- Email: [email protected]
Fabrication of Ag-WS2 composites with preferentially oriented WS2 and its anisotropic tribology behavior
Journal Pre-proofs Fabrication of Ag-WS2 composites with preferentially oriented WS2 and its anisotropic tribology behavior Yang Sun, Jisi Wu, Lei Zhang PII: DOI: Reference:
S0167-577X(19)31607-6 https://doi.org/10.1016/j.matlet.2019.126975 MLBLUE 126975
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
8 October 2019 5 November 2019 7 November 2019
Please cite this article as: Y. Sun, J. Wu, L. Zhang, Fabrication of Ag-WS2 composites with preferentially oriented WS2 and its anisotropic tribology behavior, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet. 2019.126975
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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.
© 2019 Published by Elsevier B.V.
Fabrication of Ag-WS2 composites with preferentially oriented WS2 and its anisotropic tribology behavior Yang Sun a Jisi Wu b Lei Zhang a*
*Corresponding author.
E-mail address: [email protected] (L. Zhang)
1
a
State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China b School of Aviation Manufacturing Engineering Institute, Nanchang Hangkong University, Jiangxi 330063, China
Abstract The Ag-WS2 composites with preferentially oriented WS2 flakes were successfully prepared via freeze casting and followed spark plasma sintering. The cross-sectional morphology and the XRD spectra showed that the WS2 flakes in Ag-WS2 composites were arranged in a preferred orientation along base plane (002). Based on this unique oriented structure, Ag-WS2 composites exhibited notably anisotropic tribology behavior when sliding along the direction parallel or perpendicular to the (002) plane of WS2. Compared to the parallel-plane, the friction coefficient and wear rate of the perpendicular-plane were increased by 30% and 167%, respectively. Additionally, the preferred orientation method of particles can also be applied to prepare other composites with oriented structure. Keywords: Metallic composites, Freeze casting, Preferred orientation, Wear and tribology, Anisotropy 1. Introduction Metal-WS2
composites
with
excellent
thermal
conductivity,
electrical
conductivity and lubricating property have been used for various engineering applications (such as bearings, bushings and electrical brushes) [1-5]. The outstanding lubricant property of metal-WS2 composites stems from the graphite-like layered structure of WS2 [6]. The weak interlayer bonding force in WS2 makes it easy to slide between the layers, resulting in low friction and wear. At the same time, the low shear force mainly appears along the (00n) crystal plane of WS2. For this reason, the 2
tribological performance is strongly affected by the orientation of WS2 particles. It has been found that the friction coefficient of WS2 films consisting of randomly oriented WS2 particles reached to 0.33 [7]. However, the WS2 coatings with preferentially orientated basal planes (002) exhibited a friction coefficient as low as 0.008 [8]. Therefore, it is of great scientific and practical significance to achieve a preferential orientation of WS2 basal plane (002) in metal-WS2 composites, which may effectively reduce friction and improve wear resistance of metal-WS2 composites. During the past few decades, many meaningful studies have been performed on the tribological behaviors of metal-WS2 composites, and most of which focused on the fields, such as the content/particle size of WS2 [1, 2], preparation process [3], as well as the ambient atmosphere [4]. However, few studies have been done to reveal the effect of WS2 orientation on the tribological property of metal-WS2 composites. It was attributed to the lack of an efficient approach to realize the good alignment of WS2 in the composites. As a result, WS2 particles exhibited random orientation in almost all the metal-WS2 composites which have been reported [1-5]. Scharf [8] reported a route for synthesizing WS2 coatings with high degree of (002) orientation by atomic layer deposition (ALD). However, this method had clear drawbacks that it could only be used to produce films less than 1.0 μm, severely restricting their service life and industrial applications. Recently, freeze casting has been developed to produce porous materials with lamellar structure [9-11], where some researchers found that the particles showed a 3
