Wear 263 (2007) 1253–1258 Short communication Friction and wear property of a-CNx coatings sliding against Si3N4 balls in water Fei Zhou a,∗ , Xiaolei Wang b , Koji Kato c , Zhendong Dai a 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 School of Mechanical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China c Laboratory of Tribology, School of Mechanical Engineering, Tohoku University, Sendai 980-8579, Japan Received 27 August 2006; received in revised form 15 November 2006; accepted 15 November 2006 Available online 23 May 2007 Abstract Amorphous carbon nitride coatings (a-CNx ) were deposited on Si(1 0 0) wafers and Si3 N4 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 behavior of a-CNx coating sliding against a Si3 N4 ball in water was investigated. The results indicated that the a-CNx coatings contained 12 at.% nitrogen and the major chemical bonding was sp2 C N and sp3 C–N. The nano-hardness of the a-CNx coatings was 29 GPa. At a sliding velocity of 0.16 m/s and after running-in, the mean steady-state friction coefficient varied around 0.02 when the normal load was lower than 3.5 N, and then decreased abruptly from 0.018 to 0.007 at 5 N. For self-mated Si3 N4 , the specific wear rate of a Si3 N4 ball was a little higher than that of a Si3 N4 disk, while for a-CNx /Si3 N4 , the specific wear rate of a Si3 N4 ball was slightly smaller than that of a-CNx coating. Furthermore, the specific wear rate of Si3 N4 ball sliding against a-CNx coating was reduced by a factor up to 35 in comparison to that against Si3 N4 in water. This indicated that the wear mechanism of a-CNx coating/Si3 N4 ball was the formation of a carbonaceous transfer film on the a-CNx coatings via a tribochemical reaction between a-CNx coatings and water induced by friction, while that of self-mated Si3 N4 ceramics was the formation of silica gel on the contact zone via the reaction of silicon nitride and water. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon nitride coatings; Si3 N4 ceramics; Friction; Wear; Water lubrication 1. Introduction Since Liu and Cohen  in 1989 predicted theoretically that the existence of a metastable covalent carbon nitride compound (␤-C3 N4 ) with an analogous structure to ␤-Si3 N4 , 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. But until now, nearly all CNx films grown at room temperature are amorphous mixtures of carbon and carbon nitride with x ranging from 0.1 to 0.5 [2,3]. Nitrogen incorporation in the carbon coatings decreases the fraction of sp3 carbon bonds by the formation of C–N, C N and C N bonds. Recently, ∗ Corresponding author. Tel.: +86 25 84892581 803; fax: +86 25 84892581 803. E-mail addresses: [email protected], [email protected] (F. Zhou). 0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.11.048 Zhou et al. reported that the a-CNx coatings could enhance the wear resistance of a SiC ball and shorten the running-in period as sliding against a SiC ball in water at sliding velocities in the range of 0.019–0.16 m/s [4,5] and the wear-mechanism map of the aCNx /SiC tribo-pair in water was developed . Furthermore, the friction and wear properties of the a-CNx /SiC tribo-couple in water have already been found to be better than those of the aC/SiC tribo-pair under the same experimental conditions [7,8]. At the normal load of 5 N and the sliding velocity of 0.16 m/s in water, the friction coefficients of 0.01–0.02 were obtained as the a-CNx coatings slid against SiC and Si3 N4 balls, while larger friction coefficients of 0.07–0.10 were acquired as the a-CNx coatings slid against Al2 O3 , SUS440C and SUJ2 balls . It was indicated that the amorphous carbon nitride coatings sliding against Si-based non-oxide ceramics such as SiC and Si3 N4 exhibited the lowest friction coefficient and lower wear rate. Currently, the friction, lubrication and wear mechanisms of the a-CNx /SiC tribo-pair have already been reviewed 1254 F. Zhou et al. / Wear 263 (2007) 1253–1258 in detail , however, the friction and wear property of the aCNx /Si3 N4 tribo-pair in water has not yet been studied in detail. The aim of this paper is to investigate the friction and wear properties of the a-CNx /Si3 N4 tribo-pair in water and compare their wear characteristics between the a-CNx /Si3 N4 tribo-pair and Si3 N4 /Si3 N4 tribo-pair by using ball-on-disk tribo-meters at room temperature. The influences of testing conditions on the tribological behaviors of a-CNx /Si3 N4 tribo-pair in water were analyzed. 