BMFB 3263: Materials Characterization

Dr. Mohd Warikh Bin Abd Rashid

Room: 2nd Floor, PFI, Block B

Email: warikh@utem.edu.my

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OBE – Outcome Based Education. Student-centered learning rather

than lecture based. Active Learning (AL) - Students

actively involved in the learning process. Learners activity in class.

Please read before coming to class!!!!

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Learning Outcomes

1. Explain the fundamental of materials characterization including the

theory, working principle and application.

2. Analyze the materials characterization results qualitatively and

quantitatively.

3. Summarize material characteristics based on its characterizations

results.

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Course Structure

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Course Synopsis : This course will discuss about material

characterization techniques from the theoretical aspect, instrumentations and applications.

The techniques include: Microstructural Analysis (optical microscope,

SEM, TEM and SPM) and Thermal Analysis (TGA, DTA, DSC, DTMA and

TMA). Case studies and example will be given for each

technique to show how these methods are used to characterize engineering materials.

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References: Refer to Teaching Plan Materials Characterization: Introduction To Microscopic

and Spectroscopic Methods, Yang Leng, John Wileys & Sons Microstructural Characterization of Materials by David

Brandon and Wayne D. Kaplan, John Wileys & Sons Database: www.sciencedirect.com Internet

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Topic Outcomes:

By the end of this topic, you should be: able to understand the importance of materials

characterization for materials engineers able to list down types of materials characterization know the concept of microstructure and evaluation

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Why do you think materials characterization is important for materials engineer?

In 5 minutes, discuss with 1-2 persons next to you, and write down on a piece of paper.

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Space Shuttle Columbia Disaster 2003

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The loss of the Columbia was a result of damage sustained during launch when a piece of foam insulation the size of a small briefcase broke off the Space Shuttle external tank (the main propellant tank) under the aerodynamic forces of launch. The debris struck the leading edge of the left wing, damaging the Shuttle's thermal protection system (TPS). While Columbia was still in orbit, some engineers suspected damage, but NASA managers limited the investigation on the grounds that little could be done even if problems were found

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Risk Management

NASA management failed to recognize the relevance of engineering concerns for safety failure to honour engineer requests for

imaging to inspect possible damage failure to respond to engineer requests about

status of astronaut inspection of the left wing.

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If you are given these materials, how do you inspect their properties???

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Introduction Material characterization

Physical method Mechanical tests Chemical analysis Thermal analysis Non-destructive evaluation

Physical Microstructural evaluation X-Ray Diffraction (XRD) X-Ray Fluorescence Spectroscopy (XRF) Mass Spectroscopy FTIR spectroscopy.

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Mechanical tests Tensile Compression Creep Fatigue

Chemical analysis Atomic Absorption Spectroscopy (AAS) functional group analysis.

Thermal analysis Differential thermal analysis (DTA) Differential Scanning Calorimetry (DSC ) Thermogravimetry Analysis (TGA ) Dynamic Mechanical analysis (DMA), etc.

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Non-Destructive Testing (NDT)UltrasoundRadiology liquid penetrant Eddy current, etc.

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Material Characterization Analysis depends on

application intended use.

Examples:1. Materials used for high Temperature: corrosion,

optical field, structural etc.2. Polymer : Tg point, curing T, degradation T, degree

of crystallinity. 3. Compound: melting point, phase transformation.4. Magnetic material: Curie T.5. Non-destructive Testing (NDT) – checking without

affecting usefulness. Usually inspection to finish product.

6. New materials – thorough characterization.17

Material Properties Mechanical – not a unique function of a material

but valued from test pieces e.g. response from certain mechanical loading. Tensile strength (yield & UTS), modulus, fatigue, creep.

Physical – unique properties of material. Density, thermal, electrical, magnetic and optical properties.

Thermal – thermal expansion (CTE), thermal conductivity, specific heat & thermal diffusivity.

Electrical – conductivity, thermoelectricity, charge storing capacity, dielectric loss.