well-aligned microstructure along the freeze casting direction in the hBN [10] and WS2 scaffolds [11]. This provided us a new way to drive the particles to arrange along the preferred orientation. In this paper, freeze casting technique was explored to produce Ag-WS2 composites with preferential orientation (002) of WS2. The microstructure of the green bodies and bulk composites were determined using SEM analysis. The orientation degree of WS2 was evaluated by XRD and the anisotropic tribology behavior of the Ag-WS2 composites was also studied. 2. Experimental procedure Commercially available WS2 (99.0%, d50=6.0 μm) and Ag (99.9%, d50=0.2 μm) powder were used to produced Ag-WS2 composites with WS2 volume fraction of 30%. The Ag particles had an irregular shape (Fig. 1a) and the WS2 particles exhibited typically flaky appearance (Fig. 1b). Fig. 1c showed the preparation process of Ag-WS2 composites. Firstly, gelatin (3.0 wt.% of water) as the organic binder was dispersed in distilled water, followed by adding the WS2 and Ag powder to prepare a homogeneous suspension of 8.0 vol.% solid. Then the slurry was ball-milled at 100 rpm for 12h. After ball-milling, the slurry was poured into a Polytetrafluoroethylene (PTFE) mold connected with a refrigerating system (Julabo FT 902, Germany) to produce Ag/WS2 green bodies. At the junction with the refrigeration system, PTFE mold had a 15° wedge angle which helped to construct a dual temperature gradient: vertical the cold source and parallel to the PTFE wedge. In this way, the growth direction of ice crystal was effectively controlled along the temperature gradients, as seen in Fig. 1c. After completely frozen, the ice crystal was sublimed using a freeze-dryer (Freeze 4
Dryer 8, Blocool, China). Then, the Ag/WS2 green bodies with a long-range ordered lamellar structure were obtained. To obtain the bulk Ag-WS2 composites, the green bodies were pre-compressed (10 Map) along the direction perpendicular to the layer. Thereafter, the pre-compacted bodies were sent to a vacuum furnace to remove gelatin (350 °C, 2h) and fully densified by spark plasma sintering (SPS, FCT Systeme, Germany) at 750 °C for 10 min under 35 MPa. Finally, Ag-WS2 composites with oriented WS2 were successfully fabricated. (c)
(a)
Ag/WS2 mixing
Freeze casting
15°
SPS sintering
Ice
Ag 3 μm
(b)
WS2
20 μm
Raw materials
Ag/WS2 green body
Ag/WS2 slurry
Ag-WS2 composite
Fig. 1 The raw materials (a, b) and preparation process of Ag-WS2 composites (c).
Scanning electron microscopy (SEM, FEI Quanta 250 FEG, America) was used to characterize the microstructure of Ag-WS2 green bodies and Ag-WS2 composites. The orientation of WS2 was performed on X-ray diffraction (XRD, D/max-2550, Japan). A pin-on-disc tribometer (CSM Instruments, Switzerland) was employed to evaluate the anisotropic tribology behavior of Ag-WS2 composites. 3. Results and discussion Fig. 2 showed the SEM images in the walls (a, c) and layers (b, d) of Ag/WS2 green bodies, as well as the cross-sections of Ag-WS2 composites (e, f). The ordered wall structure in Fig. 2a indicated that a long-range ordered lamellar structure had been successfully produced by freeze casting. In addition, the arrangement of WS2 5
flakes was clearly observed from the magnified images of walls and layers (Fig. 2c and 2d). Fig. 2c showed that the WS2 flakes tended to be oriented along the walls. And the orientation phenomenon of WS2 flakes was more easily observed from Fig. 2d, in which almost all WS2 flakes laid flat on the layer. The orientation behavior mainly originated from the high aspect ratio of WS2 flakes, which were prone to rotate and preferentially oriented along the direction parallel to the wall under the driving force of ice crystals [10, 11]. As a result, the Ag/WS2 green bodies exhibited a long-range ordered lamellar structure and the WS2 flakes were parallel to the layer.
