A superhard CrAlSiN superlattice coating deposited by multi-arc ion plating: I. Microstructure and mechanical properties

Surface & Coatings Technology 214 (2013) 160–167

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A superhard CrAlSiN superlattice coating deposited by multi-arc ion plating: I. Microstructure and mechanical properties Shihong Zhang a,⁎, Lei Wang a,⁎, Qimin Wang b, Mingxi Li a a b

School of Materials Science and Engineering, Anhui University of Technology, Maanshan City, Anhui Province 243002, PR China School of Mechanical and Electronic Engineering, Guangzhou University of Technology, Guangzhou 510006, PR China

a r t i c l e

i n f o

Article history: Received 14 February 2012 Accepted in revised form 15 May 2012 Available online 16 June 2012 Keywords: Microstructure Mechanical properties Cr-Al-Si-N coatings Multi-arc ion plating

a b s t r a c t A superhard CrAlSiN superlattice coating was deposited by multi-arc ion plating (M-AIP). The coating microstructure and mechanical properties were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), nano-indentation, scratch test and pin-on-disk tribo-test. The HRTEM results indicated that the CrAlSiN coating has a superlattice structure containing alternating fcc-(Cr, Al) N and h-(Al, Si) N layers with a period (λ) of 7.0 ± 0.2 nm. The coherent epitaxial growth structure was observed along {0001} plane of the h ‐(Al, Si) N phase and {111} plane of the fcc-(Cr,Al) N phase, due to the template effect of fcc-(Cr, Al) N crystal layer. The CrAlSiN coating exhibited a high hardness (52 GPa), high resistance to plastic deformation (H3/E⁎2 = 0.295 GPa), and high adhesion strength (critical load: 50 N). The wear test showed that the superhard CrAlSiN coating has a very low friction coefficient (0.1–0.2) against the counterpart material of WC/Co. The related mechanisms were discussed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Chromium nitride (CrN) coatings have been widely used in tool and mold industries due to their superior properties, such as high hardness, good adhesion, high-temperature oxidation resistance and especially excellent wear resistance [1,2]. In order to obtain more attractive properties, incorporating other elements into chromium nitride has been explored. Up to now, many ternary and quarternary coatings, such as the Cr–Al–N coating [3,4], Cr–Si–N coating [5,6], Cr–C–N coating [6,7], Cr–Mo–N coating [7], Cr–O–N coating [8], Cr-W-N coating [9,10], Cr– Al–Si–N coating [11–14] and Cr–Mo–Si–N coating [1], etc., have been investigated. Among them, quarternary Cr–Al–Si–N superlattice coatings and Cr–Al–Si–N nanocomposite coatings have drawn more attention for their excellent mechanical properties and thermal stability. In previous studies, the addition of Al and Si elements into the CrN coatings has been found to enhance their oxidation resistance, hardness, plasticity and wear resistance [13–15]. Hsien-Wei Chen et al. [16] reported that doping certain Si content could assist CrAlN coating in prolonging diffusion paths due to their reduced grain sizes, and thereby effectively inhibiting outside oxygen from penetrating into the coatings. The oxidation resistance of CrAlSiN coating was also improved due to forming dense aluminum and silicon oxide thin films on the coating surface [16–19]. The hardness and thermal stability of CrAlSiN coatings

⁎ Corresponding authors. Tel./fax: + 86 555 3553789. E-mail address: [email protected] (S. Zhang). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.144

were significantly enhanced due to the nano-crystalline microstructure and strong interfaces [20,21]. The wear behavior was improved due to the high hardness and low surface roughness [22,23]. The CrAlSiN coatings have been deposited by various processes, such as arc ion plating (AIP) [17–19], magnetron sputtering (MS) [13,24] and hybrid coating system combining AIP and MS [11,25]. The effect of deposition parameters, such as cathode arc current, bias voltage [14] and nitrogen flow on the properties of coatings have been studied. However, due to the fast and violent ionization and deposition processes, it is harder to synthesize multilayered coatings by M-AIP than MS. Among the previous studies, few studies have been conducted on the microstructures and the growth pattern of the nano-multilayered CrAlSiN coatings synthesized by M-AIP. In this study, the CrAlSiN coatings were deposited using multi-arc ion plating (M-AIP) with Cr and Al88Si12 cathodes. As is known, the deposition rate is determined by the total pressure, cathode arc current and negative bias voltage, and the bilayer period is determined by the rotated speed. In this paper, with the controlling of total pressure, cathode arc current, negative bias voltage and especially the rotated speed, the bilayer period was definite, and the nano-multilayered structure was formed. The aim of this article (Part I) was to investigate the microstructures and the growth patterns of the multilayered CrAlSiN coating by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM), and further explore the interrelationships between microstructure, hardness, adhesion, and wear behavior. We also studied the thermal stability and oxidation resistance in Part II.

