experimental aspects of nanoindentation i. why, what and how krystyn j. van vliet 3.052 spring 2003...

Experimental Experimental Aspects of Aspects of

NanoindentationNanoindentation

I. Why, What and HowI. Why, What and How

Krystyn J. Van Vliet3.052 Spring 2003February 11, 2003

Why nanoindentation?Why nanoindentation?

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.

Definition: Mechanical probing of a material surface to nm-scale depths, while simultaneously monitoring LOAD and DEPTH.

Why nanoindentation?Why nanoindentation?1. Materials engineering1. Materials engineering

Uniaxial tension/Uniaxial tension/

compressioncompression

10 g

Hardness testingHardness testing

P

LA

stress = = P/Astrain = = [Lf-Li]/Li

P = mgH = P/[D2/4]Units of stress, butNOT stress,NOT a physical measureIndicates resistance toplastic deformation D

Johannes Brinell (1880s)

y

E = /

n

Why nanoindentation?Why nanoindentation?1. Materials engineering1. Materials engineering

Indent to several depths to build stress-strain relation: = a/R P/a2

Multiaxial, NOT uniaxial stress stateRequires plastic deformation to see indentation

R

a

Why nanoindentation?Why nanoindentation?1. Materials engineering1. Materials engineering

Microelectronic thin films (early 1980s)Mechanical properties important: electromigrationQuestions: Is E a function of tf? Is y a function of tf? Is n a function of tf?Use nanoindentation to measure thin film propsConsider sharp indenters to mitigate substrate effects

tf10 nm – 1 m

Why nanoindentation?Why nanoindentation?2. Unique mechanics of small volumes2. Unique mechanics of small volumes

Are mechanical properties affected by

constraint?

Are E, y, n f(size)?

When structure dimensions approach

atomic dimensions, is

mechanical behavior quantized?

Why nanoindentation?Why nanoindentation?

By probing materials near atomic and molecular dimensions, can we learn about fundamental deformation mechanisms?

Polymers: Entropic elasticityBiologicals: 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)

3. Material physics3. Material physics

Ceramics: Fracture initiationMetals: Dislocation nucleation, material flow

What is What is nanoindentation?nanoindentation?

Show P-h response at loading/unloading. Discuss hysteresis and what it means.Introduce concepts of indirect measurement: Actually measuring voltagesEmphasize that this is a material signature, but gives no real property values

P

h

What is What is nanoindentation?nanoindentation?

Materials engineering goal: Interpret P-h data in terms of continuum scale propertiesThat is, test a small material volume to obtain parameters for a large material volumeTranslate a complicated, multiaxial stress state into uniaxial properties

What is What is nanoindentation?nanoindentation?

Show variables in response:C = loading curvaturedP/dh = unloading stiffnessPmax, hmax

Wp = plastic work of indentationWt = total work of indentation

C

dP/dh

WpWe

SiC (ceramic)

500

35 mN

16 nN

100 nmDepth

500 nm100

2 mN

What is What is nanoindentation?nanoindentation?

Load

PE (polymer)

How does How does nanoindentation work?nanoindentation work?

A. Material assumptions

Continuum: No structural length scales e.g. grain size, film thicknessHomogeneous: One phaseElastoplastic: Deforms via plastic yielding, rather than fracture, phase transformation

Spherical: * Elastic, THEN plastic * Difficult to machine diamond

Ei >> Es: Ideally, deformation occurs only in sample materialPrimarily diamond (hardest natural material)

Sharp: * Immediately elastic AND plastic * Can facet diamond into pyramids, not cones

Vickers Berkovich

B. Indenter considerations

How does How does nanoindentation work?nanoindentation work?

C. Instrumentation

TriboindenterHysitron, MN

NanoIndenterMTS, TN

NanoTestMicroMaterials, UK

How does How does nanoindentation work?nanoindentation work?

C. Instrumentation

3-plate capacitorP, h via same device

+

-

Capacitance of Parallel Plates:

How does How does nanoindentation work?nanoindentation work?

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:(1)As plates move closer, d decreases and V, C increase(2)Temperature/noise insensitive displacement meter

Inductance of a solenoid

How does How does nanoindentation work?nanoindentation work?

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)

For fixed A, a change in current I leads to:

Thus

Inductance is the

change in B(V)

created by a change

in I

Magnetic field of a solenoid:

How does How does nanoindentation work?nanoindentation work?

D. Calibration

i. P(V)

ii. h(V)

Add mass to load trainDetermine I, V to move mass (LOAD)Calculate relationship: P = VDetermine zero point: P = V +

Move indenter fixed, known distanceMeasure corresponding C, V Calculate relationship: h = VDetermine zero point: h = V +

How does How does nanoindentation work?nanoindentation work?

D. Calibration

iii. Cm= Machine Compliance

Definition: Displacement of the instrument (load train) in response to applied load

1.

2.

Importance: Need to subtract this response: Ctotal = Csample + Cm

Approach:(1.) Nanoindent standard sample

Determine Cm empirically to get “correct” E, H(2.) Compress machine with flat Measure Cm directly

How does How does nanoindentation work?nanoindentation work?

E. Environmental control

Acoustic isolation (< 1 Hz)

Thermal control (< 0.5 C)

Humidity control (< 55%)

Vibration isolation (< 1 Hz)

Hysitron MicroMaterials

How does How does nanoindentation work?nanoindentation work?

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 experiment6. Analyze data to obtain properties

light microscopetransducer

controlled positioning stageP

t

How does How does nanoindentation work?nanoindentation work?Results in bulk materials

E [GPa]

Results in composites

1. Multiphase alloys/Copolymers 2. Trabecular bone

Benzocyclobutene (BCB) [VanLandingham, NIST]

E = 3.5 GPa

[Smith, MicroMaterials] [Wyrobek, Hysitron]

How does How does nanoindentation work?nanoindentation work?

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

constanthardness

decreasinghardness

coating coating + substrate

Ni coating on Pb-Sn solder: tf = 5 m

How does How does nanoindentation work?nanoindentation work?

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

0

1000

2000

3000

4000

5000

6000

7000

8000

0 0.1 0.2 0.3 0.4 0.5

Load [

mN

]

Depth [nm]

BULK

FILM

hf

hs

h