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Comparative study of plasma oxynitriding and plasma nitriding for AISI 4140 steel
Journal of Alloys and Compounds 680 (2016) 642e645
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Comparative study of plasma oxynitriding and plasma nitriding for AISI 4140 steel Jiqiang Wu a, Han Liu a, c, Jingcai Li a, Xingmei Yang a, Jing Hu a, b, c, * a
Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou, 213164, China Materials Research and Education Center, Auburn University, AL, 36849, USA c Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou, 213164, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2016 Received in revised form 31 March 2016 Accepted 18 April 2016 Available online 20 April 2016
Plasma oxynitriding using plain air as oxygen bearing gas was developed and compared with traditional plasma nitriding for AISI 4140 steel. The treated samples are characterized by optical metallography, microhardness tester, X-ray diffraction (XRD) and electrochemical polarization. The results show that plain air can be used as oxygen bearing gas in plasma oxynitiding for AISI 4140 steel, and plasma oxynitriding owns much higher efficiency compared with plasma nitriding, thus makes thicker compound layer and effective diffusion layer under the same conditions. Meanwhile, higher surface hardness and smoother micro-hardness gradient are obtained. Furthermore, corrosion resistance can be significantly improved due to the formation of chemically stable and compact Fe3O4 oxide in the compound layer during plasma oxynitriding process, and air flow of 0.3 L/min offers the optimum performance for AISI 4140 steel due to highest ratio of Fe3O4 to Fe2O3. © 2016 Elsevier B.V. All rights reserved.
Keywords: Plasma oxynitriding X-ray diffraction Plain air Corrosion resistance
1. Introduction AISI 4140 steel is widely used for gears due to its excellent combined properties, and plasma nitriding is one of the most widely used surface modifications to improve surface hardness and wear resistance of various engineering materials [1e3], which involves the introduction of nitrogen atoms into the component surface to produce a modified layer on the surface of materials. Unfortunately, dozens of hours or even longer duration must be consumed to obtain the desired layer thickness and properties in current application, which results in low efficiency and high production cost [4]. It has been reported that gas oxynitriding by adding some plain air has effective catalysis effect on gas nitriding, since the surface layer after oxynitriding plays a crucial role on the evolution of nitriding layer [5e7]. However, there is no report on plasma oxynitriding (PON) by adding plain air. Generally, plasma oxynitriding (PON) was performed using mixture gases of hydrogen, nitrogen and oxygen. To make the process more convenient and friendly, plain air was primarily used
* Corresponding author. Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou, 213164, China. E-mail address: [email protected] (J. Hu). http://dx.doi.org/10.1016/j.jallcom.2016.04.172 0925-8388/© 2016 Elsevier B.V. All rights reserved.
as oxygen bearing gas for PON treating in this work, and the effect of air flow on treating efficiency and related properties, especially corrosion resistance was systematically investigated and compared with traditional plasma nitriding (PN) for AISI 4140 steel. 2. Experimental The material used for this investigation was AISI 4140 steel with the following chemical compositions (in wt. %): C, 0.41; Cr, 0.91; Mo, 0.18; Mn, 0.83; Si, 0.21; P, 0.014; S, 0.011 and Fe, balance. Specimens with the size of 10 mm 10 mm 5 mm were cut from a used gear. All the surfaces of samples were ground and polished by chromic oxide slurry to achieve a fine finish and ultrasonically cleaned in anhydrous ethanol prior to PN or PON treating. PON was performed in 20-KW pulsed DC plasma nitriding equipment. The entire treating process was composed of three steps: heating, plasma oxynitriding and cooling. Firstly, specimens were heated by hydrogen ion bombardment to the designed temperature of 573 K; secondly, PON was conducted at 823 K for 4 h in mixture gas of nitrogen and hydrogen with a ratio of 1/3 and different air flow at a gas pressure of 400 Pa, and three kinds of air flow (0.1 L/min, 0.3 L/min and 0.5 L/min) were applied in this study. Finally, the vacuum chamber was pumped to 10 Pa and samples were cooled down to room temperature in the furnace. The only
