Influence of Ag contents on structure and tribological properties of TiSiN-Ag nanocomposite coatings on Ti–6Al–4V

Applied Surface Science 394 (2017) 613–624

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Influence of Ag contents on structure and tribological properties of TiSiN-Ag nanocomposite coatings on Ti–6Al–4V Chaoqun Dang a,b , Jinlong Li a,∗ , Yue Wang a , Yitao Yang b , Yongxin Wang a , Jianmin Chen a a Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China b School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China

a r t i c l e

i n f o

Article history: Received 22 July 2016 Received in revised form 7 October 2016 Accepted 20 October 2016 Available online 21 October 2016 Keywords: TiSiN-Ag nanocomposite coatings Arc ion plating Structure Self-lubrication Tribological properties

a b s t r a c t TiSiN-Ag nanocomposite coatings with different Ag contents were deposited on Ti–6Al–4V using reactive co-sputtering in multi-arc ion plating system. Influence of Ag contents on structure and tribological properties of TiSiN-Ag nanocomposite coatings was investigated. The TiSiN-Ag coatings were found to have unique nanocomposite structures composed of nanocrystallite and amorphous nc-TiN/nc-Ag/aSi3 N4 . When the silver content was 1.4 at.%, the coating exhibited high hardness (36 GPa), but poor wear resistance. When the silver content was increased from 5.3 to 8.7 at.%, the coatings possessed homogeneous distribution and small variation in hardness. Although these coatings revealed obvious decrease in hardness, significantly reduced in the friction coefficient and possessed excellent tribological properties, besides, the coating with the Ag content of 5.3 at.% showed best wear resistance in artificial seawater and the coating (7.9 at.% Ag) revealed the best wear resistance in ambient air. However, with a further increased incorporation of Ag into the TiSiN-Ag coating (17.0 at.%) resulted in the formation of a large volume fraction of metallic silver, which caused a decrease both in hardness and wear resistance. The coating containing highest Ag concentration (21.0 at.%) exhibited low friction coefficient both in ambient air and artificial seawater, although possessing low hardness. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Ti–6Al–4V alloy possesses unique combination of properties including low density, high strength-to-weight ratio, fracture toughness, good fatigue behavior and excellent corrosion resistance [1]. With the rapid development of aeronautic and marine industries, an increasing number of Ti–6Al–4V are applied in many demanding applications such as: high-performance aircraft compressor casting of turbine engines, aerospace fasteners, aircraft structural components, high-performance automotive parts, marine applications and oil industry [2,3]. Furthermore, the excellent bio-compatibility has extended the medical application of Ti–6Al–4V alloy such as orthopaedics, odontology and stent catheterization [4]. However, the poor wear resistance limits the use of Ti–6Al–4V alloy in some applications [5]. Nowadays, physical vapor deposition (PVD) coatings [6–9] of surface modifications have been employed to improve the wear resistance of Ti–6Al–4V alloy.

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (J. Li). http://dx.doi.org/10.1016/j.apsusc.2016.10.126 0169-4332/© 2016 Elsevier B.V. All rights reserved.

Hard nanocomposite coatings offer an excellent combination of superior mechanical properties (such as high hardness and fracture toughness), high thermal stability and oxidation resistance, good corrosion resistance performance, as well as excellent tribological properties [10,11] have found to be the materials of choice as wearprotection [12]. Titanium nitride (TiN) with the B1-NaCl structure has been widely used as a hard protective coating since the 1980s, while its wide application is limited by its relative low hardness. If a third element Si is incorporated into TiN fabricating ternary coating, the phase formation complexity increases forming a twophase nanocomposite structure, but with that also improves the hardness of TiN coating [13]. Nanocomposite coatings consisting of nanocrystalline metal nitrides and amorphous phases, e.g. Si3 N4 incorporated in TiSiN forming a nanocrystallites/amorphous composite structure (nc-TiN/a-Si3 N4 ), which exhibit superhardness (>40 GPa) [13,14]. However, nanocomposite TiSiN coating exhibits generally high friction coefficient (0.5–0.7) [15], and thus extra selflubrication is needed in many applications. The addition of a soft metal such as Ag or Cu to nanocomposite coating is generally interesting in a variety of fields, such as tribological coatings [16,17] and antibacterial coatings [17–20], of which the friction coefficient was found to be lower and the mechanical properties of these coatings

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could be improved only when small amount of soft metal is added and is maintained [21]. In this paper, the TiSiN-Ag nanocomposite coatings have been deposited on Ti–6Al–4V by multi-arc ion plating. The relationship between the microstructure and tribological properties of TiSiNAg coatings was investigated in an effort to understand how to achieve a combination of high hardness, excellent wear resistance by controlling the Ag contents. 2. Experimental details

matic Al K␣ radiation as the X-ray source with a photon energy (h␯ = 1486.7 eV). The samples were analyzed before and after Ar ion bombardment the original surface in order to identify the original surface chemistry and the steady-state compositions. The base pressure in the analysis chamber was 5 × 10−9 Torr. Spectra were referenced to the C 1s peak of the adventitious carbon (CHx ) set at 284.8 eV. All curve-fitting procedures were carried out using a non-linear least squares fitting method employing the GaussianLorentzian function and considering the background as linear and shirley type. The microstructure was studied in details by TEM, high-resolution TEM (HR-TEM) and STEM (Tecnai F20, USA).

