Page 491 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

The next step in ceramic processing is to dry and fire Section 18.2 Shaping Ceramics 47|

l8.2.4 Drying and Firing

the part to give it the proper strength and hardness. if
Drying is a critical stage because of the tendency for TQ' j
/€,ii il a, 5 3
the part to Warp or crack from variations in moisture
content and in thickness. Control of atmospheric hu- Interparticle Pore Clay Dry
midity and of ambient temperature is important in water water particles
order to reduce warping and cracking. (H) (D) (C)
Loss of moisture during drying causes shrinkage
of the part by as much as 20% from the original, FIGURE l8.6 Shrinkage of Wet clay caused by the removal
moist size (Fig. 18.6). In a humid environment, the of vvater during drying. Shrinkage may be as much as 20%
evaporation rate is lovv, and consequently, the mois- by volume. Source: After F.H. Norton.
ture gradient across the thickness of the part is lovver
than that in a dry environment. This lovv moisture gradient, in turn, prevents a
large, uneven gradient in shrinkage from the surface to the interior during drying.
A ceramic part that has been shaped by any of the methods described thus far
is in the green state. The part can be machined in order to bring it closer to a near-
net shape. Although the green part should be handled carefully, machining it is not
particularly difficult, because of its relative softness.
Firing (also called sintering) involves heating the part to an elevated tempera-
ture in a controlled environment. Some shrinkage occurs during firing. Firing gives
the ceramic part its strength and hardness. This improvement in properties results
from (a) the development of a strong bond between the complex oxide particles in
the ceramic and (b) reduced porosity. A more recent technology (although not yet
commercialized) involves the microwave sintering of ceramics in furnaces operating
at more than 2 GHZ. Its cost-effectiveness depends on the availability of inexpensive
furnace insulation.
Nanophase ceramics (described in Section 8.2.5) can be sintered at lower tem-
peratures than those used for conventional ceramics. They are easier to fabricate,
because they can be compacted at room temperature to high densities, hot pressed
to theoretical density, and formed into net-shaped parts Without using binders or
sintering aids.

I8.2.5 Finishing Operations
Because firing causes dimensional changes, additional operations may be performed
to (a) give the ceramic part its final shape, (b) improve its surface finish and dimen-
sional tolerances, and (c) remove any surface flaws. Although they are hard and brit-
tle, major advances have been made in producing machinable ceramics and grindable
ceramics, thus enabling the production of ceramic components with high dimensional
accuracy and a good surface finish. An example is silicon carbide, which can be
machined into final shapes from sintered blanks.
o era-
The finishin s P rocesses em lo ed can be one or more of the follovvin s P
p_ y
tions, which are described in detail in various sections in Part IV of this book:
I. Grinding (using a diamond wheel)
2. Lapping and honing
3. Ultrasonic machining
4. Drilling (using a diamond-coated drill)
5. Electrical-discharge machining
6. Laser-heam machining