Quantifying Macroscopic Friction of Diamond-like Carbon Films by

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Quantifying Macroscopic Friction of Diamond-Like Carbon Films by Microscopic Adsorption and Removal of Water Molecules Jingjing Wang, Lunlin Shang, Xia Li, Zhibin Lu, and Guangan Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02613 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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Quantifying Macroscopic Friction of Diamond-Like Carbon Films by Microscopic Adsorption and Removal of Water Molecules Jingjing Wang1,2, Lunlin Shang1, Xia Li1, Zhibin Lu1,*, Guangan Zhang1,*

1. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

2. University of Chinese Academy of Sciences, Beijing 100039, China

* Corresponding Author. E-mail: [email protected] (Z.L.); [email protected] (G.Z.)

ABSTRACT

The adsorption and desorption of water molecules which affect the physical and chemical properties of the sliding interface, is critical for the friction behaviors of two solid contacts in atmosphere environment. The amount of water adsorbed on the open surface is a function of gas pressure according to adsorption equation. However, for confined sliding interface, the variation of surface fraction covered by gas molecules with water vapor pressure and its induced effects on friction have not been figured out. In this work, the macroscopic friction of diamond-like carbon (DLC) films in water

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vapor atmosphere is quantified based on microscopic adsorption and removal of water molecules. The studies correlate the fraction of water molecules adsorbed on the interface of self-mated DLC films with water vapor pressure to illustrate the direct relationship between friction coefficient and gas pressure by first principles calculations and model fitting. The calculated results revealed that chemisorption and physisorption of water molecules occur on the surfaces of hydrogen-free DLC films (ta-C) and hydrogenated DLC films (HCF). Then the relation between friction and gas pressure was built by employing fractional coverage model based on the linear adsorption equation and gas removal. The obtained model agrees well with the typically experimental results about the steady-state friction coefficient of both highly hydrogenated DLC film (HCF) and tetrahedral amorphous carbon (ta-C) film during sliding at various water vapor pressures. Besides, it gave the curves of fractional surface coverage as a function of water vapor pressure. These results show that frictional coefficient of DLC films could be predicted based on fractional surface coverage as well as the intrinsic characters on surface chemistry. We suggest that the model may be thus extended to understand and predict the friction of DLC films under a specific gas pressure at a low load and speed.

Keywords: chemisorption, physisorption, removal, water vapor pressure, DLC films, friction, fractional coverage model


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INTRODUCTION

Gas adsorption and desorption as the ubiquitous phenomena can significantly affect the physical and chemical properties of solid surfaces, and then lead to different friction behaviors for various carbonaceous materials. To understand these behaviours, it is necessary to investigate the adsorption of gas or vapor on solid surfaces qualitatively and quantitatively. In particular, water in air adsorbed chemically on the carbon based surfaces upon exposure to the ambient atmosphere greatly alters the surface chemistry irreversibly unless removed by thermal desorption at high temperature or scraping of the surfaces by extensive dusting.1-9 In addition to chemsorption, physisorption is also a common phenomenon, but reversibly changing the surface properties of solid materials due to weak interaction between surface and gas molecules such as electrostatic and van der Waals interactions.3, 10 For example, nitrogen as an inert gas can physically adsorbed on the sliding interfaces of diamond-like carbon(DLC) films to produce electronic repulsion force, which is caused by the special gas-surface interaction due to C-H bond- or π orbital-lone pair electrons of nitrogen interactions, and then resulted in ultra- or super-low friction.11-13 Vapor or gas adsorption complicating surface characteristics has recently become a research hotspot in field of tribology. It has been presumed that frictional force between carbon based materials in sliding contacts is related to the amount of gas molecules adsorbed on the interface, but the in situ chemical characterization of tribological interfaces under atmosphere conditions still remains challenging, which limits the molecular understanding of chemical reactions taking place at the interface.


