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W2N multilayer films deposited by DC magnetron sputtering
Surface & Coatings Technology 204 (2009) 470–476
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Microstructure, mechanical and tribological properties of CrN/W2N multilayer films deposited by DC magnetron sputtering R.L. Li a, J.P. Tu a,⁎, C.F. Hong a, D.G. Liu a, H.L. Sun b a b
Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Teer Films Ltd. Berry Hill Industrial Estate, Droitwich Worcestershire WR9 9AS, UK
a r t i c l e
i n f o
Article history: Received 9 January 2009 Accepted in revised form 11 August 2009 Available online 19 August 2009 Keywords: Metal nitride Multilayer Bilayer period Magnetron sputtering Mechanical property Tribological property
a b s t r a c t CrN/W2N multilayer films with various bilayer periods of 15–85 nm were deposited on high speed steel (W18Cr4V) substrates by means of DC closed field unbalanced magnetron sputtering. The morphology and microstructure of the multilayer films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The mechanical and tribological properties were evaluated using a nanoindentor, Rockwell and scratch tests and a conventional ball-on-disk tribometer, respectively. There were some transverse grains at the layer interface and the interface between the CrN and W2N layers was not so sharp owing to atom diffusion through the interface. In the bilayer period range, the microhardness, elastic modulus and adhesive strength of the CrN/W2N multilayer films increased with the decrease of bilayer period. The CrN/W2N multilayer film with a bilayer period of 15 nm showed the highest hardness (29.2 GPa), elastic modulus (376 GPa) and best adhesion strength, it also had the highest wear resistance and lowest friction coefficient. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured films have recently attracted tremendous interest due to their versatile characteristics and superior performance. Among various films, nano-multilayer or nanocomposite systems have been the focused issue [1–3]. Due to excellent properties in hardness, adhesive strength, wear and corrosion resistance, transition-metal nitrides are commonly applied as hard and protective films on tool steel [4–7]. In the nitride systems, chromium nitride films not only possess of the above performance, but also have superior oxidation resistance to 700 °C. Therefore, CrN films have been widely used as protective films for various forming and casting applications such as drawing dies, molds, etc [8,9]. Several studies have shown that the addition of the third element, such as W, Al, can optimize the overall performance of the CrN films or combine desired properties [10,11]. Also, the hard tungsten nitride films were usually used in wear resistant application [12]. Recently, a number of studies on the nitride multilayer films relating the manufacture and various characteristics have been conducted [13–24]. Wong et al. [13] and Ducros et al. [14,15] revealed that multilayer films composed of two kinds of transition-metal nitride layers often show excellent mechanical properties, as compared to single layered nitride films. As the grain boundaries in multilayer structure of the films could restrain the growth of grain size, deflect the
⁎ Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail address: [email protected] (J.P. Tu). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.08.010
cracks into a direction parallel to the interfaces and the film surface, and reduce the stress concentration, consequently it can improve the mechanical properties and wear resistance [16,17]. As for the system of CrN/WN multilayer films, Wu et al. [18] reported detailed deposition method, microstructure and mechanical properties. Also the multilayer morphology was clearly revealed through transmission electron microscopy (TEM). However, little work has been performed in the investigation of the tribological property, even though it plays an important role in determining the use of life as a protective film on tool steel. Gu et al. [19] in our research group discussed the mechanical and tribological properties of CrN/W2N and CrN/Mo2N multilayer films deposited by means of DC reactive magnetron sputtering. But the detailed discussion of hardness enhancement and wear mechanism was not conducted. In this present work, the effects of microstructure on the mechanical and tribological properties of CrN/W2N multilayer films as a function of bilayer period were further investigated.
