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Effect of Ti or TiN codeposition on the performance of MoS2-based composite coatings
Thin Solid Films 461 (2004) 288 – 293 www.elsevier.com/locate/tsf
Effect of Ti or TiN codeposition on the performance of MoS2-based composite coatings Yang Jing a,b,*, Jianbin Luo a, Siqin Pang b b
a State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China School of Mechanical Engineering Automation, Beijing Institute of Technology, Beijing 100081, China
Received 5 June 2003; received in revised form 18 February 2004; accepted 20 February 2004 Available online 23 April 2004
Abstract Properties of MoS2 coatings can be improved by the codeposition of a small amount of titanium (Ti). In this study, MoS2-based composite coatings, which consisted of Ti or titanium nitride (TiN) and molybdenum disulfide, are synthesized by unbalanced plasma plating. Two forms of MoS2-based composite coatings have been developed: TiN – MoS2/Ti and TiN – MoS2/TiN coatings. The effect of Ti or TiN and processing parameters on properties of such coatings are investigated. Scanning electron microscopy (SEM), dry drilling and turning tests are used to determine the structure, composition and tribological performance of MoS2-based coatings. Experimental results indicate that these MoS2-based composite coatings are considerably more wear-resistant and are less sensitive to atmospheric water vapor than pure MoS2 coatings. These coatings show excellent results when tested for a wide range of industrial turning and drilling applications. D 2004 Elsevier B.V. All rights reserved. PACS: 68.55.JK; 81.40.Pq Keywords: MoS2-based coatings; Unbalanced plasma plating; Titanium; Additives
1. Introduction Molybdenum disulfide is an attractive candidate for lowfriction coefficient, high bearing capacity solid lubricants for space applications [1,2]. However, the poor abrasion durability and oxidation resistance of the pure MoS2 coatings limit their wide use [3]. Many investigators have attempted to overcome these limitations. For instance, codeposition of metal (such as Ni, Au, Sb2S4 or LaF3) and MoS2 can improve the tribological performance of MoS2-based coatings in the higher temperature of 120 jC [4 –6]. Both ion beam-assisted deposition [7] and closedfield unbalanced magnetron sputter ion plating [8,9] are introduced for the deposition of dense MoS2 coatings. These MoS2 coatings are expected to show the highest durability and lowest friction when grown with the (001) planes of the
* Corresponding author. Department of Precision Instrument and Mechanology, State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China. Tel.: +86-10-62771438; fax: +86-10-62781379. E-mail address: [email protected] (Y. Jing). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.02.044
MoS2 crystallites parallel to the substrate surface (basal orientation). Furthermore, both metal/MoSx multilayer coatings and metal/MoSx quasi-amorphous coatings are successfully produced by codeposition with different metal additives [10,11]. Evaporated MoS2-based thin films have been extensively utilized on cutting tools and research on utilizing an ion plating technique in place of traditional electrolytic plating technique has been reported [12 – 14]. The columnar structure of coatings obtained using the ion plating technique often causes lubrication failure if the coating is thicker than 80 nm. In order to solve the problem cited above, many investigators have attempted to prepare the MoS2-based coatings on cutting tools by multiarc ion plating. This technique operates at a high-deposition rate, resulting in large-size droplets and relatively many surface defects in its product. Then, a new deposition method named unbalanced plasma plating is used to deposit the MoS2-based composite coatings. Multiarc ion plating is usually a balanced plasma deposition in which a metal vapor produced in a vacuum system is deposited onto a substrate whilst the substrate is
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289
Table 1 Respective deposition thickness for different MoS2-based coatings Type of coatings
TiN – MoS2 TiN – MoS2/Ti TiN – MoS2/TiN
Fig. 1. System diagram of unbalanced plasma plating.
simultaneously bombarded with ions. Ion bombardment improves both the adhesion and the structure of the coating. In the based ion plating method, however, the ions are produced in an abnormal glow discharge with the samples acting as cathode. This is an inefficient process and typically less than 1 atom in 1000 is ionized in an abnormal glow discharge. The ion current to the samples is low and is not sufficient to produce the dense coatings required in many applications, although the samples are held at a high negative potential. To solve this problem, the ionization can be increased in a closed and unbalanced magnetron with improved multiarc ion plating—unbalanced plasma plating. The extra field lines leaving the outer magnets trap electrons escaping from the magnetron discharge and prevent them from drifting to the various earthed parts of the chamber. These electrons cause ionization in the vicinity of the electrically biased substrate and the ions so formed are attracted to the substrate by the substrate bias, and the substrates receive higher ion current than in a situation where the magnetrons are balanced. Fig. 1 shows the diagram of unbalanced plasma plating system. The MoS2-based coatings deposited by this method have many advantages, e.g., longer wear life and
Total depostion thickness (Am)
TiN layer thickness (Am)
MoS2-based coating thickness (Am) MoS2/Ti layer
Ti layer
f2.10 f2.10 f2.10
f0.5 f0.5 f0.5
f1.60 f25 nm f1.60
f10 nm
higher resistance to oxidation by adjusting various parameters such as Ar/H2S pressure and titanium (Ti) additive content, etc.