S. Zhang et al. / Surface & Coatings Technology 214 (2013) 160–167

161

4

5 1 2

3 Fig. 1. The schematic structure of multi-arc ion plating. 1) Substrate, 2) cathode target, 3) entrance of gas, 4) vacuum pump, 5) substrate bias device.

2. Experimental details 2.1. Coatings deposition The CrAlSiN coatings (5.0 ±0.2 μm thick) were deposited on Si (100) wafers and cemented carbide (WC-10 at.% Co) substrates using multiarc ion plating (M-AIP) equipment. The schematic illustration of the system is shown in Fig. 1. The cathode Cr target (99.9% purity) and AlSi target (88 at.% Al, 12 at.% Si, 99.9% purity) were symmetrically installed on the opposite side of the barrel-shaped vacuum chamber wall. The cemented carbide substrates (12 mm in diameter) and the Si wafers (20 mm× 10 mm× 0.5 mm) were polished to an average surface roughness Ra ≤0.05 μm by emery paper and diamond spray, and then ultrasonically cleaned in acetone and alcohol for 15 min, respectively. Then the substrates were placed into the deposition chamber, with the substrate-cathode target distance (STD) of 180 mm, being held by a star-like rotation bracket and rotating at a speed of 4–5 rpm [14]. In the beginning of deposition, the chamber was evacuated down to a base pressure of less than 5.0× 10− 3 Pa. Meanwhile the chamber and the substrates were rapidly heated and kept at the temperature of 350 °C. Then high purity argon (Ar, 99.99% purity) with flow rate of 50 sccm was introduced to keep the chamber pressure at 5.0× 10− 1 Pa. A 15 minute plasma cleaning was conducted at the substrate negative bias voltage of −1000 V. Then Cr and CrN interlayers were deposited to enhance the adhesion strength of the coating. At last, the CrAlSiN superlattice coating was deposited using the Cr cathode and Al88Si12 cathode, simultaneously. The detailed process parameters of each layer are summarized in Table 1.

Table 1 Deposition parameters on each layer of the superlattice CrAlSiN coatings. Various deposition parameters Ar flow rates (sccm) N2 flow rates (sccm) Total pressure (Pa) Nitrogen pressure (Pa) Deposition time (min) Arc current of Cr cathode (A) Arc current of AlSi cathode (A) Bias voltage (V) Sample rotation speed (rpm)

Cr layer

CrN layer

CrAlSiN layer

22 – 0.2 – 10 60 – − 500 4–5

22 95–105 1.2 1.0 30 60 – − 150 4–5

22 125–130 2.2 2.0 100 60 60 –100 4–5

Fig. 2. Cross-sectional OM image of the CrAlSiN coating by Ball-cratering test.

