Tribological behavior of thick CrN coatings deposited by modulated pulsed power magnetron sputtering

Surface & Coatings Technology 206 (2012) 2474–2483

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Tribological behavior of thick CrN coatings deposited by modulated pulsed power magnetron sputtering Jianliang Lin a,⁎, William D. Sproul a, b, John J. Moore a a b

Advanced Coatings and Surface Engineering Laboratory (ACSEL), Metallurgical and Materials Engineering department, Colorado School of Mines, Golden, Colorado 80401, USA Reactive Sputtering, Inc, 2152 Goya Place, San Marcos, California, 92078 USA

a r t i c l e

i n f o

Article history: Received 5 August 2011 Accepted in revised form 29 October 2011 Available online 4 November 2011 Keywords: Modulated pulsed power (MPP) magnetron sputtering High power pulsed magnetron sputtering (HPPMS) High power impulse magnetron sputtering (HIPIMS) Thick coating CrN Scratch testing

a b s t r a c t Thick CrN coatings (up to 55 μm) have been deposited on different substrate materials (WC–Co, M2 steel, AISI 440C steel, AISI 304 steel, Al–Si eutectic alloy, and copper) using the modulated pulsed power (MPP) magnetron sputtering technique at a high deposition rate of 10 μm/h. The thick CrN coatings have been characterized for the microstructure, mechanical and tribological properties. The adhesion of the coatings with different thicknesses on different substrates was evaluated using progressive scratch tests. The dry sliding wear tests for the thick coatings deposited on the AISI 440C substrate were carried out using a ball-on-disk microtribometer. The wear tests were conducted on the 20 μm thick MPP CrN coatings using different ball materials (α-Al2O3, WC–Co, AISI 440C steel, AISI 302 steel, 100Cr6 steel, and brass), different applied loads (5–40 N) and sliding speeds (5–50 cm/s) for a maximum sliding distance up to 10 km. The coating wear tracks and ball wear scars were examined using scanning electron microscopy. The thick CrN coatings showed coefficients of friction in the range of 0.48–0.94 as sliding against different materials and under different test conditions. The wear rate of the coatings increased with the increases in the applied load and sliding speed, which is related to an increase in the plowing force (plastic deformation of the substrate) and an increase in the flash temperature, respectively. The wear rates of the 20 μm thick MPP CrN coating are in the range of 1 × 10 − 7 to 5.5 × 10 − 7 mm 3 N − 1 m − 1. This study has demonstrated that thicker CrN coatings exhibited larger load and sliding speed capacities, which allow them to be operated under more severe conditions, e.g. higher loads, sliding speeds, and operating temperatures. Published by Elsevier B.V.

1. Introduction As one of the hard transition metal nitride coatings, chromium nitride (CrN) coatings have attracted much attention in corrosion, oxidation and tribological applications in terms of their good mechanical properties, chemical inertness, and good thermal stability [1–4]. Magnetron sputtering is one of the state-of-art techniques for depositions of various hard coating materials, including CrN. In spite of much research and developments in the traditional magnetron sputtering of CrN coatings (e.g. dc magnetron sputtering (DCMS) and middle frequency pulsed dc magnetron sputtering), a common limitation for these techniques is that they are difficult to grow high quality thick coatings (e.g. >10 μm), due to the limited adhesion strength, large build-up strain, and lack the capability of obtaining dense nanostructure in the thick coatings. In recent years, the high power pulsed magnetron sputtering (HPPMS) technique (also known as high power impulse magnetron sputtering (HIPIMS)) has been

⁎ Corresponding author. Tel.:+1 303 273 3178; fax: + 1 303 273 3795. E-mail address: [email protected] (J. Lin). 0257-8972/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.surfcoat.2011.10.053

used to deposit CrN thin films with improved adhesion, structure and mechanical properties [5–6]. However, the low deposition rate from an HPPMS process makes it difficult for thick coating growth [7]. It is also a challenge to deposit thick coatings using magnetron sputtering on soft substrates such as Cu, Ni, Al, and some alloys, due to the large differences in the mechanical and thermal properties between the substrates and coatings [8–9]. Nevertheless, thick hard protective coatings are generally desirable to increase the life and reliability of the coatings and the substrates [10–11]. As a variation of the HPPMS technique [12–13], modulated pulsed power (MPP) magnetron sputtering has shown the capabilities to shape the high power pulses to achieve a high deposition rate and a high degree of ionization of the sputtered material [14–21]. It has shown the capability to synthesize metallic and compound coatings with dense microstructure, good adhesion, and excellent mechanical and tribological properties [18–21]. This is due to the fact that the large number of metal ions provides conditions that favor a high surface mobility of adatoms, which enables them to effectively diffuse on the grown surface and densify the coatings. Moreover, the majority of these metal ions in the MPP plasma exhibited low energies (2–5 eV), which can minimize the incorporated residual stress and defect densities in the coatings [17–18].