Fig. 2 SEM images of the wall (a, c) and layer (b, d) of Ag/WS2 green bodies, as well as the cross-sections (e, f) of Ag-WS2 composites.
Fig. 2e and 2f showed the cross-sections of the perpendicular-plane and parallel-plane in Ag-WS2 composites, respectively. The microstructure indicated that the WS2 oriented structure derived from the green bodies was well retained. A typical “brick-and-mortar” structure where the horizontally oriented WS2 flakes were uniformly embedded in the perpendicular-plane was observed from Fig. 2e, and the WS2 flakes were tiled on the parallel-plane in Fig. 2f. Obviously, a preferred 6
orientation of WS2 flakes was achieved in Ag-WS2 composites. The orientation degree of WS2 could be clearly observed from the XRD patterns of the perpendicular-plane and parallel-plane of Ag-WS2 composites in Fig. 3a. Although all the peaks in XRD spectra were matching with Ag and WS2, the intensity and position of the peaks corresponding to WS2 were quite different. Compared with the perpendicular-plane, the (00n) peaks of WS2 on the parallel-plane were very high, but the peaks at (100), (101), (103), (110) and (112) were too weak to distinguish. The difference in WS2 peaks suggested that the WS2 basal plane (002) was parallel to the parallel-plane of Ag-WS2 composites [11]. Moreover, the orientation state of WS2 was more pronounced from the ratio of the peak intensities of (002) and (100) [10, 12]. As shown in Fig. 3a, the I(002)/I(100) of the perpendicular-plane and parallel-plane were 6.16 and 332.77, respectively. The higher I(002)/I(100) of the parallel-plane strongly confirmed a preferred orientation (002) plane of WS2 throughout Ag-WS2 composites. Another point to note was that the interfacial reaction could enhance boundary wetting, which strongly affected the behavior of the composites, especially tribological behavior [1, 3, 13-15]. The XRD patterns in Fig. 3a showed that all the peaks belonged to Ag and WS2 phases, indicating that no interfacial reaction occurred between Ag and WS2. It significantly reduced the influence of boundary wetting on the tribological behavior [1, 3], which was beneficial for us to accurately evaluate the tribological behavior between the perpendicular-plane and parallel-plane of Ag-WS2 composites. As shown in Fig 3b, the Ag-WS2 composites exhibited an anisotropic tribology behavior between the perpendicular-plane and parallel-plane. When sliding along the parallel-plane, the composites exhibited a low friction coefficient (0.16) and wear rate 7
(3.08×10-5mm3/N·m). However, the friction coefficient and wear rate increased to 0.24 and 8.23×10-5mm3/N·m when sliding along the perpendicular-plane, 30% and 160% higher than that of the parallel-plane, respectively. Both the low friction coefficient and wear rate along the parallel-plane were derived from the high degree orientation (002) plane of WS2 throughout the Ag-WS2 composites. During friction tests, these oriented WS2 flakes were easily to slip along the (002) plane, which quickly developed into a low shear lubricating film, thereby improving the tribological properties of the parallel-plane [1, 8, 16].
0.24 0.24
■ WS2
■
(004) ■ 10
20
30
(110) (112)
■■
● (103)
(004) (100) (101)
● Ag
(006) ■
2θ(°)
●
■■
■
40
●
60
10
8
6
0.22
0.20
3.08
4
0.18
2
0.16
(008) ■ 50
8.23
0.26
Wear rate (×10-5mm3/N·m)
(002)
■
Intensity(a.u.)
(b)
Perpendicular-plane I002/I100=6.2 Parallel-plane I002/I100=332.8
●
Friction coefficient
(a)
0.16
70
0
80
Parallel-plane
Perpendicular-plane
Fig. 3 The XRD spectra (a) and the anisotropic tribology behavior (b) of Ag-WS2 composites.
