- Email: [email protected]
TaN multilayer coatings
Materials Characterization 58 (2007) 439 – 446
Structure, hardness and tribological properties of nanolayered TiN/TaN multilayer coatings J. An a,b,⁎, Q.Y. Zhang b a
b
Key Laboratory of Automobile Materials, Ministry of Education, Department of Materials Science and Engineering, Nanling Campus of Jilin University, Changchun 130025, People's Republic of China State Key Laboratory for Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116023, People's Republic of China Received 14 March 2006; received in revised form 5 June 2006; accepted 12 June 2006
Abstract TiN/TaN coatings, consisting of alternating nanoscaled TiN and TaN layers, were deposited using magnetron sputtering technology. The structure, hardness, tribological properties and wear mechanism were assessed using X-ray diffraction, microhardness, ball-on-disc testing and a 3-D surface profiler, respectively. The results showed that the TiN/TaN coatings exhibited a good modulation period and a sharp interface between TiN and TaN layers. In mutilayered TiN/TaN coatings, the TiN layers had a cubic structure, but a hexagonal structure emerged among the TaN layers besides the cubic structure as the modulation period went beyond 8.5 nm. The microhardness was affected by the modulation period and a maximum hardness value of 31.5 GPa appeared at a modulation period of 8.5 nm. The coefficient of friction was high and the wear resistance was improved for TiN/TaN coatings compared with a homogenous TiN coating, the wear mechanism exhibited predominantly ploughing, material transfer and local spallation. © 2006 Elsevier Inc. All rights reserved. Keywords: TiN/TaN multilayered coating; Hardness; Coefficient of friction; Wear resistance
1. Introduction Multilayers with bilayer lengths in the nanometer range exhibit significant improvements in hardness, toughness, oxidation resistance and corrosion resistance as compared to single-layered coatings. Among them, transitional nitrides/nitrides have attracted much attention recently due to their substantial strength and hardness enhancements. Mutilayers consisting of very thin
⁎ Corresponding author. State Key Laboratory for Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116023, People's Republic of China. Fax: +86 431 5095874. E-mail address: [email protected] (J. An). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.06.012
(2–10 nm) layers of nitride materials deposited by magnetron sputtering have a hardness in excess of 5000 kg/mm2 [1]. This hardness is comparable to that of cubic-BN and is second only to diamond. Therefore, the potential for the development of new hard coatings for the machine industry (for example in cutting operations) is great, using materials with good tribological properties as the individual layers of the multilayers. Up to now most of the work has been done on the mechanism for nitride superlattice hardening. A variety of mechanisms have been used to explain the enhancement, including dislocation blocking by layer interfaces, Hall–Petch strengthening and the supermodulus effect [2–4]. In recent years, hard coatings have found wide applications in cutting tools in terms of extending the
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operating life and range of conditions for which they are used. Especially hard and wear resistant TiN coatings deposited by physical vapor deposition (PVD) or by plasma assisted chemical vapor deposition (PACVD) have gained increasing importance in industry, for example in protecting cutting tools, and the tribological properties and wear mechanism have been widely investigated. On the other hand, even though several coating studies have revealed that many nitride multilayers (TiAlN/VN [5], TiN/CrN [6] and TiN/Nb [7]) can outperform the single layered TiN in terms of wear resistance, little is known about the detailed tribological behavior of the nitride multilayered coatings, such as the worn surface profile and the effect of structure and hardness on the wear mechanism. The main purpose of this paper is to investigate the structure and tribological behaviors with particular reference to the morphology of worn surfaces, and establish correlations between hardness and tribological properties of TiN/TaN multilayered coatings deposited by a reactive magnetron sputtering technique. 2. Experimental details The polycrystalline TiN/TaN nanomultilayers were deposited using a JPG450 magnetron sputtering system, which has three targets including one d.c. and two r.f. magnetron cathodes. The sputtering targets were pure Ti (99.9%) and Ta (99.9%), which were mounted on each of the r.f. cathodes. Ground and polished single-crystal silicon (111) wafers were used as substrate materials, which were chemically cleaned in an ultrasonic agitator in acetone, and absolute alcohol before being mounted in the vacuum chamber. For all coatings, the deposition sequence started with the growth of a thin (approximately 20 nm) Ti interlayer followed by a 200-nm-thick TiN layer. Both the Ti and the TiN layers were obtained with the substrates held stationary above the Ti target. After this, the substrate was rotated to the position above Ta and Ti targets alternately and was held stationary for different times to obtain a compositionally-modulated structure. The modulation ratio was obtained though exact control of the stopping times in front of the Ti and Ta targets. Typically, TiN/TaN multilayers were deposited under a base pressure of 4 × 10− 4 Pa and a total Ar + N2 gas pressure of 5.0 × 10− 1 Pa. The modulation ratio lTaN/lTiN was fixed at 3:1. The source power of Ti and Ta targets were 110 and 70 W, respectively. The total thickness of multilayers was 1.0 μm, and all the substrates were resistively heated to 723 K during deposition. Reciprocating ball-on-disc sliding experiments were performed in ambient air (60% RH) on a UMT-2 ma-