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Table 1 Properties of the substrate materials used for the thick CrN coating depositions (the coefficient of linear thermal expansion for the CrN coating is 2.3 × 10− 6/K). Substrate material

WC–Co

M2

SS440C

SS304

Cu

Al–Si

Hardness [GPa] Coefficient of linear thermal expansion [10− 6/K] Surface roughness [nm]

15 5

7.4 11

5.7 10.1

2.2 16.9

0.9 16.6

1.2 18.5

12

10

8

10

5

20

Fig. 2. XRD patterns of the 55 μm MPP CrN coatings deposited on different substrate materials (σ = residual stress).

Fig. 1. The target voltage, current and power waveforms during one modulated pulse used for the MPP CrN thick coating depositions.

These advantages make MPP an ideal technique to deposit thick coatings. Recently, the MPP technique has been successfully used to deposit thick and dense metallic (e.g. Ta) and compound coatings (e.g. CrN, Cr2N) with the thickness in the range of 10–100 μm on hard substrates with high deposition rates [20–21]. These thick MPP coatings have shown good adhesion, dense microstructure, and good mechanical properties. Most early tribological studies of the CrN coatings were carried out on thin films (e.g. 1–5 μm) [22–24]. The tribological properties of the thick CrN coatings (e.g. >10 μm) on different substrates, and under different wear conditions need further investigations and clarifications. It is anticipated that these thick coatings are attractive for wear applications in severe environments, for example, high load, high sliding speed, etc., due to the improved load carrying capacity as the coating thickness was increased. Therefore, the aims of this study include: 1) To evaluate the adhesion of thick MPP CrN coatings (up to 55 μm) on different substrate materials (from soft Al–Si alloy to hard WC–Co). 2) To investigate the wear resistance of thick MPP CrN coatings (20 μm) against different counterpart materials, and different applied loads (5–40 N) and sliding speeds (5–50 cm/s). 2. Experimental details 2.1. The coating depositions The deposition system is a closed field unbalanced magnetron sputtering system equipped with two rectangular unbalanced magnetrons

(320 mm × 127 mm) which are installed oppositely with a distance of 240 mm. A schematic diagram and detailed description of the system were reported earlier [17]. A metal Cr target (99.9% purity) was powered by a MPP generator (SOLO/AXIS-180™ Pulsed DC Plasma Generator, Zpulser LLC.), while the other magnetron was kept in the system to create a closed magnetic field. Thick CrN coatings were deposited on different substrate materials, including WC–Co, M2 tool steel, AISI 440C stainless steel (SS440C), AISI 304 stainless steel (SS304), Cu and Al–Si eutectic alloy coupons. The hardness, roughness and coefficient of linear thermal expansion (CTE) of the substrates are summarized in Table 1. The WC–Co and M2 steel substrates exhibit high hardnesses of 15 and 7.4 GPa, respectively. The SS440C and SS304 steel substrates exhibit hardnesses of 5.7 and 2.2 GPa, respectively. The Al–Si alloy and Cu have low hardnesses of 1.2 and 0.9 GPa, respectively. After mechanical polishing to a mirror finish, the substrates were ultrasonically cleaned in acetone and ethanol for 15 min, respectively. The substrates were mounted on a substrate holder and installed at a distance of 50 mm from the Cr target surface. An Ar plasma etching process was carried out at a pressure of 1.34 Pa and a pulsed dc substrate bias of −650 V (200 kHz and 1.0 μs) for 40 min. A layer of Cr (400–500 nm) was deposited for improving the adhesion between the CrN coatings and the substrate. The MPP CrN coatings with various thicknesses (5 to 55 μm) were reactively deposited by sputtering the Cr target using MPP with an average target power of 3 kW in an Ar/N2 mixture with an Ar to N2 flow ratio of 1:1. The working pressure was maintained at 0.67 Pa. A − 50 V DC substrate bias voltage was used for all depositions, as controlled by a Zpulser AXIS dc bias power supply with arc suppression capability and a maximum 50 A current control. Fig. 1 shows the MPP pulse shape used for the depositions. It was a 1000 μs pulse containing a 500 μs high ionization pulsing period. To increase the ionization degree and plasma density, the peak target current and power during the

Table 2 The deposition conditions for the Cr interlayer and CrN coatings.

