Engineering Materials and Their Categories
Why Materials Are Classified
Engineers select materials before they select processes. The OCR specification lists five main categories: metals, polymers, engineering ceramics, composites, and smart materials. Within metals, the key distinction is between ferrous metals and non-ferrous metals; within polymers, between thermoplastics and thermosets. Knowing the properties, forms of supply, and applications of each named material is essential for the exam.
Metals — Pure Metals and Alloys
An alloy is a mixture of two or more elements — at least one a metal — giving better properties than the pure metal. Pure iron is soft and rarely used directly; adding carbon produces steel. All four ferrous metals on the specification are iron alloys.
Ferrous Metals
Ferrous metals contain iron and are often magnetic, but magnetism and corrosion resistance depend on the alloy. Cast iron (~2–4% carbon) — hard, brittle, excellent in compression; engine blocks, manhole covers, machine beds. Low carbon steel (up to 0.3% C) — tough, ductile, weldable, easy to form; the most widely used structural metal for car bodies, bridges and fabrications. High carbon steel (0.6–1.4% C) — very hard, holds a sharp edge but brittle; cutting tools, springs and drill bits. Stainless steel (10.5%+ chromium) — forms a self-healing oxide layer, excellent corrosion resistance; kitchen equipment, surgical instruments and food processing. All four are supplied as bar, sheet, tube, or structural section.
The four ferrous metals in the specification are cast iron, low carbon steel, high carbon steel, and stainless steel. All contain iron. Their magnetism and corrosion resistance depend on the alloy; stainless steel's chromium content forms a self-healing protective oxide layer.
Non-Ferrous Metals
Non-ferrous metals contain no significant iron, are non-magnetic, and are more corrosion-resistant than ferrous metals. Aluminium alloys — low density, good conductors, corrosion-resistant; aircraft structures, vehicle bodies, packaging foil. Copper — excellent electrical and thermal conductor, ductile; electrical wiring, plumbing and heat exchangers. Brass (copper + zinc) — harder than copper, good machinability, corrosion-resistant in water; taps, valves and connectors. Bronze (copper + tin) — harder than brass, resists seawater, low friction; bearings and ship propellers.
Polymers — Thermoplastics
A thermoplastic softens when heated and hardens on cooling — reversible, making it suitable for injection moulding, vacuum forming, and 3D printing. Five are specified: ABS — tough, impact-resistant, easy to machine; LEGO bricks, car dashboards, 3D-printing filament. HIPS — cheap, rigid, easy to vacuum form; packaging and display. PMMA/Acrylic — optically clear, easy to cut and bend; display cases and signage. Polycarbonate — extremely tough, optically clear; safety goggles and aircraft canopies. PLA — biodegradable, made from plant starch; the most common FDM filament for prototypes and packaging.
Thermoplastics have long-chain molecules that are not permanently cross-linked — heat breaks the intermolecular bonds, allowing reshaping; cooling re-sets them. This is reversible. Thermosets are chemically cross-linked during curing; once set, the network is permanent and cannot be re-melted. If an exam question describes 'a polymer that cannot be recycled by melting', the answer is thermoset. If it describes 'a polymer used in injection moulding', the answer is thermoplastic — injection moulding requires repeated melting.
Polymers — Thermosetting Polymers
Thermosets cross-link irreversibly during curing — rigid, hard, thermally stable, cannot be re-melted. Urea formaldehyde — hard, cheap electrical insulator; electrical fittings and bottle tops. Melamine formaldehyde — hard, scratch and heat resistant; worktop laminates and tableware. Epoxy resin — strong adhesive, excellent chemical resistance; structural adhesive and carbon-fibre composite matrix. Polyester resin — cheaper than epoxy; matrix in GRP boat hulls and car panels.
Engineering Ceramics and Composites
Engineering ceramics are hard, brittle, thermally stable and chemically inert. Silicon carbide — extremely hard; abrasives and high-temperature structural components. Tungsten carbide — very hard, wear-resistant; CNC cutting tool inserts and drill tips. Silicate glass — optically transparent, hard, brittle; windows, lenses and lab equipment. Composites combine two or more materials to achieve properties neither could provide alone; unlike alloys, the constituent materials stay physically distinct. GRP (glass fibres + polyester/epoxy resin) — lightweight, corrosion-resistant; boat hulls, tanks, car panels. CRP (carbon fibres + epoxy resin) — very high strength-to-weight ratio; aerospace, racing cars and high-performance bicycles.
A smart material responds to a change in its environment (temperature, light, pressure) in a predictable, reversible way. The four specified smart materials are shape memory alloy, thermochromic pigment, photochromic pigment, and Quantum Tunnelling Composite.
Smart Materials
A smart material senses and responds to an external stimulus. Shape memory alloy (SMA / Nitinol) deforms when cold but returns to its pre-set shape on heating above the transition temperature; used in stents and orthodontic wires. Thermochromic pigment changes colour reversibly with temperature; used in thermometers and temperature-indicating packaging. Photochromic pigment changes colour reversibly with UV light; self-darkening sunglasses. Quantum Tunnelling Composite (QTC) is an insulator normally but becomes conductive under pressure; force sensors and push-button switches.
In a metal alloy the elements mix at the atomic level — you cannot separate brass into copper and zinc by eye. In a composite the constituents stay physically distinct: in GRP you can see the glass fibres in the resin matrix. Alloys are melted and re-alloyed; composites are very difficult to recycle because fibres and matrix are permanently bonded. Never call a composite an 'alloy' in an exam answer — they are fundamentally different classes of material.