2. Experimental details 2.1. Deposition processes and substrate materials The coatings are prepared by unbalanced plasma plating device using a cylindrical stainless steel chamber (base pressure: 210 3 Pa) with a vertical four-target configuration, and during deposition, the pressure rises to 4.510 1 Pa. The arrangement of magnetrons within the coating chamber is a closed-field arrangement, shown in Fig. 2. This arrangement effectively increases the ionization within the chamber by trapping more electrons and allows the deposition of multilayer coatings. The Mo target with a diameter of 60 mm has a purity of 99.97%, and the distance of the substrate-to-the-target distance is 50 cm. The substrate materials are hardened and polished with a roughness of 0.02 Am, and different MoS2-based coatings, whose thicknesses are typically 2.10 Am, are deposited on them. Furthermore, the substrate – target distance can be varied. The thickness of the coatings can be controlled by varying a combination of the deposition rate (target power), bias voltage of the substrate and deposition time. Before deposition, all the samples are cleaned ultrasonically and degassed at 593 K in vacuum chamber for 30 min. After that, the surfaces of the samples are cleaned by Ar+ sputter. At the beginning of the deposition, the reacting gas (H2S) and the working gas (Ar) are partly ionized by applying electric discharge in a high vacuum. Next, the evaporants and other reacting materials are deposited continuously on the substrates. In particular, there are three Mo and one-Ti targets that will be used for the TiN –MoS2/Ti coating. After adding titanium (Ti) atoms into MoS2 film,
Table 2 Drill testing parameters
Fig. 2. Schematic representation of a four-target deposit chamber with unbalanced plasma plating.
Drill coated
Work-piece
Hole depth (mm)
Rpm
Feed rate (mm/rev)
Coolant
Diameter: 8.5 mm
38CrNi3MoVA
20
600
0.13
No
290
Y. Jing et al. / Thin Solid Films 461 (2004) 288–293
Table 3 Turning testing parameters Turning tool coated
Work-piece
Depth (mm)
Speed (m/min)
Feed rate (mm/rev)
Coolant
YT14
38CrNi3MoVA
0.5
240
0.13
No
the structure of the coating changes from the columnar structure to a denser lamellar structure [15]. The main processing parameters are as follows: the initial temperature during the deposition is 120 jC; the pressure under argon ion bombardment is 0.5 Pa; the bias voltage of substrate under argon ion bombardment is 800 V; the time under argon ion bombardment is 30 min; the partial pressure of argon gas while depositing MoS2/Ti is 510 2 Pa; and the partial pressure of H2S gas is 410 1 Pa. The bias voltage of substrate under deposition is 200 V; the arc current is 75 A; the total deposition time is 85 min; and the respective monolayer deposition thickness for different MoS2-based coatings are shown in Table 1. There are three types of samples in the present paper. The first two types are YT14 turning tools and 8-mm HSS drills, which are utilized to test the wear-resistant properties and humid-resistant properties, and the other is 15152 mm flat samples cut from 1Cr18Ni9Ti steel plate, which are utilized to measure the fractography morphology and phases of coatings. Fig. 4. SEM cross-section showing TiN – MoS2/Ti multilayer coating.
2.2. Tribological tests The MoS2-based coatings are deposited onto drills and turning tools for testing under the conditions given inTables 2 and 3. These drills are tested under dry condition and compared with uncoated one. For evaluating the resistance to oxidation of the above coatings, the drills coated with MoS2-based coatings are stored for 1, 2, 5 and 10 days (d), respectively, in humid conditions up to 60 – 70% relative humidity (RH) before the tests are performed. Meanwhile, YT14 turning tools are coated with MoS2-based coatings to turn 38CrNi3MoVA at an ultralow temperature of 23 K in
liquid nitrogen and in an approximately 60 – 70% RH ambient air. Finally, the microstructure, composition and mechanical properties of the MoS2-based coatings are analyzed by scanning electron microscopy (SEM; JSM-5400), wavelength dispersive spectrometry (WDS). The fractograph of a SEM cross-section and a SEM micrograph of MoS2, MoS2/Ti or MoS2/TiN coating on the top of a TiN coating are shown in Figs. 3– 5.
3. Results and discussion 3.1. Characteristics of TiN – MoS2 coating
Fig. 3. SEM cross-section showing TiN – MoS2 coating.
