Ceramic Technology and Processing, King

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indentations than does the interior. The weakening is thus substantial. When the graphite mold cavity is lined with Mo foil, this zone does not appear suggesting that the diffusion is largely retarded. The molecule responsible for this effect has not been identified, but one could hypothesize that it may be sulfur.

Figure 10.9: Abraded surface, Hot-Pressed Alumina. Sand blasting reveals a weakened zone adjacent to the contact with the graphite.

Mechanical Properties

Strength (MOR) and fracture toughness will be briefly discussed in this section.

Strength

Strength, in particular, is a capricious property that depends upon intergranular phases, internal flaws, residual stresses, and surface finish. These complications make strength difficult to specifically, yet generally,

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Most of this data is for materials being investigated for hightemperature use such as in ceramic gas turbines. The curves have been redrawn and do not include all the data from the reference since this figure is intended only to illustrate general behavior. In general, strength drops off with increasing temperature. Hot-pressed silicon carbide is exceptional in its resistance to temperature.

Specific data is not especially helpful since properties change depending on materials and processing conditions. The authors of the reference indicate that high-temperature strength depends largely upon the nature of the boundaries, with grain boundary glass having a negative effect. Oxides such as alumina decrease in strength where diffusion rates start to become appreciable around 1200 °C. Here, too, glass on the grain boundaries decreases strength at elevated temperatures.

Effect of Temperature on Grain Size

It is commonly known that an increase in temperature results in grain growth. This grain growth is beneficial if the desired effect is a decrease in transmitted light scattering. However, grain growth is undesirable for conditions of higher wear resistance. A very important consideration in many ceramics is maintaining pores on the grain boundaries where they can be sintered out. Pores within grains are largely trapped. Control of pore location relates to the rate of grain boundary movement. At least four factors are important: the starting powder, green structure, heating rate, and grain growth inhibitors.

With alumina, MgO is added to control grain growth. When the starting powder is right (i.e., correct sub-micron particle size, high purity, high uniform green density, and deagglomeration), the heating rate is not as critical, but restraints still exist. Figure 10.11 illustrates the inception of uncontrolled grain growth in alumina without MgO.

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Figure 10.11: Grain Growth with Temperature, Alumina. Exaggerated grain growth occurs when the temperature reaches a threshold value.

The curve is flat until about 1600 °C where grain growth significantly increases. The examined microstructure reveals that some grains grow at the expense of others and often become faceted. While it is possible that this might increase fracture toughness, it is deleterious to other properties such as strength. To avoid discontinuous grain growth, one should examine the following: the starting powder properties, limited sintering temperature, grain growth inhibitors such as MgO or NiO for alumina, or included second phases that drag on the boundaries as they move.

Check List, Effects of Temperature on Properties

• Gas Desorption

High temperature needed Bloating

Surface decomposition

Effects of Processing on Properties 357

• Gas Absorption

From the atmosphere From the kiln

From graphite

•Effects on Mechanical Properties Strength

High temperature strength Fracture toughness

•Effect on Grain Size

Grain growth inhibitors

5.0 EFFECTS OF PRESSURE ON PROPERTIES

Pressure used to compact a green body directly affects green density which, in turn, influences firing shrinkage, specific gravity, and wear resistance.

Green Density

Green density increases as the die pressure increases, as shown in Figure 10.12.

The curve is typical for a fine powder, with green density increasing with pressure and then starting to level off at high pressures. With a rigid test machine, the curve will show one or two breaks. This is actually what occurs but is not observable with an ordinary press. When the plot is made with percent of theoretical density plotted against the logarithm of pressure, the curves become straight lines, as shown in Figure 10.13.

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Different binders or powders exhibit different slopes and displacements in the figure. In this illustration, the #1 powder compacts to a higher density than #2. Highly sinterable powders sinter to a high-fired density even at relatively low green density (45%). Less sinterable powders are more sensitive to the green density and usually require a higher green density to sinter properly. The figure shows that selection of the powder and binder are tools for improving compaction.

Compaction

The powder compacts as it is pressed. The ratio of the fill height to the compact height is the compaction ratio. For a fine powder, this ratio is around 0.4. As the part is ejected from the mold, it springs back to a larger diameter than that of the die cavity. For a fine powder, the spring back is about 0.4% depending on how hard it is pressed.

Shrinkage

Firing shrinkage varies with the molding pressure. It is intuitive that the more dense the pressing, the less shrinkage occurs. A typical curve is shown in Figure 10.14.

This curve is for a fine compact material. Coarse materials have little or no firing shrinkage. Packing is more dense in the pressing direction, so there is less shrinkage. As a general rule, higher shrinkages result in increased firing distortion, less size control, and more cracks. On the other hand, high molding pressures can result in green cracks from a variety of sources. For any system, the optimum molding pressure must be worked out.

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Figure 10.14: Firing shrinkage with Die Pressure. The width of a cylindrical part shrinks more because it is packed to a lower green density.

Fired Density

For a given firing temperature, the sintered density is a consequence of the molding pressure as shown in Figure 10.15.

This figure suggests that the particles in the compact must be close enough together to sinter well. When the molding pressure is too low, the compact does not sinter to full density, and the pore size is relatively large due to pore coalescence. The curve begins to level out at higher pressures as the particles impinge upon one another.

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Figure 10.15: Sintered Density with Die Pressure. Fired density increases with die pressure and then plateaus.

Wear

With pressure as the variable, wear resistance is affected by the fired density. High-density ceramics wear less than the same material at a lower density, as shown in Figure 10.16.

The figure shows the cumulative percentage weight loss plotted against time of the wear test. The two curves bracket an under-pressed body and a properly-pressed body. If additional pressure is used, the wear resistance levels out since adequate green density has already been achieved.

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Figure 10.16: Wear resistance with Die Pressure. Weight loss is greater when the part is molded to a lower pressure.

Check List, Effects of Pressure

•Green Density Binders Powders

•Compaction Compaction ratio Spring back

•Shrinkage

•Specific Gravity