Surface & Coatings Technology 201 (2007) 4919 – 4921 www.elsevier.com/locate/surfcoat
Plasma immersion ion implantation of nitrogen on austenitic stainless steel at variable energy for enhanced corrosion resistance K. Ram Mohan Rao a , S. Mukherjee b , S.K. Roy a , E. Richter c , W. Möller c , I. Manna a,⁎ a
Department of Metallurgical and Materials Engineering, I. I.T, Kharagpur 721302, W.B., India b FCIPT, IPR, B15-17/P, GIDC, Gandhinagar 382044, Gujarat, India c Forschungszentrum Rossendorf e.V., I.I.M, Postfach 51 01 19, Dresden, 01314, Germany Available online 23 August 2006
Abstract Plasma immersion ion implantation (PIII) of nitrogen on AISI 316L austenitic stainless steel has been investigated at four different negative implantation biases (5, 10, 15, 20 kV) and two different pulse-on-times (5 and 10 μs) for developing a nitrogen/nitride-rich corrosion resistive layer. Post implanted specimens were examined by X-ray diffraction, and subjected to potentiodynamic polarization tests in 1 wt.% NaCl solution. PIII at −15 kV shows significant and optimum improvement in corrosion resistance. © 2006 Elsevier B.V. All rights reserved. PACS: 52.77 Dq; 81.65 Ip Keywords: Plasma immersion ion implantation (PIII); Austenitic stainless steel; Nitrogen; Corrosion resistance
1. Introduction In the past, several attempts have been made to improve hardness and wear/corrosion resistance properties of austenitic stainless steel by plasma immersion ion implantation (PIII) [1–8]. As a logical continuation of our former investigation , this study is devoted to investigate the effect of implantation voltage in PIII to identify the most effective combination of process parameters for enhancement of hardness as well as corrosion resistance of austenitic stainless steel. Thus, PIII has been performed at different negative implantation bias and pulse-widths followed by microstructural studies and polarization tests to achieve the above objective. 2. Experimental Circular discs of AISI 316L austenitic stainless steel with 10 mm diameter and 3 mm thickness were used for PIII experiments performed at negative bias of 5, 10, 15 and 20 kV without external heating. PIII system has a base pressure of 2 × 10− 3 Pa,
⁎ Corresponding author. Tel.: +91 3222 283266; fax: +91 3222 282280. E-mail address: [email protected]
(I. Manna). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.177
0.4 Pa working pressure with nitrogen as the background gas, 400 W radio frequency power coupled by inductive methods to generate the plasma, 5 and 10 μs pulse duration for sample biasing and 1 h isochronal treatment time. Following PIII, the samples were subjected to phase identification/analysis by normal incidence X-ray diffraction (XRD) and glancing angle XRD (GAXRD) and polarization tests (in 1 wt.% NaCl) to asses the electrochemical behavior. 3. Results and discussion 3.1. X-ray diffraction study Glancing angle X-ray diffraction (GAXRD) analysis at 1° angle of incidence of plasma ion implanted samples exposed to 5 μs pulse-on-time shows that the surface microstructure is essentially single-phase austenite with implantation voltage up to −5 kV (Fig. 1). An identical phase evolution history is observed in the GAXRD analysis of the same set of samples after PIII with longer (10 μs) pulse-on-time. It is interesting to note that the increase in implantation voltage is accompanied both by an increase in volume fraction and interplanar spacing (d) or lattice parameter (a) of expanded austenite (Fig. 2). Perhaps, ion bombardment generated point defects and
K. Ram Mohan Rao et al. / Surface & Coatings Technology 201 (2007) 4919–4921
shorter pulse-on-time of 5 μs. Furthermore, expanded peak width of the γN(111) and γN(200) peaks with increase in applied voltage may be attributed to the effect of grain refinement (nanocrystallization) or even partial amorphization. Earlier, Samandi  obtained a thick amorphous layer within the top few micrometer of the surface of AISI 316 stainless steel after PIII. However, further evidence (say, transmission electron microscopy) is necessary to confirm the identity of this layer. The extent of peak broadening (hence grain refinement) is the maximum after PIII at −15 kV. The peaks posses greater intensity and show smaller broadening at −20 kV PIII as compared to that after implantation at −15 kV. This may be due to annealing of defects and subsequent grain growth occurring during PIII at higher implantation energy. 3.2. Electrochemical characterization Fig. 1. Glancing-angle XRD (GAXRD) of AISI 316L stainless steel at 1° angle of incidence in (a) as received condition and after PIII of nitrogen with 5 μs pulse-on-time and different levels of implantation voltage of (b) − 5 kV, (c) − 10 kV, (d) − 15 kV, and (e) −20 kV.