2. Experimental 2.1. Deposition process The quench-and-temper high speed steel (W18Cr4V, Rockwell C hardness of HRC 63), 30 mm in diameter, was used as substrate materials. The high speed steel substrate was polished to Ra ≤ 0.2 µm by using emery paper and diamond spray. All the substrates were ultrasonically cleaned in acetone for 30 min and rinsed with alcohol before being dried and placed into the deposition chamber. Silicon
R.L. Li et al. / Surface & Coatings Technology 204 (2009) 470–476
(100) wafers were gradually cleaned in hydrofluoric acid and ammonia, for getting rid of impurity and adsorption materials. The deposition of CrN/W2N multilayer films was carried out using a TEER 650 closed field unbalanced magnetron sputtering coater. Two chromium targets and two tungsten targets (380 cm × 175 cm in sizes), located about 15 cm away from the substrates, were alternative arranged in the chamber. The vacuum chamber was evacuated to a pressure lower than 5 × 10− 3 Pa. Prior to deposition, the substrates were subject to Ar+ bombardment at a bias of − 500 V for 30 min to remove the surface oxide layer. Then Cr interlayer with the thickness of 30 nm was deposited almost immediately after cleaning, in order to enhance the adhesion of the film to the substrate. After that, the Cr and W targets were co-sputtered at the working pressure of 0.2 Pa in the mixture of Ar + N2. The flux of Ar and N2 was controlled by optical emission monitor. During the deposition process, a pulsed bias of −100 V and the temperature of 200 °C were applied to the substrates. For all the experiments, the target current was fixed at 3 A for Cr and 5 A for W respectively. Through the above deposition process, CrN/W2N multilayer films with different bilayer periods, respectively 85 nm, 38 nm and 15 nm, and a CrWN composite film were obtained. The variation of the film structure was manipulated by altering the rotation velocity of the substrate holder, in the range of 0.5 to 4 rpm. In addition, the rotating substrate holder can provide a uniform exposure of the growing films. The films deposited on silicon wafer were also prepared at the same parameters for structure analysis. The detailed parameters for the deposition process are summarized in Table 1. In this work, the thicknesses of all the films were in the range of 2.7–3.0 μm. 2.2. Microstructure, mechanical and tribological properties The surface and cross-section morphologies were investigated by scanning electron microscopy (SEM, Hitachi S-4700 equipped with GENENIS 4000 EDAX director). The microstructure of the films was characterized by multifunctional high resolution transmission electron microscopy (HRTEM, JEOL CM200UT) and X-ray diffraction (XRD, Thermo X' Pert PRO X-ray diffractometer with Cu Kα1 irradiation) at a grazing angle of 4° and multifunctional high resolution transmission electron microscopy (HRTEM, JEOL CM200UT). As the existence of residual stresses, the difference of XRD patterns between CrN and W2N peak is too small to identify accurately, so step scanning was conducted. The step interval was 0.02° and the scanning rate was 0.04° min− 1. Procedures for cross-sectional TEM specimen preparation were described in detail as follows. Two coating surfaces were glued face to face and then cut to several slices. After mechanical grinding and polishing, the slice specimen was dimpled by Ar+ milling using a Gatan Precision Ion Polishing System (PIPS, Model 691) at 4.5 keV at an angle of 10°. The microhardness and elastic modulus measurement of the films were carried out by MTS Nanoindenter XP using a load of 470 mN. Adhesion tests were performed on the multilayer films through the Rockwell and scratch test. Standard scratch tests, done with a conventional scratch tester (WS-2002 equipped with an acoustic emission detector), were used to assess the transverse adhesion of the
Table 1 Parameters for the deposition of the CrN/W2N multilayer and composite films. Process
Pressure (Pa)
Substrate bias (V)
Target current (A)
Duration time (s)
Substrate cleaning Cr interlayer deposition Film deposition Refrigeration and venting
5.0×10− 3 4.0×10− 1
− 500 − 100
0 Cr: 3; W: 0.2
1800 300
2.0×10− 1 –
− 100 0
Cr: 3; W: 5 0
7200 1800
471
coating. For these scratch tests, a diamond pin (0.2 mm in radius) was drawn across the surface of film at a constant linear velocity of 10 mm min− 1, while increasing the load linearly from 10 to 85 N. The hardness, elastic modulus and critical load data are the mean of the five values performed on it. The tribological properties of the CrN/W2N multilayer films deposited on high speed steel substrate were performed on a WTM1E ball-on-disk tribometer in humid air (relative humidity 50%). A Si3N4 ceramic ball (3 mm in diameter, hardness HV = 1550) was used as the counter body, and the obtained film specimen served as the disk. The tests were carried out at a normal load of 1 N, a sliding velocity of 0.11 m s− 1. The friction coefficient was monitored continuously during the experiments by a linear variable displacement transducer and recorded on a data acquisition computer attached to the tribometer. After the number of testing cycle reached 1.05 × 104, the worn surfaces were observed by an optical microscope (OM, Nikon Eclipse ME600D). The wear rate of the CrN/W2N multilayer films were calculated from measuring the traces of surface profiles (mean of four traces per sample, every two spacing 90°) taken across the wear track using a Dektak 3 optical profilometer. Four samples for each kind of the films were chosen for the wear tests to ensure the reproducibility of the results. 