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Friction behaviors of hydrogenated diamond-like carbon film in different environment sliding against steel ball
Applied Surface Science 249 (2005) 257–265 www.elsevier.com/locate/apsusc
Friction behaviors of hydrogenated diamond-like carbon film in different environment sliding against steel ball Hongxuan Lia,b, Tao Xua, Chengbing Wanga,b, Jianmin Chena,*, Huidi Zhoua, Huiwen Liua a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate School of the Chinese Academy of Science, Beijing 100039, PR China
Received 1 September 2004; received in revised form 1 December 2004; accepted 1 December 2004 Available online 29 December 2004
Abstract Friction behaviors of hydrogenated diamond-like carbon (DLC) film sliding against steel ball were investigated on a ball-ondisk test rig in different environment. The worn surface morphology of the steel ball was observed on a scanning electron microscope (SEM), and the chemical states of some typical elements on the worn surface of DLC film were investigated by means of X-ray photoelectron spectroscopy (XPS). The result showed that the friction behaviors of hydrogenated DLC film were very sensitive to the testing environment. In dry N2, the film provided a much low and stable friction coefficient of about 0.035. When the friction tests were performed in higher relative humidity and in dry O2, higher and unstable friction coefficient of more than 0.2 was obtained. It was found that in oxygen and/or water containing environments, the transferred carbon-rich layer on the counterpart steel ball was inhibited, and the friction-induced oxidation of the DLC film surface and the chemical reactions between the DLC film and steel ball were involved in the friction processes, which accounted for the higher friction coefficient of the DLC film. The roles of environment in the friction behaviors of DLC film were discussed in terms of the friction-induced physical and chemical interactions among the DLC film, steel ball and water and/or oxygen molecules. # 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogenated DLC film; Friction behaviors; Environment; Interaction
1. Introduction In recent years, hydrogenated diamond-like carbon films have attracted increased attention due to their * Corresponding author. Tel.: +86 9314968085; fax: +86 9318277088. E-mail address: [email protected] (J. Chen).
extraordinary properties of high mechanical hardness, low friction coefficient, high wear resistance and chemical inertness. These properties are promising for a wide range of tribological applications such as magnetic hard disks, gears, bearings, cutting tools and other moving mechanical assemblies [1–3]. However, previous studies have shown that the tribological properties of hydrogenated DLC films are strongly
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.12.002
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dependent on the deposition methods and deposition conditions, and are very sensitive to the testing environment and the relative humidity (RH) [4–12]. Depending on the above-mentioned factors, the friction coefficients for the hydrogenated DLC films have been reported to broadly range from 0.003 to more than 0.3 [6–8,11,13–16]. Various theories have been proposed to explain the friction and wear behaviors of DLC films. The build-up of a friction-induced transfer film on the counterpart, followed by easy shear within the interfacial material is the most frequently observed friction-controlling mechanism for DLC films [3,10,17,18]. Liu et al. said that the steady-state low friction of DLC films was due to the wear-induced graphitization, i.e., the formation of a low friction graphitized tribolayer [6,19]. Erdemir et al. argued that the general low friction coefficient of hydrogenated DLC films was mainly attributed to the high chemical inertness of the DLC films by passivating the surface dangling bonds by hydrogen [14,20]. However, other researchers proposed that the tribochemical reactions in the tribo-system played an important role in determining the friction and wear behaviors of DLC films [3,10,17]. Therefore, these reported friction and wear mechanisms for the DLC films are the subjects to controversy and the role of the environments in the friction and wear behaviors of DLC films is still not well understood. In the present work, we investigated the friction behaviors of the hydrogenated DLC films sliding against steel balls in different environment (air, dry N2, humid N2 and dry O2), with emphasis on the friction-induced physical and chemical interactions among the DLC film, steel ball and water and/or oxygen molecules. Based on the results, it is hoped that the friction mechanisms of DLC films can be better understood.
2. Experimental details 2.1. Deposition and characterization of the hydrogenated DLC film Hydrogenated DLC film was deposited on Si (1 0 0) wafers by a PECVD technique, using CH4 plus Ar as the feedstock. The details about the deposition equipment and process were described elsewhere
Table 1 Summary of the deposition conditions and some properties of the hydrogenated DLC film Item
Parameters
CH4 gas flow rate (sccm) Ar gas flow rate (sccm) Deposition pressure (Pa) RF power (W) DC bias (V) Deposition time (h) Thickness (nm) Hardness (GPa) Young’s modulus (GPa)
11.6 11 5 300 200 3 640 15.2 120
[21,22]. Prior to deposition, the substrates were cleaned with Ar plasma sputtering at a bias voltage of 400 V for 15 min so as to remove the native oxide on the Si surface. The deposition conditions and some properties of the resulting DLC film are summarized in Table 1. During the film deposition, the substrate was only heated by the plasma and the substrate temperature was measured to be about 140 8C by a thermocouple mounted on the substrate holder. The deposited hydrogenated DLC film is dark brown in color and is extremely smooth and featureless when viewed with unaided eyes. The film thickness measured on a profilometer is about 640 nm. The hardness and Young’s modulus of the film are determined to be about 15.2 and 120 GPa, respectively, using a nano-indenter. 2.2. Friction tests The friction behaviors of the hydrogenated DLC films sliding against steel ball (AISI 52100, diameter 4 mm, surface roughness Ra 0.1 mm) were evaluated on a ball-on-disk test rig equipped with an environmental chamber with which the relative humidity and gaseous environment could be controlled. The friction tests were performed in air (RH 40%), dry N2 (RH < 5%), humid N2 (RH 100%) and dry O2 (RH < 5%), respectively, at a normal load of 2 N, a sliding velocity of about 125 m/min, to a maximum sliding duration of 60 min. The worn surface of steel ball was analyzed on a JSM-5600LV scanning electron microscope (SEM) equipped with a KEVEX SIGMA energy dispersive X-ray spectrometer (EDS). The chemical compositions and chemical states of the worn surface of the
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hydrogenated DLC film were analyzed on a PHI-5702 X-ray photoelectron spectroscope (XPS) operating with monochromated Al-K irradiation at a pass energy of 29.4 eV.
