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
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Preparation for Optical Microscopy
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
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Preparation for Optical Microscopy
Biocomposites with hydrophillic cellulosefibres present more of a challenge
Final polishing should be gentle to avoidfibre / particle pull-out and excessivedamage to reinforcement edges.
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www.struers.com/resources/elements/12/ 2434/38art4.pdf
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Applications of reflected light microscopy
Fibre orientation and lay-up structure
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Applications of reflected light microscopy
Fibre orientation and lay-up structure
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Applications of reflected light microscopy
Fibre orientation and lay-up structure
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Applications of reflected light microscopy
Inter- and Intra- laminar cracking
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Applications of reflected light microscopy
Inter- and Intra- laminar cracking
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Applications of reflected light microscopy
Fibre distribution
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Applications of reflected light microscopy
Fibre distribution with phase contrast
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Applications of reflected light microscopy
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
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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
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Scanning Electron Microscopy
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
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Scanning Electron Microscopy
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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Scanning Electron Microscopy
Back Scattered Electrons are higherenergy, so the detector needs to beabove sample.
Gives little topographic contrast.
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Detector
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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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Sample Charging – how to overcome
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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Sample Charging – how to overcome
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
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Compressive failure surface shows bands of buckling failure
Good bonding between fibres and resin49
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Bending failure of each individual fibre
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Tensile surface51
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Good fibre / matrix adhesion
Tensile fracture surface of each fibre52
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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
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56Bower et al, Appl Phys Lett. 74 (22) 1999
TEM images of carbon nanotubes
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Qian et al, Applied Physics Letters, Volume 76, Number 20, 2000
Nanotube / PS composite
Atomic Force Microscopy
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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Atomic Force Microscopy
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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)
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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)
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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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