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Spectroscopic investigation of plasma electrolytic borocarburizing on q235 low-carbon steel
Applied Surface Science 321 (2014) 348–352
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Spectroscopic investigation of plasma electrolytic borocarburizing on q235 low-carbon steel Run Liu a,b,c , Bin Wang a,b , Jie Wu a,b , Wenbin Xue a,b,∗ , Xiaoyue Jin a,b , Jiancheng Du a,b , Ming Hua a,b a Key Laboratory for Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China b Beijing Radiation Center, Beijing 100875, China c Zhenjiang Watercraft College, Zhenjiang 212000, Jiangsu, China
a r t i c l e
i n f o
Article history: Received 17 April 2014 Received in revised form 6 October 2014 Accepted 7 October 2014 Available online 14 October 2014 Keywords: Plasma electrolytic borocarburizing Optical emission spectroscopy Plasma parameters Decomposition mechanism
a b s t r a c t A plasma electrolytic borocarburizing process (PEB/C) in borax electrolyte with glycerin additive was employed to fabricate a hardening layer on Q235 low-carbon steel. Optical emission spectroscopy (OES) was utilized to investigate the spectroscopy characteristics of plasma discharge around the steel during PEB/C process. Some plasma parameters were calculated in terms of OES. The electron temperature and electron concentration in plasma discharge zone is about 3000–12,000 K and 2 × 1022 m−3 –1.4 × 1023 m−3 . The atomic ionization degrees of iron, carbon and boron are 10−16 –10−3 , and 10−23 –10−6 , 10−19 –10−4 , respectively, which depend on discharge time. The surface morphology and cross-sectional microstructure of PEB/C hardening layer were observed, and the electrolyte decomposition and plasma discharge behaviors were discussed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Plasma electrolytic saturation (PES), such as plasma electrolytic carburizing (PEC), carbonitriding (PEC/N) and borocarburizing (PEB/C), is a novel rapid surface hardening technique on steel materials [1–3]. When the critical voltage is achieved, a continuous vapor envelope around steel sample in the organic solution is broken down, resulting in the plasma discharge at the near-cathode region [1,4]. Plasma-enhanced chemical reactions and diffusion processes of C, N, B atoms occur on the steel surface, and a hardening layer with high wear resistance and corrosion resistance is fabricated. The PEB/C technique has higher efficiency than the conventional borocarburizing process treated with several hours [5]. Up to now, most of papers about PES focus on the process optimization [6,7] and property characterization of hardening layer [8–10]. The plasma discharge behaviors, which play a key role on the chemical reactions and diffusion of C, N, B atoms, are rarely studied. However, the high-temperature and high-pressure environment in the plasma envelope combined with a strong electric
∗ Corresponding author. Tel.: +86 10 62207222. E-mail address: [email protected] (W. Xue). http://dx.doi.org/10.1016/j.apsusc.2014.10.026 0169-4332/© 2014 Elsevier B.V. All rights reserved.
field provides a high flux of active diffusional species into the activated steel [7,11]. Therefore, it is necessary to evaluate the plasma parameters in the discharge zone and the kinds of the species involved in the discharge process, so that the nature of the discharge phenomena and their influence on the rapid diffusion of C, N, B atoms into steels during PES treatment can be better understood. Plasma spectroscopy is a sensitive diagnostic tool of plasma characteristics [12]. The optical emission spectroscopy (OES) was previously utilized to investigate the complex plasma discharge phenomenon during the plasma electrolytic oxidation (PEO) [13–16]. Plasma electron temperature and electron concentration in plasma zone were calculated from the OES spectral lines of PEO process on Al, Mg, Ti alloys, which was much beneficial for understanding the growth of PEO ceramic coating [14–18]. The PES process on steel as a cathode has a similar plasma discharge phenomenon in electrolyte solution to PEO process on light metal as an anode. However, the work about the spectroscopic characteristics of OES and plasma parameters in discharge zone for PES process on steels has not been reported. In this work, we measured the optical emission spectroscopy during plasma electrolytic borocarburizing (PEB/C) on Q235 lowcarbon steel and analyzed the evolution of electron temperature,
R. Liu et al. / Applied Surface Science 321 (2014) 348–352
349
and excitation energy of the two emission spectra, respectively, and kT is the thermal energy. On the other hand, the dominant line broadening in dense plasma is often due to the Stark broadening caused by the fluctuating electric field from the ions and electrons. For most diagnostic applications, the relationship between full width at half maximum intensity (FWHM, 1/2 ) in nm and electron concentration Ne is given by Eq. (2): 3/2
Ne = C(Ne , T )1/2
(2)
where the C(Ne , T) was summarized by Griem [12]. It is only a weak function of the electron concentration Ne . The H˛ line was chosen to determine plasma electron concentration in this work. According to the Saha thermal ionization equation and the Boltzmann distribution law, the atom concentration NM , univalent ion NM + and the ionization equilibrium constant ki can be respectively determined by Eqs. (3) and (4) [12].
