Friction and wear behavior of BCN coatings sliding against ceramic and steel balls in various environments

Friction and wear behavior of BCN coatings sliding against ceramic and steel balls in various environments

Wear 261 (2006) 301–310 Friction and wear behavior of BCN coatings sliding against ceramic and steel balls in various environments Fei Zhou a,∗ , Kos...

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Wear 261 (2006) 301–310

Friction and wear behavior of BCN coatings sliding against ceramic and steel balls in various environments 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 21 July 2005; received in revised form 28 September 2005; accepted 24 October 2005 Available online 27 December 2005

Abstract Ion beam assisted deposition (IBAD) techniques have been performed to synthesize ternary compound films composed of boron, carbon and nitrogen atoms (BCx Ny ). BCN coatings were deposited on Si wafers and Si3 N4 disks by evaporating B4 C target, with simultaneous N ion bombardment. The composition and chemical bonding were analyzed by XPS and the nanohardness was measured by Nano Indenter. The friction and wear property of the BCN coatings against Al2 O3 , SiC, Si3 N4 and SUS440C balls was investigated in air, N2 gas and water, respectively. The worn surfaces on the BCN coatings and the balls were observed by optical microscope. The results showed that, the atomic ratio in the BCN coatings was 49 at.% B, 42 at.% C and 9 at.% N and there were several bonding states such as B–N, B–C and C–N with B–C–N hybridization in the BCN coatings. The nanohardness of the BCN coatings was 33 GPa. As the BCN coatings slid against ceramic balls, the friction coefficients in air were largest, while those in water were smallest among three kinds of environment. But as the BCN coating slid against the SUS440C ball, the friction coefficients in N2 gas were slightly higher than those in air, whereas the friction coefficients in water also exhibited the lowest values. Among four kinds of tribo-pairs, the BCN/Al2 O3 tribo-pair showed the largest friction coefficient of 1.13 in air, but the BCN/Si3 N4 tribo-pair exhibited the smallest friction coefficient of 0.64 in N2 gas and 0.03 in water. The specific wear rates of the BCN coatings were largest in N2 gas, while lowest in water in all cases. The observation of worn surfaces on the BCN coatings and balls showed that the many roll-like materials were produced during sliding tests in air and N2 gas, while the smooth surfaces were formed in water. The experimental results indicated that the interface reaction between the BCN coatings and water or oxygen had many influences on the friction and wear property of the BCN coatings. © 2005 Elsevier B.V. All rights reserved. Keywords: Ion beam assisted deposition (IBAD); BCN coating; Friction; Wear; N2 gas; Water

1. Introduction Due to extreme hardness and excellent tribological property, the carbon-based coatings (diamond, DLC, ta-C and a-CNx ) have the high potentials for industrials applications as protective coatings on machine tools and dies. However, their poor thermal and chemical stability at elevated temperature and weak adhesion to substrate reduce their potential use in industrial field. As compared with the carbon-based coatings, the boron-based coatings such as c-BN and BCx coatings exhibit excellent thermal stability and oxidative resistance and have already been used in plastic injection molds, automotive transmission gears and



Corresponding author. Tel.: +86 25 84892581; fax: +86 25 84892581. E-mail address: [email protected] (F. Zhou).

0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.10.009

cutting tools [1]. One of the main limitations for the largescale use of the boron-based coatings is their high internal stress, which reduces the interface strength between coating and substrate and induces the delamination of coatings from a substrate [2]. One way to reduce the internal stress is to add nitrogen in B4 C [3,4] and carbon in c-BN [5]. Thus, the BCN coatings are expected to have lower residual stress and to improve the interface adhesion between coating and substrate. At present, most of the attempts to synthesize B–C–N coatings were carried out by chemical vapor deposition (CVD) or plasma assisted chemical vapor deposition (PACVD), with BCl3 –CCl4 –N2 –H2 , or BCl3 –C2 H2 –NH3 , or BCl3 –CH3 CN, or CH4 –N2 –B2 H6 , or C3 H12 B3 N3 –C7 H8 –NH3 as starting materials [6–15]. However, only soft phases with hexagonal structure (h-BCN) have been obtained in most cases. Cubic phases (cBCN) have been obtained successfully by high pressure and

