Thin Solid Films, 237 (1994) 175-180 175

Surface characterization of nitrogen-ion-implanted etched and polished AISI 316 stainless steel*

S a n j e e v A g g a r w a l , A . K . G o e l t a n d R . K . M o h i n d r a Department of Physics, Kurukshetra University, Kurukshetra 132 119 (India)

P. K . G h o s h Welding Research Laboratory, Department of Mechanical and Industrial Engineering, University of Roorkee, Roorkee 247 667 (India)

M . C. B h a t n a g a r Thin Films Laboratory, Indian Institute of Technology, New Delhi 110 016 (India)

(Received January 7, 1993; revised June 8, 1993; accepted June 22, 1993)

Abstract

The influence of nitrogen ion implantation on the surface characteristics of AISI 316 stainless steel has been examined on two different surfaces of the substrate: strain-induced (mechanically polished) and excess-strain-free (chemically etched) surfaces. Samples were implanted with 2.5 × 1021 N~" m -2 at 130 keV. The characteristics of the implanted layer and depth profile were investigated by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The indentification of the phase change and compounds formed was studied by glancing-angle X-ray diffraction. No phase transformation from the 7-austenite phase was observed as a result of mechanical polishing and after implantation. The formation of C r z N is observed in both surfaces with s o m e e-FezN-Fe3N on the polished surface only. The surface hardness is found to be enhanced after implantation in both these cases and its dependence is discussed with respect to the changes observed after implantation.

1. Introduction

Nitrogen is one of the most promising species used in ion implantation to improve the surface mechanical properties of various steels [1-9]. However, the mecha- nisms responsible for this improvement are not well understood. The modifications consist of microstruc- tural and compositional changes in the surfaces.

In the present work, the effects of high dose nitrogen ion implantation on the surface behaviour of AISI 316 austenitic stainless steel having different pre-implanta- tion surface conditions, namely strain-induced and ex- cess-strain-free surfaces (produced by mechanical polishing and the same followed by chemical etching respectively), have been examined by microhardness measurements which is an effective tool in this regard [8-12]. Glancing-angle X-ray diffraction (GXRD), X- ray photoelectron spectroscopy (XPS) and Auger elec- tron spectroscopy (AES) were used to characterize the implanted layer.

*Presented at the Twelfth International Vacuum Congress and Eighth International Conference on Solid Surfaces, The Hague, Netherlands, October 12-16, 1992.

tPresent address: Dayanand Postgraduate College, Hisar-125 001, India.

2. Experimental details

Samples of AISI 316 stainless steel were cut from a sheet 0.9 mm thick, supplied by M/S Goodfellow Metals Ltd., Cambridge, UK, having the following composi- tion: C, 0.066 wt.%; N, 0.0198 wt.%, O, 0.0075 wt.%; Mn, 1.86wt.%; Si, 0.37wt.%; Ni, 10.32wt.%; Cr, 16.48 wt.%; Mo, 2.24wt.%; Fe, balance. The samples were made optically flat by mechanical polishing with levigated alumina powder of particle size 0.014gm. Some of these were chemically etched by dipping into a solution containing 10 ml of HNO3, 20 ml of glycerol, 20 ml of HC1 and 10 ml of H202, followed by washing in running water to remove the work-hardened layer formed during mechanical polishing. Both the polished and the etched samples were ultrasonically cleaned in acetone, then in benzene and finally in isopropanol.

Ion implantation was done using a Varian DF-3000 machine equipped with a standard platen (there was almost no forced cooling of the specimen). The samples were implanted with N2 + ions at 130 keV with a dose of 2.5 × 1021 ions m -2 under a vacuum of 2 × 10 -6 Torr. The temperature during implantation was ~ 300 °C, which was measured using a thermocouple. The ion beam was electrostatically scanned over the entire sur- face at a beam current density of 3.4 × 10 -2 A m -2.

