify the surface chemistry of an alloy. PIII, which involves both the implantation and diffusion of nitrogen, seems to be effective in the temperature range of 250–500 °C for stainless steels [4]. By this process it is possible to enhance simultaneously the wear and corrosion resistance. The phase formed by PIII is called

⁎ Corresponding author. E-mail address: [email protected] (V.S. Raja). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.149

‘expanded’ austenite [5,6] or γN phase [7,8]. However, despite numerous investigations, the structure and formation of this phase is not completely understood. These results are promising in view of industrial applications, because this treatment on austenitic stainless steel can be performed with techniques using less energetic ions [9–12] such as low-energy high current density ion implantation [13,14]. Using high current density, it is possible to obtain nitrided layers several micrometers thick. Hence the present study is concerned with examining how low-energy high current density PIII as a surface modifying technique affects the corrosion and wear behavior of vacuum arc melted 316L (316LVM) stainless steel, and to have a detailed study on the phase formed. 2. Experimental 316LVM stainless steel has the composition of 17.23% Cr, 14.85% Ni, 2.42% Mo, 0.029% C and 0.067% N. For plasma immersion ion implantation the samples were polished systematically using silicon carbide emery papers starting from 120 to 4/0 grade and finally using alumina powder of 0.25 μm. The experimental set-up for PIII comprises a vacuum chamber and associated pumping systems, a substrate holder and heater, a plasma source and a high-voltage pulsed power supply. At the

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Fig. 3. Hardness profile of untreated and PIII treated 316LVM SS for 3 h at 250 °C, 380 °C and 500 °C. Fig. 1. X-ray diffractograms of 316LVM after 3 h PIII treatment at 250 °C, 380 °C and 500 °C.

center of the vacuum chamber a substrate holder usually in the form of a disc was kept. Below the substrate holder, a heater was kept connected to a 1:1 ratio transformer. The system was pumped to a base pressure of 7.0 × 10− 3 Pa. From this the operating pressure of 10− 2–10− 1 Pa was achieved by introducing nitrogen in to the system. An electrically isolated thermocouple was used for measuring substrate temperature. Before the start of implantation, the surface oxide layer was removed by sputtering with argon gas for 30 min. The substrate was negatively biased to a voltage of 1 keV, and the implantation was done at 100 mA current with the dosage of 1.52 × 1019 ions/cm2 for 3 h and 3.04 × 1019 ions/cm2 for 6 h. Implanted samples were characterized by X-ray diffraction using an Expertpro diffractometer with CuKα radiation (λ = 0.154 nm) as a source. Load dependent surface microhardness testing was carried out on samples using a Leica Vickers indenter microhardness tester with the load varying from 15 to 300 g.

Fig. 2. X-ray diffractrograms of PIII treated 316LVM for three different durations at 380 °C.

Corrosion behavior of implanted sample was studied by using the cyclic polarization technique. The electrical connections were provided by soldering copper wire on the back side of implanted samples. Thick araldite (epoxy) coating was applied excluding the implanted surface used for corrosion studies, care being taken to avoid crevice formation at the interface. Polarization experiments were performed using the EG&G Potentiostat/Galvanostat model 273 operated by m352 SoftCorrIII. Studies were carried out in 3.5% NaCl solution. For the study, platinum sheet as a counter electrode and saturated calomel electrode (SCE) as a reference electrode were used. Potential was scanned at a rate of 0.5 mV/s. The dry wear behavior of the 316LVM stainless steel was studied by means of a Ducom friction monitoring machine TR20-M-2. It is a pin-on-disc machine, where the pin of 3 mm dia was rubbed over the alumina coated 304 disc at about 200 rpm at a constant load of 3.5 N. Data such as friction coefficient and wear loss in terms of sample thickness were continuously recorded. 3. Results and discussion Fig. 1 shows the X-ray diffractograms of 316LVM SS subjected to plasma source nitrogen ion implantation for 3 h at 250 °C, 380 °C and 500 °C. In Fig. 2, the variations in X-ray diffractrograms of this alloy for various time intervals of implantation at 380 °C are displayed. In both figures, X-ray diffractograms of untreated 316LVM SS are given for comparison. From Fig. 1, it is clearly seen that the untreated alloy exhibits the austenitic phase. At 250 °C, new broad peaks appear just ahead of (111) and (200) planes of austenite. With rise in temperature only one of the two sets of peaks is present in the XRD pattern. This can be considered as either due to shift in the austenitic peaks towards lower angles or the appearance of the newly emerged peak towards the lower angles. However, a close examination of the XRD pattern of the sample treated at 380 °C reveals that the peak corresponding to (111) of austenite is very small. This indicates that the appearance of the peak is something to do with the disappearance of peaks corresponding to the austenite phase. Broadening of the new peak might be due to a possible raise in the internal strain of the austenite lattice and or a possible variation in the lattice parameter of the