Fig. 1. Schematic illustration of the apparatus for plasma electrolytic borocarburizing and optical spectra measurement on Q235 steel.
electron concentration and atomic ionization degree with the discharge time using the local thermodynamic equilibrium (LTE) model. Furthermore, the decomposition mechanism during the PEB/C process was discussed. 2. Experimental procedure As-received Q235 low-carbon steel specimens (wt.%, 0.14–0.22 C, 0.30–0.65 Mn, ≤0.30 Si, ≤0.045 P, ≤0.055 S, and Fe balance) with dimensions of 55 mm × 16 mm × 1.5 mm were polished with SiC paper up to 1000-grit size. The plasma electrolytic borocarburizing PEB/C treatment of steel samples was performed in 30% borax and 15% glycerin solution using a DC power supply at 330 V for 15 min discharge. The Q235 steel sample was used as cathode while a stainless steel plate served as anode. The detail fabrication process of PEB/C sample was described in previous paper [3]. Fig. 1 illustrates the schematic diagram of the apparatus for plasma electrolytic borocarburizing and optical spectra measurement. The optical spectra of plasma discharge around the steel sample can provide some information about the active species that emit the light. The OES spectra were collected by an optical emission spectrometer (AvaSpec-3648) with the spectral region of 200–1100 nm. This spectrometer collected the light from an optical fiber receiver, which was placed at a distance of 3 cm away from the Q235 steel sample through a quartz window. Then the spectral data were analyzed by the software of Plasus Specline 2.1. Meanwhile, the illumination in the PEB/C process was recorded when the optical emission spectrometer was replaced by a digital 1336 A light meter in Fig. 1. The surface morphology and cross-sectional microstructure of the PEB/C treated sample were observed by S-4800 scanning electron microscope (SEM). According to the theory of atomic emission spectrum, when the excited atoms transition from a high level to a low level, the energy will be released in the form of light radiation to generate atomic emission spectrometry. In the state of thermodynamic equilibrium (TE) or local thermodynamic equilibrium (LTE), when relative emission spectral line intensities of the same atomic or ionic species are obtained, the electron temperature, T, can be calculated from Eq. (1) [12].
I1 A1 g1 2 E1 − E2 = exp − I2 A2 g2 1 kT
(1)
where I1 , A1 , g1 , 1 , E1 and I2 , A2 , g2 , 2 , E2 are relative intensity, transition probability, statistical weight of upper level, wavelength
⎧ ⎨ N = Ne2 M ki ⎩ + NM = Ne
ki =
+ NM Ne
NM
=
(3)
2m kT 3/2 2z+ e h2
·
z
exp
−E
ion
kT
(4)
where Eion is the atomic ionization energy, z and z+ are the partition function of the atom and univalent ion respectively, which are given by David [19]. Then plasma atomic ionization degree ˛ can be obtained from Eq. (5): ˛=
+ NM
+ NM + NM
(5)
3. Results and discussion 3.1. Optical emission characterization Fig. 2a shows typical OES spectrum of plasma discharge during PEB/C treatment on Q235 steel in the spectral range from 185 nm to 1100 nm. Fig. 2b is a partial magnification in the range of 300–500 nm of Fig. 2a. Atomic, ionic and radical lines are identified. It is found that the plasma discharge envelope around steel sample contains iron (Fe I and Fe II) from the steel substrate, and oxygen (O I, O II), hydrogen (H I), carbon (C I, C II), boron (B I, B II and B III), sodium (Na I, Na II) and radical (CH) from the electrolyte. The spectral lines of Fe II at 569.1 nm and 818.2 nm, Na I at 589.0 nm, H␣ at 656.3 nm and C II at 590.7 nm have higher intensity. The notations of I and II refer to neutral and singly ionized atoms, respectively. It indicates that the active Na, Fe, B, O, H, C radicals and ionic components were involved in the reactions of discharge zone to fabricate the borocarburizing layer. In addition, the intensity variations of Fe II emission lines at 515.5 nm and 569.1 nm with the PEB/C treating time are shown in Fig. 2c. They rapidly increase to a maximum value within 1 min and then decrease to a lower intensity in the following 5 min. The variation of illumination intensity is depicted in Fig. 3 which shows a similar tendency to the intensity variations of Fe II emission lines in Fig. 2c. The illumination of envelope discharge quickly increases to a maximum value after 0.5 min discharge and decreases from 36 lux at 0.5 min to about 0.9 lux at 4 min. Then the illumination keeps an extreme low value. This results from the glycerol decomposition in plasma discharge envelope to produce lots of black carbon particles, which causes the reduction of light transmittance of the solution. Meanwhile, the decomposition process also leads to the decrease of the electrolyte concentration to some extent, and then the discharge gradually becomes weak.
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40 35
Illumination /lux
30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
Discharge time /min Fig. 3. Dependence of illumination intensity on discharge time during PEB/C treatment.
the Ne is in a range of 2 × 1022 m−3 to 1.4 × 1023 m−3 . It quickly increases to 1.4 × 1023 m−3 at about 1.5 min and gradually reduces to 1.0 × 1023 m−3 at about 6 min, then it keeps relatively stable till the end of discharge.
14000 (a) 12000 10000
Te /K
8000 6000 4000 2000 0
0
2
4
6
8
10
12
14
Discharge time /min 23
1.6x10
(b) 23
1.4x10 Fig. 2. (a) The OES in the range of 200–1100 nm during PEB/C treatment. (b) Magnified spectra of (a) in 300–500 nm. (c) Dependence of the spectral line intensity of Fe II on discharge time.