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Thus a large motivation for this research has been driven by the need for quantifying change in microscopic interface, namely, the change in surface fraction covered by gas species on the sliding interface, and clarifying the associated effects on macroscopic friction. Exploring the relation may shed light on the origin of environment dependence of friction and provide a way of predicting tribological behaviour under the specific environment condition.

There have been considerable progresses in understanding the relation between friction coefficient and surface fraction covered by gas species.2, 14-21 Heimberg et al.19 revealed that friction coefficient dependent on exposure time between contacts for self-mated hydrogenated DLC films is essentially relevant to the amount of gas adsorbed on the surface, and then developed a model on the relationship between the transient friction behavior and surface coverage by using an Elovich adsorption model. But the removal of the adsorbed films during mechanical sliding is not considered, leading to the model not matching experimental data. Followed by Dickrell et al.,20 they presented a model of transient and steady-state fractional coverage by combining the Langmuir adsorption and adsorbate removal, which resulted in a good fit to the experiment data of Heimberg et al..19 Besides, some revised models were successively proposed by considering other factors, such as spatial effects.17-18, 21 However，these proposed models above mainly indicated that friction coefficient varied with exposure time between sliding contacts. They did not figure out changes in the amount of adsorbates on the interfaces. According to linear adsorption equation,15 the surface coverage of gas species is a function of exposure time as well as gas or vapor pressure.


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On static flat solid surfaces, measurement of gas or vapor adsorption isotherm, which shows the number of adsorbate molecules as a function of gas pressure, was conducted by employing specific surface-sensitive techniques under adsorbate/vapor equilibrium, such as quartz crystal microbalance, ellipsometry, attenuated total reflection-adsorption infrared spectroscopy, and so on.22 However, for confined and dynamic sliding interface, those techniques will fail to detect the changes in adsorbate molecules on the interface with varying gas pressure. Therefore, employing a fractional coverage model to quantify the evolution of surface coverage with various pressures is essential to clarify the mechanism of pressure-dependent friction coefficient for sliding contacts.

Here, we consider effects of water molecular adsorption on friction coefficient of diamond-like carbon (DLC) films by varying water vapor pressure. Due to its environmental-friendly lubrication and ubiquitous in ambient air, water has recently become an important gas species of intense research in the field of tribology for carbon based materials.4, 6, 9, 22-32 As shown by Andersson et al.,23 both HCF and ta-C films in the self-mated pin-on-disc test presented a completely opposite trend of steady-state friction coefficient with increasing the pressure of water vapor. Although the results showed that the variation in friction coefficient with pressure was attributed to the numbers of water molecules adsorbed on the interface, it has not cleared the value of surface fraction covered by water at steady sliding stage under varying water vapor pressure. Thus, figuring out the quantitative relation between surface coverage of water molecules and water vapor pressure is significant for fully


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understanding the tribology mechanism of solid contacts under humidity condition.

As discussed above, this work focused on the effects of water vapor pressure on macroscopic friction of DLC films due to microscopic adsorption and removal of water molecules on the confined interface during sliding. First principles were performed to calculate the structure and energies for water molecules adsorbed on both hydrogenated diamond(111) and bare diamond(111) surfaces. And then we developed a model on the pressure dependence of friction coefficient by combining gas adsorption equation and mechanical removal theories, as well as compared the results of numerical simulation with experiment data from Andersson et al..23

FIRST PRINCIPLES CALCULATIONS

Extensive experiments have been conducted to study the mechanism about the effect of humidity on friction.9-10, 23-24, 26, 28, 33-36 To provide additional insight into the adsorption of water molecules on the surface of ta-C and HCF, we performed first principles calculations within the framework of density functional theory including long-range van der Waals interactions by CASTEP of Material Studio.37-44 We considered both diamond(111) and hydrogen terminated diamond(111) surface, to represent ta-C and HCF, respectively. The calculations involved both geometry optimizations and adsorption energy(Eads). The exchange-correlation energy functional was approximated by Generalized Gradient Approximation (GGA) in conjunction with PBE. Energy convergence of 10-6 eV/atom was obtained with a plane wave cutoff energy of 400 eV and a 8×8×1 k-points mesh. Ultrasoft


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pseudopotential

serves

as

the

plane

wave

base.