Cr layer CrN • • • •

Pa [kW]

Pp [kW]

Ia [A]

Ip [A]

Va [V]

Vp [V]

Repetition frequency [Hz]

Peak Isub [mA/cm2]

Mean Isub [mA/cm2]

Deposition rate [μm/h]

1.5 3.0

108 95

91 83

200 190

466 463

622 575

30 73

350 302

66.2 58.8

8 10

Pa and Pp are the average and peak target powers. Ia and Ip are the average and peak target currents in one pulse length. Va and Vp are the average and peak target voltages. Isub is the substrate ion current density.

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high ionization period for the Cr layer depositions were 200 A and 108 kW, while for the CrN coating depositions were 190 A and 95 kW, respectively. Other detailed MPP sputtering conditions and pulsing parameters were summarized in Table 2. The deposition rate for the MPP CrN coatings was measured to be 10 μm/h. No external substrate heating was used for the depositions. Under the current deposition conditions, the substrate temperature gradually increased to around 380 °C in 50 min and then became sable afterwards.

2.2. Microstructure characterizations The chemical composition of the coatings was measured by electron probe micro-analysis (EPMA) at a 15 kV voltage and a 10 μm beam size, with the calibration of Cr and TiN standards. The crystal structure of the coatings was determined by X-ray diffraction (XRD). The scans were carried out from 30 to 70 2θ in the conventional Bragg–Brentano mode with a Phillips X-ray X'pert diffractometer using monochromatic

Fig. 3. Cross-sectional SEM micrographs of the 20 μm thick MPP CrN coatings deposited on different substrates: (a) WC–Co, (b) AISI 440C steel, (c) Cu and (d) Al–Si alloy.

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2.3. Mechanical and tribological testing

Fig. 4. Hardness and Young's modulus of the 20 μm thick MPP CrN coatings deposited on different substrates.

Table 3 Critical loads of the thick MPP CrN coatings deposited on different substrates with different thicknesses measured using the progressive scratch test.

Cu Al–Si SS304 SS440C M2 WC–Co

5 μm

10 μm

20 μm

30 μm

40 μm

55 μm

≤5 N ≤5 N 20 N 25 N 50 N 65 N

≤5 N ≤5 N 30 N 32 N 57 N 76 N

7N 12 N 37 N 40 N 65 N 88 N

7N 15 N 40 N 58 N 75 N >100 N

12 N 20 N 55 N 75 N 87 N > 100 N

18 N 25 N 65 N 88 N 95 N >100 N

Cu K-alpha radiation (45 kV and 40 mA). The residual stress of the coatings was measured by glancing incident angle XRD (GIXRD) using a SIEMENS X-ray diffractometer (Model KRISTALLOFLEX-810) and calculated using the sin 2ψ method. The thickness and cross-sectional microstructure of the CrN coatings were characterized using a JSM-7000F field-emission scanning electron microscope (FESEM) operated at a 5 kV accelerating voltage.

The hardness and Young's modulus of the coatings were measured by an MTS nano-indenter XPII equipped with a Berkovich diamond indenter. The calculations were made by the Oliver and Pharr method from the load–displacement curve using less than 10% of the coating thickness as the indentation depth [25]. A Teer scratch tester was used to evaluate the adhesion of the coatings on different substrates at different thicknesses (5–55 μm) using a Rockwell C indent tip (tip radius R= 200 μm, and conical angle =120°). The applied load was increased from 5 N up to 100 N at a rate of 100 N/min and the scratch length was 6 mm. The critical load (Lc) was evaluated by monitoring the abrupt acoustic emission (AE) signal change during the progressiveloading process, which generally is associated with the large delamination of the coating. Since some tests didn't show abrupt AE signal change (especially for the coatings deposited on the soft substrates), the scratch tracks were examined by FESEM to verify the coating failure associated with the critical load. Sliding wear tests were conducted on the 5–20 μm CrN coatings deposited on AISI 440C substrates using a ball-on-disk microtribometer (Center for Tribology, Inc) in the ambient atmosphere (25–26 °C, 32– 35% relative humidity). The sliding counterpart is a 6 mm ball. A fresh ball and coating surface were used before each test. The selection of the AISI 440C substrate for the tests is related to the excellent corrosion resistance of the AISI 440C substrate for various applications. Firstly, the wear tests were conducted using different ball materials. Then the wear tests were carried out at different normal loads (5–40 N) and sliding speed (5–50 cm/s) using WC–Co balls. For the tests against different ball materials, α-Al2O3 (H= 26 GPa), WC–Co (20 GPa), SS440C steel (H= 5.6 GPa), SS302 steel (H= 2.5 GPa), 100Cr6 steel (annealed, H = 2.0 GPa), and brass balls (H = 1.4 GPa) were used. The hardness value of the ball materials was measured from the polished ball surface using nanoindentation. The applied load was 10 N. The sliding speed was 10 cm/s. The total sliding distance was 5000 m. However, the sliding distance for the brass ball was decreased to 1700 m due to the rapid wear of the ball material.

