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CrN nano-multilayer coatings
Surface & Coatings Technology 240 (2014) 405–412
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Influence of niobium ion implantation on the microstructure, mechanical and tribological properties of TiAlN/CrN nano-multilayer coatings Baojian Liu, Bin Deng, Ye Tao ⁎ School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China
a r t i c l e
i n f o
Article history: Received 4 June 2013 Accepted in revised form 25 December 2013 Available online 6 January 2014 Keywords: Niobium ion implantation TiAlN/CrN nano-multilayer coating Microstructure Mechanical properties Tribological properties
a b s t r a c t In this paper, TiAlN/CrN nano-multilayer coatings were deposited on the substrates of cemented carbide (WC–TiC–Co) using magnetic filtered arc ion plating (MFAIP) method and implanted with niobium ions at 40 kV with the doses of 5 × 1016, 1 × 1017 and 2 × 1017 ions/cm2 by metal vacuum vapor arc (MEVVA) ion source. The microstructure and chemical compositions of Nb-implanted coatings were investigated by highresolution transmission electron microscopy (HRTEM), grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). Meanwhile, the mechanical behavior of coatings was analyzed by nanoindentation test and the tribological properties were investigated on a ball-on-disc friction and wear tester. Nb ion irradiation induced amorphization and the formation of nanocomposite and nanocrystalline in the coatings surface. Nb ion implantation resulted in the formation of NbN, Nb2O5 and a small fraction of niobium oxynitride in the implanted zone. The nanohardness and the H3/E*2 ratio of the coatings increased remarkably, which was due to the resulting structure and the formation of NbN phases in the implanted zone caused by energetic Nb ion implantation. Nb ion implantation could effectively improve the tribological performance of TiAlN/CrN coatings, which was due to a lower friction coefficient and the increasing of the hardness and the plastic deformation resistance of the coatings. The amorphous top layer formed on the surface could act as a solid lubricant during the wear sliding. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Over the past years, in order to improve their performance in the field of cutting tools and molds, hard coatings deposited by physical vapor deposition (PVD) techniques have been developed from monolayer coatings (e.g. TiN, CrN) [1,2], multi-component coatings (e.g. TiAlN, TiAlCrN) [3,4] and multi-layered coatings (e.g. TiN/AlN, TiAlN/CrN) [5,6]. Nano-multilayer coatings are composed of multicomponent constituents and, notably, a multilayered structure often referred to as superlattice [7]. Laboratory and field studies [8–10] have shown that superlattice coatings offer a significant improvement in wear resistance over the first-generation (e.g. TiN) and secondgeneration coatings (e.g. TiAlCrN). Among the multilayer coatings, TiAlN/CrN coating has been demonstrated to exhibit good mechanical and tribological properties as well as thermal stability [11]. To further improve the comprehensive performance of the TiAlN/CrN coatings, MEVVA ion implantation could be used as an effective method for surface modification to enhance hardness and wear resistance of the coatings. There are some reports concerning the effects of ion implantation on the tribological properties and microstructure of TiN and CrN coatings implanted various ions, such as C, N, Al, Zr, V and Nb [12–17]. In all of ⁎ Corresponding author. Tel.: +86 1082317115; fax: +86 1082317125. E-mail address: [email protected] (Y. Tao). 0257-8972/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.12.065
the aforementioned works, the post-treatment of niobium ion implantation has shown that it could considerably improve the mechanical properties and the tribological behavior of hard nitride coatings. Chang et al. [17] reported that NbN and Cr–Nb–N phases were formed in the implanted zone of CrN coating, which led the hardness of the CrN film to increase from 18 GPa to 35 GPa after niobium and carbon ion implantation. In our previous studies, Nb ion implantation could improve the microhardness and wear behavior of TiAlN coatings remarkably due to the formation of the amorphous and nanocrystalline composite structure [18]. Therefore, it was speculated that niobium ion implantation on the TiAlN/CrN nano-multilayer coatings could further improve the comprehensive performance of the TiAlN/CrN coatings, yielding the increased wear resistance and lifetime. However, few papers researched the effects of ion implantation on the microstructure, mechanical and tribological properties of TiAlN/CrN nanomultilayer coatings, especially the effects of Nb ion implantation. Furthermore, there were lack of further studies on the relationship between the microstructure in implanted zone and the mechanical and tribological properties, which were very critical for TiAlN/CrN coatings with Nb implantation applied on machine parts and in relevant industries. In this work, TiAlN/CrN nano-multilayer coatings were modified by Nb implantation with the doses of 5 × 10 16 , 1 × 10 17 and 2 × 1017 ions/cm2. The microstructure and chemical compositions of Nb-implanted coatings were investigated by HRTEM, GIXRD and XPS.
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Meanwhile, the mechanical behavior of coatings was analyzed by the nano-indentation test and the tribological properties were investigated on a ball-on-disc friction and wear tester.
for the wear tracks were measured by using a 3D profiling system (MicroXAM 3D PHASE SHIFT, ADE Co., USA). Coatings specific wear rate K was calculated by using K = V/Fs, where F is the applied load, s is the total sliding distance and V is the worn volume.
2. Experimental details 3. Results and discussion The material of the substrate was YT-15 cemented carbide (WC–TiC–Co). Before the deposition, specimens with dimensions of 16 × 16 × 4 mm were polished and cleaned in an ultrasonic bath with acetone (10 min) and ethanol (10 min), respectively. Approximately 600-nm-thick TiAlN/CrN nano-multilayer coatings were deposited on the specimens by MFAIP. Before Nb implantation, all the coated samples were also degreased and cleaned in an ultrasonic bath. Niobium ions were implanted into the samples at accelerating voltage of 40 keV with the doses of 5 × 1016, 1 × 1017 and 2 × 1017 ions/cm2 by MEVVA ion source at the Institute of Low Energy Nuclear Physics in Beijing Normal University. The experimental parameters of the deposition and ion implantation process were shown in Table 1. The microstructural changes of TiAlN/CrN nano-multilayer coatings caused by ion implantation were studied by HRTEM (JEM-2100F). The depth profile of niobium ion implantation was analyzed by energydispersive X-ray spectrometer (EDS). The average projected range and the maximum implanted depth of Nb ions, calculated by a TRIM code [19], were 48.1 and 119.2 nm, respectively. The X-ray diffractometer (XRD, D/max-3A type equipment) was utilized to identify the structure of Nb-implanted coatings through the use of Cu-target Kα radiation at 40 kV and 30 mA at a low incident angle of 1° and in the scanning angular (2θ) range from 30° to 90° at 2°/min. The characteristics of composition and chemical binding of the Nb-implanted TiAlN/CrN coatings were identified by an X-ray photoelectron spectroscope (ESCALAB 250 XPS) with a monochromatic Al Kα X-ray source (1486.6 eV) operated at 200 W. In order to depict the chemical binding states of the sub-layer in the implanted zone, the samples were sputtered with Ar ions for 90 s (15 nm depth) at a voltage of 3 kV and 2 uA beam current. The nanohardness and Young's modulus of the coatings had been investigated by Nano Indenter XP (MTS, USA) with a 2 μm Berkovitch indenter tip with a maximum load of 500 mN. The instrument was operated in the continuous stiffness mode (CSM). Five areas were selected to determine the average hardness and Young's modulus of the samples. The tribological properties were investigated by dry-sliding tests at room temperature on a UMT-2 ball-on-disc tribometer. The load was 0.5 N, the test time was 5 min, the amplitude was 3 mm and the frequency of vibration was 5 Hz. All tests were performed in ambient atmosphere at a relative humidity of 35% in laboratory against a Si3N4 ceramic ball with a diameter of 4 mm. The optical 2D/3D morphologies