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high temperature method (HPHT) although segregated c-BN and diamond regions [16,17]. Recently, the c-BCN coatings have been synthesized by physical vapor deposition (PVD) with ion bombardment, such as sputtering [18–25], ion beam deposition [26–38] and pulsed laser ablation (PLA) [39–44]. After the BCN coatings were obtained, many research works have paid more attentions to the composition, structure, chemical bonding and nanohardness of BCN coatings. Only a few works related to the BCN coatings’ friction and wear property have been performed. For the tribological property of the BCN coatings deposited by ion beam assisted deposition (IBAD), Yasui et al. [26] indicated that the BCN coatings made from B4 C blocks exhibited the highest hardness and lower friction coefficients of 0.2 against steel ball in air as compared with those made from the powder mixture of boron and carbon. Albella and co-workers [32] also reported that c-BCN coatings had been synthesized successfully by the evaporation of B4 C and the simultaneous bombardment of the ions from mixture of Ar + N2 + CH4 gas. As compared with the carbon-based coatings, these BCN coatings not only showed high hardness and thermal stability temperature, but also exhibited the lowest friction coefficient of 0.05 as sliding against Al2 O3 ball at the relative humidity of 50% and the ambient temperature of 22 ◦ C. Caretti et al. [37] have recently obtained the BCx N film at room temperature with different carbon proportions. The friction coefficients of these BCN coatings varied between 0.18 and 0.35 as against WC/Co spherical tip at 3 N and 375 rev/min in air with 21% humidity and 23 ◦ C, and the specific wear rate of BCN coatings varied in the range of 10−7 –10−4 mm3 /Nm. In our previous papers [38], the BCN coatings were deposited by using ion beam assisted deposition (IBAD), and the optimum preparation parameters to produce the BCN coating with higher hardness, smooth surface and low friction coefficient in N2 gas were found. However, the friction and wear properties of the BCN coatings against Al2 O3 , SiC, Si3 N4 and SUS440C balls in various environments have not yet carried out in detail. The purpose of this paper is to investigate the friction and wear property of the BCN coatings sliding against Al2 O3 , SiC, Si3 N4 and SUS440C balls in air, N2 gas and water, and the influence of ambient environment on the friction and wear property of the BCN coatings was analyzed.

Table 1 Deposition parameters of BCN coatings

2. Experimental procedures

The BCN coatings’ surface roughness was measured by Surfcom-1500DX profilometer, and the mechanical properties were evaluated using a Nano Indenter ELIONIX ENT-1100A. The experimental results were listed in Table 2.

2.1. Coating method The IBAD machine is made by Hitachi Ltd., Japan, and its schematic diagram has already been shown in Ref. [45]. Prior to deposition, Si3 N4 disks and Si (1 0 0) wafers were ultrasonically cleaned in acetone and ethanol for 30 min. A B4 C target with purity of 99.99% was put into the electron beam evaporator and a substrate jig with a Si3 N4 disk was installed on substrate holder with two screws, and then the vacuum chamber was subsequently evacuated to lower than 2.0 × 10−4 Pa. For further cleaning, Si3 N4 disk or silicon wafer surface was bombarded for 5 minutes by nitrogen ions generated at an accelerated voltage (a.v.) of 1.0 kV and an accelerated current density (a.c.d.) of 100 ␮A/cm2 . After that, the BCN coatings were synthesized

Sputtering cleaning Nitrogen ion beam Accelerated voltage (kV) Accelerated current density (␮A/cm2 ) Gas flow Cleaning time (min)

1.0 100 N2 : 3.0 SCCM 5

Evaporation Target ˚ Deposition rate (A/s)

B4 C 5

Mixing Nitrogen ion beam Accelerated voltage (kV) Accelerated current density (␮A/cm2 ) Gas flow

2.0 60 N2 : 3.0 SCCM

Chamber pressure Background (Pa) Operating (Pa)

