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Microstructure, mechanical, and tribological properties of Ag-free and Ag-doped VCN coatings
Surface & Coatings Technology 331 (2017) 77–84
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Microstructure, mechanical, and tribological properties of Ag-free and Agdoped VCN coatings
MARK
A.V. Bondarev⁎, M. Golizadeh, N.V. Shvyndina, I.V. Shchetinin, D.V. Shtansky⁎ National University of Science and Technology “MISiS”, Leninsky prospect 4, Moscow 119049, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: VCN and VCN-Ag coatings Magnetron sputtering Microstructure Intrinsic stress Adhesion strength and fracture toughness Wear- and impact resistance
The aim of this work was a comparative study of the structure, mechanical, and tribological properties of VCN and VCN-Ag coatings. The VCN coatings were deposited by magnetron sputtering of V and C targets either in a gaseous mixture of Ar + 15%N2 or in pure nitrogen. Silver was added into such coatings by simultaneous sputtering of metallic Ag target using an additional ion source. Microstructure and elemental composition of the coatings were studied by means of X-ray diffraction, transmission and scanning electron microscopy, atomic force microscopy, energy-dispersive spectroscopy, and Raman spectroscopy. The coatings were evaluated in terms of their mechanical properties, adhesion strength, intrinsic stress, fracture toughness, room-temperature friction coefficient, as well as wear resistance and fatigue strength. Incorporation of as much as 10–11 at.% Ag was found to cause significant changes in the coating structure and properties: (i) the columnar morphology changes to equiaxial one; (ii) the coating hardness and Young's modulus decrease from 22 to 26 to 15–17 GPa and from 260 to 270 to 220 GPa, respectively; (iii) the friction coefficient and wear rate increased slightly from 0.4–0.47 to 0.51–0.53 and from 1.5–1.6 × 10− 7 to 8.2–13.3 × 10− 7 mm3/Nm, respectively; (iv) the compressive stress decreased from 1.6 to 0.3–0.5 GPa. The Ag-doped VCN coatings withstood high elastic and plastic deformation during scratching with increasing load up to 50 N without adhesive failure and applied load as high as 1000 N for 105 cycles during dynamic impact tests without brittle fracture.
1. Introduction Hard nanocomposite coatings containing Ag have been a hot topic in recent years because of their potential applications in different fields such as wear, friction, corrosion protection, medicine, optics, and so on [1]. Most of data on Ag-doped coatings deals with carbides, nitrides, and oxides of transition metals and focuses on their tribological [2–8] and antibacterial performance [4,8,9]. Although the available results regarding the influence of Ag on the tribological characteristics at room temperature are rather contradictory [3,10,11], the incorporation of Ag was shown to be an effective approach to reduce friction at elevated temperatures [2,7,12,13]. Ag-doped coatings also demonstrated active oxidation protection [3,14] and self-healing ability [3], which results from segregation of metallic Ag particles at sites of crack and oxidation. Although hardness of Ag-doped coatings was typically reported to decrease along with Ag incorporation, positive effect on material's toughness, plasticity, and residual stresses was noted [15]. Vanadium nitride coatings are a good alternative for their more common TiN, CrN, and ZrN counterparts [16–20] due to enhanced selflubricating ability [21]. The mechanical properties of VN-based
⁎
coatings can be improved by doping with carbon. For example, the hardness of VCN coatings reached 33 GPa [22], being higher than that of binary VC and VN coatings [23]. Moreover, introduction of a certain amount of carbon into VN coatings can lead to a decrease in the elastic modulus without compromising their hardness, which is known to have a positive effect on reduced intrinsic residual stress and interfacial stress between the coating and substrate material [1,23]. Another advantage of the reduced Young's modulus is the increase in the elastic strain to failure and plastic deformation described by the H/E and H3/ E2 ratios, respectively [24]. Both parameters were suggested to be indicators of enhanced tribological performance [24,25], adhesion strength [25,26], and fracture toughness [27,28] of coatings. To the best of our knowledge, VCN-Ag coatings were not investigated to date. Therefore the present study aimed at preparing, characterizing, and testing of VCN-Ag coatings with high resistance against different types of wear during sliding, scratching and dynamic impact loading. To understand processes involved into wear of such coatings, the effect of Ag on the microstructure, intrinsic stress, hardness, Young's modulus, adhesion strength, fracture toughness, and fatigue strength of the prepared VCN coatings was thoroughly studied.
Corresponding authors. E-mail addresses: [email protected] (A.V. Bondarev), [email protected] (D.V. Shtansky).
http://dx.doi.org/10.1016/j.surfcoat.2017.10.036 Received 24 July 2017; Received in revised form 9 October 2017; Accepted 10 October 2017 Available online 12 October 2017 0257-8972/ © 2017 Elsevier B.V. All rights reserved.
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Table 1 Deposition parameters and chemical composition of coatings. No.
Deposition parameters
Chemical composition (at.%)
Magnetron current, mA
1 2 3 4
Ag
V, ×103
C, × 103
– – 50 50
2 2 2 2
1 1 1 1
N2 partial pressure, %
Ag
V
C
N
(C + N)/V
100 15 100 15
– – 10 11
46 53 38 44
13 27 12 26
41 20 40 19
1.2 0.9 1.4 1.0
instruments) equipped with a diamond tip with a radius of 200 μm. During the test, a load was raised monotonously up to the maximum value of 50 N resulting in a scratch length of 5 mm. The dynamic impact tests of the VCN and VCN-Ag-coated samples deposited onto Ni-based alloys were carried out using an Impact Tester (CemeCon) at different applied loads of 250, 500, and 1000 N for as many as 105 impacts. The coatings were exposed to a range of impacts at a constant impact frequency of 50 Hz using a cemented carbide ball (WC-6% Co) with a diameter of 5 mm as counterpart material.