3. Results 3.1. Friction behaviors of the hydrogenated DLC film in different environment Fig. 1 shows the friction coefficient of hydrogenated DLC film as a function of sliding time in different environment. It is clear that the friction behaviors of the film are strongly dependent on the environment and relative humidity. In air (RH 40%), the friction coefficient of the hydrogenated DLC film was about 0.1. In dry N2 (RH < 5%), much low and stable friction coefficient of about 0.035 was obtained. As the relative humidity was increases to 100% in N2, the friction between the film and steel ball was unstable, and the friction coefficient was increased to about 0.2 in the steadystate. Particularly, when sliding in dry O2 (RH < 5%), the hydrogenated DLC film provided a much unstable and higher friction coefficient of more than 0.2. 3.2. SEM analyses of the worn surface of steel ball Fig. 2 shows the SEM pictures of the worn surface of steel ball sliding in different environment. In dry N2
Fig. 1. Friction coefficient of hydrogenated DLC film as a function of sliding time in different environment.
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(RH < 5%), the worn surface of the steel ball was covered by obvious transferred materials, which were mainly C element analyzed by EDS spectra, and this transferred carbon-rich layer was usually thick, compact and homogeneous in appearance (Fig. 2(a)). In air (RH 40%), the transferred carbon layer on the worn surface of steel ball was attenuated (Fig. 2(b)). However, when sliding at the higher relative humidity in N2 (RH 100%), the worn surface of steel ball was relatively large, and the EDS spectra showed that there was no transferred carbon layer on the worn surface of steel ball (Fig. 2(c)). It should be noted that in the dry O2 (RH < 5%) condition, the worn surface of steel ball was much larger and rougher, indicating a severe wear of the steel ball (Fig. 2(d)). 3.3. XPS analyses of the worn surface of hydrogenated DLC film Table 2 summarizes the changes of C, O and Fe concentration (at.%) determined by XPS analyses, on the original and worn surfaces of hydrogenated DLC film at varied testing conditions. The presence of O element on the original DLC film was due to air exposure. After the friction testing in dry N2 (RH < 5%), the concentration of O element had no distinct increase due to the protection action of nitrogen. However, the concentration of O element on the worn film surfaces tested in air, humid N2 and dry O2, respectively, was 18.69, 22.19 and 23.95 at.%, which was much larger than 6.13 at.%, that of the original DLC film. Moreover, when tested in the oxygen and/or water containing conditions, an increase of the concentration of Fe element was also detected on the worn film surfaces. Thus it can be inferred that the surface of the DLC film is restructured due to shearing forces and tribochemical reactions during the friction process. Fig. 3 shows the XPS spectra of C 1s and O 1s on the original surface of hydrogenated DLC film. The C 1s spectra can be fitted into three components around 284.8, 286.3, and 288.0 eV, respectively (Fig. 3(a)). The one around 284.8 eV corresponds to C–C and/or C–H bonds, and that around 286.3 eV corresponds to C–O bonds. The third peak of a much smaller intensity near 288.0 eV is assigned to C=O bond [23,24]. At the same time, the symmetrical O 1s peak at 532.1 eV is
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Fig. 2. SEM pictures of the worn surfaces of steel balls sliding in different environment: (a) in dry N2 (RH < 5%); (b) in air (RH 40%); (c) in humid N2 (RH 100%); (d) in dry O2 (RH < 5%).
