Ceramics machining approach

Most past research on grinding mechanisms for ceramics has followed either the indentation fracture mechanics" approach or machining" approach. The indentation fracture mechanics approach would seem to offer the possibility of describing both the material removal process and its influence on strength degradation in terms of the force or depth of cut at an... 

The goal of materials research is really the reverse process, the bottom-up method. In this approach, it is hoped that perfect well-controlled nanoparticles, nanostrucmres, and nanocrystals can be synthesized, which may be compacted into macroscopic nanocrystalline samples, or assembled into superlattice arrays, which may, in mrn, be used in a variety of applications such as nanoelectronic or magnetic devices. Some scientists have even envisioned a time when so-called molecular assemblers will be able to mechanically position individual atoms or molecules, one at a time, in some predefined way (Drexler, 1986). The feasibility of such machines has been hotly debated but, regardless, such systems engineering goals are not really within the scope of this chapter. At present, methods for synthesizing metal and ceramic clusters and nanoparticles fall in one of two broad categories liquid phase techniques or vapor/aerosol methods. 

The specimens of both ASTM D-897 and D-2094 can be adapted for use with materials that cannot readily be manufactured into conventional specimen shapes, by the approach shown schematically in Fig. 2c. Such materials as plates of glass or ceramics and thin polymer films can be sandwiched between the spool-like sections made of materials that can be more readily machined such as metals. Obviously, to obtain meaningful results from this test, the strength of the adhesive bond to the spool segments must be greater than that to the thin film or sheet. 

Ceramic feedstocks for extrusion can be produced either in a batch process, or continuously. Which approach is taken depends on the details of the application and the material to be prepared. In the ceramic industry at present, four different types of machines are generally used High shear mixers, roll mills, screw extruders and shear rolls. 

Brittle fracture is used for shaping and machining ceramics after they have been fired. Ceramics can be modified to make them machinable this is controlled fracture and is the approach we adopt with machinable glass-ceramics such as Macor (Chapter 26). Of course, many ceramics already are machinable and can be shaped into intricate and beautiful forms as illustrated in the carved marble sculpture shown in Figure 18.1. 

Mechanical approaches to machining ceramics include the following ... 

A synthetic or man-made diamond material is now available which is extremely tough with a hardness approaching that of natural diamond. A layer of this synthetic diamond material is bonded to a tough shock-resisting base of cemented carbide for use in the form of tips. The range of application is on non-ferrous metals such as aluminium alloys, magnesium alloys, copper, brass, bronze and zinc alloys and non-metallic materials such as ceramics, porcelain and plastics. This material will also machine fully sintered tungsten carbide. 

The material chosen as a test material is MACOR. MACOR is a machinable glass ceramic and is a fluorine rich glass with a composition approaching trisilicic fluorphlogopite mica (KMg3AlSi30ioF2) (6). The material was acquired in a rod form. 

As mentioned above, the dimensions of the samples do not change in the combustion wave. Thus, it is possible to synthesized net-shape articles directly by CS method. This approach is more important for the extremely hard silicon nitride-based ceramics, while h-BN materials are easily machinable. Figure 2.11 illustrates different net-shape articles (tubes, cylinders, screws, blocks, etc.) produced directly in the chemical reactor after minor finishing treatment. 

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