2. Experimental procedures The IBAD machine (Hitachi Ltd., Japan) consists of a cryogenically pumped chamber, a sputter deposition source, an electron beam evaporator, two ion guns for sputtering and mixing, respectively, and a substrate holder (Ref. ). The diameter of the ion beam irradiation area is about 80 mm. The substrate holder consists of a water-cooled copper plate and can be rotated at a speed of 4 rpm during deposition. Prior to the IBAD process, Si3 N4 disks (Ø 30 mm × t 8 mm) and Si(1 0 0) wafers were ultrasonically cleaned in acetone for 30 min. Then, a high purity carbon target was put into an electron beam evaporator and a substrate jig containing a Si3 N4 disk was fixed on the substrate holder. After that, the deposition chamber doors were closed and the vacuum chamber was subsequently evacuated to lower than 2.0 × 10−4 Pa. For further cleaning, the disk surface was bombarded using nitrogen ions for 5 min. After that, the a-CNx coatings were synthesized by mixing carbon vapor and energetic N ions. Energetic N ions were produced under the acceleration voltage of 1.5 kV with the acceleration current density of 90 A/cm2 . Carbon vapor was formed through heating a graphite target with an electron beam evap˚ which was controlled orator. The deposition rate was 20 A/s, by adjusting the emission current of carbon vapor. The coating thickness was 0.5 m. The deposition parameters are listed in Ref. . The composition and chemical bonding of the a-CNx coatings were determined by a scanning ESCA microprobe (Quantum 2000, Physical Electronics Inc., USA). The coatings’ surface roughness was measured by a Surfcom-1500DX profilometer, and their hardness and Young’s modulus were evaluated using a Nano Indenter ELIONIX ENT-1100A. The diameter of all Si3 N4 balls was 8 mm and the balls’ roughness was determined by a Surfcom-1500DX profilometer and its mechanical properties were obtained from the ball manufacturer. The 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 a rotating disk sliding on a stationary ball at 0.16 m/s and 1.5–5 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 Si3 N4 disks’ surface. In order to know the influence of the aCNx coatings on the wear behavior of silicon nitride ceramic, the wear behavior of self-mated Si3 N4 ceramic tribo-pairs was also studied under the same conditions. The total friction cycles were 49,200 cycles (equal to a sliding distance of 2304 m). The friction forces were detected by a LMA-A-10 N load cell (Kyowa Co. Ltd., Japan). The load cell voltage was measured by a DPM-700B strain amplifier (Kyowa Co. Ltd., Japan) and recorded by NR-110/150 data collection system (Keyence Co. Ltd., Japan) with a compatible personal computer. The diameter of the wear scar on the SiC ball under each condition was measured using a Keyence VH-8000 optical microscope (Keyence Co. Ltd., Japan). The cross-section area of the wear track on disk, A, was determined using a Tencor P-10 surface profilometer (Kurashiki Kako Co. Ltd., Japan). Thus, the specific wear rates for balls and disks were determined using the same equations in Refs. [5–9]. To know the wear mechanism of the a-CNx /Si3 N4 and self-mated Si3 N4 tribo-pairs in water, the wear scars on the balls and the wear tracks on disks were observed by optical microscopy. 