<2.0 × 10−4 7.0 × 10−3

Substrate Coating thickness (␮m) Substrate rotating speed (rpm)

Si (1 0 0) wafer and Si3 N4 disks About 0.3 4

by mixing boron vapor, carbon vapor and energetic N ions. The energetic nitrogen ions were generated at 2.0 kV (a.v.) and 60 ␮A/cm2 (a.c.d.). The boron and carbon vapors were formed through heating a B4 C target with the electron beam evapora˚ which tor. The deposition rate of the BCN coatings was 5 A/s, was controlled by adjusting the emission current of vapor. The thickness of the BCN coatings was about 0.3 ␮m. The deposition parameters are in detail listed in Table 1. 2.2. Composition and chemical bonding of BCN coatings The composition and chemical bonding of the BCN coatings was determined by a scanning ESCA microprobe (Quantum 2000, Physical Electronics Inc., USA). 2.3. Surface roughness and mechanical properties of BCN coatings

2.4. Ball-on-disk wear tests and micro-observation of worn surfaces The four types of sliding balls were SiC, Si3 N4 , Al2 O3 and SUS440C stainless steel, and their diameter was 8 mm. Their Table 2 Roughness and mechanical properties of the BCN coatings Name

Ra (␮m)

Rmax (␮m)

H (GPa)

E (GPa)

BCN

0.003

0.006

33 ± 2

450 ± 30

F. Zhou et al. / Wear 261 (2006) 301–310 Table 3 Physical properties of balls

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3. Results

Balls

SiC

Si3 N4

Al2 O3

SUS440C

Roughness, Ra (␮m) Density (g/cm3 ) Hv (GPa) E (GPa) Poisson ratio Maximum Hertzian stress, PH (GPa)

0.0885 3.2 22 430 0.16 0.5

0.0552 3.24 15.3 308 0.29 0.45

0.0528 3.88 16.5 370 0.18 0.47

0.0533 8 7.2 204 0.3 0.38

roughness and physical property was shown in Table 3. 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 (Fig. 1) at sliding speeds of 0.2 m/s and a normal load of 0.2 N and the rubbing surfaces were submerged in air with 20–30% of relative humidity, distilled water and N2 gas. All tests were carried out at 22–25 ◦ C. The contact point was 7.5 mm from the center of disk. The friction forces were detected by a strain gauge. The load cell voltage signals were recorded through A/D converter using a compatible PC. The diameter of wear scar on ball under each condition 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 rates for balls and the BCN coatings were determined using the following equation: ws,b =

3.14d 4 64RWL

(1)

ws,d =

2πrA WL

(2)

where R is the ball radius, d is the diameter of wear scar, r is the wear track radius, W is the normal load and L is the sliding distance. 2.5. Observation of the worn surface on the BCNx coating and different balls The worn surfaces on the BCN coatings and balls were observed using optical microscopy to analyze the wear characteristics of tribo-materials.

Fig. 1. Schematic diagram of ball-on-disk tester.