The cross-section of the TiN– MoS2 coating deposited on 1Cr18Ni9Ti sample is shown in Fig. 3. The TiN – MoS2 coating shows a typical porous and columnar surface structure. The results of tribological experiments indicate that these coatings, which initially nucleate along basal orientation, can convert to edge orientation once they grow beyond a certain thickness. Spalvins [16] reported that the characteristic growth mechanism led to the formation of loose columnar MoS2 platelets at the surface layer, with large amounts of voids in between. When the MoS2 layer grew beyond 80 nm, fibrous structures began to form. When exposed to humid environments, the MoS2 layers became prone to structural degradation due to oxygen penetration, and to functional
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291
degradation due to excessive wear debris from broken MoS2 platelets [16]. Owing to their mechanical strength, interfacial compatibility and high oxygen affinity, Ti and TiN were selected in this study to serve as the interrupt layer, which also supports the composite structure as well as adsorbing oxygen during contact. 3.2. Characteristics of TiN –MoS2/Ti multilayer coating Based on TiN– MoS2 coating, TiN – MoS2/Ti coating is successfully produced by codepositing Ti atoms into MoS2 while the layered microstructure of TiN –MoS2/Ti coating is formed by rotating the substrate stage between those four targets. Interlayers of MoS2/Ti are deposited sequentially on the substrates, thus forming the multilayer microstructure. The cross-section with schematic diagram of the TiN – MoS2/Ti coating is shown in Fig. 4. To enhance the structural integrity of TiN –MoS2 coatings, Ti layers are deposited sequentially after each MoS2 layer to form a reinforced composite structure. In addition to densification and strengthening effect, the Ti interlayer also reacts preferentially with oxygen to form TiO2, and thus effectively suppresses adverse of MoS2 [17]. Therefore, in humid environments, the TiN –MoS2/Ti multilayer coating is highly promising as a solid lubricant, as evidenced by its prolonged wear life and resistance to oxidation. The composition of TiN – MoS2 and TiN – MoS2/Ti coatings stored in 60 –70% RH humid air for 24 h were also analyzed by using WDS in conjunction with a scanning electron microscope. The standard reference crystals of MoS2, Ti and SiO2 were applied for quantitative analysis. As shown in Table 4, the oxygen content in the TiN –MoS2/Ti decreases from 7.26 to 2.48 at.% when the depth of the coating increases from 10 to 20 nm, while the Ti content increases from 10.74 to 28.99 at.%. It indicates that the lamellar structure of TiN – MoS2/Ti coating closes a large amount of open channels for oxygen diffusion. In addition, the Ti additive acts as an oxygen getter within the film to form TiO2, leaving the MoS2 intact despite exposure to humid or oxidizing environments. Finally, the Ti interlayer serves as a diffusion barrier to block penetration of oxygen into the film. In addition, the TiN layer (about 0.5 Am) between the lubricate layer (1.60 Am) and the substrate has provided a strong support with its very high rigidity [18,19]. Meanwhile, according to phase boundary matching princi-
Fig. 5. SEM micrograph of TiN – MoS2/TiN composite coating after electrolytic corrosion treatment.
ple [20], their favorable compatibility enhances the adhesion of the coatings to the substrate. 3.3. Characteristics of TiN –MoS2/Tin composite coating During the deposition process of TiN – MoS2/Ti lamellar coating, N2 is added into H2S gas, while the Ar/H2S/ N2 pressure is varied. Meanwhile, the other interior configurations are adjusted such as fixing Ti target on top of the vacuum chamber and adjusting the distance Ti target and sample. Therefore, the TiN – MoS2/TiN composite coating with higher abrasion durability and humid resistance is successfully produced [21]. Based on the surface morphology of the coating shown in Fig. 5, the ceramic phases, i.e., TiN superfine grains, are equably distributed in the MoS2/TiN surface layer. These grains
Table 4 Quantitative analysis of MoS2-based composite coatings by WDS Type of the coatings
Depth (nm)
Ti (at.%)
Mo (at.%)
S (at.%)
O (at.%)
TiN – MoS2/Ti
10 20 20
10.74 28.99 –
32.24 27.25 38.76
49.76 41.22 44.01
7.26 2.48 17.23
TiN – MoS2
Fig. 6. Turning tests of YT14 turning tools coated with MoS2-based coatings in an ultralow temperature.
