Radiofrequency cold plasma nitrided carbon steel: Microstructural and micromechanical characterizations

Materials Chemistry and Physics 127 (2011) 329–334

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Radiofrequency cold plasma nitrided carbon steel: Microstructural and micromechanical characterizations F.Z. Bouanis a,b , F. Bentiss c , S. Bellayer a,b , J.B. Vogt a,b , C. Jama a,b,∗ a b c

Université Lille Nord de France, F-59000 Lille, France Unité Matériaux et Transformations (UMET), Ingénierie des Systèmes Polymères, CNRS UMR 8207, ENSCL, BP 90108, F-59652 Villeneuve d’Ascq Cedex, France Laboratoire de Chimie de Coordination et d’Analytique, Faculté des Sciences, Université Chouaib Doukkali, B.P. 20, M-24000 El Jadida, Morocco

a r t i c l e

i n f o

Article history: Received 14 January 2010 Received in revised form 4 January 2011 Accepted 8 February 2011 Keywords: Cold plasma Surface Electron probe X-ray photoelectron spectroscopy Hardness

a b s t r a c t In this work, C38 carbon steel was plasma nitrided using a radiofrequency (rf) nitrogen plasma discharge on non-heated substrates. General characterizations were performed to compare the chemical compositions, the microstructures and hardness of the untreated and plasma treated surfaces. The plasma nitriding was carried out on non-heated substrates at a pressure of 16.8 Pa, using N2 gas. Surface characterizations before and after N2 plasma treatment were performed by means of the electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS) and Vickers microhardness measurements. The morphological and chemical analysis showed the formation of a uniform structure on the surface of the nitrided sample with enrichment in nitrogen when compared to untreated sample. The thickness of the nitride layer formed depends on the treatment time duration and is approximately 14 ␮m for 10 h of plasma treatment. XPS was employed to obtain chemical-state information of the plasma nitrided steel surfaces. The micromechanical results show that the surface microhardness increases as the plasma-processing time increases to reach, 1487 HV0.005 at a plasma processing time of 8 h. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Advanced surface modification processes using plasma have undergone substantial industrial development over the past several decades [1,2]. Research and development play a very active role in plasma technology. Plasma surface treatments represent around 2% of the total number of treatments in mechanical industries; however, the needs in research and development efforts are an order of magnitude higher. The main plasma treatments in metallurgical industries are plasma nitriding (34%), physical vapour deposition (PVD) (27%) and plasma spraying (36%) [3]. Plasma nitriding, especially radiofrequency (rf) plasma nitriding is one of the most effective processes at low temperature, avoiding modifications of the microstructure and the mechanical properties of the bulk. Plasma nitriding is a well-established technique for improving the performance of engineering components [4–6]. The thickness and the composition of a surface layer that develop as a result of nitriding depend on the nitriding conditions as well as on the properties of the material, such as composition, crystallographic structure and the density of various lattice defects. In particular, the thickness and the composition of the nitrided layer will be affected by the type of chemical reactions occurring at the

∗ Corresponding author. Tel.: +33 320 336 311; fax: +33 320 436 584. E-mail address: [email protected] (C. Jama). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.02.013

specimen surface as well as by the diffusivity of nitrogen in the material treated. Nitriding is proposed as a process to improve the surface properties. The diffusion of nitrogen into the bulk material produces a gradient of hardness versus depth, which generates a harder layer on the surface. Moreover, plasma nitriding allows an homogeneous treatment to the entire surface, even of complex shapes, thus reducing the treatment costs [7]. The plasma nitriding process consists of a complex mechanism of reactive diffusion leading to the formation of nitrided compounds where the thickness and composition of the formed layers are controlled by plasma process parameters such as the degree of ionization and nitrogen ion/atom flux ratio [8–10]. In general, the nitrided layer is composed of a compound sub-layer on top of a diffusion sub-layer. Plasma nitriding can enhance the diffusion of nitrogen into the metal, resulting in a thicker compound sub-layer [11]. This is due to the high concentration of active species in the plasma, high energy electrons and high ion densities [12]. The concentration of nitrogen within the nitride phases is relatively uniform, but it continuously decreases with increasing distance from the surface within the diffusion zone. In the past few decades cold plasma nitriding techniques have been widely used to improve both surface hardness and corrosion resistance of various engineering component [4,6]. In the papers published referring to low temperature plasma nitriding, the temperatures are in the range of 350–450 ◦ C. However, according to our best knowledge, there is no available work reporting on steel nitriding using a radiofrequency

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Fig. 2. SEM micrograph of the surface of C38 steel surface after chemical “nital” attack.