3. Results and discussion 3.1. Morphology and structural characterization Cross-sectional SEM morphologies of the CrN/W2N multilayer films with different bilayer periods and the CrWN composite film are shown in Fig. 1. As seen in Fig. 1a, b and c, all the multilayer films have a dense columnar structure, and are composed of a layered configuration by alternating stacking lighter chromium nitride and darker tungsten nitride phases. The CrN layer appears brighter than W2N layer due to the lower scattering factor of Cr as compared to W. The thickness of CrN and W2N bilayer, also called modulation period (λ), is evaluated, respectively 85 nm, 38 nm, and 15 nm. Interfaces between CrN and W2N layers in the CrN/W2N multilayer film with bilayer periods of 15–85 nm are clearly revealed. As seen in Fig. 1d, the multilayer structure of the film deposited at high rotation velocity of the substrate holder can hardly be found by the observation from secondary electron imaging (SEI) and backscattered electron imaging (BEI). When the bilayer period gets shorter, the effect of interface between CrN and W2N layer changes more remarkably, and the atoms in two adjacent layers diffuse to facing layer much easier. So the CrN layer gets mixed with the adjacent W2N layer, which leads to the formation of composite structure. The detailed layer morphology and crystal phase of the CrN/W2N film with a bilayer period of 15 nm are analyzed from TEM and selected area electron diffraction pattern (SAED) focused on the CrN layer. An evident dense and multilayer structure by sequentially altering of CrN and W2N layers is again verified (Fig. 2a). However, at the layer interface there are some transverse grains and the interface between the two adjacent layers is not so sharp from high resolution electron microscopy (HRTEM) in Fig. 2b, owing to atoms diffusion through the adjacent layers. The crystal CrN and W2N are obviously seen and the crystal sizes are from 5 to 10 nm. Some small CrN grains overlapped that causes formation of Moire fringes. The SAED pattern confirms the f.c.c structure and (111), (200) preferred crystal orientation of nanocrystal CrN. Fig. 3 shows the XRD patterns of the deposited films. As the existence of residual stresses, the difference between CrN peak and W2N peak is too small to identify accurately [20], so step scanning was conducted. In the CrN/W2N multilayer films with the bilayer periods of 85 nm and 15 nm, two diffraction peaks can be defined clearly at angles of 37° and 43°, corresponding to (111), (200) crystal planes of both CrN and W2N phases. This can also be demonstrated in the SAED
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pattern in Fig. 2. But in the CrWN composite film, only one diffraction peak can be distinctly defined at an angle of around 37°, corresponding to (111) crystal plane. The intensity of peak at a diffraction angle of 43° is so weak to recognize hardly, which means the crystal preferred orientation of the composite film is (111). The composite structure attributes to the diffusion of atoms in the adjacent CrN and W2N layers, which leads to the mixture of two layers. Also W may just be the incorporation as solid solution agent to the CrN crystalline structure in the deposition process. Although residual stress exists, the effect of which on microstrain (which affects the XRD peak width) would be small. Ignoring this effect, the crystallite size of CrN can be estimated using Debye– Scherrer formula [25] D=
Kλ : β cosðθÞ
ð1Þ
Where K is a constant (K = 0.91), D is the mean crystalline dimension normal to diffracting planes, λ is the X-ray wavelength (λ = 0.15406 nm), β in radian is the peak width at half-maximum height, and θ is the Bragg's angle. The crystallite sizes of CrN in all the films are calculated from the formula and shown in Table 2. With decreasing the bilayer period, the grain size becomes smaller. As for the CrN/W2N multilayer with a bilayer period of 15 nm, the crystallite size is 6.5 nm, which is in good agreement with that shown in HRTEM. 3.2. Mechanical properties The mechanical properties of the CrN/W2N multilayer films with different bilayer periods and composite film are shown in Table 2.
Both the microhardness and elastic modulus followed the same trend. The maximum values achieve to approximately 29.2 GPa and 376 GPa at the CrN/W2N multilayer film with a bilayer period of 15 nm, corresponding to the CrN crystallite size of 6.5 nm. The thickness of the single CrN or W2N layer in the multilayer films was controlled in the range of a few nanometers, thus the crystallite size was fixed to a nanometer scale by the boundaries, also demonstrated in HRTEM, resulting in hardness enhancement. Ducros et al. [14,15] revealed that the hardening by decreasing periods due to both grain size refinement and the high number of layer interface. The alternating properties of the different phases in the neighboring layers make the dislocation movement blocked [26–28]. The lattice constant of CrN and W2N is different (CrN: a = 0.414 nm and W2N: a = 0.413 nm, respectively) and the crystalline orientation at the interfaces are observed dissimilar in HRTEM (Fig. 2b), resulting in an increase of internal strain. The internal strain hampers the dislocation movement, so the hardness is enhanced. Meanwhile, the force required to move a dislocation across a phase boundary increases with the difference in shear modulus between the different phases present in the films [29]. Due to above reasons, the mechanical properties of multilayer films with the lower bilayer periods have been improved as compared to single layered or nanocomposite films. As shown in Table 2, the hardness and elastic modulus values of the CrWN composite film play down because the impact of interface does not exist and the crystal grains grow vertically without interruption. The adhesion of the CrN/W2N multilayer films to the substrate was tested through Rockwell and scratch test. The Rockwell craters and scratch traces of the films are shown in Fig. 4. The bilayer period appears to play an important role in the adhesive strength. There are obvious cracks and delamination around the Rockwell crater of the
Fig. 1. Cross-sectional SEM morphologies of CrN/W2N multilayer and composite films: (a) λ = 85 nm, (b) λ = 38 nm, (c) λ = 15 nm, (d) composite film.
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Table 2 Microstructure and mechanical properties of the CrN/W2N multilayer and composite films. Films
λ = 85 nm
λ = 38 nm
λ = 15 nm
Composite
Crystallite size of CrN (nm) E (GPa) H (GPa) H/E* H3/E*2 (GPa) Critical load (N)
9.6 304 20.5 0.061 0.077 55
7.1 323 22.0 0.062 0.085 63
6.5 376 29.2 0.071 0.146 >85
11.5 312 23.3 0.068 0.108 > 85
appear in the Rockwell and scratch craters (Fig. 4c and g). For the CrWN composite film, as shown in Fig. 4d and h, though there are no apparent delamination, the fine cracks still appear at the bottom, and then expand around of the Rockwell crater. The critical loads for these films obtained by scratch tests are also listed in Table 2.
