Tribo-surface charge and polar lubricant molecules on friction and lubrication under multiple 3D asperity contacts

Wear 332-333 (2015) 1248–1255

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Tribo-surface charge and polar lubricant molecules on friction and lubrication under multiple 3D asperity contacts H.T. Zhu a,n, X. Zheng a,b, P.B. Kosasih a, A.K. Tieu a a b

School of Mechanical Materials and Mechatronic Engineering, University of Wollongong, Northfield Avenue, Wollongong, NSW 2522, Australia Research Institute of Zhejiang University – Taizhou, Shifu Avenue, Taizhou, Zhejiang, 318000, China

art ic l e i nf o

a b s t r a c t

Article history: Received 15 September 2014 Received in revised form 22 January 2015 Accepted 2 February 2015

Lubricant is usually a mixture of polar and non-polar molecules, and tribo-metal surfaces can be oxidised and have surface atoms with charges. A molecular dynamics simulation of bcc iron with additional charge as rough tribo-surface and polarisable PEO polymer as lubricant was proposed to investigate the tribo-surface charge and polar molecules on friction and lubrication during the compression and sliding. Tribo-surface roughness is crucial for asperity interaction because it affects real contact area, friction, wear and lubrication. Random Midpoint Displacement algorithm was introduced to generate 3D multiple asperities with irregular shape at upper and lower wall surfaces. The results show that compared to non-charged tribo-surfaces where the amount of lubricant and surface roughness determine the friction and lubrication, the charged tribo-surface attracts the polar PEO molecules to form a coated like layer; thereby resulting in a different tribology behaviour. The polar PEO polymers separate the positive charged sliding tribo-surfaces and reduce the direct asperity contact; hence significantly lowering the friction force. & 2015 Elsevier B.V. All rights reserved.

Keywords: Boundary lubrication Asperity contact Polyethylene oxide Molecular dynamics simulation

1. Introduction In the boundary/mixed lubrications, two tribo-surfaces interact through an ultra-thin lubricant layer and asperity contact which results in elastic–plastic deformation and possible fracture of the asperities. However, the macroscopic laws of friction break down in nano-scale contact [1]. The influence of tribo-surface morphology on friction and wear during surface compression and sliding has attracted the significant attentions in the research of nano-tribology. Atomic tribo-surface roughness has been found to have a dramatic effect on the real contact area and contact stress [2–4]. By adopting a singleasperity model with the presence of self-assembly stearic acid layer on the iron surfaces, Eder et al. [5] proposed a smooth particle approach to define and calculate the asperity contact area and lubricant cavity volume. Zheng et al. [6] demonstrated that long chain-length alkanes significantly reduce the friction and wear between rough surfaces. However, Zheng's model only considered a regular sinusoidal asperity existed on tribo-surfaces. Molecular dynamics (MD) simulations of multi-asperity contact have been carried out by Spijker et al. [7,8] with a main conclusion that the normal load, surface roughness, and adhesion force collectively determine the contact area. However, the lubrication was ignored in Spijker's MD modelling. With the consideration of 3D rough tribo-surfaces lubricated by n-alkanes, Zheng

n

Corresponding author. Tel: þ 61 242214549. E-mail address: [email protected] (H.T. Zhu).

http://dx.doi.org/10.1016/j.wear.2015.02.045 0043-1648/& 2015 Elsevier B.V. All rights reserved.

et al. [9,10] recently proposed a MD model to investigate nano-contact mechanism of boundary/mixed film lubrication. An increase in friction force has been found for the partially-lubricated condition due to the combined contributions of asperity contact and lubricant flow resistance to sliding. But, the electrostatic influences on the tribo-surfaces and polar charges within the lubricant molecules were not considered. Real metal tribo-walls usually is oxidised and have surface atoms with charges, e.g. Fe2O3. Berro et al. [11,12] investigated tribological performance of lubricant mixture containing hexadecane and zinc dithiophospate (ZDDP) confined by smooth Fe2O3 surfaces. They found that the charge on iron and oxygen have a significant effect on the rheology behaviour of polar lubricant molecules. As the flat walls were introduced, the asperity contact and boundary lubrication mechanism were not measured in Berr's model. A new class of fluid, Ionic Liquids (ILs), is emerging in tribology research, which is being increasingly studied due to their potential to enable a step-change in lubricant design as either base lubricant or additives [13]. Metal surface in aqueous solution will acquire a charge due to either chemical dissociation of surface groups, preferential physicochemical adsorption of electrolyte ions, or, preferential desorption or adsorption of ions [14]. For common metal oxides, H þ is the potential-determining ion. The charged surface is a major determinant of the adhesion of surfaces, the transport properties of slurries, the self-assembly of amphiphiles and many other phenomena. Polyethylene oxide polymer, known as PEO, is widely used in manufacturing due to its high wetting and spreading ability. It can form a uniform coating on contact surfaces which results in low

