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Duplex treatment of plasma nitriding and plasma oxidation of plain carbon steel
ARTICLE IN PRESS
Vacuum 79 (2005) 1–6 www.elsevier.com/locate/vacuum
Duplex treatment of plasma nitriding and plasma oxidation of plain carbon steel F. Mahboubi, M. Fattah Department of Mining, Metallurgical and Petroleum Eng., Amirkabir University of Technology, Hafez Ave., PO Box 15875-4413, Tehran, Iran Received 4 January 2005; accepted 5 January 2005
Abstract Plasma nitriding is a surface treatment process which is increasingly used to improve wear, fatigue and corrosion resistance of industrial parts. Nevertheless, corrosion resistance can be further enhanced by oxidizing of nitrided components. This paper considers the duplex treatment of plasma nitriding and post-oxidation of AISI 1045 plain carbon steel which is used in manufacture of shock absorber rods in automotive industry. Plasma nitriding was carried out at 550 1C for 5 h in atmosphere having nitrogen and hydrogen with volume ratio of 3 to 1. The nitrided samples were post-oxidized at 500 1C for 1 h under O2/H2 volume ratios 1/2.5, 1/9, 1/12 and 1/20. The treated samples were characterized using metallographic techniques, XRD, SEM, micro-hardness and potentiodynamic methods. X-ray diffraction confirmed the development of gamma prime and epsilon iron nitride phases during nitriding and hematite (Fe2O3) and magnetite (Fe3O4) phases under oxidation process. Increasing the oxygen volume of the oxidizing gas led to an increase in the thickness of the oxide layer so that the thickest oxide layer (1.2 mm), consisting of mainly magnetite as well as hematite, was formed on the sample oxidized with gas composition of O2/H2: 1/2.5. This sample also displayed the highest corrosion resistance, 6 times of the nitrided sample and 15 times of the untreated one, which is believed primarily to be due to the magnetite phase formation. Microhardness measurements indicated a decrease in the surface hardness of the duplex-treated samples in comparison with the plasma nitrided one owing to the lower hardness of iron oxides than iron nitrides. r 2005 Elsevier Ltd. All rights reserved. Keywords: Plasma nitriding; Plasma oxidizing; Post-oxidation; Duplex treatment; Corrosion
1. Introduction Corresponding author. Tel.: +98 912 126 3038;
fax: +98 218 095 171 E-mail addresses: [email protected] (F. Mahboubi), [email protected] (M. Fattah).
Plasma nitriding, also known as glow discharge or ion nitriding, is a surface engineering process which effectively improves the wear, fatigue and corrosion resistance of a variety of workpieces
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.002
ARTICLE IN PRESS 2
F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
[1,2]. It uses glow discharge plasma to introduce nitrogen to the surface of the components undergoing treatment. The components are made the cathode of an electrical circuit whilst the chamber becomes the anode. Application of a voltage of 300–800 V between the two electrodes at a pressure of 1–8 mbar establishes a current-intensive glow discharge. This abnormal glow discharge covers the cathode. The components themselves become heated through the transfer of energy associated with the action of ionic bombardment. As a result nitrogen is transferred to the workpieces, which then penetrates inside by diffusion into the surface where it combines with nitride forming elements such as chromium, aluminium and vanadium in the steel, producing the classical nitrided structure, with a surface compound layer and a diffusion zone supporting it [3,4]. The compound layer is thin, typically 2–15 mm, and can be either gamma prime (g0 -Fe4N) and/or epsilon (e-Fe2–3N) iron nitrides. It is unaffected by acid etching and remains white. Hence it is also referred to as the white layer. The corrosion properties of non-alloy and low-alloy steels are improved by the thin white layer of iron nitrides. Corrosion resistance may be increased even further by post-oxidation of the compound layer as a part of the plasma nitriding treatment by changing the treatment gas mixture and temperature [5–12]. In the oxidizing process, free iron and iron nitrides are converted to a stable iron oxide such that a chemically resistant protective layer of approximately 1–2 mm thick is added to the compound layer. The important factor at the post-oxidation process is that the oxidation has to be controlled to avoid hematite (Fe2O3) formation and produce pure magnetite (Fe3O4) which is highly corrosion resistant. Chromium plating has been employed in mass production in the automotive industry for many years. Recently, duplex treatment of plasma nitriding and post-oxidation is proposed to be a successful candidate for replacement of chromiumplating process. It is reported that the duplex treatment has been applied in mass production of a wide variety of parts in automotive industry, such as piston rods, guide pins, gear selector shafts, ball joints and gas-pressure springs [13].