Pulay

of

BFGS

(Broyden-Flecher-Glodfarb-Shanno) addresses electron relaxation. Both surfaces were designed using periodic supercells with a slab of five double-layers of diamond and 10 Å vacuum, which was demonstrated to be sufficient to give converged surface energy. We put one water molecule in each (2×2) diamond surface cell, (5.058×5.058×18.776 cell). All broken carbon bonds of hydrogen terminated diamond(111) surface was added a hydrogen atom except the most central carbon bond for the purpose of placing water molecule at 1.5 Å atop the surface and for being same as bare diamond(111) surface, as shown in Figure 1(a) and 1(c). In order to ensure the slabs mimicking the bulk effect of diamond, the atoms at the bottom of slab were fixed in all three directions during structural minimization. Molecular adsorption energy Eads, which represented the most stable configurations of water adsorbed on the surfaces, was obtained from the equation as Eads= Esurf+H2O-( Esurf + EH2O) ,where Esurf+H2O was the total energy of

supercell containing the diamond surface and

adsorbed water molecule, and Esurf, EH2O were the total energies of the optimized diamond surface supercell and optimized water molecule supercell, respectively.

Figure 1 shows the side views of initial and final configuration of water molecule on the diamond(111) and H-diamond(111) surface, respectively. The Figure 1(a) and 1(b) illustrate that the distance between oxygen atom of H2O molecule and the diamond(111) surface after minimization is slightly smaller than that before minimization. In addition, the minimized structure shows the length for one of O-H bonds of water molecule increased to 2.5 Å beyond the range of bond length for


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hydrogen and oxygen, indicating that H2O have dissociated into OH and H radicals, as has been proved by experiments.4-5 Simultaneously, the oxygen atom of -OH and another hydrogen atom were directed on top of the nearest-neighbor carbon atoms of the diamond(111) surface where the bond lengths of C-OH and C-H are 1.422 and 1.096 Å, respectively. The C-OH bond length was same as the results of Qi et al..45 The calculated adsorption energy for H2O on the diamond(111) surface was 3.97 eV/molecule (382.4 kJ/mol), further demonstrating that it was a dissociation adsorption process. For H2O on the H-diamond(111) surface, as shown in Figure 1(c) and 1(d), the distance between H2O and the surface increased to 3.266 Å after minimization, suggesting that there was a weak interaction between water molecule and H-diamond(111) surface. Moreover, the minimized structure of the water molecule had no apparent change compared with the initial structure. The adsorption energy for H2O on the H-diamond(111) surface was 0.154 eV/molecule (14.83 kJ/mol) within the range of physical adsorption energy. So water molecule was adsorbed physically on the H-diamond(111) surface.


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Figure 1. Side views of H2O on the two surfaces. (a) and (b) are initial and final configuration for diamond(111) surface, respectively. (c) and (d) are initial and final configuration for H-diamond(111) surface, respectively. The colors of the spheres represent atom types: gray = carbon, red = oxygen, white = hydrogen.

The reason for the different interactions of water molecules with both surfaces is the difference in their intrinsic characters on surface chemistry. The bare diamond(111) surface with many broken carbon bonds is activated, so water molecule dissociates on the surface to reduce the surface energy by passivating the broken carbon bonds, which consists with the previous results of experiments and theoretical calculations.4, 6, 8

These most calculations were considered diamond (100) surface with dimers as the

research object. However, H-diamond(111) surface is terminated by hydrogen, which prevents water molecule from approaching the surface due to steric hindrance. So water molecule has a certain distance from H-diamond(111) surface. On the other hand, hydrogen termination results in a surface dipole layer which produces the