292
Y. Jing et al. / Thin Solid Films 461 (2004) 288–293
improve the rigidity of the coating while they destroy the intrinsic lamellar structure and result in the disorder crystal orientation. Furthermore, TiN interlayer and MoS2/TiN lubricant layer are deposited in the same chamber, which effectively averts secondary pollution in the process of deposition. In addition, according to the turning test in ultralow temperature shown in Fig. 6, the turning lives of the YT14 turning tools deposited with MoS2-based coatings are obviously increased with the continuous improvement of the deposition procedures and additional materials. In particular, the working durability of the turning tools coated with TiN – MoS2/TiN composite is the longest; its abrasion durability and resistance to low temperature are obviously improved, which conforms to the results of physical – chemical analysis of MoS2-based composite coatings. 3.4. Tribological behavior of MoS2-based coatings 3.4.1. As-deposition coatings The drilling lives of drills coated with both TiN –MoS2/Ti lamellar and TiN – MoS2/TiN composite coatings before being stored in humid air increase continuously with Ti content increasing in MoS2-based lubricant layer, as shown in Fig. 7. For lamellar coatings, the best drilling life was observed when Ti content in the coating is 12.5 at.%, while the distance between the substrate and Ti target is approximately 50 cm. For the TiN – MoS2/TiN coatings, however, the Ti content value corresponding to the best drilling life has shifted from 12.5 to 13.2 at.%. MoS2-based multilayer coatings are frequently reported [22,23]. In the present study, TiN –MoS2/Ti composite coating, which exhibits a relatively higher bearing capacity and abrasion durability, could be obtained by changing process parameters compared with the lamellar structure.
Fig. 7. Number of holes drilled in 38CrNi3MoVA using drills coated with MoS2-based coatings as a function of the Ti content in the coating: (A) TiN – MoS2/Ti lamellar coating; (B) TiN – MoS2/TiN composite coating.
Fig. 8. Number of holes drilled in 38CrNi3MoVA using drills coated with MoS2-based coatings as a function of coatings storage time in 60 – 70% RH air; (A) TiN – MoS2; (B) TiN – MoS2/Ti; (C) TiN – MoS2/TiN.
3.4.2. After being stored in humid air Before being stored in humid air, the drills coated with TiN – MoS2 have better abrasion durability and the number of holes drilled is 134 (see Fig. 8.). But after being stored with 60 – 70% RH for 5 days, its abrasion durability decreases quickly. It shows that TiN– MoS2 coating does not retain desirable properties in humid atmospheres and is not suitable for industrial turning and drilling applications. However, the working durability of drills coated with TiN– MoS2/Ti or TiN – MoS2/TiN erodes slowly with the prolonging of storage time.
4. Conclusions In this study, TiN – MoS2/Ti and TiN –MoS2/TiN coatings deposited by unbalanced plasma plating provide significantly longer wear lives than single-component MoS2 coating. For effective elimination of the columnar structure of TiN – MoS2 coating, TiN –MoS2/Ti and TiN– MoS2/TiN coatings are developed by the adapted deposition conditions. For example, by adding Ti or N2 atoms in pure MoS2 and adjusting the substrate –target distance, the TiN – MoS2/Ti and TiN – MoS2/TiN coatings with typical lamellar or composite structure are successfully produced. In particular, TiN – MoS2/Ti coating consisted of interlacing MoS2 and Ti layers. WDS quantitative analysis implies that the oxygen content decreases quickly as the coating depth increases. In addition, the growth of columnar platelets of MoS2 is effectively suppressed by the introduction of Ti or TiN additives. Furthermore, the crystallites of MoS2-based lubricants become clearly refined when Ti atoms are added in the process of deposition. The structure of the coatings becomes denser, and their bearing capacity and wear life are also obviously enhanced. Subsequently, MoS2-based composite
Y. Jing et al. / Thin Solid Films 461 (2004) 288–293
coatings are shown to be a suitable replacement for cutting fluid, and TiN – MoS2/Ti and TiN – MoS2/TiN coating shows a higher tribological property as well, after being stored in humid air.
Acknowledgements The authors would like to thank Tsinghua University and Beijing Institute of Technology for their valuable comments and support.