3.3. Tribological properties
Fig. 2. (a) TEM images and inserted SAED pattern and (b) HRTEM images of the CrN/ W2N multilayer film with a bilayer period of 15 nm.
multilayer film with a bilayer period of 85 nm (Fig. 4a). Also, some fragments can be distinctly seen around the scratch crater (Fig. 4e). The cracks and delamination of the multilayer film with a bilayer period of 38 nm decrease to some extent (Fig. 4b and f). For the CrN/ W2N multilayer film with a bilayer period of 15 nm, although it has the highest microhardness, shows the best adhesion, and no cracks
Fig. 3. XRD patterns of CrN/W2N multilayer and composite films.
The friction behavior of all the deposited films was evaluated by the pin-on-disk wear test system under the same condition. After friction test, the wear traces of these films were observed by means of SEM and optical microscope. Fig. 5 gives the friction coefficient of CrN/W2N multilayer and composite films as a function of duration time at a normal load of 1 N and a sliding velocity of 0.11 m s− 1. It is shown that the friction curves of the CrN/W2N multilayer films are stable with little fluctuation and the values of friction coefficient are all between 0.3 and 0.4. A closer look on the CrN/W2N multilayer film with a bilayer period, it exhibits that the friction coefficient increases rapidly from an initial low value then comes down gradually and at last it almost attains a constant value. Friction force is related to the surface roughness of the mating surface and the asperity inclination angle [29]. In the initial stage, as shown in Fig. 6a, the asperities on the mating surface are grinded, resulting in roughness augmentation and friction coefficient increasing rapidly. After a number of cycles, as shown in Fig. 6b, the asperities are crushed smaller, and the inclination angles of them are reduced, which make the mating surface more smooth and lead to lower friction coefficient. At the later stage, smaller wear particles fill the gaps between the asperities and provide a smooth surface for sliding, so the friction coefficient keeps at a constant value, which can be considered as the real friction coefficient value. Comparing the four friction curves, it can be seen that the friction coefficient decreases gradually with the decrease of bilayer period. When the bilayer period gets to 15 nm, the average friction coefficient of the CrN/W2N multilayer film achieved the minimum value of 0.315. However, the friction coefficient of the CrWN composite film begins to mount up. The wear trace of the two kinds of films is compared in Fig. 7. As seen in Fig. 7a, the wear trace of the multilayer film is shallow, narrow and no cracks are found around it. But for the CrWN composite film, the wear trace becomes broad and deep. Meanwhile, many cracks as well as some particles are stacking on the worn surface. It can be concluded that the wear resistance of the multilayer film with a bilayer period of 15 nm is better than that of the composite film. Fig. 8 indicates that the wear rate followed the same trend as the friction coefficient. The CrN/W2N multilayer film with a bilayer period of 15 nm has the best wear resistance. Many researches show that wear mechanism has nothing to do with hardness directly, but seems to be controlled by friction. Also, some mechanical properties like toughness or resilience have been proposed to be useful to explain the wear resistance of materials [30]. Toughness can serve as the evaluation of a material to absorb energy during deformation up to fracture, which can be estimated according to the H/E* ratio. The resilience can be defined as the resistance of plastic deformation, which is related to the H3/E*2 ratio. As shown in Table 2, the CrN/W2N
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Fig. 4. Optical images of Rockwell and scratch craters for the CrN/W2N multilayer and composite films. Rockwell craters: (a) λ = 85 nm, (b) λ = 38 nm, (c) λ = 15 nm, (d) composite film; scratch craters: (e) λ = 85 nm nm, (f) λ = 38 nm, (g) λ = 15 nm, (h) composite film.
multilayer film with a bilayer period of 15 nm has the highest values of the H/E* and H3/E*2 ratios, so it shows the best tribological properties.
The above parameters can sometimes evaluate the tribological properties, but real wear mechanism is much more complex. The performance of wear resistance depends not only on the tested material,
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Fig. 5. Friction coefficient of CrN/W2N multilayer and composite films as a function of duration time at a normal load of 1 N and a sliding velocity of 0.11 m s− 1.
but also on the environmental temperature and humidity conditions, chemical reactions at the contact region, the material of counterpart, etc. 4. Conclusions Homogeneous and compact CrN/W2N multilayer films with various bilayer periods of 15–85 nm were deposited on high speed steel (W18Cr4V) substrates by means of DC closed field unbalanced magnetron sputtering. With the decrease of bilayer period, the microhardness, elastic modulus and adhesive strength of the CrN/W2N multilayer films increase. In addition to mechanical properties, the tribological Fig. 7. Comparison of worn surface between multilayer and composite films: (a) multilayer film with a bilayer period of 15 nm, (b) composite film.
Fig. 8. Tribological properties of the CrN/W2N multilayer and composite films at a normal load of 1 N and a sliding velocity of 0.11 m s− 1.
property of the CrN/W2N multilayer films is also improved with the decrease of bilayer period. The multilayer film with a bilayer period of 15 nm has the highest wear resistance and lowest friction coefficient. Friction and wear mechanism is complex, and can be influenced by some mechanical properties like toughness or resilience. Acknowledgment Fig. 6. Different stages in the friction test procedure of CrN/W2N multilayer film with a bilayer period of 15 nm: (a) the initial stage, (b) the later stage.
