DT Section 2: Introduction to Materials and Systems

What determines what a material is useful for?

Its properties

Working properties of a material

1. Strength

2. Hardness

3. Toughness

4. Elasticity

5. Malleability

6. Ductility

Strength

Ability to withstand forces without breaking

• Rope in tug-of-war is resists pulling forces

Hardness

Ability to withstand scratching, abrasion or denting

• A very hard metal is likely to crack upon impact or force

Toughness

When a material is hard to break (it may change shape)

• The ability to absorb the force of an impact

• A tough material may scratch easily on the surface but can withstand large impacts

Elasticity

Materials that can stretch and bend and then return to their original shape

Malleability

Materials that can be bent and shaped

• Metals can be hammered into thin sheet without breaking

• Can be rolled or pressed into sheets easily but will deform if compressed

Ductility

Can be drawn out into a wire

Physical properties of materials

1. Electrical conductivity

2. Thermal conductivity

3. Fusibility

4. Density

5. Absorbency

Electrical conductors

Let electricity travel through them easily

Thermal conductors

Let heat travel through them easily

Examples of electrical insulators and conductors and thermal conductors

Fusibility

The ability of a material to be heated and joined to another material when cooled

What do materials with a high fusibility have?

Low melting points

• Only a small amount of heating is required to convert these materials to liquids

Density

A measure of its mass per unit volume

• kg/m^3

Absorbency

The more absorbent fibres and fabrics are, the better they are at soaking up moisture

Properties of absorbent materials

• Good at soaking up moisture

• Can be dyed easily

• Dry slower

• Vulnerable to stains

Examples of absorbent and non-absorbent materials

Absorbant:

• Natural fibres (wool, cotton, cellulose fibres)

Not absorbant:

• Synthetic fibres (polyester and lycra)

Metals vs Non-metals physical properties

Alloy

A mixture of 2 or more elements one of which must be a metal

• Stronger than elements in their pure form

Timber

Sawn chunks of solid wood that are used as building material

Types of wood

• Expensive hardwoods

• Cheaper softwoods

Softwood vs hardwood

Pine (softwood)

1. Yellow with brown streaks

2. Lightweight

3. Strong and cheap

4. Knotty (hard to work with)

5. Used for telegraph poles, fences and cheap furniture

Larch (softwood)

1. Attractive yellow to reddish-brown colour

2. Harder, tougher and more durable than most softwoods

3. Resistant to rot

4. Used for decking, cladding the outsides of buildings, fence posts

Spruce (softwood)

1. Reddish brown colour

2. Hard and good strength to weight ratio

3. Knotty and not durable

4. Used for structural purposes- aircraft, crates, ship masts

Oak (hardwood)

1. Light brown and attractive grain markings

2. Tough, durable, very strong

3. Finishes well

4. Corrodes steel screws and fittings

5. Used for interior panelling, flooring, furniture

Mahogany (hardwood)

1. Reddish brown colour→ beauty and warmth

2. Durable and easy to work with

3. Expensive

4. Used for good quality furniture, musical instruments, boats and interior panelling

5. But increased desire for mahogany has increased the destructive and illegal logging trade

Beech (hardwood)

1. Pinkish-brown

2. Hard enough to resist being dented, but can be bent with steam

3. Used for chairs and toys

Balsa (hardwood)

1. White or tan colour

2. Very low density and lightweight

3. Very soft → Easy to cut and shape

4. High strength to weight ratio

5. 3+4 → good for modelling

6. Balsa trees are very fast growing

   • Gives the wood a coarse, open grain

7. The living tree has large cells that fill with water

   • Gives the wood its spongy texture

8. Used for lightweight, rigid structures:

   • Prototypes, model bridges and model aircraft

Ash (hardwood)

1. Pale cream colour and attractive

2. Tough

3. Used for tool handles, wooden sports equipment and furniture

Ferrous metals

Metals or alloys made up of iron (ferrite)

• So they are magnetic

• Can corrode/rust

Properties and uses of ferrous metals

What is applied to prevent ferrous metals from rusting

Protective coatings like paint and enamel

• Except steel (resistant to rust so doesn’t need to be coated)

Non-ferrous metals

Metals and alloys that don’t contain iron

• They don’t rust

• Useful if they are exposed to moisture

Properties and uses of non-ferrous metals

Properties and uses of alloys

Polymers

• Made from synthetic materials

  • Derived from crude oil or other finite resources, such in as coal or natural gas

What are plastics made up of

Polymers

Renewable and sustainable materials used to make bio-plastics

Vegetable starches

2 types of plastic

1. Thermoforming

2. Thermosetting

Thermoforming plastics (thermoplastics)

• Doesn’t resist heat well

  • When heated becomes soft and flexible

• Easily formed into different shapes (bendy)

  • By heating, melting and remoulding

• Easy to recycle

  • Ground down, melted, re-used and remoulded

• Plastic memory

  • Each time the plastic is reheated it will try to return to its original shape

Properties and uses of thermoforming plastics

Thermosetting plastics (thermosets)

