Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Examination of a Manufactured Article Examination of a Manufactured Article (all content)

Examination of a Manufactured Article (all content)

Note: DoITPoMS Teaching and Learning Packages are intended to be used interactively at a computer! This print-friendly version of the TLP is provided for convenience, but does not display all the content of the TLP. For example, any video clips and answers to questions are missing. The formatting (page breaks, etc) of the printed version is unpredictable and highly dependent on your browser.

Contents

Aims

On completion of this TLP you should:

  • be able to identify the materials used in common objects using suitable techniques;
  • be able to identify the processes used to produce a particular shape or microstructure within components of an article;
  • be able to understand why materials and processes might be chosen to produce a given article.

Before you start

This TLP brings together considerations from a range of other topics, and so many other TLPs might be helpful in understanding the properties and techniques referred to.  There will be links to the relevant TLPs at appropriate points.

Introduction

Most manufactured items contain a large number of individual components, often using a surprisingly wide variety of materials.  Examining a manufactured item (article) can help to understand what materials are commonly used in manufacturing and why. 

It is important to keep in mind economic factors as well as the properties of materials and the specific purpose of each component.  This means it can be quite a complex procedure to identify, with confidence, the materials within an article.  It is usually done by considering a range of all appropriate factors and making use of some of a broad range of identification techniques.

This TLP suggests some factors that may be considered in attempting to identify the materials used in various articles.  It will also give some examples of where these factors may be important.

The other DoITPoMS TLPs provide a wide variety of information about the functional and mechanical behaviour of materials, as well as a range of information on techniques for examining materials.  This TLP will not aim to describe all of these, but will link to them.

Dismantling the Article

The first step to determining the materials and reasons for their use in an article is to take it apart.  This is an important stage because it is a chance to see where components fit, and what their purposes might be.

An exploded diagram of components and where they fit in relation to the rest of the article is often extremely helpful.  Once the article has been dismantled it may not be possible to reassemble it, and knowing the position of a component in the article is vital to understanding what its function might be.  The exploded diagram helps to sort out what the purpose of components are, and therefore what might be required of their properties.

The diagram should be clear and well labelled, including scale bars. 

Important properties to consider for each component could include:

  • Mechanical properties: such as strength or toughness
  • Electrical properties: conducting or insulating
  • Aesthetic properties: is the component on display, and would the appearance be important?
  • Corrosion resistance: the component will probably need to last for at least the lifetime of the product
  • Density of component: it may be important that the article is lightweight
  • Specific functions: some materials are chosen because they fulfil a specific function within the article (for example piezoelectric or ferromagnetic materials)

It is also important to consider economic factors, such as the costs of the raw materials and processing.  The cost of a material can influence choice as much as its physical properties; for example diamond is an extremely hard material, but it is not widely used due to cost.

Each component will probably be reasonably easily identifiable as belonging to one of four classes of materials:

  • Metals
  • Polymers
  • Ceramics
  • Composites

Classes of Materials

It can often be quite straightforward to tell materials classes apart by look or feel.  Metals are usually more reflective or 'metallic' looking, ceramics are commonly matte and polymers may be shiny or matte, but are typically less dense than either metals or ceramics.  Composites may be harder to immediately identify, but the surface may appear non-uniform and/or sectioning the sample may reveal fibres or particles.

It is useful to note that taking a cross-section can often be helpful in identifying materials used in components as the internal material and/or microstructure may differ from that at the edge.

Some materials may not be quite so easily identified simply from appearance and texture, but considering the factors mentioned can help to narrow down the possibilities.  The table below gives a rough guideline to the kinds of properties you would expect from each class, and a few examples:

Class Common Properties Examples
Metal Hard, ductile and conduct heat and electricity Copper (wires), stainless steel (cutlery)
Polymer Widely variable, often soft and flexible Polystyrene (cups), polycarbonate (CDs), polyethylene (plastic bags)
Ceramic Hard, brittle, resistant to corrosion, electrically non-conductive Concrete (buildings), PZT (piezoelectric used in lighters and ultrasonic transducers), porcelain (vases, teacups)

The tree-diagram below shows an overview of a variety of materials that might be encountered:

Materials tree diagram

Coatings

Many components may have some kind of coating; a covering of another material designed to improve the surface qualities of the item.  The improvement could be for many reasons including: corrosion resistance, appearance, adhesion, wear resistance and scratch resistance.