Fig. 5. SEM micrographs of the end of the scratch track for the 55 μm CrN coatings deposited on different substrates: (a) WC–Co, (b) M2, (c) AISI 440C, (d) Cu.

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the average wear volume and calculate the coating wear rate. The morphologies and chemical compositions of the wear tracks and ball scars were examined using FESEM and energy dispersive X-ray spectroscopy (EDS), respectively. The wear rate of the balls was estimated by calculating the volume of the spherical crown loss of the ball. Since the temperature at the contact area during wear sliding strongly depends on the sliding speed and the normal load, the flash temperature at the contact area was calculated using the following equations [26]: ΔT ¼  a¼

1 μPv 4 ðK 1 þ K 2 Þa P πH

1=2

Where ΔT is the flash temperature at the contact area, μ is the COF, P is the normal load, v is the sliding speed, a is the contact radius of the real contact area, H is the hardness of the coating, and K1 and K2 are the thermal conductivities of the coating and the counterpart. For the calculations, the thermal conductivities of the WC–Co ball and CrN coatings are 100 W m − 1 K − 1 [27] and 2.0 W m − 1 K − 1 [28–29], respectively. The hardness of the CrN coating use for the flash temperature calculation is 25 GPa (2.5 × 10 10 Nm − 2). 3. Results and discussion Fig. 6. The wear rates and COF values of the 20 μm CrN coatings sliding against different ball materials at a normal load of 10 N and a speed of 10 cm/s.

3.1. Microstructure and mechanical properties of the thick MPP CrN coatings

For the tests at different applied loads, the load was selected to be 5, 10, 15, 20, 30 and 40 N, which is related to an increase in the Hertzian initial point contact stress from 1.17 GPa to 2.34 GPa. The WC–Co ball material was selected because of its relatively high hardness. The sliding speed was 10 cm/s. The total sliding distance was 5000 m. For the tests at different sliding speeds, the sliding speed was increased from 5 cm/s to 50 cm/s. The counterpart material was a WC–Co ball. The normal load was maintained at 10 N. The total sliding distance was 10,000 m. The average coefficient of friction (COF) value was read from the steady sliding state during the tests. After the wear test, the wear track was examined using a Veeco 3D surface profilometer to measure

The EPMA measurements have shown that all thick CrN coatings exhibited stoichiometric composition with a N:Cr ratio close to 1:1. The O content in the coatings is below 1 at.%. Fig. 2 shows the XRD patterns of the 55 μm CrN coatings deposited on different substrates. All coatings exhibited a face centered cubic (fcc) structure. However, the thick CrN coatings deposited on different substrates show different orientations. The coatings deposited on WC–Co, M2 steel and SS440C substrates exhibited a strong (200) peak and a weak (111) peak. The intensity of the (111) peak increased for the coatings deposited on SS304, Cu and Al–Si alloy substrates. The coating on Al–Si alloy also exhibited a (220) peak. As shown in Fig. 2, the peak positions gradually shifted toward a lower diffraction angle for the coatings deposited on SS440C, SS304, Cu and Al–Si alloy substrates. Since the coatings have similar chemical

Fig. 7. SEM micrographs of the wear track of the 20 μm CrN coatings and the ball tip of different materials after the wear test at a normal load of 10 N and a speed of 10 cm/s: (a) sapphire, (b) WC–Co, (c) SS440, (d) SS302, (e) 100Cr6, and (f) brass.

