Transcript Slide 1

Experimental and Analytical
Investigation of Transient
Friction
Abdullah Alazemi
Ph.D. Research Assistant
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
2
Outline
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•
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Personal Background
Motivation
Numerical Work
Experimental Work
Summary and Future Work
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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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
Motivation
• 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
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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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.
𝒅𝒓𝒊
= 𝒗𝒊
𝒅𝒕
𝒅𝒗𝒊 𝑭𝒊
=
𝒅𝒕
𝒎𝒊
𝑭𝒊 = −𝜵𝑼(𝒓𝒊 )
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Molecular Dynamic (MD) simulation
• Nano-indentation of a single asperity
• Measuring adhesion forces during approaching and separating
• Approaching and separating speed: 1 m/s
150
1
Contact Force (nN)
100
Loading
Unloading
50
0
-50
-100
-150
-200
-10
2
-5
0
5
10
15
20
Displacement (Angstrom)
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Molecular Dynamic (MD) simulation
• Sliding friction
• Material: Pt-Au,
• Sliding velocity: 1 m/s,
• Load: 8 nN
Si3N4 tip on HOPG
[Y. Hoshi et al., 2000]
20
Load
Forward
Backward
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Sliding direction
Friction Force (nN)
10
5
0
-5
-10
-15
-20
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Displacement (nm)
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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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
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Atomic Force Microscope (AFM)
• Scanning using Si tip on
SiO2 steps on Si wafer,
• Scanning size = 5000 nm,
• Scanning speed = 10 µm/s
Forward 
 Backward
100
Lateral Force Signal (mV)
Forward
Backward
50
0
-50
-100
Surface topography
0
0.5
1
1.5
2
2.5
3
3.5
Distance in X direction ( m)
4
4.5
5
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Friction Test Rig
1.
Study friction on the micro-scale,
2.
Small ball diameter (about 5-1 mm),
3.
Small normal load (about 1 N),
4.
Using actuator with small linear displacement increment (about 100 nm),
5.
Perform the experiment in vacuum conditions.
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Friction Test Rig
Load cell
Ball
Sapphire window
Load cell
Sapphire window
Z-stage
Loading
mechanism
Actuator
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
Measurements of the Real Contact
Area (RCA)
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1. Optical observation technique [A. Ovcharenko et al., 2006]
2. Using Optical Profiling System
Before applying load
After applying load
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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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.
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013