Abrasive applications, diamond materials

Figure 6 summarizes the most important grit characteristics in grinding technology. The reactivity of diamond with transition metals such as nickel and iron limits the use of diamond to machine these metals, especially steels. However, there are some abrasive applications with ferrous materials where diamond is the tool material of choice, e.g., honing of cast iron (Marinescu et al. 2007). 

The properties of diamond, cubic boron nitride, and conventional hard materials are summarized in Table 9.1. In addition to being the hardest known substance, diamond is chemically inert to essentially aU environments below a temperature of about 500°C and is therefore uniquely qualified for many applications. Diamond has a cubic structure, with each carbon atom bonded to four nearest neighbors. Cleavage normally occurs on one of four (111) planes. In addition to its intrinsic brittleness, diamond has two important limitations. Diamond begins to oxidize and/or graphitize rapidly at temperatures above 600-700°C in air or an oxidizing atmosphere. Diamond readily dissolves in and can be graphitized by ferrous metals such as iron, steels, nickel, and nickel-based superaUoys, and therefore abrasion resistance with these metals is poor. 

The two major reasons for using diamond are the rapid stock removal and excellent finishes obtained. Diamond also can produce true flatness, and cause far less surface damage than other abrasives. And diamond properly used will keep dissimilar materials, one of which is a ceramic, at the same height. By selecting the proper diamond size and lap, a surface suitable for almost any application can be produced. 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

B4C boron carbide has a melting point of 2450 °C and a hardness somewhere between those of SiC and diamond. This makes the material a suitable abrasive. It is used in heads of sand blasting equipment, in mortars and in armour plating. For the latter application a B4C plate is provided on both sides with a plastic which has been reinforced with glass fibre. This is done to reduce the risk of splintering. Boron carbide is also used as the raw material for many other boron compounds ... 

A wide variety of materials have been implemented as abrasive particles in CMP processes. They include alumina, silica, ceria, zirconia, titania, and diamond. The effectiveness and suitability of these particles in CMP with particular applications are greatly influenced by their bulk properties (density, hardness, particle size, crystallinity etc.) and the surface properties (surface area, isoelectric electric point (lEP), OH content, etc.). This section will focus on the evaluation of alumina, silica, diamond, and ceria as the major abrasives used for the CMP of metals. 

Sodium borohydride is marketed in powdered or pellet form, and in solution, for use in fuel cells. Boron nitride can withstand temperatures of up to 650°C (1,202°E) when subjected to high pressures and temperatures, it forms cubic crystals whose hardness rivals that of diamond. Boron carbide, produced by reacting coke and boric acid at 2,600°C (4,712°E), is a highly refractory material and one of the hardest substances known. It has both abrasive and abrasion-resistant applications, and is used in nuclear shielding, see ALSO Davy, Humphry Gay-Lussac, Joseph-Louis Nuclear Chemistry. 

The extreme hardness and abrasion resistance of diamond makes this material an ideal choice for applications in which the optical components are exposed to aggressive environments. The use of diamond for optical elements for infrared seeking missiles and other military uses constitute probably the best examples of this type of application as discussed in more detail in this section. There are, however, a host of other applications in industry outside the military field in which the attributes of diamond optics are of great advantage. These include windows used in the monitoring of chemical reactions, or in the analysis of fluids which contain abrasive components. 

It is clear that the various diamond cutting tool formats are largely complementary and, with the development of electrically conductive CVD diamond, the potential application areas of the machining of highly abrasive workpiece materials becomes a reality. 

In Volume 2 ceramic hard materials are highlighted in the light of their applications. Chapter 1 of Part III concisely reviews the history of diamond and diamondlike super abrasive tools while Chapter 2 and 3 are concerned with the application of chemical vapor deposited diamond and diamond-like carbon films. These sections... 

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