Ž .Applied Surface Science 144–145 1999 272–277

Surface characterisation of plasma-nitrided iron by X-rayphotoelectron spectroscopy

Eduardo J. Miola a, Sylvio D. de Souza b, Pedro A.P. Nascente c,),Maristela Olzon-Dionysio b, Carlos A. Olivieri b, Dirceu Spinelli a

a Setor de Materiais, Escola de Engenharia de Sao Carlos, UniÕersidade de Sao Paulo, 13560-250 Sao Carlos, SP, Brazil˜ ˜b Departamento de Fısica, UniÕersidade Federal de Sao Carlos, 13565-905 Sao Carlos, SP, Brazil´ ˜

c Centro de Caracterizacao e DesenÕolÕimento de Materiais, Departamento de Engenharia de Materiais, UniÕersidade Federal de Sao˜ ˜Carlos, 13565-905 Sao Carlos, SP, Brazil

Abstract

The nitriding by plasma has the potential to improve the tribological and mechanical properties on a number ofsubstrates, and offers various advantages as compared with other methods used in the modification of surfaces. We prepared

Ž .four samples by nitriding the iron substrates in a gas mixture 80% H and 20% N under a pressure of 9.0 mbar, discharge2 2

frequency of 10 kHz and temperature of 773 K, for periods of 1, 2, 4 and 6 h. We characterised the samples by scanningŽ . Ž .electron microscopy SEM , X-ray diffraction XRD and microhardness techniques, and detected the presence of Fe N and4

Ž .Fe N phases. We employed X-ray photoelectron spectroscopy XPS to obtain chemical-state and quantitative information2 – 3

of the plasma-nitrided iron surfaces. In situ cleaning was carried out by argon ion bombardment. The Fe 2p photoelectron3r2

peaks for all samples presented three components, associated with Fe0, Fe2q and Fe3q, while the N 1s lines showed twocomponents, the main one due to iron nitride and the other is connected with oxidised nitrogen. q 1999 Elsevier ScienceB.V. All rights reserved.

PACS: 68.35.Bs; 81.65.Lp; 79.60.Dp

Keywords: Plasma nitriding; Iron; X-ray photoelectron spectroscopy; Surface; Characterisation

1. Introduction

The iron-nitride system plays an important role inthe technical applications concerning the iron-basedmaterials. The improvement of tribological proper-

) Corresponding author. Tel.: q55-16-2611229; Fax: q55-16-2611160; E-mail: nascente@power.ufscar.br

ties and corrosion resistance of these materials havew xbeen studied for many years 1–3 , essentially in

several steel types. Some nitriding procedures aredescribed in the literature which show their different

w xeffects in the final products 4,5 . However, fewstudies have been done in pure iron. We have used

Ž .the Pulsed Glow Discharge PGD nitriding tech-nique which can modify some properties of iron-based materials. The principal motivation for choos-

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00809-5

( )E.J. Miola et al.rApplied Surface Science 144–145 1999 272–277 273

ing the PGD technique is to avoid the surface sampleelectrical overcharge due to the DC current, as thevoltaic arc discharge is minimised. The other reasonsare its reproducible results and the facility in its use.

The microhardness of the surface for several steelalloys is improved with the nitriding process. Duringthis process, under gas mixture including carbonspecies, it is well known that for pure iron andcarbon steel in general, the production of carboni-trides C N increases, enhancing its microhardnessx y

w xproperties 6 . However, until now, the relationshipbetween several gas mixtures and iron matrix has notbeen well explored.

In order to correlate the microhardness with theiron nitrides formation, we have done a systematicstudy, comparing the results of this property for ironsamples with different nitriding times. For this pur-pose, we have employed scanning electron mi-

Ž . Ž .croscopy SEM , X-ray diffraction XRD and X-rayŽ .photoelectron spectroscopy XPS in order to charac-

terise the compounds.

2. Experimental procedures

The ion-nitriding apparatus used is similar to thatw xdescribed by Hudis 7 , using a gas mixture of 80

vol.% H and 20 vol.% N , at pressure of 6 mbar,2 2

for 1, 2, 4 and 6 h. Rectangular samples of pure ironŽ .99.8% were mechanically polished and insertedinto the nitriding chamber. In order to maintain thetemperature at 773 K in the cathode during thenitriding process, the pulsed voltage was adjusted tooperate at 10 kHz. Cross-section microhardness mea-surements were carried out with a 20 g load Vickersindenter.

