Alternative Friction Mechanism for Amorphous Carbon Films Sliding

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Alternative Friction Mechanism for Amorphous Carbon Films Sliding against Alumina Jingjing Wang,†,‡ Fu Wang,† Ziwen Cheng,†,§ Guangan Zhang,*,†,‡ Zhibin Lu,*,†,‡ and Qunji Xue† †

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State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China § Institute of Nanoscience and Nanotechnology, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China S Supporting Information *

ABSTRACT: Amorphous carbon films with low friction and wear become a hotspot of research and engineering application for meeting the requirements of energy savings and environmental benefits. However, the comprehensive effect of intrinsic microstructure, sp2/sp3 bonding ratio and hydrogen content, on the friction and wear of amorphous carbon film remains poorly understood. Here, basing on the experiments and first-principles calculations, the underlying mechanism between sp2 carbon lubricating and hydrogen passivation toward low friction is proposed. For the films with high sp2 carbon content, the friction coefficient increases with the hydrogen content, which demonstrates that the sp2 carbon acts as a dominant lubricating agent for low friction in vacuum. On the other hand, the decreased friction coefficient with high hydrogen content increasing is mainly attributed to hydrogen passivation. The studies will allow for significant insight into the friction mechanism in tribological contacts and provide guidance on designing high-performance tribo-systems for engineering applications. amorphous carbon-based film under inert conditions such as in vacuum remains a major challenge. Given its unique atomic structure, which comprises both sp2-hybridized carbon and hydrogen, the tribological properties of amorphous carbon can be engineered through controlling the microstructure (sp2/sp3 bonding ratio and hydrogen content) by varying the fabrication process using magnetron sputtering technique. Magnetron sputtering is a simple and industry compatible process for the scalable production of amorphous carbon films at room temperature, which can produce high sp2 carbon bonds. At present, the most popular friction mechanisms proposed are hydrogen passivation and rehybridization (sp3-to-sp2 transformation).9,12−21 Hydrogen doping in the carbon-based

1. INTRODUCTION With the increasing focus on energy savings and environmental benefits, materials with low friction and wear have become a hotspot of research. Herein, amorphous carbon films have become a potential candidate of solid lubricant with excellent mechanical properties as well as exceptional friction-reduction and wear-resistance properties,1−5 which can revolutionize many technologies, including not only precision instrument parts such as micro/nanoelectromechanical systems(MEMS/ NEMS) and AFM tips but also automotive parts and space mechanisms.6,7 However, their widespread application is limited by the dependence of this physical behavior on the chemical composition and structures. Depending on hydrogen content in the films, they are divided into two types, namely hydrogen-free amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) films. A wide range of researches have been explored to get a full understanding of the friction mechanism responsible for low friction of a-C and a-C:H films.8−11 However, the origin of friction behaviors of © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

December 8, 2018 February 19, 2019 March 5, 2019 March 5, 2019 DOI: 10.1021/acs.iecr.8b06082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

C:H films were grown with iso-butane(C4H10) gas flow of 4, 8, 12, and 16 sccm during deposition, which will be designated as a-C:H-1, a-C:H-2, a-C:H-3 and a-C:H-4 films, respectively. Meanwhile, a-C film without hydrogen was deposited in the same process as the a-C:H except that no iso-butane as a precursor gas was used during deposition. The thickness of the as-deposited five films was approximately 2.8 μm. The hardness and Young’s modulus of the as-deposited films were obtained by nanoindentation(Anton-Paar TTX-NHT2). The surface topography and the roughness were examined by atomic force microscopy (AFM, CSPM4000). The analysis of the components and structures of the as-deposited films was performed using confocal Raman spectrometer(Jobin-Yvon LabRAM HR Evolution, France). Here, visible and ultraviolet(UV) Raman spectra were measured by adopting 532 and 325 nm argon ion laser as the excitation source, respectively. The former was used to determine the hydrogen content in the films following an empirical method.18,26,40 The latter mainly detected the structural arrangement of carbon atoms, which qualitatively evaluated the fraction of sp2-hybridized carbon atoms in the a-C and a-C:H films. The measurement of friction coefficient was carried out in high vacuum (1 × 10−5 mbar) using HVTRB tribometer from Anton-paar company. The temperature and humidity of the laboratory are 25 °C and 25% RH, which corresponds to water content in vacuum (1 × 10−5 mbar) less than 9 ppm and oxygen less than 2 ppm. Ball-on-disc reciprocating sliding mode was employed with a full amplitude of 5 mm and alumina sphere with 6 mm diameter was used as counterpart. Besides, an oscillation frequency of 5 Hz inducing a maximum sliding speed of 7.85 cm/s and a load of 5, 2, and 0.5 N were applied during test. Every test last 20000 cycles. Three repeat tests were measured for every film for purpose of ensuring reliability of friction data. Here, mean friction coefficient was calculated for every friction test. The wear rate was measured by Alpha-step D-100 two-dimensional profiler.

