- Email: [email protected]
A novel duplex plasma treatment combining plasma nitrocarburizing and plasma nitriding
Accepted Manuscript A novel duplex plasma treatment combining plasma nitrocarburizing and plasma nitriding Bin Miao, Yating Chai, Kunxia Wei, Jing Hu PII:
S0042-207X(16)30218-4
DOI:
10.1016/j.vacuum.2016.07.007
Reference:
VAC 7069
To appear in:
Vacuum
Received Date: 1 June 2016 Revised Date:
6 July 2016
Accepted Date: 6 July 2016
Please cite this article as: Miao B, Chai Y, Wei K, Hu J, A novel duplex plasma treatment combining plasma nitrocarburizing and plasma nitriding, Vaccum (2016), doi: 10.1016/j.vacuum.2016.07.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
A novel duplex plasma treatment combining plasma nitrocarburizing and plasma nitriding Bin Miaoa,c, Yating Chaia,b, Kunxia Weia,c*, Jing Hua,b∗ a
RI PT
Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University,
Changzhou 213164, People’s Republic of China b
Materials Research and Education center, Auburn University, AL 36849, USA
c
Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou
SC
University, Changzhou, 213164, People’s Republic of China
Abstract
M AN U
In this study, a novel duplex treatment (DT) combining plasma nitrocarburizing (PNC) and plasma nitriding (PN) was primarily developed for AISI 1045 steel. The modified samples were investigated by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness test, and pin-on-disk tribotest. The results showed that the nitriding efficiency was remarkably improved by DT, and the compound layer was much thicker than that
TE D
treated by PNC or PN alone. Furthermore, higher cross-sectional microhardness and significant enhancement of wear resistance were achieved. The possible enhancement mechanism could be the phase transformation of γ′ to ε, and the reduced amount of cementite could be because of the activating and ionizing effect of the subsequent PN on cementite.
EP
Keywords: Surfaces; Plasma nitriding; Plasma nitrocarburizing; Microstructure; Diffusion
1. Introduction
AC C
AISI 1045 steel is a widely used type of medium carbon steel because of its low cost and excellent combined properties[1–5]. In real-time applications, however, surface modification is necessary to further enhance its properties to meet the requirements in various service environments[6–10]. Plasma nitriding (PN), plasma carburizing (PC), and plasma nitrocarburizing
(PNC) are proved to be effective surface modifications and have been adopted by most industries. In the past decades, PN and PNC have become popular surface modification techniques, as
∗
Corresponding author. Tel.: 86+0519-86330065. E-mail address: [email protected] (Jing Hu),
[email protected] (Kunxia Wei).
ACCEPTED MANUSCRIPT
they are more environment friendly than those using gas sources and solid powders[11–12]. It is well known that PN offers many advantages over traditional gas nitriding, especially in terms of gas consumption and properties control. However, it requires long cycle time[3,13]. Generally, the technique consumes several hours to yield the desired thickness and properties in most
RI PT
applications, which results in low efficiency and high production cost[14,15]. The developed PNC is a thermochemical treatment, in which nitrogen and carbon atoms diffuse simultaneously into the
workpiece surface layer to obtain a higher efficiency[16–19]. However, it has been found that the formation of a loose layer is unavoidable in the utmost surface because of its long duration, which
SC
causes side effects on surface performance such as higher brittleness[19]. Therefore, developing a new way to overcome these problems is of paramount importance.
M AN U
To make good use of the advantages of both PNC and PN, a novel duplex treatment (DT) combining PNC and PN was primarily developed for AISI 1045 steel in this study, and it was found that the developed technique could not only increase nitriding efficiency but also improve related properties. 2. Experimental procedures
TE D
The material used in this study was AISI 1045 steel with the following chemical compositions (wt%): C, 0.45; Si, 0.18; Mn, 0.52; S, 0.031; P, 0.032; and Fe: remaining. The specimens were prepared with dimensions of 10 × 10 × 5 mm, and quenched at 1123 K for 8 min, cooled in water and tempered at 833 K for 30 min, and air cooled. All the surfaces of specimens
EP
were grounded by silicon carbide paper, polished by chromic oxide slurry, and cleaned in dehydrated alcohol for 15 min in an ultrasound device before DT treatment.