Optical – refractive index, transparency, colour, etc. 18

Microstructure Characterization Visually observable – limited range of wavelength &

limited resolution. Optical microscope – 1000x magnification. TEM – wavelength of energetic electrons much lesser

than interplanar spacing in crystal potentially able to resolve crystal lattice.

SEM – usually limited by inelastic scattering under probe, & is the order of few nanometers for secondary electrons.

Resolution depends on focus of electron beam into fine probe, but beam current available decreases.

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Microstructure Characterization Achieved by allowing some form of probe to

interact carefully on prepared specimen. Probe – visible light (optical microscope), X-

ray radiation (EDX, XRD) & high energy electron beams (electron microscopy).

Resolution – ability to distinguish closely spaced features. Determined by wavelength of probe radiation, characteristic of interaction, & nature of image-forming system.

Shorter wavelength & wider acceptance angle of imaging system – better resolution.

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Microstructure Evaluation

Microstructure – identical arrangement in 3-D space of atoms & all types of non-equilibrium defects.

Very important since microstructure often affects properties. E.g different phases (diff. microstructure) in steel / iron give different properties ; pearlite, bainite & martensite.

Fracture surface, failure initiation point, defects such as pores, grain size, particle distribution & many other features can be examined.

Parameters – qualitative (shape, distribution, colour) & quantitative (grain size, % of second phase, dislocation density).

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Microstructure Evaluation

Grain size – DV, DA, DL, DASTM. Dv – average number of grains in a unit vol. DA – average number of grains intercepted per

unit area. DL – mean linear intercept. DASTM – compare

sample microstructure with ASTM Grain-size Charts.

Phase volume fraction – length of line traversing 2nd phase relative to total length. Also random grid of test points.

Optical – info obtained thru light (visible light) transmitted or reflected from matter.

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Optical microscope

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Optical Microscope Sample & sectioning – often best to have samples

from more that one orientation. E.g rolled part – sections taken perpendicular to all

3 ; rolling direction, transverse & thru-thickness. Casting – differences in cooling rate & effects of

segregation. Mounting, grinding & polishing – prepare surface

to be flat, devoid of topographical features unrelated to bulk microstructure of sample.

Polishing – mechanical, chemical & electrochemical.

Etching – selective removal of material from surface in order to develop surface features – microstructure.

Develop topography – grooving grain boundaries.24

Optical Microscope

Reflection – only surface is imaged, topology and any other features which give contrast.

Transmission – very thin specimen. In med science, bio tissues. Geology, mineral specimen thickness < 50 micron, polarized light frequently give contrast & provides info on optical properties and spatial orientation of the crystalline phases.

Metallurgical samples – reflection, Polymer – either method, Ceramic & Semiconductor – reflection.

Specimen preparation – important to have good preparation to get successful image.

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Optical Microscope

Principle components of reflection optical microscope

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Optical Microscope

3 separate system – illuminating system, specimen stage & imaging system.

Condenser lens – focus an image of the source. Condenser aperture – limits amount of light from source.

Virtual-image aperture – ensure light is not internally reflected within m/scope, leading to unwanted background intensity.

Objective lens – performance depends on its numerical aperture (NA). Not only resolution, but brightness also depends on NA.

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Optical Microscope Numerical aperture, NA – important characteristic

of objective lens system, µ sin α. Working distance of objective lens from specimen

surface decreases dramatically as NA is increased. Specially designed long-working-distance lenses

allow specimen to be imaged in hostile environment – corrosive medium, elevated or cryogenic T.

Image magnification by objective lens is insufficient to be fully resolvable by human eye – insert eyepiece, additional lens to focus on light-sensitive, photographic emulsion, or scan image in tv raster and display on monitor.