2.1. Coating 2.3. Hardness TiSiN-Ag coatings were deposited on Ti–6Al–4V alloy substrate by multi-arc ion plating in an industrial deposition system (Hauzer Flexicoat 850). The device chamber was equipped with four cathodes, one of TiSi alloy (90 at.% Ti, 10 at.% Si; purity 99.99 at.%) and the other Ti (purity 99.99 at.%) and another of Ag (purity 99.99 at.%). Ti–6Al–4V alloy (15 mm × 15 mm × 5 mm) was used as the substrate, which was ultrasonically cleaned in acetone and alcohol for 15 min respectively. All the substrates were polished to 7000-grit before cleanout. After removal from the alcohol, the samples were dried in ambient air. The substrates were mounted on a pre-cleaned substrate holder and fixed on the carousel in the deposition chamber. A base pressure of less than 4 × 10−5 mbar was reached prior to depositions. The coating deposition process comprised three steps: (i) Ar ion bombardment of the substrates for 2 min by employing a substrate bias of −900, −1100 and −1200 V separately to remove the thin oxide layer and other adherent impurities for better adhesion; (ii) A thin TiN buffer layer was deposited 10 min from Ti targets using a 65 A target current and a −30 V substrate bias to introduce a stress gradient layer which further enhance the adhesion of the TiSiN-Ag coatings; (iii) TiSiN-Ag coating depositions of about 2 ␮m in N2 . The target current of the TiSi alloy targets was fixed at 65 A, and the Ag targets at 35 A or 45 A for different Ag contents, respectively. Depositions were undertaken in N2 atmosphere and the flow rate of N2 (purity > 99.99%) was 350 sccm for the TiN buffer layers and TiSiN-Ag coatings. A bias voltage of −20 V was applied on the substrates at a temperature of 450 ◦ C for all TiSiN-Ag coating depositions. 2.2. Composition and structure The composition and structure of the coating were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDS). The XRD patterns were recorded on X-ray diffraction (Bruker D8 X-ray facility) using Cu K␣ radiation (␭ = 0.154 nm), which was carried out at 40 kV and 40 mA with grazing incidence angle 2◦ . The scanning angle was ranged from 20◦ to 100◦ at a scanning speed of 10◦ /min with a 0.02◦ step size. The elemental compositions of TiSiN-Ag coatings were identified by EDS with XPP correction and using the standard of ISO 23833. The thickness and structure of the coatings were determined by cross-sections and top-view images observed by a field emission scanning electron microscopy (SEM, Hitachi S4800, Japan) equipped with an INCA x-sight module (EDS). The chemical binding of the TiSiN-Ag coatings was studied by XPS with a Kratos spectrometer (AXIS UTLTRADLD , UK). XPS spectra were recorded on a Perkin-Elmer spectrometer using monochro-

The measurements of hardness (H) and elastic modulus (E) were performed by the load-depth-sensing nanoindentation method and the continuous stiffness measurement (CSM) mode by a MTS Nano Indenter G200 system equipped with a Berkovich indenter. Considering the rough surface of the coatings by multi-arc ion plating, the samples were polished to roughness less than 50 nm before the tests. The indentation depth was 500 nm. The measurement errors did not exceed 10%. The hardness and elastic modulus were calculated according to the Oliver-Pharr method from the loaddisplacement curves as an average from 49 indents. Hardness maps were determined, where the indents were 50 ␮m from each other in a 300 × 300 ␮m array. 2.4. Tribological property The tribological tests of the TiSiN-Ag coatings against a WC6%Co ball with 3 mm diameter were undertaken in ambient air and artificial seawater at room temperature of 20 ± 5 ◦ C and relative humidity of 80 ± 5% on a ball-on-disc Rtec instruments (Rev.1.0.0, USA) keeping the normal load of 5 N, the linear sliding speed of 20 mm/s and the wear track length of 5 mm. The wear test cycles lasted for 60 min. In this study, the artificial seawater was prepared according to standard ASTM D1141-98. The compositions of the artificial seawater shown in Table 1, where the gross concentration employed in the standard is an average of many reliable individual analyses. The wear track profiles of the coatings were measured using an Alpha-Step IQ profilometer by taking average measurements along the wear tracks. Then, based on the profiles of the wear track at several locations, the wear losses were calculated, from which the specific wear rate was obtained by normalizing the wear volume with the total sliding distance and the applied load. The wear track morphologies of the coatings were initially characterized by Zeiss large chamber scanning electron microscopy (EV018) equipped with an INCA x-sight module (EDS). 3. Results and discussion 3.1. Composition and structure Table 2 shows the compositions, thickness and roughness of the TiSiN-Ag coatings. For the TiSiN-Ag coatings, the Ag contents are 1.4, 5.3, 7.9, 8.7, 17.0 and 21.0 at.%, respectively. Furthermore, a little C and O are found in the coatings with the Ag contents of 1.4–8.7 at.%, and this is from the atmosphere. The thickness varies between 1.99 and 2.92 ␮m. With increasing Ag content, the roughness of the coatings first decreases and then increases. The