chine. 5 mm diameter hardened 52100 bearing steel balls (61HRC and 0.05 μm roughness) were employed as the counterpart, and the coatings were tested as the disc. The sliding speed was 1.0 mm/s over a stroke (track) length of 10 mm. A load of 4.0 N was used. All the experiments were run for 180 cycles. A Newview5022 3-D surface profiler was used to determine the depth profiles and hence cross-sectional areas of wear tracks. Based on the wear track diameter and depth profiles at several locations, the coating volume removed during the testing was obtained to evaluate the coating wear resistance. The modulation periods of TiN/TaN multilayers were measured by a low-angle X-ray reflectivity method using a Rigaku X-ray diffractometer (XRD) using Cu Kα radiation under conditions of 40 kV and 30 mA. The coating crystallographic structures were characterized by highangle X-ray diffraction (XRD). An atomic force microscope (AFM) was used to examine the surface morphology of the coatings. The hardness of the coatings was measured for 15 s at a load of 0.1 N using a DMH-2LS microhardness tester with a pyramidal Knoop diamond tip indenter. The morphologies of wear surfaces of coatings were examined using a Newview5022 3-D surface profiler and a JEOL8600 scanning electron microscope (SEM) with an energy dispersive X-ray spectrometer (EDS) attachment. 3. Results and discussion 3.1. Structure, surface morphology and hardness of multilayered TiN/TaN coatings Low-angle X-ray reflectivity refers to the reflection of X-rays from the interfaces between layers. Reflection peaks of different orders in the low-angle X-ray reflectivity spectra occur at 2θ positions given by the modified Bragg's law: Sin2 h ¼ ðmk=2KÞ2 þ 2d
ð1Þ
where m is the order of the reflection, λ is the X-ray wavelength, δ is related to the average reflective index n, and Λ is the modulation period of a multilayer (bilayer period). Fig. 1 represents the low-angle reflective spectra of TiN/TaN multilayers with Λ = 2.8 nm and Λ = 5.8 nm. The strong superlattice reflections indicate that the multilayered coatings deposited in this study have well-defined periodicity and abrupt interfaces, which are a beneficial characteristic for hardness enhancement. By plotting Sin2θ vs. m2 curve and fitting the data to a straight line, modulation period values can be determined from the slope of the line. All modulation periods presented in this paper were calculated using this method, and are shown in Table 1.
J. An, Q.Y. Zhang / Materials Characterization 58 (2007) 439–446
High-angle XRD analyses as shown in Fig. 2(a), (b) reveal that the single-layered TiN coating exhibits a cubic structure with a lattice constant of 0.424 nm, displays (200) preferred orientation, whereas TaN coating, mainly consists of a cubic phase with a lattice constant of 0.434 nm besides a small amount of hexagonal phase, and displays (111) preferred orientation. Fig. 2(c), (d) and (e) shows highangle X-ray diffraction spectra of TiN/TaN multilayers with different modulation periods. It was found that the TiN/TaN mutilayers were formed with a cubic structure at a modulation period less than 11.3 nm, and all the coatings exhibited first-order positive and negative satellite reflections (SR) along the (111), (200) and (311) principal reflection, thus confirming the formation of superlattice structure. Also as the modulation period was increased, the relative XRD intensity of peak (111) to that of peak (200) increased considerably. The (111) preferred orientation of hexagonal TaN0.8 became more evident as the modulation period was at 11.3 nm; meanwhile, the fraction of cubic phase of TaN decreased dramatically and a large amount of hexagonal TaN0.8 was formed. Nordin and Ericson [8] observed a similar phenomenon in a study of growth characteristics of multilayered PVD TiN/TaNx on HSS substrates. As the lamellae of TaNx grew thicker than 6 nm the fraction of the cubic phase decreased and (100) oriented hexagonal TaN0.8 appeared. This was attributed to the stabilization of the TaNx phase. Surface morphologies of the single-layered TiN, TaN coatings and multilayered TiN/TaN coatings were measured by atomic force microscopy (AFM). Fig. 3 shows the topography typical of island growth. There are two patterns seen in the islands. The first is where the lateral size decreases with an increased modulation period. The second is where the surface height (peak to valley) markedly decreases with an increased modulation period
Fig. 1. Low-angle X-ray reflection spectra of multilayered TiN/TaN coatings.
