**2. Structure of diamond**

In ultra-precision and nanometric cutting operations, the most preferred tool material is single-crystal diamond due to its extraordinary properties, including highest hardness, very strong resistance to wear, perfect chemical stability and satisfactory service life, as well as the least sharpness, when finished. Due to its extreme strength and hardness, recently diamond has become an attractive choice in engineering processes and in a variety of other technological applications. Considering the morphology and structure of diamond, its mechanical and physical properties vary not only with the different crystallographic planes but also with different crystallographic directions on a plane.

### **Figure 4.** *Crystal structure of single crystal diamond.*

Diamond is a crystal allotropic form of Carbon and has diamond cubic crystal structure with a lattice parameter of 0.3567 nm (see **Figure 4**). Each C atom has four neighboring C atoms in a tetrahedron structure with bond angle between the atoms of 109°28′. The C atoms are covalently bonded together with sharing of one of its outer four electrons with each of four other C atoms resulting in sp3 hybridization. This sharing of electrons results in a strong binding among each pair of C atoms in all three dimensions.

Diamond has some outstanding properties which makes it an obvious choice for the cutting tool material.


iii.It has highest atom-number density.

iv.At room temperature, it has highest thermal conductivity of any solid.

**Table 1** shows the mechanical properties of diamond. Diamond is categorized into four categories:


Diamond naturally occurs produced in nature at high pressure and temperature in volcanic shafts. With high pressure synthesis, it is able to produce diamond possessing the strength similar to natural diamond. Generally, the natural and synthetic diamond are used for cutting purposes. Diamonds usually exist with other allotropes

#### **Figure 5.**

*Hardness of various tooling materials.*


#### **Table 1.**

*Mechanical properties of diamond.*

of carbon and they can be characterized by knowing the structure, atomic vibration and electronic states. The diamond characteristics are usually determined using X-ray diffraction, electron microscopy and Raman spectroscopy. For diamond to be qualified, it must have the following characteristics:

i.A crystalline morphology revealed by electron microscopy

ii.Crystalline structure with a single phase identified by X-ray diffraction

iii.A sharp peak at 1332 cm−1 detected by Raman spectrum

Diamond occurs predominantly in three different forms in cubic (100), octahedron (111) and dodecahedron (110) and their combinations. These three forms are based on the three planes ((100), (110) and (111)) from a simple cubic crystal structure. **Figure 6** shows the schematic of the (100) cubic, the (110) dodecahedral and the (111) octahedral. The octahedral form of diamond crystal is the most common form.

Diamond breaks along {111} crystal planes also known as cleavage planes. Bonding is less strong perpendicular to these planes as compared to the other directions due to lower total number of bonds and hence the diamond fractures favorably along

*Diamond as a Precision Cutting Tool DOI: http://dx.doi.org/10.5772/intechopen.108557*

#### **Figure 6.**

*Schematic diagrams showing the location of {111}, {100} and {110} planes on an octahedron diamond.*


#### **Table 2.**

*Diamond classification based on impurity level [10].*

these planes. Diamonds are also categorized into four types based on the presence of nitrogen as an impurity in their crystals. These are: Ia, Ib, IIa, IIb. Majority of natural diamonds (~99%) come in type Ia. Type Ia consists of nitrogen in the form of aggregates in the crystal. All synthetic diamonds are of type Ib with an even distribution of nitrogen atoms in the crystal. Types IIa and IIb are very rare in nature but can be synthesized for industrial purposes. In all natural diamonds, there exist many impurity elements such as nitrogen, hydrogen, boron and oxygen. **Table 2** shows different classifications of diamond.