This paper is part of an investigation designed to find the optimum values of plasma nitriding plus post-oxidation process parameters employed for treatment of shock absorber rods made from AISI 1045 plain carbon steel which is widely used in automotive industry. In particular, the emphasis of this paper is on the effect of post-oxidation gas mixture on corrosion resistance of duplex-treated samples.
2. Experimental procedure The steel grade used for this study was AISI 1045 with the composition of 0.42–0.5% C, 0.5–0.8% Mn, o0.4% Si and balance Fe. The specimens were in the form of 12 mm diameter and 5 mm thick discs. The samples were ground and polished with standard metallographic techniques. The specimens were nitrided in a 5 kW DC discharge plasma nitriding unit for 5 h at 550 1C by using a gas composition of 75 vol% nitrogen and 25 vol% hydrogen at the pressure of 4 mbar. After nitriding the oxidation treatment was carried out in the same chamber by changing gas composition and treatment temperature. Postoxidizing processes were performed at 500 1C for 1 h using oxidizing gas mixtures of O2/H2 ¼ 1/2.5, 1/9, 1/12, and 1/20 under a pressure of 4 mbar. The microstructure of the cross-section of the treated samples was studied by optical metallographic techniques and scanning electron microscopy (SEM). X-ray diffraction analysis (Cu Ka radiation, l ¼ 1:5418 A˚) was carried out to identify the phases present in the surface layers. Surface hardness of the treated samples was measured by Vickers micro-hardness tester at a load of 25 gf. The hardnesses quoted are the average of at least 5 readings. Corrosion characteristic was determined in 5% NaCl solution at room temperature using the potentiodynamic technique.
3. Results and discussion The X-ray diffraction patterns of surface layers after different treatments are shown in Fig. 1. Fig. 2 presents the morphology of cross-section of
ARTICLE IN PRESS F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
Fe3O4
Fe4N
Fe2O3
Fe2-3N
Fe
Intensity
e
d c b a 32
36
40
44
48 52 56 60 Two theta (Deg.)
64
68
72
Fig. 1. X- ray pattern of plasma nitrided (a), plasma nitrided and oxidized with O2/H2 ¼ 1/2.5 (b), plasma nitrided and oxidized with O2/H2 ¼ 1/9 (c), plasma nitrided and oxidized with O2/H2 ¼ 1/12 (d) and plasma nitrided and oxidized with O2/H2 ¼ 1/20 (e).
nitrided/oxidized layers obtained by SEM. The compound layer of plasma nitrided sample is composed of iron nitrides of g0 -Fe4N and e-Fe23N phases (Fig. 1a). The thickness of the nitrided layer is 10 mm and the diffusion zone is about 100 mm thick. Surface layer of plasma nitrided and oxidized samples consists of iron nitrides, a-iron, along with iron oxides of hematite (Fe2O3) and magnetite (Fe3O4) (Fig. 1b–e). Samples treated under gas mixtures of O2/H2 ¼ 1/2.5 and 1/9 showed the highest amount of Fe3O4 phase formation (Fig. 1b and c) while samples oxidized at O2/H2 ¼ 1/12 and 1/20 exhibited the least amount of oxide phases (Fig. 1d and e). The thicknesses of the nitrided and oxidized layers are presented in Fig. 3. According to phase diagram of the Fe–O system both iron oxide phases of Fe2O3 and Fe3O4 are stable at oxidizing temperature of 500 1C. Therefore, it is possible to have both phases concurrently at post-oxidation temperature of 500 1C. However, the possibility of magnetite formation is increased by increasing the partial pressure of H2 and the chance of hematite
3