This work was supported by the National Natural Science Foundation of China (No. 50871102).
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Microstructure, mechanical and tribological properties of CrN/W2N multilayer films deposited by DC magnetron sputtering R.L. Li a, J.P. Tu a,⁎, C.F. Hong a, D.G. Liu a, H.L. Sun b a b
Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Teer Films Ltd. Berry Hill Industrial Estate, Droitwich Worcestershire WR9 9AS, UK
a r t i c l e
i n f o
Article history: Received 9 January 2009 Accepted in revised form 11 August 2009 Available online 19 August 2009 Keywords: Metal nitride Multilayer Bilayer period Magnetron sputtering Mechanical property Tribological property
a b s t r a c t CrN/W2N multilayer films with various bilayer periods of 15–85 nm were deposited on high speed steel (W18Cr4V) substrates by means of DC closed field unbalanced magnetron sputtering. The morphology and microstructure of the multilayer films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The mechanical and tribological properties were evaluated using a nanoindentor, Rockwell and scratch tests and a conventional ball-on-disk tribometer, respectively. There were some transverse grains at the layer interface and the interface between the CrN and W2N layers was not so sharp owing to atom diffusion through the interface. In the bilayer period range, the microhardness, elastic modulus and adhesive strength of the CrN/W2N multilayer films increased with the decrease of bilayer period. The CrN/W2N multilayer film with a bilayer period of 15 nm showed the highest hardness (29.2 GPa), elastic modulus (376 GPa) and best adhesion strength, it also had the highest wear resistance and lowest friction coefficient. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured films have recently attracted tremendous interest due to their versatile characteristics and superior performance. Among various films, nano-multilayer or nanocomposite systems have been the focused issue [1–3]. Due to excellent properties in hardness, adhesive strength, wear and corrosion resistance, transition-metal nitrides are commonly applied as hard and protective films on tool steel [4–7]. In the nitride systems, chromium nitride films not only possess of the above performance, but also have superior oxidation resistance to 700 °C. Therefore, CrN films have been widely used as protective films for various forming and casting applications such as drawing dies, molds, etc [8,9]. Several studies have shown that the addition of the third element, such as W, Al, can optimize the overall performance of the CrN films or combine desired properties [10,11]. Also, the hard tungsten nitride films were usually used in wear resistant application [12]. Recently, a number of studies on the nitride multilayer films relating the manufacture and various characteristics have been conducted [13–24]. Wong et al. [13] and Ducros et al. [14,15] revealed that multilayer films composed of two kinds of transition-metal nitride layers often show excellent mechanical properties, as compared to single layered nitride films. As the grain boundaries in multilayer structure of the films could restrain the growth of grain size, deflect the
⁎ Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail address: [email protected] (J.P. Tu). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.08.010
cracks into a direction parallel to the interfaces and the film surface, and reduce the stress concentration, consequently it can improve the mechanical properties and wear resistance [16,17]. As for the system of CrN/WN multilayer films, Wu et al. [18] reported detailed deposition method, microstructure and mechanical properties. Also the multilayer morphology was clearly revealed through transmission electron microscopy (TEM). However, little work has been performed in the investigation of the tribological property, even though it plays an important role in determining the use of life as a protective film on tool steel. Gu et al. [19] in our research group discussed the mechanical and tribological properties of CrN/W2N and CrN/Mo2N multilayer films deposited by means of DC reactive magnetron sputtering. But the detailed discussion of hardness enhancement and wear mechanism was not conducted. In this present work, the effects of microstructure on the mechanical and tribological properties of CrN/W2N multilayer films as a function of bilayer period were further investigated.
2. Experimental 2.1. Deposition process The quench-and-temper high speed steel (W18Cr4V, Rockwell C hardness of HRC 63), 30 mm in diameter, was used as substrate materials. The high speed steel substrate was polished to Ra ≤ 0.2 µm by using emery paper and diamond spray. All the substrates were ultrasonically cleaned in acetone for 30 min and rinsed with alcohol before being dried and placed into the deposition chamber. Silicon
R.L. Li et al. / Surface & Coatings Technology 204 (2009) 470–476
