SINTERING SCIENCE,WHAT IS SINTERING
Sintering is a processing technique used to produce density-controlled
materials and components from metal or/and ceramic powders by applying
thermal energy. Hence, sintering is categorized in the synthesis/processing
element among the four basic elements of materials science and engineering, as
shown in Figure 1.1.1 As material synthesis and processing have become crucial
in recent years for materials development, the importance of sintering is
increasing as a material processing technology.
Sintering is, in fact, one of the oldest human technologies, originating in the
prehistoric era with the firing of pottery. The production of tools from sponge
iron was also made possible by sintering. Nevertheless, it was only after the
1940s that sintering was studied fundamentally and scientifically. Since then,
remarkable developments in sintering science have been made. One of the most
important and beneficial uses of sintering in the modern era is the fabrication
of sintered parts of all kinds, including powder-metallurgical parts and bulk
ceramic components.
Figure 1.2 shows the general fabrication pattern of sintered parts. Unlike
other processing technologies, various processing steps and variables need to
be considered for the production of such parts. For example, in the shaping
step, one may use simple die compaction, isostatic pressing, slip casting,
injection moulding, etc., according to the shape and properties required for the
end product. Depending on the shaping techniques used, not only the sintering
conditions but also the sintered properties may vary considerably. In the
sintering step, too, there are various techniques and processing variables;
variations in sintered microstructure and properties can result.
Sintering aims, in general, to produce sintered parts with reproducible and,
if possible, designed microstructure through control of sintering variables.
Microstructural control means the control of grain size, sintered density, and size and distribution of other phases including pores. In most cases, the final
goal of microstructural control is to prepare a fully dense body with a fine
grain structure.
1.2 CATEGORIESOF SINTERING
Basically, sintering processes can be divided into two types: solid state sintering
and liquid phase sintering. Solid state sintering occurs when the powder
compact is densified wholly in a solid state at the sintering temperature, while
liquid phase sintering occurs when a liquid phase is present in the powder
compact during sintering. Figure 1.3 illustrates the two cases in a schematic
phase diagram.* At temperature T1, solid state sintering occurs in an A–B
powder compact with composition X1, while at temperature T3, liquid phase
sintering occurs in the same powder compact.
In addition to solid state and liquid phase sintering, other types of sintering,
for example, transient liquid phase sintering and viscous flow sintering, can be utilized. Viscous flow sintering occurs when the volume fraction of liquid is
sufficiently high, so that the full densification of the compact can be achieved
by a viscous flow of grain–liquid mixture without having any grain shape
change during densification. Transient liquid phase sintering is a combination
of liquid phase sintering and solid state sintering. In this sintering technique a
liquid phase forms in the compact at an early stage of sintering, but the liquid
disappears as sintering proceeds and densification is completed in the solid
state. An example of transient liquid phase sintering may also be found in the
schematic phase diagram in Figure 1.3 when an A–B powder compact with
composition X1 is sintered above the eutectic temperature but below a solidus
line, for example at temperature T2. Since the sintering temperature is above
the A–B eutectic temperature, a liquid phase is formed through a reaction
between the A and B powders during heating of the compact. During sintering,
however, the liquid phase disappears and only a solid phase remains because
the equilibrium phase under the given sintering condition is a solid phase.
In general, compared with solid state sintering, liquid phase sintering allows
easy control of microstructure and reduction in processing cost, but degrades
some important properties, for example, mechanical properties. In contrast,
many specific products utilize properties of the grain boundary phase and,
hence, need to be sintered in the presence of a liquid phase. Zinc oxide varistors
and SrTiO3 based boundary layer capacitors are two examples. In these cases,
the composition and amount of liquid phase are of prime importance in
controlling the sintered microstructure and properties.
Figure 1.4 shows typical microstructures of partially sintered powder
compacts without (a) and with (b) a liquid phase. In both cases, sintering
has proceeded to the final stage in which pores are isolated. Such an isolated
pore stage is generally reached quickly at usual sintering temperatures.

SINTERINGVARIABLES
The major variables which determine sinterability and the sintered microstructure
of a powder compact may be divided into two categories: material
variables and process variables (Table 1.1). The variables related to raw
materials (material variables) include chemical composition of powder
compact, powder size, powder shape, powder size distribution, degree of
powder agglomeration, etc. These variables influence the powder compressibility
and sinterability (densification and grain growth). In particular, for
compacts containing more than two kinds of powders, the homogeneity of the powder mixture is of prime importance. To improve the homogeneity, not only
mechanical milling but also chemical processing, such as sol-gel and coprecipitation
processes, have been investigated and utilized. The other variables
involved in sintering are mostly thermodynamic variables, such as temperature,
time, atmosphere, pressure, heating and cooling rate. Many previous sintering
studies have examined the effects of sintering temperature and time on sinterability
of powder compacts. It appears, however, that in real processing, the
effects of sintering atmosphere and pressure are much more complicated and
important. Unconventional processes controlling these variables have also
been intensively studied and developed.

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