4. Conclusion We proposed a new method to produce the Ag-WS2 composites with highly oriented WS2 flakes via freeze casting and followed SPS. SEM images showed Ag/WS2 green bodies with long-range ordered lamellar structure were obtained through freeze casting, in which the WS2 flakes were oriented parallel to the layer. The WS2 oriented structure derived from the green bodies was well retained in the Ag-WS2 composites, and XRD spectra confirmed that the (002) plane of WS2 was parallel to the parallel-plane throughout the composites. Moreover, the Ag-WS2 composites presented prominently anisotropic tribology behavior, which the friction 8
coefficient and wear rate of the perpendicular-plane were respectively 30% and 167% higher than that of the parallel-plane. The excellent tribological performance along the parallel-plane stemmed from the higher degree orientation (002) plane of WS2, which provided a low interfacial shear force. This efficient preferred orientation approach of WS2 flakes in this paper can also favor preparing other composites with oriented structure. Acknowledgements: The authors would like to thank the National Nature Science Foundation of China (No. 51674304, No. 51604305) and Natural Science Foundation of Hunan Province (No.2018JJ3677). Reference: [1] J. Xiao, W. Zhang, C. Zhang,Wear 412-413 (2018) 109-119. [2] J. Zhou, C. Ma, X. Kang, et al.,Trans. Nonferrous Met. Soc. China 28 (2018) 1176-1185. [3] Q. Wang, M. Chen, Z. Shan, et al.,J. Mater. Sci. Technol. 33 (2017) 1416-1423. [4] J. Wu, J. Li, L. Zhang, et al.,Trans. Nonferrous Met. Soc. China 27 (2017) 2202-2213. [5] V.B. Niste, M. Ratoi, H. Tanaka, et al.,Sci. Rep. 7 (2017) 14665. [6] T.W. Scharf, S.V. Prasad,J. Mat. Sci. 48 (2012) 511-531. [7] S. Xu, X. Gao, M. Hu, et al.,Tribol. Lett. 55 (2014) 1-13. [8] T. Scharf, S. Prasad, M. Dugger, et al.,Acta Mater. 54 (2006) 4731-4743. [9] H. Bai, Y. Chen, B. Delattre, et al.,Sci. Adv. 1 (2015) 1-8. [10] H. Shen, C. Cai, J. Guo, et al.,RSC Adv. 6 (2016) 16489-16494. [11] J. Wu, X. Liu, L. Yan, et al.,Mater. Lett. 196 (2017) 414-418. [12] H. Shen, J. Guo, H. Wang, et al.,ACS Appl. Mater. Interfaces 7 (2015) 5701-8. [13] W. Fu, A. Passerone, H. Bian, et al.,J. Mater. Sci. 54 (2018) 812-822. [14] B.B. Straumal, I. Konyashin, B. Ries, et al.,Mater. Lett. 147 (2015) 105-108. [15] B.B. Straumal, S.V. Dobatkin, A.O. Rodin, et al.,Adv. Eng. Mater. 13 (2011) 463-469. [16] Y. Sun, Z. Chai, X. Lu, et al.,Tribol. Int. 114 (2017) 478-484.
9
Declaration of Interest Statement In this paper, we proposed a new method to produce the Ag-WS2 composites with highly oriented WS2 flakes via freeze casting and followed SPS. The XRD spectra confirmed that the (002) plane of WS2 flakes was parallel to the parallel-plane throughout the Ag-WS2 composites. Moreover, the Ag-WS2 composites presented prominently anisotropic tribology property which the friction coefficient and wear rate of the perpendicular-plane were respectively 30% and 167% higher than that of the parallel-plane. The excellent tribological performance along the parallel-plane stemmed from the higher degree orientation (002) plane of WS2 flakes, which provided a low interfacial shear force. This efficient preferred orientation approach of WS2 flakes in this paper can also favor preparing other composites with oriented structure and investigating other important anisotropic properties, such as mechanical property and thermal conductivity. I hope this paper is suitable for “Materials Letters”.