AC C
All the plasma treatments were performed in the same 20-kW pulsed DC PN equipment. After placing the specimens in the equipment, it was evacuated to 10 Pa and then sputtered for 30 min by hydrogen at a flow rate and gas pressure of 500 mL/min and 300 Pa, respectively. After sputtering, PNC, PN, or DT process was subsequently run at 783 K for 4 h at a gas pressure of 400 Pa, with the gas flow rate as mentioned below: (1) PN: hydrogen flow rate of 600 mL/min and nitrogen flow rate of 200 mL/min in the whole process; (2) PNC: nitrogen flow rate of 591 mL/min, propane flow rate of 9 mL/min, and without hydrogen in the whole process; (3) DT: PNC 3 h for the first step and PN 1 h for the second step, with the same gas flow rate as in (1) or (2) for each step. The reasons for choosing the above gas flow for each process are as follows: (1) As
ACCEPTED MANUSCRIPT
reported previously, the suitable ratio of nitrogen and hydrogen in PN is 1:3 [20]; according to this ratio, hydrogen flow of 600 mL/min and nitrogen flow of 200 mL/min were used by our group [21]; (2) In our previous research, we found that the optimum concentration of propane is 1.5% in
RI PT
the PNC process [19]; thus, we used the optimum ratio in the PNC step. After each process, the nitriding furnace was pumped to 10 Pa and cooled to ambient temperature.
The surfaces of the specimens were metallographically polished and etched using ethanol solution containing 4% nitric acid. An optical microscope (DMI-3000M) was used for observing
SC
microstructures. The phases were determined by X-ray diffraction (XRD) with Cu-Ka (λ = 1.54 Å) radiation at a scan rate of 0.2°/min, 2θ ranging from 30° to 80°. The surface morphology was observed by scanning electron microscopy (SEM). In addition, the cross-sectional hardness was
M AN U
measured by a HXD-1000TMC microhardness tester with the test load of 50 g and the holding duration of 15 s. At least three measurements were carried out for each sample for assessing the hardness. Finally, a HT-1000 ball-on-disk high-temperature friction and wear tester was used to evaluate wear resistance of AISI 1045 steel at dry sliding with 50 N load and 200 rpm against GCr15 ball with a diameter of 4 mm for durations of 30 min. The wear tests were performed at
TE D
room temperature (~20°C) with a relative humidity of approximately 50%. 3. Results and discussions
3.1 Microstructure and depth analysis
The cross-sectional microstructures of samples with plasma treatment at 783 K for 4 h are
EP
shown in Figure 1a–c. The figure clearly shows the formation of the modified layer; the compound layer obtained by PN is the thinnest (Fig. 1a) and that obtained by DT is the thickest
AC C
(Fig. 1c). Especially, the outer layer (marked in red zone) of the images show that a porous layer is formed in Figure 1b after PNC, while the layer disappears and a dense surface is formed after DT. Insert Fig. 1
3.2 XRD analysis
Figure 2 shows the XRD patterns of the samples with plasma treatment at 783 K for 4 h. The figure shows that both ε-Fe2~3(C,N) and γ'-Fe4N are formed in the samples treated by PN, PNC, and DT. It can also be observed that the relative amount of cementite is decreased for DT sample because of the activating and ionizing effect of the subsequent PN step on cementite, and the
ACCEPTED MANUSCRIPT
relative amount of ε-Fe2-3N is increased because of the transformation of some γ'-Fe4N to ε-Fe2-3N with higher nitrogen content. Insert Fig. 2
RI PT
3.3 Surface morphology Figure 3 shows SEM micrographs of the surface morphologies of the samples with plasma treatment at 783 K for 4 h. Block structure with some carbon black can be seen in Figure 3a for
PNC-treated sample, while fine particles are uniformly distributed and without carbon black in
bombardment during the PN step[22].