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condition of negative electron affinity.8 For water molecules, they present a charge displacement along Oδ--Hδ+ due to the different electronegativity of the O and H elements. So, water molecules would physically interact with the surface of H-diamond(111) by forming dipole-dipole interaction. These two factors cause water molecules to be physically adsorbed on the surface of H-diamond(111), which is consistent with the results of Young et al..46 On the basis of the above results, it is speculated that different physical and tribochemical interactions of both DLC films with water molecules play a pivotal role on sliding friction. For ta-C coatings, the friction reduction when adding humidity in test environment is linked to surface broken carbon bonds passivation by –OH and –H from water dissociation adsorption,7, 47

whereas it showed high intrinsic friction in high vacuum due to high adhesion

energy from covalent bond forming by the dangling carbon bonds on the interface of ta-C.9, 48 On the contrary, HCF presented super-low friction in vacuum due to steady repulsive interaction and small adhesion energy induced by hydrogen terminations, as demonstrated by other experiments and calculations,7, 48-50 but the interface of H/H with superlubricity is replaced by the interface of H2O/H2O when water adsorbed on the H-diamond surface in water vapor atmosphere. Besides, the confined water molecules layers can establish hydrogen-bonds while hydrogenated interface cannot, which has been proved by Kajita et al..10 Thus, the adhesion of H2O/H2O interface will increase with respect to that of hydrogenated interface, leading to friction increasing with humidity.

In addition to gas adsorption, adsorbate removal caused by the breaking of the


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interatomic bonds between the adsorbate and the interface due to mechanical sliding, is also a significant effect on friction behaviors of DLC films in the atmosphere. For water molecules on the interfaces of HCF, they are easily removed by mechanical rubbing due to a weak interaction. However, for ta-C films, water molecule dissociate into -OH and -H radicals, which form C-OH and C-H bonds with surface carbon atoms, respectively. Borodich et al. have performed ab initio calculations of the bond dissociation

energies

between

carbon

atoms,

carbon-oxygen

atoms

and

carbon-hydrogen atoms using GAUSSIAN98 at the Møller-Plesset level of model chemistry.18 The bond dissociation energy found for a single carbon-carbon bond is 523 kJ/mol, for the carbon-oxygen bonds it is 1447 kJ/mol, while for the carbon-hydrogen bonds it is 295 kJ/mol. Also, these results have been published by experimental bond energies for those pairs.51 But the surface C atoms in ta-C films are bonded to their immediate neighbors with three σ bonds, so more energy is required to dissociate a C atom from the surface of hydrogen-free DLC than that of dissociating an O adatom from the surface. Therefore, H and O will be possibly removed from the surface of ta-C films by mechanical sliding.

MODELING FRICTION AND COMPARISON WITH EXPERIMENTS

Modeling Friction in Rotating Sliding Contacts

The model based on gas adsorption and fractional removal assumed that the surface coverage of the contacts before sliding was unity due to the materials being exposed to air before sliding. After run-in stage, the covered gas species were mechanically


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removed immediately so the coverage was reduced to zero. Then the studied gas began to adsorb on the generated fresh surface, which was taken as the initial surface coverage θ0 =0. For ball-on-disc sliding, it was assumed that only the gas removal occurred under the ball contact and gas adsorption on the contact was negligible, so the fractional coverage that entered into the ball (θin) was greater than the leaving fractional coverage (θout). The surface region considered was exposed to the gaseous environment after it leaved the contact area and continuously adsorbed gas molecules during the time that it took to return to contact. As a consequence, the entering fractional coverage varied from cycle to cycle until the amount of gas adsorbed on the contact was constant when the rates of adsorption and removal reached equilibrium. The most cited of the adsorption kinetics equations are followings, i.e., the Elovich equation and the linear adsorption equation.20 The model mainly adopted the latter that showed the relationship between adsorption rate and gas pressure, which was beneficial to quantitatively clarify the pressure-dependence of friction. Following the Borodich et al.,17 the amount of adsorbate removed from the surface was proportional to the probability p of the mechanical breaking the interacting bonds as well as the entering fractional coverage, so the fraction of the surface covered at the exit of the contact was defined as θout=θin(1-p), where p was between 0 and 1. For ball-on-disc rotating sliding, it was assumed that every position on the wear track for the same cycle was equivalent, which all had identical surface chemistry and fractional coverage during sliding.

