Nano- and Micromechanicsmrl.illinois.edu/sites/default/files/pdfs/Nano- and...Microindenter Tips...

© 2015 University of Illinois Board of Trustees. All rights reserved.

Nano- and Micromechanics

Kathy Walsh, Ph.D.

Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign

How does stuff respond when you poke it, squeeze it, or stretch it?

Sample


Sample

The Right Tool for the Job

• Sensitive enough to measure the sample

– Appropriate force resolution

• Spatial resolution on an interesting scale

– Lateral resolution: 10s of nm, 100s of µm, mm

– Displacement: nm, µm, mm

Choose the Right Technique for Your Sample


Small-Scale Mechanical Testing

• Nanoscale – AFM

– Nanoindenter

• Microscale – Nanoindenter

– Microindenter

• Milliscale – Rheometer (twisting)

– DMA (stretching, compressing)

DMA

Nanoindenter

AFM

Small-scale Mechanical Testing


Why Measure Nano- or Micromechanical Properties?

• Mechanical properties help define materials

– Optimize applications

– Flexibility, biomechanical compatibility

– Crack formation, wear resistance

• Local composition variations in samples

– Spatially-resolved mechanical testing

• Samples may be inherently small

– Thin films, MEMS devices, nanopillars


What Mechanical Properties Do People Measure?

• Quasistatic

– Elastic modulus

– Hardness

• Dynamic

– Time-dependent (viscoelastic) properties

– Storage modulus, loss modulus, tan delta


What Mechanical Properties Do People Measure?

• Quasistatic

– Stress vs. strain curves

– Load (force) vs. displacement curves

• Dynamic

– Properties as a function of time or frequency

– Creep or stress relaxation

Load

Displacement


How do People Measure Mechanical Properties?

• Quasistatic

– AFM force curves

– Nanoindentation, microindentation

– Stress vs. strain curves

• Dynamic

– AFM dynamic measurements

– nanoDMA, Modulus Mapping

– Dynamic Mechanical Analysis

nan

oscale m

icroscale m

illiscale


How do People Measure Mechanical Properties?

• Quasistatic

– Nanoindentation, microindentation

• Elastic (Young’s) modulus

– Related to sample stiffness

• Hardness

– Related to amount of plastic deformation


Micro- vs. Nanoindentation

• (Instrumented) microindentation is sometimes more useful

– Indents to greater depths

– Cares less about • Surface roughness

• Surface forces (adhesion)

Micro- vs. Nanoindentation


Nanoindenter

Microindenter

Optical microscope (indent positioning)

Instrumented Indentation

• Different names, same technique

– Nanoindentation

• Indentation depths shallower than a few µm

• Microindentation if deeper (some instruments)

– Instrumented Indentation

– Depth-Sensing Indentation

Record applied load and indent depth

Poke a sample and record its response


Nanoindenter Basic Parts

Tip Transducer


Nanoindenter Basic Parts

Tip Transducer

Stiff frame

Nanopositioning


Why Does the Instrument Frame Stiffness Matter?

Tip

Stiff frame © 2015 University of Illinois Board of Trustees. All rights reserved.

It’s Basically About the Tip and Sample

Tip Transducer

Stiff frame

Nanopositioning


Nanoindenter Tips

• Tips are made of diamond or sapphire

– Tip characteristics are well-known

– Tip compliance is negligible

• Variety of shapes for different applications

– Induce different deformation mechanisms

– Berkovich, Vickers, cube corner

– Flat punch, conospherical (bending, soft materials)


Nanoindenter Tips

• Nanoindentation

– Up to a few µm deep

– Up to several mN

• Most popular tip shape for nanoindentation:

– Berkovich 3-sided pyramid

Nanoindentation residual imprint Berkovich tip on aluminum foil (atomic force microscopy image)


Microindenter Tips

• Microindentation

– Many µm deep

– Up to several N

• Most popular tip shape for microindentation:

– Vickers 4-sided pyramid

– Soft materials: sphere

Microindentation residual imprints Vickers tip on steel (bright field and dark field optical microscopy images)


Contact Area

• Contact area between tip and sample

– Very, very important (crucial calibration)

– How much of your tip is applying force on how much of your sample?