films is recognized as an effective way to realize low friction from both experimental and theoretical aspects.9,12,22−28 DC.Lugo et al. have concluded that friction coefficient of aC:H films decreased with the increase in hydrogen content.26 They proposed two reasons that account for the low friction of a-C:H films: one is the hydrogen atoms saturating the dangling σ-bonds carbon at the interface, the other is the subsequent electrostatic repulsion between the two sliding surfaces.12 On the other hand, sp3-to-sp2 rehybridization is another of the common postulations to explain the low friction of carbonbased films. Chen Yinan et al. have concluded that the nanocrystalline graphene-like layer structures formed on the contact areas during compression and shear process act as a lubricating agent, resulting in low friction.9 Other works also proved that larger more ordered graphene nanoflakes formed on the interfaces is an essential factor for low friction.29−32 Several in situ TEM studies also offered strong evidence that the formation of a graphitic layer during shearing is the frictional controlling mechanism in a-C film.33,34 These results unveil that friction-induced sp2-rich carbon on the interface plays a vital role on the friction of amorphous carbon-based films. In addition, the intrinsic materials rich in the sp2 carbon, like graphite, graphene, carbon nanotubes and fullerenes, are known as good lubricant demonstrating low friction.6,35−38 However, there are few comparative studies on achieving low friction by tailoring the microstructure (sp2/sp3 bonding ratio and hydrogen content). Here, we probe the friction behaviors of amorphous carbon films with different sp2 carbon and hydrogen contents. The comprehensive effects of hydrogen passivation and sp2 carbon lubricating on the friction behavior of amorphous carbon-based films are elaborated. It would be informative to clarify the dominant friction mechanism for amorphous carbon-based films friction-reduction, and provide guidance on the design of high-performance tribosystem for engineering applications. In this letter, the roles of the sp2 carbon fraction and hydrogen content on the friction behaviors of amorphous carbon film was investigated by performing friction experiments in vacuum. Five amorphous carbon films are produced with different contents of hydrogen and sp2 carbon atoms using magnetron sputtering technique. We adopt alumina sphere as the counterpart to avoid the effect of transfer films, since we previously found that the mating Al2O3/a-C:H had intrinsically weak-interacting sliding interfaces.39 Furthermore, first principle calculations are performed to compare the interface adhesion of C (sp2)/Al2O3 and C(sp3)/Al2O3, further illustrating that unbonded sp2 carbon has a better lubricating effect compared to unpassivated sp3 carbon.

3. RESULTS AND DISCUSSION 3.1. Characterization of a-C and a-C:H Films. The asdeposited films show different mechanical properties due to different deposition parameters. Table 1 presents physical Table 1. Some Physical Properties of the As-Deposited Films

2. EXPERIMENTAL METHODS Amorphous carbon films(a-C and a-C:H) were deposited on 304 stainless steel and silicon wafer using a Teer UDP-650 unbalanced magnetron sputtering system. The flat substrates first ultrasonically cleaned in acetone for 20 min and then cleaned in alcohol for 15 min before putting into the deposition chamber. Prior to deposition, the substrates were bombarded with argon ions for 15 min to remove surface contaminants. To enhance the adhesion to substrates, a Cr interlayer was deposited first. Subsequently, the hydrogenated a-C:H films were produced on top of the Cr layer by sputtering carbon atoms from graphite targets with the current of 3.5 A at bias voltage of 70 V and duty cycle of 20% in the mixed gas of Ar and C4H10. The deposition time was 4 h. Four kinds of a-

sample

C4H10 flux (sccm)

hardness (GPa)

elastic modulus (GPa)

roughness Ra (nm)

a-C a-C:H-1 a-C:H-2 a-C:H-3 a-C:H-4

0 4 8 12 16

13.6 22.1 20.4 12.6 7.5

163.2 228.9 180.0 115.6 72.6

4.02 3.07 2.67 3.00 2.85

characteristics of the as-deposited a-C and a-C:H films. The hardness and elastic modulus of the as-deposited films increase and then decrease with increasing in flux of C4H10, which is related to hydrogen content and structures of these films. Besides, the five films deposited by magnetron sputtering technique have almost the same roughness with ∼3 nm. Figure 1 shows the surface topography of those films by AFM. It can be observed that all the films are grown smooth and uniform. Figure 2 shows the deconvoluted visible Raman spectra in G and D peak of the four a-C:H films with different flux of C4H10. B

