Friction and wear property of a-CNx coatings sliding against ceramic and steel balls in water

Friction and wear property of a-CNx coatings sliding against ceramic and steel balls in water

Diamond & Related Materials 14 (2005) 1711 – 1720 www.elsevier.com/locate/diamond Friction and wear property of a-CNx coatings sliding against cerami...

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Diamond & Related Materials 14 (2005) 1711 – 1720 www.elsevier.com/locate/diamond

Friction and wear property of a-CNx coatings sliding against ceramic and steel balls in water Fei Zhou a,*, Koshi Adachi b, Koji Kato b a

Institute of Bio-inspired Structure and Surface Engineering (IBSS), Academy of Frontier Science, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China b Laboratory of Tribology, School of Mechanical Engineering, Tohoku University, 980-8579,Sendai, Japan Received 9 December 2004; accepted 23 June 2005 Available online 28 July 2005

Abstract The amorphous carbon nitride coatings (a-CNx ) were deposited on Si3N4 disks using ion beam assisted deposition (IBAD), and their composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS). The a-CNx coatings’ hardness was measured by nano-indentation and the friction and wear property of the a-CNx coatings sliding against Si3N4, SiC, Al2O3, SUS440C and SUJ2 balls in water were investigated by using ball-on-disk tribo-meter. The worn surfaces were observed using optical microscopy and analyzed by XPS. The results of XPS analysis showed that the a-CNx coatings contained 12 at.% nitrogen and the major chemical bonding was sp2 C = N and sp3C – N. The nano-hardness of the a-CNx coatings was 29 GPa, higher than those of balls. Among five kinds of tribosystems, the lowest friction coefficient was obtained in the range of 0.01 to 0.02 for the tribo-systems with SiC and Si3N4 balls, the largest wear rate of the a-CNx coating of 1.77  10 7 mm3/Nm was obtained as sliding against Al2O3 ball, while the smallest wear rate of a-CNx coating of 1.44  10 8 mm3/Nm was gotten as sliding against Si3N4 ball. However, SUJ2 ball showed the highest wear rate of 7.0  10 7 mm3/Nm, whereas Al2O3 ball exhibited the lowest wear rate of ball of 3.55  10 9 mm3/Nm. The XPS analysis on the worn surface for the aCNx coatings displayed that the nitrogen concentration decreased and the sp2-bonding-rich structure was formed after sliding tests in water. D 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon nitride; Friction; Wear; Water lubrication; Oxidation resistance

1. Introduction Until now, man-made and nature lubrication systems are the most important lubrication methods in the world. The main difference is that the man-made lubrication systems are usually Foil-based_ while the nature lubrication systems are Fwater-based_ [1]. Oil-based lubrication systems have been widely used as the driving systems in industries. However, the leakage of oil from man-made devices can pollute the natural environments. To prevent this pollution source, nature has produced water-based lubrication systems through the process of natural selection. However, water-based lubrication systems have some technical prob-

* Corresponding author. Tel./fax: +86 25 84892581. E-mail address: [email protected] (F. Zhou). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.06.025

lems, including controllability, tribology, corrosion and reliability. Therefore, how to emulate the natural systems is one of today’s great challenges [1]. Currently, carbon coatings have been intensely studied, not only because of their effectiveness in tribological application, but also due to their potential as new and efficient semiconductor and optical thin film materials. The most promising tribological application areas are presently sliding components and tools for cutting of non-ferrous materials [2– 4]. However, adhesive problems and poor thermal and oxidation stability at elevated temperature restrict their potential use [5]. Therefore, it is imperative to look for the new carbon-based coating to replace the conventional carbon coatings. The introduction of additional elements such as Si, F, N and metals in the carbonaceous structure has been found to modify the properties of the conventional carbon coatings [6].