3.1. Composition and chemical bonding of BCN coatings According to the XPS analysis, the chemical composition of this BCN coating was 49 at.% B, 42 at.% C and 9 at.% N. Fig. 2(a)–(c) shows the XPS spectra of B 1s, C 1s and N1 s corelevel photoelectrons recorded for the present BCN coatings. In Fig. 2(a), the deconvolution of the B 1s spectrum showed the peaks at binding energies of 188.4 and 190.0 eV, the full-width at half maximum (FWHM) of the peaks were 2.2 and 2.8 eV, respectively. Berns and Cappelli [46] have reported a B 1s binding energy of 190.0 eV for h-BN and 191.0 eV for c-BN. Kunzli et al. [47] have shown that the B–C binding energies in the boron carbide of BC3.4 and B4 C are 189.4 and 188.4 eV, respectively. Therefore, the position of B 1s peak at 190.0 was attributed to B–N bonding in h-BN. However, the FWHM of the peak is 2.8 eV, which was larger than that of h-BN film (∼1 eV) [24]. This indicated that the B atoms were in different valence states. The peak at 188.4 eV was the contribution of B–C bonding, which was the same as that in B4 C. Four individual lines had to be used in order to get good agreement between the measured and the fitted C 1s spectrum (Fig. 2(b)), with peaks centered at 282.6, 284.7, 285.7 and 288.1 eV, respectively. The energy peak at 284.7 eV could be assigned to C–C bonding. The binding energy of 288.1 eV corresponded to C–O bonds. The contribution of 285.7 eV revealed that a significant number of the carbon atom was bonded to nitrogen as sp2 C–N bonds. The peak at the lowest binding energy of 282.6 was mostly attributed to carbon bonded to boron [11,24]. The deconvolution of the N 1s spectrum displayed the presence of three possible states with binding energies peaked at 397.6, 399.1 and 400.8 eV (Fig. 2(c)). The binding energy of 397.6 eV was attributed to B–N bond in hBN while the peak component at 400.8 eV could be assigned to sp2 C–N binding state [22]. Considering the change in the environment of B and N due to carbon incorporation and the consequent chemical shift, the peak at 399.1 might be assigned to sp3 C–N binding state or sp3 B–N binding state in c-BN [22]. If the B–N, B–C and C–N chemical bonds in these XPS spectra were considered, it could be concluded that the BCN coating was a compound with hybridized B–C–N bonds. 3.2. Friction behaviors of BCN coatings sliding against four kinds of balls in air, N2 gas and water In order to apply BCN coatings in MMES, it is imperative to study their friction and wear properties as sliding against ceramic and steel balls at a small normal load. As the BCN coatings slid against Al2 O3 ball in air, the friction coefficient slightly increased from 1.03 to 1.10 at initial stage, then varied in the range of 1.08–1.12 with further sliding (Fig. 3(a)). When the mating ball became SiC ceramic ball, as seen in Fig. 3(b), the initial friction coefficient was 0.6, with an increase in sliding cycles, the friction coefficient increased to 0.84, then fluctuated in the scope of 0.81–0.84. As seen in Fig. 3(c), the friction coefficient increased from initial value of 0.73 to 0.91, and varied in the range of 0.91–0.95 with an increase in sliding cycles as sliding

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Fig. 2. (a) B 1s, (b) C 1s and (c) N 1s core-level XPS spectra of BCN coating: experimental data (dots), fitted results (solid lines) and their deconvoluted components (dashed lines).

Fig. 3. Variation of friction coefficient with sliding cycles for the BCN coatings against different balls in various environments: (a) BCN/Al2 O3 ball; (b) BCN/SiC ball; (c) BCN/Si3 N4 ball; (d) BCN/SUS440C ball.