References [1] [2] [3] [4] [5] [6]
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[9] N.M. Renevier, V.C. Fox, D.G. Teer, J. Hampshire, Surf. Coat. Technol. 127 (2000) 24. [10] C. Mu¨ller, C. Menoud, M. Maillat, H.E. Hintermann, Surf. Coat. Technol. 36 (1988) 351. [11] V.C. Fox, N. Renevier, D.G. Teer, J. Hampshire, V. Rigato, Surf. Coat. Technol. 116 – 119 (1999) 492. [12] S.K. Kim, Y.H. Ahn, K.H. Kim, Surf. Coat. Technol. 160 – 170 (2003) 428. [13] G.-Z. Xu, Z.-R. Zhou, J.-J. Liu, X.-H. Ma, Wear 225 (1999) 46. [14] V. Rigato, G. Maggioni, D. Boscarino, L. Sangaletti, L. Depero, V.C. Fox, D. Teer, C. Santini, Surf. Coat. Technol. 116 – 119 (1999) 176. [15] Y. Jing, S.-Q. Pang, L.-Y. Zhou (Eds.), Proceedings of CMES Annual Conference 2001, Suzhou, Zhejiang Province, China, China Machine Press, November 21 – 24, (2001) 460. [16] T. Spalvins, Thin Solid Films 96 (1982) 17. [17] D.-Y. Wang, C.-L. Chang, Z.-Y. Chen, W.-Y. Ho, Surf. Coat. Technol. 120/121 (1999) 629. [18] G.-Z. Xu, Z.-R. Zhou, J.-J. Liu, X.-H. Ma, Wear 225 (1999) 46. [19] G.-Z. Xu, Z.-R. Zhou, J.-J. Liu, Wear 224 (1999) 211. [20] Y.-Q. Bao, Fundamentals of Metallography, Metallurgical Industry Press, Beijing BJ, 1986. In Chinese. [21] Y. Jing, S.-Q. Pang, S.-D. Zhong, L.-Y. Zhou, J. Beijing Inst. Technol. 22 (2002) 28 (in Chinese). [22] J.D. Holbery, E. Pflueger, A. Savan, Y. Gerbig, Q. Luo, D.B. Lewis, W.-D. Munz, Surf. Coat. Technol. 169/170 (2003) 716. [23] S. Mikhailov, A. Savan, E. Pflu¨ger, L. Knoblauch, R. Hauert, M. Simmonds, H.V. Swygenhoven, Surf. Coat. Technol. 105 (1998) 175.
Effect of Ti or TiN codeposition on the performance of MoS2-based composite coatings Yang Jing a,b,*, Jianbin Luo a, Siqin Pang b b
a State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China School of Mechanical Engineering Automation, Beijing Institute of Technology, Beijing 100081, China
Received 5 June 2003; received in revised form 18 February 2004; accepted 20 February 2004 Available online 23 April 2004
Abstract Properties of MoS2 coatings can be improved by the codeposition of a small amount of titanium (Ti). In this study, MoS2-based composite coatings, which consisted of Ti or titanium nitride (TiN) and molybdenum disulfide, are synthesized by unbalanced plasma plating. Two forms of MoS2-based composite coatings have been developed: TiN – MoS2/Ti and TiN – MoS2/TiN coatings. The effect of Ti or TiN and processing parameters on properties of such coatings are investigated. Scanning electron microscopy (SEM), dry drilling and turning tests are used to determine the structure, composition and tribological performance of MoS2-based coatings. Experimental results indicate that these MoS2-based composite coatings are considerably more wear-resistant and are less sensitive to atmospheric water vapor than pure MoS2 coatings. These coatings show excellent results when tested for a wide range of industrial turning and drilling applications. D 2004 Elsevier B.V. All rights reserved. PACS: 68.55.JK; 81.40.Pq Keywords: MoS2-based coatings; Unbalanced plasma plating; Titanium; Additives
1. Introduction Molybdenum disulfide is an attractive candidate for lowfriction coefficient, high bearing capacity solid lubricants for space applications [1,2]. However, the poor abrasion durability and oxidation resistance of the pure MoS2 coatings limit their wide use [3]. Many investigators have attempted to overcome these limitations. For instance, codeposition of metal (such as Ni, Au, Sb2S4 or LaF3) and MoS2 can improve the tribological performance of MoS2-based coatings in the higher temperature of 120 jC [4 –6]. Both ion beam-assisted deposition [7] and closedfield unbalanced magnetron sputter ion plating [8,9] are introduced for the deposition of dense MoS2 coatings. These MoS2 coatings are expected to show the highest durability and lowest friction when grown with the (001) planes of the
* Corresponding author. Department of Precision Instrument and Mechanology, State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China. Tel.: +86-10-62771438; fax: +86-10-62781379. E-mail address: [email protected] (Y. Jing). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.02.044
MoS2 crystallites parallel to the substrate surface (basal orientation). Furthermore, both metal/MoSx multilayer coatings and metal/MoSx quasi-amorphous coatings are successfully produced by codeposition with different metal additives [10,11]. Evaporated MoS2-based thin films have been extensively utilized on cutting tools and research on utilizing an ion plating technique in place of traditional electrolytic plating technique has been reported [12 – 14]. The columnar structure of coatings obtained using the ion plating technique often causes lubrication failure if the coating is thicker than 80 nm. In order to solve the problem cited above, many investigators have attempted to prepare the MoS2-based coatings on cutting tools by multiarc ion plating. This technique operates at a high-deposition rate, resulting in large-size droplets and relatively many surface defects in its product. Then, a new deposition method named unbalanced plasma plating is used to deposit the MoS2-based composite coatings. Multiarc ion plating is usually a balanced plasma deposition in which a metal vapor produced in a vacuum system is deposited onto a substrate whilst the substrate is
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289
Table 1 Respective deposition thickness for different MoS2-based coatings Type of coatings
TiN – MoS2 TiN – MoS2/Ti TiN – MoS2/TiN
Fig. 1. System diagram of unbalanced plasma plating.