DOI: 10.1021/acs.iecr.8b06082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Surface topography of a-C and a-C:H films by AFM.

m/IG in eq 1 is 0, which is a meaningless value for eq 1. Thus, we cannot calculate the hydrogen content of a-C:H-1 using eq 1. Here, we can only make qualitative analysis. From deposition experiment above, it is known that the only source gas for supplying hydrogen for a-C:H films is C4H10. For aC:H-1 film, though only 4 sccm C4H10 is introduced during deposition process, it is certain that a-C:H-1 films contains hydrogen. Compared with other a-C:H films, the least amount of hydrogen atoms contain in a-C:H-1 film. Since Raman spectrometry is an empirical method that approximates the hydrogen content, which only includes the bonded hydrogen atoms in the a-C:H films with C−H bonds, the actual total hydrogen content that also contains no bonded hydrogen atoms should be larger than the calculated value above, which was confirmed by the results of DC.Lugo et al.26 However, the trend of hydrogen content increasing with flux of C4H10 is constant. In the deposition process, the precursor gas C4H10 will be decomposed into various hydrocarbon plasma CHx+. When the flux of C4H10 increase, the concentration of hydrocarbon plasma in the deposition chamber will increase, which subsequently increases the probability of reaction with the graphite target. Thus, the hydrogen content of the asdeposited films will increase. To avoid the effect of large spectral background on deconvolution in D and G peak in visible Raman spectra for a-C:H films, ultraviolet Raman spectra was used to analyze the state of carbon bonding. Figure 3 presents the deconvoluted ultraviolet Raman spectra in G and D peak with Gaussian function for a-C and a-C:H films. The G band peak derives from the stretching of all pairs of sp2 carbon atoms in both chains and rings. The D band peak originates from the breathing mode of sp2 atoms in ring structure. The increase of value of ID/IG is related to the clustering of sp2 carbon in the ultraviolet Raman spectra, namely, the fraction of sp2 carbon increasing. In addition, different from visible Raman spectra, the G peak position decreases with the increasing in sp2 carbon in UV Raman.40 The Gaussian fitting results of ultraviolet Raman spectra for the as-deposited five films are summarized in Table 2. As gas flow of C4H10 increases, the ratio of ID/IG decreases and the position of G peak shifts to a higher

Figure 2. Deconvolution of visible Raman spectra for a-C:H films.

It is seen that photoluminescence (PL) background gets larger when the flux of C4H10 during deposition increases. Here, identified by the previous studies,18,26 we can estimate the hydrogen content in the film by visible Raman spectroscopy. From previous work,40 it is concluded that the visible Raman spectra of a-C:H films show a typical signature that the increasing PL background for higher H content. This is due to the hydrogen saturation of nonradiative recombination centers.41−43 And they gave a simple quantitative formula which can be empirically used as a measure of the bonded H content by the ratio between the slope m of the spectral PL background and the intensity of the G peak, m/IG, as depicted in eq 1. | l o om H (at. %) = 21.7 + 16.6 logo (μm)o m } o o o o I (1) n G ~ As shown in Figure 2, the hydrogen content calculated for aC:H films increases with flux of C4H10 apart from a-C:H-1 film. For a-C:H-1 film, the slope of spectral background is zero, so C

DOI: 10.1021/acs.iecr.8b06082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 3. Deconvolution of ultraviolet Raman spectra for a-C and a-C:H films.

Table 2. Fitted Results of Ultraviolet Raman Spectra: Peak Position (ω), the Full Width at Half Maximum (FWHM(Γ)), Intensity Ratio of D Peak to G Peak (ID/IG) D peak −1

G peak −1

−1

film

ω (cm )

Γ (cm )

ω (cm )

Γ (cm−1)