Fig. 1. Schematic representation of the experimental set-up.

(rf) nitrogen cold plasma discharge, where the process temperature is roughly ambient temperature and without heating the substrate. In a previous paper [13], we have proved that such treatment is very efficient for improving the corrosion resistance of C38 carbon steel. Indeed, it is found that the corrosion inhibition efficiency E(%) increases with the duration of plasma nitriding and attains 97% after 8 h of plasma treatment. The aim of this work is to characterize the structural modifications of the surface of plasma nitrided carbon steel without any heating of the substrate and to answer to some questions such as why such enhancement in term of corrosion resistance and what are the structural modifications obtained. For this purpose, Electron Probe Microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS) were employed. The mechanical properties were also studied using Vickers microhardness test (HV).

tions were carried out at 10 kV, 400 nA. For profiles and quantifications, a PC2 crystal was used to detect the N K␣ X-ray. Prior to analyse, the C38 steel substrates were carbon coated with a Bal-Tec SCD005 sputter coater. The chemical composition of the nitrided layer and the chemical state of the various elements were investigated by X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB 220 XL spectrometer. The monochromatized Al K␣ X-ray source (h␯ = 1486.6 eV) was operated in the CAE (constant analyser energy) mode (CAE = 150 eV for survey spectra and CAE = 30 eV for high resolution spectra), using the electromagnetic lens mode. The binding energy scale was initially calibrated using the Cu 2p3/2 (932.7 eV), Ag 3d5/2 (368.2 eV) and Au 4f7/2 (84 eV) peak positions, and internal calibration was referenced to the C 1s energy at 285 eV for aliphatic like species. Quantification of outer layers atomic composition and spectral simulation of the experimental peaks were achieved using the software provided by VG Scientific. XPS spectra were deconvoluted using a non-linear least squares algorithm with a Shirley base line and a Gaussian–Lorentzian combination. XPS Peak-Fit 4.1 software was used for all data processing. 2.4. Mechanical properties Microhardness experiments were performed on the polished untreated and nitrided surfaces, etched with 3% nital, using a microhardness tester equipped with a Vickers indentor using load of 5–100 g and a dwell time of 15 s. Five experiments were carried out performed in each case of sample and the mean value of Vickers microhardness is calculated for every substrate confirming the consistency in the results.

3. Results and discussion 3.1. Microstructure and morphology

2. Experimental 2.1. Material The material used in this study is a C38 carbon steel with chemical composition (in wt%) of 0.370% C, 0.230% Si, 0.680% Mn, 0.016% S, 0.077% Cr, 0.011% Ti, 0.059% Ni, 0.009% Co, 0.160% Cu and the remainder iron (Fe). Before nitriding, the carbon steel samples were mechanically polished by different grades of silicon carbide emery (120, 600 and 1200), and then manually polished by (6 ␮m and 0.25 ␮m) diamond paste to produce a fine surface finishing; degreased in ethanol in ultrasonic bath and then dried at room temperature before use. 2.2. Plasma nitriding Carbon steel samples were plasma nitrided using cold Radiofrequency plasma (13.56 MHz) generated by a EUROPLASMA CD 1200 set-up for different processing time on non-heated substrate. A schematic representation of the experimental setup is shown in Fig. 1. A nitrogen glow discharge was generated in an aluminium reactor chamber with a continuous out power ranging from 0 to 600 W. The chamber was pumped down to 10.7 Pa using a pump Edwards (80 m3 h−1 ), and the N2 gas was introduced into the chamber. When the pressure became constant, the generator was switched on and adjusted to a certain power value, which gave rise to a continuous glow discharge. The plasma was created using a flow of 500 sccm of nitrogen at a fixed power of 500 W and the chamber pressure was 16.8 Pa. Samples were nitrided for 6, 8 or 10 h. 2.3. Structural characterizations The microstructural analysis was carried out using a Cameca SX100 Electron Probe Microanalysis (EPMA). Secondary (SE) and back-scattered electrons (BSE) images were carried out at 15 kV, 10 nA. Nitrogen (N) X-ray profiles and quantifica-