0040-6090/94/$7.00 © 1994 -- Elsevier Sequoia. All rights reserved

176 S. Aggarwal eta/./Surface characterization of N2+-implanted AIS1 316

The depth profile of N, in the implanted AISI 316 stainless steel samples was examined using a scanning Auger microprobe (SAM-PHI model 590A) combined with a 4 keV differentially pumped Ar + source for sputter etching. The ion beam current density was kept at ~ 3 A m 2, which provides a sputter rate of

100 A min '. The Auger analysis was performed at a 3 keV beam voltage with a beam current of about 5 jaA on a scanned area of 160 I~m × 160 jam. The elements monitored during the profiling were Fe, Ni, Cr, Mo, O and C. The data of interest presented for N are in the form of relative atomic concentrations estimated from the peak-to-peak height.

XPS studies were made using a PHI model 1800 instrument with Mg K~ (1253.6eV) radiation with a cylindrical mirror analyser. An ultrahigh vacuum of 10-9Tor t was maintained throughout the measure- ments. The Au 4f7/2 line (83.8 eV) was used for calibra- tion. The overall resolution was 1.2 eV for pure Ag 3d5/2 at a pass energy of 25 eV. The XPS data have been recorded for N ls, Cr 2p3/2 , Fe 2p3/2 Ni 2p3/2 near the peak region ( ~ 85 nm) on a mechanically polished, implanted sample, in which maximum nitrogen has been detected.

G X R D (X-radiation; Cu K~; 2 = 0.154 nm) was car- ried out on a Rigaku Giegerflex D/Max-RB diffrac- tometer, to analyse the samples both prior to and after implantation. The incident angle ~ between the beam and the sample surface was kept at 0.5-3.0 ° . For each angle of incidence, the X-ray penetration depth in the sample surface can be easily calculated [13] for pure iron a s t = 2 3 n m f o r ~ = 0 . 5 ° , t = 1 0 0 n m f o r ~ = l . 5 °, and t = 250 nm for c~ = 3.0 °.

The microhardness was measured using a Knoop microhardness tester (Leitz-MM6), for an indentation time of 15 s at test loads from 19.6 to 490.3 mN. The indentation depth was calculated using the relation for the depth of indentation: T = d/30 jam where d is the long indentation diagonal.

3. Results and discussion

Figure 1 depicts the atomic concentration of N in the implanted layer of mechanically polished and chemi- cally etched AISI 316 stainless steel samples, measured by AES, combined with Ar ion sputter etching. The peak nitrogen concentration in the polished implanted sample is ~ 11 at.% at a depth of ~ 85 nm and that in the etched implanted sample is ~ 13 at.% at ~ 35 nm depth. At greater depths, implanted nitrogen diffuses and decreases to ~ 8 at.% and ~ 5 at.% at ~ 300 nm in polished and in etched samples respectively. It is believed that the enhanced nitrogen migration involves the production and migration of interstitials and vacan-

~/ ,0

z20

°i0 =E o

DEPTH (n m)

30 60 90 120 150 180 210 240 270 300 i i i i J i

3 5 n m 8 5 n m

0 t I t i i 3 G 9 12 |5 '118 2il 2L 217 30

SPUTTER TIME ( r a i n . )

Fig. 1. AES depth profile of nitrogen content in mechanically pol- ished ( ) and chemically etched ( ) AISI 316 stainless steel, implanted with 2.5 × 102] N2 + m 2 at 130keV.

cies at the implantation temperature of about 300 °C [4-6]. This accounts [14] for the extended range of the implanted depth profile and the low peak atomic con- centration of nitrogen implanted in these samples. Also the diffusion of implanted nitrogen is greater in the polished surface which has more defects than in the etched sample.