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against Si3 N4 ball in air. If the mating ball changed from ceramic ball to steel ball such as SUS440C ball, as seen in Fig. 3(d), the friction coefficient first increased from 0.85 to maximum value of 1.2, then decreased gradually to 0.83, and varied in the scope of 0.74–0.84 with further sliding. After the above data were compared with each other, the tribo-couple with the largest friction coefficient in air was found to be the BCN/Al2 O3 tribo-couple. If the rubbing surface was eventually protected via blowing nitrogen gas, the BCN/Al2 O3 and the BCN/Si3 N4 tribo-pairs had the similar friction behavior during sliding. For the BCN/Al2 O3 tribo-pair, the friction coefficient first decreased abruptly from 0.90 to 0.73, then decreased gradually from 0.70 to 0.60 with an increment of sliding cycles (Fig. 3(a)), but for the BCN/Si3 N4 tribo-couple, the friction coefficient first increased from 0.73 to 0.84 within short period of 50 cycles, then decreased to 0.67 suddenly. After that, it fluctuated in the range of 0.60–0.65 with further sliding (Fig. 3(c)). For the BCN/SiC and the BCN/SUS440C tribo-couple, the opposite phenomena have been observed, their friction coefficients first increased, then varied at higher values with an increase in sliding cycles. For the BCN/SiC tribo-pair, as seen in Fig. 3(b), the friction coefficient first increased from 0.57 to 0.70 obviously within 240 cycles, then varied in the range of 0.70–0.73 with an increment of sliding cycles. When the BCN coating slid against SUS440C ball in N2 gas, the friction coefficient increased from initial value of 0.72 to 0.90 abruptly, then fluctuated in the range of 0.83–0.89 with an increase in sliding cycles (Fig. 3(d)). The above-mentioned data indicated that the BCN/Si3 N4 tribo-pairs exhibited the excellent friction behavior in N2 gas. If the purified water was added into rubbing surface, the four kinds of tribo-pairs exhibited the similar friction tendency. With an increase in sliding cycles, the friction coefficient decreased from higher initial value to stable values. As seen in Fig. 3(a), the friction coefficient of the BCN/Al2 O3 tribo-couple first decreased from initial value of 0.24 to 0.15 within 120 cycles, then changed in the range of 0.15–0.17 with further sliding. For the BCN/SiC tribo-pair, as seen in Fig. 3(b), the friction coefficient decreased slightly from 0.19 to 0.12 at initial stage of 120 cycles, then varied in the small range of 0.11–0.13 with an increase in sliding cycles. The results from Fig. 3(c) showed that the friction coefficient of the BCN/Si3 N4 tribocouple first decreased gradually from 0.18 to 0.025 as sliding cycles increased from 0 to 875 cycles, then kept lower values of 0.021–0.03 with an increase in sliding cycles. But for the BCN/SUS440C tribo-pair, the friction coefficient decreased gradually from 0.33 to 0.26 within 700 sliding cycles, then fluctuated in the range of 0.257–0.264 with further sliding (Fig. 3(d)). The comparison results among four kinds of tribopairs pointed out that the BCN/Si3 N4 tribo-pairs displayed the lowest friction coefficient in water. As seen in Fig. 4, the mean steady-state friction coefficients of the BCN/ceramic ball tribo-pair in air all were highest, while those in water exhibited the lowest values. As compared with the data in air, nitrogen gas could decrease the friction coefficients as the BCN coating slid against ceramic balls. However, as sliding against stainless SUS440C ball, the friction coefficient in air was slightly lower than that in N2 gas. But the friction coefficients in

305

Fig. 4. Influence of mating balls on the mean steady-state friction coefficient of BCN coatings in different environments.

air and N2 gas all were further larger than those in water. These indicated that water had better effects on the friction property of the BCN coatings. 3.3. Specific wear rate of BCN coatings and balls at different environments The specific wear rates of the BCN coatings and balls in various environments were illustrated in Fig. 5. Because no measurable wear could be detected for the BCN coatings after testing in water and only a smooth wear track surface could be observed under Tencor P-10 surface profilometer, so Fig. 5 only showed the specific wear rates of the BCN coatings in air and N2 gas and four kinds of balls in air, N2 gas and water. As seen in Fig. 5, if the wear tests were done in air, the highest wear rate of the BCN coatings was found to be 9.63 × 10−6 mm3 /Nm as the mating material was SiC ball. When the counterpart materials became Al2 O3 , Si3 N4 and SUS440C balls, the specific wear rates of BCN coatings were 5.42 × 10−6 , 5.73 × 10−6 and 4.5 × 10−6 mm3 /Nm,

Fig. 5. Influence of testing environments on the specific wear rate of tribomaterials.

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respectively. But in N2 gas, the largest BCN coating’s specific wear rate of 5.1 × 10−5 mm3 /Nm was obtained as the BCN slid against Si3 N4 ball. When BCN slid against Al2 O3 , SiC and SUS440C balls, the specific wear rates of BCN coatings were 3.27 × 10−5 , 2.2 × 10−5 and 1.1 × 10−5 mm3 /Nm, respectively. The above-mentioned data indicated that the specific wear rates of the BCN coatings in N2 gas were 20 times larger than those in air. Due to layer formation on the ball wear scar surface, it was difficult to determine the accurate specific wear rates of