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compositions, the larger shift of the peaks is likely attributed to a higher residual stress in the coatings (e.g. on Cu and Al–Si alloy substrates) (as labeled in Fig. 2), which is partially attributed to the large CTE differences between these substrates and the CrN coatings (CTE(CrN) = 2.3 × 10 − 6/K [30]), as summarized in Table 1. The microstructure of the thick MPP CrNx coatings with different thicknesses up to 55 μm has been documented in Ref [21]. Fig. 3 shows the cross-sectional SEM micrographs of the 20 μm CrN coatings deposited on WC–Co, SS440C, Cu and Al–Si substrates. All 20 μm CrN coatings exhibited a dense microstructure with a small variation in the coating thickness on different substrates (less than 5%). The SEM images taken at a higher magnification depicted disordered short columnar structure for all coatings and typical transgranular fracture feature across the crystals, which can be attributed to high film density and strong boundary bonding strength between the grains [31]. The average columnar grain width as measured across the SEM micrographs slightly increased following the sequence of the coatings deposited on the WC–Co (200 nm), SS440C (250 nm), Cu (750 nm) and Al–Si alloy (840 nm) substrates (Fig. 3). The hardness and Young's modulus of the 20 μm coatings deposited on different substrates are shown in Fig. 4. For the coatings deposited from the WC–Co to the SS304 substrates, the hardness and Young's modulus of the coatings decrease slightly from 25.1 GPa to 23.8 GPa, and from 325 GPa to 307 GPa, respectively. Nevertheless, the hardness and Young's modulus of the coatings deposited on the Al–Si and Cu substrates further dropped to 20 and 19.2 GPa, respectively. The decrease in the coating hardness is mainly attributed to the increase in the grain size of the coatings deposited on the Al–Si and Cu substrates, as shown from the SEM micrographs (Fig. 3).

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coating on the SS440C substrate showed extensive cracks in the track and brittle failure at the edge, which is due to a larger deformation of the substrate and coating within the scratch track. Moreover, the coating on the soft Cu substrate showed a much wider scratch track with an almost complete removal of the coating at the end (verified by EDS), and extensive coating chipping at the edges during the scratch test. The current study has shown the possibility to deposit thick films on difficult substrates with good adhesion. The scratch test results have demonstrated that a larger coating thickness and a harder substrate generally lead to higher Lc values and a better performance of the coating during scratch tests. As summarized in Table 3, the thick MPP CrN coatings deposited on hard WC–Co and M2 steel substrates showed excellent adhesion.

3.2. Tribological behavior of the thick MPP CrN coatings 3.2.1. Adhesion The Lc values of the CrN coatings with different thicknesses deposited on different substrates were summarized in Table 3 as obtained by the scratch test using the same parameters. For the same coating thickness, the coatings deposited on the substrates with a higher hardness exhibited a higher Lc value than the coatings deposited on substrates with a lower hardness. The coatings deposited on Cu and the Al–Si alloy all showed low Lc values less than 25 N, due to the large plastic deformation of the substrates during the scratch tests. These Lc values are similar to earlier reported values associated to a good adhesion for the hard coatings deposited on soft metallic substrates [8]. However, for the coatings deposited on M2 steel and WC–Co, the Lc value is larger than 65 N and 88 N respectively for a thickness greater than 20 μm. Further increasing the coating thickness above 30 μm, the coatings deposited on the WC–Co substrate showed no failure at a maximum load of 100 N. The Lc values of the coatings deposited on the same substrate gradually increased as the thickness of the coating increased, which is related to an increase in the load capacity of the coatings. The elastic field under the indenter is a long-range field that extends into the substrate during the scratch tests. Under a similar applied load, the pull of strength at the coating–substrate interface will be lower for thick coatings as compared to thin coatings, therefore the thick coatings shows higher Lc values. Fig. 5 shows the SEM micrographs of the end of scratch tracks (which corresponds to a maximum load of 100 N) for the 55 μm CrN coatings deposited on WC–Co, M2, SS440C and Cu substrates. The track became wider following the sequence of the coatings deposited on the WC–Co, M2, SS440C and Cu substrates, which is related to a decrease in the substrate hardness. As shown in Fig. 5a, the coating deposited on the WC–Co substrate showed no coating delamination at the end of track, which confirms its excellent adhesion. The coating deposited on the M2 steel substrate showed a certain amount of coating delamination at the very end of the scratch track. In contrast, the

Fig. 8. (a) The flash temperature, and (b) the wear rates and COF values of the 20 μm CrN coatings and WC–Co ball, for the wear tests at different applied loads with a speed of 10 cm/s for a sliding distance of 5000 m.