deposition is increased by decreasing the partial pressure of H2 [14]. In gas mixture of O2/H2 ¼ 1/ 2.5, due to a decrease in hydrogen partial pressure, besides Fe3O4 formation, Fe2O3 phase also forms. A decrease in oxygen content of treatment gas results in a reduction in the thickness of the oxide layer (Fig. 3) as well as a decrease in the amount of oxide phases (Fig. 1). This is believed to be due to the lack of oxygen atoms required for oxide formation. This phenomenon is also observed and reported by other researchers [6]. The effects of post-oxidizing treatment on the thickness and the morphology of the compound layer are well described by Borgioli et al. [8]. In oxidizing treatment of nitrided layer, nitrogen atoms are released [15]. These atoms may diffuse inward into the substrate or may diffuse outward and accumulate close to the interface of nitrideoxide layers, Fig. 2. The oxidizing treatment temperature dictates the mode of diffusion. If the temperature is high enough to provide the diffusion energy of nitrogen atoms, they diffuse inward, towards the matrix, leading to a decrease in the nitrogen content of e phase and the diffusion transformation of g0 ! g0 þ ða Fe; NÞ becomes pertinent. At lower oxidizing temperature, the mobility of nitrogen atoms decreases. Thus they diffuse outward and pile up at the interface of nitride/oxide layers, leading to an increase in the nitrogen content of e phase along with the phase transformation of g0 to e. Since in this study the oxidizing treatment carried out at 500 1C, the nitrogen atoms had enough energy to diffuse towards the matrix. Therefore, the thickness of compound layer after post-oxidizing treatment is decreased and the interface of compound layer and the matrix is consists g0 and (a-Fe, N), Fig. 2. X-ray pattern of the plasma nitrided sample shows no evidence of a-iron (Fig. 2a), whereas Xray patterns of the plasma nitrided plus postoxidized samples present the peaks of a-iron (Fig. 2b–e). This can be explained by both diffusion transformation of g0 -g0 +(a-Fe, N) which produces a-iron and the fact that X-ray absorption coefficient of iron oxide is lower than that of iron nitrides, leading to penetration of X-ray more
ARTICLE IN PRESS F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
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Fig. 2. SEM cross-section image of plasma nitrided sample (a), plasma nitrided and oxidized with O2/H2 ¼ 1/2.5 (b), plasma nitrided and oxidized with O2/H2 ¼ 1/9 (c), plasma nitrided and oxidized with O2/H2 ¼ 1/12 (d) and plasma nitrided and oxidized with O2/ H2 ¼ 1/20 (e).
Oxide layer thickness (micron)
1.5 1.2 0.9 0.6 0.3 0 O2/H2:1/2.5 O2/H2:1/9 O2/H2:1/12 O2/H2:1/20 Treating gas
Fig. 3. Oxide layer thicknesses of the plasma nitrided and postoxidized samples.
deeply into the steel substrate and thus emergence of a-iron peaks [8]. Surface hardnesses of nitrided and duplextreated samples are presented in Fig. 4. A reduction in surface hardness of nitrided plus post-oxidized samples can be seen in comparison with that of nitrided sample. This can be ascribed to the lower hardness of iron oxide than iron nitrides. This reduction in surface hardness is also reported by other researchers [8]. The compound layer formed in nitriding process is usually porous (Fig. 2a) which diminishes the corrosion resistance of the nitrided part, particularly against pitting. Due to the different oxygen concentration at the valleys and tops of the surface
ARTICLE IN PRESS F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
pores, they act as local cathode and anode, respectively, [16] and promote pitting. Plasma post-oxidizing treatment of nitrided part leads to
800
hardness (HV0.025)
700 600 500 400 300 200 100 0 O2/H2:1/2.5 O2/H2:1/9 O2/H2:1/12 O2/H2:1/20
Nitriding
Treating gas
Fig. 4. Surface hardness values of the treated samples.
1 0.8 E vs. SCE (V)
0.6 0.4 0.2 0
O2/H2= 1/2.5 O2/H2=1/9 Nitriding
-0.2 -0.4 -0.6 -10
-7.5
-5 Log (I/Area)
-2.5
0
Fig. 5. Comparison of potentiodynamic polarization curves of plasma nitrided and nitrided plus oxidized at O2/H2 ¼ 1/2.5 and 1/9 samples.
