Ceramic Technology and Processing, King

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parts recovered by disassembly. It is practical to use segmented molds when one needs many identical parts. Segmented parts are in contact with the graphite and require selection to avoid unwanted reactions.

Triaxial Hot Press Design. All of the preceding molds have a monolithic shell holding the parts together. One can transfer this holding restraint outside the furnace with push rods. Figure 8.37 is of a view looking down into the furnace.

Figure 8.37: Triaxial Hot Press. Interior mold parts can be held in place with rams, placing all of the ceramic mold parts in compression.

The mold is in the center and consists of four segments restrained with four push rods that hold them in place. Two of these rods butt up against a square steel frame. The other two rods (at 90 degrees) are

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connected to hydraulic cylinders that supply the thrust holding the mold parts in place. The plungers that compact the sample come out the top and connect to a hydraulic press. In the above figure, the top was removed to expose the interior construction. The mold in Figure 8.37 makes a cylindrical sample. Figure 8.38 is of another mold with a square cross section.

Figure 8.38: Triaxial Mold Parts. The four parts are dissembled after the sample is hot pressed. They can be lined with Mo foil to prevent reactions.

The sides of the mold are identical in size and shape and fit together with rabbet joinery. The punches are rectangular blocks. While

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called a triaxial design, pressing is uniaxial with the other two axes only holding the mold parts in place. The inherent advantages to this design are as follows.

•All ceramic parts are in compression.

•A variety of ceramics can be used for the mold.

•Higher pressures can be used for hot pressing.

•There is little graphite contamination.

•Mo foil spacers are used on the walls and ends.

•The sample is recovered by disassembly.

•Triaxial designs can be scaled up.

One can make molds from recrystallized silicon carbide, titanium diboride, or other refractory materials. Push rods can be made from recrystallized silicon carbide. Silicon carbide has lower thermal conductivity than graphite and is stronger; therefore, the rod diameter is smaller. This lowers the heat loss out through the push rod.

Materials

Hot pressing is more expensive than sintering and substitutes for sintering to densify the part or to produce the necessary superior properties. Some examples will be discussed.

Oxides

There are two approaches to obtaining a very fine microstructure. Hot pressing: where pressure reduces the temperature and limits grain growth. Alternatively, very finely-divided, high-purity powders that sinter at lower temperatures can be used. The trend is toward the latter approach, but hot pressing reduces the flaw population to 10-μm cavities or larger cavities. Alumina cutting tools have been hot pressed to advantage.

Carbides/ Borides/ Nitrides

These diamond-like structures are difficult to sinter and are often hot pressed. Without use of boron and carbon additives, SiC does not

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densify during sintering.5 Otherwise, one needs to hot press SiC. Sintering and hot pressing temperatures are over 2000 °C; this adds to the difficulties and cost. Hot pressing boron carbide (B4C) yields a high density and the best properties. Sand blast nozzles and other wear parts are made from hot pressed B4C. TiC and TiN are often hot pressed. Si3N4 is usually sintered with alumina and yttria sintering aids.6 These combine with silica in the material to form an interstitial glassy phase. This may not be the best solution for making high quality parts, but many shapes are not conducive to hot pressing technology.

Composites

During sintering, it is not uncommon for one of the phases to interfere with the densification of the other. This can be overcome by force in the hot press. Maybe the best example is alumina-cutting tools reinforced with silicon carbide whiskers. Whiskers form a tangle that gets in the way of alumina sintering. These tools are very successful due to their abrasion resistance and especially their toughness. Unlike sintering, one can hot press metal inserts into ceramic structures. A good example is shown in Figure 8.39 where steel fasteners are molded into an alumina ceramic.

Figure 8.39: Hot Pressed Ceramic with Metal Fasteners. Metal inserts can be integrally hot pressed into the ceramic and used as fasteners.