higher binding energy. The intensity of C–C (or C–H) peak decreased, while the intensity of C–O and C=O peaks increased obviously ((Fig. 4(b–d)). A new peak near 290.1 eV was ascribed to carboxylic acid (HO– C=O) groups [25]. It should be noted that the Fe–C bonds at about 283.3 eV were present in the XPS C 1s spectra on the worn surfaces of hydrogenated DLC film after friction testing in humid N2 and dry O2 conditions (Fig. 4(c) and (d)) [26]. The XPS spectra for O 1s and Fe 2p states on the worn surface of the hydrogenated DLC film after friction testing in dry O2 environment are shown in Fig. 5(a) and (b), respectively. The O 1s spectrum changed from a symmetrical single band (Fig. 3(b)) to
mainly attributed to the absorbed oxygen on the film surface due to air exposure (Fig. 3(b)). Fig. 4 shows the XPS spectra for C 1s states on the worn surface of the hydrogenated DLC film after friction testing in different environment. In dry nitrogen (RH < 5%), the XPS C 1s states on the worn surface of DLC film were similar to that on the surface of the original film, which was mainly attributed to the protection actions of nitrogen (Fig. 4(a)). However, in the oxygen and/or water containing conditions, distinct changes in the C 1s spectra were observed after friction testing (Fig. 4(b– d)), as compared with the case of the original film (Fig. 3(a)). The C 1s core level spectrum shifted to a
Table 2 Changes of C, O and Fe concentration (at.%) on the surface of the hydrogenated DLC film before and after friction testing Element
The film as deposited
The films after friction testing in different environment Dry N2 (RH < 5%)
Air (RH 40%)
Humid N2 (RH 100%)
Dry O2 (RH < 5%)
C O Fe
93.87 6.13 0.00
93.34 6.66 0.00
80.03 18.69 1.28
71.63 22.19 6.18
66.29 23.95 9.76
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Fig. 3. XPS spectra for C 1s (a) and O 1s (b) states on the original surface of hydrogenated DLC film.
an asymmetrically broad band, and more than one binding states were present (Fig. 5(a)). The best Gassian fits to the O 1s spectrum resulted in five different peaks at 530.0, 531.4, 532.6, 533.9 and 535.0 eV, respectively. The peak at 530.0 eV was assigned to Fe–O bonds, suggesting the presence of the iron oxide and hydroxide on the worn surface of
DLC film (e.g. Fe2O3 at 529.9 eV, Fe3O4 at 530.0 eV and/or FeOOH at 530.2 eV) [27,28]. The second and third peaks at 531.4 and 532.6 eV corresponded to the C=O and C–O bonds, respectively [23]. The other two peaks at higher binding energy of 533.9 and 535.0 eV corresponded to the carboxylic acid (HO–C=O) groups and absorbed H2O, respectively [23,29]. As
Fig. 4. XPS spectra for C 1s states on the worn surfaces of hydrogenated DLC film tested in different environment: (a) in dry N2 (RH < 5%); (b) in air (RH 40%); (c) in humid N2 (RH 100%); (d) in dry O2 (RH < 5%).
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Fig. 5. XPS spectra for O 1s (a) and Fe 2p (b) states on the worn surface of the hydrogenated DLC film tested in dry O2.
shown in Fig. 5(b), a Fe 2p spectrum centered at 711.3 eV, which corresponded to the Fe3+ as in Fe2O3, Fe3O4 and FeOOH [30], was also detected on the worn surface of the DLC film tested in dry O2 condition.
4. Discussion Combining the above-mentioned friction results, the SEM observations and the XPS analyses, it was reasonable to conclude that the friction behaviors of hydrogenated DLC film in different environment were mainly dependent on the friction-induced physical and chemical interactions among the DLC film, steel ball and oxygen and/or water molecules. 4.1. Interfacial physical interaction According to the SEM observations in Fig. 2, the interfacial physical interaction generated during the friction process was mainly the inter-transfer between the hydrogenated DLC film and the steel ball. In dry N2 (RH < 5%), the hydrogenated DLC film was transferred on the steel ball to form the carbon-rich layer (see Fig. 2(a)). This transferred carbon-rich layer on the worn surface of steel ball helped to prevent the direct contact and the tribochemical interactions between the steel ball and the DLC film, and acted as a ‘‘lubricant’’ of a low-shear-strength at the interface to result in a much low friction coefficient (0.035) [2,3,10,17,18]. However, in the oxygen and/or water containing environments, the friction-induced chemical reactions among the DLC film, steel ball and
oxygen and/or water molecules (discussed below) would prevent the formation of the transferred carbonrich layer on the steel ball (see Fig. 2(c) and (d)) and hence result in a continuously direct contact between the DLC film and the steel ball, thus higher friction coefficient of the DLC film was observed. Moreover, since the hardness of steel ball was smaller than that of DLC film, iron was transferred on the DLC film surface due to the shearing action of friction force, resulting in the severer wear of steel ball (see Fig. 2(c) and (d)). 4.2. Interfacial chemical interactions From the above-mentioned XPS analyses of the worn surfaces of the hydrogenated DLC film, obvious changes in the element (group) compositions and chemical states between the original and worn surface of DLC film could be observed. Based on these facts, it was deduced that some kinds of tribochemical reactions among the DLC film, steel ball and oxygen and/or water molecules were involved during the friction testing in the oxygen and/or water containing environments. 4.2.1. Oxidation of film surface As seen in Table 2 and Fig. 4, after friction testing in the oxygen and/or water containing environments, the concentration of carbon element and the intensity of C–C/C–H bonds in XPS spectra on the worn surface of the DLC film decreased obviously, while the concentration of O element and the intensity of C–O and C=O bonds in XPS spectra increased, indicating
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Fig. 6. Schematic diagram of the oxidation process of the hydrogenated DLC film during the friction testing.
