Topic 4: Materials

4.1 Introducing and classifying materials

Define atom, molecule, alloy and composite.

Describe a bond as a force of attraction between atoms.
Consider and differentiate between the three main types of bond: ionic, covalent and metallic.

Describe how materials are classified into groups according to similarities in their microstructures and properties.

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.

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

Physical properties
Define density, electrical resistivity, thermal conductivity, thermal expansion and hardness.

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). Pre-packaged 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
Define tensile strength, stiffness, toughness and ductility.

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
Outline the characteristics of taste, smell, appearance, texture and colour.

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

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).

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.

Outline that conifer trees are referred to as softwoods and that these grow only in temperate regions.
Recognize the characteristics of softwood trees.

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.

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.

List two examples of composite timbers.
Consider particle board (chipboard) and plywood.

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.

Outline criteria for the selection of timber for different structural and aesthetic design contexts.
Consider timber for buildings, furniture and children’s toys.

Describe the reasons for treating or finishing wood.
Consider reducing attack by organisms and chemicals, enhancing aesthetic properties and modifying other properties.

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

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.

Explain how the movement of free electrons makes metals very good electrical and thermal conductors.

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.

Draw and describe what is meant by grain size.

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.

Define plastic deformation.

Explain how metals work-harden after being plastically deformed.

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

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.

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.

List two design criteria for superalloys.
Consider creep and oxidation resistance.

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

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).

Describe secondary bonds as weak forces of attraction between molecules.

Describe the structure and bonding of a thermoplastic.
Thermoplastics are linear chain molecules with weak secondary bonds between the chains.

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.

Explain the reversible effect of temperature on a thermoplastic, with reference to orientation of the polymer chains.
Increase in temperature causes plastic deformation.

Explain how the reversible effect of temperature on a thermoplastic contributes to the ease of recycling of thermoplastics.

Draw and describe the structure and bonding of a thermoset.
Thermosets are linear chain molecules but with strong primary bonds between adjacent polymer chains. This gives thermosets a rigid 3D structure.

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

Discuss the properties and uses of polypropene and polyethene thermoplastic materials.

Discuss the properties and uses of polyurethane and urea–formaldehyde (methanal) thermoset materials.

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

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.

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.

Describe the characteristics of glass.
Consider brittleness, transparency, hardness, unreactivity and aesthetic properties.

Explain that the desired characteristics of glass can be accurately determined by altering its composition.
Consider soda glass and Pyrex.

Outline the differences between toughened and laminated glass.
Consider their responses to being flexed and to impact.

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

Describe composites.
Composites are a combination of two or more materials that are bonded together to improve their mechanical, physical, chemical or electrical properties.

Define fibre.

Describe the matrix composition of composites.

Explain that new materials can be designed by enhancing the properties of traditional materials to develop new properties in the composite material.

Describe a smart material.
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.

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.

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.

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.

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.

Outline one application of electro-rheostatic 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.

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.

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.