Materials for engineering, 3rd Edition - (Malestrom)

4.3 Ceramic materials
Glasses and ceramics 139
It is possible to classify ceramic materials into three groups:
1. Natural ceramics. Stone is the oldest of all constructional materials and
one may distinguish limestone (largely CaCO 3 ) and granite (aluminium
silicates). Their mechanical properties are summarized in Table 4.2.
2. Domestic ceramics. Pottery, porcelain, tiles and structural and refractory
bricks are all in this category. These so-called vitreous ceramics are
made from clays which are formed in the plastic state then dried so they
lose their plasticity and acquire enough strength to be handled for firing.
Firing between 800 and 1200 °C leads to a microstructure which, on
cooling, consists of crystalline phases (mostly silicates) in a glassy matrix
based on silica (Fig. 4.2). The structure contains up to 20% porosity and
many microcracks, but a glaze may be applied to seal the surface pores.
The mechanical properties of porcelain are shown in Table 4.2.
3. Engineering Ceramics. A number of high performance engineering
ceramics are now produced. These are essentially simple compounds
Table 4.2 Mechanical properties of some ceramics
Ceramic Young’s Compressive Modulus of
modulus (GPa) strength (MPa) rupture (MPa)
Limestone 63 30–80 20
Granite 60–80 65–150 23
Porcelain 70 350 45
4.2 SEM micrograph of porcelain specimen: dark quartz crystals are
outlined in glassy regions which surround them; the fine light grey
crystals are of mullite. (Courtesy Dr P. F. Messer and H. E. Okojie.)