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Fig. 5. Pitting potential (0.6 M (3.5%) NaCl at 25 °C) versus PREN for stainless steel used in this study is located within the circled area [17]. Fig. 4. Cyclic polarization of 6h PIII treated and untreated 316LVM in 3.5% NaCl.

austenite phase along the thickness direction of the sample due to the variation in the concentration of N. Because the diffusion of N into the stainless steel (SS) at 250 °C might be very slow, it can cause a steep concentration profile with respect to N. Since N is expected to exhibit higher diffusion coefficients at 380 °C and 500 °C than at 250 °C, the former samples do not exhibit a steep concentration gradient such as that seen at 250 °C. The above propositions are supported by the following observations. While the alloy treated at 380 °C for 3 h does not show peak broadening as compared to the alloy treated at 250 °C for 3 h, the same alloy treated for 6 h at 380 °C shows a similar peak broadening. This indicates that at high temperature the sample needs to be implanted for longer time to build up a similar concentration gradient, as the N flux in the sample is high. It is possible that the N content of the austenite lattice may be too high, so that the phase should now be called as nitrides. The shape of the peak (decreasing intensity with increasing angle) indicates that the concentration of N rich phase (to be called either a nitride or a supersaturated austenite) decreases with increasing depth in the sample. This is supported by the fact that as the angle of diffraction (and incidence) increases, Xray signals come from deeper below the surface. The problem that arises in distinguishing nitrides from an expanded austenite lattice is that both exhibit a simple cubic structure. It is interesting to note that, though X-ray diffraction data of various austenitic stainless steels show similar patterns, as shown in the

present case, different authors have interpreted them differently. Thus, Samandi et al. [15] and Mukerjee et al. [5] attributed them to lattice expansion of the austenite phase, while Haen et al. [16] attributed them to mixed nitrides such as FeN, γ′-Fe4N, Fe4N. To make it simple the peaks are identified in a general way as MXN. An MXN phase can, in turn, be attributed to any of the following three phases: γ′-FeN, Fe4N, FeNiN. 4. Hardness Load dependent microhardness measurements have been carried out on the implanted samples. Fig. 3 shows the microhardness of 3 h plasma ion implanted 316LVM SS at different temperatures. For comparison, the hardness of untreated 316LVM SS is also shown. Fig. 3 shows that the implanted 316LVM SS exhibits higher hardness than that of the untreated alloy. The hardness profiles of 250 °C and 380 °C samples did not differ much, whereas, at 500 °C, implanted samples showed the highest hardness values. Further, from Fig. 3 it is clearly seen that the hardness decreases with increase in load, and at higher loads it is

Table 1 Electrochemical parameters of 3 h PIII treated and untreated 316LVM at 3.5% NaCl Alloy condition

Untreated 250 °C 380 °C 500 °C

Ecorr in mV vs SCE − 210 − 230 − 150 − 75

ipass in A/cm2 [at 100 mV vs SCE] −7

6.3 × 10 3.16 × 10− 7 3.4 × 10− 7 3.02 × 10− 7

Breakdown potentials mV vs SCE 1

2

3

108 120 120 25

– 490 500 550

– 1200 1200 1210

Fig. 6. Wear behavior of untreated and PIII treated 316LVM SS for 3 h at 250 °C 380 °C and 500 °C.

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almost constant, which suggests that nitrogen had diffused uniformly.

the surface, and it gradually decreases near the substrate. This will raise the PREN to 38, whereas earlier authors reported 5% at the surface for plasma nitriding [18].

