Susan B. Sinnott,1 Simon R. Phillpot,1 Scott Perry,1 and W. Gregory Sawyer1,2

University of Florida, 1 Materials Science and Engineering2 Mechanical and Aerospace Engineering

Design of PTFE Solid Lubricant Nanocomposites through Coupled Simulation and Experiment

Students and postdocs who carried out the work:

Peter BarryPatrick ChiuInkook JangJennifer Vale

The goal is to understand how wear and friction in PTFE depends on orientation

Objective and Motivation

2nd Generation REBO potential: Brenner et al. (2002) PTFE surfaces cross-linkedFluorocarbon extension: Jang and Sinnott (2004) Cross-linking density: 2.82 nm-3

Parallelized software: Hsu, Phillpot, and Sinnott (2007)

Computational Details

PTFE-PTFE Sliding

Parallel Sliding Perpendicular Sliding

T= 300 K

Effect of Chain Orientation: Evolution of Sliding Surface

FN

FF

Relationship Between Chain Orientation and Molecular Wear

Analysis of Chain Displacement

Experimental Details

Experimental Evidence of Oriented PTFE Films

Intermittent contact AFM image of the transfer film produced through reciprocal sliding of PTFE on a polished steel substrate. The observed oriented features are highly correlated with the direction of sliding during the generation of the PTFE transfer film.

The single line profile orthogonal to the sliding direction portrays surface features on the order of 10 nm in width and 2-3 nm in height; such features within the image strongly suggest the fibrillated and oriented nature of the transfer film.

Effect of Chain Orientation: Sliding Parallel to Chains

5 10 15 20 25 30 35 400.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

fric

tion

coef

ficie

nt

sliding distance (nm)

300 K

Effect of Chain Orientation: Sliding Perpendicular to Chains

5 10 15 20 25 30 35 400.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

fric

tion

coef

ficie

nt

sliding distance (nm)

300 K

Effect of Chain Orientation: Comparison of Parallel vs. Perpendicular

• Simulations predict anisotropic dependence of friction coefficient with orientation of PTFE (parallel versus perpendicular configurations) and illustrate the mechanisms by which the anisotropy occurs

• Experimental data validates these predictions; computational simulations explain the experimental data

ExperimentSimulation

5 10 15 20 25 30 35 400.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

perpendicular parallel

fric

tion

co

effic

ien

t

displacement (nm)

300 K

PTFE-PTFE Sliding: Parallel Orientation

-45 -30 -15 0 15 30 450

2

4

6

8

10

12

14fr

ictio

nal f

orce

(nN

)

normal force (nN)

m x (nN)0.56 0 300 K0.09 8 300 K

Additive effects of adhesion on contact pressures can be dominant, especially at the nano-scale. Efforts to report friction coefficients that are independent of load, area, and adhesive contributions to contact should use a load-ramp technique.

0.1 300 K

5 10 15 20 25 30 35 400.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

fric

tion

coef

ficie

nt

sliding distance (nm)

PTFE-PTFE Sliding at Variable Loads

0 5 10 15 20 25 30 350

4

8

12

16 perpendicular sliding parallel sliding

Ff (

nN)

Fn (nN)

Objective and Motivation

Combinatorial Simulation of Multifunctional Polymer System

System size53.4 nm in x direction12.5 nm in y direction12.5 nm in z direction

x

y

z

• 1,004,008 atoms simulated• 300 K Temperature• Top surface: Polyethylene (PE)• Bottom surface: PE-PTFE nanocomposite

– Crosslink density 3.58/nm3

– Lower substrate: stationary 14 PTFE-filled bucket configuration

Top View

Side View

PE-PE Sliding

Perpendicular

Parallel

PTFE-PE Sliding at 300K

0 8 16 24 32 40 48 560

4

8

12

16

20

24 parallel sliding perpendicular sliding

Ff (

nN)

Fn (nN)Perpendicular sliding

52 nN load

13.2 nm of sliding

13.2 nm of sliding

75 nN load

50 K temperature

PTFE-PE Composite

Top down view of lower surface during sliding

Focus on one aspect of the composite:

PTFE-PE Sliding at 300K

PE upper surface

(bottom up view)

PTFE lower surface

(top down view)

perpendicular sliding

52 nN load

13.2 nm of sliding

Conclusions

• Orientation of PTFE chains influences processes: aligned chains do not exhibit obvious wear, while misaligned chains undergo obvious molecular wear

• The combination of experimental and computational approaches has dramatically improved our ability to design multifunctional nanocomposites for tribological applications

Effect of Sliding Rate on Computational Chain Orientation Results

Perpendicular Parallel

4 8 12 16 20 240.0

0.4

0.8

1.2

1.6

2.0

fric

tion

coef

ficie

nt

sliding distance (nm)

5 m/s 10 m/s 15 m/s 20 m/s

4 8 12 16 20 240.0

0.4

0.8

1.2

1.6

2.0

sliding distance (nm)

5 m/s 10 m/s 15 m/s 20 m/s

Crosslinking Density Variation (PE)

0 4 8 12 16 20 24 28 32 36 400

4

8

12

16 3% (randomized) 6% (randomized) 12% (randomized) 19% (randomized) 19% (ordered)

Ff (

nN)

Fn (nN)