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Table 1 Modulation periods and surface roughness of TiN, TaN and multerlayered TiN/TaN coatings Coating TiN TaN TiN/TaN Coatings
Modulation period (nm)
Roughness (Ra) (nm)
2.8 3.8 5.8 7.2 8.5 11.3 14.1
0.66 5.56 2.89 1.21 1.52 1.31 1.13 0.76 0.73
and the coatings became denser and smoother. The surface roughness of multilayers is listed in Table 1. The surface roughness of multilayers is intermediate between those of the constituent single-layered TiN and TaN coatings. The roughness value of the TiN/TaN coatings at a modulation period of 2.8 nm is much higher than that of the single-layered TiN coating, but smaller than that of the single-layered TaN coating. At larger modulation periods, the surface roughness of the multilayers approaches that of the single-layered TiN coating. Based on the AFM observations and X-ray diffraction analysis, it can be speculated that this is caused by three interacting factors. The first is surface diffusion, which leads to a smooth growing surface. The second is the shadowing effect, which leads to preferred growth in a vertical direction. The third is a crystallographic orientational effect, which affects the evolution of the surface. At a small modulation period, when depositing a multilayered structure a material can “remember” its base material for some nm. This can result in an epitaxial relationship between the lamella materials in each column for a few nm and in the whole lamella if the thickness is kept small. In the present case, this means that TaN can grow a (200) orientation if grown on a (200) orientation of TiN, which is confirmed by the superlattice structure and a (200) preferred orientation from the XRD patterns shown in Fig. 2(c) and (d). However, because the deposition rate of TaN is much larger than that of TiN, the surface diffusion effect is hindered and the shadowing effect is increased, and the top TiN layer is too thin to improve the surface roughness effectively, so the surface is rather rough. At a large modulation period, the TaN layer structure changed, transformed from single phase to multiphase, in addition to this, the TiN layer thickness increases so that it can improve the surface effectively. The Knoop microhardness values for the TiN and TaN single layers are 18.0 and 22.0 Gpa, respectively. When the TiN/TaN multilayered coatings were formed,
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Fig. 2. High-angle X-ray diffraction spectra of single-layered TiN, TaN coatings and multilayered TiN/TaN coatings. (a) TiN, (b) TaN, (c) Λ = 2.8 nm, (d) Λ = 5.8 nm, (e) Λ = 11.3 nm.
J. An, Q.Y. Zhang / Materials Characterization 58 (2007) 439–446
their hardness increased considerably. Consequently, the hardness enhancement effect occurred. The variation of hardness of multilayered coatings with modulation period
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is shown in Fig. 4. The hardness of multilayered coatings initially increased rapidly with increasing modulation period, and reached a maximum value of 31.5 Gpa at a
Fig. 3. AFM images of single-layered TiN, TaN and multilayered TiN/TaN coatings. (a) TiN, (b) TaN, (c) Λ = 2.8 nm, (d) Λ = 5.8 nm, (e) Λ = 11.3 nm.
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films deposited by magnetron sputtering. The elastic modulus of TiN and TaN are 590.0 Gpa and 587.0 Gpa, respectively. Hence, it is deduced that the anomalous increase of hardness is not the contribution of an elastic modulus difference because the modulus difference is very small between TiN and TaN. While the interfacial coherency strain of the TiWN/TiN film is larger than that of the TaN/TiN film, the former produces large alternating stress fields, leading to greater hardening. The research results of Shinn et al. [10] also showed that the experimental hardness increase only equals a tenth of the hardness values calculated from Koehler's theory in the multilayer systems [2]. The hardness increase hence is considered to result from the lattice mismatch, which affects the microhardness value and the peak position for maximum hardness. Fig. 4. Variation in hardness of multilayered TiN/TaN coatings with modulation period.
modulation period of 8.5 nm. Beyond that, the hardness decreased and had a value of 23.5 Gpa at a modulation period of 14.1 nm. The maximum hardness value is 52% higher than that suggested by a simple rule for the mixture obtained from the hardness (H) and the lamellas thickness (t) of TiN and TaN, respectively, see Eq. (2): Hmultilayer ¼ HTiN tTiN =K þ HTaN tTaN =K
ð2Þ
The hardness increase around a modulation period of 8.5 nm may result from the coherency strain of lattice mismatch at the interface. According to the lattice constants of TiN and TaN, it can be calculated that lattice mismatch of TiN/TaN is about 2.3%. Xu et al. [9] obtained similar results by comparing TaN/TiN with TaWN/TiN superlattice
3.2. Tribological behaviors of multilayered coatings The variation of the coefficient of friction of TiN/TaN multilayered coatings with sliding cycles is shown in Fig. 5 (a). Based on it, the coefficient of friction can be approximately divided into two stages, i.e. a running-in stage and a steady stage. The coefficient of friction was low at the running-in stage and gradually increased to a steady value at the steady stage. The single-layered TiN coating experienced a short running-in stage, however, the TiN/ TaN multilayered coatings run about 40 cycles until the steady stage. This may be related to the higher hardness of TiN/TaN multilayered coatings. The coefficient of friction of TiN/TaN coatings was higher than that of the TiN coating; it varied in a range of 0.7–1.0 at the steady stage. The coefficient of friction increased with the increasing
Fig. 5. (a) Variation in coefficient of friction with the sliding cycles; (b) variation in wear volume loss with hardness.