the oxidation of the film surface. The processes of the oxidation of the hydrogenated DLC film were theoretically shown in Fig. 6. Under the shearing action of friction force, sufficient energy might be generated and concentrated on the surface of DLC film [26,31]. Therefore, the C– C bonds and the C–H bonds on the top layer of the hydrogenated DLC film might be mechanically broken and the macro-radicals (R and R0 ) might be produced (Fig. 6, reactions (a) and (c)) [17,26]. In dry N2, the nitrogen molecules hindered the access of water and oxygen molecules to the interface and prevented the macro-radicals to react with the water and/or oxygen molecules. The macro-radicals would be recombined to give similar macro-molecular chains to that of the original DLC film (Fig. 6, reaction (b)) [32]. This might be the main reason for the extremely low friction coefficient and wear rate of the hydrogenated DLC film in dry nitrogen as reported by many researchers. However, in the presence of oxygen and/or water, these macro-radicals would chemisorb oxygen and/or water molecules to form the peroxide radicals (ROO) (Fig. 6, reaction (d)). The peroxide radicals could abstract hydrogen atoms from other molecular chains (R0 H) of DLC film, and then the hydroperoxide groups (–COOH) might be formed (Fig. 6, reaction (e)). Moreover, the radicals (R0 ) would also react with water and/or oxygen molecules immediately and generate peroxide radicals (R0OO) (Fig. 6, reaction
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(f)), which might take part in the above reactions once again. These processes would be repeated until the carbon network was terminated. The hydroperoxide groups (–COOH) were thermally unstable, and the RO radicals could be formed due to the friction-induced thermolysis of hydroperoxide groups under the action of friction force (Fig. 6, reaction (g)). Further shearing of C–H bonds in the neighbors might donate electrons to form the C–O and C=O bonds (Fig. 6, reactions (h) and (i)), and the carbon network was terminated. These processes of the oxidation of the hydrogenated DLC film were similar to that of hydrocarbon polymer described by Scott [32]. The friction-induced oxidation of the hydrogenated DLC films changed the surface chemical states from C–H bonds to oxygen-containing groups. Subsequently, during the friction process, the bonding strength increased from about 0.08 eV per bond for the Van der Waals bonding of hydrocarbons to about 0.21 eV per bond for the hydrogen bonding at C–O and C=O sites in the presence of oxygen and/or water molecules [2,3,15], which resulted in an increase in the friction coefficient of the DLC film. 4.2.2. Reactions between hydrogenated DLC film and steel ball As mentioned above, Fe element was detected on the worn surface of DLC film and the Fe–C bonds were present in the XPS spectra tested in the oxygen and/or water containing environments. Therefore, it could be deduced that reactions between the DLC film and steel ball were present in the process of friction testing. The cracking of C–C and C–H bonds might result in the formation of active macro-radicals (R). These macro-radicals would react not only with water, oxygen and other radicals, but also with activated Fe atoms to form Fe–C bonds under the action of friction force [26]. This reaction process could be given as follows: RC þ Fe ! RCFe Similarly, the peroxide radicals (ROO) might also react with Fe atoms to form ROO–Fe bonds [26]: ROO þ Fe ! ROOFe These reactions between the DLC film and steel ball led to strong adhesive friction at the interface, and therefore caused higher and unstable friction coeffi-
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cient of the DLC film (see Fig. 1). As mentioned above, the hardness of steel ball was smaller than that of DLC film, and iron was transferred on the DLC film surface due to the strong adhesive friction, resulting in the severer wear of steel ball (see Fig. 2(c) and (d)). 4.2.3. Formation of the oxide and hydroxide of iron Fe2O3, Fe3O4 and FeOOH were found on the worn surface of the DLC film tested in the oxygen and/or water containing environments according to the XPS analyses (see Fig. 5). On the basis of a simple model for asperity temperature rise due to friction [33], Liu et al. had proposed that the temperature on the local asperity contact could reach about 100–300 8C for the metallic ball sliding against the DLC film [19]. The temperature was so high that the iron could react with oxygen and/or water:
chemical reactions between the DLC film and steel ball are involved in the friction process. These chemical reactions cause strong adhesive friction at the interface, and hence lead to higher friction coefficient of the DLC film (more than 0.2) and severer wear of steel ball.
Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant No. 59925513 and 50323007), the National Basic Research Priority Program of China (No. G1999065005), the 863 Program of China (No. 2003AA305670) and ‘‘Top Hundred Talents Program’’ of Chinese Academy of Sciences for financial support.
4Fe þ 3O2 ! 2Fe2 O3 2Fe þ 6H2 O ! 2FeðOHÞ3 þ 3H2 4Fe þ 3O2 þ 6H2 O ! 4FeðOHÞ3 However, the Fe(OH)3 products were much unstable and easy to be further oxidized. As a result, Fe2O3, Fe3O4 and FeOOH were formed on the worn surface of DLC film.
