Chemistry of engineering materials

Every structure, machine and device is made of materials whose properties come from their chemistry. Lesson 2 showed how metallic, ionic and covalent bonds give metals their ductility, ceramics their hardness and polymers their flexibility. This lesson follows that thread into the materials engineers specify every day: metals and alloys, including steel and stainless steel; ceramics and glass; Portland cement and concrete, the most widely used manufactured material on Earth; polymers; and composites. For each, the aim is to connect structure at the atomic scale to the properties, processing and failure modes that matter in design, and to the environmental cost of making them.

Crystal structures of metals

Metal atoms pack into regular, repeating three-dimensional arrangements. The smallest repeating unit is the unit cell, and three structures account for most engineering metals:

• Body-centred cubic (BCC): an atom at each corner of a cube and one in the centre, 2 atoms per cell, 68 % of space filled. Iron at room temperature (ferrite), chromium, tungsten.
• Face-centred cubic (FCC): atoms at the corners and the centre of each face, 4 atoms per cell, 74 % filled, the densest possible packing. Aluminium, copper, nickel, gold, and iron above 912 °C (austenite). FCC metals have many planes along which atoms can slip, so they stay ductile even at very low temperatures, which is why aluminium and austenitic stainless steels are used for cryogenic tanks.
• Hexagonal close-packed (HCP): also 74 % filled but with fewer slip systems, so less ductile at room temperature. Titanium, magnesium, zinc.

Knowing the structure and the size of the unit cell lets you calculate density from first principles, a good check that the atomic picture is right.

Alloys and steel

Pure metals are usually too soft for structural use. Real crystals contain defects, especially dislocations, lines of misfit atoms whose movement lets planes slip at stresses far below what a perfect crystal could bear. Anything that obstructs dislocations makes a metal stronger: foreign atoms, grain boundaries, fine particles of a second phase, or a tangle of other dislocations produced by cold working.

An alloy is a metal with other elements deliberately added. In a substitutional alloy, the added atoms replace host atoms of similar size: zinc in copper makes brass, nickel in copper makes the cupronickel used in coins and seawater piping. In an interstitial alloy, small atoms such as carbon, nitrogen or hydrogen squeeze into the gaps between host atoms.

Steel is iron with up to about 2 % carbon, usually much less: structural steels have 0.15 to 0.3 %, tool steels near 1 %. Carbon dissolves readily in FCC austenite but hardly at all in BCC ferrite, so as hot steel cools slowly, the excess carbon forms hard particles of iron carbide, Fe₃C (cementite). Cooling quickly, by quenching in water or oil, traps the carbon and produces martensite, very hard but brittle; reheating to a moderate temperature (tempering) restores toughness. The same chemistry, controlled by temperature and time, gives one alloy a range of properties from soft wire to springs and cutting tools.

Stainless steels contain at least 10.5 % chromium, which reacts with oxygen to form an invisible chromium oxide film a few nanometres thick. The film is passive: it stops further corrosion and heals itself if scratched, as Lesson 9 explained. Adding nickel stabilizes the FCC structure, giving the common austenitic grades 304 and 316; 316 also contains molybdenum, which resists the chloride ions that break down the film in seawater and road salt. Other alloy elements serve other purposes: manganese for toughness, vanadium and niobium for fine grain size and strength in modern structural steels, copper in weathering steel to form a protective rust layer.

Ceramics and glass

Ceramics are compounds of metals with non-metals, usually oxygen (alumina, Al₂O₃; zirconia, ZrO₂), carbon (silicon carbide), nitrogen (silicon nitride) or silicon and oxygen (clays, brick, porcelain). Their strong, directional ionic and covalent bonds make them hard, stiff, high-melting and chemically resistant, but they cannot deform plastically by slip. A crack therefore grows unchecked, and ceramics are brittle: their strength depends on the size of their largest flaw and varies widely from piece to piece. Engineers design ceramics in compression, and use them where heat, wear or corrosion would defeat metals: furnace linings, cutting tools, spark plug insulators, turbine coatings, artificial hip joints and armour.

Glass is a non-crystalline solid, a "frozen liquid" with no long-range order. Pure silica glass, a random network of SiO₄ tetrahedra, melts above 1,700 °C. Ordinary soda-lime window glass, about 73 % SiO₂, 14 % Na₂O and 9 % CaO, melts near 1,000 °C because the sodium oxide breaks up the network; the lime keeps it from dissolving in water. Borosilicate glass, with boron oxide, expands less on heating and resists thermal shock in laboratory ware and cookware. Tempered glass is cooled rapidly with air jets so its surfaces solidify in compression; because cracks cannot open in compressed surfaces, it is four to five times stronger than ordinary glass and shatters into small blunt pieces, as building codes require for doors and glazing near floors.