XPS measurements in conjunction with Ar + sputter- ing were employed in order to examine the compounds formed in the implanted layer at the indicated region. Figure 2 shows the X-ray photoelectron spectra of N Is, Cr 2p3/2, Fe 2p3/2 and Ni 2p3/2 levels of the im- planted samples. Figure 2(a) shows the spectrum for N ls; the peak appearing at 396.8 eV corresponds to CrN [15]. The 397.4eV peak corresponding to Cr2N [15] seems to have merged into the broad peak of the N ls spectrum. However, the peak at 576.1 eV in the Cr 2p3/2

spectrum in Fig. 2(b) reveals more about chromium nitride formation [15]. The peak at 706.8 eV in Fig. 2(c) and the peak at 852.6 eV in Fig. 2(d) correspond to metallic iron and nickel respectively [16]. It is difficult to identify iron nitride compounds by means of XPS as their binding energy difference from metallic iron is only 0.3 eV [6, 15], which is too small to be unambigu- ously detected in our spectrometer which has a resolu- tion limit of 1.2 eV.

G X R D studies were carried out to characterize the polished and etched surfaces of type 316 stainless steel samples prior to and after implantation. Figure 3 shows the behaviour of the G X RD patterns of both surfaces before implantation, for 20 (Bragg angle) values be- tween 42 ° and 45 °. Curve a is for the etched sample at c( = 3.0 °. Curves c and b are for the polished sample at

= 0.5 ° and 1.5 ° respectively. It is observed that, in all these cases only the main ?-austenite (111) peak (I/Io (Joint Committee of Powder Diffraction Standards (JCPDS) = 100) is present corresponding to 43.54 °, and no martensite line (at 44.67 °) is seen (curve a-c) . So the stress-induced 7'-martensite due to mechanical polish-

S. Aggarwal et al. / Surface characterization of Nz+-implanted AISI 316 177

a ~ CrN 096.8)

CrzN t [ ~ L

~ N l ( I s ) ~

I ~ -/-.00 -398 -396

B E ,=3, -39& -716 -712 -708

8E=:>

-58

b r-,'- CrN Cr2N I 1575.61

(576'17---'--] I Cr

I I %

-578 -574 -570 BE

-70&

r I

-865 -860

Ni (852

I -855 -850

Fig. 2. X-ray photoelectron spectra recorded in the peak region of N2÷-implanted AISI 316 stainless steel: (a) for N Is; (b) for Cr 2p3/2; (c) for Fe 2p3/2; (d) for Ni 2p3/2.

8

6

! ,,i o. 4 (111 ob ent

o f

42 2 e ( d e g r e e s ) =:>

Fig. 3. G X R D patterns of unimplanted AISI 316 stainless steel samples: curve a, chemically etched surface at ct = 3.0 °; curve b, mechanically polished surface at ct = 1.5 °; curve c, metchanically polished surface at ~ = 0.5 °.

ing of the AISI 316 stainless steel samples may not be present here.

Earlier it has been reported that the mechanically polished samples of type 304 stainless steel [5] and type 302 stainless steel [9] have been found to show the formation of ~'-martensite phase by GXRD analysis and also it has been further confirmed by conversion electron M6ssbauer spectroscopy (CEMS) [5].

The difference in the present observation from this earlier work [5, 9] regarding the formation of the ~'-martensite phase in mechanically polished AISI 316

stainless steel, as confirmed by GXRD analysis may be primarily due to its comparatively higher Ni equivalent (13.8) than observed in the type 304 stainless steel (Ni equivalent, 12.5) and in the type 302 stainless steel (Ni equivalent, 11.7) in the above earlier investigations de- scribed in refs. 5 and 9 respectively.

Figures 4 and 5 contain GXRD results for polished implanted and etched implanted type 316 stainless steel samples with ~ = 3.0 ° (t = 250 nm). In both figures, there is a family of peaks labelled 7 which correspond to the austenite phase. X-ray diffraction can reveal the presence of dissolved nitrogen through the resultant change in lattice parameter and peak shift. Split 7 peaks are present in both cases (Figs. 4 and 5) with a peak shift towards the left which indicates the presence of nitrogen in solid solution. Besides the split 7 peaks with a peak shift, there is also some contribution at the unshifted positions in both cases. As is clear from Figs. 4 and 5, there is no peak corresponding to the ~'- martensite phase; so we exclude here the transformation of ~,-austenite to ~'-martensite phase after implantation. This is in agreement with the earlier observations [17] where also no martensite was detected after nitrogen implantation in the stainless steel surfaces having a 100% austenite phase.