3. Result 3.1. Surface roughness and mechanical properties of a-CNx coatings As seen in Table 1, the arithmetic mean roughness Ra of the aCNx coating was a little smaller than that of the Si3 N4 substrate. This indicated that the energetic particle bombardment enhanced the mobility of carbon atoms on the growing surface and induced the smooth surface. Fig. 1 displays nano-indentation load versus indentation depth curves for a-CNx coatings. Based on the standard Oliver and Pharr approach , the mean values of the Table 1 Surface roughness and mechanical properties of Si3 N4 ball, a-CNx coatings and Si3 N4 disk Name Si3 N4 ball a-CNx Si3 N4 disk a Ra (m) H (GPa) E (GPa) 0.0552 0.0251 0.0280 15.3a 308a 330 ± 20 290a 29 ± 2 16a The data are from the sample manufacturer. Fig. 1. Nano-indentation load vs. indentation displacement curves for a-CNx coatings. F. Zhou et al. / Wear 263 (2007) 1253–1258 1255 elastic modulus (E) and the hardness (H) for the a-CNx film were calculated from the nano-indentation load–displacement curves (Fig. 1) and are listed in Table 1. The results in Table 1 show 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). 3.2. Composition and chemical bonding of a-CNx coatings According to the XPS analysis, the a-CNx 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 (Fig. 2). The C 1s line was also deconvoluted into three peaks at binding energies of 285.1, 286.5 and 288 eV, and the N 1s line was deconvoluted into four peaks at binding energies of 398.5, 400.3, 401.5 and 404 eV. Scharf et al.  reported that, for the a-CN0.14 coatings, the peaks at binding energies of 284.5, 285.2, 286.5 and 288.6 eV for the deconvoluted C 1s spectra were attributed to C–C, C N, C–N or C N, and C–O bonds, respectively, while the peaks at 398.6, 400.1 and 402.3 eV for the N 1s line were assigned to C–N or C N, C N and N–O bonds, respectively. Comparing the data of Ref. , the peaks at 285.1, 286.5 and 288 eV in Fig. 2a were assigned to C N, C–N or C N, and C–O bonds, respectively, while the peaks at 398.5, 400.3, 401.5 and 404 eV in Fig. 2b were marked as C–N or C N, C N and N–O bonds, respectively. The appearance of C–O and N–O bonds showed that the coatings surface was contaminated by oxygen from air. Furthermore, since the C 1s and N 1s binding energies of urotropine (N bonded to sp3 -hybridized C, C–N) and polyacrylonitrile (N bonded to sp-hybridized C, C N) molecules were nearly identical [13,14], it was difficult to unequivocally distinguish the bonding configuration of a C–N bond from that of a C N bond. However, Ref.  pointed out that, as the nitrogen concentration of the a-CNx coatings was smaller than 14 at.%, the C N bond could not be detected in the a-CNx coatings. Due to the present a-CNx coating containing 12 at.% nitrogen, the sp3 C–N Fig. 3. Variation of friction coefficient with sliding cycles as a-CNx coatings sliding against Si3 N4 ball at 160 mm/s and various normal loads in water. and sp2 C N bonds were the major CN component in the a-CNx coatings. 3.3. Friction behaviors of a-CNx coatings sliding against Si3 N4 ball in water The friction behaviors of a-CNx coatings sliding against Si3 N4 balls at 0.16 m/s with various normal loads in water lubrication are illustrated in Fig. 3. In general, the friction coefficient decreased during the early stage of the test and then approached a steady-state value. At a lower normal load of 1.5 N, the initial friction coefficient of a-CNx /Si3 N4 tribo-pair was 0.25, but when the normal load was higher than 2.5 N, the initial friction coefficient of an a-CNx /Si3 N4 tribo-pair varied in the range 0.10–0.13. As seen in Fig. 3, the running-in period of an aCNx /Si3 N4 tribo-pair varied with the normal load at a constant sliding velocity of 0.16 m/s. The running-in period was 12,500 cycles at 1.5 N, 13,000 cycles at 2.5 N, 9020 cycles at 3.5 N and 12,500 cycles at 5 N, respectively. After running-in, the friction coefficient fluctuated in the range of 0.002–0.02. Fig. 4 shows the influence of normal load on the mean steady-state friction coefficients after running-in for a-CNx /Si3 N4 tribo-pairs in water. Fig. 2. XPS spectra of the C 1s (a) and N 1s (b) photoelectron peaks for a-CNx coatings. 