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Since Liu and Cohen [7] in 1989 predicted theoretically that the existence of a meta-stable covalent carbon nitride compound (h-C3N4) with an analogous structure to h-Si3N4, and this carbon nitride compound with a high bulk modulus, might have higher hardness than diamond, many attempts at developing new processing methods to obtain carbon nitride films have been performed [8– 13], such as plasmas activated chemical vapor deposition, reactive magnetron sputtering, reactive sputtering, triode sputtering, laser ablation of graphite in the presence of atomic or molecular nitrogen source or ammonia, variations of ion-assisted physical deposition technologies and ion implantation. But until now, nearly all CNx films grown at room temperature are amorphous mixtures of carbon and carbon nitride phases with x ranging from 0.1 to 0.5 [14,15]. Nitrogen incorporation in the carbon coatings decreases the fraction of sp3 carbon bonds by the formation of C – N, C=N and CKN bonds. Previously, the micro- and macro-tribological properties of amorphous carbon nitride (a-CNx ) coatings have been already investigated when the a-CNx coatings slide against SiC, Si3N4, Al2O3 and steel in various gases [16 – 33]. Most of these studies are related to their performances in hard disk drive applications. Recently, the a-CNx coatings have been already found to enhance the SiC ceramic’s wear resistance and shorten the running-in period as the SiC ball slid against the a-CNx coating at low or high sliding speed in water [34,35]. This suggests that the a-CNx be very promising materials for the sliding parts’ coating in natural lubrication systems such as water pump or human joint. However, the detailed tribological properties of carbon nitride coatings against different materials and the mechanism of friction and wear under water lubrication have not yet been clarified. Thus, the purpose of this study is to investigate the friction and wear behaviors of the a-CNx coatings sliding against ceramic and steel balls in water at room temperature and to find the excellent tribo-couples with low friction and wear properties in water.

2. Experimental procedures 2.1. Deposition method of a-CNx coating The IBAD machine is made by Hitachi Ltd, Japan, and its schematic diagram has already been shown in Ref. [34]. Prior to IBAD process, Si3N4 disks (u30 mm  t 8 mm) were ultrasonically cleaned in acetone and ethanol for 30 min, and then the roughness of coated surface for Si3N4 disks was measured by Surfcom-1500DX profilometer. A carbon target with purity of 99.99% was put into the electron beam evaporator and a substrate jig with Si3N4 disk was installed on the substrate holder with two screws, and then the vacuum chamber was subsequently evacuated to lower than 2.0  10 4 Pa. The deposition

procedures and the deposition parameters for the a-CNx coatings also have already been in detail described and tabulated in Ref. [34]. 2.2. Composition and chemical bonding analysis of a-CNx coatings The composition and chemical bonding of the a-CNx coatings was determined by a scanning ESCA microprobe (Quantum 2000, Physical Electronics Inc, USA). 2.3. Surface roughness and mechanical properties of a-CNx coatings The coatings’ surface roughness was measured by Surfcom-1500DX profilometer, and their hardness and Young’s modulus were evaluated using a Nano Indenter ELIONIX. ENT-1100A. 2.4. Ball-on-disk wear test and microanalysis of sample surfaces The five types of sliding balls were SiC, Si3N4, Al2O3, SUS440C stainless steel and SUJ2 bearing steel, respectively. The diameter of all balls was 8 mm. The balls’ roughness was determined by Surfcom-1500DX profilometer and their mechanical properties were obtained from the ball’s company. These data are listed in Table 1. Prior to each wear test, all samples were ultrasonically cleaned in acetone and ethanol for 30 min. The experiments were performed on the ball-on-disk apparatus consisting of rotating disk sliding on stationary ball at 160 mm/s and 5 N. The rubbing surfaces were submerged in purified water. The contact point was designed at an eccentricity of 7.5 mm from the center of the rotary motion, which created a wear track of 15 mm in diameter on the a-CNx coated Si3N4 disks’ surface. The total friction cycles were 227,520 cycles (equal to the sliding distance of 10,368 m). The friction forces were detected by load cell. The load cell voltage signals were recorded through A/D converter using a compatible PC. The diameter of wear scar on the above-mentioned balls was measured using an optical microscope. The cross-section area of wear track on disk, A, was determined using Tencor P-10 surface profilometer (Kurashiki Kako Co. LTD, Japan). Thus, the specific wear Table 1 Roughness and mechanical properties of balls Ball

SiC

Al2O3

Si3N4

SUS440C SUJ2

Roughness 0.0885 0.0528 0.0552 0.0533 0.0596 R a, Am *Vickers hardness 22 16.5 15.3 7.2 7.5 H v, GPa *Young’s modulus 430 370 308 204 208 E, GPa * The data were from the balls’ company.