H.T. Zhu et al. / Wear 332-333 (2015) 1248–1255

friction. The polar charge of the PEO molecules plays an important role in the structural and thermodynamic properties of PEO polymers in the bulk and aqueous solution [15–20]. In this paper, a molecular dynamics simulation of bcc iron with additional charge as 3D rough tribo-surfaces and polarised PEO polymer as lubricant was proposed to investigate the tribo-surface charge and polar lubricant molecules on friction and lubrication during surface compression and sliding. It helps to provide a fundamental understanding of boundary film lubrication at the atomic scale.

2. Model Fig. 1 shows three types of PEO polymer with different chain length used in the current study. The TraPPE-UA force potential [21,22], which employs pseudo-atoms representing CH2 and CH3 groups, was introduced to descript the PEO polymers. The nonbonded interactions were represented by pairwise Lennard-Jones (LJ) and Coulombic potentials. Eij ¼ 4εij

Fig. 1. Three types of PEO polymer. Colours mean different UA atoms: Oxygen (red), CH3 (grey), and CH2 (purple). According to the number of oxygen, they are referred as PEO2, PEO4 and PEO6. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1 Potential parameters for PEO polymer and Fe. LJ 12-6 potentials CH3

σ (Å) 3.75

ε (eV)

Mass (g/mol)

0.008444

Charge (e) 0.25

15.0351 CH2

3.95

0.003963

0.25 14.0272

O

2.80

 0.50

0.004739 15.9994

a

Fe

2.321

0.04097

0.5 55.8450

Bond C–C C–O

Kb (eV/Å2) 22.5630 27.7658

Angle C–C–C C–C–O C–O–C

Kθ (eV/rad2) 5.3858 112 4.3345 112 5.2048 112

θ0 (deg)

Dihedral C–C–O–C O–C–C–O

C0 (eV) 0.0 0.043365

C2 (eV)  0.014110  0.021682

r 0 (Å) 1.54 1.41

C1 (eV) 0.062505 0.0

C3 (eV) 0.048102 0.086730

a The lubricant is modelled by LJ-Coulombic potentials. The interaction between Fe walls and lubricant is modelled by LJ-Coulombic potentials. The top and bottom Fe walls, and the interaction between themselves are constructed by EAM potential.

1249

"