Surface morphology investigation of the untreated C38 carbon steel surface after a 3% Nital attack, using the scanning electron microscopy (SEM), is presented in Fig. 2. It reveals the presence of two phases corresponding to its carbon content. One can easily recognize the bright grains of proeutectoidal ferrite and the grey and striated grains of perlite (bright ferrite + black zones of cementite). The grain size is about 20–50 ␮m. Fig. 3 shows secondary electrons (SE) and back-scattered electron (BSE) photographs of untreated and 8 h plasma nitrided C38 steel samples. The micrographs of the nitrided samples show a compound layer and a nitrogen diffusion zone. The thickness of the compound layer is about 10 ␮m. A white layer due to the charge effect can be seen on the edge of the nitrided sample on the SE photograph (Fig. 3c). This charge effect can be due to the presence of nitrogen. In the BSE photograph, a black layer is seen on the same edge of the nitrided steel substrate (Fig. 3d); as in BSE mode a darker colour indicates a lower atomic mass of atoms, this layer can be attributed to the presence of nitrogen. In order to complete this characterization, nitrogen (N) and iron (Fe) mappings were carried out on the nitrided steel sample (Fig. 4). Images are colour coded; white is a high concentration in the element (N or Fe) and black a low concentration of these elements. One can see the high concentration of nitrogen at the edge of the sample. For the nitrided sample (N2 , 10 h), the two layers can be also observed (Fig. 5). The thickness of the compound layer increases and is about 14 ␮m. However, electron scanning images

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Fig. 3. Cross section images of untreated and nitrided steel samples. (a) Secondary electron (SE) micrograph of untreated steel, (b) Back Scattering electron photograph (BSE) of untreated steel, (c) SE micrograph of nitrided steel and (d) BSE micrograph of nitrided steel.

Fig. 4. Mappings of the nitrided plasma steel (N2 , 8 h). (a) Nitrogen atoms and (b) iron atoms.

Fig. 5. Secondary electron micrograph of cross section of nitrided steel sample (N2 , 10 h).

for the sample treated 6 h (not shown) did not exhibit any significant nitrided layer. These results are coherent to those already published. Keddam reported that the thickness of the nitride layer increases with time of plasma treatment and the thickness of the nitride layer obtained is between 3.8 and 4.9 ␮m for a nitriding duration of 8 h at 500 ◦ C [14]. On a low alloy steel, a thickness between 6 and 8 ␮m for 8 h of treatment was obtained at 500 ◦ C [15]. Fig. 6 shows quantitative profiles of nitrogen from the inside up to the edge for untreated and treated samples at different times of treatments (6 h, 8 h and 10 h) C38 steel. For all the analysed samples treated the highest concentration of nitrogen is that of the edge. Nitrogen profiles show that from the edge to the inside, nitrogen content decreases. This decrease is faster for the sample treated 6 h in comparison to the other samples treated under identical treatment conditions. The concentration at the edge is 2% of nitrogen for the 10 h nitrided samples, the same to that of 8 h plasma treated. However, the nitrogen content remains more constant in the compound layer for the 10 h nitrided samples, whereas it decreases slightly for 8 h of treatment time. Consequently, the thickness and

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Fig. 8. The XPS deconvoluted profile for N 1s for nitrided C38 steel substrate.

Fig. 6. Quantification of nitrogen along a line from the inside of the sample to its edge for untreated and treated in pure nitrogen for different times of plasma treatment.

nitrogen content mainly depend on the nitriding time. These results are coherent to that of the literature [14–17]. 3.2. X-ray photoelectron spectroscopy (XPS) XPS analysis was used to identify the characteristic elements on the C38 steel surface before and after nitriding, its chemical state and percent on the surface electrode. XPS spectra were recorded for non-nitrided and 8 h nitrided carbon steel substrates. The high-resolution peaks for O 1s, N 1s and Fe 2p for untreated and plasma-treated surfaces (8 h nitrided) are shown in Figs. 7–9, respectively. All spectra contained complex forms, which were assigned to the corresponding species through a deconvolution fitting procedure; this way of peak identification of characteristic elements was in agreement with suitable data bases [18–20]. The obtained values of binding energy (BE) are summarised in Table 1.

Fig. 7. The XPS deconvoluted profiles for O 1s for (a) non-nitrided and (b) nitrided C38 steel substrates.

