MICROSCOPY OF COMPOSITES - welshcomposites.co.uk Webinar.pdf · Optical Microscopy With composites,...

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MICROSCOPY OF

COMPOSITESWCC WEBINAR

16th January 2011

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AIMS OF WEBINAR

To give an overview of the mostimportant microscopy methods forcomposite materials

To cover issues of sample preparationspecific to composites

Target audience is those who are:

• Familiar with composite materials but not with microscopy

• Familiar with microscopy methods, but not with composites.

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What do we use microscopy for?

Determine the size, distribution and orientation of fibres or particles

Identify and characterise any defects (voids, debonds, fibre bending)

Fractography, to determine the mode(s) of failure in:

• Composite as a whole

• Matrix

• Reinforcement

To gain information on the degree of bonding between matrix & reinforcement, or between layers

Chemical analysis and distribution

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Outline of Webinar

Optical methods• Reflected light microscopy

• Contrast modes

• Sample preparation

• Image interpretation

Digital optical microscopy

• Overview of methods

• Advantages and examples

Confocal microscopy

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Outline of Webinar

Scanning Electron Microscopy• Overview of methods

• Sample preparation

• Problems with insulating materials

• Fractography

Transmission Electron Microscopy

• Method, sample preparation and examples

Atomic Force Microscopy

• Method and examples

Compositional Microscopy• EDX mapping

• FTIR microscopy

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Optical Microscopy

With composites, main method isreflected light mode

Optical microscopy has resolutionlimited by wavelength of light (to abouta micron)

Depth of field is also limited, especiallyas magnification increases

Mainly used with flat polished sections

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Contrast in Optical Microscopy

Generally, reinforcement and matrixhave different reflectivities

May get some scattering / refraction atboundaries

Rarely need to etch samples

Most commonly use bright fieldcontrast, but can use dark field, phasecontrast or polarised light

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Bright Field Imaging

Direct reflection (or transmission)

Any absorption / scattering leads todarker regions

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Dark Field Imaging

Scattered reflection (or transmission)

Any absorption / scattering leads tolighter regions

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Phase Contrast Imaging

Phase differences on reflection ortransmission are imaged

Good for small height differences orwhere materials have similar reflectivity

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Polarised Light Imaging

Rotation of polarised light by differentphases is imaged

Good for optically active components

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Preparation for Optical Microscopy

Need flat polished surface

Soft matrix / hard reinforcements

Protection of edges can be done withsupport pieces of same material, insample holder (good with automaticpolishing)

More commonly a mounting resin isused

This also helps keep damaged / partiallybroken samples intact

Need low temp curing resin in manycases

Low viscosity resin is good to fill cracks12

Courtesy of Struers



Epoxy mounting resins have less shrinkage but are generally more viscous than polyesters and acrylics.

Grinding and polishing fairly simple with SiC papers then diamond polishing and finally alumina slurry

Important with slurry not to overload cloth – we need a sliding cutting action, rather than a rolling action.

Water is fine as lubricant in most cases. Parafin lubricants should be treated with caution with polymer-matrix composites, especially thermoplastics

13



Biocomposites with hydrophillic cellulosefibres present more of a challenge

Final polishing should be gentle to avoidfibre / particle pull-out and excessivedamage to reinforcement edges.

14

www.struers.com/resources/elements/12/ 2434/38art4.pdf


Applications of reflected light microscopy

Fibre orientation and lay-up structure

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Inter- and Intra- laminar cracking

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Inter- and Intra- laminar cracking

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Fibre distribution

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Fibre distribution with phase contrast

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Fibre distribution

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Digital Optical Microscopy

Software used to overcome some of thelimitations of optical microscopy

Images displayed on large monitor

Image stitching

Camera shake correction

Depth of field can be increased

• Multiple images taken at different focal depths

• Software used to combine parts of each image that are in focus

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Digital Optical Microscopy

Software used to overcome some of thelimitations of optical microscopy

Images displayed on large monitor

Image stitching

Camera shake correction

Depth of field can be increased

• Multiple images taken at different focal depths

• Software used to combine parts of each image that are in focus

Can be used to generate heightinformation

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Confocal Microscopy

Optical microscopy method

Allows both surface and subsurfaceimaging

Needs partially transparent material

Allows 3D structure to be built up

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Surface image Sub-surface image (10 micron deep)

3D fibre arrangement

C. Eberhardt, A. Clarke / Composites Science and

Technology 61 (2001) 1389–1400


Scanning Electron Microscopy

Higher magnifications possible

Much greater depth of field

Can also give compositional analysis(EDX)

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Backscattered electrons

Secondary electrons

Focussed electron beam

Sample



Secondary Electrons are low energy, socan be attracted to an oblique detector.