10
Highlight 1. Ag-WS2 composites with preferentially oriented WS2 were produced. 2. Obvious difference in XRD peaks showed a preferred orientation along (002) of WS2. 3 The Ag-WS2 composites exhibited notably anisotropic tribology behavior.
11
S0167-577X(19)31607-6 https://doi.org/10.1016/j.matlet.2019.126975 MLBLUE 126975
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
8 October 2019 5 November 2019 7 November 2019
Please cite this article as: Y. Sun, J. Wu, L. Zhang, Fabrication of Ag-WS2 composites with preferentially oriented WS2 and its anisotropic tribology behavior, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet. 2019.126975
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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.
© 2019 Published by Elsevier B.V.
Fabrication of Ag-WS2 composites with preferentially oriented WS2 and its anisotropic tribology behavior Yang Sun a Jisi Wu b Lei Zhang a*
*Corresponding author.
E-mail address: [email protected] (L. Zhang)
1
a
State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China b School of Aviation Manufacturing Engineering Institute, Nanchang Hangkong University, Jiangxi 330063, China
Abstract The Ag-WS2 composites with preferentially oriented WS2 flakes were successfully prepared via freeze casting and followed spark plasma sintering. The cross-sectional morphology and the XRD spectra showed that the WS2 flakes in Ag-WS2 composites were arranged in a preferred orientation along base plane (002). Based on this unique oriented structure, Ag-WS2 composites exhibited notably anisotropic tribology behavior when sliding along the direction parallel or perpendicular to the (002) plane of WS2. Compared to the parallel-plane, the friction coefficient and wear rate of the perpendicular-plane were increased by 30% and 167%, respectively. Additionally, the preferred orientation method of particles can also be applied to prepare other composites with oriented structure. Keywords: Metallic composites, Freeze casting, Preferred orientation, Wear and tribology, Anisotropy 1. Introduction Metal-WS2
composites
with
excellent
thermal
conductivity,
electrical
conductivity and lubricating property have been used for various engineering applications (such as bearings, bushings and electrical brushes) [1-5]. The outstanding lubricant property of metal-WS2 composites stems from the graphite-like layered structure of WS2 [6]. The weak interlayer bonding force in WS2 makes it easy to slide between the layers, resulting in low friction and wear. At the same time, the low shear force mainly appears along the (00n) crystal plane of WS2. For this reason, the 2
tribological performance is strongly affected by the orientation of WS2 particles. It has been found that the friction coefficient of WS2 films consisting of randomly oriented WS2 particles reached to 0.33 [7]. However, the WS2 coatings with preferentially orientated basal planes (002) exhibited a friction coefficient as low as 0.008 [8]. Therefore, it is of great scientific and practical significance to achieve a preferential orientation of WS2 basal plane (002) in metal-WS2 composites, which may effectively reduce friction and improve wear resistance of metal-WS2 composites. During the past few decades, many meaningful studies have been performed on the tribological behaviors of metal-WS2 composites, and most of which focused on the fields, such as the content/particle size of WS2 [1, 2], preparation process [3], as well as the ambient atmosphere [4]. However, few studies have been done to reveal the effect of WS2 orientation on the tribological property of metal-WS2 composites. It was attributed to the lack of an efficient approach to realize the good alignment of WS2 in the composites. As a result, WS2 particles exhibited random orientation in almost all the metal-WS2 composites which have been reported [1-5]. Scharf [8] reported a route for synthesizing WS2 coatings with high degree of (002) orientation by atomic layer deposition (ALD). However, this method had clear drawbacks that it could only be used to produce films less than 1.0 μm, severely restricting their service life and industrial applications. Recently, freeze casting has been developed to produce porous materials with lamellar structure [9-11], where some researchers found that the particles showed a 3