F. Zhou et al. / Diamond & Related Materials 14 (2005) 1711 – 1720

(a)

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(b) C1s

a-CNx coating

N1s

2

3

sp C-N sp C=N 2

Intensity (a.u.)

Intensity (a.u)

sp C=N

3

sp C-N

N-O N-O

C-O

292

290

288

286

284

282

410

408

406

Binding energy (eV)

404

402

400

398

396

394

392

Binding Energy (eV)

Fig. 1. XPS spectra of the C 1s (a) and N 1s (b) photoelectron peaks for a-CNx coatings.

rates for balls and coatings were determined using the following equation: ws;b ¼

3:14d 4 64RW L

ð1Þ

ws;d ¼

2krA WL

ð2Þ

where R is the ball radius, d is the diameter of wear scar, r is the wear track radius, W is normal load and L the sliding distance. 2.5. Observation of wear track on disk To know the wear mechanism of the a-CNx coatings in water, the wear tracks on coatings analyzed using the scanning ESCA microprobe (Quantum 2000, Physical Electronics Inc, USA) and observed by the optical microscopy.

3. Results and discussion

C = N, sp3 C – N and C – O bonds, respectively. Likewise the N 1s line was deconvoluted into four peaks at binding energies of 398.5, 400.1,401.7 and 404 eV, which were marked as C – N, C=N and N – O bonds, respectively. The appearance of C –O and N –O bonds displayed that the coatings’ surface was contaminated by oxygen from air. The results from Fig. 1 indicated that the sp3 C – N and sp2 C=N were the major bonds in the a-CNx films. 3.2. Surface roughness and mechanical properties of a-CNx coatings As seen in Table 2, the arithmetic mean roughness R a of the a-CNx coating was a little smaller than that of Si3N4 substrate. This indicated that the energetic particle bombardment enhanced the mobility of carbon atoms on the growing surface and induced the smooth surface. Fig. 2 displayed nano-indentation load vs. indentation depth curves for a-CNx coatings. Based on the standard Oliver and Pharr approach [36], the mean values of the elastic modulus (E) and the hardness (H) for the a-CNx film were calculated from the nano-indentation load-displacement

3.1. composition and chemical bonding of a-CNx coatings

Table 2 Surface roughness and mechanical properties of a-CNx coatings and Si3N4 disk Name

R a, Am

H, GPa

E, GPa

a-CNx Si3N4

0.0251 0.0280

29 T 2 16*

330 T 20 290*

* The data were from the sample’s company.

1200 1000

Fmax=980µN hmax=0.057µm

800

Load F, µN

Fig. 1 illustrated the C 1s and N 1s peaks in XPS spectra of a-CNx coatings. According to the XPS analysis, the aCNx coatings contained 12% nitrogen atoms. To know the possible chemical bonding configurations of nitrogen doped into the carbon network, the individual C 1s and N 1s lines were deconvoluted into Gaussian line shapes. The C 1s line was also deconvoluted into three peaks at binding energies of 285.1, 286.5 and 288 eV, which were assigned to sp2

600 400 200 0 0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Displacement h, µm Fig. 2. Nano-indentation load vs. indentation displacement curves for a-CNx coatings.

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F. Zhou et al. / Diamond & Related Materials 14 (2005) 1711 – 1720

(b) 0.30 o

Friction coefficient µ

0.25

In water, T=23 C, Normal load=5N, Sliding speed=160mm/s

0.20

Tribo-pairs SiC ball/a-CNx coating Al2O3 ball/a-CNx coating Si3N4 ball/a-CNx coating SUS440C ball/a-CNx coating SUJ2 ball/a-CNx coating

0.15 0.10 0.05 0.00 0.0

0.25 Tribo-pairs SiC ball/a-CNx coating Al2O3 ball/a-CNx coating Si3N4 ball/coating SUS440C ball/a-CNx coating SUJ2 ball/a-CNx coating

