Ceramic Technology and Processing, King

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top of the part was heating faster than the bottom, causing the top to shrink and bending it upward. As the bottom starts to sinter, it shrinks and causes the part to flatten out. As the sintering continues, the part then curves the other way. Sometimes, timing is everything. Apparently, there was a cold hearth or a hearth plate with too much thermal mass contributing to the problem, but the solution was simply to remove the part at the time when it was flat.

Thin parts present special problems. A thin plate of tetragonal zirconia polycrystal (TZP) that is yttria-stabilized zirconia can be die pressed and fired. The problem is that the part is thin, only 0.040 inches in thickness and 6 inches in length, and it is bound to warp. Thin pieces of TZP can be set between two 99% alumina setter plates that are one half inch in thickness and are diamond ground flat and smooth. Carefully place the pressed green strip between the two plates and fire it. The TZP part comes out of the kiln flat and intact. However, it did warp laterally probably due to an uneven die fill.

Setting Long Tubes. A short, stubby tube can be set on end, but a long tube cannot. It could be set in a V-shaped block, but there is a good chance that it will sag out of round. A better way is to hang fire the tube as in Figure 8.19.

In the kiln, a support structure at the top will hold the tube as it sinters. At first, the tube is held up by a block at the bottom. As the binder burns out, the tube gets very weak and cannot support its own weight. However, since it rests on the block, it is in compression where it is much stronger. It gets stronger as it starts to sinter. Shrinkage lowers the tube onto its conical seat, and it separates from the base support. Now, it is in tension; however, it is strong enough to withstand the stress. Being in tension, the tube straightens as it fires and retains its circular cross section. Castable alumina refractory can be used for the support structure. The tube shown in the figure has a conical top that is slip cast as an integral part of the tube. Another way is to cement a collar on the top of the tube to hold it during sintering. For thermal uniformity up the length of the furnace, there might have to be more than one tier of burners. For hang firing, the burners fire in tangentially to avoid direct flame impingement.

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Figure 8.19: Hang Firing Tubes. Hang firing keeps tubes straight and round.

Check List, Setting in Air

•Chemical compatibility

•Setting bed

•Setting plates

•Setting pack

•Supports

•Binder burnout bed

•Stacking ware

•Shrinkage plates

•Setting large parts

•Setting special shapes

•Hang firing long tubes

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Setting for Atmosphere Firing

Two aspects of setting atmosphere firing will be considered: graphite furnaces and metal furnaces.

Graphite Furnaces

Two ceramic materials are predominantly fired in graphite furnaces: SiC and Si3N4. A problem with both these ceramics is that one constituent has a much higher vapor pressure than the other. Silicon has a much higher vapor pressure than carbon, and nitrogen has a much higher vapor pressure than silicon. These ceramics will partially decompose during sintering. Setting practices can compensate for this problem in a few ways.

Enclosures. One can enclose the part in a graphite box to increase the local vapor concentration of the more volatile species. This works for SiC, especially if the box has been pre-siliconized. It also helps if one buries the part in SiC grain. One has to use coarse grain (60 mesh or so) or else the grain will sinter hard making it too difficult to remove the part later. Figure 8.20 depicts a typical enclosure.

This box is machined from a block of graphite with an interlocking lid. Ordinary graphite has impurities that can be removed by heating in Cl2, a commercially available process. One can pre-siliconize a furnace or container by first firing a bed of SiC grain, which coats the interior with SiC. It is not a good idea to use Si for this process as it will cut a hole through the bottom of the box when it melts. Silicon can be added to SiC grain as there is enough grain to contain the molten Si by capillary forces.

Figure 8.21 shows another type of enclosure.

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Figure 8.20: Graphite box Containment. Samples are often enclosed in a box to control the sintering atmosphere.

Figure 8.21: Tube Furnace Support. A firing assembly can be inserted into a tube furnace to hold the parts being sintered.

The two end members of graphite have rods of alumina that electrically isolate the support from the heating tube. Threaded rods hold the boat in the hot zone. The boat can have a lid as Figure 8.20. While the structure seems fragile, graphite is quite strong and creep resistant at high

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temperatures. For a vertical tube, one can use a structure such as that in Figure 8.22.

Figure 8.22: Vertical Support. With a vertical tube furnace, a sample holder can be inserted into the hot zone.

Fins on the support rod radiate heat back into the hot zone and help to keep the end cool. A graphite crucible holds the sample.

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Graphite plates

When the furnace is larger, one can set the part directly on a graphite hearth plate, as in Figure 8.23.

Figure 8.23: Graphite Hearth. The hearth should be raised and on a base insulated with graphite fiber.

One can also place a box on the hearth plate. Avoid cold hearths. In Figure 8.23, the plate is raised to allow for even heat distribution. Graphite fiber is an excellent insulating material; one can place it in the hearth made from thin graphite plates.

When firing SiC exclusively, it is necessary to siliconize the entire furnace in a prefiring. This can be done by firing SiC grain or SiC grain with Si mixed in. A graphitic layer forms on the SiC part after firing, if it is not set in a Si-rich atmosphere.