well-aligned microstructure along the freeze casting direction in the hBN [10] and WS2 scaffolds [11]. This provided us a new way to drive the particles to arrange along the preferred orientation. In this paper, freeze casting technique was explored to produce Ag-WS2 composites with preferential orientation (002) of WS2. The microstructure of the green bodies and bulk composites were determined using SEM analysis. The orientation degree of WS2 was evaluated by XRD and the anisotropic tribology behavior of the Ag-WS2 composites was also studied. 2. Experimental procedure Commercially available WS2 (99.0%, d50=6.0 μm) and Ag (99.9%, d50=0.2 μm) powder were used to produced Ag-WS2 composites with WS2 volume fraction of 30%. The Ag particles had an irregular shape (Fig. 1a) and the WS2 particles exhibited typically flaky appearance (Fig. 1b). Fig. 1c showed the preparation process of Ag-WS2 composites. Firstly, gelatin (3.0 wt.% of water) as the organic binder was dispersed in distilled water, followed by adding the WS2 and Ag powder to prepare a homogeneous suspension of 8.0 vol.% solid. Then the slurry was ball-milled at 100 rpm for 12h. After ball-milling, the slurry was poured into a Polytetrafluoroethylene (PTFE) mold connected with a refrigerating system (Julabo FT 902, Germany) to produce Ag/WS2 green bodies. At the junction with the refrigeration system, PTFE mold had a 15° wedge angle which helped to construct a dual temperature gradient: vertical the cold source and parallel to the PTFE wedge. In this way, the growth direction of ice crystal was effectively controlled along the temperature gradients, as seen in Fig. 1c. After completely frozen, the ice crystal was sublimed using a freeze-dryer (Freeze 4
Dryer 8, Blocool, China). Then, the Ag/WS2 green bodies with a long-range ordered lamellar structure were obtained. To obtain the bulk Ag-WS2 composites, the green bodies were pre-compressed (10 Map) along the direction perpendicular to the layer. Thereafter, the pre-compacted bodies were sent to a vacuum furnace to remove gelatin (350 °C, 2h) and fully densified by spark plasma sintering (SPS, FCT Systeme, Germany) at 750 °C for 10 min under 35 MPa. Finally, Ag-WS2 composites with oriented WS2 were successfully fabricated. (c)
(a)
Ag/WS2 mixing
Freeze casting
15°
SPS sintering
Ice
Ag 3 μm
(b)
WS2
20 μm
Raw materials
Ag/WS2 green body
Ag/WS2 slurry
Ag-WS2 composite
Fig. 1 The raw materials (a, b) and preparation process of Ag-WS2 composites (c).
Scanning electron microscopy (SEM, FEI Quanta 250 FEG, America) was used to characterize the microstructure of Ag-WS2 green bodies and Ag-WS2 composites. The orientation of WS2 was performed on X-ray diffraction (XRD, D/max-2550, Japan). A pin-on-disc tribometer (CSM Instruments, Switzerland) was employed to evaluate the anisotropic tribology behavior of Ag-WS2 composites. 3. Results and discussion Fig. 2 showed the SEM images in the walls (a, c) and layers (b, d) of Ag/WS2 green bodies, as well as the cross-sections of Ag-WS2 composites (e, f). The ordered wall structure in Fig. 2a indicated that a long-range ordered lamellar structure had been successfully produced by freeze casting. In addition, the arrangement of WS2 5
flakes was clearly observed from the magnified images of walls and layers (Fig. 2c and 2d). Fig. 2c showed that the WS2 flakes tended to be oriented along the walls. And the orientation phenomenon of WS2 flakes was more easily observed from Fig. 2d, in which almost all WS2 flakes laid flat on the layer. The orientation behavior mainly originated from the high aspect ratio of WS2 flakes, which were prone to rotate and preferentially oriented along the direction parallel to the wall under the driving force of ice crystals [10, 11]. As a result, the Ag/WS2 green bodies exhibited a long-range ordered lamellar structure and the WS2 flakes were parallel to the layer.
Fig. 2 SEM images of the wall (a, c) and layer (b, d) of Ag/WS2 green bodies, as well as the cross-sections (e, f) of Ag-WS2 composites.