2. Experimental Vanadium (V) and graphite (C) targets utilized in the present study for magnetron sputtering were manufactured by electron beam melting and hot-pressing methods, respectively. The targets, 120 mm in diameter and 6–8 mm in thickness, were sputtered either in a gaseous mixture of argon and nitrogen (Ar + 15% N2) or in pure nitrogen. The targets were sputtered in pulsed DC mode at powers of 1000–1100 W (V) and 600–650 W (C), and a frequency of 50 kHz. In order to obtain Ag-doped VCN coatings, simultaneous sputtering of metallic Ag target was carried out using an additional ion source operating at a current of 50 mA. A scheme of deposition setup was presented elsewhere [29]. During deposition, all other process parameters were kept constant as follows: substrate negative bias voltage 50 V, substrate-to-target distance 10 cm (from V and C targets) and 14 cm (from Ag target), total gas pressure 0.10–0.12 Pa, and substrate temperature 380–400 °C. The deposition time was 40 min, which resulted in coating thickness of 1.1–1.3 μm. Single crystal Si (100) and high-temperature Ni-based alloy were used as substrates. They were ultrasonically cleaned in isopropyl alcohol for 5 min and etched by Ar ions directly in the vacuum chamber for 2–10 min prior to deposition. The microstructure and elemental composition of coatings were studied by means of X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. The XRD patterns of coatings deposited onto (100) Si wafers were obtained on an Ultima IV X-ray difractometer (Rigaku) using grazing incidence (5°) geometry and CoКα radiation. The Raman spectra of as-deposited coatings were recorded at a wavelength of 473 nm using a Raman spectrometer built into a NTMDT instrument (NTEGRA Spectra) which also has an AFM module. The AFM unit was used in order to measure sample surface roughness in tapping mode. A JEM 2100 (JEOL) TEM microscope was used for microstructural studies. The coating chemical composition was determined using an EDS NORAN System 7 (Thermo Scientific). S-4800 (Hitachi) and JEM7700F (JEOL) scanning electron microscopes were utilized to determine the coating cross-section morphology and surface topography, as well as for structural analysis in the deformation zones after tribological, adhesion, and impact tests. The coating hardness and Young's modulus were evaluated using a Nano Hardness Tester (CSM Instruments) equipped with a Berkovich diamond indenter tip calibrated against fused silica. The applied load was 4 mN, and tip penetration depth did not exceed 10% of the coating thickness. The values of internal stress were calculated from the substrate-curvature radius using modified Stoney's formula. The typical size of Si samples coated with VCN and VCN-Ag films, 1.2 μm thick, used for internal stress measurements was 10 × 10 × 0.5 mm3. A ballon-disc tribometer (CSM Instruments) was employed for the evaluation of coating tribological characteristics. The coatings were tested against a 6 mm-sized Al2O3 ball at room temperature. The normal load, sliding speed, and radius of the wear tracks were 1 N, 0.1 m/s, and 4 mm, respectively. The wear track profiles were measured using a WYKO NT1100 optical profiler (Veeco). The adhesion strength of the as-deposited coatings was determined using a scratch tester (CSM
3. Results and discussion 3.1. Elemental composition The deposition experiments were carried out either in pure nitrogen or in a gaseous mixture of Ag + 15%N2 and the chemical composition of as-prepared VCN and VCN-Ag coatings is presented in Table 1. Since in N-rich coatings 1 and 3, the (C + N)/V ratio was high (≥1.2), it is reasonable to assume that excess of carbon could form an amorphous aC phase as it was observed in MoCN-Ag coatings [13]. Deposition in a gaseous mixture of Ar + 15%N2 led to a significant increase in carbon content from 12 to 13 to 26–27 at.% and, accordingly, to a decrease in nitrogen from 40 to 41 to 19–20 at.%. The observed (C + N)/V values, 0.9 for Ag-free coatings and 1.0 for Ag-doped coatings, were consistent with the stoichiometry data for the V(Cx,Ny) phase (0.72 ≤ x + y ≤ 0.98) [30]. The addition of Ag affected only the content of V, whereas the concentration of nonmetallic elements remained almost same. Reducing the amount of metallic elements with addition of Ag was reported previously for ZrCN-Ag [31] and TiCN-Ag [8] coatings. Note that although oxygen content is difficult to determine using EDS analysis due to overlapping of the lines from V and O, the presence of a small amount of oxygen cannot completely be excluded. 3.2. Structure and phase composition 3.2.1. X-ray diffraction XRD patterns of as-deposited coatings are presented in Fig. 1a. It is seen that the VCN phase with fcc structure and Fm3m space group (JCPDS 00-035-0768) is the main crystalline phase in all coatings. The phase demonstrated a weak preferential orientation along its (200) direction. Small maxima observed in the XRD patterns of Ag-free coatings in the range of 45–50° 2θ, which are particularly well seen in curve fitting (Fig. 1, inserts), suggest the presence of minor amounts of h-V2N (JCPDS 00-032-1413) and h-V2C phases (JCPDS 01-073-1320). More detailed interpretation of these phases using XRD patterns is difficult because VC and VN phases with different stoichiometry strictly depending on C and N contents were reported [32–35]. Since the position of the (111)Ag peak is very close to those of the (111)VC and (111)VN peaks, the presence of metallic Ag cannot be unambiguously confirmed from the corresponding XRD patterns. Note, however, that in case of the Ag-doped coatings, the (111) and (200) VCN maxima were broadened and shifted towards higher 2θ angles, hereby indicating the superposition of (111)Ag and (111)VCN peaks. When Ag was added into 78
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Fig. 1. (a) XRD patterns of as-deposited VCN and VCN-Ag coatings 1–4 listed in Table 1 and (b) their corresponding Raman spectra.
(a) V2C
D
VCN
V2C
G
(b)
V2N
3.2.3. Scanning and transmission electron microscopy All coatings were characterized by SEM (Fig. 2). Coatings 1 and 3 with a high value of (C + N)/V ratio, which, according to the Raman spectra, presumably contained an amorphous phase, were additionally studied by TEM (Fig. 3). According to SEM analysis, the coatings had almost the same thickness, hereby indicating that their grown rates were also similar and did not depend on chemical composition. The comparison of cross-sectional SEM micrographs presented in Fig. 2 revealed that columnar morphology became less pronounced when Ag was incorporated into the VCN coating. This observation was further confirmed by TEM. VCN coating 1 exhibited nanocolumnar grains, 15–25 nm in diameter, elongated in the direction of coating growth (Fig. 3a,b). The HRTEM image obtained from a single column revealed small nanocrystallites with interplanar spacing of 0.239 and 0.206 nm, which are fingerprints for the (111) and (200) VCN planes (Fig. 3c). Doping with Ag led to significant change in coating morphology caused by metallic atom segregation at the grain boundaries, hereby suppressing the growth of columnar grains. The size of spherical and globular VCN grains in the VCN-Ag coating was between 10 and 60 nm
the VCN coatings, the preferential (111) orientation became more pronounced and no h-V2N or h-V2C phases were observed.
3.2.2. Raman spectroscopy Since the formation of over-stoichiometric VCN phases was not reported previously [23,36], the microstructure of coatings was studied using a more carbon-sensitive method to reveal the presence of a-C. Raman spectra of all types of the VCN-(Ag) coatings are shown in Fig. 1b. Two well resolved peaks located at 1386 and 1601 cm− 1 can be ascribed to the D (disorder) and G (graphitic) absorption bands of carbon. As expected, the highest amount of highly-disordered sp2bonded carbon was observed in sample 3 with a maximum (C + N)/V value. This carbon phase was not formed in sample 2 with (C + N)/ V = 0.9. The appearance of the characteristic D and G peaks in the Raman spectrum of sample 4 with (C + N)/V = 1.0 indicates that the VCN phase is nonstoichiometric. Note that the presence of both sp2 and sp3-bonded carbon in VCN coatings with (C + N)/V < 1 was reported previously [22].
Fig. 2. Cross-sectional BSE-SEM micrographs of Ag-free coatings 1 (a) and 3 (c) and Ag-doped coatings 2 (b) and 4 (d).
(b)
(a)
500nm
VCN (1)
500nm
VCN-Ag (3)
Substrate
Substrate
(c)
(d)
500nm
VCN (2)
Substrate
500nm
VCN-Ag (4)
Substrate 79
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(a)
(b)
Fig. 3. (a,e) Bright and (b,f) dark field SEM micrographs, and (c,g,h) high resolution TEM images with (d,i) corresponding SAED patterns of coatings 1 (a–d) and 3 (e–i).
(c)
(d)
(111) (200) (220) (f)
(e)
(h)
(g)
(i)
(111) (200) (220) described by the ratio H3/E2 [39]. The material elastic recovery did not depend on the presence of Ag in the material. The values of internal stress calculated from the substrate-curvature radius using modified Stoney's formula are shown in Table 2. The stress decreased in the row: 1.6 (sample 1) → 0.5 (sample 2) → 0.3 GPa (Ag-doped samples 3 and 4). The drop in the internal stress from 1.6 to 0.3 GPa with addition of Ag can be well explained by a change in the coating morphology from columnar to equiaxial due to Ag segregation at the VCN grain boundaries. Note that the mechanical properties of the VCN-Ag coatings with 10–11 at.% of Ag were very similar to those previously reported for MoCN-Ag counterparts with Ag content in the range of 5.5–8.8 at.% [2,13].