3.1. Microstructure and chemical analysis As shown in Fig. 1, combined with the EDS analysis, the implanted depth of niobium was 120 nm with a Gaussian distribution, and the maximum niobium concentration was located at a depth of 50 nm beneath the surface in the TiAlN/CrN nano-multilayer coating at the dose of 1 × 1017 ions/cm2. It almost accorded with the result calculated by a TRIM program (119.2 nm and 48.1 nm). The depth-dependent structural evolution of the implanted TiAlN/CrN coatings was investigated by HRTEM. Fig. 2(a) and (b) show the cross-sectional bright-field image and enlarged areas in the coating surface of an as-deposited film, while Fig. 2(c) and (d) illustrate a bright-field image and enlarged areas of the implantation zone of a sample implanted to 1 × 1017 ions/cm2. From the figure, it could be obviously found that the TiAlN/CrN coating, with the thickness of approximately 0.6 μm, exhibited a growth of dense columnar structure, which was intrinsic to pure TiN or TiAlN films deposited by ion plating process [20,21]. The individual layers preserved clear separation and interface planarity, as shown in Fig. 2(b). The thicknesses of TiAlN and CrN were ~4 nm and ~1 nm, respectively. However, it was changed at the implanted zone, where the columnar structure was broken and the multilayered structure also disappeared due to Nb ion irradiation. It depicted clearly the structure consisting of an amorphous top layer (~ 40 nm), a nanocomposite transition layer (~ 20 nm) and a nanocrystalline layer (~50 nm) at the implanted zone, as shown in Fig. 2(d). Fig. 3(a–c) showed the high-resolution TEM micrographs and the corresponding selected area electron diffraction (SAED) patterns of the coating in 10 nm, 50 nm and 80 nm depth, respectively. As shown in Fig. 3(a), the damage layer was clearly identified as an amorphous layer where the atoms were arranged in disorder and atomic clusters were formed in the local position. The corresponding SAED showed a diffused ring pattern which further confirmed the amorphous structure in the Nb-implanted layer. The high-energy Nb atoms and TiAlN and CrN lattice atoms impacted together and produced cascade collision effect, which led to the high lattice distortion and even an amorphous structure. Beneath the amorphous layer, nanocrystals (~6 nm) with a clear internal crystalline structure were shown in Fig. 3(b), where the nanocrystals were separated by the boundary with a disordered structure. It indicated that after ion implantation, a nanocomposite transition layer was formed in the surface of the TiAlN/CrN coatings. Similar phenomenon had also been found in Nb-implanted TiAlN
Table 1 Operating parameters of the TiAlN/CrN coatings deposition and Nb ion implantation process. Parameters
Values
TiAlN/CrN deposition progress Base pressure (Pa) Work pressure (Pa) Ratio of gas flow Target material Bias voltage at ion cleaning stage (V) Bias voltage at coating stage (V) Substrate temperature (°C) Deposition time of TiAl interlayer (min) Deposition time of TiAlN/CrN coating (min)
6 × 10−3 0.3 40:3 (N2:Ar) Cr (99.99%), Ti50Al50 (99.99%) −1000 −80 250–300 5 50
Ion implantation progress Ion implantation doses (ions/cm2) Acceleration voltage (kV) Ion current (mA)
Nb: 5 × 1016, 1 × 1017 and 2 × 1017 40 3
Fig. 1. EDS depth profile of Nb-implanted TiAlN/CrN coating at a dose of 1 × 1017 ions/cm2.
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Fig. 2. Cross-sectional bright-field TEM images: (a) as-deposited sample, (b) enlarged areas in the coating surface, (c) sample implanted to 1 × 1017 ions/cm2, and (d) enlarged areas of the implantation zone. Region a presents an amorphous layer, the region b presents a nanocomposite transition layer and region c indicates nanocrystalline layer.
coatings, in which amorphous and nanocrystalline composite structure was formed in the implanted zone [18]. With the increasing depth, the microstructure showed the majority of equiaxed grains whose size was also about 6 nm. This was in agreement with the SAED patterns inset in Fig. 3(c), which showed a continuous and sharpening diffraction ring indicating a very fine grain structure. Meanwhile, the SAED patterns taken from the labeled nanocrystalline confirmed that the layer mainly was in fcc crystal structure. The reflections corresponded to (111), (200) and (220) planes of the TiAlN and CrN phase. In the process of atomic cascade collision, ion irradiation induced an energy transfer that provided conditions for the interaction of the surfaces and boundaries of the nanocrystals. The formation of the smaller crystallites reflected the particles size optimization for the stable structure that the internal free energy of the system had a local minimum [22,23]. In addition, the microstructure near the amorphous layer also showed apparent defect contrast due to a high density of dislocations formed by the Nb ion irradiation. The high-energy Nb implantation caused damage mainly by collision cascades, created high defect density and led to the formation of dislocation networks [24]. In addition to the TEM analysis, XRD measurements at a glancing angle of 1° was performed to investigate the microstructural changes of unimplanted TiAlN/CrN coating and Nb-implanted TiAlN/CrN coatings at different doses, as shown in Fig. 4. It was found that all the XRD patterns of the Nb-implanted films showed the same peaks of CrN and TiAlN crystal planes that both had almost similar (111), (200) and (220) diffracting planes, which were nearly identical to that of the unimplanted TiAlN/CrN coating in Fig. 4. The TiAlN and CrN had the same fcc structure and the differences between their lattice parameters were small (aCrN = 4.14 Å and aTiAlN = 4.16 Å) [11]. The (200) plane
was the preferred orientation. However, after Nb ion implantation, the intensity of (111) peak increased significantly. Also with increasing the implantation doses, the relative diffraction intensities ratio of I(111)/I(200) increased from 20.0% to 62.1%. It suggested a change of the partial texture after ion implantation, although it could not change the preferred orientation. In addition, it was clear from the figure that the (200) peak shifted to lower 2θ value with the increasing doses from Fig. 4. This was because Ti, Cr atoms in the TiAlN and CrN lattice were substituted by Nb atoms with a larger atomic radius (rNb = 0.198 nm, rTi = 0.176 nm, rCr = 0.166 nm) [25]. This led to an expansion in the lattice parameter and hence a shift in 2θ value in the XRD, which indicated that the residual stress was developed in the implantation region by niobium ion implantation. Meanwhile, implantation had caused peak broadening accompanied by a decrease in peak intensity with increasing doses due to the refined crystal structure and the formation of amorphous phase [26]. This result immediately demonstrated an excellent agreement with the HRTEM analyses. The high-resolution XPS spectra analysis of the Nb-implanted TiAlN/CrN coatings at a dose of 1 × 1017 ions/cm2 was shown in Fig. 5. The Nb 3d spectrum from the sub-layer (90s Ar ion sputtered, about 15 nm beneath the surface) was shown in Fig. 5(a). From the analysis of chemical states, the Nb3d5/2 peak located at 203.8 eV corresponded to the presence of niobium nitride [27]. The peaks centered at 207.5 eV and 210.2 eV belonged to the Nb3d5/2 and Nb3d3/2 electrons of Nb2O5 [28], respectively. The peaks observed at 204.6 eV and 205.8 eV originated from the Nb3d5/2 and Nb3d3/2 electrons of NbON [29], respectively. The N1s spectrum was shown in Fig. 5(b). The peaks situated at 397.4 eV and 396.5 eV were contributed to metal nitride. The former could be attributed to the M–N bond (CrN, TiN and AlN [30,31]) in TiAlN/CrN
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Fig. 4. GIXRD spectra of unimplanted TiAlN/CrN coating and Nb-implanted TiAlN/CrN coatings at different doses.
between the energetic Nb ions and the residual gas in the chamber during the implantation process [17].