23
1.2x10
23
3.2. Plasma electron temperature and concentration
Ne /m
-3
1.0x10
22
8.0x10
22
Fig. 4 reveals the variations of plasma parameters with discharge time. The intensity ratio of two Fe II lines at the 515.5 nm and 569.1 nm is used to calculate the plasma electron temperature Te in terms of Eq. (1). As shown in Fig. 4a, the Te is abnormally high initially, which might be due to the unstable discharge at the initial discharge stage. Then in the following 5 min, the Te drops and presents a stable value about 6000 K. After about 6 min, the average Te as shown in the fitting curve gradually decreases to 3000 K. Meanwhile, there are many spikes and some of them are over 10,000 K. As indicated in Fig. 4b,
6.0x10
22
4.0x10
22
2.0x10
0
2
4
6
8
10
12
14
Discharge time/ min Fig. 4. Dependence of plasma parameters on discharge time during PEB/C process. (a) Electron temperature. (b) Electron concentration.
Atmoic ionization degree of iron
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0.01 (a) 1E-5 1E-8 1E-11 1E-14 1E-17 1E-20
0
2
4
6
8
10
12
14
Atmoic ionization degree of carbon
Discharge time /min
1E-3 (b) 1E-6 1E-9 1E-12 1E-15 1E-18 1E-21 1E-24
0
2
4
6
8
10
12
14
Atmoic ionization degree of boron
Discharge time /min
0.01
(c) Fig. 6. SEM micrographs of the PEB/C treated steels at 330 V with 15 min. (a) Surface morphology. (b) Cross-sectional microstructure. I Q235 steel substrate, II diffusion layer, III boride layer, IV loose top layer
1E-5 1E-8 1E-11
active atoms rather than ions in discharge envelope around the steel sample.
1E-14 1E-17
3.4. Surface morphology and coating cross-section
1E-20 0
2
4
6
8
10
12
14
Discharge time /min Fig. 5. Dependence of atomic ionization degree on oxidation time. (a)–(c) Atomic ionization degree of iron, carbon and boron, respectively.
3.3. Atomic ionization degree The atomic ionization degrees of iron, carbon and boron are shown in Fig. 5a–c, respectively. Neglecting the abnormally values in the initial stage due to the instable discharge, the three curves show a similar variation tendency with discharge time. They gradually decrease with discharge time during PEB/C treatment, but keep relatively steady at the initial 6 min and show large fluctuations at the late stage of the plasma discharge. The average atomic ionization degrees of iron, carbon and boron are about 10−16 –10−3 , and 10−23 –10−6 , 10−19 –10−4 , respectively. The high plasma temperature and extreme low atomic ionization degree imply that the carbon and boron diffusing into the Q235 steel come mainly from
Fig. 6 shows the surface morphology and cross-sectional microstructure of the PEB/C treated steel with 330 V at 15 min treating time. As displayed in Fig. 6a, the surface of PEB/C sample is rough along with many grains and some pores, caused by strong plasma discharge etching. The cross-sectional microstructure in Fig. 6b reveals a multi-layer structure: one loose top layer of about 2 m, one boride layer of about 6 m and one diffusion layer of about 25 m. The PEB/C sample consists of Fe2 B and FeB phases and little Fe3 C phase [8], which identifies that the boron and carbon elements of electrolyte certainly diffuse into the steel during plasma discharge. Wang et al. [8] found that the boride layer contains Fe2 B phase and has high hardness over 1500 HV, meanwhile the carbon diffusion layer with 200–600 HV plays one role on a transition layer between the steel substrate and boride layer. 3.5. Decomposition mechanism of electrolyte and diffusion process Based on the OES spectroscopy study in the PEB/C process, the following chemical reactions in the discharge envelope take place.
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electron temperature in plasma discharge zone on Q235 lowcarbon steel is about 3000–12,000 K, and electron concentration is about 2 ×1022 m−3 –1.4 × 1022 m−3 . The atomic ionization degrees of iron, carbon and boron are about 10−16 –10−3 , and 10−23 –10−6 , 10−19 –10−4 , respectively. The plasma discharges provide high temperature environment which enhances the diffusion of active carbon and boron atoms into the steel substrate. Acknowledgments
Fig. 7. Schematic diagram of reactions in discharge envelope and diffusion process during the PEB/C treatment.
