PART I BASIS OF SINTERING SCIENCE

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1 [ :22am] [1 8] [Page No. 1] PART I BASIS OF SINTERING SCIENCE When thermal energy is applied to a powder compact, the compact is densified and the average grain size increases. The basic phenomena occurring during this process, called sintering, are densification and grain growth. To understand sintering and utilize it in materials processing, we first need to understand the fundamentals of the thermodynamics and kinetics of the two basic phenomena. In Part I, the process of sintering is briefly described and the related thermodynamics presented. The polycrystalline microstructure obtained after sintering is also characterized. The kinetics which determines the development of microstructure during sintering is the main content of this book and will be described extensively in Parts II to VI. 1

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3 [ :22am] [1 8] [Page No. 3] 1 SINTERING PROCESSES 1.1 WHAT IS 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 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. 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 molding, 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. 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. 3

4 [ :22am] [1 8] [Page No. 4] 4 CHAPTER1 SINTERING PROCESSES Figure 1.1. The four basic elements of materials science and engineering. 1 Figure 1.2. General fabrication pattern of sintered parts. 1.2 CATEGORIES OF 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 T 1, solid state sintering occurs in an A B powder compact with composition X 1, while at temperature T 3, 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 *Here, various types of sintering are explained using a schematic phase diagram, although, in most cases, the proper sintering type depends on the materials system and/or the sintering purpose.

5 [ :22am] [1 8] [Page No. 5] 1.2 CATEGORIES OF SINTERING 5 Figure 1.3. Illustration of various types of sintering. Figure 1.4. Typical microstructures observed during (a) solid state sintering (Al2O3) and (b) liquid phase sintering (98W-1Ni-1Fe(wt%)). 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.

6 [ :22am] [1 8] [Page No. 6] 6 CHAPTER1 SINTERING PROCESSES 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 DRIVING FORCE AND BASIC PHENOMENA 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 ðaþ ¼ A þ A ð1:1þ 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 basic 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 J/mole. This energy is inconsiderably small, compared with the energy change in oxide formation which is usually in the range between Figure 1.5. Basic phenomena occurring during sintering under the driving force for sintering, (A).

7 [ :22am] [1 8] [Page No. 7] 1.4 SINTERING VARIABLES 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 SINTERING VARIABLES 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 (see Section 5.6 and Section 11.6). Table 1.1. Variables affecting sinterability and microstructure Variables related to raw materials (material variables) Variables related to sintering condition (process variables) Powder: shape, size, size distribution, agglomeration, mixedness, etc. Chemistry: composition, impurity, non-stoichiometry, homogeneity, etc. Temperature, time, pressure, atmosphere, heating and cooling rate, etc.

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