Metallographic studies were performed using bothŽ . w xoptical microscopy OM and SEM techniques 8 .

The samples were cross-sectioned and then mechani-cally polished. Then, they were slightly etched with2% Nital and the thickness and microhardness pro-files of the compound layer were measured. Finally,the top view of the nitrided samples was observed bySEM to see the porosity and the precipitates whichwere formed at the vicinity of the compound layersurface.

XRD patterns were performed in a u–2u geome-Ž .try as well as glancing angle geometry GXRD by a

D5000 Siemens diffractometer equipped with acurved graphite monochromator and Co K radia-a

tion. XRD measurements were done with a scan step2u of 0.038, in the 2u range of 308 to 1008, with afixed counting time of 1 s. The GXRD patterns wereobtained with incidence angle fixed at 1.0, 3.0, 5.0,and 10.08, scan step 2u of 0.038 and counting timeof 4 s.

The XPS analyses were performed in an ultra-highvacuum using a Kratos XSAM HS spectrometer. The

Žexcitation source used was Mg K radiation hnsa

.1253.6 eV , with power given by emission of 15 mAand voltage of 15 kV. The high resolution spectrawere obtained with a pass energy of 20 eV. Thebinding energies were referred to adventitious carbon

Ž1s line set at 284.8 eV. Argon ion sputtering kineticy7 .energy of 2 keV, partial pressure of 1=10 Torr

was used to clean the surface.

3. Results

White layers were observed in all micropho-Ž .tographs Fig. 1 and is about 3 mm thick. It occurs

X w xprincipally in g -Fe N and possibly in Fe N 9 . Fig.4 2

2 shows the differences in the surface topography fordifferent nitriding times. The surfaces of the samplesare covered with non-uniform grains of several sizes.

Ž .Fig. 2 a shows smaller density, distributed through-out the surface, which increases with nitriding timew x9 .

The penetration of the Co K X-ray is about 0.1a

w x Xmm 10 . GXRD identified both g -Fe N and ´-Fe N4 3

phases for the 4 h nitrided sample, while GXRDidentified only g

X-Fe N for the 1 h nitrided sample.4

As the gX phase is cubic, the lattice parameter a can

be immediately calculated using the interplanar dis-tance. According to the Ruhl and Cohen relationw x X11,12 , the nitrogen concentration C in g -Fe N isN 4

related to the lattice constant a. As an example for aand C determination, for 1 h nitrided sample, weN

˚Ž . Ž .have found as3.798 8 A and C s28.97 5 at.%.N

In this manner, gX phase was analysed and an in-

crease in nitrogen concentration until 38–u was ob-served, including a tendency of stabilisation after thispoint.

The microhardness of the compound layer is 300Ž . ŽHV load of 20 g for all the samples 1, 2, 4 and

( )E.J. Miola et al.rApplied Surface Science 144–145 1999 272–277274

Ž .Fig. 1. SEM micrograph of nitrided layers formed on 99.8% after nitriding for 2 h.

.6 h , and is 200 HV for the diffusion layer at 200mm depth for 1 and 4 h. For 6 h nitriding the

microhardness is 200 HV at 275 mm depth, decreas-ing with the distance from the surface until it reaches

Ž . Ž . Ž . Ž . Ž .Fig. 2. Surface appearance of nitride layers formed on pure iron 99.8% after nitriding for a 1 h; b 2 h; c 4 h; d 6 h.

( )E.J. Miola et al.rApplied Surface Science 144–145 1999 272–277 275

Table 1Binding energies and atomic ratios obtained by XPS after argon ion sputtering

Ž .Iron samples Binding energy eV Atomic ratio

O 1s N 1s Fe 2p OrFe NrFe3r2

Ž .Untreated 530.1 59% – 706.8 0.20 –Ž .531.7 30% 708.2Ž .532.9 11% 710.6

Ž . Ž .Plasma nitrided for 1 h 530.1 79% 397.7 87% 706.9 0.27 0.24Ž . Ž .531.7 24% 399.5 13% 708.2

710.9

Ž . Ž .Plasma nitrided for 2 h 529.9 76% 397.5 85% 706.7 0.47 0.22Ž . Ž .531.5 24% 399.5 15% 708.8

710.8

Ž . Ž .Plasma-nitrided for 4 h 530.2 77% 397.9 87% 707.1 0.35 0.29Ž . Ž .531.7 23% 399.9 13% 708.7

711.3

the original hardness value of the material before theŽ .treatment 150 HV .