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5. Conclusions The friction behaviors of the hydrogenated DLC film sliding against steel ball are highly dependent on the testing environment, and the environment dependence of the friction behaviors of the hydrogenated DLC film is closely related with the friction-induced physical and chemical interactions among DLC film, steel ball and oxygen and/or water molecules. In dry N2, thick and compact transferred carbon-rich layer is covering on the worn surface of steel ball to prevent the direct contact and the tribochemical reactions between the DLC film and steel ball, and acts as a ‘‘lubricant’’ of a low-shear-strength at the interface to result in a much low and stable friction coefficient of about 0.035. In the oxygen and/or water containing environments, the formation of the transferred carbonrich layer on the steel ball is inhibited by the frictioninduced chemical reactions, and the friction-induced oxidation of the hydrogenated DLC film and the
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Friction behaviors of hydrogenated diamond-like carbon film in different environment sliding against steel ball Hongxuan Lia,b, Tao Xua, Chengbing Wanga,b, Jianmin Chena,*, Huidi Zhoua, Huiwen Liua a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate School of the Chinese Academy of Science, Beijing 100039, PR China
Received 1 September 2004; received in revised form 1 December 2004; accepted 1 December 2004 Available online 29 December 2004
Abstract Friction behaviors of hydrogenated diamond-like carbon (DLC) film sliding against steel ball were investigated on a ball-ondisk test rig in different environment. The worn surface morphology of the steel ball was observed on a scanning electron microscope (SEM), and the chemical states of some typical elements on the worn surface of DLC film were investigated by means of X-ray photoelectron spectroscopy (XPS). The result showed that the friction behaviors of hydrogenated DLC film were very sensitive to the testing environment. In dry N2, the film provided a much low and stable friction coefficient of about 0.035. When the friction tests were performed in higher relative humidity and in dry O2, higher and unstable friction coefficient of more than 0.2 was obtained. It was found that in oxygen and/or water containing environments, the transferred carbon-rich layer on the counterpart steel ball was inhibited, and the friction-induced oxidation of the DLC film surface and the chemical reactions between the DLC film and steel ball were involved in the friction processes, which accounted for the higher friction coefficient of the DLC film. The roles of environment in the friction behaviors of DLC film were discussed in terms of the friction-induced physical and chemical interactions among the DLC film, steel ball and water and/or oxygen molecules. # 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogenated DLC film; Friction behaviors; Environment; Interaction
1. Introduction In recent years, hydrogenated diamond-like carbon films have attracted increased attention due to their * Corresponding author. Tel.: +86 9314968085; fax: +86 9318277088. E-mail address: [email protected] (J. Chen).
extraordinary properties of high mechanical hardness, low friction coefficient, high wear resistance and chemical inertness. These properties are promising for a wide range of tribological applications such as magnetic hard disks, gears, bearings, cutting tools and other moving mechanical assemblies [1–3]. However, previous studies have shown that the tribological properties of hydrogenated DLC films are strongly
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.12.002
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dependent on the deposition methods and deposition conditions, and are very sensitive to the testing environment and the relative humidity (RH) [4–12]. Depending on the above-mentioned factors, the friction coefficients for the hydrogenated DLC films have been reported to broadly range from 0.003 to more than 0.3 [6–8,11,13–16]. Various theories have been proposed to explain the friction and wear behaviors of DLC films. The build-up of a friction-induced transfer film on the counterpart, followed by easy shear within the interfacial material is the most frequently observed friction-controlling mechanism for DLC films [3,10,17,18]. Liu et al. said that the steady-state low friction of DLC films was due to the wear-induced graphitization, i.e., the formation of a low friction graphitized tribolayer [6,19]. Erdemir et al. argued that the general low friction coefficient of hydrogenated DLC films was mainly attributed to the high chemical inertness of the DLC films by passivating the surface dangling bonds by hydrogen [14,20]. However, other researchers proposed that the tribochemical reactions in the tribo-system played an important role in determining the friction and wear behaviors of DLC films [3,10,17]. Therefore, these reported friction and wear mechanisms for the DLC films are the subjects to controversy and the role of the environments in the friction and wear behaviors of DLC films is still not well understood. In the present work, we investigated the friction behaviors of the hydrogenated DLC films sliding against steel balls in different environment (air, dry N2, humid N2 and dry O2), with emphasis on the friction-induced physical and chemical interactions among the DLC film, steel ball and water and/or oxygen molecules. Based on the results, it is hoped that the friction mechanisms of DLC films can be better understood.
2. Experimental details 2.1. Deposition and characterization of the hydrogenated DLC film Hydrogenated DLC film was deposited on Si (1 0 0) wafers by a PECVD technique, using CH4 plus Ar as the feedstock. The details about the deposition equipment and process were described elsewhere
Table 1 Summary of the deposition conditions and some properties of the hydrogenated DLC film Item
Parameters
CH4 gas flow rate (sccm) Ar gas flow rate (sccm) Deposition pressure (Pa) RF power (W) DC bias (V) Deposition time (h) Thickness (nm) Hardness (GPa) Young’s modulus (GPa)
11.6 11 5 300 200 3 640 15.2 120
[21,22]. Prior to deposition, the substrates were cleaned with Ar plasma sputtering at a bias voltage of 400 V for 15 min so as to remove the native oxide on the Si surface. The deposition conditions and some properties of the resulting DLC film are summarized in Table 1. During the film deposition, the substrate was only heated by the plasma and the substrate temperature was measured to be about 140 8C by a thermocouple mounted on the substrate holder. The deposited hydrogenated DLC film is dark brown in color and is extremely smooth and featureless when viewed with unaided eyes. The film thickness measured on a profilometer is about 640 nm. The hardness and Young’s modulus of the film are determined to be about 15.2 and 120 GPa, respectively, using a nano-indenter. 2.2. Friction tests The friction behaviors of the hydrogenated DLC films sliding against steel ball (AISI 52100, diameter 4 mm, surface roughness Ra 0.1 mm) were evaluated on a ball-on-disk test rig equipped with an environmental chamber with which the relative humidity and gaseous environment could be controlled. The friction tests were performed in air (RH 40%), dry N2 (RH < 5%), humid N2 (RH 100%) and dry O2 (RH < 5%), respectively, at a normal load of 2 N, a sliding velocity of about 125 m/min, to a maximum sliding duration of 60 min. The worn surface of steel ball was analyzed on a JSM-5600LV scanning electron microscope (SEM) equipped with a KEVEX SIGMA energy dispersive X-ray spectrometer (EDS). The chemical compositions and chemical states of the worn surface of the
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hydrogenated DLC film were analyzed on a PHI-5702 X-ray photoelectron spectroscope (XPS) operating with monochromated Al-K irradiation at a pass energy of 29.4 eV.