σ ij r ij

12




σ ij r ij

6 # þ

qi qj 4πε0 r ij

where rij ,εij ,σij , qi and qj are atom to atom separation, LJ well depth, diameter and partial charges on atom i and j, respectively. It is critical to adopt a realistic force potential for the tribo-walls in the current research. As the asperity contact involves plastic deformation, the solid iron wall was modelled by the Finnis–Sinclair (FS) EAM potentials. However, real iron surfaces are usually oxidised, which not only employs the charge on the surface but also significantly reduces the adhesion force between contact surfaces. Hence, a charge was introduced into the iron wall atoms. According to Berro's work [11,12], the charge value for Fe in an oxide state is 0.771e, and O is 0.514e. When the surface was exposed to aqueous electrolyte, the metal oxides acquire their charge by proton binding to terminal metal hydroxide groups that arise on the hydrated surface of the solid. Iron oxide is positively charged at low pH [14]. As no EAM potential has been published to model the plastic deformation of Fe2O3, only iron atom was used in the current work to construct the tribo-walls in order to consider the asperity contact and deformation in boundary lubrication. A positive charge in the range of 0, 0.25, 0.5 and 0.75e was applied to the Fe surface to investigate the tribo-surface charge and polar lubricant molecules on friction and lubrication. The interaction between tribo-walls and PEO polymers was described by LJ and Coulombic potentials to consider the electrostatic interaction. The detailed potential parameters can be found in Table 1. A schematic view of the simulation domain is shown in Fig. 2. The 3D multi-asperity simulation model consists of upper and lower iron walls with (001) surfaces (13  13 nm in the longitudinal and lateral directions), and randomly distributed PEO molecules as the lubricant. The initial system height was set large enough to relax the randomly distributed polymer molecules. The solid tribo-walls could be further divided into six layers: rigid layers (1, 2), thermostat layers (3, 4), and free deformable layers (5, 6). The two thermostat layers 3 and 4 were used to maintain the system temperature at 300 K through the Nosé–Hoover thermostat with a damping constant 100 fs  1. Periodic condition was applied to the x- and y-directions. The simulation has been carried out with the Larger-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) MD code [23]. Roughness is crucial for surface interaction because it affects real contact area, friction, wear and lubrication. In this work, Random Midpoint Displacement (RMD) algorithm was introduced [24], which has the Hurst exponent as one of its inputs, to generate a rough surface that is periodic at the surface boundary. As suggested by our previous works [9], tribo-wall surfaces were divided into 4 by 4 small squares with equal size. Within each square, the RMD was employed so that the whole surface is covered by enough asperities with random shapes under a Root Mean Square 0.462 nm, Ra 0.368 nm, Skewness  0.054 and Kurtosis 2.930. The simulation includes 2 steps: compression and sliding. During compression, the lower rigid region is fixed while the upper rigid region is free only in the z-direction. An external normal load in the range of 0.25, 0.5, 0.75 and 1.0 GPa is applied on the atoms of the upper rigid layer; thus compressing the whole system. After the compressed system reaches its equilibrium, the lower and upper

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Pz= 0.25, 0.5, 0.75, 1.0 GPa V=20m/ s rigid 1 thermostat 3

300K

z y

System Height

free deformable 5

x

free deformable 6

13

nm

thermostat 4 rigid 2

300K

13 nm

Fig. 2. Schematic view of the system set-up. For clarity, only a small amount of molecules are presented. The numbers on the left indicate the different layers.

layers are set free in the x-direction, and then slide in opposite directions along the x-direction at a velocity of 20 m/s. The absolute sliding distance for the upper and lower walls is 45.5 nm, which allows the system slides repeatedly for 7 times.

3. Results and discussion 3.1. Influence of surface charge effect on thin film lubrication To investigate the different charge strength of wall surface on the behaviour of thin film lubrication, 40 PEO polymers consisting of 2 PEO block molecules were randomly mixed in n-alkanes (n ¼4, 8, 12, 16, totally 20,000 atoms) as lubricant. A sliding velocity 10 m/s was applied on the upper and lower flat walls respectively. 0.25 GPa pressure was applied on the upper surface. Typical snapshots during sliding are shown in Fig. 3, which demonstrates how PEO2 migrated towards the surfaces. Fig. 3a shows the initial position of the PEO2 molecules when randomly mixed with the n-alkanes molecules. After 0.2 ns, some PEO2 molecules approached the surfaces, and then after sliding 0.4 ns, as shown in Fig. 3c, the PEO2 molecules stuck to the surfaces. Berro et al. [11,12] conducted a confined thin film MD model with hexadecane as the base lubricant and 5% mass concentration of C4-ZDDP as the additive. The C4-ZDDP molecules migrated towards the surfaces in a similar way observed in this work. The velocity profiles of 8-alkanes mixed with PEO2 molecules were plotted in Fig. 4. It can be found that the surface charge plays an important role on the velocity distribution of the lubricant. There is a clear boundary slip between the lubricant and solid surface when the surface charge is zero, resulting as a weak liquid solid interaction. When the positive charge was applied to the surface, the interaction between the wall and PEO2 molecules was significantly increased due to the combination effect of van der Waals force and electrostatic force. The migrated PEO polymer on the wall as shown in Fig. 3c results in less interfacial slip. Due to the reduction of the interfacial slip, the shearing momentum from the sliding surfaces can be effectively transferred to the lubricant. As shown in the Fig. 5, the shear stress remained almost