3.2. Mechanical behavior The nanohardness of the TiAlN/CrN coatings unimplanted and Nb-implanted at different doses was investigated by the continuous stiffness method, as shown in Fig. 6. It was seen that as for the untreated TiAlN/CrN coating, the maximum hardness was 25 GPa at about 25 nm. After Nb implantation, the maximum hardness was reached at approximately 40–60 nm beneath surface, which was located at the
Fig. 3. HRTEM image and the corresponding SAED pattern of the Nb-implanted TiAlN/CrN coating specimen (1 × 1017 ions/cm2): (a) surface, (b) 50 nm, and (c) 80 nm depth.
coatings, and the latter to NbN. The peaks at 400.6 eV and 398.9 eV were attributed to oxynitride [32]. As mentioned above, niobium nitrides were formed in the implanted zone after Nb ion implantation, although these could not be demonstrated in XRD results due to the low sensitivity nature of the XRD analysis in the implanted zone [33]. It was believed that the formation of niobium nitride was via a substitutional mechanism, where Ti or Cr atoms in the TiAlN and CrN lattice were partially replaced by Nb atoms. Meanwhile, the oxidation (Nb2O5) and a small fraction of niobium oxynitride were found on the sub-layer of the coatings. The formation of Nb2O5 could be caused by the contaminations produced
Fig. 5. High-resolution XPS spectra of Nb-implanted TiAlN/CrN coating at a dose of 1 × 1017 ions/cm2: (a) Nb 3d and (b) N 1s.
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Fig. 6. The nanohardness of the Nb ion implanted TiAlN/CrN coatings at different doses varied with the displacement into surface. I: amorphous layer, II: nanocomposite transition layer, III: nanocrystalline layer.
nanocomposite transition layer. Then the hardness value declined slowly, but still maintained a high hardness value in the nanocrystalline layer. With the increasing depth, the measured hardness approached a steady-state value of 20 GPa after 400 nm in depth. After Nb implantation, the maximum hardness of the coatings increased remarkably and varied with increasing implantation doses. Within the implanted zone, the maximum hardness of the Nb-implanted TiAlN/CrN coatings was improved from 25 GPa to 36 GPa. Compared with the EDS data in Fig. 1, it was worth noting that the nanohardness of Nb-implanted TiAlN/CrN coatings kept at a relatively higher level even at the depth— larger than the maximum reach of the implanted Nb. This indicated that the strengthening behavior existed not only in the Nb-implanted layer but also in the TiAlN/CrN substrate without Nb. This might be caused by the long-range effect that occurred in the TiAlN/CrN substrate. The long-range effect referred to the phenomenon that many structure defects, e.g. dislocation loops and point defects, were formed outside the implanted zone with the depth from a few to tens of micrometers and thus affected the mechanical properties of the substrate [34,35]. The mechanical behavior of hard coatings was well characterized, not only by their hardness, H, but also by their effective Young's modulus E* = E / (1 − υ2), where E was the Young's modulus, and υ was the Poisson ratio (~0.25) [36,37]. Moreover, the measured H and E values permitted simple calculation of the H3/E*2 ratio, which provided information about the material resistance to plastic deformation. The effective Young's modulus and H3/E*2 ratio of the Nb-implanted TiAlN/CrN coatings at different doses in implanted zone were shown
Fig. 7. The Young's modulus and H3/E*2 ratio of the Nb-implanted TiAlN/CrN coatings at different doses in implanted zone.
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in Fig. 7. It could be found that with the increasing ion implantation doses, the H3/E*2 ratio increased from 0.10 GPa to 0.19 GPa. It indicated that after Nb ion implantation, the resistance to plastic deformation of the TiAlN/CrN coatings increased obviously due to the fact that the hardness and Young's modulus of the TiAlN/CrN coatings were also increased after Nb ion implantation. The higher nanohardness and Young's modulus of Nb-implanted TiAlN/CrN coatings compared to unimplanted coatings can be mainly explained as follows. Firstly, the energetic Nb atoms and TiAlN and CrN lattice atoms impacted together and produced cascade collision effect, which led to the high lattice distortion and the formation of amorphous phase. The introduction of defects into a crystalline material would hinder the dislocation motion and resulted in an increase of the hardness. Second, Nb ion irradiation induced the formation of nanocomposite and nanocrystalline in the coatings surface as evidenced by TEM structural studies. The refined structures could play a critical role to improve the hardness and plastic deformation resistance. In addition, the formation of NbN in the implanted zone observed from the XPS results could cause strain hardening via lattice mismatch and further increase the hardness of the TiAlN/CrN coatings. 3.3. Tribological properties Fig. 8 showed the optical 2D/3D morphologies for the wear tracks of unimplanted TiAlN/CrN coating and Nb-implanted TiAlN/CrN coatings at different doses. From the figures, it could be obviously found that Nb ion implantation led to a remarkable decrease in the width and depth of the wear track. For the unimplanted TiAlN/CrN coating, it showed that the track profiles were very sharp in the track edge. This was the phenomenon of the coating peeling off from the substrate because of the coating break. Meanwhile, there were two obvious steps on the wear track with the width of 244.95 μm and the depth of 0.83 μm, which meant that the coating was completely worn out and cemented carbide substrate was reached in the middle of the wear track. However, after Nb-implantation at the dose of 1 × 1017 ions/cm2, the wear track became narrower and shallower with the width of 136.08 μm and the depth of 0.62 μm. The ploughing action and grooves oriented parallel to the sliding direction could be found in the middle area of the wear track due to the abraded effect of the wear particles. As a result, the wear mechanism of the TiAlN/CrN coating against Si3N4 ball was adhesive wear. Furthermore, on the wear track of the sample implanted to 2 × 1017 ions/cm2, the depth sharply decreased to 0.48 μm although the width was not changed significantly. Because the thickness of the coatings was approximately 0.6 μm as detected in TEM results, the coating was not worn out. It suggested that the Nb-implanted TiAlN/CrN coatings could reduce the friction effectively and this beneficial effect became much more significant at the high dose of Nb ions. Fig. 9 showed the wear rate of TiAlN/CrN coatings at different doses. The wear rate decreased largely with the increasing dose of Nb ions. For example, at the dose of 2 × 1017 ions/cm2, the wear rate was only 30% of that for unimplanted TiAlN/CrN coating. Above wear-testing results indicated that Nb ion implantation resulted in an excellent improvement in wear resistance of TiAlN/CrN coatings. The friction coefficient of the coatings versus wear time was shown in Fig. 10. It could be seen that the friction and wear measurements revealed remarkable differences between the unimplanted and Nb-implanted TiAlN/CrN coatings. The friction coefficient (μ) for the unimplanted sample was approximately 0.55 in the steady-state region. For the sample implanted to 5 × 1016 ions/cm2, the friction coefficient curve created a region of low friction, with μ approximately 0.12 which lasted for about 15 s, after which the coefficient reached 0.53 (approximately the steady-state unimplanted value). When the dose of Nb ion implantation was increased to 1 × 1017 ions/cm2, the duration of the low friction region was 60 s and then the friction coefficient showed a sharp ‘jump’, from approximately 0.2 to approximately 0.5. Combined with wear track morphology in Fig. 8, it indicated that the
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Fig. 8. Optical 2D/3D morphologies for the wear tracks of unimplanted TiAlN/CrN coating and Nb-implanted TiAlN/CrN coatings at different doses. a and b: as-deposited sample; c and d: sample implanted to 5 × 1016 ions/cm2; e and f: sample implanted to 1 × 1017 ions/cm2; g and h: sample implanted to 2 × 1017 ions/cm2.