In the initial stage of the PEB/C process, the following electrochemical reaction occurs on the steel cathode in borax solution with glycerin additive [7]: +
−
2H + 2e → H2
(6)
Meanwhile, a large amount of vapor appears due to the generation of Joule heat. So the vapor mixed with hydrogen gradually forms a vapor envelope around the steel sample. As the applied voltage increases, the vapor envelope is broke down to generate plasma discharge with high temperature and electron concentration as described above. Then the boiling glycerin and borax adjacent to the steel electrode quickly decomposes according to the following reactions [20]: CH2 OHCHOHCH2 OH + 6e∗ → 2 · CH2 + ·CH + 3 · OH
(7)
Na2 B4 O7 → 2Na+ + B4 O7 2−
(8)
2B4 O7 2− → 4B2 O3 + O2 + 4e−
(9)
The asterisk in Eq. (7) means excited state of electrons in plasma discharge zone and the small dots mean the breaking positions of covalent bonds. As the active radicals • CH2 and • OH are unstable, they are hard to detect by OES. The C, H radicals are further thermally decomposed into active carbon and hydrogen atoms at the cathode due to the frequent collision reactions in plasma discharge envelope. ·CH + e∗ → C + H + e ·CH2 + e∗ → C + 2H + e
(10)
In addition, sodium metaborate and active boron atoms are formed on the steel surface by: Na+ + e− → Na
(11)
3Na + 2B2 O3 → 3NaBO2 + B
(12)
The possible reactions in discharge envelope around Q235 steel sample during the PEB/C treatment are shown in Fig. 7. The plasma discharges provide a high-temperature and high-pressure environment combined with a strong electric field in envelope around the steel sample, which result in the active species such as carbon and boron atoms. Then they quickly diffuse into the steel to form the hardening layer. 4. Conclusion The measurement and spectroscopic investigation using OES provide an effective tool for the study of PEB/C process. The
This research was sponsored by the National Natural Science Foundation of China (No. 51071031), Beijing Natural Science Foundation (Nos. 2102018 and 2122017), the Fundamental Research Funds for the Central Universities (grant no. 211105562GK) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant no. 20120003110010). References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [2] P. Taheri, C. Dehganian, M. Aliofkhazraei, A.S. Rouhaghdam, Nanocrystalline structure produced by complex surface treatments: plasma electrolytic nitrocarburizing, boronitriding, borocarburizing, and borocarbonitriding, Plasma Process. Polym. 4 (2007) S721–S727. [3] B. Wang, W. Xue, J. Wu, X. Jin, M. Hua, Z. Wu, Characterization of surface hardened layers on Q235 low-carbon steel treated by plasma electrolytic borocarburizing, J. Alloys Compd. 578 (2013) 162–169. [4] J. Wu, W. Xue, X. Jin, B. Wang, J. Du, Z. Wu, Preparation and characterization of diamond-like carbon/oxides composite film on carbon steel by cathodic plasma electrolysis, Appl. Phys. Lett. 103 (2013) 031905. [5] M. Kulka, A. Pertek, Gradient formation of boride layers by borocarburizing, Appl. Surf. Sci. 254 (2008) 5281–5290. [6] M. Aliofkhazraei, C. Morillo, R. Miresmaeili, A.S. Rouhaghdam, Carburizing of low-melting-point metals by pulsed nanocrystalline plasma electrolytic carburizing, Surf. Coat. Technol. 202 (2008) 5493–5496. [7] X.M. Li, Y. Han, Porous nanocrystalline Ti(Cx N1−x ) thick films by plasma electrolytic carbonitriding, Electrochem. Commun. 8 (2006) 267–272. [8] B. Wang, X. Jin, W. Xue, Z. Wu, J. Du, J. Wu, High temperature tribological behaviors of plasma electrolytic borocarburized Q235 low-carbon steel, Surf. Coat. Technol. 262 (2013) 142–149. [9] F. Cavuslu, M. Usta, Kinetics and mechanical study of plasma electrolytic carburizing for pure iron, Appl. Surf. Sci. 257 (2011) 4014–4020. [10] M.A. Bejar, R. Henriquez, Surface hardening of steel by plasma-electrolysis boronizing, Mater. Des. 30 (2009) 1726–1728. [11] J. Wu, B.W. Xue, B. Wang, X. Jin, J. Du, Y. Li, Characterization of carburized layer on T8 steel fabricated by cathodic plasma electrolysis, Surf. Coat. Technol. 245 (2014) 9–15. [12] H.R. Griem, Plasma Spectroscopy, McGraw-Hill, Cambridge, 1964. [13] F. Mecuson, T. Czerwiec, T. Belmonte, L. Dujardin, A. Viola, G. Henrion, Diagnostics of an electrolytic microarc process for aluminium alloy oxidation, Surf. Coat. Technol. 200 (2005) 804–808. [14] M. Klapkiv, H. Nykyforchyn, V. Posuvailo, Spectral analysis of an elctrolytic plasma in the process of synthesis of aluminum oxide, Mater. Sci. 30 (1994) 333–343. [15] R.O. Hussein, X. Nie, D.O. Northwood, A.L. Yerokhin, A. Matthews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process, J. Phys. D: Appl. Phys. 43 (2010) 105203. [16] R. Liu, J. Wu, W. Xue, Y. Qu, C. Yang, B. Wang, X. Wu, Discharge behaviors during plasma electrolytic oxidation on aluminum alloy, Mater. Chem. Phys. 148 (2014) 284–292. [17] L. Wang, W. Fu, S. Wang, J. Li, Plasma electrolytic oxidation coatings in KOH electrolyte and its discharge characteristics, J. Alloys Compd. 594 (2014) 27–31. [18] R.O. Hussein, X. Nie, D.O. Northwood, A spectroscopic and microstructural study of oxide coatings produced on a Ti–6Al–4 V alloy by plasma electrolytic oxidation, Mater. Chem. Phys. 134 (2012) 484–492. [19] R.L. David (Ed.), CRC Handbook of Chemistry and Physics, 90th ed., CRC Press/Taylor and Francis, Boca Raton, FL, 2010. [20] S.H. Han, J.S. Chun, A study on the electroboronizing of steel by superimposed cyclic current, J. Mater. Sci. 15 (1980) 1379–1386.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Spectroscopic investigation of plasma electrolytic borocarburizing on q235 low-carbon steel Run Liu a,b,c , Bin Wang a,b , Jie Wu a,b , Wenbin Xue a,b,∗ , Xiaoyue Jin a,b , Jiancheng Du a,b , Ming Hua a,b a Key Laboratory for Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China b Beijing Radiation Center, Beijing 100875, China c Zhenjiang Watercraft College, Zhenjiang 212000, Jiangsu, China
