nanomechanical testing of thin polymer films kyle maner and matthew begley structural and solid...
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Nanomechanical Testing of Thin Polymer Films
Kyle Maner and Matthew Begley Structural and Solid
Mechanics Program Department of Civil Engineering University of Virginia
Uday Komaragiri (UVA) Special thanks to: Dr. Warren C. Oliver (MTS)
Prof. Marcel Utz (UConn)
Why test thin polymer films?
• Improve thermomechanical stability via self-assembly of nanostructure
• Establish connections between the nanostructure & mechanical properties
• Determine the size scale of elementary processes of plastic deformation
•Traditional nanoindentation of thin films bonded to thick substrates
• A novel freestanding film microfabrication procedure
• A novel method to probe freestanding films
Overview
Do polymers exhibit scale dependence?
Is traditional nanoindentation
sensitive enough to
detect such behavior?
3 Pure, amorphous polymers:
Poly(styrene) (PS) – Mw = 280 kD
Poly(methyl methacrylate) (PMMA) – Mw = 350 kD
Poly(phenylene oxide) (PPO) – Mw = 250 kD
2 Block co-polymers:
Poly(methyl methacrylate)-ruthenium (PMMA-Ru) – Mw = 56 kD (a metal-centered block co-polymer)
Poly(styrene)-poly(ethylene propylene) (PS-PEP) (a lamellar microphase separated block co-polymer)
Experimental Procedure
• Calibrate the tip – discard data for depths where the calibration is inaccurate
• Indent polymer films on PS substrates – 16 indents per sample to a depth of 1.0 m
• Discard rogue tests due to surface debris
• Average data to determine elastic modulus and hardness curves as a function of penetration depth
• The Berkovich diamond tip does not come to a perfect point
• The radius of the tip gradually increases with use
• The shape change alters the contact area of the indenter for a given depth
• A tip calibration determines the best-fit coefficients for the area function describing the tip
)()( 1
AECd
dPS r
Conclusions from traditional nanoindentation
• Substrate effects can be dramatically reduced if elastic mismatch is minimized
• A tip calibration can be accurate for depths greater than ~5 nm
• Scale effects indicate that elementary processes of deformation occur at depths less than ~200 nm
• Traditional nanoindentation of thin films bonded to thick substrates
• A novel freestanding film microfabrication procedure
• A novel method to probe freestanding films
Overview
A new microfabrication procedure should be:
• applicable to a wide range of materials
• easily prepared on any wet-bench
• easily integrated with existing test equipment
• easily interpreted with relatively simple mechanics models
The experimental testing of the sample created should be:
•Traditional nanoindentation of thin films bonded to thick substrates
• A novel freestanding film microfabrication procedure
• A novel method to probe freestanding films
Overview
An overview of the test method
• constant harmonic oscillation superimposed on a ramp loading
• at contact, stiffness of sample causes drop in harmonic oscillation
• mechanical properties can be extracted from load-deflection response
With the given parameters (thickness & span), what is the anticipated response??
Linear plate
Membrane
Transition
Finite element study of PPO plasticity
• Load-deflection response generated via finite elements
•Elastic-perfectly plastic stress-strain relationship
• Varied values of yield strength, elastic modulus, and pre-stretch
Conclusions
• Approximated size scale over which elementary processes of plastic deformation occur in polymers
• Developed a new microfabrication technique to create submicron freestanding polymer films
• Developed a new testing method to probe thin freestanding films and illustrated its repeatability
• Successfully used numerical models to extract mechanical properties from submicron films
• Introduction and motivation
• Description of the MTS Nanoindentation System
• Traditional nanoindentation of thin films bonded to thick substrates
• A novel freestanding film microfabrication procedure
• A novel method to probe freestanding films
Traditional methods of testing thin films
• Wafer curvature
• Bulge testing
• Nanoindentation of thin films bonded to thick substrates
• Microfabrication & probing of freestanding films
Special features of the MTS Nanoindentation System
DCM (dynamic contact measurement) module – ultra-low load indentation head with closed-loop feedback to control dynamic motion
CSM (continuous stiffness measurement) approach – measures the stiffness of the contact continuously during indentation as a function of depth by considering harmonic response of head
• Introduction and motivation
• Description of the MTS Nanoindentation System
• Traditional nanoindentation of thin films bonded to thick substrates
• A novel freestanding film microfabrication procedure
• A novel method to probe freestanding films
• Metals, metals, and more metals – deformation and scale-dependent behavior is well understood
• Plasticity in polymers – how it occurs but not how big
• Minimization of substrate effects via elastic homogeneity of film and substrate
• Probing of freestanding Si-based brittle and metal structures
The research on submicron films
A novel method to probe freestanding films should combat the problems facing
experimental testing of compliant films….
• Tip calibration errors can produce inaccurate measurements
•The surface of compliant materials is difficult to “find”
• Mechanics to extract properties is very complex
Tip Calibration Equations
• Stiffness as a function of depth, S(), is measured
• The area function, A(), is determined from the following equation:
)(2
)(
AES r
• Elastic properties of calibration sample and indenter tip must be know to calculate, :rE
i
i
s
s
r EEE
22 111
• The calculated area function is a series with geometrically decreasing exponents:
...)( 2/132
21 CCCA
Standard method: Nanoindentation of film/substrate system
• CSM stabilizes harmonic motion of the indenter head
• Probe begins to move towards surface
• Contact (1) occurs when stiffness increases
• Load (2) to a prescribed displacement
• Hold (3) at maximum load to assess creep behavior
•Unload (4) 90% of the way
• Hold (5) at 90% unload to assess thermal drift
Illustrative Theory, i.e. Math for non-Uday’s
Strain-displacement:0
2 11ˆ
Stress-strain: E
Equilibrium:
sin2
)sin(20
PF
FPFy
L
ˆ, where
By combining the strain-displacement, stress-strain, and equilibrium equations, the following equation can be found:
ˆ
1ˆ
11ˆ
22
02
EAP
For small deflections, , thus:1ˆ ...)ˆ(0ˆ2
111ˆ 322
0
The equation for load becomes:
2
03
ˆ21
1
ˆ2ˆ
EAEAP
Due to small deflections, the denominator goes to 1, and load as a function of deflection is:
EAP )ˆ2ˆ()ˆ( 03
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