Uniaxial powder pressing

Figure 7.19 Uniaxial powder pressing process (a) empty die and ram (b) powders are loaded in die (c) ram is lowered and pressure is applied, compacting the powders (d) powder compact is ejected and die is ready for the next cycle.

Figure 13.20 Schematic representation of the steps in uniaxial powder pressing, (a) The die cavity is filled with powder, (b) The powder is compacted by means of pressure apphed to the top die. (c) The compacted piece is ejected by rising action of the bottom punch. (d) The fill shoe pushes away the compacted piece, and the fill step is repeated.

An intermediate form of isostatic pressure sintering, sometimes referred to as pseudo-isostatic hot-pressing, uses uniaxial hot-pressing apparatus but immerses the object to be sintered in a refractory non-interacting powder within the die and punches. This avoids the very considerable expense of building a furnace inside a thick-walled pressure vessel but the results are inferior to those achieved with true isostatic hot-pressing. 

Starting from as-synthesized powder, dense sintered h-BN can be obtained by axial hot pressing in graphite molds at a temperature of about 2200 °C, to yield monolithic ceramic bodies of 88-97% theoretical density. Any residual oxygen in the BN at the grain boundaries will form B2O3 that, subsequently, will act as an efficient sintering aid. The typical anisotropic properties of uniaxially hot pressed... 

Powder Pressing Some ceramic pieces are formed by powder compaction uniaxial, isostatic, and hot pressing techniques are possible. 

Uniaxial pressing is the method most widely used to impart shape to ceramic powders (24). Binders, lubricants, and other additives are often incorporated into ceramic powders prior to pressing to provide strength and assist in particle compaction (25). Simple geometries such as rectangular... 

More complex shapes can be made by cold isostatic pressing (CIP). CIP uses deformable mbber molds of the required shape to contain the powder. The appHcation of isostatic pressure to the mold suspended in a pressure transfer media, such as oil, compacts the powder. CIP is not as easily automated as uniaxial pressing, but has found wide appHcation in the preparation of more complex shapes such as spark plug insulators (26). 

Polycrystalline yttria-stabilized zirconia (YSZ-8) disks were prepared using 8 mol % Y203-doped zirconia powder (TOSOH zirconia, TZ-8Y, TOSOH Corp.). The powder was pressed uniaxially into disks which were cold isostatically pressed at 40,000 psi and then sintered at 1450°C for 4 h. Pt paste (Engelhard 6926) was painted on both faces of the YSZ-8 disks. Pt wires (0.0025 cm diam) were connected to the YSZ by using Pt meshes (150 X 150 mesh, 0.005 cm wire diam, Unique Wire Weaving Co., Inc.). 

In hot pressing, powder mixtures of Si3N4 with additives are heated to high temperatures under an applied uniaxial pressure. Traditional hot pressing uses 20-30 MPa pressure which enhances both rearrangement of particles and grain boundary diffusion. The hot pressing offers the ability to fabricate dense products, but also limits the products to simple shapes.25... 

In the compaction stage, two or more different powder compositions are stacked by sequential filling of the die and uniaxially pressed in an automatic press. Packing of powders and grain size are effective parameters in controlling the composition and thus the properties. After compaction, the laminates are sintered to obtain a gradual structure. 

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