• Resist heat and fire

  • Have strong chemical bonds between the molecules, which do not separate on heating

  • Used for electrical fittings and pan handles

• Undergo a chemical change when heated and moulded to make a product

  • They are permanently hard and rigid

• Non-recyclable

  • Can’t be melted and reshaped again

  • Plastic cannot be reformed once set in to shape

Properties and uses of thermosetting plastics

Molecular structure of thermosets vs thermoplastics

Pros and cons of thermoplastics

Pros and cons of thermosets

Modern materials

Materials that have been recently developed with a specific application in mind

• Can be developed by inventing new or improved manufacturing processes

• Or by altering an existing material to perform a particular function

Graphene

1. Single, thin layer of graphite

2. Light, strong, great conductor of heat and electricity

3. Since 2004, manufacturing processes have been invented and improved to make production cheaper

4. Used in tennis rackets

5. Future applications: aerospace, vehicles, water purification, flexible electronics

Metal Foams

1. A metal that contains many gas-filled spaces

   • This makes them lightweight

   • The foam keeps some of the metals properties: stiff, tough, strong under compression

2. Developed in the 1940s

3. Used for lightweight car parts and bone implants

Titanium

1. Corrosion-resistant metal and high strength-to-weight ratio

2. Used in aerospace industry

3. Difficult and expensive to machine (drill, cut, polish)

4. Recent developments in CAD/CAM reduce machine costs and make it a more usable material

   • Bone replacements, dental implants, bikes, ships, armour

Liquid Crystal Displays LCDs

1. Used in flat-screen displays that are thin, lightweight and energy efficient

2. LC in the display are made of a mixture of chemicals

   • When an electric current is applied, the crystals shape is modified

   • This changes the image seen on the screen

3. Originally used in calculators and other small displays were black and grey

4. Developed to become high-definition, full-colour displays used in TV and computer screens

Coated metals

1. Coating metals with a different material alters their properties

2. Iron and steel can be galvanised/electroplated (coated with zinc to prevent rusting)

3. Nickel-plated steel is a cheaper, corrosion-resistant alternative to stainless steel in car parts

4. Anodised aluminium has a coating of aluminium oxide to make the surface harder and more corrosion-resistant

5. Metals can be coated with PVC used for roofing. It becomes corrosion-resistant and can be coloured

Nanomaterials

1. Made of tiny particles

2. Carbon nanotubes are tiny carbon cylinders used in electronics and tennis racquets

   • High strength to weight ratio and are good conductors of heat and electricity

   • Can be added to a material to strengthen it without adding much weight

3. Self-cleaning fabrics have a nanoparticle coating

   • Removes odour and stains upon exposure to light

4. Antibacterial fabrics use nanoparticles of silver to kill bacteria

   • Medical uses in face masks and dressings

   • Used in anti-bacterial toys and odour free socks

Smart materials

Change their properties as they respond to an external stimulus

• They usually return to their original state when the stimuli is taken away- change is reversible

Examples of external stimuli

1. Temperature

2. Light

3. Moisture

4. pH

5. Stress

Shape memory alloys SMA

1. Alloys that ‘remember‘ their original shape

2. Can be easily shaped when cool, but return to their original shape when heated above a certain temperature

3. E.g nitinol- if you bend (stress) glasses made of nitinol, you can put them in a bowl of hot water and they will return to their original shape

Photochromic pigments

1. Change colour reversibly in response to light

2. Can be put into spectacle lenses to make glasses turn into sunglasses when its sunny

3. Photochromic inks can be used to print t-shirts with designs that only show up in sunlight

Thermochromic pigments and inks

1. Used in colour changing products- they react to temperature

2. When the temperature changes, the product changes colour. The colour changes back when the object returns to its original temperature

3. Used in babies’ feeding spoons to check if the food is too hot, and in novelty mugs and t-shirts

How can biodegradable polymers (Biopol®) reduce damage to the environment

• It is a thermoplastic produced by the action of microbes on plant matter

• It biodegrades within months

• Used as packaging as it is non-toxic with a high MP

  • Suitable for take-away food and drinks

• It sinks in water which aids biodegradation and reduces risk of harm to sea life

HIPS

• Shatterproof and a good insulator

• Flexible and lightweight

  • Ideal for vacuum forming

• Impact resistant, it is suitable for food containers

• Easily mouldable and has a good gloss finish

Acrylic (Polymethyl-Methacrylate)

• Versatile and tough

• Comes in a variety of thicknesses and colours

• Scratches easily and becomes brittle if thin

• Acrylic fibres can be spun into threads for weaving and knitting

Resin identification codes

Thermoplastics must be recycled separately to enable effective processing

Polyester resin

• Viscous, clear polyester solution

• The addition of a catalyst hardens the resin

• The hardened resin is tough with high abrasion resistance which makes a high-performance coating for areas of high wear

• Clear polyester resin is suitable for embedding objects, casting clear sculpture and jewellery making

• Resin replicas such as anatomical models can also be cast and painted

Urea formaldehyde UF

• Good electrical insulator

  • With good heat resistance it is used for manufacturing electrical fittings

  • Textiles industry- treats fabrics with UF resins to for easy care properties such as anti-wrinkling

  • Paper industry- uses UF to improve tear strength

  • Timber industry- uses UF resin to bind particles in the making of manufactured boards such as MDF

Properties of plastic

Versatile material

• Plastic is self-finishing

• Easy to colour and clean

• Provides a cheap alternative to traditional materials

• Electrical insulator

• Thermal insulator

• Tough

Where are metals found?