3.4 Microhardness analysis
M AN U
Insert Fig. 3
SC
Figure 3b for DT-treated sample, which is attributed to the spallation of cementite by plasma
Figure 4 shows the microhardness profile of samples with plasma treatment at 783 K for 4 h. The figure shows that the hardness in the modified layer decreases gradually from the surface to core. Further, the hardness of samples treated by DT reaches a high level of 788 HV0.05, much
TE D
higher than that by PN and PNC. In addition, it is obvious that the modified layer by DT presents a wide hardened region as compared to that by PN and PNC, with the effective hardening depth of the order of 26, 45, and 70 µm, respectively. The higher microhardness achieved by DT can be
EP
mainly attributed to high content of ε-Fe2-3N nitride. Insert Fig. 4
AC C
3.5 Wear tracks
Micrographs of the wear tracks of specimens with plasma treatment at 783 K for 4 h are
shown in Figure 5. As shown in the figure, the largest wear scar is observed on the surface after PN, and the wear scratching of DT specimen reveals narrower and smoother appearance, which is consistent with the highest surface hardness (Fig. 4). Besides, severe scuffing and obvious parallel grooves exist on the worn surface for PN and PNC samples, confirming serious removal of debris during the wear test. Insert Fig. 5
ACCEPTED MANUSCRIPT
3.6 Mechanism analysis The most important advantage of this novel DT process is higher efficiency and hardness. As reported previously, PNC has higher efficiency than PN[19], which was confirmed in this study.
RI PT
However, the efficiency gradually decreases with time because of the formation of Fe3C on the utmost surface, which can impede nitrogen and carbon absorption and make the top layer loose (Fig. 1); this is a limitation of the PNC process. According to the observed results, it is reasonable to predict that the loose surface produced due to the formation of Fe3C in the PNC step can be
SC
activated and ionized by the subsequent PN step in DT, thus promoting the absorption and inward
diffusion of N atoms, leading to higher efficiency. Meanwhile, active N atoms can react with Fe3C to produce Fe2~3(C,N) in the following PN step, resulting in a higher surface and cross-sectional
M AN U
hardness. The possible reactions in the subsequent PN step in DT are shown as below: Fe3C→3[Fe]+[C]
[Fe]+[N]+[C]→Fe2~3(C,N) [Fe]+[N]→Fe2~3N
Fe3C+[N]→Fe2~3(C,N)
TE D
4. Conclusions
(1) The compound layer treated by DT is much thicker than that treated by PNC or PN alone, that is, the nitriding efficiency is remarkably improved. (2) The cross-sectional microhardness is significantly enhanced by DT.
EP
(3) The dominant phase transforms from γ'-Fe4N to ε-Fe2-3N, and the amount of cementite is decreased by the activating and ionizing effects of the subsequent PN step.
AC C
(4) The wear resistance can be significantly improved by DT.
Acknowledgments
This research was supported by PAPD of Jiangsu Higher Education Institutions (PAPD
2014-6), Jiangsu Province Graduate Student Innovation Fund (SCZ100431322), and Jiangsu Government Scholarship for Overseas Studies under Grant No. JS-2012-173.
References [1] K. Gao, X.P. Qin, Z. Wang, S.X. Zhu. Mater. Sci. Eng. A 2016; 651: 535-547. [2] R.L. Liu, Y.J. Qiao, M.F. Yan, Y.D. Fu. J. Mater. Sci. Technol. 2012; 28(11): 1046-1052. [3] H. Liu, J.C. Li, Y.T. Chai, W. Wei, J. Hu. Vacuum 2015; 121: 18-21.
ACCEPTED MANUSCRIPT
[4] W. Cai, F.N. Meng, X.Y. Gao, J. Hu. Appl. Surf. Sci. 2012; 261: 411-414. [5] I. Lee. Curr. Appl. Phys. 2009; 9: S257-S261. [6] A. OrjuelaG, R. Rincón, J.J. Olaya. Surf. Coat. Technol. 2014; 259: 667-675.
142-149.
RI PT
[7] B. Wang, X.Y. Jin, W.B. Xue, Z.L. Wu, J.C. Du, J. Wu. Surf. Coat. Technol. 2013; 232:
[8] J. Fernández de Ara, E. Almandoz, J.F. Palacio, G.G. Fuentes, R.J. Rodríguez, J.A. García. Surf. Coat. Technol. 2014; 258: 754-762.
SC
[9] N. Mandkarian, F. Mahboubi. Vacuum 2009; 83: 1036-1042.
