Engineering materials have the following
Key Characteristics:
- High Hardness and Brittleness: They are very hard but shatter easily under impact or tension because their strong, fixed bonds prevent dislocation movement.
- High Melting Temperatures: The strong ionic/covalent bonds require a lot of energy to break.
- Low Electrical and Thermal Conductivity: They are excellent insulators because they have no free electrons.
- Chemical Stability: Highly resistant to corrosion and chemical degradation.
Common Examples:
Alumina ($Al_2O_3$), Silicon Carbide (SiC), Zirconia ($ZrO_2$), and common window glass (silica-based).
3. Polymers (Plastics)
Polymers are long-chain organic molecules (based on carbon and hydrogen) where repeating molecular units (monomers) are joined together by covalent bonds.
Key Characteristics:
- Low Density: Generally lightweight compared to metals and ceramics.
- Low Strength and Stiffness: They are generally softer and more flexible.
- Low Electrical and Thermal Conductivity: Good insulators.
- Viscoelasticity: They exhibit both viscous (liquid-like) and elastic (solid-like) characteristics.
- Temperature Sensitivity: Their properties change significantly with temperature; they soften and melt easily (thermoplastics) or burn (thermosets).
Classification Subgroups:
- Thermoplastics: Can be melted and reformed repeatedly (e.g., polyethylene, PVC).
- Thermosets: Form permanent cross-links during curing and cannot be re-melted (e.g., epoxy, vulcanized rubber).
Common Examples:
Polyethylene (PE), Polypropylene (PP), Nylon, Bakelite, Epoxy resins.
4. Composites
Composites are materials made from two or more constituent materials with significantly different physical or chemical properties that remain separate and distinct on a macroscopic scale within the finished structure. They are designed to achieve better properties than the individual components alone.
Key Characteristics:
- High Strength-to-Weight Ratio: The primary benefit, often combining the stiffness of one material with the low density of another.
- Anisotropy: Their properties often vary with direction (e.g., stronger along the fiber direction).
- Tailorability: Properties can be precisely controlled by adjusting the type, amount, and orientation of the constituents.
Structure:
- Matrix: The continuous phase (often a polymer, metal, or ceramic).
- Reinforcement: The dispersed phase that provides strength (often fibers, particles, or sheets).
Common Examples:
Fiberglass (Glass fibers in a polymer matrix), Carbon Fiber Reinforced Polymer (CFRP), Reinforced Concrete, and advanced composites used in aircraft.