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3.2.2. Wear tests against different ball materials Fig. 6 shows the variations of the COF, and the wear rates of the coating and counterpart ball, for the 20 μm CrN coating deposited on SS440C sliding against different ball materials. The coating exhibited the lowest COF of 0.48 when it was sliding against an Al2O3 ball. The coating also showed a relatively low COF of 0.56 as sliding against WC–Co. When it was sliding against different steel balls (SS440C, SS302 and 100Cr6), relatively high COF values in the range of 0.7–0.8 were identified. The highest COF of 0.94 was found when it was sliding against brass. The low friction observed for sliding against Al2O3 and WC– Co balls might be explained by the formation of a low shear layer, for example an oxide, at the interface between the coating and the ball. When the coating was sliding against steel and brass balls, the removed materials might have less pronounced lubricant effects. Additionally, the significant removal of the steel and brass ball materials may also lead to an increase in COF. As shown in Fig. 6, the coating showed relatively higher wear rates (2 to 2.5 × 10 − 7 mm 3 N − 1 m − 1) than those of the balls (0.02 to 0.7 × 10 − 7 mm 3N − 1 m − 1) when sliding against Al2O3 and WC–Co balls. On the other hand, the coatings showed relatively lower wear rates (0.6 to 2 × 10 − 7 mm 3 N − 1 m − 1) than those of the balls (3 × 10 − 7 to 6.3 × 10 − 5 mm 3 N − 1 m − 1) as sliding against SS440C, SS302 100Cr6 and brass balls. This is mainly attributed to the much higher hardness of Al2O3 and WC–Co balls as compared to other ball materials. In these cases, the wear losses from both the coating and the ball are important. However, when the coating was sliding against much softer steel and brass balls, the major wear body switched to the ball

material. Correspondingly, the wear rate of the coatings decreased (Fig. 6). This became especially pronounced when the coating was sliding against 100Cr6 and brass, in that extremely low wear rate in the low range of 10 − 8 mm 3 N − 1 m − 1 was measured. The above observations can be further supported by Fig. 7, which shows the SEM micrographs of the wear tracks of the 20 μm CrN coatings after sliding against different ball materials. The corresponding images of the ball scar after the tests are also shown in Fig. 7. It can be seen that the width of the wear track as well as the diameter of the spherical crown of the ball increased following the sequence of Al2O3, WC–Co, SS440C, SS302, 100Cr6 to brass, which is a direct proof of the increase in the wear rate of the ball, as shown in Fig. 6. 3.2.3. Wear tests at different applied loads Fig. 8 shows the effects of the applied load on the flash temperature, and the wear rates and COF values of the 20 μm CrN coatings and the WC–Co ball. The wear rates of the 5 μm and 10 μm CrN coatings were also plotted against the load in Fig. 8b for comparison. The flash temperature at the contact area gradually increased from 77 °C to 214 °C as the applied load increased from 5 N to 40 N (Fig. 8a). The wear rates of the coatings and the ball gradually increased with an increase in the applied load. The 20 μm CrN coating performed well in the wear tests with an applied load up to 40 N. Further wear tests at larger loads (>40 N) are not applicable due to the limited range of the load suspension system in the tribometer. However, it was found that the 5 μm and 10 μm CrN coatings failed rapidly during the wear

Fig. 9. SEM micrographs of the wear scar of the WC–Co ball tip after sliding against the 20 μm CrN coatings at different normal loads with a speed of 10 cm/s for a sliding distance of 5000 m.