J. Wu et al. / Journal of Alloys and Compounds 680 (2016) 642e645
difference between PON and PN is of no air flow (0 L/min) in the second step for PN treating. The cross-sectional microstructures were observed by optical metallography and the thicknesses of compound layer were measured from the cross-sectional microstructure. Microhardness and depth measurements were made in a HXD-1000TMC microhardness tester, with the test load of 50 g and the holding duration of 15 s. At each depth, 10 measurements were taken and then the mean value was determined by averaging 8 measurements, excluding the highest and the lowest value. The phases were determined by X-ray diffraction (XRD) with Cu-Ka (l ¼ 1.54 Å) radiation. Corrosion resistance was evaluated by the potentiodynamic curves in 3.5% NaCl aerated solution using ZAHNER IM6e electrochemical workstations. Corrosion resistance was evaluated by the potentiodynamic curves in 3.5% NaCL aer-ated solution at room temperature using ZAHNER IM6e electrochemical workstations. The scanning potential was in the range of 1500 mv to 500 mv, and the potential scan rate was 1 mv/s after a delay period of 2000 s. At least three tests were carried out for each process for assessing the reproducibility. 3. Results and discussions
643
shows that there exist micropores and porosities in the compound layer for PN treated specimen, while a thin oxide layer is produced on the outermost surface for PON treated specimens, which can promote the formation of a denser compound layer as shown in Fig. 1(b). In addition, it can be found that the maximum compound layer thickness is obtained at air flow of 0.3 L/min. 3.2. XRD analysis Fig. 2 illustrates XRD patterns of samples treated at 823 K for 4 h with different air flow. Hematite (Fe2O3) and magnetite (Fe3O4) peaks are clearly visible for PON treated samples as shown in (b) ~ (d), and no oxide was formed for PN treated sample as shown in (a). Meanwhile, it can be found that the ratio of Fe3O4 to Fe2O3 is depended on the air flow, and the highest ratio of Fe3O4 to Fe2O3 is obtained at air flow of 0.3 L/min, which are the desired phase constituents in the real applications [8e10]. And it also illustrates that the amount of Fe2O3 increases with the air flow, attributing to the higher oxygen concentration [11]. In addition, the amount of g0 Fe4N is reduced and ε-Fe2-3N and CrN is increased with air flow, since the added air leads to the higher nitrogen potential in the local nitride layer during PON treating, and thus the transformation of g0 - Fe4N to the nitrogen richer ε-Fe2-3N occurs [11].
3.1. Microstructures and depth analysis 3.3. Microhardness measurements The cross-sectional microstructures of samples treated at 823 K for 4 h with different air flow are shown in Fig. 1, and the relationship between compound layer thickness and air flow are presented in Table 1. It clearly shows that the thickness of compound layer is significantly increased by adding plain air in plasma nitriding, and the compound layer of PON treating is denser as shown in Fig. 1 (b-d). The cross-sectional SEM embedded in Fig. 1(a)
The sectional microhardness of samples treated at 823 K for 4 h at different air flow is presented in Fig. 3. It can be clearly seen that higher sectional microhardness can be obtained for samples treated by PON, especially at air flow of 0.3 L/min. Hence, the thickness of effective diffusion layer is increased accordingly. The highest hardness of 790HV0.05 is achieved at air flow of 0.3 L/min, 80 HV0.05
Fig. 1. The cross sectional microstructure of AISI 4140 plasma treated at different conditions. (a) PN: Vair ¼ 0 L/min; (b)e(d) PON, (b) Vair ¼ 0.1 L/min; (c) Vair ¼ 0.3 L/min; (d) Vair ¼ 0.5 L/min.
644
J. Wu et al. / Journal of Alloys and Compounds 680 (2016) 642e645
Table 1 Compound layer thickness and surface hardness vs. air flow. Process mark
Air flow (L/min)
Compound layer thickness (mm)
Surface hardness (HV0.05)
(a) (b) (c) (d)
PN (Vair ¼ 0) PON (Vair ¼ 0.1) PON (Vair ¼ 0.3) PON (Vair ¼ 0.5)
18.5e19.0 21.5e22.0 29.0e29.5 23.0e23.5
710 745 790 730
Note: All samples were treated at the same temperature of 823 K for 4 h.