(Fig. 3e,f). The fringe contrast observed in the HRTEM image of the VCN-Ag coating perfectly matches with characteristic interplanar distances of 0.241 and 0.236 nm typical for the (111)VCN and (111)Ag, respectively (Fig. 3g,h). Note that the (111)Ag planes were often observed to be parallel or have a small misfit (< 5°) with the (111)VCN planes (Fig. 3h). This small misorientation between the adjacent phases provided low interface energy. A thin transition layer about 100 nm thick with well-defined columnar morphology was clearly seen at the film/substrate interface. For the pure VCN coating, this intermediate layer was less noticeable due to the columnar morphology of the entire coating. Selected area electron diffraction (SAED) pattern of the VCN coating demonstrated the presence of the (111), (200), and (220) diffraction rings from the fcc VCN phase with interplanar spacing of 0.236, 0.205, and 0.145 nm, respectively. In case of the Ag-doped VCN coating, the interplanar distances were 0.241, 0.209, and 0.148 nm for the (111), (200), and (220) lines, respectively. The observed increase in the lattice parameters of the VCN-Ag coating can be well explained by reduction of total compressive stresses [37,38], which is further confirmed through stress calculation reported in Section 3.3.
3.4. Tribological properties 3.4.1. Friction Roughness parameters (arithmetic average and root mean squared) of the materials are collected in Table 2. SEM surface images of asdeposited VCN and VCN-Ag coatings 1, 3, and 4 are also presented in Fig. 4. The SEM surface image of coating 2 was similar to that of coating 1 and therefore is not shown. It can be seen that the coating roughness significantly increased upon adding Ag. High surface roughness can affect friction and wear performance, especially during running-in stage. Fig. 4 illustrates how the room temperature (RT) friction coefficient (CoF) changed versus sliding distance during tribological tests of the coatings against alumina ball. Coating 1 showed an average CoF about 0.4, which is consistent with the values reported by Mitterer and
3.3. Mechanical properties The mechanical properties of the coatings are summarized in Table 2. The coating hardness was observed to decrease with an increase in both C and Ag content. Doping with Ag also led to a decrease in Young's modulus and the resistance to plastic deformation which is Table 2 Mechanical and tribological properties of VCN-(Ag) coatings. No.
1 2 3 4
Tribological properties
Roughness
Mechanical properties
CoF
WR × 10− 7, mm3/Nm
RMS, nm
Ra, nm
H, GPa
E, GPa
0.40 0.47 0.51 0.53
1.5 1.6 8.2 13.3
6 5 32 41
5 4 26 31
26 22 17 15
260 270 220 220
± ± ± ±
1.0 1.0 1.5 1.0
80
± ± ± ±
Residual stresses, GPa
10 10 15 10
W, %
H/E
H3/E2
53 56 52 47
0.10 0.08 0.07 0.06
0.21 0.11 0.08 0.05
1.6 0.5 0.3 0.3
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(a)
Fig. 4. (a) Friction coefficient of VCN and VCN-Ag samples 1–4 (Table 1) as a function of sliding distance and (b-d) SEM surface images of as-deposited coatings 1 (b), 3 (c), and 4 (d).
(b)
Coating 1 5 m (d)
(c)
Coating 4
Coating 3 5 m
2 m
Their wear tracks were mostly free from wear products. Small scratches observed at the bottom of wear tracks parallel to the sliding direction indicated abrasion wear (Fig. 5a,b,e). The wear rates rose to 8.2–13.3 × 10− 7 mm3/Nm upon adding of Ag. The bottom of wear tracks of the VCN-Ag samples was also free from wear products, although a significant amount of wear debris accumulated at the wear track edges. This result indicates that abrasion and ploughing, due to relatively low resistance to plastic deformation, were the main wear mechanisms (Fig. 5c,d,e).
coworkers for VCN coatings with similar compositions [23]. The values of CoF observed for the VCN coatings in this study were considerably lower than those previously reported for their VN counterparts [10,17,23], which indicated positive influence of carbon doping. Yet unexpectedly, coating 2 demonstrated higher CoF values than coating 1, both samples having similar surface roughness while the former having a higher C content. This discrepancy can be well explained by the lack of a free a-C phase in coating 2 whose (C + N)/V ratio was found to be low (< 1), see also the Raman spectrum presented in Fig. 1b. Both Ag-doped coatings demonstrated the highest values of CoF in the range of 0.49–0.56, although coating 3 with high (C + N)/V ratio showed slightly lower CoF values in the middle of the test. Thus our observations are in partial compliance with previously reported data showing a descending trend of CoF in C-doped VN coatings as the (C + N)/V ratio increases [23]. The relatively high CoF of the Ag-doped VCN coatings can be explained by their increased surface roughness and the formation of larger amount of wear products. It is also interesting to compare the friction coefficients of the VCN-Ag coatings with their MoCN-Ag counterparts prepared in a similar manner [2,13]. The MoCN-Ag coatings with (C + N)/Mo ratio of 0.83–1.37 demonstrated significantly lower values of RT CoF compared with VCN-Ag coatings 3 and 4 in which (C + N)/V = 1.0–1.4.
3.5. Adhesion strength and fracture toughness Comparative scratch tests of VCN and VCN-Ag coatings 2 and 4 with lower intrinsic stresses (deposited in Ar + 15%N2) were performed to investigate the influence of metallic additive on the adhesion strength and fracture toughness of the materials. The obtained SEM scratch morphology images are presented in Fig. 6. In both coatings, small semicircular tensile cracks were observed to form just behind the stylus (Fig. 6a,d, inserts). Further increase in applied load to 25 N led to an increase in the number of through-thickness cracks in both coatings (Fig. 6b,e). In addition, sample 2 revealed coating chipping inside and around the scratch edges. In case of Ag-doped coating 4, the transverse cracks were observed to change gradually to conformal cracks formed in front of the indenter when load increased. This type of plastic deformation and brittle fracture is typical for relatively ductile materials and can be taken as a sign of high adhesion strength [40–42]. As the loading reached 50 N, coating 2 was worn completely (Fig. 6c), whereas coating 4 withstood a high elastic and plastic deformation
3.4.2. Wear SEM micrographs and cross-sectional wear track depth profiles of the samples are presented in Fig. 5. The values of wear rates are collected in Table 2. Both Ag-free VCN coatings demonstrated extremely high wear resistance, their wear rates being 1.5–1.6 × 10− 7 mm3/Nm.
(a)
Fig. 5. SEM wear track images of VCN-(Ag) coatings 1 (a), 2 (b), 3 (c), and 4 (d) with (e) corresponding depth profiles evaluated with optical profiler.