The values in brackets represent the relative percentage of the peak component. The deconvolution of the O 1s spectra may be fitted into three main peaks (Fig. 7a and b). The peak located at lower binding energy (529.9 eV in non-nitrided steel and 530.5 in nitrided substrate) is attributed to O2− , and in principle could be related to the bond with Fe3+ in the Fe2 O3 and/or Fe3 O4 oxides [21]. The second peak (located at approx. 531.2 eV before nitriding and approx. 531.5 eV after nitriding) represents OH− contribution from hydrous iron oxides, such as FeOOH and/or Fe(OH)3 [21]. Finally, the last component may be assigned to oxygen of adsorbed water at 532.7 eV in non-nitrided surface and 533.1 eV in nitrided steel [22]. Fig. 8 shows the high resolution spectrum for N 1s of a nitrided substrate. Deconvolution of this spectrum suggests that it consists of three peaks. The first peak at a BE ∼ 397.3 eV is attributed to iron nitrides (Fex N) [23,24]. The second component (located at approx. 398.8 eV) is assigned to C–N bonding [25]. The last component at higher energy (located at approx. 400.7 eV) has the lowest contri-

Fig. 9. The XPS deconvoluted profiles for Fe 2p for (a) non-nitrided and (b) nitrided C38 steel substrates.

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Table 1 Binding energies (eV), relative intensity and their assignment for the major core lines observed for the untreated and the nitrided C38 steel substrates. C38 steel

N 1s

O 1s

Fe 2p

BE (eV)

Assignment

BE (eV)

Assignment

BE (eV)

Assignment

Non-nitrided

– – –

– – –

529.9 (46%) 531.2 (36%) 532.7 (18%)

Fe2 O3 FeOOH/Fe(OH)3 Adsorbed H2 O

706.1 (46%) 710.5 (9%) 714.5 (13%) 719.6 (24%) 724.5 (8%)

Fe0 Fe2 O3 /FeOOH/ Fe(OH)3 Shakeup Satellite of Fe(III) Fe 2p1/2

Nitrided

397.3 (45%) 398.8 (38%) 400.7 (17%)

Fex N CN NO

530.5 (34%) 531.5 (42%) 533.1 (24%)

Fe2 O3 FeOOH/Fe(OH)3 Adsorbed H2 O

706.7 (44%) 710.8 (13%) 715.1 (12%) 719.9 (23%) 724.2 (8%)

Fex N, Fe0 Fe2 O3 /FeOOH/ Fe(OH)3 Satellite of Fe(II) Satellite of Fe(III) Fe 2p1/2

bution and is attributed to the N–O bonding (oxidized nitrogen) [26]. In order to complete the contribution of this nitride phase, a similar XPS analysis of Fe was also performed. Fig. 9 shows the Fe 2p photoelectron spectrum for non-nitrided C38 steel substrate as compared to the nitrided one. The Fe 2p spectra depict a double peak profile located at a binding energy (BE) around 711 eV (Fe 2p3/2 ) and 725 eV (Fe 2p1/2 ) together with an associated ghost structure [27]. Deconvolution of the high resolution Fe 2p3/2 XPS spectrum from non-nitrided substrate consists in four peaks (Fig. 9a). The large peak of higher intensity located at lower binding energy (706.1 eV) can be assigned to metallic iron (Fe0 ) [28,29]. The second peak at a BE ∼ 710.5 eV was attributed to Fe3 O4 [30]. The third peak at a BE ∼ 714.5 eV may be ascribed to the satellite of Fe(II) [31]. The last peak (located at approx. 719.6 eV) is attributed to the satellites of the ferric compounds (Fe3+ ) [32]. Inspection of the Fe 2p3/2 XPS spectrum of nitrided substrate (Fig. 9b) shows that in comparison with the non-nitrided surface (Fig. 9a), the plasma nitriding causes energy shifts of +0.6 eV in the metallic Fe 2p photoelectron component. Indeed, the large component observed from the Fe 2p spectrum of nitrided substrates at a binding energy (BE) of 706.7 eV can be attributed to iron atoms in Fe0 and Fex N environments (Table 1) according to Refs. [33,34]. The second peak at a BE ∼ 710.8 eV, assigned to Fe3+ is attributed to Fe2 O3 and FeOOH [35,36]. The third peak at a BE ∼ 715.1 eV may be ascribed to the satellite of Fe(II) [32,37]. The last peak (located at approx. 719.9 eV) is attributed to the satellites of the ferric compounds (Fe3+ ) [32]. Therefore, based on the present XPS and microprobe results, we can conclude that uniform nitride layer is formed using nitrogen radiofrequency plasma discharge without heating the C38 substrate.