O

Friction coefficient µ

(a)

In water,T=23 C, 0.20 Normal load=5N, Sliding speed=160mm/s 0.15

0.10

0.05

0.00 0.2

0.4

0.6

0.8

0

1.0

4

8

12

16

20

4

4

Sliding cycle N,X10 cycles

Sliding cycles N, X10 cycles

Fig. 3. Variation of friction coefficients with sliding cycles for a-CNx coatings against different mating materials at 5 N and 160 mm/s in water. (a) Initial friction coefficients within 10 000 cycles; (b) Friction coefficients within larger cycles.

curves (Fig. 2) and listed in Table 2. The results in Table 2 showed that the a-CNx coatings offered a combination of reasonably high hardness and reduced stiffness with a remarkable elastic recovery. It indicated that the nitrogen incorporation in carbon increased the sp2 carbon bonds’ fraction so that the tribological property of the films was improved (low friction coefficient and better durability). 3.3. Friction of a-CNx coating sliding against ceramic balls and steel balls in water

0.12

10

0.06

0.03

0.00

Si3N4ball a-CNx

SiC ball Al2O3 ball SUJ2 ball SUS440C ball a-CNx a-CNx a-CNx a-CNx

Tribo-pairs Fig. 4. Mean steady-state friction coefficients after running-in for five kinds of tribo-pairs in water.

Against Si3N4 ball Against Al2O3 ball Against SiC ball

10

-7

10

-8

10

-9

Against SUJ2 ball Against SUS440C ball Normal load=5N, Sliding speed=160mm/s o In water, T=23 C

3

0.09

-6

mm /N.m

Specific wear rate of a-CNx coating

Mean steady-state friction coefficient, µ

The friction coefficients of a-CNx coatings sliding against five kinds of mating materials at a normal load of 5 N under water lubrication, as a function of sliding cycles, are illustrated in Fig. 3. The initial friction behaviors of a-CNx coatings against the different mating materials within 10 000 cycles are shown in Fig. 3(a). For the ceramic tribo-systems, the initial friction coefficient of the Al2O3/a-CNx tribo-couple was 0.17. With an increase in sliding cycles, the friction coefficient of the Al2O3/a-CNx tribo-couple decreased

rapidly, reaching a minimum value of 0.09 at 500¨1000 cycles, and then increased gradually to a constant value of 0.11. But for the Si-based ceramic balls, such as SiC, Si3N4, the initial friction coefficient of the SiC/a-CNx tribo-pairs is 0.11, lower than that of the Si3N4/a-CNx tribo-pairs (0.13). With an increase of sliding cycles, the friction coefficients reduced apparently, but the friction coefficients of the Si3N4/ a-CNx all are larger than those of the SiC/a-CNx tribo-pairs. As the a-CNx coating sliding against steel balls in water, the initial friction coefficient of the SUJ2 ball/a-CNx tribocouples was 0.18, further higher than that of the SUS440C/aCNx tribo-couples (0.12). With further sliding, the friction coefficients of the SUJ2 ball /a-CNx and the SUS440C ball /aCNx tribo-couples all decreased. But after 2000 cycles, the friction coefficients of the SUS440C ball/a-CNx tribocouples reached a stable value of 0.066, while those of the SUJ2 ball /a-CNx tribo-couples still decreased. Fig. 3(b) displays the whole range friction coefficient curves of the a-CNx coating sliding against five kinds of mating materials. With an increase in sliding cycles, the friction coefficient for the Al2O3/a-CNx tribo-couples first

a-CNx coating Fig. 5. Specific wear rates of a-CNx coatings after sliding against various balls at 5 N and 160 mm/s in water.

F. Zhou et al. / Diamond & Related Materials 14 (2005) 1711 – 1720

Specific wear rate of ball 3 mm /N.m

10

-6

Normal load=5N, Sliding speed=160mm/s O In water, T=23 C 10

-7

10

-8

10

-9

Si3N4

SiC

Al2O3 SUS440C

SUJ2

Mating balls Fig. 6. Specific wear rate of different balls after sliding against a-CNx coatings at 5 N and 160 mm/s in water.