[10] S.C. Mishra, B.B. Nayak, B.C. Mohanty. Surf. Coat. Technol. 2001; 145: 24-30. [11] M. Ebrahimi, F. Mahboubi, M.R. Naimi-Jamal. Diamond Relat. Mater. 2015; 52: 32-37.
M AN U
[12] C.J. Scheuer, R.P. Cardoso, M. Mafra, S.F. Brunatto. Surf. Coat. Technol. 2013; 214: 30-37. [13] H. Liu, J.C. Li, F. Sun, J. Hu. Vacuum 2014; 109: 170-174.
[14] J.C. Li, X.M. Yang, S.K. Wang, K.X. Wei, J. H. Mater. Lett. 2014; 116: 199-202. [15] J.Q. Wu, H. Liu, J.C. Li, X.M. Yang, J. Hu. J. Alloy Compd. 2016; 680: 642-645. [16] F.S. Chen, K.L. Wang. Surf. Coat. Technol. 2000; 132: 36-42.
TE D
[17] J. Zhang, Y. Hu, X.J. Tan, L. Guo, Q.M. Zhang. Trans. Nonferrous Met. Soc. China 2015; 25: 1525-1532.
[18] M. Karakan, A. Alsaran, A. Çelik. Mater. Des. 2004; 25: 349-353. [19] X.M. Ye, J.Q. Wu, Y.L. Zhu, J. Hu. Vacuum 2014, 110: 74-77
EP
[20] R. Mohammadzadeh, A. Akbari, M. Drouet. Surf. Coat. Technol. 2014; 258: 566-573. [21] H. Liu, J.C. Li,Y.T. Chai, W. Wei, J. Hu. Surf. Coat. Technol. 2016; 302: 22-26.
AC C
[22] R. Marchiori, A.M. Maliska, P.C. Borges, A.N. Klein, J.L.R. Muzart. Mater. Sci. Eng. A 2007; 467: 159-164.
ACCEPTED MANUSCRIPT
Figure captions
1. Fig. 1. Cross-sectional microstructure of samples treated by (a) PN, (b) PNC, and (c) DT at 783 K for 4 h.
RI PT
2. Fig. 2. XRD patterns of samples treated by different plasma techniques (783 K for 4 h).
3. Fig. 3. Surface morphologies of samples treated by (a) PNC and (b) DT at 783 K for 4 h.
4. Fig. 4. Microhardness profiles of samples treated by PN, PNC, and DT at 783 K for 4 h.
(a)
SC
5. Fig. 5. Micrographs of wear tracks of samples treated by (a) PN, (b) PNC, and (c) DT at 783 K for 4 h.
(c)
M AN U
(b)
6.8 µm
17.9 µm
50µm
50µm
21.0 µm
50µm
TE D
Fig. 1. Cross-sectional microstructure of samples treated by (a) PN, (b) PNC, and (c) DT at 783 K for 4 h.
EP
♦
AC C
Relative Intensity/a.u.
•
•
•
•
30
ε phase •
∆
Fe3C
♦
•
∆
•∆
♦
♦
γ′ phase
•
DT
♦
•
♦
∆
•
∆
∆
∆
♦
•
PNC •
•
♦ ♦
•
40
•
•
•
•
♦
50
PN
•
60
70
80
2θ/degree
Fig. 2. XRD patterns of samples treated by different plasma techniques (783 K for 4 h).
ACCEPTED MANUSCRIPT
(a)
RI PT
(b)
carbon black
5µm
5µm
M AN U
SC
Fig. 3. Surface morphologies of samples treated by (a) PNC and (b) DT at 783 K for 4 h
800
PN PNC DT
600 500
TE D
Microhardness/HV0.05
700
400 300
30
EP
0
60
90
120
Distance from surface/µm
AC C
Fig. 4. Microhardness profiles of samples treated by PN, PNC, and DT at 783 K for 4 h.
(a)
parallel grooves
500µm
(b)
(c) parallel grooves
500µm
500µm
Fig. 5. Micrographs of wear tracks of samples treated by (a) PN, (b) PNC, and (c) DT at 783 K for 4 h.
ACCEPTED MANUSCRIPT
Highlights
1. A novel duplex treatment combining plasma nitrocarburizing and plasma nitriding
RI PT
was developed.
thickness. 3. It produces higher microhardness.