Table 1 Chemical composition of artificial seawater. Compound

NaCl

MgCl2

Na2 SO4

CaCl2

KCl

NaHCO3

KBr

H3 BO3

SrCl2

NaF

Concentration (g/L)

24.53

5.20

4.09

1.16

0.695

0.201

0.101

0.027

0.025

0.003

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Table 2 EDS elemental compositions, thickness and roughness of TiSiN-Ag coatings. Samples

1.4 at.% Ag 5.3 at.% Ag 7.9 at.% Ag 8.7 at.% Ag 17.0 at.% Ag 21.0 at.% Ag

Coating composition (rel. at.%) Ti

Si

N

Ag

C

O

24.5 18.7 14.9 13.6 9.6 7.9

7.1 2.5 2.2 1.7 1.1 1.1

61.9 69.7 69.8 71.7 54.4 49.4

1.4 5.3 7.9 8.7 17.0 21.0

3.7 3.3 3.2 2.7 4.2 5.3

1.4 0.5 2.0 1.6 13.7 15.3

coating with Ag content of 7.9 at.% possesses the minimum roughness of 76.1 nm. However, the coatings with Ag contents of 17.0 and 21.0 at.% have significantly larger roughness compared with other coatings and higher concentrations of C and O are found since larger roughness results in larger area contacting with air. Thus, the Ti and Ag are oxidized which can be seen in the later XPS results (Fig. 4). Fig. 1a–f and Fig. 2a–f show the cross-sectional and top-view SEM micrographs of the TiSiN-Ag coatings, respectively. The thicknesses of each coating are measured from SEM cross-section images and the thicknesses are 1.99–2.92 ␮m. All coatings have three different zones including (i) TiSiN-Ag coating, (ii) columnar TiN interlayer and (iii) Ti–6Al–4V substrate. The cross-sectional images of 1.4–8.7 at.% Ag coatings present the homogeneous refining grain and dense structure, while the coatings become obvious coarse with the Ag contents of 17.0 and 21.0 at.%, resulting in high roughness. As shown in Fig. 2a, there are many white particles on the coating surface. The microparticles with small size distribute dispersedly on the coating, having a conical shape and protruding out of the coating surface, resulting in the rough surface of the coating. Grain size of the coatings decreases from the Ag concentration of 5.3 at.% to 7.9 at.% due to providing more nucleation sites of Ag incorporation, with the increasing of Ag concentration, the tendency for Ag segregation is strong, leading to micrometer-size precipitates, as a result, an increased deposition rate may efficiently quench the Ag surface diffusion and enable homogeneous distribution of the element embedded into the TiSiN matrix, of which the Ag content of 7.9 at.% coating exhibits minimum and homogeneous grain size of 12 nm in average determined from the top-view SEM micrograph and the smallest surface roughness of 76.1 nm (Table 2). The other three coatings (Fig. 2d–f) have a nodular morphology with precipitates and the coating with the higher Ag content exhibits the larger precipitates on the surface. The size

Thickness (␮m)

Roughness (nm)

1.99 2.00 2.05 2.85 2.80 2.92

100.1 110.3 76.1 136.4 266.6 321.2

Table 3 EDS point analyses on the particle embedded in the surface (Point A) and the matrix (Point B) acquired from Fig. 2(d). Concentration (wt.%)

Point A Point B

Ti

Si

N

Ag

C

O

– 65.1

– 5.6

– 18.9

98.2 6.5

1.8 3.2

– 0.7

of particles was measured as 0.2–0.7 ␮m. With Ag content increase, high deposition rates of Ag leading to Ag coalescences in quantity, the matrix grain and particles coarsen distinctly, and the coating with the Ag content of 21.0 at.% exhibits more and larger precipitates which also results in the largest surface roughness of 321.2 nm (Table 2). In order to confirm the particles, high-magnification SEM observation with EDS point analyses on the surface from 8.7 at.% Ag coating (Fig. 2d) was achieved and the results are listed in Table 3. EDS point analyses reveal that, the Ti, Si, C, N and Ag are detected on the surface and on one particle incorporated into the TiSiN matrix, as marked by A and B (Fig. 2d), respectively. The Ag accounting for the majority (98.2 wt%) is found on the particles whereas the EDS point on the surface reveals the paucity of Ag. It shows reliable evidence that the particles, which could lead to coarse surface, are found to be metallic silver. Fig. 3a and b shows the XRD patterns of the TiSiN-Ag coatings. All coatings have the diffraction peaks from TiN (PDF#38-1420) and Ag (PDF#04-0783). The coating that contains 1.4 at.% Ag shows intense face-centered cubic NaCl-type (111) and (200) peaks of TiN. As the Ag concentration increases from 5.3 to 21.0 at.%, the Ag (111), (200), (220) and (311) peaks gradually increase in intensity. In addition, the diffraction pattern appears to be a strong TiN (200)-preferred

Fig. 1. Cross-sectional SEM micrographs of TiSiN-Ag coatings with 1.4 at.% Ag (a), 5.3 at.% Ag (b), 7.9 at.% Ag (c), 8.7 at.% Ag (d), 17.0 at.% Ag (e) and 21.0 at.% at.% Ag (f).