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modulation period, at a modulation period of 8.5 nm, it reached up to 1.0, after that, the coefficient decreased. The variation in wear loss with the inverse hardness illustrated in Fig. 5(b) indicated that the wear resistance of multilayered coatings was better than that of TiN coating, and the wear volume increased with the decreasing hardness and exhibited a linear relationship with the inverse hardness, this is characteristic of the wear behavior of most sliding materials couples that obey the Archard relationship in which wear volume loss is inversely proportional to the hardness of the worn surface.
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3.3. Analysis of worn surfaces In order to determine the wear mechanism of the TiN/ TaN multilayered coatings, the morphology of worn surfaces with various modulation periods were analyzed using a Newview5022 3-D surface profiler. Fig. 6 shows the optical micrographs and corresponding 3-D maps of worn surfaces of TiN/TaN multilayered coatings. It is noted that the wear track mainly consists of grooves, and the width of the wear track decreased with an increase in the hardness of the multilayered coating. A small amount of
Fig. 6. Optical microphotographs and corresponding 3-D maps of worn surface of multilayered TiN/TaN coatings with different modulation: (a) 2.8 nm; (b) 2.8 nm; (c) 7.2 nm; (d) 7.2 nm; (e) 8.5 nm; (f) 8.5 nm.
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material pushed up outside the grooves can be observed on the worn track of the TiN/TaN coatings with a small modulation period, EDS analysis indicated that material transfer occurred during sliding, the materials transferred from the couple to the worn surfaces of multilayered TiN/ TaN coatings (Fig. 6(a), (b)). When the modulation period increased, local spalling of TiN/TaN coatings can be clearly observed on the worn surface (Fig. 6(c), (d)), to a certain extent, the spalling decreased the wear resistance of multilayered coatings. This may be attributed to the very high compressive stresses developed in the coating. In a paper about the wear resistance of multilayered PVD TiN/ TaN on HSS, Nordin et al. [7] revealed that the TiN/TaN coating exhibited the highest compressive stress among TiN/CrN, TiN/NbN and TiN/TaN coatings with modulation periods of about 10 nm, hence, make it suffer from poor adhesion to the substrate. With increasing modulation period, the worn surfaces become smooth (Fig. 6(e), (f )). The above observed results indicated that the dominant wear mechanisms were ploughing, material transfer and localized spalling. 4. Conclusions The study of structure and tribological properties behavior of nanolayered TiN/TaN multilayered coatings leads to the following conclusions: 1. The multilayered TiN/TaN coatings deposited by magnetron sputtering have a cubic structure, when the modulation period is less than 8.5 nm. After that, a large amount of hexagonal TaN0.8 occurred. The TiN/TaN multilayered coatings exhibited hardness enhancement effect. 2. Under the given wear conditions, the coefficient of friction of multilayered TiN/TaN coatings is higher than that of single-layered TiN coating, but the wear
resistance of the former is considerably better that of the latter. Analysis of worn surfaces revealed that the wear mechanism mainly included ploughing, material transfer and spalling. Acknowledgements The authors thank the Research Fund for the National Science Foundation of China and the National Science Foundation of Jilin Province under Grant no. 20050509. References [1] Helmersson U, Todorova S, Barnett SA, Sundgren JE, Markert LC, Greene JE. Growth of single-crystal TiN/VN strained-layer superlattices with extremely high mechanical hardness. J Appl Phys 1987;62(2):481–7. [2] Koehler JS. Attempt to design a strong solid. Phys Rev B 1970;2(2):547–51. [3] Anderson PM, Li C. Hall–Petch relation for multilayered materials. Nanostruct Mater 1995;5(3):349–62. [4] Cammarata RC, Schlesinger TE, Kim C, Qadri SB, Edelstein AS. Nanoindentation study of the mechanical properties of copper– nickel multilayered films. Appl Phys Lett 1900;56:1862–4. [5] Hovsepian PEh, Lewis DB, Munz WD. Recent progress in large scale manufacturing of multilayer/superlattice hard coatings. Surf Coat Technol 2000;133–134:166–71. [6] Nordin M, Larsson M, Hogmark S. Mechanical and tribological properties of multilayered PVD TiN/CrN. Wear 1999;232: 221–5. [7] Nordin M, Larsson M, Hogmark S. Mechanical and tribological properties of multilayered PVD TiN/CrN, TiN/MoN, TiN/NbN and TiN/TaN coatings on cemented carbide. Surf Coat Technol 1998;106:234–41. [8] Nordin M, Ericson F. Growth characteristics of multilayered physical vapour deposited TiN/TaNx on high speed steel substrate. Thin Solid Film 2001;385(1–2):174–81. [9] Xu J, Li G, Gu M. The microstructure and mechanical properties of TaN/TiN and TaWN/TiN superlattice films. Thin Solid Film 2000;370:45–9. [10] Shinn M, Hultman L, Barnett SA. Growth, structure, and microhardness of epitaxial TiN/NbN superlattice. J Mater Res 1992;7:901–11.