a r t i c l e
i n f o
Article history: Received 17 April 2014 Received in revised form 6 October 2014 Accepted 7 October 2014 Available online 14 October 2014 Keywords: Plasma electrolytic borocarburizing Optical emission spectroscopy Plasma parameters Decomposition mechanism
a b s t r a c t A plasma electrolytic borocarburizing process (PEB/C) in borax electrolyte with glycerin additive was employed to fabricate a hardening layer on Q235 low-carbon steel. Optical emission spectroscopy (OES) was utilized to investigate the spectroscopy characteristics of plasma discharge around the steel during PEB/C process. Some plasma parameters were calculated in terms of OES. The electron temperature and electron concentration in plasma discharge zone is about 3000–12,000 K and 2 × 1022 m−3 –1.4 × 1023 m−3 . The atomic ionization degrees of iron, carbon and boron are 10−16 –10−3 , and 10−23 –10−6 , 10−19 –10−4 , respectively, which depend on discharge time. The surface morphology and cross-sectional microstructure of PEB/C hardening layer were observed, and the electrolyte decomposition and plasma discharge behaviors were discussed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Plasma electrolytic saturation (PES), such as plasma electrolytic carburizing (PEC), carbonitriding (PEC/N) and borocarburizing (PEB/C), is a novel rapid surface hardening technique on steel materials [1–3]. When the critical voltage is achieved, a continuous vapor envelope around steel sample in the organic solution is broken down, resulting in the plasma discharge at the near-cathode region [1,4]. Plasma-enhanced chemical reactions and diffusion processes of C, N, B atoms occur on the steel surface, and a hardening layer with high wear resistance and corrosion resistance is fabricated. The PEB/C technique has higher efficiency than the conventional borocarburizing process treated with several hours [5]. Up to now, most of papers about PES focus on the process optimization [6,7] and property characterization of hardening layer [8–10]. The plasma discharge behaviors, which play a key role on the chemical reactions and diffusion of C, N, B atoms, are rarely studied. However, the high-temperature and high-pressure environment in the plasma envelope combined with a strong electric
∗ Corresponding author. Tel.: +86 10 62207222. E-mail address: [email protected] (W. Xue). http://dx.doi.org/10.1016/j.apsusc.2014.10.026 0169-4332/© 2014 Elsevier B.V. All rights reserved.
field provides a high flux of active diffusional species into the activated steel [7,11]. Therefore, it is necessary to evaluate the plasma parameters in the discharge zone and the kinds of the species involved in the discharge process, so that the nature of the discharge phenomena and their influence on the rapid diffusion of C, N, B atoms into steels during PES treatment can be better understood. Plasma spectroscopy is a sensitive diagnostic tool of plasma characteristics [12]. The optical emission spectroscopy (OES) was previously utilized to investigate the complex plasma discharge phenomenon during the plasma electrolytic oxidation (PEO) [13–16]. Plasma electron temperature and electron concentration in plasma zone were calculated from the OES spectral lines of PEO process on Al, Mg, Ti alloys, which was much beneficial for understanding the growth of PEO ceramic coating [14–18]. The PES process on steel as a cathode has a similar plasma discharge phenomenon in electrolyte solution to PEO process on light metal as an anode. However, the work about the spectroscopic characteristics of OES and plasma parameters in discharge zone for PES process on steels has not been reported. In this work, we measured the optical emission spectroscopy during plasma electrolytic borocarburizing (PEB/C) on Q235 lowcarbon steel and analyzed the evolution of electron temperature,
R. Liu et al. / Applied Surface Science 321 (2014) 348–352
349
and excitation energy of the two emission spectra, respectively, and kT is the thermal energy. On the other hand, the dominant line broadening in dense plasma is often due to the Stark broadening caused by the fluctuating electric field from the ions and electrons. For most diagnostic applications, the relationship between full width at half maximum intensity (FWHM, 1/2 ) in nm and electron concentration Ne is given by Eq. (2): 3/2
Ne = C(Ne , T )1/2
(2)
where the C(Ne , T) was summarized by Griem [12]. It is only a weak function of the electron concentration Ne . The H˛ line was chosen to determine plasma electron concentration in this work. According to the Saha thermal ionization equation and the Boltzmann distribution law, the atom concentration NM , univalent ion NM + and the ionization equilibrium constant ki can be respectively determined by Eqs. (3) and (4) [12].