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Fig. 2. Top-view SEM micrographs of TiSiN-Ag coatings with 1.4 at.% Ag (a), 5.3 at.% Ag (b), 7.9 at.% Ag (c), 8.7 at.% Ag (d), 17.0 at.% Ag (e) and 21.0 at.% Ag (f).

Fig. 3. (a) X-ray diffraction patterns of TiSiN-Ag coatings with Ag contents of 1.4 at.% (S1), 5.3 at.% (S2), 7.9 at.% (S3), 8.7 at.% (S4), 17.0 at.% (S5) and 21.0 at.% (S6) and (b) enlarged X-ray diffraction patterns of TiSiN-Ag coatings with Ag contents of 1.4 at.% (S1), 5.3 at.% (S2) and 7.9 at.% (S3).

orientation, which tends to shift to higher diffraction angle as the Ag content increased, may be attributed to the distortion of lattice. The peaks at 35.42◦ , 40.38◦ , 63.365◦ , 71.02◦ and 76.89◦ in all diffraction patterns are from the Ti−6Al−4V alloy substrate. The chemical binding of the TiSiN-Ag coatings was characterized by high-resolution XPS Ti 2p, N 1s, Si 2p spectra for selected coating of 5.3 at.% Ag and Ag 3d XPS core for all TiSiN-Ag coatings are presented in Fig. 4. Ti 2p, N 1s and Si 2p spectra show that the

positions of peaks are identical for all coatings. In the Ti 2p spectrum (Fig. 4a), the peaks at 455.1 and 460.7 eV, are corresponding to TiN [22], while the peaks at 456.5 and 462.1 eV are ascribed to Ti2 O3 [22] and the peaks at 458.2 and 463.8 eV are associated with TiO2 [22]. From the XRD patterns, there are no signals relating to the crystalline Si3 N4 or other titanium silicide phases. It indicates that the silicon may be an amorphous phase of either silicon nitride or silicon. In addition, Si 2p spectrum shown in Fig. 4b, where it is observed that the coating has their Si 2p peak at 101.4 eV, associating with amorphous Si3 N4 [22]. Fig. 4c shows the N 1s spectrum, where the peaks at 397.2 eV and 399.2 eV, which corresponds to TiN [23] and Si3 N4 [24], respectively. Fig. 4d shows the Ag 3d spectra, where the Ag 3d 3/2 and Ag 3d 5/2 peaks at 374.4 eV and 368.3 eV ascribing to metallic silver [22], however, two coatings show additional peaks at 367.7 and 373.7 eV associated to Ag2 O and the peaks at 369.1 and 375.3 eV ascribed to Ag clusters. This particular behavior is only observed in the coating with the Ag contents of 17.0 at.% and 21.0 at.% due to the Ag coalescence and metallic Ag oxidation with higher oxygen content. In addition, the peaks of metallic Ag increase in intensity with the Ag content increasing. No Ag Ti intermetallic bonds are observed which is in accordance with XRD results. The Ag Ag bonds convert to Ag Si bonds which shows no distinct peak variation, thus it cannot determine if there exist Ag Si bonds [25], on the other hand, Lau et al. [25,26] have pointed out that there is no formation of metastable phases at temperatures up to 400 ◦ C and from the phase diagram of Ag–Si system, mixing is extremely limited blew 835 ◦ C. Moreover, there are no crystalline silver silicides in the coatings from the XRD results, in accordance with the phase diagram of Ag–Si eutectic system [27]. Fig. 5 shows high-resolution Ti 2p (a), Si 2p (b), N 1s (c) and Ag 3d (d) XPS spectra before and after Ar+ bombardment for different time of the TiSiN-Ag coating with the Ag content of 5.3 at.% for a better understanding its bonding states. As shown in Fig. 5a, after Ar ion bombardment, the shapes of Ti 2p spectrum are strongly modified and the peaks corresponding to TiO2 and Ti2 O3 reveal an obvious decrease in intensity. The peaks corresponding to TiO2 and Ti2 O3 still exist after 65 min of bombardment, which is certainly ascribed to the high affinity of titanium towards oxygen (getter effect). Moreover, oxidation of the coating may occur in the analysis chamber, the long data acquisition time and even during transfer of the specimen from the bombardment chamber to the analysis chamber. As the bombardment time is increased, the TiO2 and Ti2 O3 account for 38.44%, 23.7%, 21.38%, 19.55% and 30.16%, 41.31%,

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Fig. 4. High-resolution Ti 2p (a), Si 2p (b), N 1s (c) XPS spectra acquired from TiSiN-Ag coating with Ag content of 5.3 at.%, and Ag 3d (d) XPS spectra acquired from all coatings, indicating TiN, TiO2 , Ti2 O3 , metallic Ag, Ag clusters, Ag2 O and amorphous Si3 N4 structures.