5
the formation and penetration of oxide phases into the pores (valleys) and closes them which in turn make an increase in the corrosion resistance of the sample. The potentiodynamic polarization curves of plasma nitrided and nitrided plus oxidized at O2/ H2 ¼ 1/2.5 and 1/9 gas mixture samples are shown in Fig. 5. The corrosion test results of untreated sample along with other treated samples are also displayed in Table 1. The untreated sample showed the lowest corrosion potential (240 mV) and the highest corrosion current (1.41 mA/cm2). Plasma nitriding and duplex treatment of the samples resulted in an improvement in corrosion resistance. This improvement was more evident for duplex treated samples. The highest corrosion resistance belongs to the sample oxidized at O2/ H2 ¼ 1/2.5 gas mixture. Corrosion current (0.094 mA/cm2) and corrosion rate (0.043 mpy) of this sample are reduced to 0.16 of nitrided sample and 0.07 of untreated sample, indicating 6 and 15 times enhancement of its corrosion resistance in comparison with plasma nitrided sample and untreated sample, respectively. The improvement of corrosion resistance is believed to be due to the type and thickness of the oxide layer formed at post-oxidizing treatment. The sample treated O2/H2 ¼ 1/2.5 gas mixture has the thickest oxide layer (Fig. 3) which consists of both Fe3O4 and Fe2O3 phases, Fig. 1. The Fe2O3 phase, developed during the postoxidation process, is a porous phase with low adherence [17] and thus has minor effect on corrosion resistance of the nitrided surface. In contrast, Fe3O4 phase which is a dense and compact layer, improves corrosion resistance of the sample more effectively [18].
Table 1 Potentiodynamic test results of plasma nitrided and duplex treated samples Treatment
Corrosion rate (mpy)
I
O2/H2 ¼ 1/2.5 O2/H2 ¼ 1/9 O2/H2 ¼ 1/12 O2/H2 ¼ 1/20 Nitrided Untreated
0.043 0.070 0.099 0.217 0.264 0.651
0.094 0.152 0.216 0.471 0.571 1.410
Corr
(mA/cm2)
Potential (mV) 158 179 411 252 169 224
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4. Conclusions This paper is part of an investigation designed to optimize the duplex process of plasma nitriding plus post-oxidation in order to improve the corrosion resistance of AISI 1045 carbon steel which is used in manufacture of shock absorber rods in automotive industry and intends to examine the feasibility of replacement of chromium plating by the duplex treatment. The following conclusions have been obtained from study. 1. The X-ray analysis revealed the formation of iron nitride phases of g0 -Fe4N and e-Fe23N during plasma nitriding and iron oxide phases of hematite (Fe2O3) and magnetite (Fe3O4) as well as a-iron through the post-oxidizing treatment. The possibility of Fe2O3 phase deposition is increased by increasing the oxygen content of the oxidizing gas mixture. 2. Corrosion resistance of the duplex treated sample is increased compared with both untreated and plasma nitrided samples. This is attributed mainly to the formation of magnetite phase during the post-oxidizing treatment of the nitrided layer. 3. Increasing the oxide layer thickness increases the corrosion resistance of the duplex treated samples so that the sample post-oxidized at O2/ H2 ¼ 1/2.5 gas mixture with 1.2 mm thick oxide layer exhibited the highest corrosion resistance which was 6 times of the nitrided sample and 15 times of the untreated one. 4. Post-oxidizing treatment reduces the thickness and changes the morphology of the iron nitride compound layer formed during plasma-nitrid-
ing treatment. This is believed to be due to the transformation of g0 ! g0 þ ða Fe; NÞ and inward diffusion of oxygen atoms. 5. Duplex-treated samples showed a decrease in surface hardness compared with plasma-nitrided sample. This is due to the lower hardness of iron oxides relative to iron nitrides.
References [1] Czerwiecu T, Renevier N, Michel H. Surf. Coat. Technol. 2000;131:267. [2] Podgornik B, Viz¡;intin J. Mater. Sci. Eng. 2001;A315:28. [3] Edenhofer B. Heat Treatment Metals 1974(part 2):59–67. [4] Kovacs W, Russell W. Ion Nitriding. In: Spalvins T, editor. ASM: Metals Park, OH; 1987. p. 9–17. [5] Hoppe S. Surf. Coat. Technol. 1998;98:1199. [6] Hong JM, Cho YR, Kim DJ, Baek JM, Lee KH. Surf. Coat. Technol. 2000;131:548. [7] Jeong BY, Kim MH. Surf. Coat. Technol. 2001;141:262. [8] Borgioli F, Galvanetto E, Fossati A, Bacci T. Surf. Coat. Technol. 2002;162:61. [9] Alsaran A, Ikmet Altun H, Karakan M, Celik A. Surf. Coat. Technol. 2004;176:344. [10] Lee I. Surf. Coat. Technol. 2004;188–189:669. [11] Zlatanovic M, Popovic N, Bogdanov Z, Zlatanovic S. Surf. Coat. Technol. 2004;177–178:277. [12] Zlatanovic M, Popovic N, Bogdanov Z, Zlatanovic S. Surf. Coat. Technol. 2003;174–175:1220. [13] Brinke T, Krug TG. Ind. Heat. 2001:61–4. [14] Rezek J, Klein IE, Yahalom J. Appl. Surf. Sci. 1997;108:159. [15] Graat PCJ, Somers MAJ, Mittemeijer EJ. Thin Solid Films 1999;353:72. [16] Zhang LZ. ASM International Heat Treatment and Surface Engineering Conference, OH, USA, 1988, pp. 343–48. [17] Senkevich JJ, Jones DA, Chatterjee I. Corrosion Sci. 2000;42:201. [18] Bell T, Sun Y, Suhadi A. Vacuum 2000;59:14.