(c)
(e)
20 m
20 m (d)
(b)
20 m
20 m
81
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a
b
VCN, 1N
c
VCN, 25N
50 m
50 m
d
VCN-Ag, 1N
50 m
e
50 m
f
VCN-Ag, 25N
VCN-Ag, 50N
50 m
50 m
internal stresses in the Ag-doped coating compared with its VCN counterpart [43]. An abrupt increase in the CoF of the VCN coating at a normal load of 45 N indicates the onset of substrate exposure (Fig. 7a). After the tests at a load of 50 N, full VCN coating spallation is also evident when comparing the difference in scratch depth of the VCN (4.7 μm) and VCN-Ag (3.8 μm) coatings (Fig. 7c,d) which is approximately equal to the coating thickness. The full removal of the VCN coating is also confirmed by SEM image (Fig. 6c). Since no chipping or spalling was observed in the VCN-Ag coating subjected to a maximum applied load of 50 N, it is reasonable to assume that doping with Ag led to significant enhancement in coating toughness and adhesion strength. In discussing results further, it is worth noting that crack initiation and propagation occurs at the substrate-coating interface when the released elastic energy exceeds the energy required for generation of new surfaces leading to spalling or buckling [42,44]. Thus the enhanced scratch resistance of the VCN-Ag coatings can be attributed to two main factors:
without adhesive failure (Fig. 6f). Additional information about coating cohesion and adhesion failure can be obtained by analyzing their CoFs and acoustic emission (AE) fluctuations during scratch tests (Fig. 7). In both coatings, the first peaks of AE were observed at relatively low values of applied load (Fig. 7a,b). As the applied load increased to 25 N, the substrate underwent significant plastic deformation evidenced by large scratch depth (~ 2.5 μm, that is 2 times the coating thickness) and swelling of material at the edges of scratch after load removal (Fig. 7c,d). The high shear stress caused by plastic deformation of substrate material led to the detachment of the coating along the scratch. Both cohesive (gray contrast) and adhesive (white contrast) types of failures were observed in corresponding SEM image (Fig. 6c). In contrast, the VCN-Ag coating completely withstood the high applied load and remained firmly adhered to the substrate (Fig. 6d–f). The lower intensity and density of AE signals in Fig. 7b indicate the higher fracture toughness and/or lower
(a)
(c)
VCN
(b)
Fig. 6. BSE-SEM surface images of samples 2 (a–c) and 4 (d–f) after scratch tests. Inserts in (a,d) are BSE-SEM images at higher magnification (same scale bars indicate 13 μm) and inserts in (b,c,e,f) are 3D optical images of scratches.
VCN, 50N
Fig. 7. (a,b) Coefficient of friction (red) and acoustic emission fluctuation (blue) curves versus normal load (lower abscissa) and scratch length (upper abscissa) for coatings 2 (a) and 4 (b). (c,d) Scratch profiles as a function of load measured on samples 2 (c) and 4 (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
VCN-Ag
(d)
82
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Fig. 8. (a–f) 3D optical images, SEM micrographs and EDS mapping (inserts) of impact cavities on the surface of coatings 2 (a–c) and 4 (d–f) after dynamic impact tests at (a,d) 250 (b,e) 500, and (c,f) 1000 N with corresponding (g,h) 2D optical depth profiles.
250 µm
500 µm
500 µm
(i) Ag which decreases residual compressive stresses and thus reduces the total level of stresses due to superposition of compressive internal and external stresses [37,44] that lead to spalling or buckling and (ii) soft Ag inclusions which can easily deform under applied load and reduce the elastic energy accumulated in the coating under the stylus tip [15].
strength compared to shear stress in the impact zone. The Ag-doped VCN coating mainly survived after 105 cycles at 1000 N with no brittle fracture observed. The impact spot only revealed small local areas of exposed substrate due to wear proceeded by a plastic deformation, which was 35 times larger than coating's thickness at this load.
3.6. Resistance to dynamic impact load
4. Conclusions
The fracture toughness and adhesion strength of coatings 2 and 4 deposited onto Ni-based alloy were additionally evaluated by dynamic impact testing using cemented carbide ball at different applied loads of 250, 500, and 1000 N for 105 impacts. The impact cavities were studied by SEM-EDS and optical profilometery. At low impact load of 250 N, no coating failure was revealed by SEM (Fig. 8a,d). The EDS analysis indicated WC-Co ball wear debris on the bottom of impact cavities. The cross-sectional depth profiles obtained after the tests at 250, 500, and 1000 N were similar for both types of coatings, hereby indicating that the coatings did not affect substrate plastic deformation (Fig. 8g,h). First considerable adhesive failure events were observed along the border of the impact cavity of coating 2 at 500 N (Fig. 8b), whereas Agdoped sample 4 withstood applied load as high as 1000 N without failure (Fig. 8f). Thus, similar to the results of scratch tests described above, the VCN-Ag coating demonstrated superior toughness and adhesion strength during dynamic impact tests. Since the maximum tensile and shear stress are usually accumulated at the boundary of impact zone [45], the VCN coating failure was observed to occur along the edge of cavities. The adhesive failure can be caused by fatigue cracks generated at the coating-substrate interface and/or by low interface
Comparative study of microstructure, mechanical, and room-temperature tribological properties of VCN and VCN-Ag coatings with 10–11 at.% of Ag deposited by magnetron sputtering of V and C (graphite) targets and simultaneous sputtering of an Ag target either in pure nitrogen or in a gaseous mixture of Ag + 15%N2 was fulfilled. It was demonstrated that the columnar morphology changes to equiaxial one when Ag incorporated into the VCN coating. The coating hardness and Young's modulus are observed to decrease from 22 to 26 to 15–17 GPa and from 260 to 270 to 220 GPa, respectively, with incorporation of Ag. In addition, the value of compressive stress decreases from 1.6 (VCN) to 0.3–0.5 GPa (VCN-Ag). The Ag-doped VCN coatings demonstrated enhanced adhesion and fatigue strength during loading. The coatings withstand a high elastic and plastic deformation during scratching with increasing load up to 50 N without adhesive failure. They also survived applied loads as high as 1000 N for 105 cycles during dynamic impact tests without brittle fracture. The tribological characteristics of the VCN-Ag coatings slightly worsened: their friction coefficient and wear rate increased from 0.4–0.47 to 0.51–0.53 and from 1.5–1.6 × 10− 7 to 8.2–13.3 × 10− 7 mm3/Nm, respectively.