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increased to maximum value 0.11, then decreased to 0.10, finally varied in the range of 0.087 to 0.11. For the Sibased ceramics/a-CNx tribo-systems, the friction coefficients decreased abruptly with an increase in sliding cycles. The friction coefficients of the Si3N4/a-CNx tribocouples reached a stable value of 0.011 after sliding 20 000 cycles, while those for the SiC/a-CNx tribo-pairs reached a minimum value of 0.01 at 10 000 cycles, and then increased a little and varied in the range of 0.013 to 0.02. When the sliding cycles exceeded 65 000 cycles, the friction coefficients of the Si3N4/a-CNx were lower than those of the SiC/a-CNx tribo-pairs. For the steel ball/a-CNx tribo-systems, the obvious difference was that, the runningin period of the SUS440C/a-CNx tribo-couples was 500 cycles, while that of the SUJ2 ball/a-CNx tribo-systems was 55000 cycles. After running-in, the stable friction

Fig. 7. Optical microstructures of wear scars on mating balls sliding against a-CNx coatings in water: (a) Al2O3 ball, (b) SiC ball, (c) Si3N4, (d) SUS440C ball, (e) SUJ2 ball.

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F. Zhou et al. / Diamond & Related Materials 14 (2005) 1711 – 1720

(a)

(b) 800 o

In water, T=23 C,F=5N, V=160mm/s Against Al2O3 ball

600

Depth of wear track,nm

400 200 0 -200 -400 -600 -800 -1000 0

200

400

600

800

1000

1200

1400

Width of wear track,µm

Fig. 8. Optical microstructure (a) and profile (b) of wear track on a-CNx coatings sliding against Al2O3 ball in water.

coefficient of the SUJ2/a-CNx tribo-pair is around 0.072, slight smaller than that of the SUS440C /a-CNx tribosystems (0.075). The mean steady-state friction coefficients for five kinds of tribo-pairs are exhibited in Fig. 4. As the a-CNx coating sliding against ceramic balls, the Al2O3/a-CNx tribo-couple had the largest mean stable friction coefficient of 0.10, while the a-CNx / Si-based non-oxide ceramic tribocouples had the lowest mean stable friction coefficient (0.013 for Si3N4/a-CNx and 0.017 for SiC/a-CNx ). For steel balls, the mean stable friction coefficient of the SUS440C/a-CNx tribopair was 0.075, slightly higher than that of the SUJ2/a-CNx tribo-pair (0.072). 3.4. Variation of specific wear rates of a-CNx coating and balls The specific wear rates of the a-CNx coatings after sliding against ceramic balls and steel balls in water are displayed in Fig. 5. Among three kinds of ceramic ball’s

(b) 800 F=5N, V=160mm/s O In water, T=23 C Against SiC ball

600

Depth of wear track,nm

(a)

tribo-pairs, the specific wear rate of the a-CNx coatings was highest as sliding against Al2O3 ball. However, when silicon nitride ball was mating materials, the specific wear rate of the a-CNx coatings was lowest. For the tribo-systems with steel balls, the specific wear rate of the a-CNx coating sliding against stainless steel SUS440C ball was two times larger than that against the bearing steel SUJ2 ball. Fig. 6 illustrates the specific wear rate variation with mating balls. For three kinds of ceramic balls, Al2O3 ball had the lowest wear rate, while Si3N4 ball had the largest wear rate. The specific wear rate of SiC ball was slightly higher than that of Al2O3 ball, but four times smaller than that of Si3N4 ball. For steel balls, the specific wear rate of SUJ2 ball was 20 times larger than that of SUS440C. If Figs. 5 and 6 were compared, it was clarified that, when Al2O3, SiC and SUS440C balls were used as mating materials, the specific wear rate of the a-CNx coating was higher than that of its mating ball. On the contrary, as Si3N4 and SUJ2 were used as mating materials, the specific wear rate of the a-CNx

400 200 0 -200 -400 -600 -800 -1000 0

200

400

600

800

1000

1200

1400

Width of wear track,µm

Fig. 9. Optical microstructure (a) and profile (b) of wear track on a-CNx coating sliding against SiC ball in water.