Structure, hardness and tribological properties of nanolayered TiN/TaN multilayer coatings J. An a,b,⁎, Q.Y. Zhang b a
b
Key Laboratory of Automobile Materials, Ministry of Education, Department of Materials Science and Engineering, Nanling Campus of Jilin University, Changchun 130025, People's Republic of China State Key Laboratory for Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116023, People's Republic of China Received 14 March 2006; received in revised form 5 June 2006; accepted 12 June 2006
Abstract TiN/TaN coatings, consisting of alternating nanoscaled TiN and TaN layers, were deposited using magnetron sputtering technology. The structure, hardness, tribological properties and wear mechanism were assessed using X-ray diffraction, microhardness, ball-on-disc testing and a 3-D surface profiler, respectively. The results showed that the TiN/TaN coatings exhibited a good modulation period and a sharp interface between TiN and TaN layers. In mutilayered TiN/TaN coatings, the TiN layers had a cubic structure, but a hexagonal structure emerged among the TaN layers besides the cubic structure as the modulation period went beyond 8.5 nm. The microhardness was affected by the modulation period and a maximum hardness value of 31.5 GPa appeared at a modulation period of 8.5 nm. The coefficient of friction was high and the wear resistance was improved for TiN/TaN coatings compared with a homogenous TiN coating, the wear mechanism exhibited predominantly ploughing, material transfer and local spallation. © 2006 Elsevier Inc. All rights reserved. Keywords: TiN/TaN multilayered coating; Hardness; Coefficient of friction; Wear resistance
1. Introduction Multilayers with bilayer lengths in the nanometer range exhibit significant improvements in hardness, toughness, oxidation resistance and corrosion resistance as compared to single-layered coatings. Among them, transitional nitrides/nitrides have attracted much attention recently due to their substantial strength and hardness enhancements. Mutilayers consisting of very thin
⁎ Corresponding author. State Key Laboratory for Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116023, People's Republic of China. Fax: +86 431 5095874. E-mail address: [email protected] (J. An). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.06.012
(2–10 nm) layers of nitride materials deposited by magnetron sputtering have a hardness in excess of 5000 kg/mm2 [1]. This hardness is comparable to that of cubic-BN and is second only to diamond. Therefore, the potential for the development of new hard coatings for the machine industry (for example in cutting operations) is great, using materials with good tribological properties as the individual layers of the multilayers. Up to now most of the work has been done on the mechanism for nitride superlattice hardening. A variety of mechanisms have been used to explain the enhancement, including dislocation blocking by layer interfaces, Hall–Petch strengthening and the supermodulus effect [2–4]. In recent years, hard coatings have found wide applications in cutting tools in terms of extending the
440
J. An, Q.Y. Zhang / Materials Characterization 58 (2007) 439–446
operating life and range of conditions for which they are used. Especially hard and wear resistant TiN coatings deposited by physical vapor deposition (PVD) or by plasma assisted chemical vapor deposition (PACVD) have gained increasing importance in industry, for example in protecting cutting tools, and the tribological properties and wear mechanism have been widely investigated. On the other hand, even though several coating studies have revealed that many nitride multilayers (TiAlN/VN [5], TiN/CrN [6] and TiN/Nb [7]) can outperform the single layered TiN in terms of wear resistance, little is known about the detailed tribological behavior of the nitride multilayered coatings, such as the worn surface profile and the effect of structure and hardness on the wear mechanism. The main purpose of this paper is to investigate the structure and tribological behaviors with particular reference to the morphology of worn surfaces, and establish correlations between hardness and tribological properties of TiN/TaN multilayered coatings deposited by a reactive magnetron sputtering technique. 2. Experimental details The polycrystalline TiN/TaN nanomultilayers were deposited using a JPG450 magnetron sputtering system, which has three targets including one d.c. and two r.f. magnetron cathodes. The sputtering targets were pure Ti (99.9%) and Ta (99.9%), which were mounted on each of the r.f. cathodes. Ground and polished single-crystal silicon (111) wafers were used as substrate materials, which were chemically cleaned in an ultrasonic agitator in acetone, and absolute alcohol before being mounted in the vacuum chamber. For all coatings, the deposition sequence started with the growth of a thin (approximately 20 nm) Ti interlayer followed by a 200-nm-thick TiN layer. Both the Ti and the TiN layers were obtained with the substrates held stationary above the Ti target. After this, the substrate was rotated to the position above Ta and Ti targets alternately and was held stationary for different times to obtain a compositionally-modulated structure. The modulation ratio was obtained though exact control of the stopping times in front of the Ti and Ta targets. Typically, TiN/TaN multilayers were deposited under a base pressure of 4 × 10− 4 Pa and a total Ar + N2 gas pressure of 5.0 × 10− 1 Pa. The modulation ratio lTaN/lTiN was fixed at 3:1. The source power of Ti and Ta targets were 110 and 70 W, respectively. The total thickness of multilayers was 1.0 μm, and all the substrates were resistively heated to 723 K during deposition. Reciprocating ball-on-disc sliding experiments were performed in ambient air (60% RH) on a UMT-2 ma-