the same for the case of 4-alkane mixed with PEO at the charges ranging from 0 to 0.75e. As the boundary layers of 4-alkane lubricant can completely stick to the sliding surfaces, the adsorbed PEO2 molecules by charge did not increase the shear stress. In contrast, the shear stress of longer chain n-alkane increased with tribo-surface charges. The adsorbed PEO2 molecules (extra surface roughness) as shown in Fig. 3c helped the surfaces to shear the lubricant (no interfacial slip), thereby reaching the maximum shear stress. When the larger charges (0.5 and 0.75e) were applied, a greater increase was observed.

3.2. Boundary lubrication under multiple 3D asperity contacts To investigate the influence of molecular chain length of the polymer on boundary lubrication, three different types of PEO polymer, consisting of 2, 4 and 6 PEO blocks in one molecule are employed in the current work. The charge of unit atoms of CH3, CH2 groups and oxygen atom are listed in Table 1. According to the oxygen number in the backbone, three types of polar polymers are referred as PEO2, PEO4, and PEO6. The amount of lubricant determines the extent of asperity contact [9]. To generate a realistic analysis, the simulation system needs to have sufficient number of lubricant molecules. In the current work, a total 4000 PEO blocks are used to construct PEO polar polymer, which consists of 2000 PEO2 molecules, 1000 PEO4 molecules and 667 PEO6 molecules, respectively.

3.2.1. Compression The compression consists of two sub-steps. After a normal load is applied on the upper rigid wall layer, initially there is no supporting force and the upper wall is lowered quickly. This is unfavourable for the tribo-surface contact as it brings high momentum before two surfaces contact. It may deform the prominent asperities under different initial wall separations even if the normal load is the same. In order to control this effect, the lowering speed of the upper body is limited by a maximum displacement in the z-direction (approximately 0.125 m/s). With this limit, the upper surface contacts the lower surface with a constant small momentum. The compression

H.T. Zhu et al. / Wear 332-333 (2015) 1248–1255

t=0.0 ns

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t=0.2 ns

t=0.4 ns

Fig. 3. Side views of the thin film model at different sliding time. (only PEO molecules was shown for easy clarification). 9.0 10.0

Surface Charge (e) 0.0 0.25 0.50 0.75

Velocity profile (m/s)

5.0 2.5

8.8 Non-Charged Surfaces Dry Contact at 250MPa PEO2 at 250MPa PEO4 at 250MPa PEO6 at 250MPa

8.6

System Height (nm)

7.5

0.0 -2.5 -5.0 -7.5

8.4 8.2 8.0 7.8 free compression

limited compression

-10.0

7.6 2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

0.0

z (nm)

0.2

0.4

0.6

0.8

1.0

1.2

Time (ns) Fig. 4. Velocity profiles of 8-alkanes and PEO2 confined by charged surface. Fig. 6. System heights of dry contact and non-charged surfaces with different types of PEO polymer under 0.25 GPa.

18

Shear Stress (MPa)

15

4 alkane+PEO2 8 alkane+PEO2 12 alkane+PEO2 16 alkane+PEO2

12

9

6

3

0 0.00

0.25

0.50

0.75

Charge (e) Fig. 5. Shear stress as a function of surface charge values.

lasts for 1.2 ns with the first 0.4 ns under the displacement limitation, and next 0.8 ns for free compression. The models ignoring the charge effect on tribo-wall surfaces have been carried out for comparison. As shown in Fig. 6, PEO molecules are initially placed between upper and lower iron wall