F. Zhou et al. / Diamond & Related Materials 14 (2005) 1711 – 1720

(a)

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(b) 800 F=5N, V=160mm/s O In water, T=23 C Against Si3N4 ball

Depth of wear track,nm

600 400 200 0 -200 -400 -600 -800 -1000 0

200

400

600

800

1000

1200

1400

Width of wear track,µm

Fig. 10. Optical microstructure (a) and profile (b) of wear track on a-CNx coating sliding against Si3N4 ball in water.

coating was smaller than that of its mating ball. This indicated that the specific wear rate of the a-CNx coatings was mainly governed by the chemical properties of mating materials. When the ball materials had an excellent antioxidation ability, the main wear was occurred in the a-CNx coatings. For easily oxidative materials such as SUJ2, the main oxidative wear was occurred in SUJ2 ball. 3.5. Observation and analysis of worn surface Fig. 7 shows the wear scar morphologies of the five types of mating balls after sliding against a-CNx coating in water. The worn surface of Al2O3 ball was covered with many scratch lines and black pits, furthermore there were many wear particles and deposits around the rim of worn scars (Fig. 7(a)). As seen in Fig. 8(a, b), the a-CNx coatings were peeled off completely, the maximum depth of wear track on a-CNx coatings was 0.85 Am, larger than the thickness of aCNx coatings (0.5Am). When SiC ball was used as mating materials, the wear scar of the ball (Fig. 7(b) ) and the wear

(a)

track on the disk (Fig. 9(a)) exhibited many deep waviness bands parallel to sliding direction. Many original pits could be observed on the flat wear scar surface on SiC ball, while the worn surface on disk became smooth. The maximum depth of wear track on a-CNx coatings was 0.27 Am, smaller than the a-CNx coatings’ original thickness (0.5 Am) (Fig. 9(b)). For the Si3N4/a-CNx tribo-system, besides micro-pits, there were some shallow scratches on the flat worn scar surface of Si3N4 ball (Fig. 7(c)). As seen in Fig. 10(a), the wear track surface exhibited light, flat and smooth under optical microscope, and the maximum wear track depth was 0.15 Am, further lower than the a-CNx coatings’ original thickness. As stainless steel SUS440C ball was used in here, the wear scar surface of SUS440C ball exhibited many scratches parallel to sliding direction (Fig. 7(d)). This indicated that the plastic deformation also occurred on SUS440C ball during sliding against the a-CNx coatings in water. Fig. 11(a) and (b) show the wear track of a-CNx coatings became light and smooth, its depth was very shallow. But for the SUJ2/a-CNx tribo-system, the wear scar

(b) 800 F=5N, V=160mm/s O In water, T=23 C Against SUS440C ball

Depth of wear track,nm

600 400 200 0 -200 -400 -600 -800 -1000 0

200

400

600

800

1000

1200

1400

Width of wear track,µm

Fig. 11. Optical microstructure (a) and profile (b) of wear track on a-CNx coating sliding against SUS440C ball in water.

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F. Zhou et al. / Diamond & Related Materials 14 (2005) 1711 – 1720

(a)

(b) 800 F=5N, V=160mm/s O In water, T=23 C Against SUJ2 ball

Depth of wear track,nm

600 400 200 0 -200 -400 -600 -800 -1000 0

200

400

600

800

1000

1200

1400

Width of wear track,µm

Fig. 12. Optical microstructure (a) and profile (b) of wear track on a-CNx coating sliding against SUJ2 ball in water.

surface of SUJ ball became flat and was covered with a large number of micro-pits (Fig. 7(e)). These pits indicated that iron on sliding surface had reacted with water and the oxides layer was delaminated from SUJ2 ball. The wear track on the a-CNx coating became light, flat and smooth (Fig. 12(a)). The profile of wear track in Fig. 12(b) showed that the width of wear track was largest, but the depth of wear track was smallest among five kinds of tribo-systems. This indicated that the corrosion wear occurred on the wear scar of SUJ2 ball due to its poor oxidative resistance in water. Fig. 13 shows the XPS spectrum of N 1s on the worn surface for the a-CNx coatings after sliding against SiC ball in water. The XPS analysis results in Table 3 showed that the surface nitrogen concentration of the wear track at 5 N decreased in comparison to that of the original a-CNx coating surface. As compared with Fig. 1(b), the intensity ratio between sp2 C=N and sp3 C –N for the a-CNx coatings increased after sliding against SiC ball in water. This indicated that the surface structure of the a-CNx coating was

