Ceramic Technology and Processing, King
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10
Effects of Processing on Properties
1.0 INTRODUCTION
Crafting a ceramic is similar to building a pyramid. It starts with a base, which, in this case, is the starting grain or powders. Everything depends on the base, and when it is insecure, the pyramid will not stand.
Both microstructure and superior properties derive from the starting materials, provided that these materials are handled with respect throughout the remaining processing. Preceding chapters addressed how careful handling is accomplished. Remaining to be addressed is the important task of understanding the results.
This chapter describes how materials selection and processing affects the properties of the final ceramic.
2.0 SELECTION OF MATERIALS
The characteristics of the starting materials are crucial and complex: there are many alternatives that depend on what is to be accomplished. Addressed below are three important factors: physical
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properties of the phases, chemical properties of the phases, and the final microstructure of results from the materials selected at the beginning. These preceding factors serve as guiding restraints; the final selection comes from experience.
Physical Properties of the Phases
Different materials have unique intrinsic properties that derive from the chemical bonding and crystal structure of the lattice. The initial choice of powdered and granular materials depend upon these properties.
Melting Point
The melting point of the phase limits the material’s temperature applications.
Thermal Conductivity
High conductivity reduces thermal shock and conducts heat. Materials with low thermal conductivity are useful thermal insulators.
Thermal Expansion
High expansion increases the tendency for thermal shock to occur.
Heat Capacity
High heat capacity retards heating and cooling.
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Electronic Conductivity
Different applications require either conductors or insulators.
Ionic Conductors
Such conductors are used in oxygen sensors and fuel cells.
Hardness
As a general rule, hard materials are more wear resistant. However, exceptions to this rule do exist.
Chemical Properties of the Phases
Chemical properties determine which phase will be stable at a particular temperature as well as the reactivity of a given application.
Thermodynamics
The free energy of formation predicts reactions occurring at elevated temperatures. Thermodynamics also predicts the phases that are stable at different temperatures. Many phase diagrams exist and are useful aids when selecting the starting raw materials.
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Solubility
A material can dissolve when it contacts a liquid. Because most phase diagrams do not provide solubility information, a material's solubility must often be determined experimentally.
Impurities
Impurities in the starting materials can greatly affect the final ceramic. So much as cost is not a limiting factor, the use of clean materials is much preferred.
Microstructure
Of concern is the effect of the starting materials on final microstructure after sintering. There do exist fine and coarse-grained materials. However, other considerations include sinterability, final grain size, shrinkage, porosity, and flaw populations. These properties are often characteristics of the starting materials.
Sinterability
Very fine powders sinter at lower temperatures and produce a finer grain intercept. Sinterability is also influenced by particle shape. Irregular particle shapes result in excess pore coalescence and a decreased sintered density. Blocky shapes pack well and sinter in an orderly fashion.
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Firing Shrinkage
Fine powders exhibit higher firing shrinkages for the following two reasons: these powders sinter to higher densities, and as the particle size becomes increasingly fine, surface forces between particles prevent packing to a high green density and result in a larger volume change upon densification.
Flaw Populations
Flaws in the green body greatly influence the fired strength. Strength increases and strength distribution narrows as flaws in the starting powder are removed. This effect is illustrated in Figure 10.1 as adapted from the literature.1
Figure 10.1: Strength Effect of Removing Flaws. Strength increases as flaws are removed from the starting powder and surface grinding.
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While the figure is schematic, it illustrates the strength behavior as flaw populations are eliminated: strength increases and the distribution narrows. Two options for removing flaws are to select a better powder or clean up the existing powder. Cleanup is accomplished by milling, leaching, classifying, and calcining.
Agglomerates in the starting powder often result in flaws in the fired body, an example of which is shown in Figure 10.2. Fracture origins are usually seen on fracture surfaces for fine-grained ceramics. This origin is an area where grain growth occurred, probably due to an impurity.
The relatively smooth area around the inclusion is called the mirror. In a homogenous material such as glass, the mirror, like its common-day counterpart, can be featureless. The mirrors on fine-grained polycrystalline ceramic fracture surfaces are less distinct, but still observable. Coarse-grained ceramics may not exhibit a fracture origin due to their heterogeneity. Most fracture surfaces exhibit a crack origin with the accompanying mirror. Sometimes these origins are inclusions, as depicted previously, but they can also be voids, areas of coarse grain size, or surface flaws caused by machining.
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A
B
Figure 10.2: Fracture origin. A scale bar 1000 μm, B scale bar 100 μm.
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Check List, Selection of Materials
• Physical Properties Melting point Thermal conductivity Thermal expansion Heat capacity
Electronic conductivity Ionic conductivity Hardness
•Chemical Properties Thermodynamics Solubility Impurities
•Microstructure Sinterability Firing shrinkage Flaw populations
Clean up of flaw populations
3.0EFFECTS OF TEMPERATURE AND PRESSURE ON PROPERTIES
Since temperature and pressure often interact, these properties are discussed together below.
Density, Die Pressed TZP
Figure 10.3, presented as a contour map, shows the typical effect of temperature and pressure on the fired density.
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Figure 10.3: Fired density contours as influenced by temperature and molding pressure, TZP zirconia. A maximum density occurs with a drop-off to the left of the diagram.
Figure 10.3 presents more information than first apparent by cursory review. Quite evident is the locus of equal densities depicted by the contours with a maximum in the area shown. The figure is a summary of a statisticallydesigned, factorial experiment where the interaction is evident from the contours sloping down to the left. An increase in pressure allows the ceramic to sinter to an equivalent density, but at a lower temperature. This is reasonable since the particles are packed closer together. Ordinarily, one maximizes the density. However, other criteria such as fracture toughness, can take precedent.
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Somewhat unexpected is that the contours curve around the maximum, where over-firing leads to a reduction of sintered density. This effect can be produced by bloating which will be discussed later. Additionally, the contours are more closely spaced at lower die pressures; that is the slope is steeper. One could conclude that low pressures make pressings where densification is insensitive to temperature. Lacking sufficient green density where the particles are mainly in contact, the material does not shrink as it normally would.
Also from the figure, one can observe that the contours slope around the maximum at the bottom, becoming almost horizontal. Here the sintering almost becomes independent of pressure, whereas on the left side of the figure, density was independent of temperature (both conclusions limited to this part of the diagram). Why the reversal? It is possible that at higher forming pressures, the particles are so close together that they sinter at a lower temperature resulting in the flat response. Of course, at even lower temperatures they would not sinter at all. However, for this data, the temperature was high enough for sintering to occur.
Density, Die Pressed Alumina
Alumina acts similarly to zirconia. However, when the powder is less reactive, the maximum does not loop around. This effect is potentially due to the higher sintering temperature that desorbs the gases so that bloating does not occur. This is shown in Figure 10.4.
At lower temperatures, the contours level out as observed in the TZP data. The reason for the independence of density and temperature is probably the same, at least for this temperature regime. Again, the contours are more closely packed on the left side of the figure, indicative of a strong dependence on temperature as the green density is increased.
For a given material and with other processing variables constant, contour maps, such as the preceding two figures, allow one to predict sintering behavior.