Fig. 5. High-resolution Ti 2p (a), Si 2p (b), N 1s (c) and Ag 3d (d) XPS spectra before and after Ar+ bombardment for different time acquired from TiSiN-Ag coating with Ag content of 5.3 at.%.

38.63%, 23.69% of the total Ti in the coating before and after Ar ion bombardment for 0, 1, 5 and 65 min, respectively. It is obvious that the TiO2 exhibits a gradually decrease, while the Ti2 O3 increases after 1 min of bombardment and maintains a high. The Ti2 O3 decreases after 65 min of bombardment, however, a couple additional peaks at 455.8 and 461.5 eV corresponding to TiO, and the Ti2 O3 and TiO correspond to 50.36%. The above reason is that preferential sputtering oxygen atoms results in anionic deficien-

cies that the reduction of Ti4+ to Ti3+ and Ti2+ . This phenomenon is observed for all kinds of TiO2 [28]. On the contrary, as the titanium oxides decrease by ion bombardment, the TiN increases. The Si 2p spectra slightly decrease in the intensity by ion bombardment (Fig. 5b). A steady-state is reached, and the N 1s shape does not change further after ion bombardment (Fig. 5c). For Ag 3d spectra shown in Fig. 5d, metallic Ag is observed on the raw coating surface, however, additional perks at 369.1 and 375.2 eV ascribed to Ag clusters appears after 1 min of bombardment due to the Ag coalescence. Using XPS analyses, relative elemental compositions in the TiSiN-Ag coatings with the Ag contents of 1.4 (a), 5.3 (b) and 21.0 at.% (c) as a founction of the bombardment time are shown in Fig. 6. From the composition results shown in Fig. 6a and b reveal the coatings are rich in Ag on the surface. The TiSiN-Ag coatings with the Ag contents of 1.4 and 5.3 at.% reach a stable composition after 5 min of bombardment and the Ag content is 1 ± 0.1 and 4 ± 0.1 at.%, respectively. In addition, as shown in Fig. 6c, the TiSiNAg coating with the Ag content of 21.0 at.% suggests a more distinct silver enrichment on the surface, of which the Ag content reaches a very high value of 93.3 at.% on the raw surface ascribed to Ag coalescence on the surface as shown in Fig. 2f. The Ag content still maintains a high value of 35.0 at.% after 65 min of bombardment. In order to investigate the structure in more details, HR-TEM was conducted, together with the corresponding selected area electron diffraction (SAED) pattern of the TiSiN-Ag coating with Ag content of 5.3 at.%, are depicted in Fig. 7. The SAED pattern in Fig. 7c reveals the existence of polycrystalline phases of TiN and metallic Ag, in which the (111), (200), (220) and (311) reflections are identified. The TEM results are consistent with the XRD results. The coating exhibits a typical nanocomposite structure, in which

618

60 50 40 30 20 10 0 50 40 30 20 10 0

N (a)

4

Ti

3 2

Ag Si N (b) Ti

Ag Si

0

1 0 30 25 20 15 10 5 0

Composition (at. %)

Composition (at. %)

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10 20 30 40 50 60 70

Ion bombardment (min)

Composition (at. %)

(c) 100 Ti N Si Ag

80 60 40 20 0 0

5 15 35 1 2 Ion bombardment (min)

65

Fig. 6. Element compositions obtained by XPS on the original and sputtered surfaces of TiSiN-Ag coatings with Ag contents of 1.4 (a), 5.3 (b) and 21.0 at.% (c).

the plate-like TiN (200) with width ∼5 nm and length 10–26 nm, 7–11 nm TiN (111), 7 nm TiN (220) and 5–12 nm Ag (111) nanocrystalline compounds are embedded in the amorphous Si3 N4 matrix,

where amorphous Si3 N4 around nanocrystallites TiN and metallic Ag boundaries exhibits a fine-grained crystalline. Fig. 8 shows cross-sectional STEM, HR-TEM images and an EDX mapping of the TiSiN-Ag coating with Ag content of 7.9 at.%, and schematic diagram of nanocomposite TiSiN-Ag coatings design for microstructure. As shown in Fig. 8a, the coating has two different zones including homogeneous TiSiN-Ag coating and columnar TiN buffer layer. The columnar TiN buffer layer can be significantly determined by EDS mapping. Ti, Si and N are evenly distributed over the coating thickness. Ag, however, increases in concentration along the coating thickness, which is consistent with the previous XPS results. From the HR-TEM image, the coating exhibits a typical nanocomposite structure. The plate-like TiN (200) with width 4–8 nm and length 7–15 nm, TiN (111) with width 5 nm and length 11 nm, and 2–10 nm Ag (111) nanocrystalline compounds are embedded in the amorphous Si3 N4 matrix. In addition, the amorphous Si3 N4 around nanocrystallites TiN and Ag boundaries exhibits a fine-grained crystalline. Liu et al. [29,30] have pointed out that the crystal grain boundaries prefer to be stuck by amorphous phase, and thus amorphous phase hinders the growth of crystal grains and locates at the crystal grain boundaries which can lead to a remarkable decrease in crystalline grain size, which is ascribed to the energy difference between the grain boundary energy and the crystallite/amorphous phase interfacial energy. For TiSiN-Ag coatings, the strong interfaces between nc-TiN, nc-Ag and amorphous Si3 N4 that increase the cohesive energy of the interface boundaries between the nanocrystalline and amorphous phases, which effectively restrains the grain boundary sliding. This phenomenon is responsible for the grain growth restriction of the nanocrystallites. According to the aforementioned analyses, the microstructure of the TiSiN-Ag coating is the result of a competitive grain growth between Ag, TiN and Si3 N4 segregation. Once TiN or Ag crystallite nucleate and grow, which stop growing when buried under amorphous Si3 N4 due to the bulk diffusion of Ag and TiN is limited at the present temperature [31]. Furthermore, as the increasing of Ag content, for this competitive growth, together with Ag grow and coalescences on the coating surface. This similar competition of grain growth has been reported by J. Lauridsen [32]. The microstructure of the TiSiN-