SC
2. The duplex treatment can increase the nitriding efficiency and compound layer
AC C
EP
TE D
M AN U
4. The duplex treatment can significantly improve wear resistance.
S0042-207X(16)30218-4
DOI:
10.1016/j.vacuum.2016.07.007
Reference:
VAC 7069
To appear in:
Vacuum
Received Date: 1 June 2016 Revised Date:
6 July 2016
Accepted Date: 6 July 2016
Please cite this article as: Miao B, Chai Y, Wei K, Hu J, A novel duplex plasma treatment combining plasma nitrocarburizing and plasma nitriding, Vaccum (2016), doi: 10.1016/j.vacuum.2016.07.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
A novel duplex plasma treatment combining plasma nitrocarburizing and plasma nitriding Bin Miaoa,c, Yating Chaia,b, Kunxia Weia,c*, Jing Hua,b∗ a
RI PT
Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University,
Changzhou 213164, People’s Republic of China b
Materials Research and Education center, Auburn University, AL 36849, USA
c
Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou
SC
University, Changzhou, 213164, People’s Republic of China
Abstract
M AN U
In this study, a novel duplex treatment (DT) combining plasma nitrocarburizing (PNC) and plasma nitriding (PN) was primarily developed for AISI 1045 steel. The modified samples were investigated by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness test, and pin-on-disk tribotest. The results showed that the nitriding efficiency was remarkably improved by DT, and the compound layer was much thicker than that
TE D
treated by PNC or PN alone. Furthermore, higher cross-sectional microhardness and significant enhancement of wear resistance were achieved. The possible enhancement mechanism could be the phase transformation of γ′ to ε, and the reduced amount of cementite could be because of the activating and ionizing effect of the subsequent PN on cementite.
EP
Keywords: Surfaces; Plasma nitriding; Plasma nitrocarburizing; Microstructure; Diffusion
1. Introduction
AC C
AISI 1045 steel is a widely used type of medium carbon steel because of its low cost and excellent combined properties[1–5]. In real-time applications, however, surface modification is necessary to further enhance its properties to meet the requirements in various service environments[6–10]. Plasma nitriding (PN), plasma carburizing (PC), and plasma nitrocarburizing
(PNC) are proved to be effective surface modifications and have been adopted by most industries. In the past decades, PN and PNC have become popular surface modification techniques, as
∗
Corresponding author. Tel.: 86+0519-86330065. E-mail address: [email protected] (Jing Hu),
[email protected] (Kunxia Wei).
ACCEPTED MANUSCRIPT
they are more environment friendly than those using gas sources and solid powders[11–12]. It is well known that PN offers many advantages over traditional gas nitriding, especially in terms of gas consumption and properties control. However, it requires long cycle time[3,13]. Generally, the technique consumes several hours to yield the desired thickness and properties in most
RI PT
applications, which results in low efficiency and high production cost[14,15]. The developed PNC is a thermochemical treatment, in which nitrogen and carbon atoms diffuse simultaneously into the
workpiece surface layer to obtain a higher efficiency[16–19]. However, it has been found that the formation of a loose layer is unavoidable in the utmost surface because of its long duration, which
SC
causes side effects on surface performance such as higher brittleness[19]. Therefore, developing a new way to overcome these problems is of paramount importance.
M AN U
To make good use of the advantages of both PNC and PN, a novel duplex treatment (DT) combining PNC and PN was primarily developed for AISI 1045 steel in this study, and it was found that the developed technique could not only increase nitriding efficiency but also improve related properties. 2. Experimental procedures
TE D
The material used in this study was AISI 1045 steel with the following chemical compositions (wt%): C, 0.45; Si, 0.18; Mn, 0.52; S, 0.031; P, 0.032; and Fe: remaining. The specimens were prepared with dimensions of 10 × 10 × 5 mm, and quenched at 1123 K for 8 min, cooled in water and tempered at 833 K for 30 min, and air cooled. All the surfaces of specimens
EP
were grounded by silicon carbide paper, polished by chromic oxide slurry, and cleaned in dehydrated alcohol for 15 min in an ultrasound device before DT treatment.
