Topic 4

Topic 4

Materials Total Study Time (17 hours)

4.1 Introducing and classifying materials 1 hour

4.1.1 Define atom, molecule, alloy and composite.

4.1.2 Describe a bond as a force of attraction between atoms.

Consider and differentiate between the three main types of bond: ionic,

covalent and metallic.

4.1.3 Describe how materials are classified into groups according to

similarities in

their microstructures and properties.

4.1.4 Explain that several classifications are recognized but that no single

classification is “perfect”.

It is convenient to be able to classify materials into categories (albeit crude in nature) that

have characteristic combinations of properties

4.1.5 Describe that, for this course, materials are classified into groups:

timber, metals, plastics, ceramics, food and composites; and that some

of these groups have subdivisions.

In each group there can be subdivisions, for example, for timber (natural wood and

man-made), metals (ferrous and non-ferrous), plastics (thermoplastics, thermosets),

ceramics (earthenware, porcelain, stoneware, glass), textile

fibres (natural or synthetic), food (vegetable or animal origin) and composites (difficult to

classify due to variability and continual development of

new composite materials). Food is included here for completeness, although it is dealt with in

detail as an option.

4.2 Properties of materials 3hours

Physical properties

4.2.1 Define density, electrical resistivity, thermal conductivity, thermal

expansion and hardness.4.2.2 Explain a design context where each of the properties in 4.2.1 is an

important consideration.

Density is important in relation to product weight and size (for example, for portability).

Prepackaged food is sold by weight or volume, and a particular consistency is required.

Electrical resistivity is particularly important in

selecting materials as conductors or insulators. Thermal conductivity is important for objects

that will be heated or must conduct or insulate against heat. Thermal expansion (expansivity)

is important where two dissimilar materials are joined. These may then experience large

temperature changes while staying joined.

Hardness is important where resistance to penetration or scratching is required. Ceramic floor

tiles are extremely hard and resistant to scratching. Mechanical properties

Mechanical properties

4.2.3 Define tensile strength, stiffness, toughness and ductility.

4.2.4 Explain a design context where each of the properties in 4.2.3 is an

important consideration.

Tensile strength is important in selecting materials for ropes and cables, for example, for an

elevator. Stiffness is important when maintaining shape is crucial to performance, for

example, an aircraft wing. Toughness is important where abrasion and cutting may take

place. Ductility is important when metals are extruded (not to be confused with malleability,

the ability to be shaped plastically).

Aesthetic characteristics

4.2.5 Outline the characteristics of taste, smell, appearance, texture and

colour.

4.2.6 Explain a design context where each of the characteristics in 4.2.5 is

an important consideration.

Some of these properties are only relevant to food, while others can be applied to more than

one material group. Although these properties activate people’s senses, responses to them

vary from one individual to another, and they are difficult to quantify scientifically, unlike the

other properties.4.3 Timber 3hours

4.3.1 Describe the structure of natural timber.

Natural timber is a natural composite material comprising cellulose fibres in a lignin matrix. The

tensile strength of timber is greater along the grain (fibre) than across the grain (matrix).

4.3.2 Outline that timber can be classified according to the conditions

needed for tree growth.

Consider temperate and tropical conditions. A general knowledge of the geographical

distribution of world timber resources is required.

4.3.3 Outline that conifer trees are referred to as softwoods and that these

grow only in temperate regions.

Recognize the characteristics of softwood trees.

4.3.4 Outline that deciduous trees are referred to as hardwoods and that

these grow in both temperate and tropical regions.

Recognize the characteristics of hardwood trees.

4.3.5 Discuss the issues relating to the consideration of timber as a

renewable resource.

Consider time to reach maturity, soil erosion, greenhouse effect and extinction of species.

The issues should be placed in local, national and international contexts.

4.3.6 List two examples of composite timbers.

Consider particle board (chipboard) and plywood.

4.3.7 Compare the characteristics of particle board, laminated woods

(for example, plywood), pine wood (a softwood) and mahogany (a

hardwood).

Consider composition, hardness, tensile strength, resistance to damp environments, longevity

and the aesthetic properties of grain, colour and texture. The ability to produce sketches

showing cross-sectional views of the structure of the

materials is expected.4.3.8 Outline criteria for the selection of timber for different structural and

aesthetic design contexts.

Consider timber for buildings, furniture and children’s toys.