3. Results 3.1. Friction behaviors of the hydrogenated DLC film in different environment Fig. 1 shows the friction coefficient of hydrogenated DLC film as a function of sliding time in different environment. It is clear that the friction behaviors of the film are strongly dependent on the environment and relative humidity. In air (RH 40%), the friction coefficient of the hydrogenated DLC film was about 0.1. In dry N2 (RH < 5%), much low and stable friction coefficient of about 0.035 was obtained. As the relative humidity was increases to 100% in N2, the friction between the film and steel ball was unstable, and the friction coefficient was increased to about 0.2 in the steadystate. Particularly, when sliding in dry O2 (RH < 5%), the hydrogenated DLC film provided a much unstable and higher friction coefficient of more than 0.2. 3.2. SEM analyses of the worn surface of steel ball Fig. 2 shows the SEM pictures of the worn surface of steel ball sliding in different environment. In dry N2
Fig. 1. Friction coefficient of hydrogenated DLC film as a function of sliding time in different environment.
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(RH < 5%), the worn surface of the steel ball was covered by obvious transferred materials, which were mainly C element analyzed by EDS spectra, and this transferred carbon-rich layer was usually thick, compact and homogeneous in appearance (Fig. 2(a)). In air (RH 40%), the transferred carbon layer on the worn surface of steel ball was attenuated (Fig. 2(b)). However, when sliding at the higher relative humidity in N2 (RH 100%), the worn surface of steel ball was relatively large, and the EDS spectra showed that there was no transferred carbon layer on the worn surface of steel ball (Fig. 2(c)). It should be noted that in the dry O2 (RH < 5%) condition, the worn surface of steel ball was much larger and rougher, indicating a severe wear of the steel ball (Fig. 2(d)). 3.3. XPS analyses of the worn surface of hydrogenated DLC film Table 2 summarizes the changes of C, O and Fe concentration (at.%) determined by XPS analyses, on the original and worn surfaces of hydrogenated DLC film at varied testing conditions. The presence of O element on the original DLC film was due to air exposure. After the friction testing in dry N2 (RH < 5%), the concentration of O element had no distinct increase due to the protection action of nitrogen. However, the concentration of O element on the worn film surfaces tested in air, humid N2 and dry O2, respectively, was 18.69, 22.19 and 23.95 at.%, which was much larger than 6.13 at.%, that of the original DLC film. Moreover, when tested in the oxygen and/or water containing conditions, an increase of the concentration of Fe element was also detected on the worn film surfaces. Thus it can be inferred that the surface of the DLC film is restructured due to shearing forces and tribochemical reactions during the friction process. Fig. 3 shows the XPS spectra of C 1s and O 1s on the original surface of hydrogenated DLC film. The C 1s spectra can be fitted into three components around 284.8, 286.3, and 288.0 eV, respectively (Fig. 3(a)). The one around 284.8 eV corresponds to C–C and/or C–H bonds, and that around 286.3 eV corresponds to C–O bonds. The third peak of a much smaller intensity near 288.0 eV is assigned to C=O bond [23,24]. At the same time, the symmetrical O 1s peak at 532.1 eV is
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Fig. 2. SEM pictures of the worn surfaces of steel balls sliding in different environment: (a) in dry N2 (RH < 5%); (b) in air (RH 40%); (c) in humid N2 (RH 100%); (d) in dry O2 (RH < 5%).
higher binding energy. The intensity of C–C (or C–H) peak decreased, while the intensity of C–O and C=O peaks increased obviously ((Fig. 4(b–d)). A new peak near 290.1 eV was ascribed to carboxylic acid (HO– C=O) groups [25]. It should be noted that the Fe–C bonds at about 283.3 eV were present in the XPS C 1s spectra on the worn surfaces of hydrogenated DLC film after friction testing in humid N2 and dry O2 conditions (Fig. 4(c) and (d)) [26]. The XPS spectra for O 1s and Fe 2p states on the worn surface of the hydrogenated DLC film after friction testing in dry O2 environment are shown in Fig. 5(a) and (b), respectively. The O 1s spectrum changed from a symmetrical single band (Fig. 3(b)) to
mainly attributed to the absorbed oxygen on the film surface due to air exposure (Fig. 3(b)). Fig. 4 shows the XPS spectra for C 1s states on the worn surface of the hydrogenated DLC film after friction testing in different environment. In dry nitrogen (RH < 5%), the XPS C 1s states on the worn surface of DLC film were similar to that on the surface of the original film, which was mainly attributed to the protection actions of nitrogen (Fig. 4(a)). However, in the oxygen and/or water containing conditions, distinct changes in the C 1s spectra were observed after friction testing (Fig. 4(b– d)), as compared with the case of the original film (Fig. 3(a)). The C 1s core level spectrum shifted to a
Table 2 Changes of C, O and Fe concentration (at.%) on the surface of the hydrogenated DLC film before and after friction testing Element
The film as deposited
The films after friction testing in different environment Dry N2 (RH < 5%)
Air (RH 40%)
Humid N2 (RH 100%)
Dry O2 (RH < 5%)
C O Fe
93.87 6.13 0.00
93.34 6.66 0.00
80.03 18.69 1.28
71.63 22.19 6.18
66.29 23.95 9.76
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Fig. 3. XPS spectra for C 1s (a) and O 1s (b) states on the original surface of hydrogenated DLC film.
