Sintering polycrystalline diamond

A wide range of cutting-tool materials is available. Properties, performance capabilities, and cost vary widely (2,7). Various steels (see Steel) cast cobalt alloys (see Cobalt and cobalt alloys) cemented, cast, and coated carbides (qv) ceramics (qv), sintered polycrystalline cubic boron nitride (cBN) (see Boron compounds) and sintered polycrystalline diamond tbin diamond coatings on cemented carbides and ceramics and single-crystal natural diamond (see Carbon) are all used as tool materials. Most tool materials used in the 1990s were developed during the twentieth century. The tool materials of the 1990s... 

Sintered polycrystalline diamond tools are much more expensive than conventional cemented-carbide or ceramic tools because of the high cost of the processing technique and the finishing methods used. Diamond tools, however, are economical on an overall-cost-per-part basis for certain applications because of long life and increased productivity. 

In the diamond stmcture, carbon atoms are present in sp hybridization, with a tetrahedral stereochemistry and a face-centered cubic stmcture that is shown in Fig. 2.1. Besides natural diamond, synthetic diamond has been produced since General Electric first announced its successful high-pressure synthesis in 1955. Sintered polycrystalline diamond, different types of diamond films, and diamondlike carbon are other types of diamond-related synthetic materials, some of which are noncrystalline [13, 19] these solids have their own terminology [10, 20]. Unhke other carbonaceous solids, diamond has a rather limited and specific relevance to adsorption. Indeed, ever since the publication of a pioneering work... 

In 1958, Hall [141] discussed the desirability of preparing a cemented diamond composition analogous to WC and hinted that experiments to produce polycrystalline diamonds were underway. But it was not until 1970, when he reported details of his procedures [142], that he established experimentally practical pressure and temperature fields where pure diamond powder can be sintered within times ranging from several days down to about one second. He mentions hard refractory materials like borides, carbides, nitrides and oxides as suitable binders. 

Polycrystalline diamond or PCD is produced by sintering micron diamond powders under ultrahigh pressure (>5 GPa) in the presence of a metal catalyst such as cobalt. These materials are commercially available in a range of grain sizes and typically are... 

Fullerene crystals can be produced at high yield. By counter diffusion from fullerene solution to pure isopropyl alcohol solvent, fullerene single crystal fibers with needle shape were formed. Needle diameters were found to be 2-100 pm and their lengths were 0.15 = 5 mm. Buckyball-based sintered carbon materials can be transformed into polycrystalline diamonds at less severe conditions using powder metallurgy methods. 

Diamond and cBN powders produced by milling are essentially monocrystalline and dominate the market. However, polycrystalline diamond powder can also be produced by shock synthesis. Under suitable conditions, shock waves produced by explosively driven projectiles can produce HPHT conditions in confined volumes for a sufficient duration to achieve partial conversion of graphite into nanometer-sized diamond grains which can also sinter into micrometer-sized, polycrystalline partieles." This process was commercialized by DuPont to produce a polycrystalline DMP (trade name Mypolex ) that is more friable than monocrystalline DMP and is well suited to fine polishing applications. Hexagonal (graphite-hke) BN will also react under shock-synthesis conditions, but the dense, nanometersized particles that are produced are of the wurtzite phase (wBN) rather than the cubic phase. So far, nano-wBN has not achieved much commercial importance. 

PCD (polycrystalline diamond) is obtained by sintering synthetic diamond powder in the presence of a metal binder (Co, Ni or Fe a low percentage by volume), at 1,350-1,500°C imder 5 GPa pressure. One may also sinter a layer of diamonds (0.5 mm thick) on a sintered hard metal substrate, the cobalt of the substrate thus participating in the sintering of the diamond and the adherence of the PCD on the substrate. Inserts up to 72 mm diameter and hardness of 5,000 to 8,000 HV may thus be attained. 

Limited supply, increasing demand, and high cost have led to an intense search for an alternative, dependable source of diamond. This search led to the high pressure (ca 5 GPa (0.5 x 106 psi)), high temperature (ca 1500°C) (HP—HT) synthesis of diamond from graphite in the mid-1950s (153—155) in the presence of a catalyst—solvent material, eg, Ni or Fe, and the subsequent development of polycrystalline sintered diamond tools in the late 1960s (156). 

Sintered Diamond. A natural polycrystalline form of diamond called carbonado is known. It is much tougher... 

Experiments so far have clearly indicated that these natural forms of sintered diamond are truly polycrystalline ceramics, exhibiting transgranular fracture as a result of diamond to diamond bonding. 

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