Experimental Aspects of Nanoindentation

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Experimental Aspects of Nanoindentation

• I. Why, What and How

Krystyn J. Van Vliet 3.052 Spring 2003 February
11, 2003
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Why nanoindentation?
Definition Mechanical probing of a material
surface to nm-scale depths, while
simultaneously monitoring LOAD and DEPTH.

1. Materials engineering Optimizing material
   composition, structure, processing for particular
   applications (bulk and thin films)
2. Mechanics of small volumes Understanding whether
   thin films, lines, dots have different mechanical
   properties than bulk counterparts
3. Material physics Measuring deformation processes
   such as dislocation nucleation, crack growth, etc.

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Why nanoindentation?

• 1. Materials engineering

P
10 g
Johannes Brinell (1880s)
stress s P/A strain e Lf-Li/Li
P mg
H P/pD2/4 Units of stress, but NOT
stress, NOT a physical measure Indicates
resistance to plastic deformation
A
L
sy
n
D
E s/e
Uniaxial tension/ compression
Hardness testing
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Why nanoindentation?

• 1. Materials engineering

• Indent to several depths to build stress-strain
  relation
• a/R
• s a P/pa2

R
Multiaxial, NOT uniaxial stress state Requires
plastic deformation to see indentation
a
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Why nanoindentation?

• 1. Materials engineering

Microelectronic thin films (early
1980s) Mechanical properties important
electromigration Questions Is E a function
of tf? Is sy a function of tf? Is n a
function of tf? Use nanoindentation to measure
thin film props Consider sharp indenters to
mitigate substrate effects
tf
10 nm 1 mm
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Why nanoindentation?

• 2. Unique mechanics of small volumes

Are mechanical properties affected by
constraint? Are E, sy, n f(size)? When
structure dimensions approach atomic
dimensions, is mechanical behavior quantized?
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Why nanoindentation?

• 3. Material physics

Ceramics Fracture initiation
Metals Dislocation nucleation, material flow
By probing materials near atomic and molecular
dimensions, can we learn about fundamental
deformation mechanisms? Polymers Entropic
elasticity Biologicals Ligand/receptor
binding Progress due to rapid increases in
microelectronics (data acquisition, signal
processing) Utility due to rapid progress in
computational modeling (same time/length scales)
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What is nanoindentation?
P
h
Show P-h response at loading/unloading. Discuss
hysteresis and what it means. Introduce concepts
of indirect measurement Actually measuring
voltages Emphasize that this is a material
signature, but gives no real property values
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What is nanoindentation?
Materials engineering goal Interpret P-h data in
terms of continuum scale properties That is, test
a small material volume to obtain parameters for
a large material volume Translate a complicated,
multiaxial stress state into uniaxial properties
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What is nanoindentation?
dP/dh
Show variables in response C loading
curvature dP/dh unloading stiffness Pmax,
hmax Wp plastic work of indentation Wt total
work of indentation
C
We
Wp
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What is nanoindentation?
35 mN
Load
2 mN
SiC (ceramic)
100
500 nm
16 nN
500
PE (polymer)
100 nm
Depth
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How does nanoindentation work?
A. Material assumptions
Continuum No structural length scales e.g. grain
size, film thickness Homogeneous One
phase Elastoplastic Deforms via plastic
yielding, rather than fracture, phase
transformation
B. Indenter considerations
Ei gtgt Es Ideally, deformation occurs only in
sample material Primarily diamond (hardest
natural material)
Spherical Elastic, THEN plastic Difficult
to machine diamond
Sharp Immediately elastic AND plastic Can
facet diamond into pyramids, not cones
Vickers
Berkovich
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How does nanoindentation work?
C. Instrumentation
NanoIndenter MTS, TN
Triboindenter Hysitron, MN
NanoTest MicroMaterials, UK
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How does nanoindentation work?
C. Instrumentation

-
3-plate capacitor P, h via same device
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How does nanoindentation work?
Capacitance of Parallel Plates
Electric field between two parallel plates E
V/d
Voltage difference between two plates as q moves
from positive to negative plate
Capacitance is the amount of charge Q stored per
unit voltage V applied to plates

• Relevance to nanoindentation
• As plates move closer, d decreases and V, C
  increase
• Temperature/noise insensitive displacement meter

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How does nanoindentation work?
Inductance of a solenoid
Inductance is the change in B(V) created by a
change in I
For fixed A, a change in current I leads to
Magnetic field of a solenoid
Thus
Relevance to nanoindentation (1) Can be used
to exact motion (change in I) (2) Can be used
to measure load (change in I or V)
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How does nanoindentation work?
D. Calibration
i. P(V)
Add mass to load train Determine I, V to move
mass (LOAD) Calculate relationship P
aV Determine zero point P aV b
ii. h(V)
Move indenter fixed, known distance Measure
corresponding C, V Calculate relationship h
aV Determine zero point h aV b
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How does nanoindentation work?
1.
D. Calibration
iii. Cm
Machine Compliance
Definition Displacement of the instrument (load
train) in response to applied load
Importance Need to subtract this response
Ctotal Csample Cm
2.
Approach (1.) Nanoindent standard sample
Determine Cm empirically to get
correct E, H (2.) Compress machine with flat
Measure Cm directly
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How does nanoindentation work?
E. Environmental control
Humidity control (lt 55)
Acoustic isolation (lt 1 Hz)
Thermal control (lt 0.5 C)
Vibration isolation (lt 1 Hz)
Hysitron
MicroMaterials
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How does nanoindentation work?
light microscope
transducer
Procedure 1. Prepare sample surface Flat,
Parallel faces, Smooth 2. Find region of
interest on surface 3. Program load
profile 4. Program spatial matrix 5.
Execute experiment 6. Analyze data to obtain
properties
P
t
controlled positioning stage
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How does nanoindentation work?
Results in bulk materials
E 3.5 GPa
Benzocyclobutene (BCB) VanLandingham, NIST
Results in composites
2. Trabecular bone
1. Multiphase alloys/Copolymers
E GPa
Smith, MicroMaterials
Wyrobek, Hysitron
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How does nanoindentation work?

• Results in thin films
• Substrate affects indentation response
• Complicated relationship due to n
• Empirical rule hmax lt 10 tf for continuum
  analysis to hold

Ni coating on Pb-Sn solder tf 5 mm
coating substrate
coating
decreasing hardness
constant hardness
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How does nanoindentation work?

• Results in thin films
• Substrate affects indentation response
• Complicated relationship due to n
• Empirical rule hmax lt 10 tf for continuum
  analysis to hold

FILM
BULK
Load mN
Depth nm