2.2. Mechanical properties Hardness (H) and Young modulus (E⁎ = E/(1 − ν 2)) of the coatings were measured by a nano-indenter G300 using the continuous stiffness measurement (CSM) method at a target frequency of 45 Hz and a maximum indenter load of 800 mN. The maximum depth of indentation was limited under 2000 nm. The drift rate was measured to be below 0.05 nms − 1. The adhesion strength was measured by a scratch tester at a loading speed of 100 N/min and a scratch speed of 4 mm/min. A standard Rockwell-C indentation tester (Wilson/Rockwell B503-R) using a load of 150 kgf was also utilized to characterize the adhesion property of the coatings. The tribological properties were evaluated by a pin-on-disk (POD) wear apparatus using the counterpart material WC/Co. The wear tests were conducted at the temperature of 32 °C. The experimental parameters were as follows: a considerable high load of 20 N, a rotating speed of 637.0 r/min with the orbit diameter of 6 mm and a total testing time of 1800 s. 2.3. Characterization The thickness of CrAlN coatings deposited on cemented carbide was measured using a ball-erosion method [26]. Surface optical microscopy (OM) images of the coatings were obtained using an Eclipse LV150 Upright Metallurgical Microscope (Nikon Co. Ltd. Japan) for the investigation of macroparticle statistics by the Image-Pro Plus software program, by means of which, the number and size of the macroparticles in the images can be automatically counted [27]. Surface morphology of the samples was observed using scanning electron microscopy (SEM, JSM-6490LV). The crystallographic phases of the coatings were analyzed by an X-ray diffractometer (XRD, Bruker D8 Advance) using a Cu-Kα (wavelength λ =0.15406 nm) radiation source operated at 40 kV and 40 mA, the scans recorded in a range from 20 to 80° with a 0.02° step size and a 0.5 s dwell time. The bonding structure was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) with an Al-Kα (hν = 1486.6 eV) radiation source operated at 150 W. The XPS spectra were obtained after plasma cleaning by in situ sputtering with Ar+ ions, and were calibrated by carbon peak C 1 s at

Table 2 The thickness, deposition time and deposition rate of each layer.

Deposition time (min) Layer thickness (μm) Deposition rate (μm/h)

Cr layer

CrN layer

CrAlSiN layer

10 ~ 0.1 ~ 0.6

30 0.59 1.18

100 4.64 2.78

162

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Fig. 3. Typical SEM surface morphology images of CrAlSiN coatings, (a) 1000 times and (b) 5000 times.

284.5 eV. The nanostructure and grain growth features of the coatings were analyzed by a high-resolution transmission electron microscopy (HRTEM, FEI TECNAI G2S-TWIN) equipped with an EDX. Chemical composition of the coatings was analyzed using energy dispersive X-ray spectroscopy (EDX). For the HRTEM observation, the samples were prepared by milling in an focused-ion-beam (FIB) system with incident angle of 3°–8° and ion energy of 4.5 keV, then the samples were ground and polished using a rapid etching system (RES), and transferred onto a copper grid.

3. Results and discussion 3.1. Deposition rate and surface morphology Fig. 2 presents the ball crater micrograph of CrAlSiN coating. The thickness and the deposition rate of each layer are listed in Table 2. As seen in Table 2, the thickness of the Cr interlayer (0.1 μm) was much lower than that of CrN layer (0.59 μm) due to the etching phenomenon

of high bias voltages [28–30]. In addition, the etching phenomenon on the Si substrate can be observed in Fig. 4. Fig. 3 presents the typical surface SEM images of CrAlSiN coatings deposited by M-AIP. As seen in Fig. 3(a), many macroparticles and defects were observed on the surfaces, such as the solid droplets, liquid droplets, meshy shallow craters (MSCs) and pinholes. As shown in Fig. 3(b), the MSCs were observed clearly. Three factors could account for the formation of MSCs: 1) The solid droplets peeled off from the surface for the ion bombardment and etching. 2) The liquid droplets shrank due to the rapidly heating/cooling during deposition. 3) The solid droplets bombarded into liquid droplets [27–30]. Previous studies have explained the reduction mechanism of macroparticles in the AIP process [27,30]. The effect of the macroparticles on the microhardness and adhesion strength has been investigated in our previous study [31]. 3.2. Chemical composition and microstructure Fig. 4 shows the cross-sectional TEM image of the CrAlSiN coatings and the EDX spectra at various locations. It was observed that the

Fig. 4. Cross-sectional TEM image of CrAlSiN coatings and the related EDX spectra at various locations: (a) top, (b) middle, (c) down.