Metallic minerals are found naturally in rock or ore in the Earths crust

How are ores and the metals within obtained?

• Ore- mining

• Metals within- extraction

  • The method used for extraction depends on the metal’s reactivity with air, water or acids

What is the link between the reactivity of a metal and the cost to extract it?

The more reactive the metal, the more expensive it is \n to extract

Examples of extraction processes

• Most metals are smelted in a blast furnace

• Electrolysis is used to extract metals such as aluminium

Smelting

• The process of extracting metals from their ores in a blast furnace

• Metals are drawn off in a liquid state (‘hot metal’)

  • The blast furnace reaches about 1,700°C to extract iron from iron ore

• The impurities are removed

  • Used for industrial metals like iron and copper

Physical structure of a metal

• Crystalline structure of metals can be modified by heat treatments

• The larger the grains; the tougher and more ductile the material

Molecular structure of a metal

• Strong bonds between the ions and electrons give metals strength and high MP

• Positive ions in a sea of delocalised electrons make metals good electrical conductors

Rust

• Compound called iron oxide

• Formed when iron and oxygen react in the presence of moisture

Cause of oxidisation

By corrosion or weather exposure over a period of time

Patina

The thin layer of tarnish that appears on the surface of the metal

Do non-ferrous metals rust or oxidise?

• Oxidise

• Examples are copper and bronze

Verdigris

The green-turquoise patina occurring on copper

Conductivity of metals

Why are non-ferrous metals more expensive than ferrous metals?

• Due to their desirable properties:

  • Lightweight

  • Good conductivity

  • Ductile and malleable

  • Resistant to corrosion

Brass properties and uses

Copper and zinc alloys with differing properties:

• Decorative properties: it is hard, ductile, resistant to wear and is antimicrobial

• Used for low-friction applications such as padlocks, gears, valves, bearings and musical instruments

How can the hardness, machinability and corrosion resistance be adjusted for brass?

• Hardness- Ratio of copper to zinc

• Machinability- Lead

• Corrosion resistance- Aluminium

Aluminium

• Very versatile material used in many shapes and forms

  • A light grey metal, efficient thermal insulator and doesn’t degrade when recycled

Where is aluminium ore found?

Bauxite is in the Earth’s crust

Why is wood a useful and versatile material

• Aesthetically pleasing

• Good insulator

• Durable and tough

• Good strength to weight ratio

Felling

• A tree is ‘felled’ when it is cut down

• Agricultural logging uses machinery with large chainsaw attachments.

  • These can fell, de-branch and log a tree in one swift action

What equipment is used in felling

• Traditional- Saws and axes

• Modern- Chainsaws

Seasoning

Involves reducing the moisture content of the timber to between 10-20%

Processing and seasoning

• Once felled, natural timber is processed into standard sizes and seasoned

• Timber is left to dry naturally or dried artificially in kilns

How can you identify manufactured and natural timbers?

• Manufactured timbers- sheets, fibres or chips from which they are made are visible

• Natural timbers- only the grain on natural timbers will connect seamlessly to the end grain

Coniferous trees

Evergreen and have needle-like leaves

Softwood

• Most softwood trees (evergreen) grow faster than hardwood trees

  • So it is relatively cheap and readily available

• The grain is wider making it more absorbent

Deciduous trees

Drop their leaves in the autumn and new leaves grow in spring

Hardwood

• Sourced from deciduous trees

• Hardwood is usually slower growing and so is more expensive

• Comes in variety of colours and grains

  • It has good aesthetical and physical properties

• Most have a closer grain, making them more dense and hardwearing

Which hardwoods are tough or hard?

• Beech- tough, very durable with a fine finish

• Oak- tough, hard and durable with a variable grain

• Mahogany- durable and fairly hard with a distinctive grain

• Cedar- very tough, but also flexible and shock resistant

Cedar (softwood)

Contains natural oils which offer natural water resistance

Tonewood

Woods that possess tonal properties that make them ideal for use in instruments:

1. Spruce- violins, piano and guitars

   • High stiffness to weight ratio → ideal for the soundboard

2. Cedar- a richer tone for classical guitars

   • Less dense than spruce

3. Mahogany- necks and sides of acoustic guitars, the body of electric guitars

Manufactured boards

1. Made from sawmill scraps, recycled wood, low grade timbers and sawdust

2. Wood pieces are bound together with adhesives to make man-made / manufactured board

3. Susceptible to moisture

4. Boards are rigid, stable and supplied in large sheets

Manufactured board structure

• Plywood- made up of an odd number of thin layers (or ply) arranged with the grain running in alternating directions at 90º

• MDF- made from compressed sawdust and adhesive