Thus, considering a differential element of contact region at the wear track, an


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expression for the average fractional coverage of the considered region at the entrance of n cycle was given by Eq.(1), which was proposed by Dickrell et al.,20

, = , + (1 − , )

(1)

Where α is the adsorption ratio that depends on the time between contacts, temperature and gas atmosphere. And at the exit of n cycle the corresponding coverage was expressed by Eq.(2).

, = , (1 − )

(2)

Here, it was supposed that adsorption ratio α and the probability p was constant between cycles. The model of the fractional coverage is recursive, and has a closed-form solution for entering fraction at any cycle n, as shown in Eq(3).

, = [(1 − )(1 − ) +

[()() ()()

(3)

It is considered that friction reaches steady state when the number of cycle is large enough, so the steady-state coverage is obtained by taking limit of Eq.(3) as n approaching infinity, as given in Eq.(4).

, =

()()

(4)

Combining the linear adsorption equation, which indicates that the rate of adsorption is proportional to the bare surface area fraction (1-θ) and the gas pressure, and the results from Dickrell et al.,20 an expression about adsorption ratio α and gas pressure was given in Eq.(5).

α = 1 − e

(5)

Where v is adsorption coefficient, T is the exposure time of one cycle，which is constant for every position on wear track for rotating sliding. Thus, substituting the


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Eq.(5) into Eq.(4) can get Eq.(6), which is a pressure-dependent steady-state solution for the entering fractional coverage.

, =

! "#

%$()! "#

%$(6)

Following Blanchet and Sawyer,14 who proposed the relationship between the average fractional coverage(̅ ) under the pin and the entering coverage(θin), the average coverage for steady state can be expressed as a function of gas pressure, as given by Eq.(7). "#

%$(! ) ̅ = [ '()[()!"#$%

(7)

As shown by Dickrell et al.,20 the friction coefficient functioned as the average coverage in a linear rule-of-mixtures. Therefore, the relationship between friction coefficient and gas pressure is given by Eq(8). )! "#$% *

( = ( + [ +,()[()! "#$% (( − ( )

(8)

Therein, µ0 and µ1 are assumed to correspond to the friction coefficients of clean or nascent surface and a nearly saturated or covered surface, respectively.

Comparison with Experiments

Previously, an experiment of pressure-dependent friction of both hydrogentated and hydrogen-free DLC films has been studied with a pin-on-disc machine in self-mated rotating sliding.23 In the experiment, both of steel balls and steel discs were deposited with HCF and ta-C films, which were produced by PE-CVD and arc-PVD, respectively. With low speed and contact pressure, the wear of these films is without consideration. Varying water vapor pressure from nearly zero to 2500 Pa, the friction


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coefficient of HCF and ta-C films in the self-mated pairs showed an inverse trend, though they gave a similar frictional response only when it was closed to the water condensation pressure. Here, the model proposed above compared with the experimental data of steady-state friction with varying pressure. The results for ta-C films and HCF are shown in Figure 2 and Figure 3, respectively.

In Figure 2, it gave a numerical simulation of the ta-C on ta-C contact test. Taking µ0=0.53, µ1=0.057, v=0.00162Pa-1s-1, p=0.002, and T=1s, one can observe an excellent agreement between the simulated results and experimental data over the entire of pressure, with R2=0.95615. Substituting these parameters into the Eq.(7), then we got the curve of the average fractional coverage varying with water vapor pressure from the experimental data for ta-C films, as shown in Figure 4(a).