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test as the applied load was increased to 30 N and 40 N respectively. It is also noted that the wear rate of the coating decreased as the thickness of the coating increased for the same applied load. These results are consistent with various earlier reports that a thicker hard coating can assist a softer substrate in carrying a higher load under severe wear conditions and thus decrease the contact area [32]. Fig. 9 shows the SEM micrographs of the WC–Co ball scars after the wear tests at different applied loads for the 20 μm CrN coating. The wear track in the CrN coatings exhibited similar smooth morphologies without damages, and the width of the wear tracks matches the diameter of the wear scar on the ball (not shown here). The wear volume loss of the ball increased as the applied load was increased for the same sliding distance of 5000 m. This increase is also related to an increase in the wear rate of the coatings (Fig. 8b). For the tested load range, the wear rates of the 20 μm CrN coating (in the range of 1.1 to 5.6 × 10 − 7 mm 3 N − 1 m − 1) are always higher than the wear rates of the WC–Co ball (in the range of 7.6 × 10 − 9 mm 3 N − 1 m − 1 to 3.2 × 10 − 7 mm 3 N − 1 m − 1) for the same load, as shown in Fig. 8b. For the 20 μm coatings, the COF value of the tests decreased from 0.62 to 0.53 as the applied load was increased from 5 N to 15 N, and then gradually increased to 0.79 with an increase in the load up to 40 N. Given the large thickness of the MPP CrN coatings used for the wear test (20 μm), the load carrying capacity of the coatings is much larger than the thin films. Therefore, the substrate deformation should be considered small when the applied load is low (e.g. 5 N to 15 N). Under these circumstances, the friction mainly comes from the elastic deformation of the surface. Therefore the plowing or hysteresis effects due to the plastic deformation can be considered small, which decrease the friction [33]. As the applied load was increased to 15 N above, the plastic deformation of the substrate may become significant, which led to a rapid increase in the area at the contact interface (Fig. 9). Under these circumstances, the plowing force due to the plastic deformation became significant. Given the relatively small flash temperature changes as the load increased from 15 N to 40 N (Fig. 8a), the tribochemical effects can be considered similar for the tests. Consequently, the rapidly increasing contact area (Fig. 9) after 15 N greatly increases shear stress at the contact interface, and therefore increases the COF. 3.2.4. Wear tests at different sliding speeds Fig. 10 shows the effects of the sliding speed on the flash temperature, the wear rates and COF values of the 20 μm CrN coatings and the WC–Co ball. The wear rates of the 5 μm and 10 μm CrN coatings were also plotted against the sliding speed in Fig. 10b for comparisons. As compared to the effects of the applied load, the sliding speed showed a more significant effect on the flash temperature at the contact area. The flash temperature of the tests increased rapidly from 54 °C to 562 °C as the sliding speed increased from 5 cm/s to 50 cm/s (Fig. 10a). As shown in Fig. 10b, for the 20 μm CrN coatings, the COF of the tests gradually increased from 0.55 to 0.94 as the sliding speed was increased from 5 cm/s to 50 cm/s. The wear rate of the coating starts at 1 × 10− 7 mm3 N− 1 m− 1 at a sliding speed of 5 cm/s, and remains in the range of 1–1.3 × 10− 7 mm3 N − 1 m − 1 when the speed is below 31.4 cm/s. However, when the sliding speed increased from 31.4 cm/s to 50 cm/s, which corresponds to an increase in the flash temperature from 350 °C to 562 °C, the wear rate of the coating increased up to 3.5 × 10− 7 mm3 N − 1 m − 1. Nevertheless, the 5 μm and 10 μm CrN coatings failed rapidly during the wear tests as the sliding speed was increased to above 31.4 cm/s and 44 cm/s, respectively. Fig. 11 shows the SEM micrographs of the WC–Co ball scar after sliding against a 20 μm CrN coating at different sliding speeds. Since the applied load (10 N) and the total sliding distance (10,000 m) are the same, the wear losses (wear rates) of the balls are close (4.1 × 10 − 8 mm 3 N − 1 m − 1 to 5.8 × 10 − 8 mm 3 N − 1 m − 1) (Fig. 10b and Fig. 11), which are smaller than the wear rates of the coating for the same sliding speed. By examining the wear scar of the ball at

Fig. 10. (a) The flash temperature, and (b) the wear rates and COF values of the 20 μm CrN coatings and WC–Co ball, for the wear tests at different sliding speeds with an applied load of 10 N for a sliding distance of 10 km.

a higher magnification, small dark regions can be identified. As shown in Fig. 11, the areas of the dark region on the ball scar gradually increased as the sliding speed was increased. As shown in Fig. 12, further EDS analyses of the dark and white regions of the WC–Co ball scar (for a sliding speed of 20 cm/s) confirmed that the white grains are WC–Co ball material. In contrast, the dark regions contain a large amount of O and W, and with the appearance of Cr. This result indicates that oxides were formed in these dark regions from the tribochemical reactions between the CrN coating and the WC–Co ball. Based on the EDS results, the oxides contain chromium oxides and possibly tungsten oxides, which tend to be formed more easily at higher sliding speeds because of the larger flash temperature at the contact area (Fig. 10a).