AC C
All the plasma treatments were performed in the same 20-kW pulsed DC PN equipment. After placing the specimens in the equipment, it was evacuated to 10 Pa and then sputtered for 30 min by hydrogen at a flow rate and gas pressure of 500 mL/min and 300 Pa, respectively. After sputtering, PNC, PN, or DT process was subsequently run at 783 K for 4 h at a gas pressure of 400 Pa, with the gas flow rate as mentioned below: (1) PN: hydrogen flow rate of 600 mL/min and nitrogen flow rate of 200 mL/min in the whole process; (2) PNC: nitrogen flow rate of 591 mL/min, propane flow rate of 9 mL/min, and without hydrogen in the whole process; (3) DT: PNC 3 h for the first step and PN 1 h for the second step, with the same gas flow rate as in (1) or (2) for each step. The reasons for choosing the above gas flow for each process are as follows: (1) As
ACCEPTED MANUSCRIPT
reported previously, the suitable ratio of nitrogen and hydrogen in PN is 1:3 [20]; according to this ratio, hydrogen flow of 600 mL/min and nitrogen flow of 200 mL/min were used by our group [21]; (2) In our previous research, we found that the optimum concentration of propane is 1.5% in
RI PT
the PNC process [19]; thus, we used the optimum ratio in the PNC step. After each process, the nitriding furnace was pumped to 10 Pa and cooled to ambient temperature.
The surfaces of the specimens were metallographically polished and etched using ethanol solution containing 4% nitric acid. An optical microscope (DMI-3000M) was used for observing
SC
microstructures. The phases were determined by X-ray diffraction (XRD) with Cu-Ka (λ = 1.54 Å) radiation at a scan rate of 0.2°/min, 2θ ranging from 30° to 80°. The surface morphology was observed by scanning electron microscopy (SEM). In addition, the cross-sectional hardness was
M AN U
measured by a HXD-1000TMC microhardness tester with the test load of 50 g and the holding duration of 15 s. At least three measurements were carried out for each sample for assessing the hardness. Finally, a HT-1000 ball-on-disk high-temperature friction and wear tester was used to evaluate wear resistance of AISI 1045 steel at dry sliding with 50 N load and 200 rpm against GCr15 ball with a diameter of 4 mm for durations of 30 min. The wear tests were performed at
TE D
room temperature (~20°C) with a relative humidity of approximately 50%. 3. Results and discussions
3.1 Microstructure and depth analysis
The cross-sectional microstructures of samples with plasma treatment at 783 K for 4 h are
EP
shown in Figure 1a–c. The figure clearly shows the formation of the modified layer; the compound layer obtained by PN is the thinnest (Fig. 1a) and that obtained by DT is the thickest
AC C
(Fig. 1c). Especially, the outer layer (marked in red zone) of the images show that a porous layer is formed in Figure 1b after PNC, while the layer disappears and a dense surface is formed after DT. Insert Fig. 1
3.2 XRD analysis
Figure 2 shows the XRD patterns of the samples with plasma treatment at 783 K for 4 h. The figure shows that both ε-Fe2~3(C,N) and γ'-Fe4N are formed in the samples treated by PN, PNC, and DT. It can also be observed that the relative amount of cementite is decreased for DT sample because of the activating and ionizing effect of the subsequent PN step on cementite, and the
ACCEPTED MANUSCRIPT
relative amount of ε-Fe2-3N is increased because of the transformation of some γ'-Fe4N to ε-Fe2-3N with higher nitrogen content. Insert Fig. 2
RI PT
3.3 Surface morphology Figure 3 shows SEM micrographs of the surface morphologies of the samples with plasma treatment at 783 K for 4 h. Block structure with some carbon black can be seen in Figure 3a for
PNC-treated sample, while fine particles are uniformly distributed and without carbon black in
bombardment during the PN step[22].