surfaces with enough room for them to relax and approach the surfaces. Under a normal load, the upper and lower surfaces move against each other under displacement limitation. As expected for boundary lubrication, this amount of polymer could not form a full lubricant film, and only fill the asperity valleys between upper and lower surfaces, which results in the same system height at equilibrium. The step-like behaviour of the system heights of PEO4 and PEO6 comes from the flowing behaviour of lubricant around the contacting asperities. The PEO molecules are not evenly distributed on the upper and lower surfaces before the contact occurred. When the dominant asperities bring into contact, PEO2 molecules are more easily squeezed out from the contacting interface than PEO4 and PEO6 molecules. It results in a smaller reduction rate of the system height for longer chain PEO after dominant asperities come to contact, which is shown as steplike behaviour. It needs to be pointed out that the 3D confinement is different with the 2D model suggested by Zheng et al. [6] and Sivebaek et al. [25]. The 3D surface asperities form a more reasonable contacting geography which could weaken the effect of molecular chain length on asperity contact. When the lubricant is confined by 3D multi asperities, it can be squeezed out both in x and y directions, whereas the 2D model [6] only allows lubricant molecules to move away from contacting interface in x-direction. When the charged tribo-wall surfaces are used, the contact behaviour changes significantly. The strong electrostatic interaction

H.T. Zhu et al. / Wear 332-333 (2015) 1248–1255

3.2.2. Lubricated asperity sliding After the wall surfaces reaches the equilibrium under the normal load, a sliding velocity of 20 m/s was applied on the upper and lower rigid layers, which results in a shearing speed at 40 m/s, as shown in Fig. 2. The upper and lower walls were slid 7 cycles to guarantee a stabilized friction coefficient and contact behaviour. The sliding of dry contact and non-charged tribo-wall surfaces were also carried out for comparison. Fig. 9 shows the contact area as a function of sliding distance. During sliding, tribo-wall surface asperities are flattened by the direct asperity contact, resulting in an increase of contact area, especially at the first 2 sliding cycles. With the introduction of PEO6 molecules into the sliding of non-charged wall surface, the direct asperity contact is reduced; and the normal load is partly supported by polymer molecules, which results in a smaller contact area compared with dry sliding. The difference of contact area between the dry sliding and lubricated sliding of non-charged 9.0 Non-Charged Surfaces PEO2 at 0.25 GPa Charged Surfaces PEO2 at 0.25 GPa PEO2 at 0.50 GPa PEO2 at 0.75 GPa PEO2 at 1.00 GPa

8.8

System Height (nm)

8.6 8.4 8.2 8.0 7.8 7.6 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Time (ns) Fig. 7. System heights of charged wall surfaces with polar PEO2 under different normal loads.

8.8

PEO2 PEO4 PEO6

8.6

System Height (nm)

between the charged wall atoms and polar polymer molecules enables the PEO polymers to be attached on wall surfaces, and avoid direct asperity contact. Fig. 7 clearly shows that the charged wall surfaces are separated more than non-charged wall surfaces. Under 0.25 GPa normal pressure, the charged wall surfaces increases the system height by around 0.6 nm from that of the non-charged surfaces, which means there is at least one small layer of PEO between the contacting interfaces. As the normal load increases, the system height at compression equilibrium decreases. The system height with consideration of wall charge under the largest load, 1.0 GPa, is even larger than that of non-charged surfaces under smallest load 0.25 GPa. The charge of tribo-wall surfaces plays a critical role to separate the contact interfaces. The effect of molecular chain length of PEO polymer on the system height after compression is shown in Fig. 8. The system height due to different types of polymer vary significantly when the load is small at 0.25 GPa; whereas the system height under larger loads more than 0.5 GPa keeps nearly same. The main reason is that the thin layer of PEO6 molecules locates at dominant asperities tends to remain intact because the longer PEO chain have more negative charged sites (oxygen atoms) attracted by positive charged iron wall surfaces, which stabilizes the whole chain. However, PEO2 and PEO4 molecules are more likely to be squeezed out because it has less oxygen atoms to attach at the contacting interface under 0.25 GPa. When a large load is applied, PEO6 molecules are also squeezed out, so the system height becomes almost the same.