chine. 5 mm diameter hardened 52100 bearing steel balls (61HRC and 0.05 μm roughness) were employed as the counterpart, and the coatings were tested as the disc. The sliding speed was 1.0 mm/s over a stroke (track) length of 10 mm. A load of 4.0 N was used. All the experiments were run for 180 cycles. A Newview5022 3-D surface profiler was used to determine the depth profiles and hence cross-sectional areas of wear tracks. Based on the wear track diameter and depth profiles at several locations, the coating volume removed during the testing was obtained to evaluate the coating wear resistance. The modulation periods of TiN/TaN multilayers were measured by a low-angle X-ray reflectivity method using a Rigaku X-ray diffractometer (XRD) using Cu Kα radiation under conditions of 40 kV and 30 mA. The coating crystallographic structures were characterized by highangle X-ray diffraction (XRD). An atomic force microscope (AFM) was used to examine the surface morphology of the coatings. The hardness of the coatings was measured for 15 s at a load of 0.1 N using a DMH-2LS microhardness tester with a pyramidal Knoop diamond tip indenter. The morphologies of wear surfaces of coatings were examined using a Newview5022 3-D surface profiler and a JEOL8600 scanning electron microscope (SEM) with an energy dispersive X-ray spectrometer (EDS) attachment. 3. Results and discussion 3.1. Structure, surface morphology and hardness of multilayered TiN/TaN coatings Low-angle X-ray reflectivity refers to the reflection of X-rays from the interfaces between layers. Reflection peaks of different orders in the low-angle X-ray reflectivity spectra occur at 2θ positions given by the modified Bragg's law: Sin2 h ¼ ðmk=2KÞ2 þ 2d
ð1Þ
where m is the order of the reflection, λ is the X-ray wavelength, δ is related to the average reflective index n, and Λ is the modulation period of a multilayer (bilayer period). Fig. 1 represents the low-angle reflective spectra of TiN/TaN multilayers with Λ = 2.8 nm and Λ = 5.8 nm. The strong superlattice reflections indicate that the multilayered coatings deposited in this study have well-defined periodicity and abrupt interfaces, which are a beneficial characteristic for hardness enhancement. By plotting Sin2θ vs. m2 curve and fitting the data to a straight line, modulation period values can be determined from the slope of the line. All modulation periods presented in this paper were calculated using this method, and are shown in Table 1.
J. An, Q.Y. Zhang / Materials Characterization 58 (2007) 439–446
High-angle XRD analyses as shown in Fig. 2(a), (b) reveal that the single-layered TiN coating exhibits a cubic structure with a lattice constant of 0.424 nm, displays (200) preferred orientation, whereas TaN coating, mainly consists of a cubic phase with a lattice constant of 0.434 nm besides a small amount of hexagonal phase, and displays (111) preferred orientation. Fig. 2(c), (d) and (e) shows highangle X-ray diffraction spectra of TiN/TaN multilayers with different modulation periods. It was found that the TiN/TaN mutilayers were formed with a cubic structure at a modulation period less than 11.3 nm, and all the coatings exhibited first-order positive and negative satellite reflections (SR) along the (111), (200) and (311) principal reflection, thus confirming the formation of superlattice structure. Also as the modulation period was increased, the relative XRD intensity of peak (111) to that of peak (200) increased considerably. The (111) preferred orientation of hexagonal TaN0.8 became more evident as the modulation period was at 11.3 nm; meanwhile, the fraction of cubic phase of TaN decreased dramatically and a large amount of hexagonal TaN0.8 was formed. Nordin and Ericson [8] observed a similar phenomenon in a study of growth characteristics of multilayered PVD TiN/TaNx on HSS substrates. As the lamellae of TaNx grew thicker than 6 nm the fraction of the cubic phase decreased and (100) oriented hexagonal TaN0.8 appeared. This was attributed to the stabilization of the TaNx phase. Surface morphologies of the single-layered TiN, TaN coatings and multilayered TiN/TaN coatings were measured by atomic force microscopy (AFM). Fig. 3 shows the topography typical of island growth. There are two patterns seen in the islands. The first is where the lateral size decreases with an increased modulation period. The second is where the surface height (peak to valley) markedly decreases with an increased modulation period
Fig. 1. Low-angle X-ray reflection spectra of multilayered TiN/TaN coatings.
