Material families and their structure
Materials science studies how the inside of a material controls what it can do.
- Metals (iron, aluminium, copper): metallic bonds, a sea of free electrons. Conduct heat and electricity, can bend (ductile) and be hammered (malleable).
- Polymers (polythene, nylon, rubber): long chain molecules made of repeating units (monomers). Light, insulating, often flexible. Thermoplastics melt and can be reshaped; thermosets do not.
- Ceramics (clay, porcelain, glass, cement): ionic and covalent bonds. Hard, heat-resistant, but brittle.
- Composites (concrete, fibreglass, carbon fibre, plywood): two or more materials together, each doing a job.
Crystals, grains and defects
In a crystal atoms repeat in a pattern (a lattice). Real metals are made of many small crystals called grains. Smaller grains usually mean a stronger metal. Tiny faults (dislocations) let layers slip, which is why metals bend. In an amorphous solid like glass there is no long-range order. An alloy mixes a metal with other atoms (steel = iron + a little carbon); the extra atoms block slipping, so alloys are harder.
Testing materials: stress, strain, hardness and toughness
Tensile test
A bar is pulled until it breaks while force and extension are measured.
- Stress σ = F ÷ A (unit: pascal, Pa = N/m²; 1 MPa = 1 N/mm²).
- Strain ε = ΔL ÷ L₀ (no unit).
- Young's modulus E = σ ÷ ε: stiffness. Steel ≈ 200 GPa, aluminium ≈ 70 GPa, nylon ≈ 3 GPa.
- The elastic limit / yield point: below it the bar springs back; above it the bar stays stretched (plastic).
- Ultimate tensile strength (UTS): the highest stress before breaking.
Other tests
- Hardness (Brinell, Vickers, Rockwell): press a hard ball or diamond into the surface and measure the dent. Smaller dent = harder.
- Impact test (Charpy, Izod): a swinging hammer breaks a notched bar; energy absorbed shows toughness.
- Fatigue: repeated small loads can crack a part over time.
- Non-destructive tests: ultrasound, X-ray, dye penetrant find hidden cracks without damage.
Treatments and manufacturing
Heat treatments (mainly steel)
- Annealing: heat, then cool slowly → softer, easier to shape.
- Quenching (hardening): heat, then cool fast in water or oil → very hard but brittle.
- Tempering: reheat a hardened part gently → less brittle, tougher.
- Case hardening: hard surface, tough core.
Surface treatments
Painting, galvanising (zinc coat), anodising aluminium, electroplating: stop corrosion and improve looks.
Shaping processes
Casting (pour liquid metal into a mould), forging and rolling (shape hot metal by force), machining (cut away material), moulding of plastics (injection, extrusion), joining (welding, adhesives), and additive manufacturing (3D printing layer by layer). Engineers choose the process by cost, shape, number of parts and the material.
New materials and the chemistry of art materials
New and advanced materials
- Composites: carbon fibre and glass fibre in resin; light and strong (aircraft, sports gear).
- Nanomaterials: built at 1–100 nm; graphene (one layer of carbon atoms), nanosilver (kills germs).
- Smart materials: change with conditions — shape-memory alloys, thermochromic pigments, piezoelectric crystals.
- Biomaterials and bioplastics: titanium hip joints, plastics made from plants that can break down.
- Semiconductors: silicon for chips and solar cells.
Art materials
Paint = pigment (coloured powder, often a metal oxide or organic dye) + binder (oil, egg, gum, acrylic polymer) + solvent. Ceramics harden when clay is fired. Glass is mostly silica with added oxides for colour. Bronze (copper + tin) is used for statues. Conservators use chemistry to clean and protect artworks.
Try it: bend a paperclip back and forth until it breaks (fatigue). Then compare a rubber band, a plastic ruler and a pencil lead: which bends, which stretches, which snaps?
Key formulas and definitions
- Stress σ = F ÷ A (Pa or N/m²; 1 MPa = 1 N/mm²)
- Strain ε = ΔL ÷ L₀ (no unit)
- Young's modulus E = σ ÷ ε
- Density ρ = m ÷ V
- Specific strength = strength ÷ density
- Steel = iron + up to about 2% carbon
Worked examples
1. A steel wire of area 2 mm² carries a load of 400 N. Find the stress.
σ = F ÷ A = 400 N ÷ 2 mm² = 200 N/mm² = 200 MPa.
2. A 2 m rod stretches by 1 mm. What is the strain?
ε = ΔL ÷ L₀ = 0.001 m ÷ 2 m = 0.0005 (no unit).
3. Using the two answers above, find Young's modulus of the steel.
E = σ ÷ ε = 200 MPa ÷ 0.0005 = 400 000 MPa = 400 GPa. (Real steel is about 200 GPa, so this would be an unusual sample or a measuring error.)
4. A blacksmith heats a chisel red-hot and drops it in water, then reheats it gently. Name both treatments and the result.
Quenching makes it very hard but brittle. Tempering then reduces brittleness, giving a hard, tough chisel.
Common mistakes
- Thinking strong and hard mean the same. Strength resists breaking under load; hardness resists scratching or denting.
- Forgetting to change mm² to m² (or using N/mm² = MPa directly). 1 mm² = 10⁻⁶ m².
- Thinking brittle means weak. Ceramics can be very strong in squeezing but break suddenly without bending.
- Believing glass is a crystal. Ordinary glass is amorphous: its atoms have no repeating pattern.