S. Zhang et al. / Surface & Coatings Technology 214 (2013) 160–167

compositions at the locations (a) and (b) were almost the same, 40.5Cr-7.9Al-2.2Si-49.4N (at.%) determined by XPS. The compositions at the location (c) were mainly composed of Cr and N atoms, indicating the CrN interlayers. The Cr/(Al + Si) ratio was 3.5 ± 0.1, which indicated that the evaporation rate of pure metal Cr cathode was higher than that of AlSi alloy cathode at the same deposition parameters. Fig. 5(a) presents the XRD pattern of the CrAlSiN coating. The main phase in the coating was fcc-CrN, with diffraction peaks of (111), (200) and (222), and (111) preferred orientation centered at 37.1° being observed. The h ‐AlN phase was also observed with orientations of (002), (101) and (103), centered at 36.04°, 37.89° and 65.7°, respectively. The diffraction peaks relating to CrxSi or Si3N4 were not detected in the XRD patterns. Fig. 5(b) presents the details of superposed CrN (111), AlN (101) and AlN (002) peaks. Compared with the standard PDF card, the XRD peaks of fcc-CrN crystals tended to shift a little toward low angle due to the larger Al atoms' (0.143 nm) being substituted for the smaller Cr atoms (0.127 nm) in CrN lattice [15]. Thus, the fcc-(Cr, Al) N solid-solution phase might exist in the CrAlSiN coating. Moreover, assuming the substitution of smaller Si atoms (0.134 nm) for larger Al atoms happened, the lattice parameter of h-AlN would decrease and the peak of h-AlN crystal should shift toward high angle. However, owing to the coherent epitaxial growth (as the detailed discussion below), the lattice parameter of h-AlN increased to match that of fcc-(Cr, Al) N. Thus, the peaks of h-AlN crystal tended to shift toward lower angle obviously.

(a) (111) (002 )

700

f-CrN h-Al N

(101)

s

500

s 400 300

s

200

s

s (103)

s s s

(222 )

(200)

Intensity (a.u.)

600

s -WC/Co substrate

100 30

40

50

60

70

80

2 Theta (deg.)

(b) f-(Cr,Al)N f-CrN h-AlN

700

f-CrN PDF-031157 h-AlN PDF-031144

Intensity (a.u.)

600 500 400 300 200 100

(002)

34

35

36

(111) (101)

37

38

39

40

2 Theta (deg.) Fig. 5. X-ray diffraction patterns of CrAlSiN coatings deposited on cemented carbide substrate (a) and the details of CrN (111) and AlN (002) peaks ranging from 33.8 to 40°.