Fig. 10. Vickers microhardness of untreated and treated C38 steel at different nitriding-treatment times with a contact load of 5 g.

10, 25, 50, and 100 g. Fig. 11 shows the microhardness values of the untreated and the treated plasma samples for 8 h of nitrogen plasma treatment as a function of the applied load. Each point shown in Fig. 11 is the average of five measurements at different places on the sample. The uncertainties shown are derived from the variation in the five values, and so include a component arising from the variation in hardness over the surface. One can observe that the microhardness values rapidly decrease with increasing applied load for the nitrided sample. While it remains stable for the untreated one. This result is in accordance with the findings of other researchers [38,39,41–43]. The measured surface

3.3. Hardness of nitrided layer The microhardness values of the untreated and nitrided plasma C38 steel substrates with a contact load of 5 g applied for 15 s are shown in Fig. 10. One can see from this figure that, as the plasma treatment time increases, higher hardness values are obtained compared to the untreated sample. Even for the sample that was treated for only 1 h, a microhardness value of more than 265 HV0.005 was obtained, this value is higher compared to that of the untreated sample (247.5 HV0.005 ). A maximum hardness value of approximately 1487 HV0.005 is obtained for the sample treated during 8 h. The increase in microhardness with increasing of the plasma processing time can be explained by the increase of nitrogen concentration and the formation of a thicker nitrided layer on the steel surface improving therefore the mechanical properties of the studied carbon steel [38–40]. In order to accomplish the microhardness measurements, the microhardness value were determined using different loads of 5,

Fig. 11. Variation of the Vickers microhardness of the untreated and 8 h nitrided C38 steel as function of the applied loads.

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hardness is an average value resulting from two contributions: one due to the nitrided part (high hardness) and the second due to the unmodified part (low hardness) of the substrate. The contribution of the unmodified part of the substrate to the measured hardness increases with increasing the applied load leading to the observed decrease in the hardness values. Such hardness decrease with the applied load is known as the “indentation size effect” (ISE) [44]. 4. Conclusions C38 carbon steel samples were plasma nitrided using a radiofrequency (rf) nitrogen plasma discharge on non-heated substrates. The main results and conclusions are as follows: • The morphological and chemical analysis, using Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA) and X-ray Photoelectron Spectroscopy (XPS), showed the formation of a uniform structure on the surface of the nitrided C38 steel. • XPS analysis revealed Fe2 O3 , FeOOH and/or Fe(OH)3 for oxygen atoms environments and Fex N structure was also evidenced from nitrogen and iron atoms environments. • Microhardness values of the nitrided samples increase as plasma processing time increases to reach a value of 1487HV0.005 for samples treated 8 h. This behaviour was interpreted by the formation of a thicker nitrided layer on the steel surface. Acknowledgments The authors wish to thank the FEDER for its financial support in getting the Cameca SX100 Electron Probe MicroAnalyser (EPMA). References [1] J.R. Roth, Industrial Plasma Engineering Principles, IOP Publishing Ltd., Bristol, 1995. [2] M. Ohring, The Material Science of Thin Films, Academic Press, San Diego, 1992. [3] J.P. Peyre, Workshop: International Conference on Plasma Surface Engineering, Garmisch Germany, 1996. [4] T. Morita, H. Takahashi, M. Shimizu, K. Kawasaki, Fatigue Fract. Eng. Mater. Struct. 20 (1997) 85. [5] F.M. El-Hossary, N.Z. Negm, S.M. Khalil, M. Raaif, Appl. Surf. Sci. 239 (2005) 142. [6] F.M. El-Hossary, N.Z. Negm, S.M. Khalil, M. Raaif, Thin Solid Films 497 (2006) 196. [7] V. Fouquet, L. Pichon, M. Drouet, A. Straboni, Appl. Surf. Sci. 221 (2004) 248. [8] Y. Sun, T. Bell, Mater. Sci. Eng., A: Struct. Mater.: Prop. Microstruct. Process 140 (1991) 419.

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