Fig. 1. Schematic illustration of the apparatus for plasma electrolytic borocarburizing and optical spectra measurement on Q235 steel.
electron concentration and atomic ionization degree with the discharge time using the local thermodynamic equilibrium (LTE) model. Furthermore, the decomposition mechanism during the PEB/C process was discussed. 2. Experimental procedure As-received Q235 low-carbon steel specimens (wt.%, 0.14–0.22 C, 0.30–0.65 Mn, ≤0.30 Si, ≤0.045 P, ≤0.055 S, and Fe balance) with dimensions of 55 mm × 16 mm × 1.5 mm were polished with SiC paper up to 1000-grit size. The plasma electrolytic borocarburizing PEB/C treatment of steel samples was performed in 30% borax and 15% glycerin solution using a DC power supply at 330 V for 15 min discharge. The Q235 steel sample was used as cathode while a stainless steel plate served as anode. The detail fabrication process of PEB/C sample was described in previous paper [3]. Fig. 1 illustrates the schematic diagram of the apparatus for plasma electrolytic borocarburizing and optical spectra measurement. The optical spectra of plasma discharge around the steel sample can provide some information about the active species that emit the light. The OES spectra were collected by an optical emission spectrometer (AvaSpec-3648) with the spectral region of 200–1100 nm. This spectrometer collected the light from an optical fiber receiver, which was placed at a distance of 3 cm away from the Q235 steel sample through a quartz window. Then the spectral data were analyzed by the software of Plasus Specline 2.1. Meanwhile, the illumination in the PEB/C process was recorded when the optical emission spectrometer was replaced by a digital 1336 A light meter in Fig. 1. The surface morphology and cross-sectional microstructure of the PEB/C treated sample were observed by S-4800 scanning electron microscope (SEM). According to the theory of atomic emission spectrum, when the excited atoms transition from a high level to a low level, the energy will be released in the form of light radiation to generate atomic emission spectrometry. In the state of thermodynamic equilibrium (TE) or local thermodynamic equilibrium (LTE), when relative emission spectral line intensities of the same atomic or ionic species are obtained, the electron temperature, T, can be calculated from Eq. (1) [12].
I1 A1 g1 2 E1 − E2 = exp − I2 A2 g2 1 kT
(1)
where I1 , A1 , g1 , 1 , E1 and I2 , A2 , g2 , 2 , E2 are relative intensity, transition probability, statistical weight of upper level, wavelength
⎧ ⎨ N = Ne2 M ki ⎩ + NM = Ne
ki =
+ NM Ne
NM
=
(3)
2m kT 3/2 2z+ e h2
·
z
exp
−E
ion
kT
(4)
where Eion is the atomic ionization energy, z and z+ are the partition function of the atom and univalent ion respectively, which are given by David [19]. Then plasma atomic ionization degree ˛ can be obtained from Eq. (5): ˛=
+ NM
+ NM + NM
(5)
3. Results and discussion 3.1. Optical emission characterization Fig. 2a shows typical OES spectrum of plasma discharge during PEB/C treatment on Q235 steel in the spectral range from 185 nm to 1100 nm. Fig. 2b is a partial magnification in the range of 300–500 nm of Fig. 2a. Atomic, ionic and radical lines are identified. It is found that the plasma discharge envelope around steel sample contains iron (Fe I and Fe II) from the steel substrate, and oxygen (O I, O II), hydrogen (H I), carbon (C I, C II), boron (B I, B II and B III), sodium (Na I, Na II) and radical (CH) from the electrolyte. The spectral lines of Fe II at 569.1 nm and 818.2 nm, Na I at 589.0 nm, H␣ at 656.3 nm and C II at 590.7 nm have higher intensity. The notations of I and II refer to neutral and singly ionized atoms, respectively. It indicates that the active Na, Fe, B, O, H, C radicals and ionic components were involved in the reactions of discharge zone to fabricate the borocarburizing layer. In addition, the intensity variations of Fe II emission lines at 515.5 nm and 569.1 nm with the PEB/C treating time are shown in Fig. 2c. They rapidly increase to a maximum value within 1 min and then decrease to a lower intensity in the following 5 min. The variation of illumination intensity is depicted in Fig. 3 which shows a similar tendency to the intensity variations of Fe II emission lines in Fig. 2c. The illumination of envelope discharge quickly increases to a maximum value after 0.5 min discharge and decreases from 36 lux at 0.5 min to about 0.9 lux at 4 min. Then the illumination keeps an extreme low value. This results from the glycerol decomposition in plasma discharge envelope to produce lots of black carbon particles, which causes the reduction of light transmittance of the solution. Meanwhile, the decomposition process also leads to the decrease of the electrolyte concentration to some extent, and then the discharge gradually becomes weak.
350
R. Liu et al. / Applied Surface Science 321 (2014) 348–352
40 35
Illumination /lux
30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
Discharge time /min Fig. 3. Dependence of illumination intensity on discharge time during PEB/C treatment.
the Ne is in a range of 2 × 1022 m−3 to 1.4 × 1023 m−3 . It quickly increases to 1.4 × 1023 m−3 at about 1.5 min and gradually reduces to 1.0 × 1023 m−3 at about 6 min, then it keeps relatively stable till the end of discharge.
14000 (a) 12000 10000
Te /K
8000 6000 4000 2000 0
0
2
4
6
8
10
12
14
Discharge time /min 23
1.6x10
(b) 23
1.4x10 Fig. 2. (a) The OES in the range of 200–1100 nm during PEB/C treatment. (b) Magnified spectra of (a) in 300–500 nm. (c) Dependence of the spectral line intensity of Fe II on discharge time.