Vacuum 79 (2005) 1–6 www.elsevier.com/locate/vacuum
Duplex treatment of plasma nitriding and plasma oxidation of plain carbon steel F. Mahboubi, M. Fattah Department of Mining, Metallurgical and Petroleum Eng., Amirkabir University of Technology, Hafez Ave., PO Box 15875-4413, Tehran, Iran Received 4 January 2005; accepted 5 January 2005
Abstract Plasma nitriding is a surface treatment process which is increasingly used to improve wear, fatigue and corrosion resistance of industrial parts. Nevertheless, corrosion resistance can be further enhanced by oxidizing of nitrided components. This paper considers the duplex treatment of plasma nitriding and post-oxidation of AISI 1045 plain carbon steel which is used in manufacture of shock absorber rods in automotive industry. Plasma nitriding was carried out at 550 1C for 5 h in atmosphere having nitrogen and hydrogen with volume ratio of 3 to 1. The nitrided samples were post-oxidized at 500 1C for 1 h under O2/H2 volume ratios 1/2.5, 1/9, 1/12 and 1/20. The treated samples were characterized using metallographic techniques, XRD, SEM, micro-hardness and potentiodynamic methods. X-ray diffraction confirmed the development of gamma prime and epsilon iron nitride phases during nitriding and hematite (Fe2O3) and magnetite (Fe3O4) phases under oxidation process. Increasing the oxygen volume of the oxidizing gas led to an increase in the thickness of the oxide layer so that the thickest oxide layer (1.2 mm), consisting of mainly magnetite as well as hematite, was formed on the sample oxidized with gas composition of O2/H2: 1/2.5. This sample also displayed the highest corrosion resistance, 6 times of the nitrided sample and 15 times of the untreated one, which is believed primarily to be due to the magnetite phase formation. Microhardness measurements indicated a decrease in the surface hardness of the duplex-treated samples in comparison with the plasma nitrided one owing to the lower hardness of iron oxides than iron nitrides. r 2005 Elsevier Ltd. All rights reserved. Keywords: Plasma nitriding; Plasma oxidizing; Post-oxidation; Duplex treatment; Corrosion
1. Introduction Corresponding author. Tel.: +98 912 126 3038;
fax: +98 218 095 171 E-mail addresses: [email protected] (F. Mahboubi), [email protected] (M. Fattah).
