In this module, we will be learning about the properties of ceramics
Initially we will look at mechanical properties, which limit their possible
applications.The primary limitation ceramics and glasses have is the fact
that they are very brittle, which means that they fracture easily.
Ceramics and glasses have a well-defined modulus, like metals, but their modulus is generally higher than metals, because the bonding (either ionic or covalent) is stronger than metallic bonding. Glasses have a lower modulus than most metals because they have a non-uniform structure, unlike other ceramics.
Ceramics and glasses are the hardest type of solid. Many ceramics are used as abrasives, in cutting, grinding or polishing (eg silicon carbide and diamond). Their hardness is so high because unlike metals, it is extremely difficult for dislocations to move through the atomic lattice, because of the localised or ordered bonding between atoms.
At room temperature, both ceramics and glasses will undergo fast fracture
in a tensile test before any plastic deformation has occurred. Ceramics
have a fracture toughness about fifty times less than metals, even though
their bonding forces are high. The fracture strength of a material which
fails by fast fracture is given by:
where σf is applied stress, KIc is the plane strain fracture toughness,
Y is a dimensionless constant, and a is the length of a surface crack
or half the length of an internal crack. It can be seen that the tensile
strength of a ceramic is determined by the length of the longest flaw.