163

Fig. 6 illustrates the fitted Al 2p, Cr 2p, Si 2p and N 1s XPS spectra of CrAlSiN coatings. As shown in Fig. 6(a), the peaks of Al 2p spectra centered at 74.2 eV and 77.8 eV were recognized as Al 2p and Cr 3s peaks respectively. The main peak of the Al 2p spectrum centered at 74.2 eV corresponding to the Cr\Al\N bonds, indicating the presence of (Cr,Al) N phase [19], and the peak appearing at 77.8 eV was recognized as Cr-N bonds with the assignment of CrN [32]. As shown in Fig. 6(b), the peaks of the Cr 2p spectra centered at 575.5 eV and 584.8 eV were recognized as Cr 2p3/2 and Cr 2p1/2 respectively, which were in agreement with Cr\Al\N bonds [19]. As shown in Fig. 6(c), the fitted Si 2p spectrum centered at 101.9 eV was recognized as Si\N bonds corresponding to Si3N4 (101.8 eV) [16,19]. The fitted N 1s spectrum showed three binding energy components at 396.5 eV, 396.8 eV and 397.2 eV, which were recognized as the AlN, (Cr,Al) N and Si3N4 phase respectively. Based on the XRD and XPS spectra results, the CrAlSiN coatings were mainly comprised of fcc-CrN and fcc-(Cr,Al) N solid-solution structures. Fig. 7 presents the cross-sectional HRTEM images of CrAlSiN coatings at the region (b) of Fig. 4. Fig. 7(a) revealed the multilayer structure of the superlattice CrAlSiN coatings deposited by M-AIP. The bilayer thickness was about 8 nm due to the controlling of rotated speed (4–5 rpm). Previous reports pointed out that the sharp interfaces between the two layers throughout the whole coating exhibited a superior hardness due to that the sharp interfaces minimized the intermixing of the two materials at the interfaces [14,25,33]. Fig. 7(b) presents the selected area electron diffraction (SAED) pattern in the middle region of the multilayer coatings. The SAED pattern was constituted of polycrystalline rings superimposed with spots, which indicated the existence of preferred orientation growth of crystals in the coating [14]. As seen in the SAED pattern, the fcc-CrN and h-AlN phases corresponding to the JCPDS-numbers 76–2494 and 79–2497 can be identified [18]. The strong (111), (200), (220) and weak (222) rings were observed, which were consistent with the results of Figs. 6(d) and 5. In addition, the crystal pattern of Si3N4 phase was not found in the SAED pattern. Combining the results of XRD, SAED pattern and XPS spectra, it can be inferred that the Si3N4 phase might form as a solid solution of h-(Al,Si) N and/or amorphous phase in the CrAlSiN coatings [16,26]. Fig. 7(c) shows the HRTEM image in the middle regions of Fig. 7(a). Some research showed that the dark layers were crystalline fcc-(Cr,Al) N and the light layers were amorphous (Al,Si)N [14,18] or the h-(Al, Si) N solid solution phase ref. [19]. In this study, it was acceptable that the light layers were composed of the h-(Al,Si) N solid solution phase. According to the thermodynamic formula ET = Ei + t⁎Eb, where ET, Ei and Eb represented the total energy, the interface energy and the volume energy of per unit area, respectively, t⁎ represented the thickness of the layer. It could be concluded that the interface energy has more impact on the total energy than volume energy when the thickness of the layers (t⁎) was very low. Geyang Li et al. [34] investigated the coherent epitaxial growth of c-TiN/h-TiB2 nanomultilayers and found that when the layer thickness of metastable phases TiB2 was b0.9 nm, the template effect happened and the normally amorphous TiB2 layers crystallized into a hexagonal structure. Other research also found that the template effect might happen when the thickness of one layer was less than 2 nm or much lower than another layer [35]. In this study, the thickness of h-(Al,Si) N layers were less than 1.5 nm, and it was easy for h-(Al,Si) N layers to grow coherently and epitaxially to reduce the interfacial energy. As seen in Fig. 7(c), the coherent epitaxial growth structure was observed clearly along {0001} crystal plane of h-(Al,Si) N phase and {111} crystal plane of fcc-(Cr,Al) N phase [34]. That was why the peaks and the diffraction rings of the h-AlN phases were almost overlapping with fcc-(Cr,Al)N phases as shown in Figs. 5 and 7(b). The orientation relationship of two layers was {0001}h-(Al,Si)N // {111}fcc-(Cr,Al)N. Concerning the epitaxial growth, alternating stress field from the lattice mismatch and the structural barriers could restrict the motion of dislocations across the interfaces. Thus, the hardness of CrAlSiN coatings was enhanced significantly and the fracture toughness was improved.

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(b)

Intensity (arb.units)

2k

(Cr,AI) N

1k

A1 2p

Cr 3s

Intensity (arb.units)

(a)

(Cr,AI) N

20.0k 15.0k

(Cr,AI) N

10.0k 5.0k 0.0

0 68

70

72

74

76

78

80

82

570

575

Binding energy (eV)

580

585

590

595

Binding energy (eV)

(d)

350 300 250 200 150 100 50 0 -50

Si3N4

96

Si 2p

98 100 102 104 106 108 110

Binding energy (eV)

Intensity (arb.units)

(c) Intensity (arb.units)

Cr 2p

16.0k 14.0k 12.0k 10.0k 8.0k 6.0k 4.0k 2.0k 0.0

N 1s (Cr,AI) N AIN Si3N4

394

396

398

400

402

Binding energy (eV)

Fig. 6. The fitted (a) Al 2p, (b) Cr 2p, (c) Si 2p and (d) N 1s XPS spectra of the CrAlSiN coatings. The left and right peaks of the Al 2p spectrum were Al 2p3/2 and Cr 3s respectively in (a), the left and right peaks of the Cr 2p spectrum were Cr 2p3/2 and Cr 2p1/2 respectively in (b).

Fig. 7. (a) Cross sectional HRTEM images of CrAlSiN coating in the (2) region of finger 3. (b) The SAED pattern in the middle region of the multilayer coatings. (c) The HRTEM image in the middle region of (a).