3.4 Microhardness analysis
M AN U
Insert Fig. 3
SC
Figure 3b for DT-treated sample, which is attributed to the spallation of cementite by plasma
Figure 4 shows the microhardness profile of samples with plasma treatment at 783 K for 4 h. The figure shows that the hardness in the modified layer decreases gradually from the surface to core. Further, the hardness of samples treated by DT reaches a high level of 788 HV0.05, much
TE D
higher than that by PN and PNC. In addition, it is obvious that the modified layer by DT presents a wide hardened region as compared to that by PN and PNC, with the effective hardening depth of the order of 26, 45, and 70 µm, respectively. The higher microhardness achieved by DT can be
EP
mainly attributed to high content of ε-Fe2-3N nitride. Insert Fig. 4
AC C
3.5 Wear tracks
Micrographs of the wear tracks of specimens with plasma treatment at 783 K for 4 h are
shown in Figure 5. As shown in the figure, the largest wear scar is observed on the surface after PN, and the wear scratching of DT specimen reveals narrower and smoother appearance, which is consistent with the highest surface hardness (Fig. 4). Besides, severe scuffing and obvious parallel grooves exist on the worn surface for PN and PNC samples, confirming serious removal of debris during the wear test. Insert Fig. 5
ACCEPTED MANUSCRIPT
3.6 Mechanism analysis The most important advantage of this novel DT process is higher efficiency and hardness. As reported previously, PNC has higher efficiency than PN[19], which was confirmed in this study.
RI PT
However, the efficiency gradually decreases with time because of the formation of Fe3C on the utmost surface, which can impede nitrogen and carbon absorption and make the top layer loose (Fig. 1); this is a limitation of the PNC process. According to the observed results, it is reasonable to predict that the loose surface produced due to the formation of Fe3C in the PNC step can be
SC
activated and ionized by the subsequent PN step in DT, thus promoting the absorption and inward
diffusion of N atoms, leading to higher efficiency. Meanwhile, active N atoms can react with Fe3C to produce Fe2~3(C,N) in the following PN step, resulting in a higher surface and cross-sectional
M AN U
hardness. The possible reactions in the subsequent PN step in DT are shown as below: Fe3C→3[Fe]+[C]
[Fe]+[N]+[C]→Fe2~3(C,N) [Fe]+[N]→Fe2~3N
Fe3C+[N]→Fe2~3(C,N)
TE D
4. Conclusions
(1) The compound layer treated by DT is much thicker than that treated by PNC or PN alone, that is, the nitriding efficiency is remarkably improved. (2) The cross-sectional microhardness is significantly enhanced by DT.
EP
(3) The dominant phase transforms from γ'-Fe4N to ε-Fe2-3N, and the amount of cementite is decreased by the activating and ionizing effects of the subsequent PN step.
AC C
(4) The wear resistance can be significantly improved by DT.
Acknowledgments
This research was supported by PAPD of Jiangsu Higher Education Institutions (PAPD
2014-6), Jiangsu Province Graduate Student Innovation Fund (SCZ100431322), and Jiangsu Government Scholarship for Overseas Studies under Grant No. JS-2012-173.
References [1] K. Gao, X.P. Qin, Z. Wang, S.X. Zhu. Mater. Sci. Eng. A 2016; 651: 535-547. [2] R.L. Liu, Y.J. Qiao, M.F. Yan, Y.D. Fu. J. Mater. Sci. Technol. 2012; 28(11): 1046-1052. [3] H. Liu, J.C. Li, Y.T. Chai, W. Wei, J. Hu. Vacuum 2015; 121: 18-21.
ACCEPTED MANUSCRIPT
[4] W. Cai, F.N. Meng, X.Y. Gao, J. Hu. Appl. Surf. Sci. 2012; 261: 411-414. [5] I. Lee. Curr. Appl. Phys. 2009; 9: S257-S261. [6] A. OrjuelaG, R. Rincón, J.J. Olaya. Surf. Coat. Technol. 2014; 259: 667-675.
142-149.
RI PT
[7] B. Wang, X.Y. Jin, W.B. Xue, Z.L. Wu, J.C. Du, J. Wu. Surf. Coat. Technol. 2013; 232:
[8] J. Fernández de Ara, E. Almandoz, J.F. Palacio, G.G. Fuentes, R.J. Rodríguez, J.A. García. Surf. Coat. Technol. 2014; 258: 754-762.
SC
[9] N. Mandkarian, F. Mahboubi. Vacuum 2009; 83: 1036-1042.
[10] S.C. Mishra, B.B. Nayak, B.C. Mohanty. Surf. Coat. Technol. 2001; 145: 24-30. [11] M. Ebrahimi, F. Mahboubi, M.R. Naimi-Jamal. Diamond Relat. Mater. 2015; 52: 32-37.