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remarkably improve the nanohardness and the H3/E*2 ratio of the coatings, resulting in the increase of the plastic deformation resistance and wear resistance. Consequently, the Nb-implanted TiAlN/CrN coatings showed the better wear behavior than the unimplanted TiAlN/CrN coating. 4. Conclusions In this paper, the TiAlN/CrN nano-multilayer coatings, deposited on cemented carbide by magnetic filtered arc ion plating, were implanted with niobium ions at different doses by MEVVA ion source. The major conclusions could be summarized as follows:
Fig. 9. Wear rate of Nb-implanted TiAlN/CrN coatings at different doses.
implanted zone was totally worn out. After further increasing the dose to 2 × 1017 ions/cm2, the trace extended the low friction region to about 125 s. It was interesting to notice that after Nb implantation the friction coefficients of the Nb-implanted coatings possessed the similar feature: the curve started with a region of low friction, with a coefficient of 0.1–0.2, and after a relatively short region of transition, the coefficient reached a stable value. The effect of a lower friction coefficient after ion implantation was attributed to the formation of an amorphous top layer formed on the surface. This effect resulted in lowering the shear stress at the contact point, thus lowering the friction coefficient [38], causing the implanted layer to act as a solid lubricant. In the steady phase, inspection of the wear tracks revealed that after the implanted layer had worn off, the number of particles increased between the ball and the coatings, so the coefficient of friction stabilized at a high value. With the increasing ion implantation doses, the time of the implanted layer worn off increased, and low friction coefficient phase was extended, which was crucial to improve the friction properties of the coatings. As stated above, Nb ion implantation could effectively improve the wear behavior of TiAlN/CrN coating and a higher dose was greatly beneficial to the decrease of the friction coefficient. The improvement of the wear behavior of TiAlN/CrN coating probably resulted from a lower friction coefficient and the increasing of the hardness and elastic modulus of the coatings as a result of energetic Nb ion bombardment. The amorphous top layer formed on the surface of the coating after Nb ion implantation from the TEM results could act as a solid lubricant during the wear sliding and resulted in the drop of friction coefficient, which could improve the wear behavior of the TiAlN/CrN coatings. Meanwhile, the formation of amorphization, nanocomposite and nanocrystalline detected in TEM results in the implanted zone, could
Fig. 10. Friction coefficient of Nb-implanted TiAlN/CrN coatings at different doses versus wear time.
(1) Nb ion irradiation induced amorphization and the formation of nanocomposite and nanocrystalline in the coatings surface. For the sample implanted to 1 × 1017 ions/cm2, the structure consisting of an amorphous top layer (~40 nm), a nanocomposite transition layer (~20 nm) and a nanocrystalline layer (~50 nm) was formed in the implanted zone. (2) Nb ion implantation resulted in the formation of niobium nitrides, niobium oxides and a small fraction of niobium oxynitride in the implanted zone. NbN phases were formed via a substitutional mechanism and Nb2O5 phases were formed as a result of oxidation during the implantation process. (3) The nanohardness and the H3/E*2 ratio of the coatings increased remarkably due to the resulting structure and the formation of NbN phases in the implanted zone caused by the energetic Nb ion implantation. With the increasing ion implantation doses, the hardness value increased from 24 GPa of the TiAlN/CrN coating to 35 GPa and the H3/E*2 ratio increased from 0.10 GPa to 0.19 GPa. (4) Nb ion implantation could effectively improve the tribological performance of TiAlN/CrN coatings in terms of friction coefficient and wear rate due to a lower friction coefficient and the increasing of the hardness and plastic deformation resistance of the coatings. The amorphous top layer formed on the surface could act as a solid lubricant during the wear sliding. References [1] O. Kessler, Th. Herding, F. Hoffmann, P. Mayr, Surf. Coat. Technol. 182 (2004) 184–191. [2] Z.K. Chang, X.S. Wan, Z.L. Pei, J. Gong, C. Sun, Surf. Coat. Technol. 205 (2011) 4690–4696. [3] H.G. Prengel, A.T. Santhanam, R.M. Penich, P.C. Jindal, K.H. Wendt, Surf. Coat. Technol. 94–95 (1997) 597–602. [4] G.S. Fox-Rabinovich, K. Yamomoto, S.C. Veldhuis, A.I. Kovalev, G.K. Dosbaeva, Surf. Coat. Technol. 200 (2005) 1804–1813. [5] S.H. Yao, W.H. Kao, Y.L. Su, T.H. Liu, Mater. Sci. Eng. A 386 (2004) 149–155. [6] Yin-Yu. Chang, Da-Yung Wang, Chi-Yung Hung, Surf. Coat. Technol. 200 (2005) 1702–1708. [7] X.T. Zeng, Surf. Coat. Technol. 113 (1999) 75–79. [8] Harish C. Barshilia, M. Surya Prakash, Anjana Jain, K.S. Rajam, Vacuum 77 (2005) 169–179. [9] Q. Luo, W.M. Rainforth, W.-D. Münz, Wear 225–229 (1999) 74–82. [10] Q. Luo, W.M. Rainforth, L.A. Donohue, I. Wadsworth, W.-D. Münz, Vacuum 53 (1999) 123–126. [11] P.L. Sun, C.H. Hsu, S.H. Liu, C.Y. Su, C.K. Lin, Thin Solid Films 518 (2010) 7519–7522. [12] Chi-Lung Chang, Da-Yung Wang, Nucl. Inst. Methods Phys. Res. B 194 (2002) 463–468. [13] D. Sansom, J.L. Viviente, F. Alonso, J.J. Ugarte, J.I. Oñate, Surf. Coat. Technol. 84 (1996) 519–523. [14] Jerzy Narojczyk, Zbigniew Werner, Dmitrij Morozow, Waldemar Tuszyński, Vacuum 81 (2007) 1275–1277. [15] K.P. Purushotham, Liam.P. Ward, Narelle Brack, Paul J. Pigram, Peter Evans, Hans Noorman, Rafael R. Manory, Wear 257 (2004) 901–908. [16] Ko-Wei Weng, Tai-Nan Lin, Da-Yung Wang, Thin Solid Films 516 (2008) 1012–1019. [17] Yin-Yu. Chang, Da-Yung Wang, Wu. WeiTe, Surf. Coat. Technol. 177–178 (2004) 441–446. [18] Bin Deng, Ye Tao, Hongliang Liu, Peiying Liu, Surf. Coat. Technol. 228 (2013) S554–S557. [19] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping Powers and Range of Ions in Mater, Pergamon Press, New York, 1985.