8.4

8.2

8.0

7.8 0.25

0.50

0.75

1.00

Load (GPa) Fig. 8. System height of the charged model as a function of load after compression. 20 18 16 14

Contact Area (%)

1252

12 10 8

Dry sliding at 0.25 GPa Non-charged at 0.25 GPa Charged at 0.25 GPa

6 4

Averaging values Dry sliding at 0.25 GPa Non-charged at 0.25 GPa

2 0 -2 0

13

26

39

52

65

78

91

104

Sliding Distance (nm) Fig. 9. Contact areas as a function of sliding distance. The charged and non-charged wall surfaces were lubricated by PEO6 polymer. The lines with symbols are instantaneous values while the thick line represents the averaging value.

surfaces becomes large with the increase of sliding distance. However, the charged wall surfaces lubricated by PEO6 polymer exhibits a totally different behaviour. The contact area decreases dramatically to nearly zero, which indicated that the rough wall surfaces are almost separated by PEO6 polymer during sliding at the load of 0.25 GPa, and direct asperity contact is almost prevented. A comparison of typical snapshots of charged and non-charged wall surfaces lubricated by PEO6 polymer during the sliding is shown in Fig. 10. For the sliding of non-charged wall, the lubricant molecules just fills the gap between the surfaces. Due to the relatively weak adhesion of PEO molecules to iron walls by LJ potential without electrostatic interaction, the polymers are squeezed and accumulated in large valleys between the surfaces as shown in Fig. 10(b). In contrast, the charged wall surfaces can firmly attract PEO6 molecules by the combined contribution of LJ and Coulombic interactions. As shown in Fig. 10(c), the polar PEO molecules behave like a coated layer to prevent the surface contact. Moreover, the top view of this layer indicates that these molecules distributed evenly on the surfaces, as indicated in Fig. 10(d). Because the charged wall surfaces are almost separated by the lubricant, the friction between the walls would be significantly affected by this polymer layer. The variation of friction force with the sliding distance (corresponding to the contact area in Fig. 9) was plotted in Fig. 11. As Zheng et al. [9] found in multi-asperity

H.T. Zhu et al. / Wear 332-333 (2015) 1248–1255

1253

Fig. 10. A snapshot of (a) non-charged model and (c) charged model lubricated by PEO6 polymer during sliding. The corresponding top views of the lubricant molecules were plotted in (b) and (d), respectively.

32 28 24

Friction Force (nN)

sliding lubricated by limited number of n-alkanes, the sliding of non-charged walls exhibits a relative larger friction force than the dry sliding. As indicated by Zheng et al. [9], the friction force comes from two components, including asperity contact and pulling force from the lubricant at the conditions of low load and insufficient lubricant molecules. The lubricant is likely to remain in an unlubricated state where the contact of asperity results in the similar force like the dry sliding. Meanwhile, the flow resistance from the chain molecules itself exerts a comparable force against the sliding, which results in the relative large friction force compared with dry sliding. With the increase of the normal load and amount of lubricant, this special phenomenon will be disappeared. In contrast, the sliding of charged walls displays a very small friction force due to the high adhesion strength and uniform coated PEO polymer on tribo-walls as indicated in the Fig. 10 (c) and (d). Moreover, the sliding of charged walls shows an obvious and stable periodicity during the repeated sliding. For the sliding of non-charged wall, this periodicity decays with the sliding distance because the sliding flattens the surface asperities, and then the friction becomes smaller. The influence of various normal loads, 0.25, 0.5, 0.75 and 1.0 GPa, on the friction force during the sliding of the charged walls can be found in Fig. 12. With the increase of normal load, not only the friction force increases, but also the variation of friction force becomes obvious. However, the strong adhesion of polar PEO polymer on charged wall surfaces prevents the damage of wall topography; hence the periodicity remains the same. The molecular structure of the lubricant, such as chain length, shape and branch atoms, plays an important role on the tribological performance of confined lubrication [6,25–27]. The effect of molecular chain length of PEO polymer on the sliding of the non-charged

Dry Sliding at 0.25 GPa Non-charged at 0.25 GPa Charged at 0.25 GPa

20 16 12 8 4 0 -4 0

13

26

39

52

65

78

91

104

Sliding Distance (nm) Fig. 11. Friction force as a function of sliding distance, which is corresponding to Fig. 9.

and charged surfaces has been investigated and shown in Fig. 13. It can be found that the non-charged surfaces lubricated by PEO polymer exhibits a large friction force. For non-charged surfaces, the friction force increased with the chain length as shown by the Arrow 1 in the Fig. 13, which is consistent with the previous observation from thin film lubrication that a longer chain has a high shear stress [28]. As the adsorbed PEO molecule layer separating the surfaces, the friction force of the charged surfaces is very small; and it slightly increases with the higher normal load. More importantly,

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H.T. Zhu et al. / Wear 332-333 (2015) 1248–1255

rough tribo-pair walls and polarisable PEO polymer as lubricant has been proposed. Compared to non-charged tribo-surfaces where the amount of lubricant and surface roughness determine the friction and lubrication, the charged tribo-surface attracts polar PEO molecules to form a coated like layer; thereby resulting in a different tribology behaviours. The polar PEO polymers separate the positive charged sliding tribo-surfaces and reduce the direct asperity contact; hence significantly lowering the friction force.

