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PARTIBASISOFSINTERINGSCIENCE

When thermal energy is applied to a powder compact, the compact is densified and theaverage grain size increases. The basic phenomena occurring during this process, calledsintering, are densification and grain growth. To understand sintering and utilize it inmaterials processing, we first need to understand the fundamentals of the thermodynamicsand kinetics of the two basic phenomena. In Part I, the process of sintering is brieflydescribed and the related thermodynamics presented. The polycrystalline microstructureobtained after sintering is also characterized. The kinetics which determines thedevelopment of microstructure during sintering is the main content of this book andwill be described extensively in Parts II to VI.

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1SINTERINGPROCESSES

1.1 WHATIS SINTERING?

Sintering is a processing technique used to produce density-controlled materials andcomponents from metal or/and ceramic powders by applying thermal energy. Hence,sintering is categorized in the synthesis/processing element among the four basic elementsof materials science and engineering, as shown in Figure 1.1.1 As material synthesis andprocessing have become crucial in recent years for materials development, the importanceof sintering is increasing as a material processing technology.

Sintering is, in fact, one of the oldest human technologies, originating in the prehistoricera with the firing of pottery. The production of tools from sponge iron was also madepossible by sintering. Nevertheless, it was only after the 1940s that sintering was studiedfundamentally and scientifically. Since then, remarkable developments in sintering sciencehave been made. One of the most important and beneficial uses of sintering in the modernera is the fabrication of sintered parts of all kinds, including powder-metallurgical partsand bulk ceramic components.

Figure 1.2 shows the general fabrication pattern of sintered parts. Unlike otherprocessing technologies, various processing steps and variables need to be considered forthe production of such parts. For example, in the shaping step, one may use simple diecompaction, isostatic pressing, slip casting, injection molding, etc., according to the shapeand 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. Inthe sintering step, too, there are various techniques and processing variables; variations insintered 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 controlmeans the control of grain size, sintered density, and size and distribution of other phasesincluding pores. In most cases, the final goal of microstructural control is to prepare afully dense body with a fine grain structure.

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1.2 CATEGORIESOF SINTERING

Basically, sintering processes can be divided into two types: solid state sintering and liquidphase sintering. Solid state sintering occurs when the powder compact is densified whollyin a solid state at the sintering temperature, while liquid phase sintering occurs when aliquid phase is present in the powder compact during sintering. Figure 1.3 illustrates thetwo cases in a schematic phase diagram.* At temperature T1, solid state sintering occurs inan A–B powder compact with composition X1, while at temperature T3, liquid phasesintering occurs in the same powder compact.

In addition to solid state and liquid phase sintering, other types of sintering, forexample, transient liquid phase sintering and viscous flow sintering, can be utilized.Viscous flow sintering occurs when the volume fraction of liquid is sufficiently high, sothat the full densification of the compact can be achieved by a viscous flow of grain–liquidmixture without having any grain shape change during densification. Transient liquidphase sintering is a combination of liquid phase sintering and solid state sintering. In thissintering technique a liquid phase forms in the compact at an early stage of sintering, butthe 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

Figure1.1. The four basic elements ofmaterials science and engineering.1

Figure1.2. General fabrication pattern of sintered parts.

*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.

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diagram in Figure 1.3 when an A–B powder compact with composition X1 is sinteredabove 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 isformed 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 remainsbecause the equilibrium phase under the given sintering condition is a solid phase.

In general, compared with solid state sintering, liquid phase sintering allows easycontrol of microstructure and reduction in processing cost, but degrades some importantproperties, for example, mechanical properties. In contrast, many specific products utilizeproperties of the grain boundary phase and, hence, need to be sintered in the presence of aliquid phase. Zinc oxide varistors and SrTiO3 based boundary layer capacitors are twoexamples. In these cases, the composition and amount of liquid phase are of primeimportance in controlling the sintered microstructure and properties.

Figure1.3. Illustration of various types of sintering.

Figure 1.4. Typical microstructures observed during (a) solid state sintering (Al2O3) and (b) liquid phasesintering (98W-1Ni-1Fe(wt%)).

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Figure 1.4 shows typical microstructures of partially sintered powder compactswithout (a) and with (b) a liquid phase. In both cases, sintering has proceeded to the finalstage in which pores are isolated. Such an isolated pore stage is generally reached quicklyat usual sintering temperatures. The elimination of isolated pores is more time consumingand utilizes almost all of the sintering time.

1.3 DRIVINGFORCEANDBASICPHENOMENA

The driving force of sintering is the reduction of the total interfacial energy. The totalinterfacial 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 ofthe total energy can be expressed as

� �Að Þ ¼ ��Aþ ��A ð1:1Þ

Here, the change in interfacial energy (��) is due to densification and the change ininterfacial area is due to grain coarsening. For solid state sintering, �� is related to thereplacement of solid/vapour interfaces (surface) by solid/solid interfaces. As schematicallyshown in Figure 1.5, the reduction in total interfacial energy occurs via densification andgrain growth, the basic phenomena of sintering.

In general, the size of powders for sintering is in the range between 0.1 and 100 mm; thetotal surface energy of the powder is 100–0.1 J/mole. This energy is inconsiderably small,compared with the energy change in oxide formation which is usually in the range between

Figure1.5. Basic phenomena occurring during sintering under the driving force for sintering, �(�A).

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300 and 1500 kJ/mole. If the desired microstructure of the sintered body is to be achievedby the use of such a very small amount of energy, it is necessary to understand and controlthe variables involved in the sintering processes.

1.4 SINTERINGVARIABLES

The major variables which determine sinterability and the sintered microstructure of apowder compact may be divided into two categories: material variables and processvariables (Table 1.1). The variables related to raw materials (material variables) includechemical composition of powder compact, powder size, powder shape, powder sizedistribution, degree of powder agglomeration, etc. These variables influence the powdercompressibility and sinterability (densification and grain growth). In particular, forcompacts containing more than two kinds of powders, the homogeneity of the powdermixture is of prime importance. To improve the homogeneity, not only mechanical millingbut also chemical processing, such as sol-gel and coprecipitation processes, have beeninvestigated and utilized. The other variables involved in sintering are mostly thermo-dynamic variables, such as temperature, time, atmosphere, pressure, heating and coolingrate. Many previous sintering studies have examined the effects of sintering temperatureand time on sinterability of powder compacts. It appears, however, that in real processing,the effects of sintering atmosphere and pressure are much more complicated andimportant. Unconventional processes controlling these variables have also been intensivelystudied and developed (see Section 5.6 and Section 11.6).

Table1.1. Variables affecting sinterability andmicrostructure

Variables related to raw

materials (material variables)

Powder:

shape, size, size distribution, agglomeration,

mixedness, etc.

Chemistry:

composition, impurity, non-stoichiometry,

homogeneity, etc.

Variables related to sintering

condition (process variables)

Temperature, time, pressure, atmosphere, heating and

cooling rate, etc.

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