Why are ceramics generally brittle while metals are ductile, given that both can have crystalline structures?
Think about your answer, then reveal below.
Model answer: Ductility requires dislocation motion — dislocations glide through the crystal under stress, allowing permanent shape change without fracture. In metals, the non-directional metallic bond allows atoms to slide past each other without breaking specific bonds. In ceramics, the strong directional ionic/covalent bonds resist dislocation motion because displacing atoms changes the local bonding environment. In ionic ceramics, dislocation glide would bring like charges into contact (cation next to cation), creating enormous electrostatic repulsion. The result: ceramics fracture before they can deform plastically.
This brittleness is the central engineering limitation of ceramics. The same strong bonding that gives ceramics their hardness and thermal stability makes them susceptible to catastrophic failure from small cracks. Fracture toughness (resistance to crack propagation) is typically 1-5 MPa-m^(1/2) for ceramics vs. 50-200 for metals. Transformation toughening (partially stabilized ZrO2) and fiber reinforcement (SiC fibers in ceramic matrices) are strategies to improve ceramic toughness.
Question 2 Multiple Choice
BaTiO3 is a perovskite ceramic used in capacitors because of its extremely high dielectric constant. This high dielectric constant arises from which structural feature?
AThe large unit cell size, which provides more space for charge separation
BThe displacement of the Ti4+ ion from the center of its oxygen octahedron, creating a permanent electric dipole that can align with an applied field
CThe high ionic conductivity of Ba2+ ions through the lattice
DThe metallic bonding character of the Ti-O bonds
Below its Curie temperature (120 C), BaTiO3 is ferroelectric: the Ti4+ ion sits off-center in its octahedral cage of O2- ions, creating a spontaneous electric dipole. In an applied electric field, these dipoles align cooperatively, producing polarizations far larger than what electronic or ionic displacement alone could achieve. The dielectric constant can exceed 10,000 near the Curie temperature. This ferroelectric behavior is a direct consequence of crystal chemistry — the tolerance factor and ion sizes in the perovskite structure determine whether the distortion occurs.
Question 3 True / False
Sintering a ceramic powder at high temperature increases its density without melting the material.
TTrue
FFalse
Answer: True
Sintering densifies a powder compact through solid-state diffusion: atoms migrate from grain surfaces and boundaries to fill the pores between particles. The driving force is reduction of surface energy — the total surface area decreases as small pores shrink and particles merge into larger grains. Typical sintering temperatures are 50-80% of the melting point (in Kelvin). No liquid phase is needed for solid-state sintering, though liquid-phase sintering (adding a low-melting additive) can accelerate densification. The key process variables are temperature, time, atmosphere, and starting particle size.
Question 4 Short Answer
Why is silicon carbide (SiC) used in high-temperature structural applications where metals would fail?
Think about your answer, then reveal below.
Model answer: SiC has predominantly covalent bonding (88% covalent character), giving it extreme hardness (9.5 Mohs), high thermal conductivity (120 W/m-K), and a decomposition temperature above 2700 C — far above the melting points of most structural metals. It also has excellent oxidation resistance because it forms a protective SiO2 layer at the surface. Unlike metals, SiC maintains its strength up to very high temperatures because covalent bonds do not weaken as rapidly as metallic bonds with increasing temperature.
SiC exemplifies how bonding chemistry dictates application. Its covalent network structure (similar to diamond but with alternating Si and C atoms) provides the mechanical and thermal properties, while the ability to form protective oxide scales provides chemical durability. The tradeoff is processability: SiC is extremely difficult to sinter (requiring temperatures above 2000 C and sintering aids like B and C) and nearly impossible to machine after densification.