M AN U
[12] C.J. Scheuer, R.P. Cardoso, M. Mafra, S.F. Brunatto. Surf. Coat. Technol. 2013; 214: 30-37. [13] H. Liu, J.C. Li, F. Sun, J. Hu. Vacuum 2014; 109: 170-174.
[14] J.C. Li, X.M. Yang, S.K. Wang, K.X. Wei, J. H. Mater. Lett. 2014; 116: 199-202. [15] J.Q. Wu, H. Liu, J.C. Li, X.M. Yang, J. Hu. J. Alloy Compd. 2016; 680: 642-645. [16] F.S. Chen, K.L. Wang. Surf. Coat. Technol. 2000; 132: 36-42.
TE D
[17] J. Zhang, Y. Hu, X.J. Tan, L. Guo, Q.M. Zhang. Trans. Nonferrous Met. Soc. China 2015; 25: 1525-1532.
[18] M. Karakan, A. Alsaran, A. Çelik. Mater. Des. 2004; 25: 349-353. [19] X.M. Ye, J.Q. Wu, Y.L. Zhu, J. Hu. Vacuum 2014, 110: 74-77
EP
[20] R. Mohammadzadeh, A. Akbari, M. Drouet. Surf. Coat. Technol. 2014; 258: 566-573. [21] H. Liu, J.C. Li,Y.T. Chai, W. Wei, J. Hu. Surf. Coat. Technol. 2016; 302: 22-26.
AC C
[22] R. Marchiori, A.M. Maliska, P.C. Borges, A.N. Klein, J.L.R. Muzart. Mater. Sci. Eng. A 2007; 467: 159-164.
ACCEPTED MANUSCRIPT
Figure captions
1. Fig. 1. Cross-sectional microstructure of samples treated by (a) PN, (b) PNC, and (c) DT at 783 K for 4 h.
RI PT
2. Fig. 2. XRD patterns of samples treated by different plasma techniques (783 K for 4 h).
3. Fig. 3. Surface morphologies of samples treated by (a) PNC and (b) DT at 783 K for 4 h.
4. Fig. 4. Microhardness profiles of samples treated by PN, PNC, and DT at 783 K for 4 h.
(a)
SC
5. Fig. 5. Micrographs of wear tracks of samples treated by (a) PN, (b) PNC, and (c) DT at 783 K for 4 h.
(c)
M AN U
(b)
6.8 µm
17.9 µm
50µm
50µm
21.0 µm
50µm
TE D
Fig. 1. Cross-sectional microstructure of samples treated by (a) PN, (b) PNC, and (c) DT at 783 K for 4 h.
EP
♦
AC C
Relative Intensity/a.u.
•
•
•
•
30
ε phase •
∆
Fe3C
♦
•
∆
•∆
♦
♦
γ′ phase
•
DT
♦
•
♦
∆
•
∆
∆
∆
♦
•
PNC •
•
♦ ♦
•
40
•
•
•
•
♦
50
PN
•
60
70
80
2θ/degree
Fig. 2. XRD patterns of samples treated by different plasma techniques (783 K for 4 h).
ACCEPTED MANUSCRIPT
(a)
RI PT
(b)
carbon black
5µm
5µm
M AN U
SC
Fig. 3. Surface morphologies of samples treated by (a) PNC and (b) DT at 783 K for 4 h
800
PN PNC DT
600 500
TE D
Microhardness/HV0.05
700
400 300
30
EP
0
60
90
120
Distance from surface/µm
AC C
Fig. 4. Microhardness profiles of samples treated by PN, PNC, and DT at 783 K for 4 h.
(a)
parallel grooves
500µm
(b)
(c) parallel grooves
500µm
500µm
Fig. 5. Micrographs of wear tracks of samples treated by (a) PN, (b) PNC, and (c) DT at 783 K for 4 h.
ACCEPTED MANUSCRIPT
Highlights
1. A novel duplex treatment combining plasma nitrocarburizing and plasma nitriding
RI PT
was developed.
thickness. 3. It produces higher microhardness.
SC
2. The duplex treatment can increase the nitriding efficiency and compound layer
AC C
EP
TE D
M AN U
4. The duplex treatment can significantly improve wear resistance.