simultaneously bombarded with ions. Ion bombardment improves both the adhesion and the structure of the coating. In the based ion plating method, however, the ions are produced in an abnormal glow discharge with the samples acting as cathode. This is an inefficient process and typically less than 1 atom in 1000 is ionized in an abnormal glow discharge. The ion current to the samples is low and is not sufficient to produce the dense coatings required in many applications, although the samples are held at a high negative potential. To solve this problem, the ionization can be increased in a closed and unbalanced magnetron with improved multiarc ion plating—unbalanced plasma plating. The extra field lines leaving the outer magnets trap electrons escaping from the magnetron discharge and prevent them from drifting to the various earthed parts of the chamber. These electrons cause ionization in the vicinity of the electrically biased substrate and the ions so formed are attracted to the substrate by the substrate bias, and the substrates receive higher ion current than in a situation where the magnetrons are balanced. Fig. 1 shows the diagram of unbalanced plasma plating system. The MoS2-based coatings deposited by this method have many advantages, e.g., longer wear life and
Total depostion thickness (Am)
TiN layer thickness (Am)
MoS2-based coating thickness (Am) MoS2/Ti layer
Ti layer
f2.10 f2.10 f2.10
f0.5 f0.5 f0.5
f1.60 f25 nm f1.60
f10 nm
higher resistance to oxidation by adjusting various parameters such as Ar/H2S pressure and titanium (Ti) additive content, etc.
2. Experimental details 2.1. Deposition processes and substrate materials The coatings are prepared by unbalanced plasma plating device using a cylindrical stainless steel chamber (base pressure: 210 3 Pa) with a vertical four-target configuration, and during deposition, the pressure rises to 4.510 1 Pa. The arrangement of magnetrons within the coating chamber is a closed-field arrangement, shown in Fig. 2. This arrangement effectively increases the ionization within the chamber by trapping more electrons and allows the deposition of multilayer coatings. The Mo target with a diameter of 60 mm has a purity of 99.97%, and the distance of the substrate-to-the-target distance is 50 cm. The substrate materials are hardened and polished with a roughness of 0.02 Am, and different MoS2-based coatings, whose thicknesses are typically 2.10 Am, are deposited on them. Furthermore, the substrate – target distance can be varied. The thickness of the coatings can be controlled by varying a combination of the deposition rate (target power), bias voltage of the substrate and deposition time. Before deposition, all the samples are cleaned ultrasonically and degassed at 593 K in vacuum chamber for 30 min. After that, the surfaces of the samples are cleaned by Ar+ sputter. At the beginning of the deposition, the reacting gas (H2S) and the working gas (Ar) are partly ionized by applying electric discharge in a high vacuum. Next, the evaporants and other reacting materials are deposited continuously on the substrates. In particular, there are three Mo and one-Ti targets that will be used for the TiN –MoS2/Ti coating. After adding titanium (Ti) atoms into MoS2 film,
Table 2 Drill testing parameters
Fig. 2. Schematic representation of a four-target deposit chamber with unbalanced plasma plating.
Drill coated
Work-piece
Hole depth (mm)
Rpm
Feed rate (mm/rev)
Coolant
Diameter: 8.5 mm
38CrNi3MoVA
20
600
0.13
No
290
Y. Jing et al. / Thin Solid Films 461 (2004) 288–293
Table 3 Turning testing parameters Turning tool coated
Work-piece
Depth (mm)
Speed (m/min)
Feed rate (mm/rev)
Coolant
YT14
38CrNi3MoVA
0.5
240
0.13
No
the structure of the coating changes from the columnar structure to a denser lamellar structure [15]. The main processing parameters are as follows: the initial temperature during the deposition is 120 jC; the pressure under argon ion bombardment is 0.5 Pa; the bias voltage of substrate under argon ion bombardment is 800 V; the time under argon ion bombardment is 30 min; the partial pressure of argon gas while depositing MoS2/Ti is 510 2 Pa; and the partial pressure of H2S gas is 410 1 Pa. The bias voltage of substrate under deposition is 200 V; the arc current is 75 A; the total deposition time is 85 min; and the respective monolayer deposition thickness for different MoS2-based coatings are shown in Table 1. There are three types of samples in the present paper. The first two types are YT14 turning tools and 8-mm HSS drills, which are utilized to test the wear-resistant properties and humid-resistant properties, and the other is 15152 mm flat samples cut from 1Cr18Ni9Ti steel plate, which are utilized to measure the fractography morphology and phases of coatings. Fig. 4. SEM cross-section showing TiN – MoS2/Ti multilayer coating.