Gives best topographic contrast.

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Detector

Large signal – appears

bright

+ve



Secondary Electrons are low energy, socan be attracted to an oblique detector.

Gives best topographic contrast.

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Detector

Small signal – appears

dark+ve


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Back Scattered Electrons are higherenergy, so the detector needs to beabove sample.

Gives little topographic contrast.

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Detector


Secondary electrons – oblique detector

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Secondary electrons – vertical detector

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Back-scattered electrons

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Sample preparation

Sample preparation for SEM can beminimal

Fracture surfaces may need cutting tosize

Composites often have complex failurewith many cracks and no single fracturesurface.

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Beam Damage

Polymer-based materials are susceptibleto damage from electron beam

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Sample Charging

All insulating samples can be prone tospecimen charging

Build-up of negative charge on sampledeflects incoming beam or gives highersignal

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Sample Charging

Worse with glass-fibre composites

Not so bad with carbon fibres and nano-tubes

Isolated fibres more prone to charging

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Sample Charging – how to overcome

Coat samples with thin layer of gold,aluminium or carbon, by evaporation orsputtering

Slows sample preparation

Increases cost

Reduces resolution at very highmagnifications

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Use “Controlled Pressure” mode

SEM operated under partial vacuum

Negative charge removed by collisionswith inert gas atoms

Can only image back-scatteredelectrons

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Use lower accelerating voltage orcurrent

Resolution can be reduced somewhat

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Fractography

Interpretation of fracture surfaces

Identification of failure modes

Determination of the order of differentfailure modes

Quality of bonding between matrix andreinforcement

Strength of interlaminar bonds

For more details see: “Failure Analysis and Fractography of Polymer Composites”, E. Greenhalgh, MJ Hiley & CB Meeks

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Flexural failure of carbon-fibre / epoxy composite

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Compressive failure

Tensilefailure


Compressive failure surface shows bands of buckling failure

Good bonding between fibres and resin49


Bending failure of each individual fibre

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Tensile surface51


Good fibre / matrix adhesion

Tensile fracture surface of each fibre52


Fracture can be followed from initiation point on edge of fibre

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Transmission Electron Microscopy

Uses a beam of electrons transmitted

through an ultra-thin sample – typically

100nm

Image is a 2D projection of the sample

Advantages:

• X500,000 magnification

• Very high resolution - down to 0.1nm enabling study at

atomic level

Disadvantages:

• 3D information only available through repeated images

at different tilt angles

• Difficult sample preparation

• Beam damage


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TEM Sample Preparation Issues with Composites

Microtomy – difficult due to large

difference in mechanical properties

between phases.

Solution casting – almost impossible with

all but nano-composites.

Chemical etching & Ion beam milling –

difficult due to different material

components.

Good for individual nano-fibre imaging


56Bower et al, Appl Phys Lett. 74 (22) 1999

TEM images of carbon nanotubes


57

Qian et al, Applied Physics Letters, Volume 76, Number 20, 2000

Nanotube / PS composite


X

Y

ZPIEZO

SCANNER

SAMPLE

CANTILEVER

PSPD

DETECTOR

LIGHT

MICROSCOPELASER

DIODE

FEEDBACK

LOOP

SEM image of a high aspect ratio

tip



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Chemical Mapping - EDX

X-rays produced by electron beam inelectron microscope

Energy of X-rays depend on elementswith which they interact

X-rays come from an “interactionvolume”, typically a few microns in size

Limits spatial resolution of image

Good method to chemically identifyphases seen with other imaging, but nota good imaging method on its own

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SEM Image of MMC

EDX Map (blue Si; red = Al)


Chemical Mapping – FTIR Mapping

Infra-red spectroscopy allows identification of organic chemical bonds

Mapping allows polymer composition to be mapped

Resolution of about 1 – 2 microns

Quite slow technique

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Toughened thermosetmatrix (image is 100

microns across)


CONCLUSIONS

Wide range of microscopy methods forobservation of structure and fracturebehaviour

Optical microscopy and SEM dominate

New advances in digital microscopy,AFM, mapping and easier samplepreparation are providing improvements

Interpretation of images and avoidanceof artefacts are still important.

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MICROSCOPY OF COMPOSITES - welshcomposites.co.uk Webinar.pdf · Optical Microscopy With composites,...

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