an asymmetrically broad band, and more than one binding states were present (Fig. 5(a)). The best Gassian fits to the O 1s spectrum resulted in five different peaks at 530.0, 531.4, 532.6, 533.9 and 535.0 eV, respectively. The peak at 530.0 eV was assigned to Fe–O bonds, suggesting the presence of the iron oxide and hydroxide on the worn surface of
DLC film (e.g. Fe2O3 at 529.9 eV, Fe3O4 at 530.0 eV and/or FeOOH at 530.2 eV) [27,28]. The second and third peaks at 531.4 and 532.6 eV corresponded to the C=O and C–O bonds, respectively [23]. The other two peaks at higher binding energy of 533.9 and 535.0 eV corresponded to the carboxylic acid (HO–C=O) groups and absorbed H2O, respectively [23,29]. As
Fig. 4. XPS spectra for C 1s states on the worn surfaces of hydrogenated DLC film tested in different environment: (a) in dry N2 (RH < 5%); (b) in air (RH 40%); (c) in humid N2 (RH 100%); (d) in dry O2 (RH < 5%).
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Fig. 5. XPS spectra for O 1s (a) and Fe 2p (b) states on the worn surface of the hydrogenated DLC film tested in dry O2.
shown in Fig. 5(b), a Fe 2p spectrum centered at 711.3 eV, which corresponded to the Fe3+ as in Fe2O3, Fe3O4 and FeOOH [30], was also detected on the worn surface of the DLC film tested in dry O2 condition.
4. Discussion Combining the above-mentioned friction results, the SEM observations and the XPS analyses, it was reasonable to conclude that the friction behaviors of hydrogenated DLC film in different environment were mainly dependent on the friction-induced physical and chemical interactions among the DLC film, steel ball and oxygen and/or water molecules. 4.1. Interfacial physical interaction According to the SEM observations in Fig. 2, the interfacial physical interaction generated during the friction process was mainly the inter-transfer between the hydrogenated DLC film and the steel ball. In dry N2 (RH < 5%), the hydrogenated DLC film was transferred on the steel ball to form the carbon-rich layer (see Fig. 2(a)). This transferred carbon-rich layer on the worn surface of steel ball helped to prevent the direct contact and the tribochemical interactions between the steel ball and the DLC film, and acted as a ‘‘lubricant’’ of a low-shear-strength at the interface to result in a much low friction coefficient (0.035) [2,3,10,17,18]. However, in the oxygen and/or water containing environments, the friction-induced chemical reactions among the DLC film, steel ball and
oxygen and/or water molecules (discussed below) would prevent the formation of the transferred carbonrich layer on the steel ball (see Fig. 2(c) and (d)) and hence result in a continuously direct contact between the DLC film and the steel ball, thus higher friction coefficient of the DLC film was observed. Moreover, since the hardness of steel ball was smaller than that of DLC film, iron was transferred on the DLC film surface due to the shearing action of friction force, resulting in the severer wear of steel ball (see Fig. 2(c) and (d)). 4.2. Interfacial chemical interactions From the above-mentioned XPS analyses of the worn surfaces of the hydrogenated DLC film, obvious changes in the element (group) compositions and chemical states between the original and worn surface of DLC film could be observed. Based on these facts, it was deduced that some kinds of tribochemical reactions among the DLC film, steel ball and oxygen and/or water molecules were involved during the friction testing in the oxygen and/or water containing environments. 4.2.1. Oxidation of film surface As seen in Table 2 and Fig. 4, after friction testing in the oxygen and/or water containing environments, the concentration of carbon element and the intensity of C–C/C–H bonds in XPS spectra on the worn surface of the DLC film decreased obviously, while the concentration of O element and the intensity of C–O and C=O bonds in XPS spectra increased, indicating
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Fig. 6. Schematic diagram of the oxidation process of the hydrogenated DLC film during the friction testing.