23
1.2x10
23
3.2. Plasma electron temperature and concentration
Ne /m
-3
1.0x10
22
8.0x10
22
Fig. 4 reveals the variations of plasma parameters with discharge time. The intensity ratio of two Fe II lines at the 515.5 nm and 569.1 nm is used to calculate the plasma electron temperature Te in terms of Eq. (1). As shown in Fig. 4a, the Te is abnormally high initially, which might be due to the unstable discharge at the initial discharge stage. Then in the following 5 min, the Te drops and presents a stable value about 6000 K. After about 6 min, the average Te as shown in the fitting curve gradually decreases to 3000 K. Meanwhile, there are many spikes and some of them are over 10,000 K. As indicated in Fig. 4b,
6.0x10
22
4.0x10
22
2.0x10
0
2
4
6
8
10
12
14
Discharge time/ min Fig. 4. Dependence of plasma parameters on discharge time during PEB/C process. (a) Electron temperature. (b) Electron concentration.
Atmoic ionization degree of iron
R. Liu et al. / Applied Surface Science 321 (2014) 348–352
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0.01 (a) 1E-5 1E-8 1E-11 1E-14 1E-17 1E-20
0
2
4
6
8
10
12
14
Atmoic ionization degree of carbon
Discharge time /min
1E-3 (b) 1E-6 1E-9 1E-12 1E-15 1E-18 1E-21 1E-24
0
2
4
6
8
10
12
14
Atmoic ionization degree of boron
Discharge time /min
0.01
(c) Fig. 6. SEM micrographs of the PEB/C treated steels at 330 V with 15 min. (a) Surface morphology. (b) Cross-sectional microstructure. I Q235 steel substrate, II diffusion layer, III boride layer, IV loose top layer
1E-5 1E-8 1E-11
active atoms rather than ions in discharge envelope around the steel sample.
1E-14 1E-17
3.4. Surface morphology and coating cross-section
1E-20 0
2
4
6
8
10
12
14
Discharge time /min Fig. 5. Dependence of atomic ionization degree on oxidation time. (a)–(c) Atomic ionization degree of iron, carbon and boron, respectively.
3.3. Atomic ionization degree The atomic ionization degrees of iron, carbon and boron are shown in Fig. 5a–c, respectively. Neglecting the abnormally values in the initial stage due to the instable discharge, the three curves show a similar variation tendency with discharge time. They gradually decrease with discharge time during PEB/C treatment, but keep relatively steady at the initial 6 min and show large fluctuations at the late stage of the plasma discharge. The average atomic ionization degrees of iron, carbon and boron are about 10−16 –10−3 , and 10−23 –10−6 , 10−19 –10−4 , respectively. The high plasma temperature and extreme low atomic ionization degree imply that the carbon and boron diffusing into the Q235 steel come mainly from
Fig. 6 shows the surface morphology and cross-sectional microstructure of the PEB/C treated steel with 330 V at 15 min treating time. As displayed in Fig. 6a, the surface of PEB/C sample is rough along with many grains and some pores, caused by strong plasma discharge etching. The cross-sectional microstructure in Fig. 6b reveals a multi-layer structure: one loose top layer of about 2 m, one boride layer of about 6 m and one diffusion layer of about 25 m. The PEB/C sample consists of Fe2 B and FeB phases and little Fe3 C phase [8], which identifies that the boron and carbon elements of electrolyte certainly diffuse into the steel during plasma discharge. Wang et al. [8] found that the boride layer contains Fe2 B phase and has high hardness over 1500 HV, meanwhile the carbon diffusion layer with 200–600 HV plays one role on a transition layer between the steel substrate and boride layer. 3.5. Decomposition mechanism of electrolyte and diffusion process Based on the OES spectroscopy study in the PEB/C process, the following chemical reactions in the discharge envelope take place.
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electron temperature in plasma discharge zone on Q235 lowcarbon steel is about 3000–12,000 K, and electron concentration is about 2 ×1022 m−3 –1.4 × 1022 m−3 . The atomic ionization degrees of iron, carbon and boron are about 10−16 –10−3 , and 10−23 –10−6 , 10−19 –10−4 , respectively. The plasma discharges provide high temperature environment which enhances the diffusion of active carbon and boron atoms into the steel substrate. Acknowledgments
Fig. 7. Schematic diagram of reactions in discharge envelope and diffusion process during the PEB/C treatment.