Different kinds of coating will have different processing methods.  It is often possible to deduce the method from the composition (of the coating and the bulk of the component) and the shape of the component. 

Common coating methods include:

Hot dip coating – a method used for coating metals (commonly ferrous alloys) with a low melting point alloy.  The component is dipped in a bath of the molten coating alloy. For example zinc is often hot dipped onto steel (called ‘galvanising’).  This also offers sacrificial corrosion protection and gives a distinctive ‘spangled’ appearance (which can be prevented by including particles to encourage nucleation in the electrolyte).

Image of surface of steel coated with zinc

Steel coated with zinc

Electroplating – reduction of cations in an electrolytic solution onto conducting components. For example silver plated cutlery.

Anodising – commonly used for aluminium components, an electrochemical cell is set up which drives the oxidation of the metal, increasing the thickness of the protective oxide layer. 

Vacuum deposition – also known as PVD – physical vapour deposition.  For example, ‘evaporation’ involves the heating of the coating metal in a vacuum, so that it evaporates and is deposited onto the surface of the component that is positioned above.  This process is used to make mirrors, depositing a thin layer of metal, usually aluminium.

Thermal Spraying - powder particles are fed into a high temperature torch (combustion or plasma), where they melt and are accelerated against the substrate. It is mainly used to produce relatively thick ceramic and metallic layers.

Enamelling - a powder is distributed on a surface, which is then heated so that the powder melts and bonds to the substrate. The resultant layer is usually glassy. Originally developed in ancient Egypt, and extensively used for jewelry, it is also widely employed for cooking utensils and various domestic items, especially those subjected to high temperature.

One example of a coated metal is shown below; a drawing pin, which appears to be brass, was found to be magnetic and so the surface was abraded, revealing a grey metal within – steel.  Steel, which has very good mechanical properties, is covered with brass for aesthetic reasons and also protects the surface from corrosion.

Magnetic drawing pin Abraded pin revealing steel

Metals

Metals are extremely widely used in manufacturing, often for mechanical or electrical properties. They are often easily shaped and have good mechanical properties so they may be used in ‘structural’ elements of an article, and they also have good conduction (electrical and thermal) and so may also be used, for example, in electrical wiring. Familiar metals can often be reasonably well identified by eye (see examples below), but there are more complex methods of metal identification available too.

Image of copper and brass

Techniques for the Identification of Metals

One very easy test for a metal is to see if the component is magnetic, this narrows down the possible materials to those that are ferromagnetic (most commonly iron or nickel).

Simple corrosion tests involving immersing a scratched sample (to remove any coating) in water (or some other electrolyte) can be helpful. Leaving a sample in water overnight might reveal rusting. For example a scratched zinc coated steel sample would not rust due to the zinc offering sacrificial protection. However, a scratched tin coated steel sample would rust, because the tin is supposed to act as a barrier between the steel and air.This is especially useful for ferrous alloys as corrosion resistance is very often a concern for these (and they are very common).

Optical Microscopy

This involves looking at mounted, polished and etched samples under a light microscope. It reveals the microstructure of the sample; this can give information on both the composition and processing of the component.

See Optical Microscopy and Specimen Preparation TLP for more information on how to go about this.

Image showing Al-Cu Eutectic composition Image of cold rolled zinc showing deformation twins
Al-Cu Eutectic composition
This is an Al-Cu alloy showing a very clear eutectic lamellar microstructure.
(See micrograph library entry for more information)
Cold rolled zinc showing deformation twins
This is zinc, it has been cold rolled as can be seen from lenticular deformation twins

See the DoITPoMS Micrograph Library for further examples.

