Quantitative Nano-Mechanical Characterization of ... · Quantitative Nano-Mechanical...
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June 29, 2012 1
www.bruker.com
Quantitative Nano-Mechanical Characterizationof Composites and Thin Films
‐Combo TMT/AFM Webinar on Nanoindentation and Peak‐force on June 26‐
Ilja Hermann Ben Ohler
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• Motivation• Theory of Nano‐indentation• Experimental aspects • Example Results• Conclusion
Outline
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MotivationWhy do we need mechanical properties ?
June 29, 2012 3
Mechanical Properties
Failure Analysis Virtual Tests
Materials design Tribology
Wear
Understand and prevent irreversible deformation
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June 29, 2012 4
MotivationFundamentals of mechanical properties
Stress and Strain :
AF
Normal Stress
ll
Normal Strain
AF
Shear Stress
ll
tan
Shear Strain
... mnklijklmnklijklij SS
Constitutive Equation :
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June 29, 2012 5
MotivationMeasuring mechanical properties
Test & specimen:•Uniaxial tension/compression •Bending•Accoustic wave
=> Mechanical properties on macroscopic scale
Hooke:
11
12
11
11
E
klijklij S
homogenious, isotropic:
Properties:• E, , K, G, • Y0, K1
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MotivationHardness test
28544.1dFHV
[Applied load] = kgf
[Diagonal] = mm
VICKERS-Hardnessnumber
23
32
21
1 015.0c
FHE
laK
Fracture-
Toughness
F…Applied load (N)H…Hardness (Gpa)E…Young’s Modulus (Gpa)c,a,l…distances from img (m)
Pros:• Simple application• Small specimen• Easy to scale
Cons:• HV difficult co-relate to physical properties• Scaling limited by imaging resolution• No elastic properties
Concept: 1. Apply load. 2. Analysis of residual imprint after load removal
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MotivationInstrumented indentation
Concept: 1. Accurate measurement of compliance curve 2. Elastic analysis of tri-axial stress-state in contact
Nano-indentation QNM
Elastic-plastic Elastic
Typical load: ~10 mN ~1 nN
Typical depth: ~100 nm ~10 nm
Property access: E, H, Y, K1, Rep(Rep) E, E(x,y)=>Mechanical properties on mesoscopic scale
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June 29, 2012 8
Theory of Nano‐indentation
Fundamentals of contact mechanics
p(r,)
i, i
drdrpE
uz ),(1 2
Point-contact solution
Superposition
Obtaining contact stress field and surface deformation
, = ?
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June 29, 2012 9
Theory of Nano‐indentation
Elastic surface deformation
Contact Geometry
Pressure distribution
Mean pressure Surface Deflection (r=0)
Surface Deflection (r=a)
Flat Punch Sneddon:
Sphere(Hertz)
Cone(Sneddon) 2
12
tan21
rEFh
21
2
2
123
ar
pm
z
ra
pm
z 1cosh
21
2
2
121
ar
pm
z FaE
hr2
1 2
32
43
REFh
r he FS
pm HB 4Er
3
aR
f (r) Brn
m 1 1
m(2m 2)
m(2m1)
with
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June 29, 2012 10
Theory of Nano‐indentation
Hertzian contact stress
222222 *621
zyxzxyzzxxyyzzyyxxMisesvon
Prin
cipa
l stre
ss
1(a
.u.)
Von
Mis
es S
tress
(a.u
.)
Tensile stress Von Mises stress
Hertzian cone fracture Residual imprint by sub-surface plasticity
Related material failure modes
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June 29, 2012 11
Theory of Nano‐indentation
Elastic-plastic indentation
i R
aR cos()
Ideal cone/Pyramid: elastic-plastic transition occurs instantly
0.07 0.5
Stress field during unloading altered by elastic constraint plastic zone
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hc hmax FS f(r)=Brn
r
“Effective indenter”
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Theory of Nano‐indentation
Oliver & Pharr
AFH
F a(hh0 )m
1.2 m 1.6
hc hmax FS
hR FS
Ac Ac (hc )
1rE
1 i2
iE
1 p2
pE
Er
2 Ac
S
Young’s modulus E
S dFdh Fmax
2 Ac
Er
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Experimental aspects
Typical setup an test
Calibrations:• Tip-shape• Frame compliance• Offset in-line toolsSpecimen:• Smooth• Alignment • Bonding
Nano-HeadMicro-Head
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June 29, 2012 14
ResultsLoading types
• Constant• Linear• Parabolic• Harmonic• Combinations
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ResultsRaw-data post processing - Zero-point correction
F=c(h-h0)3/2
h0i
1. Hertzian fit for small depths2. Auto-correction applied to all curves
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Experimental aspects
Raw-data post-processing - other automatic corrections
• Thermal drift
• Frame Compliance: hframe F
Si (F)
hth ht driftholdtime
t
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Hardness and Young’s Modulus on SiO2NanoHead results at 10 different surface positions
Force-Displacement curve (raw-data)
Oliver&Pharr analysis results:
Experimental aspects
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June 29, 2012
Results
Hardness of steel SRM 724HV1
Ac,AFM 1 AC
AC
Ac,OP
HV 725.011.4 Kpmm2 H 7.110.11 GPaStatic :
Instrumented : H 9.82 0.17 GPaPoor agreement !