aided in extending nitrogen concentration beyond the thermodynamic equilibrium by several orders [9–11]. Fig. 2 shows that interplanar spacing (d ) steadily increases with implantation voltage. It may be pointed out that the d spacing (=0.224 nm) calculated in the present study after PIII at −20 kV implantation voltage for 1 h is higher than that (0.218 nm) reported by Collins et al.  obtained after PIII (at 45 kV, 3 h). Thus, the degree of nitrogen incorporation in the present experiment appears greater than that in an earlier study . Ion dose, calculated as per the equation by Qiu et al. , increases with the increase in implantation voltage and thereby increases the volume fraction of expanded austenite and interplanar spacing (d) or lattice parameter (a) (Fig. 2). PIII with longer pulse-on-time of 10 μs allows greater dissolution of nitrogen. The integrated intensity of the expanded austenite peaks after PIII with 10 μs pulse-width are marginally greater than that after PIII with
Potentiodynamic polarization tests of post PIII samples, performed in 1% NaCl solution, show the breakdown potential for unimplanted and post-PIII specimen at 5, 10, 15 and 20 kV implantation to be around 431.52, 430.58, 388.62, 496.27, and 307.15 mV, respectively (Fig. 3). At − 15 kV, there is a significant improvement in corrosion resistance which can be seen from the shifting of breakdown potential towards higher potential as compared to that in unimplanted condition. At − 20 kV, corrosion resistance could possibly decrease due to the growth of some anti-passivating phase (as evidenced by a sharp and intense peak of expanded austenite in Fig. 1) and the creation of local galvanic cell. The grain size values, calculated from broadening of the most intense peak of expanded austenite after PIII at −15 kV using Voigt method [15,16], lie in the nanometric range (b30 nm). At this small and uniform grain size, though the interfacial/grain boundary area significantly increases, the galvanic corrosion activity are significantly reduced due to homogeneity in composition and microstructure, and hence, uniform electrochemical potential in the matrix.
Fig. 2. Variation of lattice parameter (□) and interplanar spacing (■) of expanded austenite as a function of implantation voltage during PIII of N+2 on AISI 316L stainless steel with 5 μs pulse-on-time.
Fig. 3. Potentiodynamic polarization of AISI 316 austenitic stainless steel in 1 wt.% NaCl solution with a scan rate of 1 mV/s in unimplanted condition and after PIII of N+2 with 10 μs pulse-on-time at variable implantation voltages of −5 kV (curve S1), − 10 kV (S2), − 15 kV (S3),and − 20 kV (S4).
K. Ram Mohan Rao et al. / Surface & Coatings Technology 201 (2007) 4919–4921
Thus, nanocrystallization reduces corrosion after PIII at − 15 kV. On the other hand, the above advantages on small and uniform grain size disappear after PIII at − 20 kV due to grain coarsening (evidenced by reduction in peak width) and results in deterioration in corrosion resistance. 4. Conclusions (a) Volume fraction of expanded austenite increases with increase in implantation voltage. The microstructure at − 20 kV implantation voltage is predominantly single phase expanded austenite. (b) Increase in pulse-on-time increases the ion dose and nitrogen incorporation in the austenite lattice. (c) Both interplanar spacing and lattice parameter (up to 0.224 nm after PIII at −20 kV for 1 h) of expanded austenite increase with the increase in implantation voltage. This expansion is attributed to dissolution of nitrogen in expanded austenite. (d) Corrosion resistance of AISI 316L stainless steel improves with increase in implantation voltage up to −15 kV, but deteriorates at higher implantation voltage of −20 kV possibly due to grain coarsening at 20 kV. Acknowledgement Partial financial support from the DST, New Delhi (Grant No.: SP/S2/K-17/98) and Tata Steel, Jamshedpur (project code ‘SPI’) is gratefully acknowledged.
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