Page 99 - Mechanical Behavior of Materials

Section 3.6 Ceramics and Glasses 97

economical to use in construction and has the important advantage that it can be poured as a slurry
into forms and hardened in place into complex shapes. Improved concretes continue to be developed,
including some exotic varieties with quite high strength achieved by minimizing the porosity or by
adding substances such as metal or glass particles or fibers.

3.6.2 Engineering Ceramics
The processing of engineering ceramics composed of simple chemical compounds involves
first obtaining the compound. For example, alumina (Al 2 O 3 ) is made from the mineral bauxite
(Al 2 O 3 –2H 2 O) by heating to remove the hydrated water. Other engineering ceramics, such as
ZrO 2 , are also obtainable directly from naturally available minerals. But some, such as WC, SiC,
and Si 3 N 4 , must be produced by appropriate chemical reactions, starting from constituents that are
available in nature. After the compound is obtained, it is ground to a fine powder if it is not already
in this form. The powder is then compacted into a useful shape, typically by cold or hot pressing. A
binding agent, such as a plastic, may be used to prevent the consolidated powder from crumbling.
The ceramic at this stage is said to be in a green state and has little strength. Green ceramics are
sometimes machined to obtain flat surfaces, holes, threads, etc., that would otherwise be difficult
to achieve.
The next and final step in processing is sintering, which involves heating the green ceramic,
typically to around 70% of its absolute melting temperature. This causes the particles to fuse and
form a solid that contains some degree of porosity. Improved properties result from minimizing the
porosity—that is, the volume percentage of voids. This can be done by using a gradation of particle
sizes or by applying pressure during sintering. Small percentages of other ceramics may be added to
the powder to improve response to processing. Also, small to medium percentages of other ceramics
may be mixed with a given compound to tailor the properties of the final product.
One variation on the sintering process that aids in minimizing voids is hot isostatic pressing.
This involves enclosing the ceramic in a sheet metal enclosure and placing this in a vessel that is
pressurized with a hot gas. Some additional methods of processing that are sometimes used are
chemical vapor deposition and reaction bonding. The former process involves chemical reactions
among hot gases that result in a solid deposit of ceramic material onto the surface of another
material. Reaction bonding combines the chemical reaction that forms the ceramic compound with
the sintering process.
Engineering ceramics typically have high stiffness, light weight, and very high strength in
compression. Although all are relatively brittle, their strength in tension and fracture toughness
may be sufficiently high that their use in high-stress structural applications is not precluded if the
limitations of the material are considered in the details of the component design. Increased use of
ceramics in the future is likely, due to their high-temperature capability.

3.6.3 Cermets; Cemented Carbides

A cermet is made from powders of a ceramic and a metal by sintering them together. The metal
surrounds the ceramic particles and binds them together, with the ceramic constituent providing
high hardness and wear resistance. Cemented carbides, as made into cutting tools, are the most
important cermets. In this case, tungsten carbide (WC) is sintered with cobalt metal in amounts