1. Susan B. Sinnott, 1 Simon R. Phillpot, 1 Scott Perry, 1 and W. Gregory Sawyer 1,2 University of Florida, 1 Materials Science and Engineering 2 Mechanical and Aerospace Engineering Design of PTFE Solid Lubricant Nanocomposites through Coupled Simulation and Experiment Students and postdocs who carried out the work: Peter Barry Patrick Chiu Inkook Jang Jennifer Vale

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

3. Objective and Motivation

4. 2 nd Generation REBO potential: Brenner et al. (2002)PTFE surfaces cross-linked Fluorocarbon extension: Jang and Sinnott (2004) Cross-linking density: 2.82 nm -3 Parallelized software: Hsu, Phillpot, and Sinnott (2007) Computational Details

5. PTFE-PTFE Sliding Parallel Sliding Perpendicular Sliding T= 300 K

6. Effect of Chain Orientation: Evolution of Sliding Surface FNFFFNFFF

7. Relationship Between Chain Orientation and Molecular Wear

8. Analysis of Chain Displacement

9. Experimental Details

10. 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.

11. Effect of Chain Orientation: Sliding Parallel to Chains 300 K

12. Effect of Chain Orientation: Sliding Perpendicular to Chains 300 K

13. 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 Experiment Simulation 300 K

14. PTFE-PTFE Sliding: Parallel Orientation (nN) 0.56 0 300 K 0.09 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

15. PTFE-PTFE Sliding at Variable Loads

16. Objective and Motivation

17. Combinatorial Simulation of Multifunctional Polymer System System size 53.4 nm in x direction 12.5 nm in y direction 12.5 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/nm 3 –Lower substrate: stationary 14 PTFE-filled bucket configuration Top View Side View

18. PE-PE Sliding Perpendicular Parallel

19. PTFE-PE Sliding at 300K Perpendicular sliding 52 nN load 13.2 nm of sliding

20. 75 nN load 50 K temperature PTFE-PE Composite Top down view of lower surface during sliding Focus on one aspect of the composite:

22. 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

23. 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

24. Effect of Sliding Rate on Computational Chain Orientation Results Perpendicular Parallel

25. Crosslinking Density Variation (PE)