N1s 2

sp C=N 3

Intensity (a.u.)

sp C-N

transferred via the chemical reaction between the a-CNx coatings and water. 3.6. Discussion Generally, the friction and wear properties of carbonbased film in air are influenced by contact stress and mating materials hardness [37]. As seen Table 1, SiC ball shows the highest roughness, while the initial friction coefficient of the SiC/a-CNx tribo-pairs is further lower among the five kinds of tribo-pairs. In fact, the ball’s oxidative wear and the mechantribochemical wear of the a-CNx coating were simultaneously existed in all tribo-systems. For ceramic balls in water, tribochemical reaction would be occurred at contact surface between tribo-materials and water. In distilled water, the wear of ceramic ball was the tribo-chemical dissolution of materials via the formation of hydrate, namely Al2 O3 þ 3H2 O ¼ 2AlðOHÞ3

ð5Þ

¼ 16:36 kJ=mol DG298 f

ð6Þ

SiC þ 4H2 O ¼ SiðOHÞ4 þ CH4

ð7Þ

¼  598:91 kJ=mol DG298 f

ð8Þ

Si3 N4 þ 12H2 O ¼ 3SiðOHÞ4 þ 4NH3

ð9Þ

¼  1268:72 kJ=mol DG298 f

ð10Þ

where DG298 is the reaction Gibbs free energy of formation at f 298 K. From Eqs. (6) (8) and (10), we could conclude that the

N-O N-O

410

408

406

404

402

400

398

396

394

392

Binding Energy (eV) Fig. 13. XPS spectrum of N 1s photoeletron peaks on worn surface for the a-CNx coatings against SiC ball at 5 N and 160 mm/s in water.

Table 3 Chemical composition of original surface and worn surface of a-CNx coatings (at.%) Name

C

N

I sp2 / I sp3

Original surface Worn surface

88 94

12 6

0.96 1.09

F. Zhou et al. / Diamond & Related Materials 14 (2005) 1711 – 1720

hydration reaction between alumina and water was not occurred at room temperature, but silicon nitride was more easily hydrated than silicon carbide. Because of no hydration reaction of Al2O3 in water, the mechanical wear and the soft effect of a-CNx coatings’ surface hardness induced by structure transformation were occurred at contact surface between Al2O3 ball and the a-CNx coating. But for the tribosystems with Si-based ceramic balls, the tribo-oxidatively formed amorphous hydrate Si(OH)4 is then either dissolved into water, known as tribo-chemical wear, or removed from the interface. Because silicon nitride more easily reacted with water than silicon carbide did, the wear scar surface of silicon nitride ball became more flat and smooth than that of SiC. These results indicated that the lower friction coefficient and lower wear rate of a-CNx coating in the Si3N4/a-CNx tribosystems was caused by the formation of hydrodynamic lubrication at the contact surface. Due to partial the tribooxidation of SiC in water, the mechan-tribochemical wear was happened on ball and disk. Thus, there were many scratch lines on wear scars (Figs. 7(b) and 9(a)). It is known that the carbon nitride coatings are hydrophilic, and the physisorption of water seems to have a hydrogen-bonded mechanism, by formation of hydrogen bonds between water molecules and nitrogen atoms [38]. It pointed out that nitrogen atoms were removed easily form the a-CNx coating by reaction with water. After nitrogen was removed from aCNx coating, friction induced the a-CNx coating surface structure transformation, as seen in Table 3. The surface chemistry of this easy-shear transfer film was determined in previous publications on CNx films as formed by C sp2-bonding-rich structure [31 –33], so the soft carbonaceous possessed lower shear strength, which was responsible for the decrease of friction coefficient and wear rate for the mating balls. For the steel ball tribo-systems, though they had similar contact stress and hardness, due to their different oxidation resistance in water, their wear rate difference was very larger. Because of low hardness and high anti-oxidative ability for SUS440C, the plastic deformation occurred easily during sliding against a-CNx coating. Thus, the ball wear scar surface exhibited some grooves and oxides. The wear track surface on disk becomes smooth and was covered with some scratches parallel to sliding direction (Fig. 11(a)). This indicates that wear mechanism is mechanical wear. For SUJ2 ball/a-CNx tribo-system, owing to the poor antioxidation of SUJ2 ball, SUJ2 ball was easily worn off via oxidation reaction between SUJ2 and water, there were many oxidative pits on the flat wear scar surface on SUJ2 ball. Whereas the wear track surface on disk becomes light and the wear track depth is so far shallower (Fig. 12(b)). Many iron oxides were observed on the wear track surface of the a-CNx coating. As compared with SUS440C/a-CNx tribo-system, it was found that the friction and wear behaviors of steel ball/a-CNx tribo-systems in water were more strongly influenced by the anti-oxidative ability of steel balls in water than by their mechanical properties.