higher than that of PN treated sample as shown in Table 1. This may be attributed to the formation of a much denser compound layer under this condition as shown in Fig. 1. 3.4. Corrosion behavior The effect of air flow on the corrosion behavior for AISI 4140 steel is shown in Fig. 4, and the corresponding data related to the corrosion resistance are presented in Table 2. It is obvious that the
PN treated sample owns the lowest corrosion potential, in other words, the corrosion resistance of all the PON treated samples is improved, and the highest corrosion potential of 300 mV and lowest corrosion current of 0.267 mA/cm2 are achieved for PON treated sample at air flow of 0.3 L/min, i.e. the optimum corrosion resistance is achieved at air flow of 0.3 L/min, which can be ascribed to the highest ratio of Fe3O4 to Fe2O3 as shown in Fig. 2(c). In order to confirm the corrosion behavior, typical surface images of the corroded specimens treated by PN and PON after polarization test are taken and compared in Fig. 5. The image of PN treated sample in Fig. 5(a) clearly shows that there exist many corrosion pits distributed on the surface due to the pores existed in the compound layer. While there shows no corrosion pit in the image of PON treated sample shown in Fig. 5(b) due to the protection from chemically stable and compact Fe3O4 formed on the outermost surface, corresponding to the optimum corrosion resistance. 4. Discussions The main enhancement mechanism can be explained from the
Fig. 2. XRD patterns of AISI 4140 specimens plasma treated at 823 K for 4 h with different air flow. (a) PN: Vair ¼ 0 L/min; (b)e(d) PON, (b) Vair ¼ 0.1 L/min; (c) Vair ¼ 0.3 L/min; (d) Vair ¼ 0.5 L/min.
Fig. 4. Current density of the samples treated with different plain flow. (a) PN; Vair ¼ 0 L/min (b)e(d) PON, (b) Vair ¼ 0.1 L/min; (c) Vair ¼ 0.3 L/min; (d) Vair ¼ 0.5 L/ min.
Table 2 The related corrosion resistance data corresponding to Fig. 4. Process mark
Fig. 3. Microhardness of AISI 4140 steel treated with different air flow. (a) PN: Vair ¼ 0 L/min; (b)e(d) PON, (b) Vair ¼ 0.1 L/min; (c) Vair ¼ 0.3 L/min; (d) Vair ¼ 0.5 L/ min.
(a) (b) (c) (d)
Air flow (L/min)
PN (Vair ¼ 0) PON (Vair ¼ 0.1) PON (Vair ¼ 0.3) PON (Vair ¼ 0.5)
Corrosion resistance Ecorr(mV)
Icorr(mA/cm2)
980 400 300 350
0.267 0.254 0.207 0.304
Note: All samples were treated at the same temperature of 823 K for 4 h.
J. Wu et al. / Journal of Alloys and Compounds 680 (2016) 642e645
645
Fig. 5. SEM images of corroded specimens. (a) PN: Vair ¼ 0 L/min; (b) PON: Vair ¼ 0.3 L/min.
