In this section we will consider the typical properties of ceramics.

Ceramics are typically hard and brittle. While their strength in compression is very high, they are not suitable for loading in tension. Their brittle qualities mean that they fracture very easily. In compression, a crack is not easily propagated, but in tension the crack is free to grow.

 

Elastic Modulus

The elastic modulus (Young's Modulus) of ceramics is usually higher than for metals, because ceramics are bonded either covalently or ionically. This bonding is stronger than metallic bonding. Glasses have a lower modulus than other ceramics due to their non-uniform (amorphous) atomic structure.

 

Hardness

Ceramics and glasses are the hardest known materials. Many ceramics are often used as abrasives for this reason. They are hard because of their ordered structure, it is very difficult for dislocations to move through the atomic lattice.

 

Fracture Toughness

At room temperature, both ceramics and glasses will undergo fast fracture in a tensile test before any plastic deformation has occurred. Fast fracture is where the specimen breaks by the rapid initiation, growth and failure of a crack.

Ceramics have a fracture toughness about fifty times less than metals, even though their bonding forces are higher. Because of their susceptibility to cracking, consideration of the strength of ceramics requires consideration of their cracking behaviour. This quite often involves design and analysis with the use of fracture mechanics.

Ceramics are particularly susceptible to cracking because they usually contain many inherent small flaws or cracks. The tensile strength of a ceramic is determined by the length of the longest flaw.

It is because of the inherently low fracture toughness of most ceramics that they are usually loaded in compression. Ceramics have compressive strengths about ten times higher than their tensile strengths.