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Acknowledgments The authors gratefully acknowledge the financial support from the Russian Scientific Foundation (Agreement No. 14-19-00273-П) and the Ministry of Education and Science of the Russian Federation within the framework of Increased Competitiveness Program of NUST “MISiS” (No. K2-2016-073) in the part of dynamic impact tests. References [1] S. Calderon Velasco, A. Cavaleiro, S. Carvalho, Functional properties of ceramic-Ag nanocomposite coatings produced by magnetron sputtering, Prog. Mater. Sci. 84 (2016) 158–191. [2] A.V. Bondarev, P.V. Kiryukhantsev-Korneev, D.A. Sidorenko, D.V. Shtansky, A new insight into hard low friction MoCN–Ag coatings intended for applications in wide temperature range, Mater. Des. 93 (2016) 63–72. [3] A.V. Bondarev, P.V. Kiryukhantsev-Korneev, E.A. Levashov, D.V. Shtansky, Tribological behavior and self-healing functionality of TiNbCN-Ag coatings in wide temperature range, Appl. Surf. Sci. 396 (2017) 110–120. [4] P.J. Kelly, H. Li, P.S. Benson, K.A. Whitehead, J. Verran, R.D. Arnell, I. Iordanova, Comparison of the tribological and antimicrobial properties of CrN/Ag, ZrN/Ag, TiN/Ag, and TiN/Cu nanocomposite coatings, Surf. Coat. Technol. 205 (2010) 1606–1610. [5] C.P. Mulligan, D. Gall, CrN–Ag self-lubricating hard coatings, Surf. Coat. Technol. 200 (2005) 1495–1500. [6] P. Basnyat, B. Luster, Z. Kertzman, S. Stadler, P. Kohli, S. Aouadi, J. Xu, S.R. Mishra, O.L. Eryilmaz, A. Erdemir, Mechanical and tribological properties of CrAlN-Ag selflubricating films, Surf. Coat. Technol. 202 (2007) 1011–1016. [7] S.M. Aouadi, D.P. Singh, D.S. Stone, K. Polychronopoulou, F. Nahif, C. Rebholz, C. Muratore, A.A. Voevodin, Adaptive VN/Ag nanocomposite coatings with lubricious behavior from 25 to 1000 °C, Acta Mater. 58 (2010) 5326–5331. [8] J.C. Sánchez-López, M.D. Abad, I. Carvalho, R. Escobar Galindo, N. Benito, S. Ribeiro, M. Henriques, A. Cavaleiro, S. Carvalho, Influence of silver content on the tribomechanical behavior on Ag-TiCN bioactive coatings, Surf. Coat. Technol. 206 (2012) 2192–2198. [9] D.-H. Song, S.-H. Uhm, S.-E. Kim, J.-S. Kwon, J.-G. Han, K.-N. Kim, Synthesis of titanium oxide thin films containing antibacterial silver nanoparticles by a reactive magnetron co-sputtering system for application in biomedical implants, Mater. Res. Bull. 47 (2012) 2994–2998. [10] H. Guo, M. Han, W. Chen, C. Lu, B. Li, W. Wang, J. Jia, Microstructure and properties of VN/Ag composite films with various silver content, Vacuum 137 (2017) 97–103. [11] C.P. Mulligan, T.A. Blanchet, D. Gall, CrN–Ag nanocomposite coatings: tribology at room temperature and during a temperature ramp, Surf. Coat. Technol. 204 (2010) 1388–1394. [12] S.M. Aouadi, H. Gao, A. Martini, T.W. Scharf, C. Muratore, Lubricious oxide coatings for extreme temperature applications: a review, Surf. Coat. Technol. 257 (2014) 266–277. [13] D.V. Shtansky, A.V. Bondarev, P.V. Kiryukhantsev-Korneev, T.C. Rojas, V. Godinho, A. Fernández, Structure and tribological properties of MoCN-Ag coatings in the temperature range of 25–700 °C, Appl. Surf. Sci. 273 (2013) 408–414. [14] A.A. Voevodin, J.S. Zabinski, Nanocomposite and nanostructured tribological materials for space applications, Compos. Sci. Technol. 65 (2005) 741–748. [15] N.K. Manninen, F. Ribeiro, A. Escudeiro, T. Polcar, S. Carvalho, A. Cavaleiro, Influence of Ag content on mechanical and tribological behavior of DLC coatings, Surf. Coat. Technol. 232 (2013) 440–446. [16] M.A. Auger, J.J. Araiza, C. Falcony, O. Sánchez, J.M. Albella, Hardness and tribology measurements on ZrN coatings deposited by reactive sputtering technique, Vacuum 81 (2007) 1462–1465. [17] N. Fateh, G.A. Fontalvo, G. Gassner, C. Mitterer, Influence of high-temperature oxide formation on the tribological behaviour of TiN and VN coatings, Wear 262 (2007) 1152–1158. [18] E. Santecchia, A.M.S. Hamouda, F. Musharavati, E. Zalnezhad, M. Cabibbo, S. Spigarelli, Wear resistance investigation of titanium nitride-based coatings, Ceram. Int. 41 (2015) 10349–10379.
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Microstructure, mechanical, and tribological properties of Ag-free and Agdoped VCN coatings
MARK
A.V. Bondarev⁎, M. Golizadeh, N.V. Shvyndina, I.V. Shchetinin, D.V. Shtansky⁎ National University of Science and Technology “MISiS”, Leninsky prospect 4, Moscow 119049, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: VCN and VCN-Ag coatings Magnetron sputtering Microstructure Intrinsic stress Adhesion strength and fracture toughness Wear- and impact resistance
The aim of this work was a comparative study of the structure, mechanical, and tribological properties of VCN and VCN-Ag coatings. The VCN coatings were deposited by magnetron sputtering of V and C targets either in a gaseous mixture of Ar + 15%N2 or in pure nitrogen. Silver was added into such coatings by simultaneous sputtering of metallic Ag target using an additional ion source. Microstructure and elemental composition of the coatings were studied by means of X-ray diffraction, transmission and scanning electron microscopy, atomic force microscopy, energy-dispersive spectroscopy, and Raman spectroscopy. The coatings were evaluated in terms of their mechanical properties, adhesion strength, intrinsic stress, fracture toughness, room-temperature friction coefficient, as well as wear resistance and fatigue strength. Incorporation of as much as 10–11 at.% Ag was found to cause significant changes in the coating structure and properties: (i) the columnar morphology changes to equiaxial one; (ii) the coating hardness and Young's modulus decrease from 22 to 26 to 15–17 GPa and from 260 to 270 to 220 GPa, respectively; (iii) the friction coefficient and wear rate increased slightly from 0.4–0.47 to 0.51–0.53 and from 1.5–1.6 × 10− 7 to 8.2–13.3 × 10− 7 mm3/Nm, respectively; (iv) the compressive stress decreased from 1.6 to 0.3–0.5 GPa. The Ag-doped VCN coatings withstood high elastic and plastic deformation during scratching with increasing load up to 50 N without adhesive failure and applied load as high as 1000 N for 105 cycles during dynamic impact tests without brittle fracture.
1. Introduction Hard nanocomposite coatings containing Ag have been a hot topic in recent years because of their potential applications in different fields such as wear, friction, corrosion protection, medicine, optics, and so on [1]. Most of data on Ag-doped coatings deals with carbides, nitrides, and oxides of transition metals and focuses on their tribological [2–8] and antibacterial performance [4,8,9]. Although the available results regarding the influence of Ag on the tribological characteristics at room temperature are rather contradictory [3,10,11], the incorporation of Ag was shown to be an effective approach to reduce friction at elevated temperatures [2,7,12,13]. Ag-doped coatings also demonstrated active oxidation protection [3,14] and self-healing ability [3], which results from segregation of metallic Ag particles at sites of crack and oxidation. Although hardness of Ag-doped coatings was typically reported to decrease along with Ag incorporation, positive effect on material's toughness, plasticity, and residual stresses was noted [15]. Vanadium nitride coatings are a good alternative for their more common TiN, CrN, and ZrN counterparts [16–20] due to enhanced selflubricating ability [21]. The mechanical properties of VN-based
⁎
coatings can be improved by doping with carbon. For example, the hardness of VCN coatings reached 33 GPa [22], being higher than that of binary VC and VN coatings [23]. Moreover, introduction of a certain amount of carbon into VN coatings can lead to a decrease in the elastic modulus without compromising their hardness, which is known to have a positive effect on reduced intrinsic residual stress and interfacial stress between the coating and substrate material [1,23]. Another advantage of the reduced Young's modulus is the increase in the elastic strain to failure and plastic deformation described by the H/E and H3/ E2 ratios, respectively [24]. Both parameters were suggested to be indicators of enhanced tribological performance [24,25], adhesion strength [25,26], and fracture toughness [27,28] of coatings. To the best of our knowledge, VCN-Ag coatings were not investigated to date. Therefore the present study aimed at preparing, characterizing, and testing of VCN-Ag coatings with high resistance against different types of wear during sliding, scratching and dynamic impact loading. To understand processes involved into wear of such coatings, the effect of Ag on the microstructure, intrinsic stress, hardness, Young's modulus, adhesion strength, fracture toughness, and fatigue strength of the prepared VCN coatings was thoroughly studied.
Corresponding authors. E-mail addresses: [email protected] (A.V. Bondarev), [email protected] (D.V. Shtansky).
http://dx.doi.org/10.1016/j.surfcoat.2017.10.036 Received 24 July 2017; Received in revised form 9 October 2017; Accepted 10 October 2017 Available online 12 October 2017 0257-8972/ © 2017 Elsevier B.V. All rights reserved.
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Table 1 Deposition parameters and chemical composition of coatings. No.