Si3N4 is fired in a N2 atmosphere, often at a high pressure to suppress nitrogen loss. About 200-1000 psi is in a usable range depending upon the temperature. Nitrogen loss is a more serious problem as the temperature increases, as this requires a higher pressure. One can do several things with the setting.

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Pack the part in S3N4 grain, in a graphite box lined with BN plates, or in a box coated with a BN wash. BN is hot pressed and is very expensive.

Metal Furnaces

There are two classes of metal furnaces: Ni/Cr alloys are for low temperatures, and Mo is predominantly for high temperatures. With Ni/Cr, one can buy muffles that slip into preexisting furnaces and can be connected to vacuum/atmosphere manifolds. Binder burnout of materials, where this is inadvisable in the sintering furnace, can be done in the muffle.

One can use Tungsten as heating elements whenever very high temperatures are necessary because it embrittles. Tantalum (Ta) also embrittles but it is very expensive.

Thin sheets of Mo or Ta can be used as setters, often just once. One can also use refractory ceramic plates such as alumina, magnesia, or zirconia as setters, but make a nondestructive trial run as a precaution. As mentioned earlier, free energy data can be used to predict reactions between materials. If one is not familiar with this, an expert should be consulted.

Check List, Firing in Atmosphere

•Volatilization

•Purified graphite

•Support structures

•Pressurized atmosphere

•Metals, Ni/Cr, Mo

•Metal foil setters

•Setting on graphite

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4.0 FIRING PROCEDURES

After proper milling, slip preparation, and part formation to a uniform high density, the difficult tasks will have been done. With good equipment in place and the parts set properly, firing is usually straightforward. There are four steps in firing for most ceramics: binder burnout, ramp up, soak, and ramp down. Each step will be considered, as it was before for firing in air.

Binder Burnout

There are two potential problems, bursting the part and, with injection or wax molding, the part may slump. With bursting, the problem is having permeability high enough to allow the binder decomposition products to escape without building up excessive internal pressure. Obviously, this is less critical with coarse materials than with fine. Ceramics with low thermal conductivity are also more difficult because of thermal gradients. Modern programmers make burnout easy, and ramps of about one half degree per minute up to 500 °C are typical. If there is trouble, just adjust the heating rate. Most binders are essentially gone by 300 °C to 400 °C, which can be determined by thermal gravimetric analysis.

Ramp Up

With the binder burnout, the heating rate can now be steeper. Ramps of 4-5 °C/hr are typical for fine-grained, dense ceramics where thermal shock is a problem. Fire the other materials on a faster ramp. During ramp up, porosity is coarsening and sintering starts. High rates of grain boundary movement can result in pores being trapped within the grains, which suggests that the upper part of the ramp might be too fast for fine-grained, dense ceramics. Atmosphere in the pores can also have an

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effect. Gases that can diffuse along grain boundaries help to attain a high density. These include oxygen, hydrogen, and vacuum for ceramics such as alumina. Gases such as nitrogen and argon are more difficult to remove.

Soak

Determine the soak temperature and time experimentally. This is another good place for a statistically designed experiment, where the independent variables are milling time, die pressure, soak temperature, and time. The dependant variables are chosen to optimize the properties of the ceramic. These can be density, strength, permeability, or whatever is needed. Lab firing cycles generally have a soak time of about one to two hours. Pure oxides soak between 1400 and 1800 °C, depending on the particle size and composition.

Ramp Down

The most common ramp down is just to turn the power off and let the furnace cool at its own rate. The first part of the cool down is steep, and it might be necessary to fire down for the initial part of the ramp. Generally, there is little difficulty associated with ramp down, except the newer fiber lined kilns where the ramp could be too steep, causing thermal shock.

Multiphased Ceramics

These materials can present special firing problems. Two systems will be considered as challenging: MgO stabilized zirconia, and alumina with an interstitial glassy phase.

Partially Stabilized Zirconia

Zirconia by itself undergoes a phase change at 1200 °C, from monoclinic at lower temperatures to tetragonal at higher temperatures.

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There is a big increase in density, which disrupts the ceramic to where it falls apart. To avoid this, the zirconia is stabilized. CaO, Y2O3, CeO2, or MgO are the most common stabilizers. There are other less important molecules such as the rare earths. The ZrO2-MgO system is especially interesting. It is a commercial material that shows the use of ceramic crafts. The phase diagram is as in Figure 8.24.

Figure 8.24: ZrO2-MgO Phase Diagram. The part sintered at A is in the cubic solid solution region. When cooled, it will precipitate out small inclusions of tetragonal zirconia and impart toughness. The part sintered at B will contain both tetragonal and cubic phases. This imparts a lower thermal expansion and improves thermal shock.

Along the horizontal axis is the amount of magnesium oxide added to the batch. When this batch is heated to an elevated temperature, several particular phases form. The diagram shows these phases: C-SS cubic solid solution, T-SS tetragonal solid solution, and M-SS monoclinic solid solution.

Solid solutions are where the material crystallizes in a specific