balls in air and N2 gas. As four kinds of balls slid against BCN coatings in air, the specific wear rates of balls were 2.00 × 10−7 mm3 /Nm for Si3 N4 ball, 2.30 × 10−7 mm3 /Nm for SUS440C ball, 1.34 × 10−6 mm3 /Nm for SiC ball and 6.55 × 10−7 mm3 /Nm for Al2 O3 ball. These data indicated that Si3 N4 ball had excellent wear resistance as sliding against the BCN coatings in air. However, as the above wear tests were done in N2 gas, the specific wear rates of balls were 1.34 × 10−6 mm3 /Nm for Si3 N4 ball, 2.77 × 10−6 mm3 /Nm for SUS440C ball, 5.27 × 10−7 mm3 /Nm for SiC ball and

Fig. 6. Optical micrographs of worn surface on various balls in different environments.

F. Zhou et al. / Wear 261 (2006) 301–310

2.44 × 10−6 mm3 /Nm for Al2 O3 ball. It was clear that except for SiC ball, the specific wear rates of balls in N2 gas were further larger than those in air. If the distilled water was used, the wear rates of balls were 2.41 × 10−7 mm3 /Nm for Si3 N4 ball, 1.65 × 10−7 mm3 /Nm for SUS440C ball, 1.65 × 10−7 mm3 /Nm for SiC ball and 1.27 × 10−7 mm3 /Nm for Al2 O3 ball. In comparison of the above values of wear rate for coatings and balls, it is obvious that water was beneficial to decreasing the wear rate of the BCN coating and balls. This indicated that the BCN coatings had some superior tribological properties in water. 3.4. Observation on worn surface of balls sliding against BCN coatings in various environments Fig. 6 illustrated the observation differences of worn surface on ball sliding against the BCN coatings in different environments. For Al2 O3 balls, the worn surface in air became flat and exhibited many fracture pits, while in N2 gas, many needle-like particles and wrinkly layer formation were observed on the

307

worn surface of Al2 O3 balls. Furthermore, lots of needle-shaped particles were distributed at edge of wear scar in above two kinds of situations. But in water, the worn surface was covered with many grooves and original voids, and large size fracture pits were observed at edge of wear scar. When SiC ball slid against the BCN coatings in air, besides many original voids caused by incomplete densification, there were many various size scratch lines on the bright zone. But in N2 gas, the worn surface was covered with many needle-like particles besides many original voids. Many worn particles were accumulated at the edge of wear scar. If the testing environment became water, the worn surface became flat besides many original voids. Moreover, the large size fracture void was displayed at edge of wear scar, which was similar to that of Al2 O3 balls in water. This indicated that before tribochemical wear, the severe mechanical wear occurred. As Si3 N4 ball slid against the BCN coating in air and N2 gas, there were many fine scratch lines and needle-like particles on the smooth worn surface. In meanwhile, a large number of particles were assembled at the primary sliding side of wear

Fig. 7. Optical micrographs of BCN coatings sliding against Al2 O3 balls in various environments: (a) in air, (b) in N2 gas and (c) in water.