XPS wide and narrow scans were recorded forfour samples: one of them with no nitriding, and theothers which were nitrided for 1, 2 and 4 h. Theresults of binding energies and atomic ratios ob-tained after sputtering are summarised in Table 1.

The values in parenthesis represent the relative per-centage of the peak components.

The O 1s peak has three components for thenon-nitrided iron, and only two for the nitridedsamples. The component at binding energy of about530 eV is attributed to iron oxide, while the othertwo could be due to the cleaning solvent residue,

Ž . Ž . Ž . Ž .Fig. 3. Fe 2p X-ray photoelectron spectra for a non-nitrided, b 1 h, c 2 h and d 4 h nitrided samples.

( )E.J. Miola et al.rApplied Surface Science 144–145 1999 272–277276

about 531.5 and 532–533 eV for double and singleC–O bonds, respectively. The O 1s component at

y w xabout 531.5 eV could be due to OH as well 13 .The N 1s photoelectron spectra contain two com-

ponents: the most intense one in the binding energyrange of 397.5 to 397.9 eV, which corresponds to a

w xnitride layer 13,14 , and the other at 399.5–399.9w xeV, which is attributed to an oxidised nitrogen 13 .

The Fe 2p photoelectron spectra are shown in Fig. 3Ž . Ž .for non-nitrided bottom , 1, 2 and 4 h top nitride

samples. The most prominent component of the Fe2p peak for all three spectra, at binding energy of3r2

about 707 eV, is associated with Fe0. The overlap-ping peaks for Fe2q and Fe3q appear as a broadshoulder at higher binding energy, being more pro-nounced for longer nitriding treatments.

4. Discussion

The GXRD analysis indicates that the compoundlayers produced by PGD consists of g

X-Fe N and a4

small amount of ´-Fe N. The increase of nitrogen3

concentration inside the sample compared to thesurface is caused by the formation of unstable ni-trides: part is sputtered by the hydrogen-ion bom-bardment and another part diffuses into the sampleforming stable nitrides, Fe N and Fe N. This mecha-3 4

nism explains the reduction of nitrogen concentrationC in the surface.N

Cross-section micrographs shows the morphologyX Ž . w xof g -Fe N ‘needle-like’ 15 at the vicinity of the4

surface for 2 h sample. In agreement with theseresults, the GXRD spectra also show the g

X-Fe N4

phase. Also corroborating with these results, the XPSsurface NrFe atomic ratios presented in Table 1 areclose to the ideal value of 0.25 for Fe N.4

5. Conclusions

The combined microhardness, SEM, GXRD andXPS study of nitriding plasma low-carbon steel gaverise to the following findings.

–The hardness layer produced by nitriding plasmain a mixture of the gas 80% vol. H and 20% vol. N2 2

in the surface consists of gX-Fe N and ´-Fe N4 2 – 3

phases. The increase in nitrogen concentration,meaning more g

X-Fe N phase, as the GXRD results4

indicates, reaches a maximum for all the samples at38–u , which is probably determined by the plasmaconditions such as the gas mixture and nitrogenpartial pressure.

–The concentration of the nitrogen C close toN

the surface for all samples is lower than the concen-tration in the bulk. This effect occurs due to thepresence of the H in the gas mixture used for2

cleaning the surface during the plasma nitriding,which not only cleans the surface but also removesnitrogen through sputtering.

–The phases formed in the nitride samples changecompletely with the different nitriding gas mixture.

w xUsing a mixture of Ar and N 16 , most of the2

nitride obtained is in the ´-Fe N phase. In this work,3

in spite of using the same material but different gasmixtures of H and N , the formation in the surface2 2

the of phases ´-Fe N and gX-Fe N was observed as3 4

well.

Acknowledgements

This work has been supported by CNPq andFAPESP of Brazil. The authors would like to thankR. Machado for his assistance in the GXRD experi-ments. We would like to express our recognition toProfessor L.C. Casteletti for allowing us to use thePGD apparatus.

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