1 WHAT IS SINTERING

1.1 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 finegrain 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 The elimination of isolated pores is more time consuming and utilizes almost all of the sintering time

1.3 DRIVINGFORCEANDBASICPHENOMENA
The driving force of sintering is the reduction of the total interfacial energy. The total interfacial energy of a powder compact is expressed as _A, where _ is the specific surface (interface) energy and A the total surface (interface) area of the compact. The reduction of the total energy can be expressed as
_ً_Aق ¼ __A ‏ _ _A ً1:1ق
Here, the change in interfacial energy (__) is due to densification and the change in interfacial area is due to grain coarsening. For solid state sintering __ is related to the replacement of solid/vapour interfaces (surface) by solid/solid interfaces. As schematically shown in Figure 1.5, the reduction in total interfacial energy occurs via densification and grain growth, the asic
phenomena of sintering.
In general, the size of powders for sintering is in the range between 0.1 and 100 mm; the total surface energy of the powder is 500–0.5 J/mole. This energy is inconsiderably small , compared with the energy change in oxide formation which is usually in the range between 300 and 1500 kJ/mole. If the desired microstructure of the sintered body is to be achieved by the use of such a very small amount of energy, it is necessary to understand and control the variables involved in the sintering processes.

1.4 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