Natural diamond The cubic form of carbon is renowned for its

10.4.6.2 Natural diamond The cubic form of carbon is renowned for its

hardness, strength and beauty. The carbon–carbon bond lengths are all the same (0.1555 nm) so that any sections taken through the tetrahedrally coordinated, highly symmetrical structure cut a large number of

these strong bonds; accordingly, there are no planes of easy cleavage. In commercial terms, there are three main qualities of mined diamonds: gemstones, industrial stones and boart (bort, bortz). Rough stones of gemstone quality are an exclusively natural product and their appearance is enhanced by a highly-demanding cutting procedure which involves cleavage on meticulously-selected planes, such as the

f 111g, and sawing/grinding/polishing with a mixture of diamond paste and olive oil. During cutting at least half of the mass is lost. 1 The symmetrical faceted shapes produced by the lapidary comply with standard mathematically-substantiated patterns (cuts) which optimise the optical effects of internal reflection (‘life’) and refraction (‘fire’). Three cuts currently favoured are brilliant, emerald or baguette.

Single industrial stones of near-gem quality are used as tools for trueing and dressing grinding wheels, engraving, rock-drilling and as dies for drawing wires of copper, steel, tungsten, etc. In the important pro- cess of trueing and dressing, a tool holding a single diamond is held against the working face of the grind- ing wheel as it rotates. The wheel profile is corrected and grit particles are left projecting slightly above the bonding matrix. Natural single crystals are used for drawing the smaller sizes of wire (e.g. <0.2 mm). These dies have a crystallographically-oriented aper- ture which is typically capable of passing 10 000 km of wire before dimensional tolerances are exceeded.

Diamond indenters are used in macrohardness and microhardness testing machines (Section 5.2.2.4). Sur- face textures of machined metals are commonly char- acterized by profilometers which traverse a sampling length with a fine diamond stylus; interpretation of the resultant high-resolution trace should be tempered by the observation that the moving stylus tends to plough

a furrow in the test surface. Boart is generally <1.5 mm in size, badly flawed

and imperfectly crystallized. For many years, boart was virtually unsaleable. Then, in the 1930s, its use as an abrasive grit or powder for impregnating the working faces of grinding wheels was fostered. Thereafter it found increasing application on a large scale for saw- ing, drilling and machining operations and became an accepted abrasive medium for non-ferrous metals and alloys, hard carbides, concrete, rock, glass, polymers, etc. The working face of a bonded grinding wheel that is being propelled into the workpiece is a compos- ite structure, consisting of abrasive particles of grit (diamond, cobalt-bonded tungsten carbide, alumina or silicon carbide) set in a bonding matrix. The matrix can be sintered metal, electroplate, vitreous or resinoid. Selection of the best combination of grit and bond can draw from a large pool of practical experience and broadly depends upon the material being machined and upon the machining conditions. It is reasonable

1 For diamonds, the carat (ct) is a unit of mass, with 1 ct D

0.2 g: for alloyed gold, the carat is one twenty-fourth part by mass of gold.

338 Modern Physical Metallurgy and Materials Engineering to regard each particle of diamond grit as a single cut-

ting tool. The tendency of some qualities of diamond to fracture and regenerate new cutting edges, rather than be torn out of the matrix, can be advantageous in certain machining operations. The high thermal con- ductivity of diamond (which is greater than that of copper) and low coefficient of friction minimize the generation and dissipation of heat from each of a myr- iad of such tools. Characteristics such as these have encouraged the trend toward higher rates of feed and greater peripheral wheel speeds, to the benefit of pro- ductivity, dimensional accuracy and surface texture. In simple terms, a higher cutting speed means that each particle is subjected to stress and heat effects for shorter periods of time. (The availability of synthetic grits has aided this particular development in grinding practice.)

Lapping and polishing operations require diamond powders; under these fine-scale cutting operations, heat generation is not usually a problem. In the metallo- graphic polishing of metallic and ceramic specimens,

a progression from coarse to sub-micron powders smoothes the surface as well as gradually reducing the depth of unwanted surface distortion.