In Fig. 4 for the GXRD pattern of the polished implanted AISI 316 stainless steel sample, the highest intensity peak observed at 42.7 ° corresponds to the major peaks ( I / I o (JCPDS) = 100) of Cr2N and/or (Cr, Fe, Ni)2N ] _ x. The formation of Cr2N is also confirmed by the presence of its other intensity peaks at 37.3 ° and

178 S. Aggarwal et al./ Surface characterization of Ne+-implanted AISI 316

1.2

U1 n C.)

o

~r (J

o "o o c o o ~"~ I

g : s

~ o L . z c ~ z g . u .

m . . - ~ TM 6~

. i r ~ . J

20 40

o

Z

e. .

u~ 04 i (.3

, ,~ ~ r ~

60 80 90 2 0 (degrees) =C>

Fig. 4. GXRD pattern of N2+-implanted, mechanically polished AISI 316 stainless steel at ~ = 3'

1'2

g

c7 z ~

c~ o_ ;, -o c~ 0 ~ , ~ 0 ~ ~

m ¢ , ~ I i ¢~ -.it ¢~ , i , c a d e . .

. ' i i ,

20 40 60 80 90 2 8 (degrees) =~>

Fig. 5. GXRD pattern of N:+-implanted, chemically etched AISI 316 stainless steel at ~ = 3 ~

67.7 ° . The peaks observed at 38.8 ° and 39.8 ° corre- spond to the main lines (I/Io = 100) o f hexagonal ni- tride E - F e 2 N - F e 3 N and o f FeNiN. The peak present at 35.5 ° corresponds to the formation of an oxide, which may be either Fe304 or Cr302 as the strongest intensity peaks at 35.43 ° and 35.58 ° respectively ( from JCPDS) are very close to this observed peak at 35.5 ° . The formation of such an oxide containing Fe304 and/or Cr302 has also been shown earlier [7, 9] after N2 + implantation o f type 304 stainless steel and type 302 stainless steel respectively.

In the G X R D pattern o f the etched implanted AISI 316 stainless steel (Fig. 5), again the formation o f CrzN is clear because o f the presence o f its strongest intensity peak at 42.5 ° and other intensity peaks at 37.4 ° and 65.7 °. Here, the intensity o f the Cr2N peak is found to be comparatively less than that obseved in the case o f the polished implanted sample (Fig. 4). This may be due to a higher atomic concentration of implanted nitrogen (at t = 250 nm) in the polished implanted sam- ple than in the etched implanted sample as shown by AES (Fig. 1). The presence o f a higher atomic concen-

S. Aggarwal et al./ Surface characterization of N2+-implanted AISI 316 179

tration of implanted nitrogen in the highly strained stainless steel surface than in a less-strained surface at the same depth (t = 250 nm) has also been shown by Shrivastava et al. [8]. Besides this, the presence of a more damaged region in the polished implanted sample may promote the formation of Cr2N over a thicker layer than for the etched surface.

Also the highest intensity peak of Fe304 and/or C r 3 0 2 is present at 35.9 °. In the present work at :~ = 3.0 ° (t = 250 nm) the main peak of CrN at 43.8 ° may have merged into the main peaks of CrzN and the e-austenite phase. The formation of CrN has already been observed [13, 18] in the upper layers of the im- planted region and, in the surface region > 60 nm, Cr2N increases and CrN diminishes. In the present work the formation of CrN and Cr2N is observed in the peak region (85 nm) using XPS, as already discussed.

