Ceramics
introduction
properties
crack sensitivity
thermal shock
ceramics
glasses

Properties 1

In this module, we will be learning about the properties of ceramics and glasses.

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.

Modulus
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.
modulus of different materials
hardness of different materials Hardness
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.

Strength
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:

equation

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.

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