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Optical Microscope Or different height of neighbouring grain

surface. Different phases (second phases,

reinforcement, inclusion, etc). Thermal etching – usually for material which

inert to chemical attack in etching. Image contrast – developed thru proper

polishing and etching. Most metals absorb significant portion of

incident light. E.g Cu & gold absorb blue, so reflected light appear reddish or yellow.

Angle of incidence – reflected, transmitted or absorbed. 29

(i) Cu-4Ti : cast, cold worked & aged.

(ii) Cu-5Ni-2.5Ti : cast, cold worked & aged.

(iii) high N2, high Mn, austenitic stainless steel.

(i) (ii)

(iii)

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Optical Microscope Common magnification 10x –

1500x, resolution limit about 0.2 microns.

Imaging modes – transmitted & reflected light, polarized light, bright-field, dark field, differential interference contrast, and phase contrast.

In transmission mode, thickness no more than 5 microns.

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How to prepare samples for optical microscopy observation?

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How to prepare metal sample for optical microscope observation??

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Sample preparation for metalCut the sample Mounting in resin

Grinding with SiC paper

LappingPolishingEtching

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Cutting a specimen Specimen from a larger piece of material

ensure that it is representative of the features found in the larger sample

it contains all the information required to investigate a feature of interest.

Problem: preparation of the specimen could change the microstructure of the material e.g. through heating, chemical attack, or mechanical damage. The amount of damage depends on the method by which the specimen is cut and the material itself.

Cutting with abrasives → high amount of damage Cutting with low-speed diamond saw → lessen the

problems.

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Mounting Mounting of specimens:

necessary to allow them to be handled easily. Minimise the amount of damage likely to be caused to the specimen

itself. Mounting material

not influence the specimen as a result of chemical reaction or mechanical stresses.

should adhere well to the specimen if the specimen is to be electropolished later in the preparation then

the mounting material should also be electrically conducting. Hot mounting: (about 150°C) using a mounting press

either in a thermosetting plastic, e.g. phenolic resin, or a thermosoftening plastic e.g. acrylic resin.

Cold mounting: e.g. epoxy, acrylic or polyester resin. Porous materials must be impregnated by resin before

mounting or polishing to prevent grit, polishing media or etchant being trapped in the

pores, and to preserve the open structure of the material.

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A mounted specimen usually has a thickness of about half its diameter, to prevent rocking during grinding and polishing.

The edges of the mounted specimen should also be rounded to minimise the damage to grinding and polishing discs.

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Grinding Grinding- remove surface layers damaged by cutting Mounted specimens are ground with rotating discs of abrasive

paper, for example wet silicon carbide paper → COARSER to FINER.

The coarseness of the paper is indicated by a number: the number of grains of silicon carbide per square inch. So, for example, 180 grit paper is coarser than 1200.

The grinding procedure involves several stages, using a finer paper (higher number) each time. Each grinding stage removes the scratches from the previous coarser

paper. Easily achieved by orienting the specimen perpendicular to the previous

scratches. Between each grade the specimen is washed thoroughly with soapy

water to prevent contamination from coarser grit present on the specimen surface.

Typically, the finest grade of paper used is the 1200, and once the only scratches left on the specimen are from this grade, the specimen is thoroughly washed with water, followed by alcohol and then allowed to dry. The drying can be made quicker using a hot air drier.

Cleaning specimens in an ultrasonic bath can also be helpful, but is not essential.

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Copper specimen after series of grinding

180 grit 400 grit

800 grit1200 grit

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Lapping The lapping process is an alternative to grinding, in

which the abrasive particles are not firmly fixed to paper. Lapping process: applied a paste and lubricant to the

surface of a disc. Surface roughness from coarser preparation steps is

removed by the micro-impact of rolling abrasive particles.

Lapping machine

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Polishing Polishing discs are covered with soft cloth impregnated

with abrasive diamond particles and an oily lubricant. Particles of two different grades are used :

a coarser polish - typically with diamond particles 6 microns in diameter: remove the scratches produced from the finest grinding stage

finer polish – typically with diamond particles 1 micron in diameter, to produce a smooth surface. Before using a finer polishing wheel the specimen should be washed thoroughly with warm soapy water followed by alcohol to prevent contamination of the disc.