Fig. 7. HR-TEM image (a), enlarged view (b) and SAED pattern (c) of TiSiN-Ag coating with Ag content of 5.3 at.%.

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Fig. 8. Cross-sectional STEM (a), HR-TEM (c) images and an EDX mapping (b) of TiSiN-Ag coating with Ag content of 7.9 at.%, and schematic diagram of nanocomposite TiSiN-Ag coatings design for microstructure.

Fig. 9. Hardness map (a), hardness and elastic modulus (b) of TiSiN-Ag coatings. The scale bar of harness map is in GPa.

Ag coatings are illustrated in Fig. 8d, which presents a schematic of a nanocomposite coating design that exhibits nanocrystallites and amorphous microstructure of nc-TiN, nc-Ag and amorphous Si3 N4 , where amorphous Si3 N4 around nanocrystallites TiN boundaries and the metallic silver is embedded in the matrix. As shown

in Fig. 8d, a non-uniform Ag distribution along the coating thickness, Ag-rich on the surface is driven by the decrease in the surface energy and strain energy associated with the Ag surface diffusion. Ag segregation diffuse to the surface has a high driving force which is ascribed to the high Ag atom mobility in the TiSiN matrix, the

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high diffusion coefficient, comparatively high cohesive energy of silver and the low interaction with the TiSiN matrix [33]. This phenomenon of Ag-rich on the surface is significantly found when the coating exhibits higher Ag content, which can be seen from the surface morphology. 3.2. Hardness and tribological properties The hardness, elastic modulus and hardness map of the TiSiN-Ag coatings are presented in Fig. 9. The hardness and elastic modulus of TiSiN-Ag coatings linearly decreases from 36 to 4.7 GPa and 468 to 130 GPa, respectively, as the Ag content in the coatings was increased from 1.4 to 21.0 at.%. Each coating exhibits a variation in hardness, as shown by the hardness map in Fig. 9a. When the indent is over an area where the concentration of Ag, low hardness down to 1.1 GPa is revealed (21.0 at.%), since Ag has a very low hardness of 0.5 GPa, and vice versa: if the indent is over an area where enrich nanocrystalline/amorphous composite structure, in which TiN crystallites are embedded in a Si3 N4 amorphous matrix [34,35], hardness up to 45 GPa is obtained (1.4 at.%), because TiSiN has a superhardness over 40 GPa [14]. Generally speaking, the hardness map of the coating with the Ag content of 1.4 at.% exhibits large variation ranged from 9 to 45 GPa in hardness, although it possesses the highest mean hardness of 36 GPa. Large same color area together with a few area of low hardness (5.3, 7.9 and 8.7 at.%), accounting for that these coatings exhibit homogeneous hardness and small variation in hardness due to the uniform distribution of the metallic silver. The coating with Ag content of 17.0 at.% reveals many and large low hardness blue area (1.5–2.8 GPa) and higher hardness of 5–11 GPa due to large Ag agglomerates on coating surface. The coating with highest Ag content possesses lowest hardness range from 1.1 to 8 GPa and inhomogeneous distribution in hardness as the same. The tribological properties were studied in ambient air and artificial seawater separately, and they are shown in Fig. 10, as a function of the Ag contents. The measured friction coefficient presented at the end of the test in ambient air is found to be obvious decrease along with the increasing of Ag content. The decreasing friction coefficient is as a result of the lubricious soft silver. Nevertheless, the measured friction coefficient in artificial seawater does not present the same pronounced variation as in ambient air because of the main lubrication of seawater. The best wear resistance with the low wear rate of 4.7 × 10−7 mm3 /Nm in ambient air is found for the coating with the Ag content of 7.9 at.%, offering an excellent combination of high hardness of 20 GPa and lubricious soft metals. While further Ag content increasing to 17.0 at.%, accompanied by decreasing of hardness, leading to a continuous degradation of the wear resistance due to the concomitant softening of the coating. However, the coating with a very high Ag content of 21.0 at.%, a number of metallic Ag segregates to the surface acting as a solid lubricant which reduced friction and wear, particularly when a continuous Ag interlayer was formed [35]. In artificial seawater, the wear rate reaches a minimum 3.55 × 10−6 mm3 /Nm at a silver content of 5.3 at.%, offering an excellent combination of higher hardness of 25.5 GPa and main lubrication of seawater, as shown in Fig. 10b. Afterwards, the wear rate increases gradually with increasing silver concentration to 17.0 at.% probably due to the concomitant softening of the coatings and the aggravated corrosion of the grain coarsening. Nevertheless, the coating with highest Ag content of 21.0 at.%, a number of metallic Ag segregate to the surface acting as a solid lubricant which reduces friction, and wear and prevents further corrosion particularly when a continuous interlayer consisted of Ag was formed [36]. In order to study the wear mechanisms and the effect of Ag contents on the tribological properties of TiSiN-Ag coatings, the wear tracks for these coatings were observed using SEM equipped with