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4. Conclusions The friction and wear of a-CNx coating sliding against SiC, Si3N4, Al2O3, SUS440, SUJ2 balls in water has been investigated by using ball-on-disk tribo-meter at a normal load of 5 N and a sliding velocity of 160 mm/s. The conclusions are summarized as: (1) The a-CNx coatings contained 12 at.% nitrogen and the major chemical bonding of sp2 C=N and sp3 C–N. The nano-hardness of the a-CNx coatings was 29 GPa. (2) For five kinds of tribo-systems, the lower friction coefficients were obtained in the range of 0.01 to 0.02 for the tribo-systems with SiC and Si3N4 balls, whereas the highest friction coefficients in the range of 0.07¨0.10 were obtained for Al2O3, SUS440, SUJ2 balls’ tribo-systems. (3) Among five kinds of tribo-systems, the largest wear rate of the a-CNx coating of 1.77  10 7 mm3/Nm was obtained as sliding against Al2O3 ball, while the smallest wear rate of the a-CN x coating of 1.44  10 8 mm3/Nm was gotten as sliding against Si3N4 ball. But SUJ2 ball showed the highest wear rate of 7.0  10 7 mm3/Nm, whereas Al2O3 ball had the lowest wear rate of 3.55  10 9 mm3/Nm. Acknowledgement The authors would like to express their appreciation to Mrs. Yujiro Matsumoto, Naoya Sodeyama and Daisuke Kawase for their helps in CNx coating preparation and water lubrication tests. One of the authors (F. Zhou) also gratefully acknowledges Japan Society for the Promotion of Science (JSPS) for giving him a Post-Doc. fellowship. References [1] M. Urbakh, J. Klafter, D. Gourdon, J. Israelachvili, Nature 430 (2004) 525. [2] A. Erdemir, G.R. Fenske, J. Terry, P. Wilbur, Surf. Coat. Technol. 94/95 (1997) 525. [3] W. Zhang, A. Tanaka, K. Wazumi, Y. Koga, Tribol. Lett. 14 (2003) 123. [4] H. Ronkainen, S.V. Holmberg, Wear 249 (2001) 267. [5] A. Grill, Surf.Coat.Technol. 94/95 (1997) 507. [6] C. Donnet, Surf.Coat.Technol. 100/101 (1998) 180. [7] A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. [8] R. Kaltofen, T. Sebald, J. Schulte, G. Weise, Thin Solid Films 347 (1999) 31. [9] J.L. He, W.L. Chang, Thin Solid Films 312 (1998) 86. [10] C. Niu, Y.Z. Lu, C.M. Lieber, Science 261 (1993) 334. [11] M.G. Krishna, K.R. Gunasekhar, S. Mohan, J. Mater. Res. 10 (1995) 1083. [12] Y. Aoi, K. Ong, E. Kamijo, J. Appl. Phys. 86 (1999) 2318. [13] M. Kohzai, A. Matsumuro, T. Hayashi, M. Muramatsu, K. Yamaguchi, Thin Solid Films 308/309 (1997) 239. [14] Z.J. Zhang, J. Huang, S. Fan, C.M. Lieber, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 209 (1996) 5.

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