Comparing properties and selecting suitable materials
Using resources • Using materials
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Definitions of material classes
Glass and clay ceramics are inorganic, non-metallic solids formed by heating then cooling or firing. These materials are generally hard, chemically inert and electrically insulating but brittle. Polymers are long-chain organic molecules formed by polymerisation. Polymers are generally low density, poor electrical and thermal conductors and often ductile. Metals are crystalline, metallically bonded materials with high electrical and thermal conductivity, high density and good ductility and toughness. Composites combine two or more distinct materials to produce properties not present in the individual components, such as increased strength-to-weight ratio or tailored stiffness.
Quantitative density comparisons
Density controls mass for a given volume and influences suitability where weight matters. Typical densities: common glass ≈ 2500 kg·m⁻3, clay ceramics ≈ 2000–2600 kg·m⁻3, aluminium ≈ 2700 kg·m⁻3, steel ≈ 7700–7900 kg·m⁻3, polymers (e.g. polyethylene) ≈ 900–1400 kg·m⁻3. Composites vary widely: fibreglass composites ≈ 1500–2000 kg·m⁻3, carbon-fibre composites ≈ 1500–1800 kg·m⁻3. Lower density causes lighter components for the same volume, benefiting transport and handheld applications.
Mechanical strength, hardness and brittleness
Mechanical behaviour defines load-bearing and failure modes. Metals combine high tensile strength and ductility; typical structural steel tensile strength ≈ 400–800 MPa while aluminium alloys ≈ 200–600 MPa. Ceramics and glass show high hardness but low tensile strength and low fracture toughness, causing brittle fracture under tensile stress. Polymers show much lower tensile strength (tens of MPa to low hundreds of MPa) and higher ductility for many thermoplastics. Composites produce high tensile or flexural strength with designable stiffness; carbon-fibre composites reach several hundred to over 1000 MPa in fibre direction depending on lay-up. High tensile strength and ductility cause resistance to permanent deformation; brittleness causes sudden fracture without prior plastic deformation.
Thermal and electrical conductivity and melting behaviour
Thermal and electrical conductivity determine heating, cooling and electrical applications. Metals show high thermal conductivity (e.g. copper ≈ 400 W·m⁻1·K⁻1, steel ≈ 15–60 W·m⁻1·K⁻1) and high electrical conductivity, so metals serve as heat sinks and electrical conductors. Ceramics and most polymers show low thermal conductivity (<1–30 W·m⁻1·K⁻1 for ceramics; ≈0.1–0.5 W·m⁻1·K⁻1 for polymers) and act as thermal or electrical insulators. Melting and decomposition points vary: metals and glass melt at high temperatures; thermoplastics soften and melt at relatively low temperatures; thermosetting polymers char or decompose rather than melt. Low thermal conductivity causes thermal insulation; high conductivity causes efficient heat transfer.
Corrosion, chemical resistance and durability
Corrosion and chemical resistance determine longevity in corrosive environments. Many metals corrode in the presence of water and oxygen unless alloyed (stainless steels) or coated. Glass and ceramics present strong chemical resistance and maintain properties in acidic or alkaline environments, causing long-term durability in pipes, tiles and containers. Polymers show varied chemical resistance; some resist acids and bases while others degrade under UV or solvents. Composites inherit chemical resistance from the matrix and the fibres; resin selection controls environmental durability.
Selecting materials by property→use links
High electrical conductivity → selection of copper or aluminium for electrical wiring because electrons move freely and energy loss is low. High thermal conductivity and corrosion resistance → selection of stainless steel or copper for cookware and heat exchangers because heat transfers quickly and surfaces resist corrosive food acids. Low density and high strength → selection of aluminium alloys or carbon-fibre composites for aerospace and transportation because weight reduction increases efficiency. Brittleness and hardness → selection of ceramics or glass for scratch resistance, high-temperature applications and electrical insulation because materials resist wear and do not conduct electricity. Polymers provide insulation, corrosion resistance and flexibility → selection for packaging, insulating components and lightweight housings.
Key notes
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