point of the nitrogen potential in nitriding atmosphere, as is well known that faster nitriding is attributed to higher nitrogen potential. Since there exist mixture gases of nitrogen, hydrogen and oxygen in the chamber during the PON process, the following chemical reactions occur [12]. 2H2 þ O2 / 2H2O
(1)
N2 þ O2 / 2NO
(2)
3Fe þ 2O2 / Fe3O4
(3)
4Fe3O4 þ O2 / 6Fe2O3
(4)
The standard nitrogen potential for nitriding atmosphere can be calculated by the following equation [11]:
KPN2 3
=
N¼
PH22
(5)
Here, N is the nitrogen potential in nitriding atmosphere, k is the equilibrium constant of given concentration, PN2 and PH2 are the decomposed pressure of hydrogen and nitrogen, respectively. It can be seen from formula (5) that the value of nitrogen potential increases with the increase of decomposed pressure of nitrogen and the decrease of decomposed pressure of hydrogen. In PON process, the mixed gases were first sputtered into reactive nitrogen, hydrogen and oxygen, and as is well known that the affinity between oxygen and hydrogen is greater than that between oxygen and nitrogen, therefore, hydrogen prefers to combine with oxygen to form H2O as shown in reaction (1), and thus the decomposed pressure of hydrogen is decreased in the atmosphere, resulting in higher nitrogen potential and hence leading to higher efficiency of nitriding. Meanwhile, active oxygen can also combine with Fe to preferentially form compact Fe3O4 in the compound layer as shown in formula (3), which brings out a significantly increase in corrosion resistance. And Fe3O4 will further transform to Fe2O3 as shown in
formula (4) with the increase of air flow, which has opposite effect on the corrosion resistance. Therefore, there exists an optimum air flow in PON treating to obtain the oxide constituents with the highest ratio of Fe3O4 to Fe2O3, which is proved to be 0.3 L/min in this research. 5. Conclusions Plain air is primarily adopted as oxygen bearing gas for plasma oxynitriding, which is more convenient than normally used oxygen. The thickness of compound layer and effective diffusion layer is significantly increased by plasma oxynitriding, and the highest efficiency is obtained at air flow of 0.3 L/min. At this optimum air flow, highest sectional microhardness of 790HV0.05 is achieved, which is 80 HV0.05 higher than that of PN treated sample, and best corrosion resistance with no corrosion pits after polarization test is obtained due to highest ratio of Fe3O4 to Fe2O3. Acknowledgement This research was supported by National Natural Science Foundation of China (51271039) and PAPD of Jiangsu Higher Education Institutions (PAPD-2014016). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
H. Liu, J.C. Li, F. Sun, J. Hu, Vacuum 109 (2014) 170e174. S.K. Wang, W. Cai, J.C. Li, W. Wei, J. Hu, Mater. Lett. 105 (2013) 47e49. Y. Li, L. Wang, D. Zhang, L. Shen, Appl. Surf. Sci. 256 (2010) 4149e4152. J.C. Li, X.M. Yang, S.K. Wang, K.X. Wei, J. Hu, Mater. Lett. 116 (2014) 199e202. J.Q. Wu, X.M. Ye, H. Li, J. Hu, Vacuum 111 (2014) 110e113. Shih-Hsien Chang, Te-Chien Tang, Kuo-Tsung, M.L. Chung, Surf. Coat. Technol. 261 (2015) 331e336. X.M. Ye, J.Q. Wu, Y.L. Zhu, J. Hu, Vacuum 110 (2014) 74e77. Abedi, H.R. Salehi, M. Yazdkhasti, Vacuum 85 (2010) 443e447. J.Q. Wu, H. Liu, X.M. Ye, Y.T. Chai, J. Hu, J. Alloy Compd. 632 (2015) 397e401. Z.S. Zhou, M.Y. Dai, Z.Y. Shen, J. Hu, J. Alloy Compd. 623 (2015) 261e265. R.Y. Chen, W.Y.D. Yuen, Oxid. Metals 56 (2001) 89e118. A.C. Rovani, R.R. Fischer, F. Cemin, Scr. Mat. 62 (2010) 863e866.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Comparative study of plasma oxynitriding and plasma nitriding for AISI 4140 steel Jiqiang Wu a, Han Liu a, c, Jingcai Li a, Xingmei Yang a, Jing Hu a, b, c, * a
Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou, 213164, China Materials Research and Education Center, Auburn University, AL, 36849, USA c Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou, 213164, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2016 Received in revised form 31 March 2016 Accepted 18 April 2016 Available online 20 April 2016
Plasma oxynitriding using plain air as oxygen bearing gas was developed and compared with traditional plasma nitriding for AISI 4140 steel. The treated samples are characterized by optical metallography, microhardness tester, X-ray diffraction (XRD) and electrochemical polarization. The results show that plain air can be used as oxygen bearing gas in plasma oxynitiding for AISI 4140 steel, and plasma oxynitriding owns much higher efficiency compared with plasma nitriding, thus makes thicker compound layer and effective diffusion layer under the same conditions. Meanwhile, higher surface hardness and smoother micro-hardness gradient are obtained. Furthermore, corrosion resistance can be significantly improved due to the formation of chemically stable and compact Fe3O4 oxide in the compound layer during plasma oxynitriding process, and air flow of 0.3 L/min offers the optimum performance for AISI 4140 steel due to highest ratio of Fe3O4 to Fe2O3. © 2016 Elsevier B.V. All rights reserved.