Plasma nitriding, also known as glow discharge or ion nitriding, is a surface engineering process which effectively improves the wear, fatigue and corrosion resistance of a variety of workpieces
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.002
ARTICLE IN PRESS 2
F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
[1,2]. It uses glow discharge plasma to introduce nitrogen to the surface of the components undergoing treatment. The components are made the cathode of an electrical circuit whilst the chamber becomes the anode. Application of a voltage of 300–800 V between the two electrodes at a pressure of 1–8 mbar establishes a current-intensive glow discharge. This abnormal glow discharge covers the cathode. The components themselves become heated through the transfer of energy associated with the action of ionic bombardment. As a result nitrogen is transferred to the workpieces, which then penetrates inside by diffusion into the surface where it combines with nitride forming elements such as chromium, aluminium and vanadium in the steel, producing the classical nitrided structure, with a surface compound layer and a diffusion zone supporting it [3,4]. The compound layer is thin, typically 2–15 mm, and can be either gamma prime (g0 -Fe4N) and/or epsilon (e-Fe2–3N) iron nitrides. It is unaffected by acid etching and remains white. Hence it is also referred to as the white layer. The corrosion properties of non-alloy and low-alloy steels are improved by the thin white layer of iron nitrides. Corrosion resistance may be increased even further by post-oxidation of the compound layer as a part of the plasma nitriding treatment by changing the treatment gas mixture and temperature [5–12]. In the oxidizing process, free iron and iron nitrides are converted to a stable iron oxide such that a chemically resistant protective layer of approximately 1–2 mm thick is added to the compound layer. The important factor at the post-oxidation process is that the oxidation has to be controlled to avoid hematite (Fe2O3) formation and produce pure magnetite (Fe3O4) which is highly corrosion resistant. Chromium plating has been employed in mass production in the automotive industry for many years. Recently, duplex treatment of plasma nitriding and post-oxidation is proposed to be a successful candidate for replacement of chromiumplating process. It is reported that the duplex treatment has been applied in mass production of a wide variety of parts in automotive industry, such as piston rods, guide pins, gear selector shafts, ball joints and gas-pressure springs [13].
This paper is part of an investigation designed to find the optimum values of plasma nitriding plus post-oxidation process parameters employed for treatment of shock absorber rods made from AISI 1045 plain carbon steel which is widely used in automotive industry. In particular, the emphasis of this paper is on the effect of post-oxidation gas mixture on corrosion resistance of duplex-treated samples.
2. Experimental procedure The steel grade used for this study was AISI 1045 with the composition of 0.42–0.5% C, 0.5–0.8% Mn, o0.4% Si and balance Fe. The specimens were in the form of 12 mm diameter and 5 mm thick discs. The samples were ground and polished with standard metallographic techniques. The specimens were nitrided in a 5 kW DC discharge plasma nitriding unit for 5 h at 550 1C by using a gas composition of 75 vol% nitrogen and 25 vol% hydrogen at the pressure of 4 mbar. After nitriding the oxidation treatment was carried out in the same chamber by changing gas composition and treatment temperature. Postoxidizing processes were performed at 500 1C for 1 h using oxidizing gas mixtures of O2/H2 ¼ 1/2.5, 1/9, 1/12, and 1/20 under a pressure of 4 mbar. The microstructure of the cross-section of the treated samples was studied by optical metallographic techniques and scanning electron microscopy (SEM). X-ray diffraction analysis (Cu Ka radiation, l ¼ 1:5418 A˚) was carried out to identify the phases present in the surface layers. Surface hardness of the treated samples was measured by Vickers micro-hardness tester at a load of 25 gf. The hardnesses quoted are the average of at least 5 readings. Corrosion characteristic was determined in 5% NaCl solution at room temperature using the potentiodynamic technique.
3. Results and discussion The X-ray diffraction patterns of surface layers after different treatments are shown in Fig. 1. Fig. 2 presents the morphology of cross-section of
ARTICLE IN PRESS F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
Fe3O4
Fe4N
Fe2O3
Fe2-3N
Fe
Intensity
e
d c b a 32
36
40
44
48 52 56 60 Two theta (Deg.)
64
68
72
Fig. 1. X- ray pattern of plasma nitrided (a), plasma nitrided and oxidized with O2/H2 ¼ 1/2.5 (b), plasma nitrided and oxidized with O2/H2 ¼ 1/9 (c), plasma nitrided and oxidized with O2/H2 ¼ 1/12 (d) and plasma nitrided and oxidized with O2/H2 ¼ 1/20 (e).