Fig. 10. The average friction coefficient and wear rate of TiSiN-Ag coatings as a function of Ag contents after ball-on-disc test in ambient air (a) and in artificial seawater (b).

EDS after tribological tests. Low (Fig. 11a, c and e) and high (Fig. 11b, d and f) magnification SEM micrographs of wear track developed on the silver contents of 1.4 at.% (Fig. 11a and b), 7.9 at.% (Fig. 11c and d) and 21.0 at.% (Fig. 11e and f) against WC-6%Co ball with 3 mm diameter at an applied load of 5 N in ambient air are shown in Fig. 11. As shown in Fig. 11b, the wear track at high magnification reveals furrows and localized plastic deformation. The presence of furrows and oxidation due to wear-loss are also observed following wear. Localized plastic deformation and oxidation are attributed to very high concentration of pressure at the point of contact of the mating surface causing local rise in temperature and therefore, oxidation during wear. The mean width and height of wear track are measured as ∼ 329.62 and 12.89 ␮m (Table 4), the track depth is larger than the thickness of the coating, moreover, EDS composition analyses reveal that high concentration of substrate elements of Ti, Al and V. These results demonstrate that the coating possesses bad wear resistance. Fig. 11c and d shows SEM images of wear track on the coating with Ag content of 7.9 at.%. The flake pits and micro cracks (shown by arrows) are observed on the track. The crack formation is attributed to micro fragmentation of oxide scale developed due to fretting motion during wear tests [37].The depth of wear track and wear rate are significantly low, although many flake pits are observed. A large numbers of pits are observed in the wear track which might be due to removal of very fine hard grain during wear test. It is obvious that surface damage is due to abrasion including scratches, oxide scale and material removal by micro crack propagating [38–40]. The EDS analysis result (Table 5) shows that the wear track mainly contains Ti, Si, C, O, Ag, W and Co elements, high concentration of O element indicating that oxidation occurred dur-

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Fig. 11. Low (a, c and e) and high (b, d and f) magnification SEM images of wear track developed on 1.4 at.% Ag (a and b), 7.9 at.% Ag (c and d) and 21.0 at.% Ag (e and f) coatings after ball-on-disc test in ambient air. Table 4 Mean width and depth of wear track from the ball-on-disc wear tests of TiSiN-Ag coatings in ambient air and artificial seawater. In ambient air

1.4 at.% Ag 5.3 at.% Ag 7.9 at.% Ag 8.7 at.% Ag 17.0 at.% Ag 21.0 at.% Ag

In artificial seawater

Mean width of wear track (␮m)

Mean depth of wear track (␮m)

Mean width of wear track (␮m)

Mean depth of wear track (␮m)

329.62 206.54 145.56 156.63 157.99 187.07

12.89 1.42 0.59 0.68 2.11 0.92

204.14 179.40 187.62 187.84 193.87 196.90

2.84 1.89 2.10 2.64 2.52 2.23

ing sliding. W and Co elements are also identified on the wear track, which indicates that a transfer of elements from ball to coating occurred. Fig. 11e and f shows the worn surface of the coating with silver content of 21.0 at.%. Closer inspection of the worn surface confirms that there are a number of micro cracks, but few flake pits. In addition, a smooth surface can be observed due to high concentration of silver formed continuous Ag interlayer [36], which also can be found that the smooth worn surface possesses high concentration of silver (31.0 at.%) from the EDS analysis in Table 5, at the same

time, a large number of furrows are observed on the wear track which can be attributed to the soft metallic silver phase leading to low hardness. Higher concentration of W and Co elements is also identified on the worn surface, which indicates that more transfer of elements from ball to coating occurred. A large amount of deformation causes partial fragmentation of large nitride particles on the rough surface and its removal resulting in the formation of fine micro cracks along with the presence of large area fraction of oxides of very small particle size [41,42]. From the above analyses, excellent wear resistance of the coating with the Ag content

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Table 5 EDS results of the area marked by rectangle on the worn surface from Fig. 11 and 12. In ambient air

Ti (at.%) Si (at.%) N (at.%) Ag (at.%) C (at.%) O (at.%) Al (at.%) V (at.%) Na (at.%) S (at.%) Cl (at.%) Mg (at.%) Ca (at.%) W (at.%) Co (at.%)