24 0.25 GPa 0.50 GPa 0.75 GPa 1.00 GPa

20

Friction Force (nN)

16

12

8

4

Acknowledgements

0

We would like to thank the Information Management & Technology Services (IMTS) at the University of Wollongong for computing time on the UOW High Performance Computing Cluster.

-4 0

13

26

39

52

65

78

91

Sliding Distance (nm)

References

Fig. 12. Friction force as a function of sliding distance. Four different loads are applied on to the charged wall surfaces lubricated by PEO6 polymer.

40 35

1

Friciton Force (nN)

30 25 Non-charged PEO2-4000 PEO4-4000 PEO6-4000 Charged PEO2-4000 PEO4-4000 PEO6-4000

20 15 10 5

2

0 0.25

0.50

0.75

1.00

Load (GPa) Fig. 13. Averaging friction forces at equilibrium as a function of normal load. The arrows indicate the different effects of chain length on charge and non-charge models.

the chain length of PEO molecules displays a different effect on the friction force of charged tribo-surfaces. PEO6 polymer exhibits a smaller friction force than PEO4 and PEO2 polymer, particularly under a large load as shown by the Arrow 2 in Fig. 13. As indicated in the Fig. 10 (c) and (d), the sliding of the charged surfaces actually contact through PEO polymer layers. Since the PEO6 molecules strongly attract to the charged tribo-surfaces, they are more invulnerable against asperity squeezing, which reduces direct asperity contact and then decrease friction. On contrast, the direct asperity contact may occur under a large load with short chain PEO polymer, resulting in slightly larger friction force.

4. Conclusions Lubricant is usually a mixture of polar and non-polar molecules, and tribo-wall may have surface atoms with charges. To provide a fundamental understanding of the electrostatic interaction between confining tribo-surface and polar lubricant on friction and lubrication, a molecular dynamics simulation of bcc iron with additional charge as