S. Zhang et al. / Surface & Coatings Technology 214 (2013) 160–167

3.3. Mechanical properties of the coatings

700

3.3.1. Hardness and elastic modulus Generally, for measuring the hardness of M-AIP hard films [31], the indentation depth should be above 50 nm to avoid the effect of surface macroparticles and zero drift. In addition, there was no limit to the maximum indentation depth as the use of the continuous stiffness measurement (CSM) method. Thus, the hardness values at the indentation depth in the range of 80 nm–150 nm were close to the real value, which was acceptable as the hardness of the Cr39.6Al8.9Si2.5N48.9 coating (also CrN/h-AlSiN). Fig. 8 illustrates the typical microhardness (H) and Young's modulus (E⁎ = E/(1-ν 2)) curves of the Cr39.6Al8.9Si2.5N48.9 coating and the Cr38.1Al14.2N47.7 coating reported in our previous paper [31]. In addition, the hardness of CrAlSiN coatings reported by other researchers was referred in Fig. 8. The hardness of the CrAlSiN and CrAlN coatings were in the range of 50 ± 2 GPa and 38 ± 2 GPa, respectively. The hardness of the CrAlSiN coating in the present study was much higher than the CrAlN coating and other CrAlSiN coatings referenced [14], [19] and [36]. Three factors could account for the hardening effects in the Cr39.6Al8.9Si2.5N48.9 coating: 1) The solid solution hardening effect. 2) As the addition of Al and Si elements, the critical fracture stress (σc =σ0 + k/d½, d is the crystallite size) of grain boundary was increased due to the grain refinement and strong cohesive energy (σ0) [14,37]. 3) The epitaxial growth induced an alternating stress field in the multilayers and increased the crystal boundary energy, which could restrict the motion of dislocations and enhance the hardness of the coating [34]. Fig. 9 shows the typical loading–unloading curves of the CrAlSiN and CrAlN coatings, from which the quantities such as the hardness (H), effective Young's modulus (E⁎) and elastic recovery (We) can be evaluated. The values of elastic recovery We were 56.25% for the CrAlSiN coating and 51.5% for the CrAlN coating calculated from curves in Fig. 9. The H3/E⁎2 ratio were 0.295 GPa (H =50 GPa and E⁎ = 650 GPa) for the CrAlSiN coating and 0.277 GPa (H =40 GPa and E⁎ = 480 GPa) for the CrAlN coating. As previous studies stated, the H3/E⁎2 ratio was proportional to the resistance of coatings to plastic deformation, thus, the larger the H3/E⁎2 ratio, the better the plastic deformation and toughness [37,38]. In conclusion, compared with the CrAlN coatings, the CrAlSiN coatings possessed higher hardness and fracture toughness.

600

3.3.2. Adhesion strength Adhesion strength between coating and substrate, as an important property of superhard coatings, directly determined the service life of

60

(b)

30

(c)

Load (mN)

500 400 300 200 100 0 0

200

400

600

800

1000

1200

Displacement Into Surface (nm) Fig. 9. Typical loading–unloading curves of CrAlSiN and CrAlN coatings measured by nano-indentation tester.

coating products. Up to now, Rockwell indentation and scratch tests were used to assess the adhesion strength of coatings to substrates. Fig. 10 presents the optical micrograph of Rockwell indentation of the CrAlSiN coating. A few small cracks occurred around the edge of the indentation but no flake was found. Thus, the adhesion strength can be classified under the highest group (HF1) according to Rockwell indentation test VDI 3198 standard [15]. Fig. 11 shows the typical friction-load curve and optical micrographs of scratch measured by scratch tester. Generally, the scratch test includes three stages: the fine cracks stage (critical load denoted as Lc 1) with an oscillation of the friction; the peeling off stage (critical load denoted as Lc 2) with a rapid increase of the friction; and the complete peeling off stage (critical load denoted as Lc 3). The second stage was regarded as a sign of coating adhesion failure, which was defined as adhesion strength [39]. As shown in Fig. 11, Lc 1 was found to be 40 N. Fig. 11 (Lc1) shows the optical images of the scratch scar at the Lc1 point, from which the fine cracks could be seen in the enlarged picture. With further increasing load, the inflection point of the friction-load curve and the rapid increase of the friction appeared at the Lc2 of 50 N. Fig. 11 (Lc2) shows the critical point of spallation and peeling. The high oscillation and high slope of friction-load curve appeared at the position of Lc3. Fig. 11 (Lc3) shows the scratch scar picture at the Lc3 point, where the intermittent fragments are observed on both sides of the scratch. As a result, the adhesion strength of the Cr39.6Al8.9Si2.5N48.9 coating was 50 N at the position of Lc2.