The benefits of this method are that optical micrographs can reveal a large amount of information about a metallographic sample, and it is possible to find known examples (see above links) to compare your work to. After an initial examination by eye and consideration of properties, optical microscopy is an important step in the characterisation of metals. It can reveal many things that the initial examination does not.

Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy uses a focussed beam of high-energy electrons to form images of samples. Electron Microscopy is not limited by the wavelength of light, so very closely spaced features can be resolved, so this method gives very clear high magnification images when set up correctly. It also gives a large depth of field, so rough surfaces can still be in focus.

The SEM can give very good high magnification images, again revealing more than optical microscopy could. One limitation is that the sample must be electrically conducting; the mounting polymer and the sample must both conduct electricity.

Energy Dispersive X-ray Spectroscopy (EDS)

This is a technique often used in conjunction with the SEM, with an electron beam of ~20 keV. The beam strikes the sample resulting in X-rays being emitted; the X-rays are collected and the intensities and energies examined. The results can determine the atomic composition of the sample at the point of beam-sample interaction.

Examples: It can reveal the composition of a very thin coating; for example by taking a linescan. The diagram below shows results for a linescan taken across a coating, showing a layer of copper and a finer layer of nickel on an iron alloy (all other elements were ignored). This may not be easy to tell from optical microscopy alone.

Linescan across a nickel/copper coating on iron alloy

EDS can be a very helpful method of characterisation, but it is not always absolutely reliable. Elements of low atomic number (less than about 11, i.e. below sodium) are difficult to detect by EDS. There is often contamination of elements like carbon from the environment. It is important to use common sense when interpreting the EDS results; most of the time is it unlikely that very heavy elements like uranium are actually present. Results may be very good qualitatively, but care must be taken when trying to obtain and interpret quantitative results. The quantitative analysis results depend upon things like the set-up of the SEM and the geometry of the sample.

Examples of Fabrication Processes

The ease with which metals are shaped leads to a wide range of processing techniques for different end products; any coatings will also be applied by one from a range of processes. It may be possible to deduce the method of processing from the shape and the properties of the metal. The microstructure may also give further clues:

Process Description Features
Deformation Includes a variety of techniques including forging, extruding and drawing, see the TLP Introduction to Deformation Processes for more information May expect to see directionality or squashed grains in places that have been stressed (see the animation)
Machining Includes, for example, laser cutting and water-jet cutting as well as more conventional methods like sawing or grinding. These methods would not result in larger scale microstructural directionality, but may show localised deformation.
Can give a very good finish
Casting The molten metal is set in a cast of some kind, see the Casting TLP for further details on the different kinds of casting. There is a lot of variety of microstructure from cast products, which ranges from single crystal components to those with clear chill, columnar and equiaxed zones.
Carburisation – a surface heat treatment A surface treatment in which carbon is diffused into the surface of a steel object above the ferrite-austenite transition temperature (~ 740 °C). This is done by heating the steel in a C-rich atmosphere, (e.g. in CO gas). The result is a hard, high carbon surface several hundreds of microns thick, surrounding a tough, low carbon interior. To improve the hardness, the surface may be quenched, which helps the production of martensite. See micrograph below

Micrograph number 271 from micrograph library

Micrograph #271
An example of carburisation; see the description in the micrograph library for further information.

Polymers

Polymers are very widely used in many areas today. They have a range of properties that can often be controlled by additives, blending or copolymerisation. Many structures and chemical compositions are seen in polymers, but they can be separated into three main groups:

Thermoplastics: These are the most widely used polymers due to the ease of processing (especially for injection molding). Thermoplastics can, once they have been set (solidified) for the first time, be re-melted and remoulded (unlike thermosets). Some examples of thermoplastics are: polyethylene, polystyrene and PET.

Thermosets: These differ from thermoplastics in that they do not re-melt after they have been set (or cured). This is due to the long polymer chains forming cross links on curing. One example is melamine formaldehyde, which is used in domestic electrical plugs.