Deposition parameters
Chemical composition (at.%)
Magnetron current, mA
1 2 3 4
Ag
V, ×103
C, × 103
– – 50 50
2 2 2 2
1 1 1 1
N2 partial pressure, %
Ag
V
C
N
(C + N)/V
100 15 100 15
– – 10 11
46 53 38 44
13 27 12 26
41 20 40 19
1.2 0.9 1.4 1.0
instruments) equipped with a diamond tip with a radius of 200 μm. During the test, a load was raised monotonously up to the maximum value of 50 N resulting in a scratch length of 5 mm. The dynamic impact tests of the VCN and VCN-Ag-coated samples deposited onto Ni-based alloys were carried out using an Impact Tester (CemeCon) at different applied loads of 250, 500, and 1000 N for as many as 105 impacts. The coatings were exposed to a range of impacts at a constant impact frequency of 50 Hz using a cemented carbide ball (WC-6% Co) with a diameter of 5 mm as counterpart material.
2. Experimental Vanadium (V) and graphite (C) targets utilized in the present study for magnetron sputtering were manufactured by electron beam melting and hot-pressing methods, respectively. The targets, 120 mm in diameter and 6–8 mm in thickness, were sputtered either in a gaseous mixture of argon and nitrogen (Ar + 15% N2) or in pure nitrogen. The targets were sputtered in pulsed DC mode at powers of 1000–1100 W (V) and 600–650 W (C), and a frequency of 50 kHz. In order to obtain Ag-doped VCN coatings, simultaneous sputtering of metallic Ag target was carried out using an additional ion source operating at a current of 50 mA. A scheme of deposition setup was presented elsewhere [29]. During deposition, all other process parameters were kept constant as follows: substrate negative bias voltage 50 V, substrate-to-target distance 10 cm (from V and C targets) and 14 cm (from Ag target), total gas pressure 0.10–0.12 Pa, and substrate temperature 380–400 °C. The deposition time was 40 min, which resulted in coating thickness of 1.1–1.3 μm. Single crystal Si (100) and high-temperature Ni-based alloy were used as substrates. They were ultrasonically cleaned in isopropyl alcohol for 5 min and etched by Ar ions directly in the vacuum chamber for 2–10 min prior to deposition. The microstructure and elemental composition of coatings were studied by means of X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. The XRD patterns of coatings deposited onto (100) Si wafers were obtained on an Ultima IV X-ray difractometer (Rigaku) using grazing incidence (5°) geometry and CoКα radiation. The Raman spectra of as-deposited coatings were recorded at a wavelength of 473 nm using a Raman spectrometer built into a NTMDT instrument (NTEGRA Spectra) which also has an AFM module. The AFM unit was used in order to measure sample surface roughness in tapping mode. A JEM 2100 (JEOL) TEM microscope was used for microstructural studies. The coating chemical composition was determined using an EDS NORAN System 7 (Thermo Scientific). S-4800 (Hitachi) and JEM7700F (JEOL) scanning electron microscopes were utilized to determine the coating cross-section morphology and surface topography, as well as for structural analysis in the deformation zones after tribological, adhesion, and impact tests. The coating hardness and Young's modulus were evaluated using a Nano Hardness Tester (CSM Instruments) equipped with a Berkovich diamond indenter tip calibrated against fused silica. The applied load was 4 mN, and tip penetration depth did not exceed 10% of the coating thickness. The values of internal stress were calculated from the substrate-curvature radius using modified Stoney's formula. The typical size of Si samples coated with VCN and VCN-Ag films, 1.2 μm thick, used for internal stress measurements was 10 × 10 × 0.5 mm3. A ballon-disc tribometer (CSM Instruments) was employed for the evaluation of coating tribological characteristics. The coatings were tested against a 6 mm-sized Al2O3 ball at room temperature. The normal load, sliding speed, and radius of the wear tracks were 1 N, 0.1 m/s, and 4 mm, respectively. The wear track profiles were measured using a WYKO NT1100 optical profiler (Veeco). The adhesion strength of the as-deposited coatings was determined using a scratch tester (CSM
3. Results and discussion 3.1. Elemental composition The deposition experiments were carried out either in pure nitrogen or in a gaseous mixture of Ag + 15%N2 and the chemical composition of as-prepared VCN and VCN-Ag coatings is presented in Table 1. Since in N-rich coatings 1 and 3, the (C + N)/V ratio was high (≥1.2), it is reasonable to assume that excess of carbon could form an amorphous aC phase as it was observed in MoCN-Ag coatings [13]. Deposition in a gaseous mixture of Ar + 15%N2 led to a significant increase in carbon content from 12 to 13 to 26–27 at.% and, accordingly, to a decrease in nitrogen from 40 to 41 to 19–20 at.%. The observed (C + N)/V values, 0.9 for Ag-free coatings and 1.0 for Ag-doped coatings, were consistent with the stoichiometry data for the V(Cx,Ny) phase (0.72 ≤ x + y ≤ 0.98) [30]. The addition of Ag affected only the content of V, whereas the concentration of nonmetallic elements remained almost same. Reducing the amount of metallic elements with addition of Ag was reported previously for ZrCN-Ag [31] and TiCN-Ag [8] coatings. Note that although oxygen content is difficult to determine using EDS analysis due to overlapping of the lines from V and O, the presence of a small amount of oxygen cannot completely be excluded. 3.2. Structure and phase composition 3.2.1. X-ray diffraction XRD patterns of as-deposited coatings are presented in Fig. 1a. It is seen that the VCN phase with fcc structure and Fm3m space group (JCPDS 00-035-0768) is the main crystalline phase in all coatings. The phase demonstrated a weak preferential orientation along its (200) direction. Small maxima observed in the XRD patterns of Ag-free coatings in the range of 45–50° 2θ, which are particularly well seen in curve fitting (Fig. 1, inserts), suggest the presence of minor amounts of h-V2N (JCPDS 00-032-1413) and h-V2C phases (JCPDS 01-073-1320). More detailed interpretation of these phases using XRD patterns is difficult because VC and VN phases with different stoichiometry strictly depending on C and N contents were reported [32–35]. Since the position of the (111)Ag peak is very close to those of the (111)VC and (111)VN peaks, the presence of metallic Ag cannot be unambiguously confirmed from the corresponding XRD patterns. Note, however, that in case of the Ag-doped coatings, the (111) and (200) VCN maxima were broadened and shifted towards higher 2θ angles, hereby indicating the superposition of (111)Ag and (111)VCN peaks. When Ag was added into 78
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Fig. 1. (a) XRD patterns of as-deposited VCN and VCN-Ag coatings 1–4 listed in Table 1 and (b) their corresponding Raman spectra.
(a) V2C
D
VCN
V2C
G
(b)
V2N
3.2.3. Scanning and transmission electron microscopy All coatings were characterized by SEM (Fig. 2). Coatings 1 and 3 with a high value of (C + N)/V ratio, which, according to the Raman spectra, presumably contained an amorphous phase, were additionally studied by TEM (Fig. 3). According to SEM analysis, the coatings had almost the same thickness, hereby indicating that their grown rates were also similar and did not depend on chemical composition. The comparison of cross-sectional SEM micrographs presented in Fig. 2 revealed that columnar morphology became less pronounced when Ag was incorporated into the VCN coating. This observation was further confirmed by TEM. VCN coating 1 exhibited nanocolumnar grains, 15–25 nm in diameter, elongated in the direction of coating growth (Fig. 3a,b). The HRTEM image obtained from a single column revealed small nanocrystallites with interplanar spacing of 0.239 and 0.206 nm, which are fingerprints for the (111) and (200) VCN planes (Fig. 3c). Doping with Ag led to significant change in coating morphology caused by metallic atom segregation at the grain boundaries, hereby suppressing the growth of columnar grains. The size of spherical and globular VCN grains in the VCN-Ag coating was between 10 and 60 nm
the VCN coatings, the preferential (111) orientation became more pronounced and no h-V2N or h-V2C phases were observed.