1256 F. Zhou et al. / Wear 263 (2007) 1253–1258 Fig. 4. Influence of normal load on mean steady-state friction coefficients after running-in for a-CNx /Si3 N4 tribo-pairs in water. It is clear that the mean steady-state friction coefficient varied around 0.02 or so when the normal load was lower than 3.5 N. At the highest normal load of 5 N, the mean steady-state friction coefficient decreased abruptly from 0.018 to 0.007. The above friction coefficients indicate that the a-CNx /Si3 N4 tribo-pair has excellent friction characteristics in water lubrication. Fig. 5. Variation in specific wear rate at various normal loads for two kinds of tribo-pairs in water. 3.4. Wear behaviors of two kinds of tribo-pairs In order to know the influence of a-CNx coating on the specific wear rate of Si3 N4 in water, the wear behavior of a-CNx /Si3 N4 tribo-pairs was compared with that of self-mated Si3 N4 ceramics. The experimental results are illustrated in Fig. 5. It is clear Fig. 6. Wear scar on Si3 N4 ball (a and c) and wear track surface on a-CNx coatings (b and d) after sliding in water: (a and b) 2.5 N; (c and d) 5 N at 0.16 m/s. F. Zhou et al. / Wear 263 (2007) 1253–1258 1257 Fig. 7. Wear scar on Si3 N4 ball (a and c) and wear track surface on Si3 N4 disks (b and d) after sliding in water: (a and b) 2.5 N; (c and d) 5 N at 0.16 m/s. that, with an increase in normal load, the specific wear rate of all tribo-materials decreased gradually as the normal load was in the range of 1.5–5 N. For the a-CNx /Si3 N4 tribo-pair, the specific wear rate of the a-CNx coatings varied in the range of 3.89 × 10−8 to 7.89 × 10−8 mm3 /Nm, a little larger than that of Si3 N4 balls. Moreover, the specific wear rate of the aCNx coatings and the Si3 N4 balls all were at a lowest level of 10−8 mm3 /Nm. But for self-mated Si3 N4 ceramics, the specific wear rate of Si3 N4 balls fluctuated in the range of 1.28 × 10−6 to 2.79 × 10−6 mm3 /Nm, and was approximately twice as large as that of the Si3 N4 disk, whose specific wear rate varied in the range of 6.18 × 10−7 to 9.64 × 10−7 mm3 /Nm. The results in Fig. 5 also show that the specific wear rate of Si3 N4 ball sliding against a-CNx coating was reduced by a factor up to 35 in comparison to that against Si3 N4 in water. This indicates that the a-CNx coatings can enhance the wear behavior of silicon nitride ceramics in water. surface with some shallower grooves except for some original micro-voids (Fig. 6b and d). But for the self-mated Si3 N4 ceramic tribo-pair, the wear scar surface on the Si3 N4 ceramic ball became much smoother and the wear track surface on the Si3 N4 ceramic disks also became smoother and flatter (Fig. 7). As compared with the unworn surface on the Si3 N4 ceramic disk, it is clear that the original micro-pits disappeared during sliding of a Si3 N4 ball against a Si3 N4 disk in water. If Fig. 7 is compared with Fig. 6, it is evident that the wear scar diameter for a Si3 N4 ball in the a-CNx /Si3 N4 tribo-pair was smaller than that in the self-mated Si3 N4 ceramic tribo-pair, but the wear track surface on a Si3 N4 disk was smoother than that on the a-CNx coatings. This indicated that the friction and wear mechanisms of a self-mated Si3 N4 ceramic tribo-pair were different from those of an a-CNx /Si3 N4 tribo-pair in water. 