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Influence of niobium ion implantation on the microstructure, mechanical and tribological properties of TiAlN/CrN nano-multilayer coatings Baojian Liu, Bin Deng, Ye Tao ⁎ School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China
a r t i c l e
i n f o
Article history: Received 4 June 2013 Accepted in revised form 25 December 2013 Available online 6 January 2014 Keywords: Niobium ion implantation TiAlN/CrN nano-multilayer coating Microstructure Mechanical properties Tribological properties
a b s t r a c t In this paper, TiAlN/CrN nano-multilayer coatings were deposited on the substrates of cemented carbide (WC–TiC–Co) using magnetic filtered arc ion plating (MFAIP) method and implanted with niobium ions at 40 kV with the doses of 5 × 1016, 1 × 1017 and 2 × 1017 ions/cm2 by metal vacuum vapor arc (MEVVA) ion source. The microstructure and chemical compositions of Nb-implanted coatings were investigated by highresolution transmission electron microscopy (HRTEM), grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). Meanwhile, the mechanical behavior of coatings was analyzed by nanoindentation test and the tribological properties were investigated on a ball-on-disc friction and wear tester. Nb ion irradiation induced amorphization and the formation of nanocomposite and nanocrystalline in the coatings surface. Nb ion implantation resulted in the formation of NbN, Nb2O5 and a small fraction of niobium oxynitride in the implanted zone. The nanohardness and the H3/E*2 ratio of the coatings increased remarkably, which was due to the resulting structure and the formation of NbN phases in the implanted zone caused by energetic Nb ion implantation. Nb ion implantation could effectively improve the tribological performance of TiAlN/CrN coatings, which was due to a lower friction coefficient and the increasing of the hardness and the plastic deformation resistance of the coatings. The amorphous top layer formed on the surface could act as a solid lubricant during the wear sliding. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Over the past years, in order to improve their performance in the field of cutting tools and molds, hard coatings deposited by physical vapor deposition (PVD) techniques have been developed from monolayer coatings (e.g. TiN, CrN) [1,2], multi-component coatings (e.g. TiAlN, TiAlCrN) [3,4] and multi-layered coatings (e.g. TiN/AlN, TiAlN/CrN) [5,6]. Nano-multilayer coatings are composed of multicomponent constituents and, notably, a multilayered structure often referred to as superlattice [7]. Laboratory and field studies [8–10] have shown that superlattice coatings offer a significant improvement in wear resistance over the first-generation (e.g. TiN) and secondgeneration coatings (e.g. TiAlCrN). Among the multilayer coatings, TiAlN/CrN coating has been demonstrated to exhibit good mechanical and tribological properties as well as thermal stability [11]. To further improve the comprehensive performance of the TiAlN/CrN coatings, MEVVA ion implantation could be used as an effective method for surface modification to enhance hardness and wear resistance of the coatings. There are some reports concerning the effects of ion implantation on the tribological properties and microstructure of TiN and CrN coatings implanted various ions, such as C, N, Al, Zr, V and Nb [12–17]. In all of ⁎ Corresponding author. Tel.: +86 1082317115; fax: +86 1082317125. E-mail address: [email protected] (Y. Tao). 0257-8972/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.12.065
the aforementioned works, the post-treatment of niobium ion implantation has shown that it could considerably improve the mechanical properties and the tribological behavior of hard nitride coatings. Chang et al. [17] reported that NbN and Cr–Nb–N phases were formed in the implanted zone of CrN coating, which led the hardness of the CrN film to increase from 18 GPa to 35 GPa after niobium and carbon ion implantation. In our previous studies, Nb ion implantation could improve the microhardness and wear behavior of TiAlN coatings remarkably due to the formation of the amorphous and nanocrystalline composite structure [18]. Therefore, it was speculated that niobium ion implantation on the TiAlN/CrN nano-multilayer coatings could further improve the comprehensive performance of the TiAlN/CrN coatings, yielding the increased wear resistance and lifetime. However, few papers researched the effects of ion implantation on the microstructure, mechanical and tribological properties of TiAlN/CrN nanomultilayer coatings, especially the effects of Nb ion implantation. Furthermore, there were lack of further studies on the relationship between the microstructure in implanted zone and the mechanical and tribological properties, which were very critical for TiAlN/CrN coatings with Nb implantation applied on machine parts and in relevant industries. In this work, TiAlN/CrN nano-multilayer coatings were modified by Nb implantation with the doses of 5 × 10 16 , 1 × 10 17 and 2 × 1017 ions/cm2. The microstructure and chemical compositions of Nb-implanted coatings were investigated by HRTEM, GIXRD and XPS.
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Meanwhile, the mechanical behavior of coatings was analyzed by the nano-indentation test and the tribological properties were investigated on a ball-on-disc friction and wear tester.
for the wear tracks were measured by using a 3D profiling system (MicroXAM 3D PHASE SHIFT, ADE Co., USA). Coatings specific wear rate K was calculated by using K = V/Fs, where F is the applied load, s is the total sliding distance and V is the worn volume.
2. Experimental details 3. Results and discussion The material of the substrate was YT-15 cemented carbide (WC–TiC–Co). Before the deposition, specimens with dimensions of 16 × 16 × 4 mm were polished and cleaned in an ultrasonic bath with acetone (10 min) and ethanol (10 min), respectively. Approximately 600-nm-thick TiAlN/CrN nano-multilayer coatings were deposited on the specimens by MFAIP. Before Nb implantation, all the coated samples were also degreased and cleaned in an ultrasonic bath. Niobium ions were implanted into the samples at accelerating voltage of 40 keV with the doses of 5 × 1016, 1 × 1017 and 2 × 1017 ions/cm2 by MEVVA ion source at the Institute of Low Energy Nuclear Physics in Beijing Normal University. The experimental parameters of the deposition and ion implantation process were shown in Table 1. The microstructural changes of TiAlN/CrN nano-multilayer coatings caused by ion implantation were studied by HRTEM (JEM-2100F). The depth profile of niobium ion implantation was analyzed by energydispersive X-ray spectrometer (EDS). The average projected range and the maximum implanted depth of Nb ions, calculated by a TRIM code [19], were 48.1 and 119.2 nm, respectively. The X-ray diffractometer (XRD, D/max-3A type equipment) was utilized to identify the structure of Nb-implanted coatings through the use of Cu-target Kα radiation at 40 kV and 30 mA at a low incident angle of 1° and in the scanning angular (2θ) range from 30° to 90° at 2°/min. The characteristics of composition and chemical binding of the Nb-implanted TiAlN/CrN coatings were identified by an X-ray photoelectron spectroscope (ESCALAB 250 XPS) with a monochromatic Al Kα X-ray source (1486.6 eV) operated at 200 W. In order to depict the chemical binding states of the sub-layer in the implanted zone, the samples were sputtered with Ar ions for 90 s (15 nm depth) at a voltage of 3 kV and 2 uA beam current. The nanohardness and Young's modulus of the coatings had been investigated by Nano Indenter XP (MTS, USA) with a 2 μm Berkovitch indenter tip with a maximum load of 500 mN. The instrument was operated in the continuous stiffness mode (CSM). Five areas were selected to determine the average hardness and Young's modulus of the samples. The tribological properties were investigated by dry-sliding tests at room temperature on a UMT-2 ball-on-disc tribometer. The load was 0.5 N, the test time was 5 min, the amplitude was 3 mm and the frequency of vibration was 5 Hz. All tests were performed in ambient atmosphere at a relative humidity of 35% in laboratory against a Si3N4 ceramic ball with a diameter of 4 mm. The optical 2D/3D morphologies