Portland cement and concrete

Portland cement is made by heating limestone and clay or shale to about 1,450 °C in a rotary kiln. Calcination (Lessons 5 and 6) drives off CO₂, and the lime combines with silica, alumina and iron oxide to form clinker, which is ground with a little gypsum. Cement chemists use a shorthand for its main compounds: C₃S (tricalcium silicate, Ca₃SiO₅, about 55 to 65 %), C₂S (dicalcium silicate, 15 to 25 %), C₃A (tricalcium aluminate) and C₄AF (an aluminoferrite).

When mixed with water, these compounds hydrate. The key reaction is:

The calcium silicate hydrate, called C-S-H, is a poorly crystalline gel of nanometre-scale sheets that binds the sand and stone together and provides most of the strength. C₃S hydrates within days and gives early strength; C₂S reacts over weeks and months. The calcium hydroxide keeps the pore water strongly alkaline, at pH 12.5 to 13.5, which passivates the steel reinforcement. Hydration consumes water and releases heat, so concrete must be kept moist and at a suitable temperature while it cures (Lesson 7).

Concrete's durability is a chemistry problem. Carbon dioxide from the air slowly reacts with calcium hydroxide (carbonation), lowering the pH near the surface to about 9, where steel is no longer passive. Chloride ions from de-icing salt or seawater break down the passive film even at high pH. Either way, the reinforcement corrodes, and because rust occupies several times the volume of steel, it cracks and spalls the cover, the main cause of deterioration in parking garages and bridges (Lesson 9). Sulfates in soil and groundwater react with hydrated aluminates to form expansive crystals; some aggregates contain reactive silica that swells in the alkaline pore solution (alkali–silica reaction). Durable concrete uses a low water-to-cement ratio, adequate cover, supplementary materials and, where needed, sulfate-resistant cement and air entrainment for freeze–thaw resistance.

Polymers

Polymers are giant molecules made by linking thousands of small molecules, monomers, into chains. In addition polymerization, monomers with a carbon–carbon double bond open it and join end to end: ethylene (C₂H₄) becomes polyethylene, vinyl chloride becomes PVC, propylene becomes polypropylene. In condensation polymerization, two different monomers join and release a small molecule, usually water: nylons for fibres, gears and bearings, and polyesters such as PET for bottles and fibres.

The size of the chains is described by the degree of polymerization, the number of repeat units per chain. Longer chains tangle more and give a stronger, tougher material that is harder to melt and mould.

Polymers fall into three families by how their chains are connected:

• Thermoplastics (polyethylene, polypropylene, PVC, nylon, polycarbonate) have separate chains held together only by intermolecular forces. They soften when heated and can be melted, moulded, extruded and recycled.
• Thermosets (epoxy, phenolics, polyester resins, many polyurethanes) are cured into a single three-dimensional network by covalent cross-links. Once set, they do not melt but char, and cannot be remoulded. They make strong, heat-resistant adhesives, coatings and composite matrices.
• Elastomers (natural and synthetic rubbers, silicones) are lightly cross-linked, so the coiled chains can stretch and spring back. Vulcanizing rubber with sulfur forms the cross-links.

Polymer properties depend strongly on temperature. Below its glass transition temperature, T_g, a polymer is rigid and glassy; above it, the chains can move and it becomes leathery or rubbery. PVC (T_g about 80 °C) is rigid at room temperature, but adding plasticizers lowers T_g to make flexible cable jackets and hoses. Polyethylene's T_g is far below room temperature, but its partly crystalline regions keep it solid. Many polymers also creep, slowly deforming under steady load, so plastic pipes and structural parts are designed for long-term strength at service temperature. Sunlight and oxygen break polymer chains over time (Lesson 7); carbon black and stabilizers are added to products that must last outdoors.

Composites

A composite combines two or more materials to get properties neither has alone. Fibres carry load; a matrix holds them in place, transfers stress between them and protects them. Glass-fibre-reinforced plastic (fibreglass) makes boat hulls, tanks and wind turbine blades; carbon-fibre composites, stronger and stiffer per unit weight than steel, make aircraft structures, racing bicycles and strengthening wraps for bridge columns. Reinforced concrete is itself a composite: concrete carries compression, steel carries tension, and they work together because they expand almost equally with temperature and because the alkaline concrete protects the steel. Wood is a natural composite of cellulose fibres in a lignin matrix.

Lesson 11 turns from materials to the chemistry of fuels, emissions, the atmosphere and water, and Lesson 12 applies the course to integrated case studies, including a reinforced concrete structure exposed to de-icing salt.