9.38C - Comparing ceramics, polymers, composites and metals
Compare glass, clay ceramics, polymers, composites and metals using precise property descriptions and supplied data. Learn why strength, hardness and toughness mean different things, and when a comparison needs more evidence.
Separate Chemistry.
What a property measures
A physical property is a feature that can be measured or observed without forming a different substance. Numerical (quantitative) data and descriptive (qualitative) evidence both help us compare materials.
| Property | Meaning |
|---|---|
| Density | Mass per unit volume; lower density means less mass for the same volume |
| Tensile strength | Maximum tensile stress a material withstands before failure; stress is pulling force per unit cross-sectional area |
| Stiffness | Resistance to elastic deformation; it is different from strength |
| Hardness | Resistance to scratching or indentation |
| Toughness | Ability to absorb energy before breaking; impact toughness concerns a sudden blow |
| Brittleness | Tendency to fracture with little deformation |
| Thermal conductivity | How readily heat is transferred through the material |
| Electrical conductivity | How readily charge flows through the material |
A hard material can still be brittle: glass resists scratching but can shatter. A strong material need not be stiff; strength and bending/deformation are different measurements. Ductile means able to be drawn into wires; malleable means able to be hammered or rolled into shape.
Check that numerical comparisons use the same property and compatible units. A strength measured at yield (the onset of permanent deformation) cannot simply be treated as the breaking strength. Nor can strength values by themselves compare breaking forces unless cross-sectional areas are the same.
Patterns in material families
The family of a material suggests likely properties, but particular samples can differ.
| Family | Common properties and examples |
|---|---|
| Glass | Usually hard and brittle; many glasses are transparent and poor electrical conductors. Soda-lime glass is used in windows. |
| Clay ceramics | Clay is shaped then fired. Bricks and many ceramic tiles are hard, brittle, heat resistant and poor electrical conductors. They are often strong in compression but weaker in tension. Most are opaque. |
| Polymers | Long-chain molecules. Many everyday polymers have low density and are poor thermal and electrical conductors. Flexibility, toughness and heat resistance vary; some are rigid. |
| Metals | Usually good thermal and electrical conductors. Many are malleable and ductile. Strength and density differ: aluminium is less dense than steel. |
| Composites | Two or more constituent materials remain distinct, combining useful properties. A matrix holds reinforcement in place and transfers forces. |
For example, glass-fibre reinforced polymer combines glass fibres with a polymer matrix. The fibres can increase strength and stiffness; the matrix binds and protects them and passes forces between them.
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