Hot press the alumina at a low enough temperature to avoid reactions with the steel. This can be done by using a submicron powder, a glass interstitial phase, or a sintering aid such as titania. Steel inserts are set

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on MgO pressed powder discs to isolate the steel from the graphite mold and to prevent their reaction. MgO does not sinter at low temperatures and is easily removed after hot pressing. Since steel (iron) has a higher thermal expansion than alumina, the insert is loose after cooling, but it is locked in place by its shape. One can drill and tap the steel inserts making the composite suitable for bolting to a structure. Figure 8.40 shows another example of a hot-pressed composite. It shows the placement of a finegrained facing on a coarse-grained backing.

Figure 8.40: Hot Pressed Ceramic with Dense Facing. Two different materials can be hot pressed together. Here, a dense facing was pressed on a porous backing.

By facing an alumina refractory block with a dense, fine-grained, alumina ceramic, the surface becomes impervious and wear resistant. This could not be done by sintering because of the huge difference in shrinkage during firing. This is not a problem in hot pressing. Another technique is to mold a metal reinforcement into a ceramic structure, seen in Figure 8.41.

The figure is schematic as there are limits on the amount of Mo metal that can be successfully incorporated because of the lower thermal expansion. One example is wire or perforated sheet in hot pressed alumina. Alumina penetrates the spaces between the metal during hot pressing and forms a composite structure. This would not be possible with sintering since the ceramic shrinks and the metal does not. Interestingly, boron has a thermal expansion very close to alumina up to 1000 °C and boron fibers have been made. This could make an interesting composite.

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Figure 8.41: Hot Pressed Ceramic with Metal Reinforcing. A Mo mesh can be hot pressed in a ceramic to provide reinforcement.

While not a composite, there is another interesting thing that one can do with hot pressing. Coin a preform out of graphite powder with a particular surface contour, which will be reproduced on the ceramic surface. Look at Figure 8.42.

Reproduce the surface of the penny by placing it in a die, pour in a fine graphite powder, and press. Place the graphite form in a hot pressing die, pour in fine alumina powder and level it. Recover the alumina reproduction after hot pressing. While this is not a good way to make money, the technique could be useful for more reasonable things.

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Figure 8.42: Coined Alumina Penny. By pressing graphite powder over a contoured face, a reproduction can be made with a ceramic part.

Check List, Hot Pressing

•Decision to hot press

•Equipment choices: resistance, induction, vacuum, insertion, triaxial

•Equipment design

•Molds: graphite type, monolithic, segmented, foil separators

•Special cases: low expansion ceramics, tubes, cones

•Reactions with graphite

•Ceramic materials: oxides, carbides, nitrides, borides

•Composites: matrix+whiskers, metal embedded fasteners, metal reinforcements, hard facings

•Coining

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Starting from the outside and working one's way in, the press end closures are secured with a wire-wound yoke. Wire-wound structures are tougher than solid steel, which is why they are used for suspension bridges. Some HIPs have breach block end closures that also work well. Pressure vessels are heat-treated, alloy steel with a good safety factor, and they are pressure tested above the operating specifications. The vessel is water cooled with a jacket. For graphite heaters, the thermal insulation is graphite fiber molded into shapes. Slotted graphite heating elements are common. The base of the heater is a structure that supports the sample and element. It also connects to the power supply. When the heating element is a refractory metal, the thermal insulators are radiant heat shields. The internal structure is similar to that in vacuum furnaces. When using metal construction, the atmosphere is a lot cleaner. Graphite turns a white sample dark grey. Temperature is sensed with a thermocouple. Pt-Pt/Rh is preferable if the temperature does not exceed 1650 °C, otherwise use W/Re.

Enclosures

It is common practice to isolate HIPs for safety reasons. When cramped for space, surround the HIP with sliding panels that run on both bottom tracks and hang from rails supported by stanchions. Panels are made to manufacturer's specifications, 1/4-inch steel with 2-inch plywood fastened on the HIP side of the panels. Plywood traps the floating shrapnel, while steel prevents penetration, not that this is likely to occur. A big advantage is that by sliding the panels around, access is available to all sides of the apparatus.

Difficulties

Some HIP designs result in difficulties, which is of interest as it is a warning to the pitfalls in design that should be avoided.