**3. Fabrication of diamond tools**

Predominantly there are two methods used for fabrication of single crystal diamond tools. The diamond tool fabrication is performed in such a way that the tool edge radius is maintained in the order of nanometers and a very smooth rake and flank surface are generated. These methods include mechanical lapping, thermochemical polishing, polishing with ion beam, damage-free tribochemical polishing, chemical-assisted mechanical polishing and planarization, chemical polishing with plasma, oxidative etching, and laser ablation.

Mechanical lapping is the most conventional and popular method to mechanically polish the single crystal diamond. Various mechanisms have been proposed in different studies such as: 1) Micro-cleavage or fracture along the (111) plane; 2) thermal wear; 3) Burnout or carbonization will take place at elevated temperature; 4) fracture or micro-chip in hard direction and plastic deformed layer in soft direction; 5) diamond phase transformation; 6) Plastic deformation as a result of brittleductile transition. Mechanical lapping is the simplest and most cost effective process method which enables to reach a cutting edge radius of about 70–80 nm [11]. It is

very challenging task to manufacture a diamond tool with cutting edge radius smaller than 50 nm. In order to produce very fine cutting edge profile for diamond tool, it is very important to understand the material removal mechanism in lapping of the diamond cutting tool. It will help in optimizing and help controlling the cutting edge sharpness.

Zong et al. [12] put forward the brittle–ductile transition theory to indicate the material removal nature in lapping of diamond surface layer. It explains that plastic deformation is responsible for the dominant removal mode of the lapped surface layer of the single crystal diamond in both the soft and hard directions on the diamond crystal planes. Plastic deformation in diamond crystal takes place when the embedding depth of diamond grit into the lapped surface layer is less than the corresponding critical depth of the cut, which leads to brittle–ductile transition.

In mechanical lapping, control of contact accuracy is extremely important between the high speed rotating scaife and lapped diamond tool. Accurately controlling the contact helps in better cutting edge sharpness. Lapping set-up is shown by schematic in **Figure 7**. The set up consists of cast iron scaife mounted on air bearing. Diamond tool is placed against the rotating cast iron scaife. Speed of cast iron scaife can be varied from 300 to 3000 rpm. Diamond grits of approximate size of 0.5 μm are used in the scaife for lapping. The relative velocity between scaife and the tool face is approx. 30 m/s. The contact accuracy should be ensured otherwise there will be impacts on the cutting edge and subsequent damages the cutting edge sharpness. Since the variations of the external load will change the number of grits in contact. In fact, only the material removal rate will change, and the mean force of single grit has almost no variations. So the changes of external applied load result in minute influence on the maximal groove depth left on lapped surfaces and are ignored in all experiments. Other variants of lapping are thermo-mechanical lapping, thermochemical polishing and chemically assisted mechanical lapping etc. which can also be employed to fabricate diamond tool.

In mechanical lapping, the removal rate takes place in the nanometric level that results in a sharp cutting edge radius ranging from 35 nm to 50 nm. Another common variant of the lapping of diamond tools is thermo-mechanical lapping [13]. Thermomechanical lapping is a development over mechanical lapping by using steel scaife in

**Figure 7.** *Schematic representation of the lapping set up [11].*