Keywords: Plasma oxynitriding X-ray diffraction Plain air Corrosion resistance
1. Introduction AISI 4140 steel is widely used for gears due to its excellent combined properties, and plasma nitriding is one of the most widely used surface modifications to improve surface hardness and wear resistance of various engineering materials [1e3], which involves the introduction of nitrogen atoms into the component surface to produce a modified layer on the surface of materials. Unfortunately, dozens of hours or even longer duration must be consumed to obtain the desired layer thickness and properties in current application, which results in low efficiency and high production cost [4]. It has been reported that gas oxynitriding by adding some plain air has effective catalysis effect on gas nitriding, since the surface layer after oxynitriding plays a crucial role on the evolution of nitriding layer [5e7]. However, there is no report on plasma oxynitriding (PON) by adding plain air. Generally, plasma oxynitriding (PON) was performed using mixture gases of hydrogen, nitrogen and oxygen. To make the process more convenient and friendly, plain air was primarily used
* Corresponding author. Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou, 213164, China. E-mail address: [email protected] (J. Hu). http://dx.doi.org/10.1016/j.jallcom.2016.04.172 0925-8388/© 2016 Elsevier B.V. All rights reserved.
as oxygen bearing gas for PON treating in this work, and the effect of air flow on treating efficiency and related properties, especially corrosion resistance was systematically investigated and compared with traditional plasma nitriding (PN) for AISI 4140 steel. 2. Experimental The material used for this investigation was AISI 4140 steel with the following chemical compositions (in wt. %): C, 0.41; Cr, 0.91; Mo, 0.18; Mn, 0.83; Si, 0.21; P, 0.014; S, 0.011 and Fe, balance. Specimens with the size of 10 mm 10 mm 5 mm were cut from a used gear. All the surfaces of samples were ground and polished by chromic oxide slurry to achieve a fine finish and ultrasonically cleaned in anhydrous ethanol prior to PN or PON treating. PON was performed in 20-KW pulsed DC plasma nitriding equipment. The entire treating process was composed of three steps: heating, plasma oxynitriding and cooling. Firstly, specimens were heated by hydrogen ion bombardment to the designed temperature of 573 K; secondly, PON was conducted at 823 K for 4 h in mixture gas of nitrogen and hydrogen with a ratio of 1/3 and different air flow at a gas pressure of 400 Pa, and three kinds of air flow (0.1 L/min, 0.3 L/min and 0.5 L/min) were applied in this study. Finally, the vacuum chamber was pumped to 10 Pa and samples were cooled down to room temperature in the furnace. The only
J. Wu et al. / Journal of Alloys and Compounds 680 (2016) 642e645
difference between PON and PN is of no air flow (0 L/min) in the second step for PN treating. The cross-sectional microstructures were observed by optical metallography and the thicknesses of compound layer were measured from the cross-sectional microstructure. Microhardness and depth measurements were made in a HXD-1000TMC microhardness tester, with the test load of 50 g and the holding duration of 15 s. At each depth, 10 measurements were taken and then the mean value was determined by averaging 8 measurements, excluding the highest and the lowest value. The phases were determined by X-ray diffraction (XRD) with Cu-Ka (l ¼ 1.54 Å) radiation. Corrosion resistance was evaluated by the potentiodynamic curves in 3.5% NaCl aerated solution using ZAHNER IM6e electrochemical workstations. Corrosion resistance was evaluated by the potentiodynamic curves in 3.5% NaCL aer-ated solution at room temperature using ZAHNER IM6e electrochemical workstations. The scanning potential was in the range of 1500 mv to 500 mv, and the potential scan rate was 1 mv/s after a delay period of 2000 s. At least three tests were carried out for each process for assessing the reproducibility. 3. Results and discussions
643
shows that there exist micropores and porosities in the compound layer for PN treated specimen, while a thin oxide layer is produced on the outermost surface for PON treated specimens, which can promote the formation of a denser compound layer as shown in Fig. 1(b). In addition, it can be found that the maximum compound layer thickness is obtained at air flow of 0.3 L/min. 3.2. XRD analysis Fig. 2 illustrates XRD patterns of samples treated at 823 K for 4 h with different air flow. Hematite (Fe2O3) and magnetite (Fe3O4) peaks are clearly visible for PON treated samples as shown in (b) ~ (d), and no oxide was formed for PN treated sample as shown in (a). Meanwhile, it can be found that the ratio of Fe3O4 to Fe2O3 is depended on the air flow, and the highest ratio of Fe3O4 to Fe2O3 is obtained at air flow of 0.3 L/min, which are the desired phase constituents in the real applications [8e10]. And it also illustrates that the amount of Fe2O3 increases with the air flow, attributing to the higher oxygen concentration [11]. In addition, the amount of g0 Fe4N is reduced and ε-Fe2-3N and CrN is increased with air flow, since the added air leads to the higher nitrogen potential in the local nitride layer during PON treating, and thus the transformation of g0 - Fe4N to the nitrogen richer ε-Fe2-3N occurs [11].