However, for HCF contact test, as shown in Figure 3, because of different surface chemistry, the employed parameters changed, which were µ0=0.017, µ1=0.12,

ν=0.000135, p=0.3, and T=1s. The model also matched experimental data with R2=0.90938. Similarly, Figure 4(b) gave changes in average coverage at every water vapor pressure based on Eq.(7).


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Figure 2. Plot of the steady-state friction coefficient(µ) versus water vapor pressure(P). Data points ( ) are steady-state experimental data (from Anderrsson et al.23) for ta-C on ta-C contact. Solid line represents data obtained using numerical simulation for the steady-state friction coefficient based on Eq.(8), with µ0=0.53, µ1=0.057, ν=0.00162 Pa-1s-1, p=0.002, and T=1s. The insert shows the friction coefficient under the water vapor pressure lower than 15 Pa.


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Figure 3. Plot of the steady-state friction coefficient(µ) versus water vapor pressure(P). Data points ( ) are steady-state experimental data (from Anderrsson et al.23) for HCF on HCF contact. Solid line represents data obtained using numerical simulation for the steady-state friction coefficient based on the Eq.(8), with µ0=0.017, µ1=0.12, ν=0.000135 Pa-1s-1, p=0.7, and T=1s.


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Figure 4. Plot of average fractional surface coverage versus water vapor pressure based on Eq.(7): (a)for ta-C on ta-C contact; (b)for HCF on HCF contact. The insert of (a) shows the average fractional surface coverage under the water vapor pressure lower than 15 Pa.

DISCUSSION

According to the present simulation, it indicated that the steady-state friction coefficient of ta-C films was highly sensitive to water vapor pressure. When water vapor pressure was lower than 100 Pa, the steady-state friction coefficient of ta-C films was reduced substantially, while as pressure exceeded 100 Pa, the friction was constant independent of pressure and the average coverage approached unity simultaneously. So, it was speculated that friction essentially depended on the average fractional coverage, in other words, when the surfaces were fully covered by the gas species in the absence of serious wear, the friction would not vary with water vapor pressure. Besides, the low value of the probability p showed that the adsorbed gas species on the interface were difficultly removed, indicating the strong tribochemical


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interaction between water molecules and the surface of ta-C films, namely covalent bonds, which was consistent with the results of ab intio calculation for water on diamond(111), as well as it explained why the surface coverage soon reached unity at very low pressure. Additionally, adsorption coefficient v of simulation results from ta-C film was one order of magnitude larger than that for HCF. The reason for the rapid increase in surface coverage with pressure for ta-C film was that the later dissociative adsorption of water molecules would be activated by initial dissociative processes, which was consistent with the theoretical results for diamond proposed by other researchers. Manelli et al. highlighted initial dissociative adsorption processes of water molecules could give rise to chains of dissociative processes that produced a quick surface passivation by performing first principles calculations based on DFT with QUANTUM ESPRESSO software.3 When the carbon dangling bonds on the interface of ta-C are terminated by chemisorpted species, namely –H and –OH, few or no chemical bonds will form across the interface, so the adhesion can be dramatically reduced. It is known that friction decrease with adhesion energy of interface.6-7, 52 Therefore, friction coefficient of ta-C films decreased with coverage of water molecules increasing.

For HCF, the results of simulation showed a slow increasing trend of friction coefficient with water vapor pressure. Simultaneously, the fractional surface coverage increased slowly with pressure until it reached about 0.5 when pressure is beyond 2500 Pa, because the intrinsic characters of low-energy stable surface due to hydrogen passivation and the steric effect of hydrogen on the surface of HCF both prevented the


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adsorption of water molecules. The low increase rate of coverage also confirmed that the adsorption of water molecules on the HCF interface is a physisorption process with weak dipole-dipole interaction, which was consistent with results of calculation for water on H-diamond(111). Besides, the large probability p standing for adsorbate removal also demonstrated a weak interaction between HCF surface and water molecules, inducing that the coverage did not reach 1 at saturated vapor pressure. For self-mated sliding interfaces of HCF, additional hydrogen-bonds interaction in waters adsorbed on the counterfaces would increase adhesion energy, thus friction increased. As the coverage of water molecules on the HCF increase, adhesion energy increase.3, 10