– Depends on depth indented into sample

– Depends on roughness


Vickers indent on steel (AFM image)

Contact Area

• To ensure well-defined contact area between tip and sample…

• make your sample as smooth as possible

– Polishing

– But beware of work hardening


vs.

smooth rough

The 5% Rule

• Contact area between tip and sample

• Sample roughness should be ≤ 5% of indent depth…

• … indent 20x deeper than surface roughness

• Can get tricky for thin samples because of the 10% rule


This indent is not deep enough to get good data

The 10% Rule

• The substrate effect

• Indent depth should be ≤ 10% of sample thickness

• Indent too deep, start measuring the substrate


feels like foam feels stiffer

vs.

sample

indent here

substrate

The 5% and 10% Rules

• 5% rule and 10% rule are just rules of thumb… the actual values are sample-dependent

– Compliant sample on stiff substrate: can probably go deeper than 10%

– Stiff sample on compliant substrate: probably see substrate effect at depths shallower than 10%


Performing an Indent Th

is is actually a n

on

-instru

me

nted

micro

ind

enter

can hold to do a creep/stress relaxation test


Example Nanoindentation Data (Quartz)

Load P (few µN to few mN)

Displacement h (few tens of nm to few µm)

“Load—displacement curve”




loading curve

unloading curve

(optional) hold segment

to measure creep




unloading curve

fit for analysis


Getting “the Answer”

Reduced modulus Hardness


(Actually, , but just look up “fundamental equation of nanoindentation.”)

Contact area: area of tip in contact with sample at a given depth

Why Does Contact Area Matter, Again?

Reduced modulus

Hardness


Elastic from Reduced Modulus

Elastic (Young’s) modulus

Already known (diamond tips)

Poisson’s ratio of the sample

Measure using nanoindentation


Many people just quote this value

(Quasi)static vs. Dynamic Testing

• Many materials are somewhat viscoelastic

– Time-dependent mechanical behavior

• Creep or stress relaxation

– Hold a constant load or displacement for a long time

– Beware of drift

• Dynamic testing

creep

relaxation

creep and stress relaxation tests on nitrile glove

(Quasi)static vs. Dynamic Testing


Nanomechanical Properties as a Function of Depth

hardness

and

(reduced) modulus


indentation depth (tens of nm to few µm typical)

5% rule

surface effects and calibrations matter more for shallower indents

Nanomechanical Properties as a Function of Depth

hardness

and

(reduced) modulus


indentation depth (tens of nm to few µm typical)

10% rule

may start to measure substrate properties

Nanoindentation Gives Nanomechanical Properties as a Function of Depth and Location

hardness

and

(reduced) modulus


These indents were done at different places on the sample as well as at different depths

Nanoindenters Are (Usually) Not Made for Soft Materials

• Nanoindenters are usually made for the “engineering materials” community

– Metals, composites, non-porous materials

• Using a traditional nanoindenter to study soft, compliant, porous, or sticky materials

– Usually doesn’t go deep enough

– Usually doesn’t have awesome enough force resolution

Measurements are Harder on Soft Materials


~MPa comfortable modulus for nanoindentation (try AFM-based techniques for more compliant samples)

Practical Concerns for Biomaterials

• Fixative changes mechanical properties!

– Fixative makes things stiffer

– If that’s the only way you can study your sample…

• Do a comparative study (no absolute numbers)

• Make control samples with just the fixative

• May need a heating cell/stage to stay at biorelevant temperatures

• May need to work in fluid

Practical Considerations for Biomaterials


Keeping the “Hydro” in Hydrogel

• Keep wet samples wet

– Drying and rewetting can change properties

– Samples may dry out during measurements

• Don’t get fluid (or vapors) into the instrument

• Petri dishes, closed fluid cells, special tips

Keeping the “Hydro” in Hydrogel

standard

fluid-compatible

the tip is at the end


Sample Preparation

• Know what your surface looks like first

– Look at it under an optical microscope

– Check sample roughness

• Mounting the sample

– Can’t measure mechanical properties of something that’s floating

– Compliant glue affects results

Sample Preparation


Sample Mounting

• Glue (usually superglue)

– Porous samples may get partially filled with glue

– Some glue is designed for biological use

• Alternative sample mounting strategies

– Cast gels directly onto substrate

• Glass slide, Petri dish

– Wrap or clamp samples

• Don’t stress the area you want to measure

Sample Mounting


How to Approach Your Data

• Oliver—Pharr model

– Elastoplastic materials

• But your samples may be…

– Sticky, compliant

– Poroelastic

– Viscoelastic

– Poroviscoelastic

– Thin films

Try continuous stiffness measurements


JKR model

Do dynamic testing, creep/relaxation tests

Most people start here

TA Instruments Q800 DMA clamps: dual cantilever, tension

Nano/micromechanical Testing Facilities

Asylum Cypher 5µm z range

30µm x 30µm scan size

Asylum MFP-3D-SA (x2) 15µm z range

90µm x 90µm scan size

Hysitron TI-950 TriboIndenter transducers: standard, nanoDMA,

high load (2.8N), AE, nanoECR

Nano/micromechanical Testing Facilities at MRL

DMA (milliscale) AFM (nanoscale)

Nanoindentation and Microindentation (nano/microscale)


Recommended Reading

• Nanoindentation (3rd ed., 2011) – Anthony C. Fischer-Cripps

– Classic text on nanoindentation

• Handbook of Nanoindentation with Biological Applications – Michelle L. Oyen

– Soft materials people: read this one

• Both books are available for free online through the U of I library

Useful Books about Nanoindentation


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