nitrided/oxidized layers obtained by SEM. The compound layer of plasma nitrided sample is composed of iron nitrides of g0 -Fe4N and e-Fe23N phases (Fig. 1a). The thickness of the nitrided layer is 10 mm and the diffusion zone is about 100 mm thick. Surface layer of plasma nitrided and oxidized samples consists of iron nitrides, a-iron, along with iron oxides of hematite (Fe2O3) and magnetite (Fe3O4) (Fig. 1b–e). Samples treated under gas mixtures of O2/H2 ¼ 1/2.5 and 1/9 showed the highest amount of Fe3O4 phase formation (Fig. 1b and c) while samples oxidized at O2/H2 ¼ 1/12 and 1/20 exhibited the least amount of oxide phases (Fig. 1d and e). The thicknesses of the nitrided and oxidized layers are presented in Fig. 3. According to phase diagram of the Fe–O system both iron oxide phases of Fe2O3 and Fe3O4 are stable at oxidizing temperature of 500 1C. Therefore, it is possible to have both phases concurrently at post-oxidation temperature of 500 1C. However, the possibility of magnetite formation is increased by increasing the partial pressure of H2 and the chance of hematite
3
deposition is increased by decreasing the partial pressure of H2 [14]. In gas mixture of O2/H2 ¼ 1/ 2.5, due to a decrease in hydrogen partial pressure, besides Fe3O4 formation, Fe2O3 phase also forms. A decrease in oxygen content of treatment gas results in a reduction in the thickness of the oxide layer (Fig. 3) as well as a decrease in the amount of oxide phases (Fig. 1). This is believed to be due to the lack of oxygen atoms required for oxide formation. This phenomenon is also observed and reported by other researchers [6]. The effects of post-oxidizing treatment on the thickness and the morphology of the compound layer are well described by Borgioli et al. [8]. In oxidizing treatment of nitrided layer, nitrogen atoms are released [15]. These atoms may diffuse inward into the substrate or may diffuse outward and accumulate close to the interface of nitrideoxide layers, Fig. 2. The oxidizing treatment temperature dictates the mode of diffusion. If the temperature is high enough to provide the diffusion energy of nitrogen atoms, they diffuse inward, towards the matrix, leading to a decrease in the nitrogen content of e phase and the diffusion transformation of g0 ! g0 þ ða Fe; NÞ becomes pertinent. At lower oxidizing temperature, the mobility of nitrogen atoms decreases. Thus they diffuse outward and pile up at the interface of nitride/oxide layers, leading to an increase in the nitrogen content of e phase along with the phase transformation of g0 to e. Since in this study the oxidizing treatment carried out at 500 1C, the nitrogen atoms had enough energy to diffuse towards the matrix. Therefore, the thickness of compound layer after post-oxidizing treatment is decreased and the interface of compound layer and the matrix is consists g0 and (a-Fe, N), Fig. 2. X-ray pattern of the plasma nitrided sample shows no evidence of a-iron (Fig. 2a), whereas Xray patterns of the plasma nitrided plus postoxidized samples present the peaks of a-iron (Fig. 2b–e). This can be explained by both diffusion transformation of g0 -g0 +(a-Fe, N) which produces a-iron and the fact that X-ray absorption coefficient of iron oxide is lower than that of iron nitrides, leading to penetration of X-ray more
ARTICLE IN PRESS F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
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Fig. 2. SEM cross-section image of plasma nitrided sample (a), plasma nitrided and oxidized with O2/H2 ¼ 1/2.5 (b), plasma nitrided and oxidized with O2/H2 ¼ 1/9 (c), plasma nitrided and oxidized with O2/H2 ¼ 1/12 (d) and plasma nitrided and oxidized with O2/ H2 ¼ 1/20 (e).
Oxide layer thickness (micron)
1.5 1.2 0.9 0.6 0.3 0 O2/H2:1/2.5 O2/H2:1/9 O2/H2:1/12 O2/H2:1/20 Treating gas
Fig. 3. Oxide layer thicknesses of the plasma nitrided and postoxidized samples.
deeply into the steel substrate and thus emergence of a-iron peaks [8]. Surface hardnesses of nitrided and duplextreated samples are presented in Fig. 4. A reduction in surface hardness of nitrided plus post-oxidized samples can be seen in comparison with that of nitrided sample. This can be ascribed to the lower hardness of iron oxide than iron nitrides. This reduction in surface hardness is also reported by other researchers [8]. The compound layer formed in nitriding process is usually porous (Fig. 2a) which diminishes the corrosion resistance of the nitrided part, particularly against pitting. Due to the different oxygen concentration at the valleys and tops of the surface
ARTICLE IN PRESS F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
pores, they act as local cathode and anode, respectively, [16] and promote pitting. Plasma post-oxidizing treatment of nitrided part leads to
800
hardness (HV0.025)
700 600 500 400 300 200 100 0 O2/H2:1/2.5 O2/H2:1/9 O2/H2:1/12 O2/H2:1/20
Nitriding
Treating gas
Fig. 4. Surface hardness values of the treated samples.