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Fig. 11. SEM micrographs of the wear scar of the WC–Co ball tip after sliding against the 20 μm CrN coatings at different sliding speeds with an applied load of 10 N for a sliding distance of 10 km.

As a result, the increase in the COF between the CrN coatings and the WC–Co ball as the sliding speed was increased is the consequence of the increased hard debris trapped in the contact area, which increases the roughness, shear stress and COF. This effect became significantly pronounced when the sliding speed was increased to above 31.5 cm/s, which is related to a high flash temperature of 350 °C. The increase in the wear rate at higher sliding speeds for the CrN coatings is consistent with the increase in the COF, which can be attributed to several aspects. Firstly, the significantly increased flash temperature at the contact area at higher sliding speed decreased the wear resistance of the CrN coatings. It has been well documented that the hexagonal Cr2N phase (h-Cr2N) starts to form in the CrN coatings as early as 400–500 °C in the ambient air [34]. When the sliding speed is higher than 31.4 cm/s, the high flash temperature (400–500 °C) could possibly lead to the locally formation of the h-Cr2N phase in the contact region, which may compromise the wear resistance

of the CrN coatings, since the h-Cr2N coatings exhibited degraded tribological properties as compared to the cubic CrN coatings [21,24]. Secondly, the increase of sliding speed led to an increase in the sliding surface roughness due to the formation of tribochemical products between the two sliding surfaces as shown in Fig. 11, which is probably also responsible for an increase in the COF. At last, the increase in the wear rate of the coating may also be related to the micro vibration generated during the test at high speeds, which could introduce impact damage to the two sliding surface [35–36]. Overall, the wear test results obtained at different applied loads and sliding speeds have demonstrated that the thickness of the CrN coatings plays a critical role in determining the wear resistance of the coatings. The thicker coatings exhibited a larger load capacity than the thin coatings, which allow them to be operated under more severe conditions, e.g. higher loads and sliding speeds, as well as higher operating temperatures.

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Fig. 12. SEM micrograph and the EDS analysis of the WC–Co ball worn surface after sliding against a 20 μm CrN coating at a sliding speed of 20 cm/s.

4. Conclusions The current study has demonstrated the possibility to deposit thick tribological coatings with good adhesion on various substrates with different mechanical and thermal properties using the MPP technique. In this study, thick CrN coatings (up to 55 μm) have been deposited using the MPP technique at a high deposition rate (10 μm/ h) on different substrates. Scratch tests have shown that the critical load of the CrN coatings deposited on the same substrate increased as the thickness of the coating increased. For the same coating thickness, the coatings deposited on the harder substrates exhibited a higher critical load than the coatings deposited on the softer substrates. The wear tests at different applied loads (5 N to 40 N) and sliding speeds (5 cm/s to 50 cm/s) have demonstrated that the wear rate of the thick MPP CrN coating increased at higher applied load and sliding speed, which is related to an increase in the plowing force (plastic deformation of the substrate) and an increase in the flash temperature, respectively. The wear rates of the thick MPP CrN coatings are in the range of 1 × 10− 7 to 5.5× 10− 7 mm3 N− 1 m− 1 for the applied loads and speeds. This study has demonstrated that the thicker MPP CrN coatings exhibited a larger load capacity than the thin coatings, which allows them to be operated under more severe conditions, e.g. higher loads, sliding speeds, and operating temperatures. Acknowledgment The authors are grateful for financial support of this research program from DOE-OIT, ATI, and the North American Die Casting Association (NADCA). References [1] X.M. He, N. Baker, B.A. Kehler, K.C. Walter, M. Nastasi, J. Vac. Sci. Technol., A 18 (1) (2000) 30. [2] I. Milošev, J.M. Abels, H.H. Strehblow, B. Navinšek, M. Metikoš‐Hukovič, J. Vac. Sci. Technol., A 14 (4) (1996) 2527. [3] P.H. Mayrhofer, G. Tischler, C. Mitterer, Surf. Coat. Technol. 142 (2001) 78. [4] J. Lin, Z.L. Wu, X.H. Zhang, B. Mishra, J.J. Moore, W.D. Sproul, Thin Solid Films 517 (2009) 1887.

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