3.2.2. Raman spectroscopy Since the formation of over-stoichiometric VCN phases was not reported previously [23,36], the microstructure of coatings was studied using a more carbon-sensitive method to reveal the presence of a-C. Raman spectra of all types of the VCN-(Ag) coatings are shown in Fig. 1b. Two well resolved peaks located at 1386 and 1601 cm− 1 can be ascribed to the D (disorder) and G (graphitic) absorption bands of carbon. As expected, the highest amount of highly-disordered sp2bonded carbon was observed in sample 3 with a maximum (C + N)/V value. This carbon phase was not formed in sample 2 with (C + N)/ V = 0.9. The appearance of the characteristic D and G peaks in the Raman spectrum of sample 4 with (C + N)/V = 1.0 indicates that the VCN phase is nonstoichiometric. Note that the presence of both sp2 and sp3-bonded carbon in VCN coatings with (C + N)/V < 1 was reported previously [22].
Fig. 2. Cross-sectional BSE-SEM micrographs of Ag-free coatings 1 (a) and 3 (c) and Ag-doped coatings 2 (b) and 4 (d).
(b)
(a)
500nm
VCN (1)
500nm
VCN-Ag (3)
Substrate
Substrate
(c)
(d)
500nm
VCN (2)
Substrate
500nm
VCN-Ag (4)
Substrate 79
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(a)
(b)
Fig. 3. (a,e) Bright and (b,f) dark field SEM micrographs, and (c,g,h) high resolution TEM images with (d,i) corresponding SAED patterns of coatings 1 (a–d) and 3 (e–i).
(c)
(d)
(111) (200) (220) (f)
(e)
(h)
(g)
(i)
(111) (200) (220) described by the ratio H3/E2 [39]. The material elastic recovery did not depend on the presence of Ag in the material. The values of internal stress calculated from the substrate-curvature radius using modified Stoney's formula are shown in Table 2. The stress decreased in the row: 1.6 (sample 1) → 0.5 (sample 2) → 0.3 GPa (Ag-doped samples 3 and 4). The drop in the internal stress from 1.6 to 0.3 GPa with addition of Ag can be well explained by a change in the coating morphology from columnar to equiaxial due to Ag segregation at the VCN grain boundaries. Note that the mechanical properties of the VCN-Ag coatings with 10–11 at.% of Ag were very similar to those previously reported for MoCN-Ag counterparts with Ag content in the range of 5.5–8.8 at.% [2,13].
(Fig. 3e,f). The fringe contrast observed in the HRTEM image of the VCN-Ag coating perfectly matches with characteristic interplanar distances of 0.241 and 0.236 nm typical for the (111)VCN and (111)Ag, respectively (Fig. 3g,h). Note that the (111)Ag planes were often observed to be parallel or have a small misfit (< 5°) with the (111)VCN planes (Fig. 3h). This small misorientation between the adjacent phases provided low interface energy. A thin transition layer about 100 nm thick with well-defined columnar morphology was clearly seen at the film/substrate interface. For the pure VCN coating, this intermediate layer was less noticeable due to the columnar morphology of the entire coating. Selected area electron diffraction (SAED) pattern of the VCN coating demonstrated the presence of the (111), (200), and (220) diffraction rings from the fcc VCN phase with interplanar spacing of 0.236, 0.205, and 0.145 nm, respectively. In case of the Ag-doped VCN coating, the interplanar distances were 0.241, 0.209, and 0.148 nm for the (111), (200), and (220) lines, respectively. The observed increase in the lattice parameters of the VCN-Ag coating can be well explained by reduction of total compressive stresses [37,38], which is further confirmed through stress calculation reported in Section 3.3.
3.4. Tribological properties 3.4.1. Friction Roughness parameters (arithmetic average and root mean squared) of the materials are collected in Table 2. SEM surface images of asdeposited VCN and VCN-Ag coatings 1, 3, and 4 are also presented in Fig. 4. The SEM surface image of coating 2 was similar to that of coating 1 and therefore is not shown. It can be seen that the coating roughness significantly increased upon adding Ag. High surface roughness can affect friction and wear performance, especially during running-in stage. Fig. 4 illustrates how the room temperature (RT) friction coefficient (CoF) changed versus sliding distance during tribological tests of the coatings against alumina ball. Coating 1 showed an average CoF about 0.4, which is consistent with the values reported by Mitterer and
3.3. Mechanical properties The mechanical properties of the coatings are summarized in Table 2. The coating hardness was observed to decrease with an increase in both C and Ag content. Doping with Ag also led to a decrease in Young's modulus and the resistance to plastic deformation which is Table 2 Mechanical and tribological properties of VCN-(Ag) coatings. No.
1 2 3 4
Tribological properties
Roughness
Mechanical properties
CoF
WR × 10− 7, mm3/Nm
RMS, nm
Ra, nm
H, GPa
E, GPa
0.40 0.47 0.51 0.53
1.5 1.6 8.2 13.3
6 5 32 41
5 4 26 31
26 22 17 15
260 270 220 220
± ± ± ±
1.0 1.0 1.5 1.0
80
± ± ± ±
Residual stresses, GPa
10 10 15 10
W, %
H/E
H3/E2
53 56 52 47
0.10 0.08 0.07 0.06
0.21 0.11 0.08 0.05
1.6 0.5 0.3 0.3
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(a)
Fig. 4. (a) Friction coefficient of VCN and VCN-Ag samples 1–4 (Table 1) as a function of sliding distance and (b-d) SEM surface images of as-deposited coatings 1 (b), 3 (c), and 4 (d).
(b)
Coating 1 5 m (d)
(c)
Coating 4
Coating 3 5 m
2 m
Their wear tracks were mostly free from wear products. Small scratches observed at the bottom of wear tracks parallel to the sliding direction indicated abrasion wear (Fig. 5a,b,e). The wear rates rose to 8.2–13.3 × 10− 7 mm3/Nm upon adding of Ag. The bottom of wear tracks of the VCN-Ag samples was also free from wear products, although a significant amount of wear debris accumulated at the wear track edges. This result indicates that abrasion and ploughing, due to relatively low resistance to plastic deformation, were the main wear mechanisms (Fig. 5c,d,e).
coworkers for VCN coatings with similar compositions [23]. The values of CoF observed for the VCN coatings in this study were considerably lower than those previously reported for their VN counterparts [10,17,23], which indicated positive influence of carbon doping. Yet unexpectedly, coating 2 demonstrated higher CoF values than coating 1, both samples having similar surface roughness while the former having a higher C content. This discrepancy can be well explained by the lack of a free a-C phase in coating 2 whose (C + N)/V ratio was found to be low (< 1), see also the Raman spectrum presented in Fig. 1b. Both Ag-doped coatings demonstrated the highest values of CoF in the range of 0.49–0.56, although coating 3 with high (C + N)/V ratio showed slightly lower CoF values in the middle of the test. Thus our observations are in partial compliance with previously reported data showing a descending trend of CoF in C-doped VN coatings as the (C + N)/V ratio increases [23]. The relatively high CoF of the Ag-doped VCN coatings can be explained by their increased surface roughness and the formation of larger amount of wear products. It is also interesting to compare the friction coefficients of the VCN-Ag coatings with their MoCN-Ag counterparts prepared in a similar manner [2,13]. The MoCN-Ag coatings with (C + N)/Mo ratio of 0.83–1.37 demonstrated significantly lower values of RT CoF compared with VCN-Ag coatings 3 and 4 in which (C + N)/V = 1.0–1.4.
