Page 219 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 8 Ceramics, Graphite, Diamond, and Nanomaterials: Structure, General Properties, and Applications

than metallic bonds. Consequently, properties such as hardness and thermal and
electrical resistance are significantly higher in ceramics than in metals (Tables 3.1
and 3.2). Ceramics are available in single-crystal or in polycrystalline form. Grain
size has a major influence on the strength and properties of ceramics; the finer the
grain size (hence the term fine ceramics), the higher the strength and toughness.

8.2.I Raw Materials
Among the oldest of the raw materials used for making ceramics is clay, which has a
fine-grained sheet-like structure. The most common example is lzaolinite (from
Kaoling, a hill in China), a white clay consisting of silicate of aluminum with alter-
nating weakly bonded layers of silicon and aluminum ions. When added to kaolin-
ite, water attaches itself to the layers (adsorption). This makes the layers slippery
and gives wet clay both its well-known softness and the plastic properties (hydro-
plasticity) that make it formable.
Other major raw materials for ceramics that are found in nature are flint
(a rock composed of very fine grained silica, SiO2) and feldspar (a group of crys-
talline minerals consisting of aluminum silicates and potassium, calcium, or sodium).
Porcelain is a white ceramic composed of kaolin, quartz, and feldspar; its largest use
is in appliances and kitchen and bath ware. In their natural state, these raw materials
generally contain impurities of various kinds, which have to be removed prior to fur-
ther processing of the materials into useful products with reliable performance.

8.2.2 Oxide Ceramics
There are two major types of oxide ceramics: alumina and zirconia (Table 8.1).

Alumina. Also called corundum or emery, alumina (aluminum oxide, Al2O3) is the
most widely used oxide ceramic, either in pure form or as a raw material to be blended
with other oxides. It has high hardness and moderate strength. Although alumina
exists in nature, it contains varying levels of impurities and possesses nonuniform
properties; as a result, its performance also varies. Aluminum oxide, silicon carbide,
and many other ceramics are now manufactured almost totally synthetically, so that
their quality can be controlled. First made in 1893, synthetic aluminum oxide is ob-
tained from the fusion of molten bauxite (an aluminum-oxide ore that is the principal
source of aluminum), iron filings, and coke in electric furnaces. The cooled product is
crushed and graded by size by passing the resulting particles through standard screens.
Aluminum oxide can be blended with small amounts of other ceramics, such as titanium
oxide and titanium carbide.
Structures containing alumina and various other oxides are known as rnullite
and spinel; they are used as refractory materials for high-temperature applications.
The mechanical and physical properties of alumina are suitable particularly in
applications such as electrical and thermal insulation and in cutting tools and
abrasives.

Zirconia. Zirconia (zirconium oxide, ZrO2, white in color) has good toughness;
good resistance to thermal shock, wear, and corrosion; low thermal conductivity;
and a low friction coefficient. Partially stabilized zirconia (PSZ) has higher strength
and toughness and better reliability in performance than does zirconia. It is obtained
by doping zirconia with oxides of calcium, yttrium, or magnesium. This process
forms a material with fine particles of tetragonal zirconia in a cubic lattice. Typical
applications include dies for the hot extrusion of metals, and zirconia beads used as
grinding and dispersion media for aerospace coatings, for automotive primers and
topcoats, and for fine glossy print on flexible food packaging.