3.1. Microstructure and chemical analysis As shown in Fig. 1, combined with the EDS analysis, the implanted depth of niobium was 120 nm with a Gaussian distribution, and the maximum niobium concentration was located at a depth of 50 nm beneath the surface in the TiAlN/CrN nano-multilayer coating at the dose of 1 × 1017 ions/cm2. It almost accorded with the result calculated by a TRIM program (119.2 nm and 48.1 nm). The depth-dependent structural evolution of the implanted TiAlN/CrN coatings was investigated by HRTEM. Fig. 2(a) and (b) show the cross-sectional bright-field image and enlarged areas in the coating surface of an as-deposited film, while Fig. 2(c) and (d) illustrate a bright-field image and enlarged areas of the implantation zone of a sample implanted to 1 × 1017 ions/cm2. From the figure, it could be obviously found that the TiAlN/CrN coating, with the thickness of approximately 0.6 μm, exhibited a growth of dense columnar structure, which was intrinsic to pure TiN or TiAlN films deposited by ion plating process [20,21]. The individual layers preserved clear separation and interface planarity, as shown in Fig. 2(b). The thicknesses of TiAlN and CrN were ~4 nm and ~1 nm, respectively. However, it was changed at the implanted zone, where the columnar structure was broken and the multilayered structure also disappeared due to Nb ion irradiation. It depicted clearly the structure consisting of an amorphous top layer (~ 40 nm), a nanocomposite transition layer (~ 20 nm) and a nanocrystalline layer (~50 nm) at the implanted zone, as shown in Fig. 2(d). Fig. 3(a–c) showed the high-resolution TEM micrographs and the corresponding selected area electron diffraction (SAED) patterns of the coating in 10 nm, 50 nm and 80 nm depth, respectively. As shown in Fig. 3(a), the damage layer was clearly identified as an amorphous layer where the atoms were arranged in disorder and atomic clusters were formed in the local position. The corresponding SAED showed a diffused ring pattern which further confirmed the amorphous structure in the Nb-implanted layer. The high-energy Nb atoms and TiAlN and CrN lattice atoms impacted together and produced cascade collision effect, which led to the high lattice distortion and even an amorphous structure. Beneath the amorphous layer, nanocrystals (~6 nm) with a clear internal crystalline structure were shown in Fig. 3(b), where the nanocrystals were separated by the boundary with a disordered structure. It indicated that after ion implantation, a nanocomposite transition layer was formed in the surface of the TiAlN/CrN coatings. Similar phenomenon had also been found in Nb-implanted TiAlN
Table 1 Operating parameters of the TiAlN/CrN coatings deposition and Nb ion implantation process. Parameters
Values
TiAlN/CrN deposition progress Base pressure (Pa) Work pressure (Pa) Ratio of gas flow Target material Bias voltage at ion cleaning stage (V) Bias voltage at coating stage (V) Substrate temperature (°C) Deposition time of TiAl interlayer (min) Deposition time of TiAlN/CrN coating (min)
6 × 10−3 0.3 40:3 (N2:Ar) Cr (99.99%), Ti50Al50 (99.99%) −1000 −80 250–300 5 50
Ion implantation progress Ion implantation doses (ions/cm2) Acceleration voltage (kV) Ion current (mA)
Nb: 5 × 1016, 1 × 1017 and 2 × 1017 40 3
Fig. 1. EDS depth profile of Nb-implanted TiAlN/CrN coating at a dose of 1 × 1017 ions/cm2.
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Fig. 2. Cross-sectional bright-field TEM images: (a) as-deposited sample, (b) enlarged areas in the coating surface, (c) sample implanted to 1 × 1017 ions/cm2, and (d) enlarged areas of the implantation zone. Region a presents an amorphous layer, the region b presents a nanocomposite transition layer and region c indicates nanocrystalline layer.
coatings, in which amorphous and nanocrystalline composite structure was formed in the implanted zone [18]. With the increasing depth, the microstructure showed the majority of equiaxed grains whose size was also about 6 nm. This was in agreement with the SAED patterns inset in Fig. 3(c), which showed a continuous and sharpening diffraction ring indicating a very fine grain structure. Meanwhile, the SAED patterns taken from the labeled nanocrystalline confirmed that the layer mainly was in fcc crystal structure. The reflections corresponded to (111), (200) and (220) planes of the TiAlN and CrN phase. In the process of atomic cascade collision, ion irradiation induced an energy transfer that provided conditions for the interaction of the surfaces and boundaries of the nanocrystals. The formation of the smaller crystallites reflected the particles size optimization for the stable structure that the internal free energy of the system had a local minimum [22,23]. In addition, the microstructure near the amorphous layer also showed apparent defect contrast due to a high density of dislocations formed by the Nb ion irradiation. The high-energy Nb implantation caused damage mainly by collision cascades, created high defect density and led to the formation of dislocation networks [24]. In addition to the TEM analysis, XRD measurements at a glancing angle of 1° was performed to investigate the microstructural changes of unimplanted TiAlN/CrN coating and Nb-implanted TiAlN/CrN coatings at different doses, as shown in Fig. 4. It was found that all the XRD patterns of the Nb-implanted films showed the same peaks of CrN and TiAlN crystal planes that both had almost similar (111), (200) and (220) diffracting planes, which were nearly identical to that of the unimplanted TiAlN/CrN coating in Fig. 4. The TiAlN and CrN had the same fcc structure and the differences between their lattice parameters were small (aCrN = 4.14 Å and aTiAlN = 4.16 Å) [11]. The (200) plane
was the preferred orientation. However, after Nb ion implantation, the intensity of (111) peak increased significantly. Also with increasing the implantation doses, the relative diffraction intensities ratio of I(111)/I(200) increased from 20.0% to 62.1%. It suggested a change of the partial texture after ion implantation, although it could not change the preferred orientation. In addition, it was clear from the figure that the (200) peak shifted to lower 2θ value with the increasing doses from Fig. 4. This was because Ti, Cr atoms in the TiAlN and CrN lattice were substituted by Nb atoms with a larger atomic radius (rNb = 0.198 nm, rTi = 0.176 nm, rCr = 0.166 nm) [25]. This led to an expansion in the lattice parameter and hence a shift in 2θ value in the XRD, which indicated that the residual stress was developed in the implantation region by niobium ion implantation. Meanwhile, implantation had caused peak broadening accompanied by a decrease in peak intensity with increasing doses due to the refined crystal structure and the formation of amorphous phase [26]. This result immediately demonstrated an excellent agreement with the HRTEM analyses. The high-resolution XPS spectra analysis of the Nb-implanted TiAlN/CrN coatings at a dose of 1 × 1017 ions/cm2 was shown in Fig. 5. The Nb 3d spectrum from the sub-layer (90s Ar ion sputtered, about 15 nm beneath the surface) was shown in Fig. 5(a). From the analysis of chemical states, the Nb3d5/2 peak located at 203.8 eV corresponded to the presence of niobium nitride [27]. The peaks centered at 207.5 eV and 210.2 eV belonged to the Nb3d5/2 and Nb3d3/2 electrons of Nb2O5 [28], respectively. The peaks observed at 204.6 eV and 205.8 eV originated from the Nb3d5/2 and Nb3d3/2 electrons of NbON [29], respectively. The N1s spectrum was shown in Fig. 5(b). The peaks situated at 397.4 eV and 396.5 eV were contributed to metal nitride. The former could be attributed to the M–N bond (CrN, TiN and AlN [30,31]) in TiAlN/CrN
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Fig. 4. GIXRD spectra of unimplanted TiAlN/CrN coating and Nb-implanted TiAlN/CrN coatings at different doses.
between the energetic Nb ions and the residual gas in the chamber during the implantation process [17].