(100) wafers were gradually cleaned in hydrofluoric acid and ammonia, for getting rid of impurity and adsorption materials. The deposition of CrN/W2N multilayer films was carried out using a TEER 650 closed field unbalanced magnetron sputtering coater. Two chromium targets and two tungsten targets (380 cm × 175 cm in sizes), located about 15 cm away from the substrates, were alternative arranged in the chamber. The vacuum chamber was evacuated to a pressure lower than 5 × 10− 3 Pa. Prior to deposition, the substrates were subject to Ar+ bombardment at a bias of − 500 V for 30 min to remove the surface oxide layer. Then Cr interlayer with the thickness of 30 nm was deposited almost immediately after cleaning, in order to enhance the adhesion of the film to the substrate. After that, the Cr and W targets were co-sputtered at the working pressure of 0.2 Pa in the mixture of Ar + N2. The flux of Ar and N2 was controlled by optical emission monitor. During the deposition process, a pulsed bias of −100 V and the temperature of 200 °C were applied to the substrates. For all the experiments, the target current was fixed at 3 A for Cr and 5 A for W respectively. Through the above deposition process, CrN/W2N multilayer films with different bilayer periods, respectively 85 nm, 38 nm and 15 nm, and a CrWN composite film were obtained. The variation of the film structure was manipulated by altering the rotation velocity of the substrate holder, in the range of 0.5 to 4 rpm. In addition, the rotating substrate holder can provide a uniform exposure of the growing films. The films deposited on silicon wafer were also prepared at the same parameters for structure analysis. The detailed parameters for the deposition process are summarized in Table 1. In this work, the thicknesses of all the films were in the range of 2.7–3.0 μm. 2.2. Microstructure, mechanical and tribological properties The surface and cross-section morphologies were investigated by scanning electron microscopy (SEM, Hitachi S-4700 equipped with GENENIS 4000 EDAX director). The microstructure of the films was characterized by multifunctional high resolution transmission electron microscopy (HRTEM, JEOL CM200UT) and X-ray diffraction (XRD, Thermo X' Pert PRO X-ray diffractometer with Cu Kα1 irradiation) at a grazing angle of 4° and multifunctional high resolution transmission electron microscopy (HRTEM, JEOL CM200UT). As the existence of residual stresses, the difference of XRD patterns between CrN and W2N peak is too small to identify accurately, so step scanning was conducted. The step interval was 0.02° and the scanning rate was 0.04° min− 1. Procedures for cross-sectional TEM specimen preparation were described in detail as follows. Two coating surfaces were glued face to face and then cut to several slices. After mechanical grinding and polishing, the slice specimen was dimpled by Ar+ milling using a Gatan Precision Ion Polishing System (PIPS, Model 691) at 4.5 keV at an angle of 10°. The microhardness and elastic modulus measurement of the films were carried out by MTS Nanoindenter XP using a load of 470 mN. Adhesion tests were performed on the multilayer films through the Rockwell and scratch test. Standard scratch tests, done with a conventional scratch tester (WS-2002 equipped with an acoustic emission detector), were used to assess the transverse adhesion of the
Table 1 Parameters for the deposition of the CrN/W2N multilayer and composite films. Process
Pressure (Pa)
Substrate bias (V)
Target current (A)
Duration time (s)
Substrate cleaning Cr interlayer deposition Film deposition Refrigeration and venting
5.0×10− 3 4.0×10− 1
− 500 − 100
0 Cr: 3; W: 0.2
1800 300
2.0×10− 1 –
− 100 0
Cr: 3; W: 5 0
7200 1800
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coating. For these scratch tests, a diamond pin (0.2 mm in radius) was drawn across the surface of film at a constant linear velocity of 10 mm min− 1, while increasing the load linearly from 10 to 85 N. The hardness, elastic modulus and critical load data are the mean of the five values performed on it. The tribological properties of the CrN/W2N multilayer films deposited on high speed steel substrate were performed on a WTM1E ball-on-disk tribometer in humid air (relative humidity 50%). A Si3N4 ceramic ball (3 mm in diameter, hardness HV = 1550) was used as the counter body, and the obtained film specimen served as the disk. The tests were carried out at a normal load of 1 N, a sliding velocity of 0.11 m s− 1. The friction coefficient was monitored continuously during the experiments by a linear variable displacement transducer and recorded on a data acquisition computer attached to the tribometer. After the number of testing cycle reached 1.05 × 104, the worn surfaces were observed by an optical microscope (OM, Nikon Eclipse ME600D). The wear rate of the CrN/W2N multilayer films were calculated from measuring the traces of surface profiles (mean of four traces per sample, every two spacing 90°) taken across the wear track using a Dektak 3 optical profilometer. Four samples for each kind of the films were chosen for the wear tests to ensure the reproducibility of the results. 3. Results and discussion 3.1. Morphology and structural characterization Cross-sectional SEM morphologies of the CrN/W2N multilayer films with different bilayer periods and the CrWN composite film are shown in Fig. 1. As seen in Fig. 1a, b and c, all the multilayer films have a dense columnar structure, and are composed of a layered configuration by alternating stacking lighter chromium nitride and darker tungsten nitride phases. The CrN layer appears brighter than W2N layer due to the lower scattering factor of Cr as compared to W. The thickness of CrN and W2N bilayer, also called modulation period (λ), is evaluated, respectively 85 nm, 38 nm, and 15 nm. Interfaces between CrN and W2N layers in the CrN/W2N multilayer film with bilayer periods of 15–85 nm are clearly revealed. As seen in Fig. 1d, the multilayer structure of the film deposited at high rotation velocity of the substrate holder can hardly be found by the observation from secondary electron imaging (SEI) and backscattered electron imaging (BEI). When the bilayer period gets shorter, the effect of interface between CrN and W2N layer changes more remarkably, and the atoms in two adjacent layers diffuse to facing layer much easier. So the CrN layer gets mixed with the adjacent W2N layer, which leads to the formation of composite structure. The detailed