Experimental and Analytical Investigation of Transient Friction

1. Experimental and Analytical Investigation of Transient Friction Abdullah Alazemi Ph.D. Research Assistant

2. Outline • Personal Background • Motivation • Numerical Work • Experimental Work • Summary and Future Work

3. Motivation Personal Background • Joined METL – Jan, 2013. • M.S. in Mechanical Engineering, Kuwait University (June, 2009)- GPa 4.0 • B.S. in Mechanical Engineering, Kuwait University (June, 2006)- GPa 3.85 • Lack of clearly and completely understanding of the friction nature and behavior • Dry friction of a particular surface is caused by normal force, adhesion force, and interlocking between asperities . • Bowden and Tabor model: Ff = τ∙A

4. Numerical Work • MD simulation represents friction on a single asperity. • Finite element modeling can be used to study friction on a larger scale where each asperity force interaction is taken from MD simulation.

5. Molecular Dynamic (MD) simulation • Nano-indentation of a single asperity • Measuring adhesion forces during approaching and separating • Approaching and separating speed: 1 m/s 1 2

6. Load Molecular Dynamic (MD) simulation Sliding direction • Sliding friction • Material: Pt-Au, • Sliding velocity: 1 m/s, • Load: 8 nN • Si3N4tip on HOPG • [Y. Hoshi et al., 2000]

7. Atomic Force Microscope (AFM) • Experimentally study single asperity friction • Tip diameter: 5 µm - 10 nm • A laser beam that is reflected off the cantilever into a photodiode monitors the bending and twisting of the cantilever. Source: www.appnano.com

8. Atomic Force Microscope (AFM) • Scanning using Si tip on SiO2 steps on Si wafer, • Scanning size = 5000 nm, • Scanning speed = 10 µm/s  Backward Forward  Surface topography

9. Friction Test Rig • Study friction on the micro-scale, • Small ball diameter (about 5-1 mm), • Small normal load (about 1 N), • Using actuator with small linear displacement increment (about 100 nm), • Perform the experiment in vacuum conditions.

10. Friction Test Rig Ball Load cell Sapphire window Load cell Sapphire window Loading mechanism Actuator Z-stage

11. Measurements of the Real Contact Area (RCA) • Optical observation technique [A. Ovcharenko et al., 2006] • Using Optical Profiling System Before applying load After applying load

12. Summary and Future Work • Transient friction experimental investigations can be done on the Nano-scale using AFM and on the Micro-scale using Friction test rig. • Transient friction analytical investigations can be done on the Nano-scale using MD simulation and then connected to FE modeling to investigate Micro-scale friction. • Future AFM experiments are friction on smooth substrate and substrate with low roughness. • Future MD simulations are sliding friction of different surface geometries, normal loads, and speeds.