3.2. Mechanical behavior The nanohardness of the TiAlN/CrN coatings unimplanted and Nb-implanted at different doses was investigated by the continuous stiffness method, as shown in Fig. 6. It was seen that as for the untreated TiAlN/CrN coating, the maximum hardness was 25 GPa at about 25 nm. After Nb implantation, the maximum hardness was reached at approximately 40–60 nm beneath surface, which was located at the
Fig. 3. HRTEM image and the corresponding SAED pattern of the Nb-implanted TiAlN/CrN coating specimen (1 × 1017 ions/cm2): (a) surface, (b) 50 nm, and (c) 80 nm depth.
coatings, and the latter to NbN. The peaks at 400.6 eV and 398.9 eV were attributed to oxynitride [32]. As mentioned above, niobium nitrides were formed in the implanted zone after Nb ion implantation, although these could not be demonstrated in XRD results due to the low sensitivity nature of the XRD analysis in the implanted zone [33]. It was believed that the formation of niobium nitride was via a substitutional mechanism, where Ti or Cr atoms in the TiAlN and CrN lattice were partially replaced by Nb atoms. Meanwhile, the oxidation (Nb2O5) and a small fraction of niobium oxynitride were found on the sub-layer of the coatings. The formation of Nb2O5 could be caused by the contaminations produced
Fig. 5. High-resolution XPS spectra of Nb-implanted TiAlN/CrN coating at a dose of 1 × 1017 ions/cm2: (a) Nb 3d and (b) N 1s.
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Fig. 6. The nanohardness of the Nb ion implanted TiAlN/CrN coatings at different doses varied with the displacement into surface. I: amorphous layer, II: nanocomposite transition layer, III: nanocrystalline layer.
nanocomposite transition layer. Then the hardness value declined slowly, but still maintained a high hardness value in the nanocrystalline layer. With the increasing depth, the measured hardness approached a steady-state value of 20 GPa after 400 nm in depth. After Nb implantation, the maximum hardness of the coatings increased remarkably and varied with increasing implantation doses. Within the implanted zone, the maximum hardness of the Nb-implanted TiAlN/CrN coatings was improved from 25 GPa to 36 GPa. Compared with the EDS data in Fig. 1, it was worth noting that the nanohardness of Nb-implanted TiAlN/CrN coatings kept at a relatively higher level even at the depth— larger than the maximum reach of the implanted Nb. This indicated that the strengthening behavior existed not only in the Nb-implanted layer but also in the TiAlN/CrN substrate without Nb. This might be caused by the long-range effect that occurred in the TiAlN/CrN substrate. The long-range effect referred to the phenomenon that many structure defects, e.g. dislocation loops and point defects, were formed outside the implanted zone with the depth from a few to tens of micrometers and thus affected the mechanical properties of the substrate [34,35]. The mechanical behavior of hard coatings was well characterized, not only by their hardness, H, but also by their effective Young's modulus E* = E / (1 − υ2), where E was the Young's modulus, and υ was the Poisson ratio (~0.25) [36,37]. Moreover, the measured H and E values permitted simple calculation of the H3/E*2 ratio, which provided information about the material resistance to plastic deformation. The effective Young's modulus and H3/E*2 ratio of the Nb-implanted TiAlN/CrN coatings at different doses in implanted zone were shown
Fig. 7. The Young's modulus and H3/E*2 ratio of the Nb-implanted TiAlN/CrN coatings at different doses in implanted zone.
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in Fig. 7. It could be found that with the increasing ion implantation doses, the H3/E*2 ratio increased from 0.10 GPa to 0.19 GPa. It indicated that after Nb ion implantation, the resistance to plastic deformation of the TiAlN/CrN coatings increased obviously due to the fact that the hardness and Young's modulus of the TiAlN/CrN coatings were also increased after Nb ion implantation. The higher nanohardness and Young's modulus of Nb-implanted TiAlN/CrN coatings compared to unimplanted coatings can be mainly explained as follows. Firstly, the energetic Nb atoms and TiAlN and CrN lattice atoms impacted together and produced cascade collision effect, which led to the high lattice distortion and the formation of amorphous phase. The introduction of defects into a crystalline material would hinder the dislocation motion and resulted in an increase of the hardness. Second, Nb ion irradiation induced the formation of nanocomposite and nanocrystalline in the coatings surface as evidenced by TEM structural studies. The refined structures could play a critical role to improve the hardness and plastic deformation resistance. In addition, the formation of NbN in the implanted zone observed from the XPS results could cause strain hardening via lattice mismatch and further increase the hardness of the TiAlN/CrN coatings. 3.3. Tribological properties Fig. 8 showed the optical 2D/3D morphologies for the wear tracks of unimplanted TiAlN/CrN coating and Nb-implanted TiAlN/CrN coatings at different doses. From the figures, it could be obviously found that Nb ion implantation led to a remarkable decrease in the width and depth of the wear track. For the unimplanted TiAlN/CrN coating, it showed that the track profiles were very sharp in the track edge. This was the phenomenon of the coating peeling off from the substrate because of the coating break. Meanwhile, there were two obvious steps on the wear track with the width of 244.95 μm and the depth of 0.83 μm, which meant that the coating was completely worn out and cemented carbide substrate was reached in the middle of the wear track. However, after Nb-implantation at the dose of 1 × 1017 ions/cm2, the wear track became narrower and shallower with the width of 136.08 μm and the depth of 0.62 μm. The ploughing action and grooves oriented parallel to the sliding direction could be found in the middle area of the wear track due to the abraded effect of the wear particles. As a result, the wear mechanism of the TiAlN/CrN coating against Si3N4 ball was adhesive wear. Furthermore, on the wear track of the sample implanted to 2 × 1017 ions/cm2, the depth sharply decreased to 0.48 μm although the width was not changed significantly. Because the thickness of the coatings was approximately 0.6 μm as detected in TEM results, the coating was not worn out. It suggested that the Nb-implanted TiAlN/CrN coatings could reduce the friction effectively and this beneficial effect became much more significant at the high dose of Nb ions. Fig. 9 showed the wear rate of TiAlN/CrN coatings at different doses. The wear rate decreased largely with the increasing dose of Nb ions. For example, at the dose of 2 × 1017 ions/cm2, the wear rate was only 30% of that for unimplanted TiAlN/CrN coating. Above wear-testing results indicated that Nb ion implantation resulted in an excellent improvement in wear resistance of TiAlN/CrN coatings. The friction coefficient of the coatings versus wear time was shown in Fig. 10. It could be seen that the friction and wear measurements revealed remarkable differences between the unimplanted and Nb-implanted TiAlN/CrN coatings. The friction coefficient (μ) for the unimplanted sample was approximately 0.55 in the steady-state region. For the sample implanted to 5 × 1016 ions/cm2, the friction coefficient curve created a region of low friction, with μ approximately 0.12 which lasted for about 15 s, after which the coefficient reached 0.53 (approximately the steady-state unimplanted value). When the dose of Nb ion implantation was increased to 1 × 1017 ions/cm2, the duration of the low friction region was 60 s and then the friction coefficient showed a sharp ‘jump’, from approximately 0.2 to approximately 0.5. Combined with wear track morphology in Fig. 8, it indicated that the
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Fig. 8. Optical 2D/3D morphologies for the wear tracks of unimplanted TiAlN/CrN coating and Nb-implanted TiAlN/CrN coatings at different doses. a and b: as-deposited sample; c and d: sample implanted to 5 × 1016 ions/cm2; e and f: sample implanted to 1 × 1017 ions/cm2; g and h: sample implanted to 2 × 1017 ions/cm2.