Topographical pile-up analysis
Correction:
AC
AC
26%StaticIIT
Excellent agreement with static test after pile-up – correction
-V +V -Ac
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June 29, 2012
Results
Stress-strain curve
rep HB2.88
rep 0.2 aR
Tabor:
Load-multiple-partial-unload test(LMPU) with 12.5 um radius tip
Micro-structure – oriented positioning structure-sensitive mechanical response
Insights into constitutive curve
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June 29, 2012
Results
Hardness of 100 nm DLC on Steel
H Hs
H f
Hs
1
1B hd
D
Hu-Lawn:
H=(32.4+12.7) GPa
Rms>50nm
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June 29, 201221
[1] D. S. Harding, W. C. Oliver and G. M. Pharr: Mat. Res. Soc. Symp. Proc., 356, 1995, 663.[2] R. Dukino and M.V. Swain, J. Am. Ceram. Soc. 75 12, 1992, 3299
Step 2: Produce a series of indentations at different loads and surface locationsStep 3: Image indents to find critical load for fully developed set of lead corner radial cracks
AFM
Step 4: Calculate Fracture toughness :
These parameters must be known:E…Young’s modulus H…hardnessP…test loada, c, l….distances from image
Berkovich [2]:
Cube Corner [1]:
Example: Si02 K1=0.604 Mpa m1/2
Results
Fracture toughness
Step 1: Measure Hardness and Young’s modulus of the specimen
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June 29, 2012 22
ResultsSoft materials
• 9 indents on polycarbonate can be clearly distinguished from each other on an are of 5x5 um
• The required test force was under 10 uN, and all resulting Force-displacement curves are nicely repeatable
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• 16 Scratches: 1xMarker + 15 Scratches 0.1‐4mN in Fused silica• Repeatable scratch test over 40 um length• The deepest scratch is <50 nm ; the shallowest is just 1.2 nm deep
June 29, 201223
3‐D view (AFM‐topography)
Results
Nano-Scratch
Application: Sclero-hardness measurement on very thin films
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June 29, 201224
Results
Zig-Zag-Scratch
Application: Adhesion study
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Conclusions
June 29, 2012 25
• Nano-Indentation is a tool for quantitative characterization of elastic, plastic, and brittle behavior of thin films and composites
• Nano-indentation accuracy depends on a careful consideration of the most important parasitic influences
• Nano-indentation is an ideal tool to test new contact mechanical models, which will expand the field of applications for contact tests in the future.
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Introduction to PeakForce QNM Ben Ohler
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PeakForce QNM: Quantitative Nanomechanics
• PeakForce QNM is an AFM-based technique for high-resolution mapping of quantitative nanomechanical properties
• PeakForce QNM can measure: • Topography • Young’s modulus (~10 kPa - ~100 GPa) • Adhesion (pN-µN) • Deformation (nm) • Dissipation (eV)
• PeakForce QNM enables:
• High-resolution property mapping (true nanoscale resolution) • High-speed property mapping (normal AFM imaging rates) • Non-destructive operation (typically avoid plastic deformation)
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Atomic Force Microscopy
• Imaging technique based on the mechanical interactions between a very sharp microfabricated probe and sample surface.
• Probe raster scans, line by line, across the surface, tracing its topography
Different AFM modes differ in how the surface is detected:
• Contact mode • Probe drags in contact with the
surface at a constant force • TappingMode
• Probe oscillates at its resonance, intermittently contacting the surface. Amplitude held constant.