3.1. Microstructures and depth analysis 3.3. Microhardness measurements The cross-sectional microstructures of samples treated at 823 K for 4 h with different air flow are shown in Fig. 1, and the relationship between compound layer thickness and air flow are presented in Table 1. It clearly shows that the thickness of compound layer is significantly increased by adding plain air in plasma nitriding, and the compound layer of PON treating is denser as shown in Fig. 1 (b-d). The cross-sectional SEM embedded in Fig. 1(a)
The sectional microhardness of samples treated at 823 K for 4 h at different air flow is presented in Fig. 3. It can be clearly seen that higher sectional microhardness can be obtained for samples treated by PON, especially at air flow of 0.3 L/min. Hence, the thickness of effective diffusion layer is increased accordingly. The highest hardness of 790HV0.05 is achieved at air flow of 0.3 L/min, 80 HV0.05
Fig. 1. The cross sectional microstructure of AISI 4140 plasma treated at different conditions. (a) PN: Vair ¼ 0 L/min; (b)e(d) PON, (b) Vair ¼ 0.1 L/min; (c) Vair ¼ 0.3 L/min; (d) Vair ¼ 0.5 L/min.
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J. Wu et al. / Journal of Alloys and Compounds 680 (2016) 642e645
Table 1 Compound layer thickness and surface hardness vs. air flow. Process mark
Air flow (L/min)
Compound layer thickness (mm)
Surface hardness (HV0.05)
(a) (b) (c) (d)
PN (Vair ¼ 0) PON (Vair ¼ 0.1) PON (Vair ¼ 0.3) PON (Vair ¼ 0.5)
18.5e19.0 21.5e22.0 29.0e29.5 23.0e23.5
710 745 790 730
Note: All samples were treated at the same temperature of 823 K for 4 h.
higher than that of PN treated sample as shown in Table 1. This may be attributed to the formation of a much denser compound layer under this condition as shown in Fig. 1. 3.4. Corrosion behavior The effect of air flow on the corrosion behavior for AISI 4140 steel is shown in Fig. 4, and the corresponding data related to the corrosion resistance are presented in Table 2. It is obvious that the
PN treated sample owns the lowest corrosion potential, in other words, the corrosion resistance of all the PON treated samples is improved, and the highest corrosion potential of 300 mV and lowest corrosion current of 0.267 mA/cm2 are achieved for PON treated sample at air flow of 0.3 L/min, i.e. the optimum corrosion resistance is achieved at air flow of 0.3 L/min, which can be ascribed to the highest ratio of Fe3O4 to Fe2O3 as shown in Fig. 2(c). In order to confirm the corrosion behavior, typical surface images of the corroded specimens treated by PN and PON after polarization test are taken and compared in Fig. 5. The image of PN treated sample in Fig. 5(a) clearly shows that there exist many corrosion pits distributed on the surface due to the pores existed in the compound layer. While there shows no corrosion pit in the image of PON treated sample shown in Fig. 5(b) due to the protection from chemically stable and compact Fe3O4 formed on the outermost surface, corresponding to the optimum corrosion resistance. 4. Discussions The main enhancement mechanism can be explained from the
Fig. 2. XRD patterns of AISI 4140 specimens plasma treated at 823 K for 4 h with different air flow. (a) PN: Vair ¼ 0 L/min; (b)e(d) PON, (b) Vair ¼ 0.1 L/min; (c) Vair ¼ 0.3 L/min; (d) Vair ¼ 0.5 L/min.