5. Electrochemical corrosion studies 6. Wear studies Fig. 4 shows cyclic polarization curves of 316LVM SS implanted for 3 h at different temperatures as well as that for untreated material. Table 1 summarizes the data. Generally the treated alloy shows a slight increase in passivity (as exhibited by lower passive current density than that of the untreated alloy, as seen in Table 1) and a marked increase in pitting resistance. The following observations can be made on the data. The untreated material shows passive current density (ipass) of about 6.3 × 10− 7 A/cm2, but the anodic current shows the fluctuation even at low applied potential. The sample treated at 250 °C shows a marginal decrease in corrosion potential as compared to that of the untreated material. They have three break down potentials, which suggest that the alloy might have three different passive layers. Their final break down potential is found to be 1210 mV. The passive current density (ipass) decreased from 6.3 × 10− 7 A/cm2 (untreated) to 3.16 × 10− 7 A/ cm2 (treated). For the alloy treated at 380 °C, a shift in corrosion potential (Ecorr) from − 210 mV untreated to − 150 mV was observed. Further, all its break down potentials (380 °C) are almost the same as those observed in case of 250 °C treated material, eventhough a slight increase in passive current density as compared to both 250 °C was noticed. 500 °C treated material exhibited a slightly lower passive current density as compared to 250 °C and 380 °C. But its corrosion potential remained the same as that of 250 °C and 380 °C treated samples. There were three breaks in the passive region. The first break is lower than that of untreated material and other two break down potentials are the same as those of the other two treatment temperatures. At all treatment temperatures, the first break down might be due to nitride formed at the surface. From XRD analysis it is clearly seen that a nitride phase dominates over the austenitic phase, and this should lead to a decrease in the break down potential. Among various alloying elements added to stainless steel, only Cr, Mo, and N were found to be effective to promote localized corrosion resistance [17]. Pitting resistance of an alloy is indexed using the pitting resistance equivalent number (PREN). The relation between PREN and pitting potential is shown in Fig. 5, to show how effective N is in enhancing the pitting resistance of the stainless steel. Since untreated 316LVM SS contains 17.23% Cr, 2.42% Mo and 0.067% N, its PREN turns out to be 26.28. From the figure the corresponding pitting potential of 0.6 M (∼ 3.5%) NaCl solution is around 100 mV (SCE) for the above composition. This is in good agreement with the experimental results shown in Fig. 4, where a sudden increase in current density occurred at a potential of about 107 mV. As the plasma ion implanted sample shows a pitting potential of around 1 V, it should correspond to a PREN value of 38. Since only nitrogen has been added at the surface, the increase in the PREN value is attributable to enrichment of nitrogen at the surface. The nitrogen content on the sample surface can be around 12% in

Fig. 6 shows the wear rate of untreated and 3h PIII treated 316LVM for different temperatures. It is evident that PIII treatment has considerably improved the wear resistance of 316LVM SS. The samples treated at 500 °C and 380 °C for 3 h show better wear resistance than that of the sample treated at 250 °C for 3 h. At the initial stage, the 250 °C treated sample also exhibited good wear resistance, but soon it started behaving like the untreated one. This suggests that only a thinner coating existed on the sample treated at 250 °C, compared with that on the 380 and 500 °C treated samples. Samples treated at 500 °C and 380 °C show good wear resistance for longer duration, possibly because of greater depth of nitride. 7. Conclusion Low energy PIII treatment (1 keV) on 316LVM SS has been carried out at different temperatures to implant nitrogen. The XRD results reveal the existence of nitrides along with the expanded austenite lattice in the implanted alloys. Broad XRD peaks seen in the X-ray diffractograms are attributed to the presence of mixed nitrides such as FeN, γ′-FeN, Fe4N. Hardness of 316LVM SS has been found to increase with increasing treatment temperature. This has been attributed to increase in thickness of the nitrided layer. Corrosion resistance 316LVM SS increases drastically when implanted for 3 h. PIII treated 316LVM shows better wear resistance than that of untreated material, and the extent of wear resistance increases with increasing temperature of the sample during implantation. Acknowledgements The authors acknowledge Department of Science and Technology for providing financial support. Our thanks are due to Dr. S Ramadurai, Mishra Dhatu Nigam Ltd. Hyderabad for supplying the 316LVM sample. References [1] C. Allen, A. Ball, B.E. Protheroe, Wear 74 (1981) 287. [2] D.J. Mills, R.D. Knutsen, Wear 215 (1998) 83. [3] J.R. Conard, R.A. Dodd, F.J. Worzala, X. Qiu, Surf. Coat. Technol. 36 (1988) 927. [4] W. Ensinger, Surf. Coat. Technol. 100–101 (1998) 341. [5] S. Mukherjee, J. Chakraborty, S. Gupta, P.M. Raole, P.I. John, K.R.M. Rao, I. Manna, Surf. Coat. Technol. 186 (2004) 282. [6] D.L. Williamson, L. Wang, R. Wei, P.J. Wilbur, Materials Letter vol. 9 (1990). [7] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, X. LI, M. Samandi, Surf. Coat. Technol. 74–75 (1988) 417. [8] M.P. Fewell, D.R.G. Mitchell, J.M. Priest, K.T. Short, G.A. Collins, Surf. Coat. Technol. 131 (2000) 300. [9] T. Bacci, F. Borgioli, E. Galvanetto, G. Pradelli, Surf. Coat. Technol. 139 (2001) 251.

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