In the initial stage of the PEB/C process, the following electrochemical reaction occurs on the steel cathode in borax solution with glycerin additive [7]: +
−
2H + 2e → H2
(6)
Meanwhile, a large amount of vapor appears due to the generation of Joule heat. So the vapor mixed with hydrogen gradually forms a vapor envelope around the steel sample. As the applied voltage increases, the vapor envelope is broke down to generate plasma discharge with high temperature and electron concentration as described above. Then the boiling glycerin and borax adjacent to the steel electrode quickly decomposes according to the following reactions [20]: CH2 OHCHOHCH2 OH + 6e∗ → 2 · CH2 + ·CH + 3 · OH
(7)
Na2 B4 O7 → 2Na+ + B4 O7 2−
(8)
2B4 O7 2− → 4B2 O3 + O2 + 4e−
(9)
The asterisk in Eq. (7) means excited state of electrons in plasma discharge zone and the small dots mean the breaking positions of covalent bonds. As the active radicals • CH2 and • OH are unstable, they are hard to detect by OES. The C, H radicals are further thermally decomposed into active carbon and hydrogen atoms at the cathode due to the frequent collision reactions in plasma discharge envelope. ·CH + e∗ → C + H + e ·CH2 + e∗ → C + 2H + e
(10)
In addition, sodium metaborate and active boron atoms are formed on the steel surface by: Na+ + e− → Na
(11)
3Na + 2B2 O3 → 3NaBO2 + B
(12)
The possible reactions in discharge envelope around Q235 steel sample during the PEB/C treatment are shown in Fig. 7. The plasma discharges provide a high-temperature and high-pressure environment combined with a strong electric field in envelope around the steel sample, which result in the active species such as carbon and boron atoms. Then they quickly diffuse into the steel to form the hardening layer. 4. Conclusion The measurement and spectroscopic investigation using OES provide an effective tool for the study of PEB/C process. The
This research was sponsored by the National Natural Science Foundation of China (No. 51071031), Beijing Natural Science Foundation (Nos. 2102018 and 2122017), the Fundamental Research Funds for the Central Universities (grant no. 211105562GK) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant no. 20120003110010). References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [2] P. Taheri, C. Dehganian, M. Aliofkhazraei, A.S. Rouhaghdam, Nanocrystalline structure produced by complex surface treatments: plasma electrolytic nitrocarburizing, boronitriding, borocarburizing, and borocarbonitriding, Plasma Process. Polym. 4 (2007) S721–S727. [3] B. Wang, W. Xue, J. Wu, X. Jin, M. Hua, Z. Wu, Characterization of surface hardened layers on Q235 low-carbon steel treated by plasma electrolytic borocarburizing, J. Alloys Compd. 578 (2013) 162–169. [4] J. Wu, W. Xue, X. Jin, B. Wang, J. Du, Z. Wu, Preparation and characterization of diamond-like carbon/oxides composite film on carbon steel by cathodic plasma electrolysis, Appl. Phys. Lett. 103 (2013) 031905. [5] M. Kulka, A. Pertek, Gradient formation of boride layers by borocarburizing, Appl. Surf. Sci. 254 (2008) 5281–5290. [6] M. Aliofkhazraei, C. Morillo, R. Miresmaeili, A.S. Rouhaghdam, Carburizing of low-melting-point metals by pulsed nanocrystalline plasma electrolytic carburizing, Surf. Coat. Technol. 202 (2008) 5493–5496. [7] X.M. Li, Y. Han, Porous nanocrystalline Ti(Cx N1−x ) thick films by plasma electrolytic carbonitriding, Electrochem. Commun. 8 (2006) 267–272. [8] B. Wang, X. Jin, W. Xue, Z. Wu, J. Du, J. Wu, High temperature tribological behaviors of plasma electrolytic borocarburized Q235 low-carbon steel, Surf. Coat. Technol. 262 (2013) 142–149. [9] F. Cavuslu, M. Usta, Kinetics and mechanical study of plasma electrolytic carburizing for pure iron, Appl. Surf. Sci. 257 (2011) 4014–4020. [10] M.A. Bejar, R. Henriquez, Surface hardening of steel by plasma-electrolysis boronizing, Mater. Des. 30 (2009) 1726–1728. [11] J. Wu, B.W. Xue, B. Wang, X. Jin, J. Du, Y. Li, Characterization of carburized layer on T8 steel fabricated by cathodic plasma electrolysis, Surf. Coat. Technol. 245 (2014) 9–15. [12] H.R. Griem, Plasma Spectroscopy, McGraw-Hill, Cambridge, 1964. [13] F. Mecuson, T. Czerwiec, T. Belmonte, L. Dujardin, A. Viola, G. Henrion, Diagnostics of an electrolytic microarc process for aluminium alloy oxidation, Surf. Coat. Technol. 200 (2005) 804–808. [14] M. Klapkiv, H. Nykyforchyn, V. Posuvailo, Spectral analysis of an elctrolytic plasma in the process of synthesis of aluminum oxide, Mater. Sci. 30 (1994) 333–343. [15] R.O. Hussein, X. Nie, D.O. Northwood, A.L. Yerokhin, A. Matthews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process, J. Phys. D: Appl. Phys. 43 (2010) 105203. [16] R. Liu, J. Wu, W. Xue, Y. Qu, C. Yang, B. Wang, X. Wu, Discharge behaviors during plasma electrolytic oxidation on aluminum alloy, Mater. Chem. Phys. 148 (2014) 284–292. [17] L. Wang, W. Fu, S. Wang, J. Li, Plasma electrolytic oxidation coatings in KOH electrolyte and its discharge characteristics, J. Alloys Compd. 594 (2014) 27–31. [18] R.O. Hussein, X. Nie, D.O. Northwood, A spectroscopic and microstructural study of oxide coatings produced on a Ti–6Al–4 V alloy by plasma electrolytic oxidation, Mater. Chem. Phys. 134 (2012) 484–492. [19] R.L. David (Ed.), CRC Handbook of Chemistry and Physics, 90th ed., CRC Press/Taylor and Francis, Boca Raton, FL, 2010. [20] S.H. Han, J.S. Chun, A study on the electroboronizing of steel by superimposed cyclic current, J. Mater. Sci. 15 (1980) 1379–1386.