Fig. 2e and 2f showed the cross-sections of the perpendicular-plane and parallel-plane in Ag-WS2 composites, respectively. The microstructure indicated that the WS2 oriented structure derived from the green bodies was well retained. A typical “brick-and-mortar” structure where the horizontally oriented WS2 flakes were uniformly embedded in the perpendicular-plane was observed from Fig. 2e, and the WS2 flakes were tiled on the parallel-plane in Fig. 2f. Obviously, a preferred 6
orientation of WS2 flakes was achieved in Ag-WS2 composites. The orientation degree of WS2 could be clearly observed from the XRD patterns of the perpendicular-plane and parallel-plane of Ag-WS2 composites in Fig. 3a. Although all the peaks in XRD spectra were matching with Ag and WS2, the intensity and position of the peaks corresponding to WS2 were quite different. Compared with the perpendicular-plane, the (00n) peaks of WS2 on the parallel-plane were very high, but the peaks at (100), (101), (103), (110) and (112) were too weak to distinguish. The difference in WS2 peaks suggested that the WS2 basal plane (002) was parallel to the parallel-plane of Ag-WS2 composites [11]. Moreover, the orientation state of WS2 was more pronounced from the ratio of the peak intensities of (002) and (100) [10, 12]. As shown in Fig. 3a, the I(002)/I(100) of the perpendicular-plane and parallel-plane were 6.16 and 332.77, respectively. The higher I(002)/I(100) of the parallel-plane strongly confirmed a preferred orientation (002) plane of WS2 throughout Ag-WS2 composites. Another point to note was that the interfacial reaction could enhance boundary wetting, which strongly affected the behavior of the composites, especially tribological behavior [1, 3, 13-15]. The XRD patterns in Fig. 3a showed that all the peaks belonged to Ag and WS2 phases, indicating that no interfacial reaction occurred between Ag and WS2. It significantly reduced the influence of boundary wetting on the tribological behavior [1, 3], which was beneficial for us to accurately evaluate the tribological behavior between the perpendicular-plane and parallel-plane of Ag-WS2 composites. As shown in Fig 3b, the Ag-WS2 composites exhibited an anisotropic tribology behavior between the perpendicular-plane and parallel-plane. When sliding along the parallel-plane, the composites exhibited a low friction coefficient (0.16) and wear rate 7
(3.08×10-5mm3/N·m). However, the friction coefficient and wear rate increased to 0.24 and 8.23×10-5mm3/N·m when sliding along the perpendicular-plane, 30% and 160% higher than that of the parallel-plane, respectively. Both the low friction coefficient and wear rate along the parallel-plane were derived from the high degree orientation (002) plane of WS2 throughout the Ag-WS2 composites. During friction tests, these oriented WS2 flakes were easily to slip along the (002) plane, which quickly developed into a low shear lubricating film, thereby improving the tribological properties of the parallel-plane [1, 8, 16].
0.24 0.24
■ WS2
■
(004) ■ 10
20
30
(110) (112)
■■
● (103)
(004) (100) (101)
● Ag
(006) ■
2θ(°)
●
■■
■
40
●
60
10
8
6
0.22
0.20
3.08
4
0.18
2
0.16
(008) ■ 50
8.23
0.26
Wear rate (×10-5mm3/N·m)
(002)
■
Intensity(a.u.)
(b)
Perpendicular-plane I002/I100=6.2 Parallel-plane I002/I100=332.8
●
Friction coefficient
(a)
0.16
70
0
80
Parallel-plane
Perpendicular-plane
Fig. 3 The XRD spectra (a) and the anisotropic tribology behavior (b) of Ag-WS2 composites.