1 0.8 E vs. SCE (V)
0.6 0.4 0.2 0
O2/H2= 1/2.5 O2/H2=1/9 Nitriding
-0.2 -0.4 -0.6 -10
-7.5
-5 Log (I/Area)
-2.5
0
Fig. 5. Comparison of potentiodynamic polarization curves of plasma nitrided and nitrided plus oxidized at O2/H2 ¼ 1/2.5 and 1/9 samples.
5
the formation and penetration of oxide phases into the pores (valleys) and closes them which in turn make an increase in the corrosion resistance of the sample. The potentiodynamic polarization curves of plasma nitrided and nitrided plus oxidized at O2/ H2 ¼ 1/2.5 and 1/9 gas mixture samples are shown in Fig. 5. The corrosion test results of untreated sample along with other treated samples are also displayed in Table 1. The untreated sample showed the lowest corrosion potential (240 mV) and the highest corrosion current (1.41 mA/cm2). Plasma nitriding and duplex treatment of the samples resulted in an improvement in corrosion resistance. This improvement was more evident for duplex treated samples. The highest corrosion resistance belongs to the sample oxidized at O2/ H2 ¼ 1/2.5 gas mixture. Corrosion current (0.094 mA/cm2) and corrosion rate (0.043 mpy) of this sample are reduced to 0.16 of nitrided sample and 0.07 of untreated sample, indicating 6 and 15 times enhancement of its corrosion resistance in comparison with plasma nitrided sample and untreated sample, respectively. The improvement of corrosion resistance is believed to be due to the type and thickness of the oxide layer formed at post-oxidizing treatment. The sample treated O2/H2 ¼ 1/2.5 gas mixture has the thickest oxide layer (Fig. 3) which consists of both Fe3O4 and Fe2O3 phases, Fig. 1. The Fe2O3 phase, developed during the postoxidation process, is a porous phase with low adherence [17] and thus has minor effect on corrosion resistance of the nitrided surface. In contrast, Fe3O4 phase which is a dense and compact layer, improves corrosion resistance of the sample more effectively [18].
Table 1 Potentiodynamic test results of plasma nitrided and duplex treated samples Treatment
Corrosion rate (mpy)
I
O2/H2 ¼ 1/2.5 O2/H2 ¼ 1/9 O2/H2 ¼ 1/12 O2/H2 ¼ 1/20 Nitrided Untreated
0.043 0.070 0.099 0.217 0.264 0.651
0.094 0.152 0.216 0.471 0.571 1.410
Corr
(mA/cm2)
Potential (mV) 158 179 411 252 169 224
ARTICLE IN PRESS 6
F. Mahboubi, M. Fattah / Vacuum 79 (2005) 1–6
4. Conclusions This paper is part of an investigation designed to optimize the duplex process of plasma nitriding plus post-oxidation in order to improve the corrosion resistance of AISI 1045 carbon steel which is used in manufacture of shock absorber rods in automotive industry and intends to examine the feasibility of replacement of chromium plating by the duplex treatment. The following conclusions have been obtained from study. 1. The X-ray analysis revealed the formation of iron nitride phases of g0 -Fe4N and e-Fe23N during plasma nitriding and iron oxide phases of hematite (Fe2O3) and magnetite (Fe3O4) as well as a-iron through the post-oxidizing treatment. The possibility of Fe2O3 phase deposition is increased by increasing the oxygen content of the oxidizing gas mixture. 2. Corrosion resistance of the duplex treated sample is increased compared with both untreated and plasma nitrided samples. This is attributed mainly to the formation of magnetite phase during the post-oxidizing treatment of the nitrided layer. 3. Increasing the oxide layer thickness increases the corrosion resistance of the duplex treated samples so that the sample post-oxidized at O2/ H2 ¼ 1/2.5 gas mixture with 1.2 mm thick oxide layer exhibited the highest corrosion resistance which was 6 times of the nitrided sample and 15 times of the untreated one. 4. Post-oxidizing treatment reduces the thickness and changes the morphology of the iron nitride compound layer formed during plasma-nitrid-
ing treatment. This is believed to be due to the transformation of g0 ! g0 þ ða Fe; NÞ and inward diffusion of oxygen atoms. 5. Duplex-treated samples showed a decrease in surface hardness compared with plasma-nitrided sample. This is due to the lower hardness of iron oxides relative to iron nitrides.
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