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Table 1 Modulation periods and surface roughness of TiN, TaN and multerlayered TiN/TaN coatings Coating TiN TaN TiN/TaN Coatings
Modulation period (nm)
Roughness (Ra) (nm)
2.8 3.8 5.8 7.2 8.5 11.3 14.1
0.66 5.56 2.89 1.21 1.52 1.31 1.13 0.76 0.73
and the coatings became denser and smoother. The surface roughness of multilayers is listed in Table 1. The surface roughness of multilayers is intermediate between those of the constituent single-layered TiN and TaN coatings. The roughness value of the TiN/TaN coatings at a modulation period of 2.8 nm is much higher than that of the single-layered TiN coating, but smaller than that of the single-layered TaN coating. At larger modulation periods, the surface roughness of the multilayers approaches that of the single-layered TiN coating. Based on the AFM observations and X-ray diffraction analysis, it can be speculated that this is caused by three interacting factors. The first is surface diffusion, which leads to a smooth growing surface. The second is the shadowing effect, which leads to preferred growth in a vertical direction. The third is a crystallographic orientational effect, which affects the evolution of the surface. At a small modulation period, when depositing a multilayered structure a material can “remember” its base material for some nm. This can result in an epitaxial relationship between the lamella materials in each column for a few nm and in the whole lamella if the thickness is kept small. In the present case, this means that TaN can grow a (200) orientation if grown on a (200) orientation of TiN, which is confirmed by the superlattice structure and a (200) preferred orientation from the XRD patterns shown in Fig. 2(c) and (d). However, because the deposition rate of TaN is much larger than that of TiN, the surface diffusion effect is hindered and the shadowing effect is increased, and the top TiN layer is too thin to improve the surface roughness effectively, so the surface is rather rough. At a large modulation period, the TaN layer structure changed, transformed from single phase to multiphase, in addition to this, the TiN layer thickness increases so that it can improve the surface effectively. The Knoop microhardness values for the TiN and TaN single layers are 18.0 and 22.0 Gpa, respectively. When the TiN/TaN multilayered coatings were formed,
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Fig. 2. High-angle X-ray diffraction spectra of single-layered TiN, TaN coatings and multilayered TiN/TaN coatings. (a) TiN, (b) TaN, (c) Λ = 2.8 nm, (d) Λ = 5.8 nm, (e) Λ = 11.3 nm.
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their hardness increased considerably. Consequently, the hardness enhancement effect occurred. The variation of hardness of multilayered coatings with modulation period
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is shown in Fig. 4. The hardness of multilayered coatings initially increased rapidly with increasing modulation period, and reached a maximum value of 31.5 Gpa at a
Fig. 3. AFM images of single-layered TiN, TaN and multilayered TiN/TaN coatings. (a) TiN, (b) TaN, (c) Λ = 2.8 nm, (d) Λ = 5.8 nm, (e) Λ = 11.3 nm.
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films deposited by magnetron sputtering. The elastic modulus of TiN and TaN are 590.0 Gpa and 587.0 Gpa, respectively. Hence, it is deduced that the anomalous increase of hardness is not the contribution of an elastic modulus difference because the modulus difference is very small between TiN and TaN. While the interfacial coherency strain of the TiWN/TiN film is larger than that of the TaN/TiN film, the former produces large alternating stress fields, leading to greater hardening. The research results of Shinn et al. [10] also showed that the experimental hardness increase only equals a tenth of the hardness values calculated from Koehler's theory in the multilayer systems [2]. The hardness increase hence is considered to result from the lattice mismatch, which affects the microhardness value and the peak position for maximum hardness. Fig. 4. Variation in hardness of multilayered TiN/TaN coatings with modulation period.
modulation period of 8.5 nm. Beyond that, the hardness decreased and had a value of 23.5 Gpa at a modulation period of 14.1 nm. The maximum hardness value is 52% higher than that suggested by a simple rule for the mixture obtained from the hardness (H) and the lamellas thickness (t) of TiN and TaN, respectively, see Eq. (2): Hmultilayer ¼ HTiN tTiN =K þ HTaN tTaN =K
ð2Þ
The hardness increase around a modulation period of 8.5 nm may result from the coherency strain of lattice mismatch at the interface. According to the lattice constants of TiN and TaN, it can be calculated that lattice mismatch of TiN/TaN is about 2.3%. Xu et al. [9] obtained similar results by comparing TaN/TiN with TaWN/TiN superlattice
3.2. Tribological behaviors of multilayered coatings The variation of the coefficient of friction of TiN/TaN multilayered coatings with sliding cycles is shown in Fig. 5 (a). Based on it, the coefficient of friction can be approximately divided into two stages, i.e. a running-in stage and a steady stage. The coefficient of friction was low at the running-in stage and gradually increased to a steady value at the steady stage. The single-layered TiN coating experienced a short running-in stage, however, the TiN/ TaN multilayered coatings run about 40 cycles until the steady stage. This may be related to the higher hardness of TiN/TaN multilayered coatings. The coefficient of friction of TiN/TaN coatings was higher than that of the TiN coating; it varied in a range of 0.7–1.0 at the steady stage. The coefficient of friction increased with the increasing
Fig. 5. (a) Variation in coefficient of friction with the sliding cycles; (b) variation in wear volume loss with hardness.