the oxidation of the film surface. The processes of the oxidation of the hydrogenated DLC film were theoretically shown in Fig. 6. Under the shearing action of friction force, sufficient energy might be generated and concentrated on the surface of DLC film [26,31]. Therefore, the C– C bonds and the C–H bonds on the top layer of the hydrogenated DLC film might be mechanically broken and the macro-radicals (R and R0 ) might be produced (Fig. 6, reactions (a) and (c)) [17,26]. In dry N2, the nitrogen molecules hindered the access of water and oxygen molecules to the interface and prevented the macro-radicals to react with the water and/or oxygen molecules. The macro-radicals would be recombined to give similar macro-molecular chains to that of the original DLC film (Fig. 6, reaction (b)) [32]. This might be the main reason for the extremely low friction coefficient and wear rate of the hydrogenated DLC film in dry nitrogen as reported by many researchers. However, in the presence of oxygen and/or water, these macro-radicals would chemisorb oxygen and/or water molecules to form the peroxide radicals (ROO) (Fig. 6, reaction (d)). The peroxide radicals could abstract hydrogen atoms from other molecular chains (R0 H) of DLC film, and then the hydroperoxide groups (–COOH) might be formed (Fig. 6, reaction (e)). Moreover, the radicals (R0 ) would also react with water and/or oxygen molecules immediately and generate peroxide radicals (R0OO) (Fig. 6, reaction
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(f)), which might take part in the above reactions once again. These processes would be repeated until the carbon network was terminated. The hydroperoxide groups (–COOH) were thermally unstable, and the RO radicals could be formed due to the friction-induced thermolysis of hydroperoxide groups under the action of friction force (Fig. 6, reaction (g)). Further shearing of C–H bonds in the neighbors might donate electrons to form the C–O and C=O bonds (Fig. 6, reactions (h) and (i)), and the carbon network was terminated. These processes of the oxidation of the hydrogenated DLC film were similar to that of hydrocarbon polymer described by Scott [32]. The friction-induced oxidation of the hydrogenated DLC films changed the surface chemical states from C–H bonds to oxygen-containing groups. Subsequently, during the friction process, the bonding strength increased from about 0.08 eV per bond for the Van der Waals bonding of hydrocarbons to about 0.21 eV per bond for the hydrogen bonding at C–O and C=O sites in the presence of oxygen and/or water molecules [2,3,15], which resulted in an increase in the friction coefficient of the DLC film. 4.2.2. Reactions between hydrogenated DLC film and steel ball As mentioned above, Fe element was detected on the worn surface of DLC film and the Fe–C bonds were present in the XPS spectra tested in the oxygen and/or water containing environments. Therefore, it could be deduced that reactions between the DLC film and steel ball were present in the process of friction testing. The cracking of C–C and C–H bonds might result in the formation of active macro-radicals (R). These macro-radicals would react not only with water, oxygen and other radicals, but also with activated Fe atoms to form Fe–C bonds under the action of friction force [26]. This reaction process could be given as follows: RC þ Fe ! RCFe Similarly, the peroxide radicals (ROO) might also react with Fe atoms to form ROO–Fe bonds [26]: ROO þ Fe ! ROOFe These reactions between the DLC film and steel ball led to strong adhesive friction at the interface, and therefore caused higher and unstable friction coeffi-
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cient of the DLC film (see Fig. 1). As mentioned above, the hardness of steel ball was smaller than that of DLC film, and iron was transferred on the DLC film surface due to the strong adhesive friction, resulting in the severer wear of steel ball (see Fig. 2(c) and (d)). 4.2.3. Formation of the oxide and hydroxide of iron Fe2O3, Fe3O4 and FeOOH were found on the worn surface of the DLC film tested in the oxygen and/or water containing environments according to the XPS analyses (see Fig. 5). On the basis of a simple model for asperity temperature rise due to friction [33], Liu et al. had proposed that the temperature on the local asperity contact could reach about 100–300 8C for the metallic ball sliding against the DLC film [19]. The temperature was so high that the iron could react with oxygen and/or water:
chemical reactions between the DLC film and steel ball are involved in the friction process. These chemical reactions cause strong adhesive friction at the interface, and hence lead to higher friction coefficient of the DLC film (more than 0.2) and severer wear of steel ball.
Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant No. 59925513 and 50323007), the National Basic Research Priority Program of China (No. G1999065005), the 863 Program of China (No. 2003AA305670) and ‘‘Top Hundred Talents Program’’ of Chinese Academy of Sciences for financial support.
4Fe þ 3O2 ! 2Fe2 O3 2Fe þ 6H2 O ! 2FeðOHÞ3 þ 3H2 4Fe þ 3O2 þ 6H2 O ! 4FeðOHÞ3 However, the Fe(OH)3 products were much unstable and easy to be further oxidized. As a result, Fe2O3, Fe3O4 and FeOOH were formed on the worn surface of DLC film.
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5. Conclusions The friction behaviors of the hydrogenated DLC film sliding against steel ball are highly dependent on the testing environment, and the environment dependence of the friction behaviors of the hydrogenated DLC film is closely related with the friction-induced physical and chemical interactions among DLC film, steel ball and oxygen and/or water molecules. In dry N2, thick and compact transferred carbon-rich layer is covering on the worn surface of steel ball to prevent the direct contact and the tribochemical reactions between the DLC film and steel ball, and acts as a ‘‘lubricant’’ of a low-shear-strength at the interface to result in a much low and stable friction coefficient of about 0.035. In the oxygen and/or water containing environments, the formation of the transferred carbonrich layer on the steel ball is inhibited by the frictioninduced chemical reactions, and the friction-induced oxidation of the hydrogenated DLC film and the
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