Fig. 4. Current density of the samples treated with different plain flow. (a) PN; Vair ¼ 0 L/min (b)e(d) PON, (b) Vair ¼ 0.1 L/min; (c) Vair ¼ 0.3 L/min; (d) Vair ¼ 0.5 L/ min.
Table 2 The related corrosion resistance data corresponding to Fig. 4. Process mark
Fig. 3. Microhardness of AISI 4140 steel treated with different air flow. (a) PN: Vair ¼ 0 L/min; (b)e(d) PON, (b) Vair ¼ 0.1 L/min; (c) Vair ¼ 0.3 L/min; (d) Vair ¼ 0.5 L/ min.
(a) (b) (c) (d)
Air flow (L/min)
PN (Vair ¼ 0) PON (Vair ¼ 0.1) PON (Vair ¼ 0.3) PON (Vair ¼ 0.5)
Corrosion resistance Ecorr(mV)
Icorr(mA/cm2)
980 400 300 350
0.267 0.254 0.207 0.304
Note: All samples were treated at the same temperature of 823 K for 4 h.
J. Wu et al. / Journal of Alloys and Compounds 680 (2016) 642e645
645
Fig. 5. SEM images of corroded specimens. (a) PN: Vair ¼ 0 L/min; (b) PON: Vair ¼ 0.3 L/min.
point of the nitrogen potential in nitriding atmosphere, as is well known that faster nitriding is attributed to higher nitrogen potential. Since there exist mixture gases of nitrogen, hydrogen and oxygen in the chamber during the PON process, the following chemical reactions occur [12]. 2H2 þ O2 / 2H2O
(1)
N2 þ O2 / 2NO
(2)
3Fe þ 2O2 / Fe3O4
(3)
4Fe3O4 þ O2 / 6Fe2O3
(4)
The standard nitrogen potential for nitriding atmosphere can be calculated by the following equation [11]:
KPN2 3
=
N¼
PH22
(5)
Here, N is the nitrogen potential in nitriding atmosphere, k is the equilibrium constant of given concentration, PN2 and PH2 are the decomposed pressure of hydrogen and nitrogen, respectively. It can be seen from formula (5) that the value of nitrogen potential increases with the increase of decomposed pressure of nitrogen and the decrease of decomposed pressure of hydrogen. In PON process, the mixed gases were first sputtered into reactive nitrogen, hydrogen and oxygen, and as is well known that the affinity between oxygen and hydrogen is greater than that between oxygen and nitrogen, therefore, hydrogen prefers to combine with oxygen to form H2O as shown in reaction (1), and thus the decomposed pressure of hydrogen is decreased in the atmosphere, resulting in higher nitrogen potential and hence leading to higher efficiency of nitriding. Meanwhile, active oxygen can also combine with Fe to preferentially form compact Fe3O4 in the compound layer as shown in formula (3), which brings out a significantly increase in corrosion resistance. And Fe3O4 will further transform to Fe2O3 as shown in
formula (4) with the increase of air flow, which has opposite effect on the corrosion resistance. Therefore, there exists an optimum air flow in PON treating to obtain the oxide constituents with the highest ratio of Fe3O4 to Fe2O3, which is proved to be 0.3 L/min in this research. 5. Conclusions Plain air is primarily adopted as oxygen bearing gas for plasma oxynitriding, which is more convenient than normally used oxygen. The thickness of compound layer and effective diffusion layer is significantly increased by plasma oxynitriding, and the highest efficiency is obtained at air flow of 0.3 L/min. At this optimum air flow, highest sectional microhardness of 790HV0.05 is achieved, which is 80 HV0.05 higher than that of PN treated sample, and best corrosion resistance with no corrosion pits after polarization test is obtained due to highest ratio of Fe3O4 to Fe2O3. Acknowledgement This research was supported by National Natural Science Foundation of China (51271039) and PAPD of Jiangsu Higher Education Institutions (PAPD-2014016). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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