1500

1000

20

AlCrSiN AlCrN Modulus (GPa)

10

Modulus (GPa)

Hardness (GPa)

(a) 40

CrAlSiN CrAlN

2000

AlCrSiN AlCrN Hardness

50

165

500

0 -10 0

50

100

150

200

250

300

Displacement into surface (nm) Fig. 8. The hardness and Young's modulus curves of the CrAlSiN and CrAlN coatings as a function of displacement into surface, and the hardness of (a) reference [14], (b) reference [19], (c) reference [36].

Fig. 10. A typical micrograph of Rockwell indentation on the CrAlSiN coating.

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Lc 1 Friction (N)

30

Lc 3

20

Lc 2 Lc 1

10 0

20

40

60

80

Load (N)

Lc 3

Lc 2

Fig. 11. Typical friction–load curve and optical micrographs of scratch scar measured by scratch tester.

3.4. Tribological behaviors Fig. 12 shows the friction coefficient of the CrN/AlSiN coatings as a function of test time. For comparison, the mean value of friction coefficient of the Cr30.2Al10Si9.8N coatings (constituted by a lot of nanocrystalline (Cr,Al, Si) N embedded in the Si3N4 matrix) was marked from ref. [40] in Fig. 12. As shown in the curves, the friction coefficient of the CrN/AlSiN coating was in the range of 0.1–0.2, which was much lower than that of the Cr30.2Al10Si9.8N nanocomposite coating. Thus, compared with the columnar crystal structure of the CrAlSiN coating in ref. [40], the superlattice structure of the fcc-CrN/h-(Al, Si) N coating in this study was more advantageous to the reduction of the friction coefficient. Fig. 13 shows the SEM images of wear track and the compositions of the wear debris after wear tests. As seen in Fig. 13 (2), the Cr and Al

4. Conclusions In this work, the deposition rate, surface morphology, chemical compositions, microstructures and mechanical properties of CrAlSiN superlattice coatings were investigated. From the study, the following conclusions can be obtained.

0.5

0.4

Friction coefficient

atoms in the middle of wear tracks contents increased, while the N atoms decreased, and the O atoms were also involved in the scar. These results indicated that formation of thin oxide films on the coating surface during the sliding stage could reduce the friction coefficient of the coatings. In addition, W atoms were found at a low content (b0.8%), which might come from the WC/Co counterpart material. It is believed that the superior wear resistance of the CrAlSiN coatings could be attributed to the multilayered crystalline structure of the coatings, which could restrain the growth of the cracks into a parallel direction to the interfaces and the coating surface. In addition, the superior wear resistance might also be due to the super-hardness and excellent resistance to oxidation [15].

ref. [40] 0.3

0.2

0.1

0.0 0

200

400

600

800

1000 1200 1400 1600 1800

Time (s) Fig. 12. Friction coefficients of the CrAlSiN coatings as a function of test time (the mean value of friction coefficient of the CrAlSiN nanocomposite coating from referenced [40] was marked).

(1) In the CrAlSiN superlattice coatings, the coherent epitaxial growth of the h-(Al,Si) N crystal layers was observed. The h-(Al,Si) N layers (following the {0001}(Al,Si)N crystal face) grew along the {111}(Cr,Al)N crystal face of fcc-(Cr,Al) N crystal layers for the thin thickness of Al–Si layers (less than 2.0 nm). (2) The hardness was enhanced by the solid–solution strengthening phases, such as fcc-(Al,Cr) N and h-(Al,Si) N, and the strong cohesive energy in inter-phase boundaries. In addition, epitaxial growth induced an alternating stress field, which restricted the motion of dislocations and enhanced the hardness of the coating. (3) The low elastic recovery (W) and H 3/E⁎2 showed that the CrAlSiN superhard coatings had a sufficient plastic and toughness, which improved the adhesion strength of the CrAlSiN coatings.

S. Zhang et al. / Surface & Coatings Technology 214 (2013) 160–167

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Fig. 13. SEM morphologies of wear tracks and EDS results of the wear debris after sliding test.

Acknowledgments This work was supported by the key research project of Anhui Educational Administration Office (KJ2011A041) and the International S&T Cooperation Projects of Anhui Province (10080703019). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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