• Peak Force Tapping mode
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PeakForce QNM uses Peak Force Tapping
• Peak Force Tapping is a new Bruker AFM imaging technology • Probe is oscillated, but sub-resonance (typically 2 kHz) • Probe intermittently contacts the surface • Full force-distance interaction for each tap is measured • Feedback based on the peak force
• PeakForce QNM analyzes these force-distance interactions • Calculate modulus based on indentation models: DMT or Sneddon • Other properties measured directly from the force-distance data
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PeakForce QNM enables truly nanoscale nanomechanical measurements
• Typical instrumented nanoindenters • Indents 100’s nm deep and ~µm wide • Plastic deformation common
• Typical AFM-based nanoindentation
• Indents 10’s nm deep and 10’s or 100’s nm wide
• Plastic deformation common • Non-normal load results in asymmetric
pile-up around indents
• PeakForce QNM • Indentations several nm deep and
several nm wide • Deformation typically remains elastic • Also, mapping vs. discrete points
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0.5µm
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Why? Sharper tips and lower loads
• Typical Berkovich nanoindentation tip • Included angle 142.3° • Typical apex end radius >100nm • Loads typically in mN range
• Conventional AFM-based nanoindentation probe • Included angle 90° • Typical end radius ~40nm • Loads typically in µN range
• Typical PeakForce QNM probe • Half angle <20° • Typical end radius <10nm • Loads can vary from pN to µN range
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TappingMode Phase Imaging
• Phase contrast is proportional to energy dissipation: • Low tapping force, phase contrast dominated by adhesion • At higher tapping forces, the elasticity may dominate
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Harder tapping
Lighter tapping
Lighter tapping
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PeakForce QNM vs. Phase Imaging
• Comparison of the adhesion and phase images clearly shows that the phase contrast is primarily due to adhesion, whereas one might more commonly assume that it reflects modulus variations
• Section plot illustrates ability to measure the modulus across the polymer layers
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PeakForce QNM can quantitatively and unambiguously identify modulus and
adhesion variations. Phase imaging and multifrequency imaging techniques cannot.
PFQNM-Height
PFQNM-Adhesion
PFQNM-Modulus
Tapping-Height
Tapping-Phase
Multilayered polymer film, 10 µm scans Left: PeakForce QNM Right: TappingMode
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A typical example: polymer blend
• Many polymer materials are blends of two or more components, formulated to obtain more desirable properties than those attainable with a single component.
• PeakForce QNM can assist with identifying components in microtomed sections of the blends
• Here, a polymer blend clearly shows three components: • Two higher modulus
polymers, ~1.8 GPa and 2.6 Gpa.
• A softer, rubbery material, ~200 Mpa.
6/29/2012 9
Modulus map of a multi-component polymer blend imaged using PeakForce QNM. There are three different components clearly present, the light blue component (A), the darker blue component (B), and the red/black component (C). (7 µm scan)
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Nanoscale Tribofilm Properties
• PeakForce QNM reveals complex nanoscale structure in both topography and modulus data
• Nanoindentation measures only discreet points, averaging over a much larger volume and missing the bigger picture
6/29/2012 10
Samples: Tribofilms with wear scars PeakForce QNM modulus maps overlaid on topography, 2µm scan areas
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Nanoscale mechanical properties at polymer interphases
• Example: Three layer polymer film used for heat sealed bags • Barrier layer: provides strength and gas impermeability • Sealant layer: adheres during heat sealing • Tie layer: preserves adhesion between Barrier and Sealant layers
• What’s happening in the interphase regions? 6/29/2012 11
Barrier layer: Nylon
Tie layer: Ultra low density
polyethylene
Sealant layer: Metallocene polyethylene/
LLDPE blend (85-15)
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PeakForce QNM clearly visualizes interphase properties
• Compatibility of polymer layers has a strong influence on interphase properties
• Barrier-Tie interphase • Lamella do not cross the
interface, but grow epitaxially from the Barrier layer into the Tie layer.
• Main modulus transition occurs over ~100 nm, but the whole interphase is ~330 nm wide.
• Tie-Sealant interphase • Lamella from Tie layer act as
nucleation sites or penetrate into the Sealant layer resulting in a more ordered region ~1 µm from the interface.
• Main modulus transition occurs over ~250 nm, but the whole interphase is ~1 µm wide. 6/29/2012 12
DMTModulus DMTModulus
Tie-Sealant interphase (more compatible polymers)
Barrier-Tie interphase (less compatible polymers)
Bar
rier
Tie
Tie
Seal
ant
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PeakForce QNM combined with other techniques for a more complete picture
• SEM, EDS and PeakForce QNM can be used together to understand the composition and structure of nanocomposite materials • EDS clearly shows the carbon-rich epoxy filled voids and the
calcium-rich calcium hydroxide regions • PeakForce QNM reveals that the calcium hydroxide regions
vary from ~30 GPa to ~50 GPa
6/29/2012 13
Cement paste sample (epoxy embedded)
PeakForce QNM
SEM / EDS
AFM- Topography AFM- Modulus AFM- Adhesion
SEM EDS- Carbon EDS- Calcium Modulus overlaid on SEM
Modulus of calcium hydroxide regions
Trtik et al. Cement and Concrete Research. 42:215-21 (2012).
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Conclusions
• PeakForce QNM is a powerful technique for characterizing nanomechanical properties on a variety of sample types • Unambiguously separates modulus, adhesion, dissipation variations • Wide modulus operating range: 10 kPa – 100 GPa • True nanoscale lateral resolution: same as normal AFM • Fast and convenient: same as normal AFM
• PeakForce QNM can be complementary to nanoindentation • Enables mapping instead of measurements at discrete points • Enables investigation at length scales too small for nanoindentation
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