[1] B. Luan, M.O. Robbins, The breakdown of continuum models for mechanical contacts, Nature 435 (2005) 929–932. [2] B. Luan, M.O. Robbins, Contact of single asperities with varying adhesion: comparing continuum mechanics to atomistic simulations, Phys. Rev. E 74 (2006) 26111 (1–17). [3] Y. Mo, K.T. Turner, I. Szlufarska, Friction laws at the nanoscale, Nature 457 (2009) 1116–1119. [4] Y. Mo, I. Szlufarska, Roughness picture of friction in dry nanoscale contacts, Phy. Rev. B 81 (2010) 35405 (1–17). [5] S. Eder, A. Vernes, G. Vorlaufer, G. Betz, Molecular dynamics simulations of mixed lubrication with smooth particle post-processing, J Phy.: Condens. Matter 23 (2011) 175004. [6] X. Zheng, H.T. Zhu, P.B. Kosasih, A. Kiet Tieu, A molecular dynamics simulation of boundary lubrication: the effect of n-alkanes chain length and normal load, Wear 301 (2013) 62–69. [7] P. Spijker, G. Anciaux, J.F Molinari, Dry sliding contact between rough surfaces at the atomistic scale, Tribol. Lett. 44 (2011) 279–285. [8] P. Spijker, G. Anciaux, J.F. Molinari, The effect of loading on surface roughness at the atomistic level, Computat. Mech. 50 (2012) 273–283. [9] X. Zheng, H.T. Zhu, A.K. Tieu, B. Kosasih, Roughness and lubricant effect on 3D atomic asperity contact, Tribol. Lett. 53 (2013) 215–223. [10] X. Zheng, H.T. Zhu, A.K. Tieu, P.B. Buyung, A molecular dynamics simulation of 3D lubricated contact, Tribol. Int. 67 (2013) 217–221. [11] H. Berro, A Molecular Dynamics Approach to Nano-Scale Lubrication, INSA de Lyon, Villeurbanne, 2010, Phd thesis. [12] H. Berro, N. Fillot, P. Vergne, Molecular dynamics simulation of surface energy and ZDDP effects on friction in nano-scale lubricated contacts, Tribol. Int. 43 (2010) 1811–1822. [13] A. García, R. González, A. Hernández Battez, J.L. Viesca, R. Monge, A. Fernández-González, M. Hadfield, Ionic liquids as a neat lubricant applied to steel–steel contacts, Tribol. Int. 72 (2014) 42–50. [14] I. Larson, P. Attard, Surface charge of silver iodide and several metal oxides. Are all surfaces Nernstian? J. Colloid Interface Sci. 227 (2000) 152–163. [15] J. Kolafa, M. Ratner, Oligomers of poly(ethylene oxide): molecular dynamics with a polarisable force field, Mol. Simul. 21 (1998) 1–26. [16] G.D. Smith, R.L. Jaffe, D.Y. Yoon, Force field for simulations of 1,2-dimethoxyethane and poly(oxyethylene) based upon ab initio electronic structure calculations on model molecules, J. Phys. Chem. 97 (1993) 12752–12759. [17] J. Fischer, D. Paschek, A. Geiger, G. Sadowski, Modeling of aqueous poly (oxyethylene) solutions: 1 atomistic simulations, J. Phys. Chem. B 112 (2008) 2388–2398. [18] O. Borodin, G.D. Smith, R. Bandyopadhyaya, O. Byutner, Molecular dynamics study of the influence of solid interfaces on poly(ethylene oxide) structure and dynamics, Macromolecules 36 (2003) 7873–7883. [19] D. Bedrov, M. Pekny, G.D. Smith, Quantum-chemistry-based force field for 1,2dimethoxyethane and poly(ethylene oxide) in aqueous solution, J. Phys. Chem. B 102 (1998) 996–1001. [20] S. Hezaveh, S. Samanta, G. Milano, D. Roccatano, Structure and dynamics of 1,2-dimethoxyethane and 1,2-dimethoxypropane in aqueous and nonaqueous solutions: a molecular dynamics study, J. Chem. Phys. 135 (2011) 164501. [21] J.M. Stubbs, J.J. Potoff, J.I. Siepmann, Transferable potentials for phase equilibria 6. united-atom description for ethers, glycols, ketones, and aldehydes, J. Phys. Chem. B 108 (2004) 17596–17605. [22] M.H. Ketko, J. Rafferty, J.I. Siepmann, J.J. Potoff, Development of the TraPPE-UA force field for ethylene oxide, Fluid Ph. Equilib. 274 (2008) 44–49. [23] S. Plimpton, Fast parallel algorithms for short-range molecular dynamics, J Comput. Physics. 117 (1995) 1–19. [24] R.F. Voss, Fundamental Algorithms in Computer Graphics, Springer, Berlin, 1985.

H.T. Zhu et al. / Wear 332-333 (2015) 1248–1255

[25] I.M. Sivebaek, V.N. Samoilov, B.N.J. Persson, Squeezing molecular thin alkane lubrication films between curved solid surfaces with long-range elasticity: layering transitions and wear, J. Chem. Phy. 119 (2003) 2314–2321. [26] A. Jabbarzadeh, J.D. Atkinson, R.I. Tanner, The effect of branching on slip and rheological properties of lubricants in molecular dynamics simulation of couette shear flow, Tribol. Int. 35 (2002) 35–46.

1255

[27] A. Jabbarzadeh, J.D. Atkinson, R.I. Tanner, Effect of molecular shape on rheological properties in molecular dynamics simulation of star, H, comb, and linear polymer melts, Macromolecules 36 (2003) 5020–5031. [28] D. Savio, N. Fillot, P. Vergne, M. Zaccheddu, A model for wall slip prediction of confined n-alkanes: effect of wall-fluid interaction versus fluid resistance, Tribol. Lett. 46 (2012) 11–22.