Elastomers: These polymers have a glass transition temperature below room temperature (see The Glass Transition in Polymers TLP). Rubbers are examples of commonly used elastomers (for more information see the Stiffness of Rubber TLP)

Techniques for identifying polymers

Polymer tests

The polymer tests are a simple way to identify polymers, or at least to narrow down the possibilities. Some of the steps rely on recognising smells, which can be difficult, and it is important to remember that some tests (for example transparency) can be unhelpful due to additives like dyes.

The polymer identification chart goes through a series of simple tests, which should be carried out on a small sample of the polymer. Below is an interactive version of the identification chart. It is important to connect the results of the test to the function and cost of the item.

Infra Red (IR)

In an IR spectrometer IR radiation excites covalent bonds, causing them to vibrate at their resonant frequency. This frequency depends on the exact nature of the bonds (e.g. single/double and the atomic masses of the elements involved). The output is a graph of intensities at different wavelengths (and therefore energies) of infrared radiation. This plot shows the transmitted intensities, so at resonant frequencies, where the energy is absorbed, there is a peak. This allows the bonds to be identified and therefore the sample identified.

Here is a collection of IR spectra for some common polymers:

IR spectrometry is often a very quick method of polymer identification (depending on the equipment available). Preparing a sample for IR spectroscopy may be very simple. A small sample of the polymer with any coatings removed should be placed in the IR machine, and analysed. If it is likely that the plastic contains plasticizers and colours, placing the polymer in ether for an hour and then fully drying it may remove them prior to carrying out IR spectroscopy on the sample. (Test this with a small piece of your sample polymer first though, as some polymers are soluble in ether).

Differential Scanning Calorimetry (DSC)

DSC measures specific heat capacity and how it varies with temperature. As a polymer is heated through its glass transition point, it experiences a sudden change in heat capacity, as chain rotation allows it to take up more energy. This means that DSC allows us to identify the glass transition temperature of a polymer. It can also aid the interpretation of the type of a copolymer (e.g. block copolymer, random copolymer, graft copolymer).

See Macrogalleria’s page on DSC for an explanation of this technique.

Examples of processes

Polymers are usually processed by moulding methods:

Process

Description

Features

Injection moulding

Polymer granules are melted and forced into a mould. This is extremely widely used to mass produce small, precise polymer components.

It gives a good finish and the injection points where excess material has been cut off are often visible. In transparent polymers a residual stress field may be visible under crossed polars (see the Introduction to Photoelasticity TLP).

Blow moulding

Cylinders of polymer are inserted into a die and hot air is forced in, pushing the polymer out to the walls of the die.

Gives hollow components, such as bottles or containers. It is only used for thermoplastics.

For further examples see plastics processes on Plastipedia.

Additives and Blends:

Polymers very often have some form of additive, even if it is simply to add colour. These may or may not impair the ability to identify the polymer. When identifying any material it is important to think about the cost and properties, but blending or additives can change the properties of a polymer.

One very common example of a polymer commonly found both with and without additives is polyvinylchloride (PVC). This polymer is used in its rigid, un-plasticized form in plastic guttering and water and gas piping, but is also often found with added plasticizers in a variety of applications from clothing to coating electrical wires.

Ceramics and Composites

Ceramics

Ceramics cover a very wide range of materials from structural materials like concrete to technical ceramics like PZT – a piezoelectric.  Usually they are defined as solids with a mixture of metallic or semi-metallic and non-metallic elements (often, although not always, oxygen), that are quite hard, non-conducting and corrosion-resistant.

Techniques for identifying ceramics

It is effectively impossible to identify ceramics by eye. Optical microscopy will allows the examination of the microstructure to identify the method of processing, however, it does not allow the identification of different phases.

The most useful technique for finding the composition of a ceramic is energy dispersive x-ray spectroscopy (EDS).  Note that for non-conducting ceramics the surface of the sample must be covered with a metallic coating (often gold) to prevent charge build-up.

Here is an example EDS for PZT – a piezoelectric ceramic: Pb[ZrxTi1-x]O3, this data gives the formula to be: Pb0.7[Zr0.49Ti0.44]O3.  For the piezoelectric ceramic we would expect to have x ~ 0.52.