2.2. Tribological tests The MoS2-based coatings are deposited onto drills and turning tools for testing under the conditions given inTables 2 and 3. These drills are tested under dry condition and compared with uncoated one. For evaluating the resistance to oxidation of the above coatings, the drills coated with MoS2-based coatings are stored for 1, 2, 5 and 10 days (d), respectively, in humid conditions up to 60 – 70% relative humidity (RH) before the tests are performed. Meanwhile, YT14 turning tools are coated with MoS2-based coatings to turn 38CrNi3MoVA at an ultralow temperature of 23 K in
liquid nitrogen and in an approximately 60 – 70% RH ambient air. Finally, the microstructure, composition and mechanical properties of the MoS2-based coatings are analyzed by scanning electron microscopy (SEM; JSM-5400), wavelength dispersive spectrometry (WDS). The fractograph of a SEM cross-section and a SEM micrograph of MoS2, MoS2/Ti or MoS2/TiN coating on the top of a TiN coating are shown in Figs. 3– 5.
3. Results and discussion 3.1. Characteristics of TiN – MoS2 coating
Fig. 3. SEM cross-section showing TiN – MoS2 coating.
The cross-section of the TiN– MoS2 coating deposited on 1Cr18Ni9Ti sample is shown in Fig. 3. The TiN – MoS2 coating shows a typical porous and columnar surface structure. The results of tribological experiments indicate that these coatings, which initially nucleate along basal orientation, can convert to edge orientation once they grow beyond a certain thickness. Spalvins [16] reported that the characteristic growth mechanism led to the formation of loose columnar MoS2 platelets at the surface layer, with large amounts of voids in between. When the MoS2 layer grew beyond 80 nm, fibrous structures began to form. When exposed to humid environments, the MoS2 layers became prone to structural degradation due to oxygen penetration, and to functional
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degradation due to excessive wear debris from broken MoS2 platelets [16]. Owing to their mechanical strength, interfacial compatibility and high oxygen affinity, Ti and TiN were selected in this study to serve as the interrupt layer, which also supports the composite structure as well as adsorbing oxygen during contact. 3.2. Characteristics of TiN –MoS2/Ti multilayer coating Based on TiN– MoS2 coating, TiN – MoS2/Ti coating is successfully produced by codepositing Ti atoms into MoS2 while the layered microstructure of TiN –MoS2/Ti coating is formed by rotating the substrate stage between those four targets. Interlayers of MoS2/Ti are deposited sequentially on the substrates, thus forming the multilayer microstructure. The cross-section with schematic diagram of the TiN – MoS2/Ti coating is shown in Fig. 4. To enhance the structural integrity of TiN –MoS2 coatings, Ti layers are deposited sequentially after each MoS2 layer to form a reinforced composite structure. In addition to densification and strengthening effect, the Ti interlayer also reacts preferentially with oxygen to form TiO2, and thus effectively suppresses adverse of MoS2 [17]. Therefore, in humid environments, the TiN –MoS2/Ti multilayer coating is highly promising as a solid lubricant, as evidenced by its prolonged wear life and resistance to oxidation. The composition of TiN – MoS2 and TiN – MoS2/Ti coatings stored in 60 –70% RH humid air for 24 h were also analyzed by using WDS in conjunction with a scanning electron microscope. The standard reference crystals of MoS2, Ti and SiO2 were applied for quantitative analysis. As shown in Table 4, the oxygen content in the TiN –MoS2/Ti decreases from 7.26 to 2.48 at.% when the depth of the coating increases from 10 to 20 nm, while the Ti content increases from 10.74 to 28.99 at.%. It indicates that the lamellar structure of TiN – MoS2/Ti coating closes a large amount of open channels for oxygen diffusion. In addition, the Ti additive acts as an oxygen getter within the film to form TiO2, leaving the MoS2 intact despite exposure to humid or oxidizing environments. Finally, the Ti interlayer serves as a diffusion barrier to block penetration of oxygen into the film. In addition, the TiN layer (about 0.5 Am) between the lubricate layer (1.60 Am) and the substrate has provided a strong support with its very high rigidity [18,19]. Meanwhile, according to phase boundary matching princi-
Fig. 5. SEM micrograph of TiN – MoS2/TiN composite coating after electrolytic corrosion treatment.