Copper specimen polished to 6 micron level

Copper specimen polished to 1 micron level. Ideally there should be no scratches after polishing, but it is often hard to completely remove them all.

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Etching Etching is used to reveal the microstructure of the metal through

selective chemical attack. In alloys with more than one phase: etching creates contrast

between different regions through differences in topography or the reflectivity of the different phases.

The rate of etching is affected by crystallographic orientation, so contrast is formed between grains, for example in pure metals.

The reagent will also preferentially etch high energy sites such as grain boundaries. This results in a surface relief that enables different crystal orientations, grain boundaries, phases and precipitates to be easily distinguished.

The specimen is etched using a reagent. For example, etching stainless steel or copper and its alloys: a saturated

aqueous solution of ferric chloride, containing a few drops of hydrochloric acid is used. This is applied using a cotton bud wiped over the surface a few of times The specimen should then immediately be washed in alcohol and dried.

metal: Nital 5-10% (nitric acid in alcohol)

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Following the etching process there may be numerous small pits present on the surface. These are etch pits caused by localised chemical attack, and in most cases they do not represent features of the microstructure. They may occur preferentially in regions of high local disorder, for example where there is a high concentration of dislocations.

If the specimen is over etched, ie. etched for too long, these pits tend to grow, and obscure the main features to be observed - as seen in the images below:

Etched copper specimen Over etched copper specimen 43

Effect of Etching

Unetched Steel200 X

Etched Steel200 X

Unetched Brass200 X

Etched Brass200 X

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Surface requirement: Flat and level. If not, then as the viewing area is

moved across the surface it will pass in and out of focus.

In addition, it will make it difficult to have the whole of the field of view in focus - while the centre is focused, the sides will be out of focus.

Use a specimen levelling press – overcome this problem

Press the mounted specimen into plasticene on a microscope slide, making it level.

Use a small piece of paper or cloth covers the surface of the specimen to avoid scratching.

Specimen levelling press

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Ceramic SamplesThin Sections

To prepare ceramic specimens, a thin slice, approximately 5 mm thick, is cut using a diamond saw or cutting wheel.

One surface is then lapped using liquid suspensions of successively finer silicon carbide powders. Between stages in the process the specimen must be thoroughly cleaned. After final washing and drying the ground surface is bonded to a microscope slide with resin.

A cut off saw is used on the exposed face to reduce the thickness to about 0.7 mm. The specimen is then lapped to take it to the required thickness – usually about 30 mm, although some ceramic specimens are thinned to as little as 10 mm, due to their finer grain size. The slide is checked for thickness under the microscope, and then hand finished.

Polished sections These differ from ordinary thin sections in that the upper surface of

the specimen is not covered with a cover slip, but is polished. Care must be taken to prevent the specimen breaking. Sections may be examined using both transmitted and reflected light microscopy, which is particularly useful if some constituents are opaque.

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Polymer SamplesThin sections Thin sections of organic polymers are prepared from solid material by cutting slices

using a microtome. They must be cut at a temperature below the glass transition temperature of the polymer.

A cut section curls up during cutting and must be unrolled and mounted on a microscope slide and covered with a cover slip. A few drops of mounting adhesive wet the specimen and must be compatible with it. The mounting temperature must not affect the microstructure of the specimen.

The thickness of cut slices of polymer tends to lie in the range 2 to 30 mm depending on the type of material.

Harder polymers can be prepared in the same way as thin ceramic specimens.

Polished sections These are prepared in the same way as metallographic specimens. Elastomers are more difficult to polish than thermosetting polymers and require

longer polishing times. Lubricants used during polishing must not be absorbed by the specimen.

As crystalline regions are attacked more slowly than amorphous ones, etching of polymer specimens can produce contrast revealing the polymer structure.

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