Question

Consider the properties of ceramics. Which of the following is a reason for their brittleness?

• A. High thermal expansion
• B. High ionic bond strength
• C. Covalent bonding networks
• D. Presence of amorphous phases

Hint:

Ceramic Brittleness. Caused by strong ionic and/or directional covalent bonds that resist dislocation motion. Fracture occurs before significant plastic deformation (slip).

Answer 1

The Correct Option is C

Ceramics are typically brittle, meaning they fracture with little or no plastic deformation.
This behavior stems from their bonding and structure.
- Ceramics often exhibit strong ionic bonds (Option 2) and/or directional covalent bonds (Option 3).
- Ionic bonds are strong but non-directional.
Movement of ions relative to each other disrupts the charge balance, leading to repulsion and fracture.
- Covalent bonds are strong and highly directional.
Dislocation movement, the primary mechanism for plastic deformation in metals, is very difficult in covalently bonded networks because it requires breaking and reforming these strong, specific bonds.
The energy required is high, and fracture often occurs before significant slip.
- Therefore, both strong ionic bonding and extensive covalent bonding networks contribute to brittleness by restricting dislocation motion.
Option (3) focusing on covalent networks often represents materials like silicon carbide or diamond, which are extremely brittle due to the rigid, directional bonding.
Option (2) applies to ionic ceramics like MgO.
Given the options, both strong ionic and covalent bonding are reasons, but covalent networks often imply a higher degree of restricted plasticity.
- High thermal expansion (1) relates to thermal shock resistance, not directly to mechanical brittleness at room temperature.
Amorphous phases (4) (like in glass) lack long-range order and dislocation mechanisms, also contributing to brittleness, but crystalline ceramics are also brittle due to bonding.
Covalent bonding networks (3) is a major reason for restricted dislocation motion leading to brittleness.