EDS data for PZT
Element

Weight%

Atomic%

O K 17.26 63.49
Ti K 7.58 9.32
Zr L 16.15 10.42
Pb M 59.01 16.77

Totals

100.00

100.00

Another appropriate method is X-ray diffraction. This allows you to detect the phase or phases present as well as measuring lattice parameter(s) in order to specify precise compositions.

Processing techniques for ceramics

Ceramics are mostly made by powder processing techniques, for example sintering. It may be possible to identify the kind of processing from directionality or porosity in the sample.

Composites

Composites are often used in applications that require specific ‘conflicting’ properties such as a high strength and high toughness. The properties may be conflicting because having a high yield stress sometimes relies on trapping and tangling dislocations, but these reduce the ductility and toughness of the material.  Composites often consist of a matrix and fibres or particles that affect the properties (see the TLP on the Mechanics of Fibre-Reinforced Composites).

Usually for composites, once they have been identified as such, it is better to treat each part of the composite as a separate material, and then subsequently look at costs of manufacture and processing.

One important distinction to make is the structure of the two parts that make up the composite – i.e. is it a matrix with long, aligned fibres? Or a matrix with particles? etc

Example Article

The best way to understand the concepts in this TLP is to try analysing something. Here is a 'virtual' article, which can be clicked through:

Summary

Many articles can be analysed using the relatively simple techniques described here.  This can help with determining the types of material used for different components, their composition, and also processing history.  The examination of an article can start to put into use the methods and theory of materials science.  Looking at the mechanical, thermal and aesthetic properties of materials can help materials scientists design similar items.

The range of techniques available today is very large, but often a reasonable amount of understanding can be gained from fairly simple techniques and using some common sense.  It is essential not to forget the importance of stepping back from results and considering whether or not they are logical; do they fulfil the requirements in terms of mechanical, thermal, aesthetic and economic properties?

Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

  1. Which of these methods of characterisation would be helpful in identifying a ceramic?

    a Infra red spectroscopy
    b Differential Scanning Calorimetry
    c Energy Dispersive X-ray Spectroscopy
    d Scanning Electron Microscopy

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

  1. Match the material to the properties, try to think of reasons why the material may have been chosen.
    Material 1 has a shiny 'spangled' appearance due to large dendritic grains on the surface, it is a structural component within the article and must withstand relatively high stresses, and may experience wear in service, it must not corrode in warm air and is in a low cost article.

    a Tinned Steel
    b Stainless Steel
    c Aluminium
    d Galvanised Steel

  2. Match the material to the properties, try to think of reasons why the material may have been chosen.
    Material 2 is a brightly coloured, low-density component, it must be tough, rigid and non-toxic, the recycling mark is number 7.

    a Polyethylene (PE)
    b Acrylonitrile-butadiene-styrene (ABS)
    c Polyethylene terephthalate (PET)
    d Polytetrafluoroethylene (PTFE)

  3. Match the material to the properties, try to think of reasons why the material may have been chosen.
    Material 3 is an electrical component, it forms a contact that is subjected to a reasonable amount of wear, it must not corrode in air.

    a Low carbon steel
    b Copper
    c Brass: Copper/Zinc alloy
    d Stainless steel

Going further

Websites

Material Selection and Processing
Information on selection, design and processing
Identifying Metals
Some simple ways to identify metals
How to Identify Plastics
Plastic Materials Identification Chart
Some simple ways to identify polymers
Introduction to Deformation Processes
This TLP covers the fundamentals of metal forming processes
Differential Scanning Calorimetry
Macrogalleria's explanation of using DSC for sudying thermal transitions of polymers
Plastipedia
"Probably" the Web's largest plastics encyclopedia, including plastics processes
MetPrep Support
Metallography advice, including sample preparation methods and choice of standard etchants

 

Academic consultant: Zoe Barber, John Durrell and Noel Rutter (University of Cambridge)
Content development: Catriona Yeoh
Photography and video: Brian Barber
Web development: Lianne Sallows and David Brook

This DoITPoMS TLP was funded by the UK Centre for Materials Education and the Department of Materials Science and Metallurgy, University of Cambridge