ID/IG

a-C a-C:H-1 a-C:H-2 a-C:H-3 a-C:H-4

1395 1375 1379 1390 1430

367 383 372 375 351

1562 1566 1571 1575 1582

147 164 139 115 88

0.95 0.51 0.38 0.37 0.27

wavenumber, indicating sp2 carbon decreasing. All in all, the results of visible and ultraviolet Raman spectra indicate that the fraction of sp2 carbon atoms decreases when hydrogen content in the films increases. 3.2. Friction and Wear Analysis of a-C and a-C:H Films. The friction coefficient as a function of sliding laps recorded from the five films against alumina sphere at 0.5, 2, and 5 N are shown in Figure S1. The friction curves at three loads for five films share a common feature: the friction coefficient reach its steady-state after a run-in period (ca. 400 laps) due to similar roughness, because it is usually thought that the run-in behavior is a process of smoothing the initial surface roughness.44 The mean friction coefficients of these five films in vacuum at different loads are obtained by calculating the average of three replicates, including run-in and steadystate period except for films failure stages, as shown in Figure 4. The specific values are listed in Table S1. For a-C:H films, mean friction coefficient decreases with hydrogen content increasing. It is obviously accepted that hydrogen containing in the amorphous carbon films plays a vital role in friction reduction in vacuum, which is related to carbon dangling bonds at the sliding interfaces passivated by hydrogen. However, mean friction coefficient of a-C film is lower than a-C:H-1 film with a few hydrogen atoms at load of 2 and 5 N. At 0.5 N, the values of mean friction coefficient of both films are almost same. This may be due to the fact that the effect of the large amount of sp2 carbon contained in the a-C film suppresses that of a small amount of hydrogen incorporated in the a-C:H-1 film on the friction at higher loads. Since a-C films

Figure 4. Variation of mean friction coefficient of the five films at different loads.

has more sp2 than a-C:H-1, the sp3 friction of a-C is lower than that of a-C:H-1 film. In other words, sp2 carbon has better lubricity than sp3 carbon. J. Andersson have found the friction coefficient of self-mated ta-C films reaches 0.6, which is twice than that of our results for a-C film.45 Besides, mean friction coefficient increases with load, which is compatible with the previous studies.46,47 Figure 5 shows optical micrographs of wear track and wear scar after sliding under different loads. It is observed that wear tracks on the films gets wider and appear more obvious grooves when the applied load increased. No remarkable transfer film was found at the contact region on the counterpart sphere, which is also determined by Raman analysis presenting alumina rather than carbon characteristic peaks, as shown in Figure S2. However, obvious wear debris accumulates on the edge of wear scar, especially at the load of 2 and 5 N. The contact area on wear scar increased with load. Also, the areas on the wear track and wear scar for a-C:H-1 film are larger than that for a-C film. At 5 N, a-C:H-4 film is worn out and the steel substrate is bared on the wear track in Figure 5c. Figure 6 compares the wear rates of a-C and a-C:H films after sliding with Al2O3 in vacuum at three loads. It is found that wear rates are all very small on the same order of magnitude at three loads except for a-C:H-4 film. Wear rate of a-C:H-4 film is largest compared with other films, since the D

DOI: 10.1021/acs.iecr.8b06082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 7. ID/IG ratio and G peak position in UV Raman spectra of wear track on the five films at three loads.

content in vacuum. However, for a-C and a-C:H-1 films, because of low hydrogen content, adhesion of the interface will be an overwhelming cause of high friction. In this case, it is guessed that the more sp2 carbon in the a-C film has weaker adhesion to alumina than a-C:H-1 film with more sp3 carbon. Therefore, we calculate the interface adhesion between the interfaces of Al2O3/C(sp2) and Al2O3/C(sp3), respectively. Here, it is supposed that a very small amount of hydrogen atoms contained in the a-C:H-1 film are removed due to sliding. The first-principles calculation was performed using density functional theory by CASTEP.48 From the Raman results of wear tracks (Figure 7), it is found that the G peak position shifts to higher wavenumber and ID/IG ratio increases for a-C film compared with a-C:H-1 film at three loads after sliding, which indicates that the fraction of the sp2 carbon in the rings in the a-C film is more than that in the a-C:H-1 film. Thus, we need to compare the adhesion of sp2 carbon in the ring with sp3 carbon to alumina. Here, we adopted diamond (111) and graphite (001) to mimic sp3 carbon and sp2 carbon in the ring, respectively, namely, Al2O3(001)/diamond (111) and Al2O3(001)/graphite(001). The both interfaces are modeled by a slab of atoms which are infinite in extent in directions parallel to the interfaces containing 5 carbon layers (5 layers) and one alumina layer for Al2O3(001)/graphite (001), and 10 carbon layers (5 bilayers) and one alumina layer for Al2O3(001)/diamond (111). The cell dimensions are (4.86 Å × 4.86 Å) and (4.93 Å × 4.93 Å) for Al2O3(001)/graphite (001) and Al2O3(001)/diamond (111) interface, respectively. Lattice mismatch for building both interfaces is less than 5%. Adhesion was studied by computing Wsep of two interfaces. Perdew−Burke−Ernzerhof (PBE)49 augmented with the Grimme vdW corrections50 in the generalized gradient approximation (GGA) was used for the exchange-correlation functional for geometry optimization of interface. The cutoff energy of 340 eV was adopted for the plane-wave expansion and the k-point of Brillouin zone was sampled by 6 × 6 × 1 Monkhorst−Pack grid after convergence tests. The energy selfconsistent convergence precision of 10−6 eV/atom between two steps was chosen, all atomic positions were optimized until the maximum of Hellmann−Feynman force acting on each carbon atom was less than 0.03 eV/Å. The optimized structures are shown in Figure 8. The Wsep is defined as

Figure 5. Optical micrographs of wear track on the films and wear scar on the counterpart at a load of (a) 0.5, (b) 2, and (c) 5 N.