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remarkably improve the nanohardness and the H3/E*2 ratio of the coatings, resulting in the increase of the plastic deformation resistance and wear resistance. Consequently, the Nb-implanted TiAlN/CrN coatings showed the better wear behavior than the unimplanted TiAlN/CrN coating. 4. Conclusions In this paper, the TiAlN/CrN nano-multilayer coatings, deposited on cemented carbide by magnetic filtered arc ion plating, were implanted with niobium ions at different doses by MEVVA ion source. The major conclusions could be summarized as follows:
Fig. 9. Wear rate of Nb-implanted TiAlN/CrN coatings at different doses.
implanted zone was totally worn out. After further increasing the dose to 2 × 1017 ions/cm2, the trace extended the low friction region to about 125 s. It was interesting to notice that after Nb implantation the friction coefficients of the Nb-implanted coatings possessed the similar feature: the curve started with a region of low friction, with a coefficient of 0.1–0.2, and after a relatively short region of transition, the coefficient reached a stable value. The effect of a lower friction coefficient after ion implantation was attributed to the formation of an amorphous top layer formed on the surface. This effect resulted in lowering the shear stress at the contact point, thus lowering the friction coefficient [38], causing the implanted layer to act as a solid lubricant. In the steady phase, inspection of the wear tracks revealed that after the implanted layer had worn off, the number of particles increased between the ball and the coatings, so the coefficient of friction stabilized at a high value. With the increasing ion implantation doses, the time of the implanted layer worn off increased, and low friction coefficient phase was extended, which was crucial to improve the friction properties of the coatings. As stated above, Nb ion implantation could effectively improve the wear behavior of TiAlN/CrN coating and a higher dose was greatly beneficial to the decrease of the friction coefficient. The improvement of the wear behavior of TiAlN/CrN coating probably resulted from a lower friction coefficient and the increasing of the hardness and elastic modulus of the coatings as a result of energetic Nb ion bombardment. The amorphous top layer formed on the surface of the coating after Nb ion implantation from the TEM results could act as a solid lubricant during the wear sliding and resulted in the drop of friction coefficient, which could improve the wear behavior of the TiAlN/CrN coatings. Meanwhile, the formation of amorphization, nanocomposite and nanocrystalline detected in TEM results in the implanted zone, could
Fig. 10. Friction coefficient of Nb-implanted TiAlN/CrN coatings at different doses versus wear time.
(1) Nb ion irradiation induced amorphization and the formation of nanocomposite and nanocrystalline in the coatings surface. For the sample implanted to 1 × 1017 ions/cm2, the structure consisting of an amorphous top layer (~40 nm), a nanocomposite transition layer (~20 nm) and a nanocrystalline layer (~50 nm) was formed in the implanted zone. (2) Nb ion implantation resulted in the formation of niobium nitrides, niobium oxides and a small fraction of niobium oxynitride in the implanted zone. NbN phases were formed via a substitutional mechanism and Nb2O5 phases were formed as a result of oxidation during the implantation process. (3) The nanohardness and the H3/E*2 ratio of the coatings increased remarkably due to the resulting structure and the formation of NbN phases in the implanted zone caused by the energetic Nb ion implantation. With the increasing ion implantation doses, the hardness value increased from 24 GPa of the TiAlN/CrN coating to 35 GPa and the H3/E*2 ratio increased from 0.10 GPa to 0.19 GPa. (4) Nb ion implantation could effectively improve the tribological performance of TiAlN/CrN coatings in terms of friction coefficient and wear rate due to a lower friction coefficient and the increasing of the hardness and plastic deformation resistance of the coatings. The amorphous top layer formed on the surface could act as a solid lubricant during the wear sliding. References [1] O. Kessler, Th. Herding, F. Hoffmann, P. Mayr, Surf. Coat. Technol. 182 (2004) 184–191. [2] Z.K. Chang, X.S. Wan, Z.L. Pei, J. Gong, C. Sun, Surf. Coat. Technol. 205 (2011) 4690–4696. [3] H.G. Prengel, A.T. Santhanam, R.M. Penich, P.C. Jindal, K.H. Wendt, Surf. Coat. Technol. 94–95 (1997) 597–602. [4] G.S. Fox-Rabinovich, K. Yamomoto, S.C. Veldhuis, A.I. Kovalev, G.K. Dosbaeva, Surf. Coat. Technol. 200 (2005) 1804–1813. [5] S.H. Yao, W.H. Kao, Y.L. Su, T.H. Liu, Mater. Sci. Eng. A 386 (2004) 149–155. [6] Yin-Yu. Chang, Da-Yung Wang, Chi-Yung Hung, Surf. Coat. Technol. 200 (2005) 1702–1708. [7] X.T. Zeng, Surf. Coat. Technol. 113 (1999) 75–79. [8] Harish C. Barshilia, M. Surya Prakash, Anjana Jain, K.S. Rajam, Vacuum 77 (2005) 169–179. [9] Q. Luo, W.M. Rainforth, W.-D. Münz, Wear 225–229 (1999) 74–82. [10] Q. Luo, W.M. Rainforth, L.A. Donohue, I. Wadsworth, W.-D. Münz, Vacuum 53 (1999) 123–126. [11] P.L. Sun, C.H. Hsu, S.H. Liu, C.Y. Su, C.K. Lin, Thin Solid Films 518 (2010) 7519–7522. [12] Chi-Lung Chang, Da-Yung Wang, Nucl. Inst. Methods Phys. Res. B 194 (2002) 463–468. [13] D. Sansom, J.L. Viviente, F. Alonso, J.J. Ugarte, J.I. Oñate, Surf. Coat. Technol. 84 (1996) 519–523. [14] Jerzy Narojczyk, Zbigniew Werner, Dmitrij Morozow, Waldemar Tuszyński, Vacuum 81 (2007) 1275–1277. [15] K.P. Purushotham, Liam.P. Ward, Narelle Brack, Paul J. Pigram, Peter Evans, Hans Noorman, Rafael R. Manory, Wear 257 (2004) 901–908. [16] Ko-Wei Weng, Tai-Nan Lin, Da-Yung Wang, Thin Solid Films 516 (2008) 1012–1019. [17] Yin-Yu. Chang, Da-Yung Wang, Wu. WeiTe, Surf. Coat. Technol. 177–178 (2004) 441–446. [18] Bin Deng, Ye Tao, Hongliang Liu, Peiying Liu, Surf. Coat. Technol. 228 (2013) S554–S557. [19] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping Powers and Range of Ions in Mater, Pergamon Press, New York, 1985.
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