4.3.9 Describe the reasons for treating or finishing wood.

Consider reducing attack by organisms and chemicals, enhancing aesthetic properties and

modifying other properties.

4.3.10 Explain three differences in the selection of timbers for flooring

if it were made of a hardwood, a softwood or a composite material.

Consider durability, ease of maintenance and

aesthetics.

4.4 Metals 3hours

4.4.1 Draw and describe a metallic bond.

Metals are often described as positively charged nuclei in a sea of electrons. The outer

electrons of the metal atom nuclei are free and can flow through the crystalline structure.

The bonding is caused by attraction between the positively charged metallic atom nuclei and

the negatively charged cloud of free electrons. Specific arrangements of metal atoms are not

required.

4.4.2 Explain how the movement of free electrons makes metals very good

electrical and thermal conductors.

4.4.3 State that metals (pure or alloyed) exist as crystals.

Crystals are regular arrangements of particles (atoms, ions or molecules). Details of types of

crystals are not required.

4.4.4 Draw and describe what is meant by grain size.

4.4.5 Explain how grain size can be controlled and modified by the rate of

cooling of the molten metal, or by heat treatment after solidification.

Reheating a solid metal or alloy allows material to diffuse between neighbouring grains and

the grain structure to change. Slow cooling allows larger grains to form; rapid cooling

produces smaller grains. Directional properties in the structure may be achieved by

selectively cooling one area of the solid.4.4.6 Define plastic deformation.

4.4.7 Explain how metals work-harden after being plastically deformed.

4.4.8 Describe how the tensile strength of a metal is increased by alloying.

4.4.9 Explain the effect of alloying on malleability and ductility.

The presence of “foreign” atoms in the crystalline structure of the metal interferes with the

movement of atoms in the structure during plastic deformation.

4.4.10 Describe a superalloy.

The strength of most metals decreases as the temperature is increased. Superalloys are

metallic alloys that can be used at high temperatures, often in excess of 0.7 of their absolute

melting temperature.

4.4.11 List two design criteria for superalloys. 1 Consider creep and

oxidation resistance.

4.4.12 Identify applications for superalloys.

Superalloys can be based on iron, cobalt or nickel. Nickel-based superalloys are particularly

resistant to temperature and are appropriate materials for use in aircraft engines and other

applications that require high performance at high temperatures, for example, rocket engines,

chemical plants.

4.5 Plastics 3hours

4.5.1 Describe a covalent bond.

In a covalent bond the outer electrons of some atoms come close enough to overlap and are

shared between the nuclei, forming a covalent bond. Each pair of electrons is called a

covalent bond. Mention of sigma (σ), pi (π), double or triple

bonds is not required. Covalent bonds are strong bonds and examples of primary bonds (as

are metallic and ionic bonds).

4.5.2 Describe secondary bonds as weak forces of attraction between

molecules.

4.5.3 Describe the structure and bonding of a thermoplastic.Thermoplastics are linear chain molecules with

weak secondary bonds between the chains.

4.5.4 Describe the effect of load on a thermoplastic with reference to

orientation of the polymer chains.

Deformation occurs in two ways:

• elastic, in which initially coiled chains are stretched and the material returns to its original

size and shape when the load is removed.

• plastic, when at higher loads the secondary bonds between the chains weaken and allow

the molecular chains to slide over each other, and the material does not return to its original

size and shape when the load is removed.Creep and flow are important. No quantitative

details are required.

4.5.5 Explain the reversible effect of temperature on a thermoplastic, with

reference to orientation of the polymer chains.

Increase in temperature causes plastic deformation.

4.5.6 Explain how the reversible effect of temperature on a thermoplastic

contributes to the ease of recycling of thermoplastics.

4.5.7 Draw and describe the structure and bonding of a thermoset.

Thermosets are linear chain molecules with strong primary bonds between adjacent polymer

chains. This gives thermosets a rigid 3D structure.

4.5.8 Explain the non-reversible effect of temperature on a thermoset.

4.5.9 Discuss the properties and uses of polypropene and polyethene

thermoplastic materials.

4.5.10 Discuss the properties and uses of polyurethane and

urea–formaldehyde

(methanal) thermoset materials.

4.5.11 Discuss the issues associated with the disposal of plastics, for

example, polyvinyl chloride (PVC).Although PVC disposal is problematic, PVC is still

widely used as a structural material, for example, in

windows and for guttering and drainpipes.

