The process flow for the powder route is shown in Fig. 10.7. A uniform fine dispersion of the reinforcement and matrix powder helps in minimizing voids in the consolidated composite. When the constituents are not effectively packed, densification becomes more difficult, requiring higher pressures and longer times. Optimum packing occurs when the particle size distribution contains about 30% by volume of small particles and 70% by volume of large particles.12 Constituent mixing is often accomplished by ball milling.
Short fibers or whiskers are often mixed with a ceramic powder slurry, dried, and hot pressed. Short fibers and whiskers often undergo some orienting during hot pressing. Whisker agglomeration in the green body is a major problem. Mechanical stirring and adjustment of the pH level of the suspension (whiskers and matrix powder in water) can help. Addition of whiskers to a slurry can result in unacceptable increases in viscosity. Also, whiskers with a large aspect
Green Body Fabrication
Green Body Machining
Green Body Fabrication
Agglomeration due to surface chemical forces, electrostatic forces, or solid bridging. Ball milling is frequently used.
Organic binders used. Green body fabricated by cold pressing, injection molding, etc.
Consolidation and Densification
Typical methods include sintering, hot pressing, and HIP.
Fig. 10.7. Fabrication Sequence for Powder Processing ratio (>50) tend to form bundles and clumps. Obtaining well-separated and deagglomerated whiskers is important for a uniform composite. Organic dispersants and techniques, such as agitation mixing assisted by ultrasonics, can be used along with deflocculation by proper pH control.13
Organic binders are usually mixed with the reinforcement and matrix so that near net shaped parts can be produced by cold forming processes, such as uniaxial pressing, cold isostatic pressing, tape casting, extrusion, and injection molding. After cold consolidation, the green body can be handled and even machined without damage. All organic binders must be burned out either before or during the consolidation process.
While sintering without pressure is often used to fabricate monolithic ceramics, the presence of reinforcements significantly hinders the sintering process. Although SiC whisker reinforced alumina, with whisker contents less than 10% by volume, can be pressure-less sintered to greater than 95% theoretical densities, higher volume fractions will result in unacceptable porosity levels.14 To improve consolidation and provide acceptable levels of porosity, very fine ceramic particle sizes are used as well as pressure applied by hot pressing or hot isostatic pressing (HIP).15 Both hot pressing and HIP have limitations. Hot pressing is restricted by press size and tonnage and is not practical for complex shapes, while HIP can be expensive and difficult if the part must be "canned" with a metal bag for parts containing porosity open to the surface. Hot pressing is used to make cutting tools, such as those shown in Fig. 10.8, used to machine
Fig. 10.8. SiCW/Äl2O3 Composite Cutting Tools Source: Greenleaf Corporation difficult-to-machine metals, such as nickel based superalloys. The SiCw/Al2O3 composites can be hot pressed at 2700-3450° F at pressures of 3-6 ksi.
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