3.5. Adhesion strength and fracture toughness Comparative scratch tests of VCN and VCN-Ag coatings 2 and 4 with lower intrinsic stresses (deposited in Ar + 15%N2) were performed to investigate the influence of metallic additive on the adhesion strength and fracture toughness of the materials. The obtained SEM scratch morphology images are presented in Fig. 6. In both coatings, small semicircular tensile cracks were observed to form just behind the stylus (Fig. 6a,d, inserts). Further increase in applied load to 25 N led to an increase in the number of through-thickness cracks in both coatings (Fig. 6b,e). In addition, sample 2 revealed coating chipping inside and around the scratch edges. In case of Ag-doped coating 4, the transverse cracks were observed to change gradually to conformal cracks formed in front of the indenter when load increased. This type of plastic deformation and brittle fracture is typical for relatively ductile materials and can be taken as a sign of high adhesion strength [40–42]. As the loading reached 50 N, coating 2 was worn completely (Fig. 6c), whereas coating 4 withstood a high elastic and plastic deformation
3.4.2. Wear SEM micrographs and cross-sectional wear track depth profiles of the samples are presented in Fig. 5. The values of wear rates are collected in Table 2. Both Ag-free VCN coatings demonstrated extremely high wear resistance, their wear rates being 1.5–1.6 × 10− 7 mm3/Nm.
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Fig. 5. SEM wear track images of VCN-(Ag) coatings 1 (a), 2 (b), 3 (c), and 4 (d) with (e) corresponding depth profiles evaluated with optical profiler.
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internal stresses in the Ag-doped coating compared with its VCN counterpart [43]. An abrupt increase in the CoF of the VCN coating at a normal load of 45 N indicates the onset of substrate exposure (Fig. 7a). After the tests at a load of 50 N, full VCN coating spallation is also evident when comparing the difference in scratch depth of the VCN (4.7 μm) and VCN-Ag (3.8 μm) coatings (Fig. 7c,d) which is approximately equal to the coating thickness. The full removal of the VCN coating is also confirmed by SEM image (Fig. 6c). Since no chipping or spalling was observed in the VCN-Ag coating subjected to a maximum applied load of 50 N, it is reasonable to assume that doping with Ag led to significant enhancement in coating toughness and adhesion strength. In discussing results further, it is worth noting that crack initiation and propagation occurs at the substrate-coating interface when the released elastic energy exceeds the energy required for generation of new surfaces leading to spalling or buckling [42,44]. Thus the enhanced scratch resistance of the VCN-Ag coatings can be attributed to two main factors:
without adhesive failure (Fig. 6f). Additional information about coating cohesion and adhesion failure can be obtained by analyzing their CoFs and acoustic emission (AE) fluctuations during scratch tests (Fig. 7). In both coatings, the first peaks of AE were observed at relatively low values of applied load (Fig. 7a,b). As the applied load increased to 25 N, the substrate underwent significant plastic deformation evidenced by large scratch depth (~ 2.5 μm, that is 2 times the coating thickness) and swelling of material at the edges of scratch after load removal (Fig. 7c,d). The high shear stress caused by plastic deformation of substrate material led to the detachment of the coating along the scratch. Both cohesive (gray contrast) and adhesive (white contrast) types of failures were observed in corresponding SEM image (Fig. 6c). In contrast, the VCN-Ag coating completely withstood the high applied load and remained firmly adhered to the substrate (Fig. 6d–f). The lower intensity and density of AE signals in Fig. 7b indicate the higher fracture toughness and/or lower
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Fig. 6. BSE-SEM surface images of samples 2 (a–c) and 4 (d–f) after scratch tests. Inserts in (a,d) are BSE-SEM images at higher magnification (same scale bars indicate 13 μm) and inserts in (b,c,e,f) are 3D optical images of scratches.
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Fig. 7. (a,b) Coefficient of friction (red) and acoustic emission fluctuation (blue) curves versus normal load (lower abscissa) and scratch length (upper abscissa) for coatings 2 (a) and 4 (b). (c,d) Scratch profiles as a function of load measured on samples 2 (c) and 4 (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. (a–f) 3D optical images, SEM micrographs and EDS mapping (inserts) of impact cavities on the surface of coatings 2 (a–c) and 4 (d–f) after dynamic impact tests at (a,d) 250 (b,e) 500, and (c,f) 1000 N with corresponding (g,h) 2D optical depth profiles.
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(i) Ag which decreases residual compressive stresses and thus reduces the total level of stresses due to superposition of compressive internal and external stresses [37,44] that lead to spalling or buckling and (ii) soft Ag inclusions which can easily deform under applied load and reduce the elastic energy accumulated in the coating under the stylus tip [15].
strength compared to shear stress in the impact zone. The Ag-doped VCN coating mainly survived after 105 cycles at 1000 N with no brittle fracture observed. The impact spot only revealed small local areas of exposed substrate due to wear proceeded by a plastic deformation, which was 35 times larger than coating's thickness at this load.
3.6. Resistance to dynamic impact load
4. Conclusions
The fracture toughness and adhesion strength of coatings 2 and 4 deposited onto Ni-based alloy were additionally evaluated by dynamic impact testing using cemented carbide ball at different applied loads of 250, 500, and 1000 N for 105 impacts. The impact cavities were studied by SEM-EDS and optical profilometery. At low impact load of 250 N, no coating failure was revealed by SEM (Fig. 8a,d). The EDS analysis indicated WC-Co ball wear debris on the bottom of impact cavities. The cross-sectional depth profiles obtained after the tests at 250, 500, and 1000 N were similar for both types of coatings, hereby indicating that the coatings did not affect substrate plastic deformation (Fig. 8g,h). First considerable adhesive failure events were observed along the border of the impact cavity of coating 2 at 500 N (Fig. 8b), whereas Agdoped sample 4 withstood applied load as high as 1000 N without failure (Fig. 8f). Thus, similar to the results of scratch tests described above, the VCN-Ag coating demonstrated superior toughness and adhesion strength during dynamic impact tests. Since the maximum tensile and shear stress are usually accumulated at the boundary of impact zone [45], the VCN coating failure was observed to occur along the edge of cavities. The adhesive failure can be caused by fatigue cracks generated at the coating-substrate interface and/or by low interface
Comparative study of microstructure, mechanical, and room-temperature tribological properties of VCN and VCN-Ag coatings with 10–11 at.% of Ag deposited by magnetron sputtering of V and C (graphite) targets and simultaneous sputtering of an Ag target either in pure nitrogen or in a gaseous mixture of Ag + 15%N2 was fulfilled. It was demonstrated that the columnar morphology changes to equiaxial one when Ag incorporated into the VCN coating. The coating hardness and Young's modulus are observed to decrease from 22 to 26 to 15–17 GPa and from 260 to 270 to 220 GPa, respectively, with incorporation of Ag. In addition, the value of compressive stress decreases from 1.6 (VCN) to 0.3–0.5 GPa (VCN-Ag). The Ag-doped VCN coatings demonstrated enhanced adhesion and fatigue strength during loading. The coatings withstand a high elastic and plastic deformation during scratching with increasing load up to 50 N without adhesive failure. They also survived applied loads as high as 1000 N for 105 cycles during dynamic impact tests without brittle fracture. The tribological characteristics of the VCN-Ag coatings slightly worsened: their friction coefficient and wear rate increased from 0.4–0.47 to 0.51–0.53 and from 1.5–1.6 × 10− 7 to 8.2–13.3 × 10− 7 mm3/Nm, respectively.
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