Probably the advantages in application of this model are as follows. On one hand, the model quantifies the relationship between friction coefficient and gas pressure during sliding contacts. On the other hand, the model can be applied in different carbon-based materials. From the fitting results above, the variable friction behaviors of both DLC films with water vapor pressure can be simulated with the same model except different values of some parameters that described the intrinsic properties of various materials. For example, friction coefficient µ0 represents the intrinsic friction of both films in vacuum, such as 0.53 and 0.017 for ta-C films and HCF, respectively. The same trend of friction coefficient and fractional coverage with varying water vapor pressure also demonstrated that the essential factor determining friction is the evolution of factional coverage of sliding interfaces. Besides, the model may be work for gas species, such as the effects of organic vapor adsorbed on carbon-based surface


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on friction.53-54 However, the disadvantages about the model cannot be ignored. The model may be well applied only in the friction conditions under low speed and contact pressure. Otherwise, there are some defects for results of the model fitting with experimental data, which are attributed to temperature rise and third body during sliding. For friction with low sliding speed and small contact pressure, it would be a better model of quantifying the effects of gas or vapor adsorption on friction behaviors. Furthermore, the model assumed the maximum coverage on the sliding interfaces was unity, so it would not be used in multilayer adsorption case where the coverage exceeded unity.

CONCLUSIONS

We quantified the evolution of interfaces during sliding in water atmosphere with various pressures by figuring out fractional coverage of adsorbates on the interfaces. In particular, theoretical calculation and numerical modeling have been conducted to investigate the physical and tribochemical interaction between DLC films and water molecules. First principles calculations showed that water molecules adsorbed on diamond(111) and H-diamond(111) surfaces were chemical and physical processes, respectively. It revealed that chemisorption and physisorption of water molecules occur on the surfaces of ta-C and HCF, respectively. The reason for the different interactions of water molecules with both surfaces is the difference in their intrinsic characters

on

surface

chemistry.

A

fractional

coverage

model

of

gas

pressure-dependence of friction in rotating ball-on-disc sliding contact was developed


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by employing the linear adsorption equation and gas removal. The data of steady-state friction coefficient with varying water vapor pressures for both DLC films from Andersson et al.23 were compared to the friction curves relevant to pressure generated from the model. The results showed a distinct dependence of friction on water vapor pressure for both HCF and ta-C films because of different interaction of the sliding interfaces with water molecules. For HCF, water molecules physically interact with the sliding interfaces due to dipole-dipole interaction. The instead H2O/H2O interface as well as hydrogen bonds induced by the confined water molecules layers lead to increase in adhesion energy of sliding interface, thus in friction of HCF. In contrast, the activated surface of ta-C film is passivated by –OH and –H radicals from dissociative adsorption of water molecules, decreasing adhesion induced by covalent bonds between dangling carbon bonds, which results in friction decreasing with water vapor pressure. As well, the good fitting results suggest that changes in friction coefficient with pressure are attributed to the increase in surface fractional coverage, which leads to district changes in physics and chemistry of interfaces for both films.

Finally, it is suggested that the model could be used to predict the friction of DLC films under a specific water vapor pressure except for some situations, such as multilayer adsorption, temperature shift on the contacts and severe wear. Thus, it is necessary to further develop a refined model for better understanding the effects of vapor or gas adsorption on friction behaviours of DLC films in future study.


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AUTHOR INFORMATION Corresponding Author Zhibin Lu, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail: [email protected];

Guangan Zhang, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for financial support from National Natural Science Foundation of China (Grant No. 51775535) and Key Program of the Chinese Academy of Sciences (No. QYZDY-SSW-JSC009).

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Table of Contents (TOC)


Quantifying Macroscopic Friction of Diamond-like Carbon Films by

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