ple [20], their favorable compatibility enhances the adhesion of the coatings to the substrate. 3.3. Characteristics of TiN –MoS2/Tin composite coating During the deposition process of TiN – MoS2/Ti lamellar coating, N2 is added into H2S gas, while the Ar/H2S/ N2 pressure is varied. Meanwhile, the other interior configurations are adjusted such as fixing Ti target on top of the vacuum chamber and adjusting the distance Ti target and sample. Therefore, the TiN – MoS2/TiN composite coating with higher abrasion durability and humid resistance is successfully produced [21]. Based on the surface morphology of the coating shown in Fig. 5, the ceramic phases, i.e., TiN superfine grains, are equably distributed in the MoS2/TiN surface layer. These grains
Table 4 Quantitative analysis of MoS2-based composite coatings by WDS Type of the coatings
Depth (nm)
Ti (at.%)
Mo (at.%)
S (at.%)
O (at.%)
TiN – MoS2/Ti
10 20 20
10.74 28.99 –
32.24 27.25 38.76
49.76 41.22 44.01
7.26 2.48 17.23
TiN – MoS2
Fig. 6. Turning tests of YT14 turning tools coated with MoS2-based coatings in an ultralow temperature.
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improve the rigidity of the coating while they destroy the intrinsic lamellar structure and result in the disorder crystal orientation. Furthermore, TiN interlayer and MoS2/TiN lubricant layer are deposited in the same chamber, which effectively averts secondary pollution in the process of deposition. In addition, according to the turning test in ultralow temperature shown in Fig. 6, the turning lives of the YT14 turning tools deposited with MoS2-based coatings are obviously increased with the continuous improvement of the deposition procedures and additional materials. In particular, the working durability of the turning tools coated with TiN – MoS2/TiN composite is the longest; its abrasion durability and resistance to low temperature are obviously improved, which conforms to the results of physical – chemical analysis of MoS2-based composite coatings. 3.4. Tribological behavior of MoS2-based coatings 3.4.1. As-deposition coatings The drilling lives of drills coated with both TiN –MoS2/Ti lamellar and TiN – MoS2/TiN composite coatings before being stored in humid air increase continuously with Ti content increasing in MoS2-based lubricant layer, as shown in Fig. 7. For lamellar coatings, the best drilling life was observed when Ti content in the coating is 12.5 at.%, while the distance between the substrate and Ti target is approximately 50 cm. For the TiN – MoS2/TiN coatings, however, the Ti content value corresponding to the best drilling life has shifted from 12.5 to 13.2 at.%. MoS2-based multilayer coatings are frequently reported [22,23]. In the present study, TiN –MoS2/Ti composite coating, which exhibits a relatively higher bearing capacity and abrasion durability, could be obtained by changing process parameters compared with the lamellar structure.
Fig. 7. Number of holes drilled in 38CrNi3MoVA using drills coated with MoS2-based coatings as a function of the Ti content in the coating: (A) TiN – MoS2/Ti lamellar coating; (B) TiN – MoS2/TiN composite coating.
Fig. 8. Number of holes drilled in 38CrNi3MoVA using drills coated with MoS2-based coatings as a function of coatings storage time in 60 – 70% RH air; (A) TiN – MoS2; (B) TiN – MoS2/Ti; (C) TiN – MoS2/TiN.
3.4.2. After being stored in humid air Before being stored in humid air, the drills coated with TiN – MoS2 have better abrasion durability and the number of holes drilled is 134 (see Fig. 8.). But after being stored with 60 – 70% RH for 5 days, its abrasion durability decreases quickly. It shows that TiN– MoS2 coating does not retain desirable properties in humid atmospheres and is not suitable for industrial turning and drilling applications. However, the working durability of drills coated with TiN– MoS2/Ti or TiN – MoS2/TiN erodes slowly with the prolonging of storage time.
4. Conclusions In this study, TiN – MoS2/Ti and TiN –MoS2/TiN coatings deposited by unbalanced plasma plating provide significantly longer wear lives than single-component MoS2 coating. For effective elimination of the columnar structure of TiN – MoS2 coating, TiN –MoS2/Ti and TiN– MoS2/TiN coatings are developed by the adapted deposition conditions. For example, by adding Ti or N2 atoms in pure MoS2 and adjusting the substrate –target distance, the TiN – MoS2/Ti and TiN – MoS2/TiN coatings with typical lamellar or composite structure are successfully produced. In particular, TiN – MoS2/Ti coating consisted of interlacing MoS2 and Ti layers. WDS quantitative analysis implies that the oxygen content decreases quickly as the coating depth increases. In addition, the growth of columnar platelets of MoS2 is effectively suppressed by the introduction of Ti or TiN additives. Furthermore, the crystallites of MoS2-based lubricants become clearly refined when Ti atoms are added in the process of deposition. The structure of the coatings becomes denser, and their bearing capacity and wear life are also obviously enhanced. Subsequently, MoS2-based composite
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coatings are shown to be a suitable replacement for cutting fluid, and TiN – MoS2/Ti and TiN – MoS2/TiN coating shows a higher tribological property as well, after being stored in humid air.
Acknowledgements The authors would like to thank Tsinghua University and Beijing Institute of Technology for their valuable comments and support.
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