In artificial seawater

1.4 at.% Ag

7.9 at.% Ag

21.0 at.% Ag

1.4 at.% Ag

5.3 at.% Ag

52.8 – – – 7.9 31.9 5.3 2.1 – – – – – – –

16.3 1.6 – 8.3 9.0 62.1 – – – – – – – 2.3 0.4

10.4 – – 31.0 1.1 52.0 – – – – – – – 4.6 0.9

22.7 0.7 – 0.2 5.0 67.3 1.0 0.6 1.2 0.8 0.2 – – 0.3 –

17.4 2.0 2.3 3.6 8.1 63.9 – – 1.0 – 0.3 0.4 0.7 – 0.3

8.3 1.0 – 21.5 0.6 62.8 – – 1.7 – – 0.9 0.8 – 2.4

Fig. 12. Low (a, c and e) and high (b, d and f) magnification SEM images of wear track developed on 1.4 at.% Ag (a and b), 5.3 at.% Ag (c and d) and 21.0 at.% Ag (e and f) coatings after ball-on-disc test in artificial seawater.

of 7.9 at.% compared with other TiSiN-Ag coatings is attributed to higher hardness, Ag lubrication, uniformity in microstructure and better micro-structural homogeneity.

Low (Fig. 12a, c and e) and high (Fig. 12b, d and f) magnification SEM images of wear track developed on the silver contents of 1.4 at.% (Fig. 12a and b), 5.3 at.% (Fig. 12c and d) and 21.0 at.% (Fig. 12e and f) against WC-6%Co ball with 3 mm diameter at an

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applied load of 5 N in artificial seawater are shown in Fig. 12. As shown in Fig. 12a and b, it is apparently observed that large area expose attributed to micro cracks propagation, whereas the wear rate is about one order of magnitude lower than the wear rate in ambient air due to seawater lubrication. Moreover, from the EDS result (Table 5), Al and V elements were identified which indicates that exposed substrate was developed during sliding and corrosion in seawater. At the same time, the result shows that the wear track mainly contains Ti, and O elements, indicating that oxidation occurred during wear test. Na, Cl, S, C, and W elements are also identified on the wear track, which indicates that a transfer of elements from ball to coating occurred and the deposition of elements in the artificial seawater during tests. Fig. 12 c–f shows SEM images of the worn surface of the coatings with the Ag contents of 5.3 and 21.0 at.%, respectively. As shown in Fig. 12c and d, some parallel furrows, flake pits and micro cracks can be observed on the worn surface. The SEM morphologies of wear track and the EDS composition analyses (Table 5) show a mechanism of adhesive wear, wherein a transfer layer was formed in the counter surface of wear track, because W and Co elements were found in the EDS results. In addition, Mg and Ca were identified which implies that CaCO3 and Mg(OH)2 might form during tests in artificial seawater. These products of CaCO3 and Mg(OH)2 were known to play a role as a good lubricating medium or protection layer [43]. Whereas, as shown in Fig. 12e and f, only micro cracks and furrows can be observed on the worn surface of the coating with Ag content of 21.0 at.%. Furthermore, a smooth surface covered with Ag interlayer is observed. EDS results (Table 5) of the area marked by rectangle on the worn surface shown in Fig. 12f indicates that the wear track contains O, Ag, Ti, C, Si, Na, Mg, Ca and Co elements. It implies that the smooth interlayer containing silver accompanied by some CaCO3 and Mg(OH)2 was probably formed on the part surface owing to large amount of metallic silver emerging on the surface during wear tests, which is beneficial to the tribological performance of the coating against WC-6%Co ball pairs [43,44]. As a result, the tribological properties can be affected by the structure, mechanical properties and lubrication. Moreover, the wear behavior of deposited coatings is a combined function of friction coefficient, friction environment and hardness.

4. Conclusion Nanocomposite TiSiN-Ag coatings with different silver contents were synthesized by arc ion plating. The coating was characterized as a mixture of nano-crystallites and amorphous composite microstructures of the TiN and metallic Ag embedded in the amorphous Si3 N4 matrix. The coating containing the silver content of 1.4 at.% exhibited high hardness of 36 GPa, but showed poor wear resistance. Improvements in the wear resistance were achieved together with a decrease in the coating hardness to 17.4–25.5 GPa as the Ag contents in the coatings increased from 5.3 to 8.7 at.%, whereas the coatings possessed homogeneous distribution and small variation in hardness attributed to uniform distribution of the metallic silver in hardness map. However, excessive incorporation of Ag into the TiSiN-Ag coating (17.0 at.%) resulted in the formation of a large volume fraction of metallic silver, which led to a decrease in both hardness and wear resistance. The coating with highest concentration of Ag (21.0 at.%) emerged massive metallic silver on the surface, which could form a smooth continuous interlayer composed of Ag, resulting in low friction coefficient both in ambient air and artificial seawater and as the barrier of the corrosion from the seawater tested in the artificial seawater, although possessing low hardness. As a result, quaternary TiSiN-Ag coatings controlled the Ag contents of 5.3–7.9 at.% with high hardness,

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