Material classes
Before you compare grades, decide the class. Bonding sets the ceiling on what a material
can do — a ceramic will not become tough because you want it to, and a polymer will not
hold its modulus at 400 °C. These 5 classes cover essentially everything
an engineer specifies.
Metallic bonding — delocalized electron gas; crystalline (mostly FCC, BCC, HCP).
Strengths - High strength and stiffness with excellent toughness/ductility
- Electrically and thermally conductive
- Well-understood processing, joining (welding, brazing) and recycling
- Properties tunable via alloying and heat treatment
Limitations - Prone to corrosion (except noble/passivated metals)
- Relatively dense
- Fatigue and creep at elevated temperature
Ionic/covalent bonding; crystalline (oxides, carbides, nitrides) or amorphous (glasses).
Strengths - Very high hardness and compressive strength
- Excellent temperature and wear resistance
- Chemically inert; good electrical insulators (some are ionic/electronic conductors)
Limitations - Brittle — low fracture toughness, sensitive to flaws
- Difficult to machine; joining is challenging
- Thermal-shock sensitivity (except low-CTE grades)
Covalent chains with secondary (van der Waals/H-bond) interactions; amorphous or semi-crystalline.
Strengths - Low density, low cost, easy mass production
- Corrosion resistant; electrical insulators
- Huge design freedom (injection molding), transparency, elasticity (elastomers)
Limitations - Low stiffness/strength vs. metals; creep under load
- Limited service temperature (typically < 150–300 °C)
- UV/chemical degradation, flammability (grade-dependent)
Two or more constituents (matrix + reinforcement) combined at macro/micro scale.
Strengths - Outstanding specific strength/stiffness (CFRP)
- Anisotropy can be designed into the laminate
- Fatigue and corrosion resistance superior to many metals
Limitations - Costly raw materials and slow processing
- Complex failure modes (delamination, impact damage)
- Difficult joining/repair and recycling
Covalent (Si, GaAs) or ionic-covalent; band gap defines electronic behavior.
Strengths - Tunable electronic/optical properties via doping and heterostructures
- Foundation of electronics, photonics, sensors, PV
- Functional effects: piezo-, ferro-, thermo-electricity, magnetism
Limitations - Brittle; extreme purity/processing requirements
- Expensive single-crystal growth
- Sensitive to defects and contamination