4.6 Ceramics 2hours

4.6.1 Describe the composition of glass.

Glass is composed primarily of silicon dioxide together with some sodium oxide and

calcium oxide and small quantities of a few other chemicals.

4.6.2 Explain that glass is produced from sand, limestone and sodium

carbonate, and requires large quantities of energy for its manufacture.

Scrap glass is added to new raw materials to make the process more

economical.

4.6.3 Describe the characteristics of glass. 2 Consider brittleness,

transparency, hardness, unreactivity and aesthetic properties.

4.6.4 Explain that the desired characteristics of glass can be accurately

determined by altering its composition.

Consider soda glass and Pyrex®.

4.6.5 Outline the differences between toughened and laminated glass.

Consider their responses to being flexed and to impact.

4.6.6 Explain why glass is increasingly used as a structural material.

Consider the use of plate glass and glass bricks as wall and flooring materials. Consider

material properties, for example, resistance to tensile and compressive forces, thermal

conductivity and transparency. Consider aesthetic properties and psychological benefits:

allows natural light into buildings and can visually link spaces, creating more interesting

interiors.4.7 Composites 2hours

4.7.1 Describe composites.

Composites are a combination of two or more materials that are bonded together to improve

their mechanical, physical, chemical or electrical properties.

4.7.2 Define fibre.

4.7.3 Describe the matrix composition of composites.

4.7.4 Explain that new materials can be designed by enhancing the

properties of traditional materials to develop new properties in the

composite material.

4.7.5 Describe a smart material.

Asses Smart materials have one or more properties that can be dramatically altered, for

example, viscosity, volume, conductivity. The property that can be altered influences the

application of the smart material.Syllabus details

4.7.6 Identify a range of smart materials.

Smart materials include piezoelectric materials, magneto-rheostatic materials,

electro-rheostatic materials, and shape memory alloys. Some everyday items are already

incorporating smart materials (coffee pots, cars, the International Space Station,

eye-glasses), and the number of applications for them is growing steadily.

4.7.7 Describe a piezoelectric material.

When a piezoelectric material is deformed, it gives off a small electrical discharge. When an

electric current is passed through it, it increases in size (up to a 4% change in volume). They

are widely used as sensors in different environments. Specific details of crystalline structure

are not required.

4.7.8 Outline one application of piezoelectric materials.

Piezoelectric materials can be used to measure the force of an impact, for example, in the

airbag sensor on a car. The material senses the force of an impact on the car and sends an

electric charge to activate the airbag.4.7.9 Describe electro-rheostatic and magneto-rheostatic materials.

Electro-rheostatic (ER) and magneto-rheostatic (MR) materials are fluids that can undergo

dramatic changes in their viscosity. They can change from a thick fluid to a solid in a fraction

of a second when exposed to a magnetic (for MR materials) or electric (for ER materials)

field, and the effect is reversed when the field is removed.

4.7.10 Outline one application of electrorheostatic materials and one

application of magneto-rheostatic materials.

MR fluids are being developed for use in car shock absorbers, damping washing machine

vibration, prosthetic limbs, exercise equipment, and surface polishing of machine parts.

ER fluids have mainly been developed for use in clutches and valves, as well as engine

mounts designed to reduce noise and vibration in vehicles.

4.7.11 Describe shape memory alloys (SMAs).

SMAs are metals that exhibit pseudo-elasticity and shape memory effect due to

rearrangement of the molecules in the material. Pseudo-elasticity occurs without a change in

temperature. The load on the SMA causes molecular rearrangement, which reverses when

the load is decreased and the material springs back to its original shape. The shape memory

effect allows severe deformation of a material, which can then be returned to its original

shape by heating it.

4.7.12 Identify applications of SMAs.

Applications for pseudo-elasticity include eye-glasses

frames, medical tools and antennas for mobile

phones. One application of shape memory effect is

for robotic limbs (hands, arms and legs). It is difficult

to replicate even simple movements of the human

body, for example, the gripping force required to

handle different objects (eggs, pens, tools). SMAs

are strong and compact and can be used to create

smooth lifelike movements. Computer control

of timing and size of an electric current running

through the SMA can control the movement of an

artificial joint. Other design challenges for artificial

joints include development of computer software to

control artificial muscle systems, being able to create

large enough movements and replicating the speed

and accuracy of human reflexes