Nanomechanical Testing of Thin Polymer Films Kyle Maner
- Slides: 52
Nanomechanical Testing of Thin Polymer Films Kyle Maner and Matthew Begley Structural and Solid Mechanics Program Department of Civil Engineering University of Virginia Special thanks to: Uday Komaragiri (UVA) 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
Overview • Traditional nanoindentation of thin films bonded to thick substrates • A novel freestanding film microfabrication procedure • A novel method to probe freestanding films
Do polymers exhibit scale dependence? Is traditional nanoindentation sensitive enough to detect such behavior?
3 Pure, amorphous polymers: Poly(styrene) (PS) – Mw = 280 k. D Poly(methyl methacrylate) (PMMA) – Mw = 350 k. D Poly(phenylene oxide) (PPO) – Mw = 250 k. D 2 Block co-polymers: Poly(methyl methacrylate)-ruthenium (PMMA-Ru) – Mw = 56 k. D (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 mm • 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
Quartz, E = 72 GPa
Nanostructured lamellar block co-polymer
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
Overview • Traditional nanoindentation of thin films bonded to thick substrates • A novel freestanding film microfabrication procedure • A novel method to probe freestanding films
A new microfabrication procedure should be: • applicable to a wide range of materials • easily prepared on any wet-bench The experimental testing of the sample created should be: • easily integrated with existing test equipment • easily interpreted with relatively simple mechanics models
The short answer… Spin-casting Etching Testing
Spin-cast polymer film onto glass plate with etchable fibers
The short answer… Spin-casting Etching Testing
2% HCl FRONT-LIGHTING BACK-LIGHTING
Mechanical properties via nanoindentation before and after acid bath
The short answer… Spin-casting Etching Testing
Overview • Traditional nanoindentation of thin films bonded to thick substrates • A novel freestanding film microfabrication procedure • A novel method to probe freestanding films
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 loaddeflection response
Probing of freestanding films: surface find
Probing of freestanding films: test flow
Stiffness scan
With the given parameters (thickness & span), what is the anticipated response? ? Linear plate Transition Membrane
PMMA Mw = 120 k. D thickness = 350 nm span = 30 mm
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
PPO Mw = 250 k. D thickness = 750 nm span = 30 mm
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
Questions? Thank you.
• 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
Nanoindentation Probe
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
The research on submicron 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 question of contact
Film thickness before and after acid bath
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
Sensitivity of the Method PMMA: ~350 nm thick, 30 mm span E = 3. 0 GPa e 0 = 0. 0026
Tip Calibration Equations • Stiffness as a function of depth, S(d), is measured • The area function, A(d), is determined from the following equation: • Elastic properties of calibration sample and indenter tip must be know to calculate, : • The calculated area function is a series with geometrically decreasing exponents:
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
Parameters of Spin-Casting
Surface Characterizations PS substrate PMMA film on PS substrate
Illustrative Theory, i. e. Math for non-Uday’s Strain-displacement: Stress-strain: Equilibrium: , where
By combining the strain-displacement, stress-strain, and equilibrium equations, the following equation can be found: 0 For small deflections, , thus: The equation for load becomes: Due to small deflections, the denominator goes to 1, and load as a function of deflection is:
Sensitivity of the method: very shallow depths PMMA: ~350 nm thick, 30 mm span E = 3. 0 GPa e 0 = 0. 0026
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