Figure 6. Wear rates of a-C and a-C:H films at 0.5, 2, and 5 N, respectively.

film is removed as seen in Figure 5c. It is attributed to the poor mechanical properties of a-C:H-4 film, such as lowest hardness. UV Raman spectra recorded from the wear tracks on the films are compared in Figure 7. The G peak position in higher wavenumber and the lower ID/IG ratio indicate the decrease of sp2 carbon clusters in wear tracks compared with the asdeposited films (Table 2). Meanwhile, the ID/IG ratio of wear track decreases with increasing in hydrogen contents in the films, indicating the highest sp2 content for the wear track on aC film. Finally, it is concluded that there is no graphitization on the wear tracks compared to the original films (Table 2). 3.3. Interface Adhesion Calculations by First-Principles. Following the above results, it indicates that hydrogen passivation is the main reason that friction coefficient increases with hydrogen content for a-C:H films with higher hydrogen E

DOI: 10.1021/acs.iecr.8b06082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Jariwala et al. explained the hydrogenation of sp2 sites using the Eley−Rideal type mechanism, which concluded that hydrogenation reaction was exothermic in nature and decreased the bond energies of sp2 carbon atoms.52 This demonstrates that most of the sp3 sites are hydrogen terminated and the films with high hydrogen content tend to be polymer-like aC:H(PLCH). Thus, the hardness of a-C:H films decreases with hydrogen content increasing. The optical micrographs and Raman spectra show that no transfer films are found on the wear scar on alumina sphere. While, it also shows that no graphitization occurs on the wear track by comparing the results of UV Raman spectra of wear tracks and original films. Thus, we speculated that the differences in tribological behavior of these films mainly origin from the difference in the fraction of sp2 carbon and the hydrogen content in the film. The hydrogen passivation mechanism is less controversial and supported by many experiments and simulations. Therefore, it is naturally accepted that a-C:H-4 with highest hydrogen content behaves lowest friction compared with other three a-C:H films, as shown in Figure 5. It may be also speculated that all a-C:H films exhibited lower friction than a-C film. However, the opposite trend is found in our friction experiments, namely, the friction coefficient of a-C:H-1 film is slightly larger than that of a-C film at 2 and 5 N. This probably implies that sp2 carbon lubricating is more effective than hydrogen passivation to achieve low friction when the both films have little hydrogen. The a-C film has richer sp2 carbon than a-C:H-1 film. In other words, sp2 hybridized carbon in the film is more beneficial for achievement of low friction than sp3 carbon. The results are confirmed by our first principles calculations. It is found that the adhesion between Al2O3 and sp2 hybrid carbon is lower than that between Al2O3 and sp3 hybrid carbon, which further indicates that sp2 carbon plays an important role on friction in vacuum. Besides, friction coefficients of five films increase with load, while the increase rate of friction coefficient for a-C film is slower than that for a-C:H films. For a-C:H films, sliding interface passivated by hydrogen atoms is an effective way to obtain low friction in vacuum. With load increasing, the rates of dangling bonds formation will increase, inducing insufficient passivation of surface of a-C:H films. The gaps between the rate of formation and saturation of dangling bonds becomes larger with loads increasing, thereby increasing friction coefficient. In addition, the contact areas increase with load, as shown in Figure 5, thus enhancing friction. For a-C films with highest sp2 carbon, due to weaker adhesion between sp2 carbon and Al2O3 than that between sp3 carbon and Al2O3, the formation rate of dangling bonds is much lower than that for aC:H films with more sp3 carbon when load increases. Thus, when load increases, friction coefficient of a-C film increases at a slower rate than that of a-C:H films. Furthermore, it explains why the friction coefficient of a-C film is almost equal to that of a-C:H-1 film at 0.5N. At low load, the formation rate of the dangling bonds is greatly reduced. At this time, the more dangling bonds that a-C:H-1 film produces than a-C film can be passivated by hydrogen atom containing in a-C:H-1 film. As a result, the friction coefficients of both films behave equal at 0.5 N. Here, we propose that the friction behavior of the five films is the competing result of sp2 hybridized carbon atoms lubrication and hydrogen passivation. For the films with low hydrogen content (