4. Near-surface hardness

The effect of N2 ÷ ion implantation on the surface hardness of the mechanically polished and the chemi- cally etched samples is shown in Fig. 6 as a function of the depth of indentation corresponding to different applied loads. The depth of indentation gives a semi- quantitative evaluation of the flow behaviour of the

I~00

1200

1ooo

800

600 2: r,,

zOO

200

a

b

c

0 I I ; J 0 500 1000 15 0 2000

DEPTH OF INDENTATION (rim)

Fig. 6. Variation in Knoop hardness in kilograms-force per square millimetre against depth of indentation for AISI 316 stainless steel (dose, 2.5 × 1021 N2 ÷ m -2 at 130keV): curve a, mechanically pol- ished implanted; curve b, etched implanted; curve c, mechanically polished unimplanted; curve d, etched unimplanted. The indentations were made at 19.62, 49.05, 98.1, 245.25 and 490.5rnN loads and correspond to increasing depth of indentation.

implanted layer. The measured hardness is not the true hardness of the implanted layer itself, but it projects an average hardness of the surface layer modified by the ion implantation. Curve c in Fig. 6 shows the hardness behaviour of a mechanically polished sample before implantation. The observed microhardness near the sur- face is comparatively higher, owing to the presence of strained layers formed during mechanical polishing at the surface. After chemical etching, these layers are removed completely, thus lowering the microhardness as shown by curve d in Fig. 6.

Nitrogen implantation has increased the Knoop sur- face hardness at a load of 19.62 mN by 884 HK and by 697 HK in the case of the mechanically polished and the chemically etched samples of type 316 stainless steel (curves a and b in Fig. 6) respectively. At higher loads the probe depth of the indenter exceeds the depth of the implanted layer, resulting in a large contribution from the substrate. Because of this, the hardness values fall to that of the unimplanted samples with increasing depth of indentation corresponding to different applied loads.

Nitrogen is interstitial in type 316 stainless steel and produces solid solution hardening. Additional strength- ening mechanisms are associated with the formation of nitrides and oxides. GXRD analysis has revealed the formation of Cr2N in both types of implanted surfaces, but its formation is greater and it is extended over a thicker layer in the mechanically polished surface to- gether with the formation of E-FezN-Fe3N and FeNiN, which are responsible for the higher increase in relative surface hardness in this case compared with etched implanted surface.

5. Conclusions

The main results come from the comparison of pol- ished and etched surfaces before and after N2 + implan- tation and after microhardness measurements.

GXRD studies of the polished surface before implan- tation show the absence of the c(-martensite phase in the work-hardened surface and only the 7-austenite phase exists in the same surface before and after chemical etching. However, for more clarification about the ab- sence of the ~' phase on the mechanically polished type 316 stainless steel surface, CEMS will be carried out.

After implantation with a dose of 2.5 × 102~ N2 + m -2 (130 keV), the observations are in agreement with our earlier conclusions [9] and with other workers concern- ing (i) the stability of the 7-austenite phase after N2 + implantation in surfaces having the same phase (100%) [17], (ii) the preferential reaction of N2 + ions with Cr rather than with Fe [17] and (iii) the formation of Cr2N due to a sufficiently high dose N2 + implantation [ 13, 17].

180 S. Aggarwal et al. / Surface characterization o f N2+-implanted AISI 316

The interstitial nitrogen solution in the austenite phase produces an observable change in the X-ray diffraction peaks. At an implantation temperature of ~ 300 °C, the diffusion of implanted nitrogen occurs to larger depths which will produce further solid solution strengthening. N2 + implantation does produce a further hardening because of the significant presence of Cr2N (hardness,

1500 kgf mm-2). Thus both solid solution strengthen- ing and CrzN may be associated with the increase in microhardness after implantation of both samples. The additional increase in hardness of polished sample may be due to the presence of e-Fe2 N-Fe3 N and FeNiN with a large amount of Cr2N, which are all heterogeneous in terms of hardness.

Acknowledgments

The authors are grateful to Professor L. K. Malhotra, Thin Films Laboratory, for liberal use of his laboratory facilities, to Dr. Ami Chand, Centre for Applied Re- search in Electronics, for providing the ion implantation facilities and to Dr. R. D. Tarey, Thin Films Laboratory for his generous help with the GXRD analysis, all of the Indian Institute of Technology, New Delhi, India. The authors also thank the Alexander von Humboldt- Stiftung, Germany for providing the funds (to PKG) necessary for attending the conference. Thanks are due to the University Grants Commission, New Delhi, India, for funding this major research project.

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