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modulation period, at a modulation period of 8.5 nm, it reached up to 1.0, after that, the coefficient decreased. The variation in wear loss with the inverse hardness illustrated in Fig. 5(b) indicated that the wear resistance of multilayered coatings was better than that of TiN coating, and the wear volume increased with the decreasing hardness and exhibited a linear relationship with the inverse hardness, this is characteristic of the wear behavior of most sliding materials couples that obey the Archard relationship in which wear volume loss is inversely proportional to the hardness of the worn surface.
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3.3. Analysis of worn surfaces In order to determine the wear mechanism of the TiN/ TaN multilayered coatings, the morphology of worn surfaces with various modulation periods were analyzed using a Newview5022 3-D surface profiler. Fig. 6 shows the optical micrographs and corresponding 3-D maps of worn surfaces of TiN/TaN multilayered coatings. It is noted that the wear track mainly consists of grooves, and the width of the wear track decreased with an increase in the hardness of the multilayered coating. A small amount of
Fig. 6. Optical microphotographs and corresponding 3-D maps of worn surface of multilayered TiN/TaN coatings with different modulation: (a) 2.8 nm; (b) 2.8 nm; (c) 7.2 nm; (d) 7.2 nm; (e) 8.5 nm; (f) 8.5 nm.
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material pushed up outside the grooves can be observed on the worn track of the TiN/TaN coatings with a small modulation period, EDS analysis indicated that material transfer occurred during sliding, the materials transferred from the couple to the worn surfaces of multilayered TiN/ TaN coatings (Fig. 6(a), (b)). When the modulation period increased, local spalling of TiN/TaN coatings can be clearly observed on the worn surface (Fig. 6(c), (d)), to a certain extent, the spalling decreased the wear resistance of multilayered coatings. This may be attributed to the very high compressive stresses developed in the coating. In a paper about the wear resistance of multilayered PVD TiN/ TaN on HSS, Nordin et al. [7] revealed that the TiN/TaN coating exhibited the highest compressive stress among TiN/CrN, TiN/NbN and TiN/TaN coatings with modulation periods of about 10 nm, hence, make it suffer from poor adhesion to the substrate. With increasing modulation period, the worn surfaces become smooth (Fig. 6(e), (f )). The above observed results indicated that the dominant wear mechanisms were ploughing, material transfer and localized spalling. 4. Conclusions The study of structure and tribological properties behavior of nanolayered TiN/TaN multilayered coatings leads to the following conclusions: 1. The multilayered TiN/TaN coatings deposited by magnetron sputtering have a cubic structure, when the modulation period is less than 8.5 nm. After that, a large amount of hexagonal TaN0.8 occurred. The TiN/TaN multilayered coatings exhibited hardness enhancement effect. 2. Under the given wear conditions, the coefficient of friction of multilayered TiN/TaN coatings is higher than that of single-layered TiN coating, but the wear
resistance of the former is considerably better that of the latter. Analysis of worn surfaces revealed that the wear mechanism mainly included ploughing, material transfer and spalling. Acknowledgements The authors thank the Research Fund for the National Science Foundation of China and the National Science Foundation of Jilin Province under Grant no. 20050509. References [1] Helmersson U, Todorova S, Barnett SA, Sundgren JE, Markert LC, Greene JE. Growth of single-crystal TiN/VN strained-layer superlattices with extremely high mechanical hardness. J Appl Phys 1987;62(2):481–7. [2] Koehler JS. Attempt to design a strong solid. Phys Rev B 1970;2(2):547–51. [3] Anderson PM, Li C. Hall–Petch relation for multilayered materials. Nanostruct Mater 1995;5(3):349–62. [4] Cammarata RC, Schlesinger TE, Kim C, Qadri SB, Edelstein AS. Nanoindentation study of the mechanical properties of copper– nickel multilayered films. Appl Phys Lett 1900;56:1862–4. [5] Hovsepian PEh, Lewis DB, Munz WD. Recent progress in large scale manufacturing of multilayer/superlattice hard coatings. Surf Coat Technol 2000;133–134:166–71. [6] Nordin M, Larsson M, Hogmark S. Mechanical and tribological properties of multilayered PVD TiN/CrN. Wear 1999;232: 221–5. [7] Nordin M, Larsson M, Hogmark S. Mechanical and tribological properties of multilayered PVD TiN/CrN, TiN/MoN, TiN/NbN and TiN/TaN coatings on cemented carbide. Surf Coat Technol 1998;106:234–41. [8] Nordin M, Ericson F. Growth characteristics of multilayered physical vapour deposited TiN/TaNx on high speed steel substrate. Thin Solid Film 2001;385(1–2):174–81. [9] Xu J, Li G, Gu M. The microstructure and mechanical properties of TaN/TiN and TaWN/TiN superlattice films. Thin Solid Film 2000;370:45–9. [10] Shinn M, Hultman L, Barnett SA. Growth, structure, and microhardness of epitaxial TiN/NbN superlattice. J Mater Res 1992;7:901–11.