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scar. As compared with that in air, the thickness of friction layer in N2 gas became much larger. But in water, the worn surface of Si3 N4 ball was covered with some scratch lines and became more smooth and flat, and no fracture pits were observed. In comparison of those of SiC ball under the same conditions, the worn surface of Si3 N4 ball was more flat and smooth. When SUS440C ball slid against the BCN coating in air, many grooves and iron oxides were observed on the worn surface of ball, while in N2 gas, besides grooves, many fracture pits were observed on the worn surface. But in water, the worn surface exhibited many scratch lines and iron oxides. These grooves for SUS440C ball indicated that the plastic deformation occurred during sliding tests. 3.5. Observation on worn surface of BCN coatings As compared with the worn surface microstructure images of ball, the differences of worn surface images on the BCN coatings were not obvious as sliding different balls. As seen in Fig. 7, the optical micrograph of the BCN coatings’ wear track as sliding against Al2 O3 ball in air exhibited many fine particles on the smooth surface. As sliding in N2 , there were some large-size needle-like particles distributed on the smooth worn surface. At edge of wear track, the particles were gotten together. This indicated that the wear mechanism of the BCN coatings in air and N2 is micro-ploughing. But after testing in water, it is difficult to observe the worn surface of the BCN coating under optical microscope (OM). The blue light reflected the smooth surface under OM. As the BCN coatings slid against SiC, Si3 N4 and SUS440C balls, the worn surfaces’ optical micrographs of the BCN coatings were similar to those against Al2 O3 ball under the same ambient conditions. 4. Discussion 4.1. Influence of air and nitrogen gas The results in Figs. 4 and 5 showed that the mating materials and ambient conditions had many influences on the friction and wear property of the BCN coatings. For the BCN/ceramic balls tribo-pairs, the friction coefficients in air was larger than those in N2 , while the specific wear rates of BCN coatings in N2 were higher than those in air. The phenomena indicated that oxygen or water had some influences on the friction and wear properties of BCN coatings. In here, the BCN coatings were prepared by evaporating B4 C with nitrogen ion bombardment assistance. According to the XPS results, the nitrogen content in the BCN coatings was not higher than 0.1. Thus, the BCN coatings could be described as B4 C:N. Erdemir et al. [48,49] have already reported that B4 C is not a low-friction materials before it is annealed. Whereas Reigada and Freire [50] indicated that the incorporation of nitrogen in BCx coatings would raise the friction coefficient of BCx coatings. In here, the BCN coating/ceramic tribo-pairs possessed higher friction coefficients. This indicated that the smaller nitrogen content in BCN coatings had some negative influences on the friction property of matrix materials B4 C in air. If the slid-

ing surfaces were protected through blowing N2 gas, the friction coefficient of the BCN/ceramic ball tribo-couples decreased owing to the blowing effect and N2 gas lubrication [51], however, the largest specific wear rate of the BCN coatings indicated that severe wear was occurred due to lack of oxygen or water on contact surface. In comparison of the experimental data in N2 gas, the specific wear rate of the BCN coating in air was lower up to the factor of 10. All above comparisons indicated that the friction and wear property of the BCN coatings against various balls were mainly governed by the tribochemical reaction at contact surface. It is known that chemical reactions involving the oxidation of BCN to B2 O3 , some of which was transformed to H3 BO3 , depending on the degree of access to water [52]. As seen in Figs. 6 and 7, the amount of coarse particles on the worn surface of ball and the BCN coatings in N2 gas was further larger than that in air. Generally, the pace of the surface oxidation process mainly controlled the rate of materials removal. Although the flowing phase of B2 O3 has been reported to be able to provide very low friction, a too low viscosity reduced the load carrying capacity and gave rise to high friction values. However, to our surprise, the wear rates of the BCN coatings and most of balls in air were smaller than those in N2 gas, as seen in Fig. 5. This may be attributed to an inhibition of any further oxidation at the sliding contact by the viscous phase. This indicated that the friction was caused by shearing of glassy boron oxides but not disrupting it completely and the large coarse particles were partially removed from contact surface or the high-friction transfer layer was formed by reaction with oxygen in air, which were probably responsible for the higher friction coefficient and lower wear rates of BCN coating/ceramic ball tribo-pairs in air. Whereas in N2 gas, the needle-like particles were kept up on the sliding surface and had the roll-bearing effect, furthermore, a friction film was formed by the agglomeration, compaction and the smearing of the very fine particles, which induced the lower friction behavior and higher wear rates of BCN/ceramic ball in N2 . For BCN coating/SUS440C tribo-pair, on the worn surface of SUS440C ball (Fig. 6), the iron oxide and grooves were observed in air, while many fatigue fracture pits were observed in N2 gas. This indicated that some tribochemical reactions occurred in air, while only mechanical wear was happened in N2 gas. Thus, the friction coefficients of BCN coating/SUS440C ball tribo-pair in air were smaller than those in N2 gas. In meanwhile, the wear rate of SUS440C ball induced by mechanical wear was higher than that caused by mix wear. 4.2. Influence of water In water, the friction coefficients for the BCN/ceramic ball tribo-couples and the specific wear rates of the BCN coatings and the balls were all smaller than those in air and N2 gas. In fact, the friction and wear properties of the BCN coatings in water were largely the results of the tribochemical surface polishing, the polishing process led to the development of smooth surface, which also facilitated the occurrence of hydrodynamic lubrication [53]. For the BCN coating/balls tribo-couples, the tribochemical reaction occurred not only on BCN coating, but