layer morphology and crystal phase of the CrN/W2N film with a bilayer period of 15 nm are analyzed from TEM and selected area electron diffraction pattern (SAED) focused on the CrN layer. An evident dense and multilayer structure by sequentially altering of CrN and W2N layers is again verified (Fig. 2a). However, at the layer interface there are some transverse grains and the interface between the two adjacent layers is not so sharp from high resolution electron microscopy (HRTEM) in Fig. 2b, owing to atoms diffusion through the adjacent layers. The crystal CrN and W2N are obviously seen and the crystal sizes are from 5 to 10 nm. Some small CrN grains overlapped that causes formation of Moire fringes. The SAED pattern confirms the f.c.c structure and (111), (200) preferred crystal orientation of nanocrystal CrN. Fig. 3 shows the XRD patterns of the deposited films. As the existence of residual stresses, the difference between CrN peak and W2N peak is too small to identify accurately [20], so step scanning was conducted. In the CrN/W2N multilayer films with the bilayer periods of 85 nm and 15 nm, two diffraction peaks can be defined clearly at angles of 37° and 43°, corresponding to (111), (200) crystal planes of both CrN and W2N phases. This can also be demonstrated in the SAED
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pattern in Fig. 2. But in the CrWN composite film, only one diffraction peak can be distinctly defined at an angle of around 37°, corresponding to (111) crystal plane. The intensity of peak at a diffraction angle of 43° is so weak to recognize hardly, which means the crystal preferred orientation of the composite film is (111). The composite structure attributes to the diffusion of atoms in the adjacent CrN and W2N layers, which leads to the mixture of two layers. Also W may just be the incorporation as solid solution agent to the CrN crystalline structure in the deposition process. Although residual stress exists, the effect of which on microstrain (which affects the XRD peak width) would be small. Ignoring this effect, the crystallite size of CrN can be estimated using Debye– Scherrer formula [25] D=
Kλ : β cosðθÞ
ð1Þ
Where K is a constant (K = 0.91), D is the mean crystalline dimension normal to diffracting planes, λ is the X-ray wavelength (λ = 0.15406 nm), β in radian is the peak width at half-maximum height, and θ is the Bragg's angle. The crystallite sizes of CrN in all the films are calculated from the formula and shown in Table 2. With decreasing the bilayer period, the grain size becomes smaller. As for the CrN/W2N multilayer with a bilayer period of 15 nm, the crystallite size is 6.5 nm, which is in good agreement with that shown in HRTEM. 3.2. Mechanical properties The mechanical properties of the CrN/W2N multilayer films with different bilayer periods and composite film are shown in Table 2.
Both the microhardness and elastic modulus followed the same trend. The maximum values achieve to approximately 29.2 GPa and 376 GPa at the CrN/W2N multilayer film with a bilayer period of 15 nm, corresponding to the CrN crystallite size of 6.5 nm. The thickness of the single CrN or W2N layer in the multilayer films was controlled in the range of a few nanometers, thus the crystallite size was fixed to a nanometer scale by the boundaries, also demonstrated in HRTEM, resulting in hardness enhancement. Ducros et al. [14,15] revealed that the hardening by decreasing periods due to both grain size refinement and the high number of layer interface. The alternating properties of the different phases in the neighboring layers make the dislocation movement blocked [26–28]. The lattice constant of CrN and W2N is different (CrN: a = 0.414 nm and W2N: a = 0.413 nm, respectively) and the crystalline orientation at the interfaces are observed dissimilar in HRTEM (Fig. 2b), resulting in an increase of internal strain. The internal strain hampers the dislocation movement, so the hardness is enhanced. Meanwhile, the force required to move a dislocation across a phase boundary increases with the difference in shear modulus between the different phases present in the films [29]. Due to above reasons, the mechanical properties of multilayer films with the lower bilayer periods have been improved as compared to single layered or nanocomposite films. As shown in Table 2, the hardness and elastic modulus values of the CrWN composite film play down because the impact of interface does not exist and the crystal grains grow vertically without interruption. The adhesion of the CrN/W2N multilayer films to the substrate was tested through Rockwell and scratch test. The Rockwell craters and scratch traces of the films are shown in Fig. 4. The bilayer period appears to play an important role in the adhesive strength. There are obvious cracks and delamination around the Rockwell crater of the
Fig. 1. Cross-sectional SEM morphologies of CrN/W2N multilayer and composite films: (a) λ = 85 nm, (b) λ = 38 nm, (c) λ = 15 nm, (d) composite film.
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Table 2 Microstructure and mechanical properties of the CrN/W2N multilayer and composite films. Films
λ = 85 nm
λ = 38 nm
λ = 15 nm
Composite
Crystallite size of CrN (nm) E (GPa) H (GPa) H/E* H3/E*2 (GPa) Critical load (N)
9.6 304 20.5 0.061 0.077 55
7.1 323 22.0 0.062 0.085 63
6.5 376 29.2 0.071 0.146 >85
11.5 312 23.3 0.068 0.108 > 85
appear in the Rockwell and scratch craters (Fig. 4c and g). For the CrWN composite film, as shown in Fig. 4d and h, though there are no apparent delamination, the fine cracks still appear at the bottom, and then expand around of the Rockwell crater. The critical loads for these films obtained by scratch tests are also listed in Table 2.