3.5. Observation of friction surfaces For the a-CNx /Si3 N4 tribo-couple, the Si3 N4 ball hardness is 15.3 GPa, and the a-CNx coatings’ hardness is 29 GPa. According to normal friction and wear theory, the wear rate of a Si3 N4 ball should be larger than that of the a-CNx coatings. But here, the wear rate of the Si3 N4 ball was a little smaller than that of the a-CNx coating. A similar phenomenon has already been reported For the a-CNx /Si3 N4 tribo-pair, at a low normal load of 2.5 N, the wear scar surface on a Si3 N4 ball displayed a smooth surface with some shallow scratch lines (Fig. 6a and c). The wear track surface of a-CNx coatings exhibited a smoother and flatter 4. Discussion 1258 F. Zhou et al. / Wear 263 (2007) 1253–1258 by Jia et al.  when they studied the tribology of a DLC coating sliding against a Si3 N4 ball. The wear rate of a DLC film was higher than that of a Si3 N4 ball, which was related to transfer of a tribo-layer from the disk to the ball. Furthermore, the specific wear rate of a Si3 N4 ball sliding against an a-CNx coating was reduced by a factor up to 35 in comparison to that against Si3 N4 in water. This indicated that the a-CNx could enhance the anti-wear ability of silicon nitride ceramics in water. For selfmated Si3 N4 ceramic tribo-pairs, the wear track surface became smooth and flat, as seen in Fig. 7. When a Si3 N4 or SiC ceramic slides against itself in water, tribochemical reactions occurred as follows: SiC + 4H2 O = Si(OH)4 +CH4 (1) G298 = −598.91 kJ/mol f (2) Si3 N4 + 12H2 O = 3Si(OH)4 + 4NH3 (3) G298 = −1268.72 kJ/mol f (4) G298 f where is the reaction Gibbs free energy of formation at 298 K. From Eqs. (2) and (3), we could conclude that silicon nitride is more easily hydrated than silicon carbide. The tribooxidatively formed amorphous hydrate Si(OH)4 is then either dissolved water, known as tribo-chemical wear, or removed from the interface, and an ultra flat contact surface is obtained easily. Furthermore, water lubrication of ceramics is thin film hydrodynamic lubrication, and the water during friction is an electrolyte solution, so these two ceramics were in charged states during friction, and the effect of an electric double layer formed by electrostatic charges may play a role in the hydrodynamic lubrication of the two ceramics . Thus, the self-mated Si3 N4 ceramic tribo-pair exhibited the higher specific wear rate and lower friction coefficient. It is well-known that the a-CNx coatings offered higher values of H/E and a combination of reasonably high hardness and suitable stiffness, so the a-CNx coatings possess excellent tribological properties . If the Si3 N4 disk was covered with amorphous carbon nitride coatings, friction transforms the surface layer of the a-CNx coating and gives it lower shear strength, which is responsible for low friction and the transfer of material. As Tanaka et al. [17–20] studied the water lubrication of DLC coatings, they indicated that the structure of transferred materials was very different from that of the original DLC film and similar to that of polymer-like carbon, which is softer in comparison to DLC film. The amount of transferred material with the polymer-like structure was larger in water than that in air. But now, the surface chemistry of this easy-shear transfer film was determined in previous publications on CNx films to be formed by C sp2 -bonding-rich structures [8,9,21,22]. Moreover, the aCNx coatings are hydrophilic, and the physisorption of water seems to cause the formation of hydrogen bonds between water molecules and nitrogen atoms . Nitrogen atoms are removed easily from the CNx coating by reaction with water. Hellgren et al.  have indicated that if operated in the presence of oxygen or hydrogen, those elements would react with a-CNx film and promote decomposition. After the nitrogen atoms are removed from a-CNx coatings, a carbonaceous transfer film will form on the wear scar surface and carbon bonds can be terminated with OH− in water, which is responsible for low friction for the aCNx /Si3 N4 tribo-couple, and low wear rate of the Si3 N4 ball in water. 5. Conclusions (1) The a-CNx coatings contained 12 at.% nitrogen and the major chemical bonding was sp2 C N and sp3 C–N. 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