F. Zhou et al. / Wear 261 (2006) 301–310

also on balls. For BCN coating in water, it reacted with water as: BCx Ny + H2 O → (NH4 )H2 BO3 + CO2

(3)

Erdemir et al. indicated that ultralow friction coefficient of annealed B4 C surface is directly related to the formation of H3 BO3 film on the friction surface [49]. At the same time, the ceramic ball materials were also reacted with water as:

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air, whereas the friction coefficients in water also exhibited the lowest values. Among four kinds of tribo-pairs, the BCN/Al2 O3 tribo-pair showed the largest friction coefficient of 1.13, but in N2 gas and water, the BCN/Si3 N4 tribo-pair exhibited the smallest friction coefficient of 0.64 and 0.03, respectively. (3) The specific wear rates of the BCN coatings were largest in N2 gas, 10 times larger than those in air, but lowest in water in all cases.

Al2 O3 + 3H2 O = 2Al(OH)3

(4)

= 16.362 kJ/mol G298 f

(5)

Acknowledgement

SiC + 4H2 O = Si(OH)4 + CH4

(6)

= −598.91 kJ/mol

(7)

The authors would like to express their appreciation to Mrs. Yujiro Matsumoto, Naoya Sodeyama and Kazuhiko Sakaguchi for their helps in BCNx coating preparation,N2 lubrication tests and XPS analysis. One of the authors (F. Zhou) also gratefully acknowledges Japan Society for the Promotion of Science (JSPS) for giving him a Post-doctoral fellowship.

G298 f

Si3 N4 + 12H2 O = 3Si(OH)4 + 4NH3

(8)

= −1268.72 kJ/mol G298 f

(9)

is the reaction Gibbs free energy of formation at where G298 f 298 K. From Eqs. (5), (7) and (9), we could conclude that the hydration reaction between alumina and water was not occurred at room temperature, but silicon nitride was more easily hydrated than silicon carbide. Based on above-mentioned data, the rank of tribochemical reaction in water was arranged from easy to difficulty: Si3 N4 ball > SiC ball > Al2 O3 ball. Thus, the specific wear rate of Si3 N4 ball was higher than that of SiC ball, and then the wear rate of SiC was larger than that of Al2 O3 ball (Fig. 5). Because the tribochemical reaction easily occurred for the BCN/Si3 N4 ball tribo-pair and the sliding surfaces for the BCN coatings and Si3 N4 ball became flat and smooth easily (Fig. 6), which induced the occurrence of hydrodynamic lubrication, so the BCN/Si3 N4 tribo-pair exhibited the lowest friction coefficient of 0.03 among three kinds of ceramic balls’ tribo-pairs (0.12 for SiC/BCN and 0.16 for Al2 O3 /BCN). For BCN/SUS440C ball tribo-pair, as seen in Fig. 6, the worn surface was covered with iron oxides and scratch lines. This indicated that the wear mechanism was mix wear. Thus, the BCN/SUS440C tribo-pair possessed higher friction coefficient of 0.27 in water. 5. Conclusions The friction and wear property of the BCN coatings sliding against SiC, Si3 N4 , Al2 O3 and SUS440 balls in air, N2 and water have been investigated by using ball-on-disk tribometer at 0.2 N and 0.2 m/s. The conclusions are summarized as: (1) The atomic ratio in the BCN coatings was 49 at.% B, 42 at.% C and 9 at.% N and there were several bonding states such as B–N, B–C and C–N with B–C–N hybridization in the BCN coatings. (2) As the BCN coatings slid against ceramic balls, the friction coefficients in air were largest, while those in water were smallest among three kinds of environment. But as the BCN coating sliding against SUS440C ball, the friction coefficients in N2 gas were slightly higher than those in

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