3.3. Tribological properties
Fig. 2. (a) TEM images and inserted SAED pattern and (b) HRTEM images of the CrN/ W2N multilayer film with a bilayer period of 15 nm.
multilayer film with a bilayer period of 85 nm (Fig. 4a). Also, some fragments can be distinctly seen around the scratch crater (Fig. 4e). The cracks and delamination of the multilayer film with a bilayer period of 38 nm decrease to some extent (Fig. 4b and f). For the CrN/ W2N multilayer film with a bilayer period of 15 nm, although it has the highest microhardness, shows the best adhesion, and no cracks
Fig. 3. XRD patterns of CrN/W2N multilayer and composite films.
The friction behavior of all the deposited films was evaluated by the pin-on-disk wear test system under the same condition. After friction test, the wear traces of these films were observed by means of SEM and optical microscope. Fig. 5 gives the friction coefficient of CrN/W2N multilayer and composite films as a function of duration time at a normal load of 1 N and a sliding velocity of 0.11 m s− 1. It is shown that the friction curves of the CrN/W2N multilayer films are stable with little fluctuation and the values of friction coefficient are all between 0.3 and 0.4. A closer look on the CrN/W2N multilayer film with a bilayer period, it exhibits that the friction coefficient increases rapidly from an initial low value then comes down gradually and at last it almost attains a constant value. Friction force is related to the surface roughness of the mating surface and the asperity inclination angle [29]. In the initial stage, as shown in Fig. 6a, the asperities on the mating surface are grinded, resulting in roughness augmentation and friction coefficient increasing rapidly. After a number of cycles, as shown in Fig. 6b, the asperities are crushed smaller, and the inclination angles of them are reduced, which make the mating surface more smooth and lead to lower friction coefficient. At the later stage, smaller wear particles fill the gaps between the asperities and provide a smooth surface for sliding, so the friction coefficient keeps at a constant value, which can be considered as the real friction coefficient value. Comparing the four friction curves, it can be seen that the friction coefficient decreases gradually with the decrease of bilayer period. When the bilayer period gets to 15 nm, the average friction coefficient of the CrN/W2N multilayer film achieved the minimum value of 0.315. However, the friction coefficient of the CrWN composite film begins to mount up. The wear trace of the two kinds of films is compared in Fig. 7. As seen in Fig. 7a, the wear trace of the multilayer film is shallow, narrow and no cracks are found around it. But for the CrWN composite film, the wear trace becomes broad and deep. Meanwhile, many cracks as well as some particles are stacking on the worn surface. It can be concluded that the wear resistance of the multilayer film with a bilayer period of 15 nm is better than that of the composite film. Fig. 8 indicates that the wear rate followed the same trend as the friction coefficient. The CrN/W2N multilayer film with a bilayer period of 15 nm has the best wear resistance. Many researches show that wear mechanism has nothing to do with hardness directly, but seems to be controlled by friction. Also, some mechanical properties like toughness or resilience have been proposed to be useful to explain the wear resistance of materials [30]. Toughness can serve as the evaluation of a material to absorb energy during deformation up to fracture, which can be estimated according to the H/E* ratio. The resilience can be defined as the resistance of plastic deformation, which is related to the H3/E*2 ratio. As shown in Table 2, the CrN/W2N
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Fig. 4. Optical images of Rockwell and scratch craters for the CrN/W2N multilayer and composite films. Rockwell craters: (a) λ = 85 nm, (b) λ = 38 nm, (c) λ = 15 nm, (d) composite film; scratch craters: (e) λ = 85 nm nm, (f) λ = 38 nm, (g) λ = 15 nm, (h) composite film.
multilayer film with a bilayer period of 15 nm has the highest values of the H/E* and H3/E*2 ratios, so it shows the best tribological properties.
The above parameters can sometimes evaluate the tribological properties, but real wear mechanism is much more complex. The performance of wear resistance depends not only on the tested material,
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Fig. 5. Friction coefficient of CrN/W2N multilayer and composite films as a function of duration time at a normal load of 1 N and a sliding velocity of 0.11 m s− 1.
but also on the environmental temperature and humidity conditions, chemical reactions at the contact region, the material of counterpart, etc. 4. Conclusions Homogeneous and compact CrN/W2N multilayer films with various bilayer periods of 15–85 nm were deposited on high speed steel (W18Cr4V) substrates by means of DC closed field unbalanced magnetron sputtering. With the decrease of bilayer period, the microhardness, elastic modulus and adhesive strength of the CrN/W2N multilayer films increase. In addition to mechanical properties, the tribological Fig. 7. Comparison of worn surface between multilayer and composite films: (a) multilayer film with a bilayer period of 15 nm, (b) composite film.
Fig. 8. Tribological properties of the CrN/W2N multilayer and composite films at a normal load of 1 N and a sliding velocity of 0.11 m s− 1.
property of the CrN/W2N multilayer films is also improved with the decrease of bilayer period. The multilayer film with a bilayer period of 15 nm has the highest wear resistance and lowest friction coefficient. Friction and wear mechanism is complex, and can be influenced by some mechanical properties like toughness or resilience. Acknowledgment Fig. 6. Different stages in the friction test procedure of CrN/W2N multilayer film with a bilayer period of 15 nm: (a) the initial stage, (b) the later stage.
This work was supported by the National Natural Science Foundation of China (No. 50871102).
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