4. Conclusion We proposed a new method to produce the Ag-WS2 composites with highly oriented WS2 flakes via freeze casting and followed SPS. SEM images showed Ag/WS2 green bodies with long-range ordered lamellar structure were obtained through freeze casting, in which the WS2 flakes were oriented parallel to the layer. The WS2 oriented structure derived from the green bodies was well retained in the Ag-WS2 composites, and XRD spectra confirmed that the (002) plane of WS2 was parallel to the parallel-plane throughout the composites. Moreover, the Ag-WS2 composites presented prominently anisotropic tribology behavior, which the friction 8
coefficient and wear rate of the perpendicular-plane were respectively 30% and 167% higher than that of the parallel-plane. The excellent tribological performance along the parallel-plane stemmed from the higher degree orientation (002) plane of WS2, which provided a low interfacial shear force. This efficient preferred orientation approach of WS2 flakes in this paper can also favor preparing other composites with oriented structure. Acknowledgements: The authors would like to thank the National Nature Science Foundation of China (No. 51674304, No. 51604305) and Natural Science Foundation of Hunan Province (No.2018JJ3677). Reference: [1] J. Xiao, W. Zhang, C. Zhang,Wear 412-413 (2018) 109-119. [2] J. Zhou, C. Ma, X. Kang, et al.,Trans. Nonferrous Met. Soc. China 28 (2018) 1176-1185. [3] Q. Wang, M. Chen, Z. Shan, et al.,J. Mater. Sci. Technol. 33 (2017) 1416-1423. [4] J. Wu, J. Li, L. Zhang, et al.,Trans. Nonferrous Met. Soc. China 27 (2017) 2202-2213. [5] V.B. Niste, M. Ratoi, H. Tanaka, et al.,Sci. Rep. 7 (2017) 14665. [6] T.W. Scharf, S.V. Prasad,J. Mat. Sci. 48 (2012) 511-531. [7] S. Xu, X. Gao, M. Hu, et al.,Tribol. Lett. 55 (2014) 1-13. [8] T. Scharf, S. Prasad, M. Dugger, et al.,Acta Mater. 54 (2006) 4731-4743. [9] H. Bai, Y. Chen, B. Delattre, et al.,Sci. Adv. 1 (2015) 1-8. [10] H. Shen, C. Cai, J. Guo, et al.,RSC Adv. 6 (2016) 16489-16494. [11] J. Wu, X. Liu, L. Yan, et al.,Mater. Lett. 196 (2017) 414-418. [12] H. Shen, J. Guo, H. Wang, et al.,ACS Appl. Mater. Interfaces 7 (2015) 5701-8. [13] W. Fu, A. Passerone, H. Bian, et al.,J. Mater. Sci. 54 (2018) 812-822. [14] B.B. Straumal, I. Konyashin, B. Ries, et al.,Mater. Lett. 147 (2015) 105-108. [15] B.B. Straumal, S.V. Dobatkin, A.O. Rodin, et al.,Adv. Eng. Mater. 13 (2011) 463-469. [16] Y. Sun, Z. Chai, X. Lu, et al.,Tribol. Int. 114 (2017) 478-484.
9
Declaration of Interest Statement In this paper, we proposed a new method to produce the Ag-WS2 composites with highly oriented WS2 flakes via freeze casting and followed SPS. The XRD spectra confirmed that the (002) plane of WS2 flakes was parallel to the parallel-plane throughout the Ag-WS2 composites. Moreover, the Ag-WS2 composites presented prominently anisotropic tribology property which the friction coefficient and wear rate of the perpendicular-plane were respectively 30% and 167% higher than that of the parallel-plane. The excellent tribological performance along the parallel-plane stemmed from the higher degree orientation (002) plane of WS2 flakes, which provided a low interfacial shear force. This efficient preferred orientation approach of WS2 flakes in this paper can also favor preparing other composites with oriented structure and investigating other important anisotropic properties, such as mechanical property and thermal conductivity. I hope this paper is suitable for “Materials Letters”.
10
Highlight 1. Ag-WS2 composites with preferentially oriented WS2 were produced. 2. Obvious difference in XRD peaks showed a preferred orientation along (002) of WS2. 3 The Ag-WS2 composites exhibited notably anisotropic tribology behavior.
11