**4.1 Experimental procedures and materials characterization**

For the synthesis, characterization and their applications of NPD materials, see the review journal paper in [12]. This chapter will focus on our own work on the basis of micro-polycrystalline diamond (MPD) or catalyst-free polycrystalline diamond (CFPCD), based on a newly developed hinged six-sided cubic press [18]. Drilling very hard, abrasive and sandwich formations presents a significant challenge for today's PDC drill bits. Current PDC tools on drill bits do not provide sufficient wear, impact, or thermal stability to withstand this drilling environment, resulting in low penetration rates (ROP) and short drill life. The weakness of existing PDC cutters is due to the inevitable use of cobalt catalysts to bond diamond grains manufactured by traditional HPHT techniques with a combined capacity of about 5.5 GPa and a temperature of about 1400°C. In our study, ultra-high pressure and high temperature (UHPHT) technology was developed by an innovative two-stage multiple anvil that is capable of producing ultra-high pressure of 14–35 GPa, which is three to seven times that of traditional PDC cutting machine manufacturing techniques. In addition, extreme heat from 1900–2300°C was achieved. Using this UHPHT technology, new PDC materials with super strength and no catalyst at two high pressures of 14 and 16 GPa have been successfully produced in order to study the different responses of material properties under different processing parameters. As the starting material for the experiment, the average particle size of commercially available high-purity diamond powder is 10 microns, and the particle size distribution is 8 to 12 microns. Then these

diamond powders were pressed in the newly developed and innovative UHPHT press. Details of the processing can be found in the references [13]. The preparation of NPD from graphite involves a phase transition mechanism [15]. This results in a significant volume change of more than 35% due to density changes (under ambient conditions, graphite density ~ 2.25 g/cc to diamond density ~ 3.52 g/cc). Large shrinkage is known to make pressure and temperature control more unstable or difficult (heater deformation), especially under extreme HPHT conditions. On the other hand, our CFPCD synthesis or fabrication process uses diamond powders with micron, sub-micron or nanoscale grains available on the market as raw materials to solve all of the above problems and challenges, as there is no phase transition when pressing diamond powder into diamond blocks under UHPHT. As a result, the microstructure of CFPCD materials and the resulting mechanical properties are more controllable between hardness and toughness. This is important for large-scale industrial tool manufacturing applications. Essentially, this work provides a new strategy or approach to developing the next generation of super-hard or super-hard diamonds. **Figure 7(a)** and **(b)** show the typical microstructure of a PDC tool with pressures of 14-GPa and 16-GPa, respectively, showing the full detection of diamond structures without a metal binder.

Scanning electron microscopy (SEM) was used to study the morphology and microstructure of polished samples. Samples prepared at 16 GPa and 2300° C are characterized by transmission electron microscopy (TEM) with an acceleration voltage of 200,000 volts for microstructures at high magnifications. **Figure 8** shows the HRTEM nanostructure features in a 16-GPa CFPCD material that are produced by severe plastic deformation of a single diamond grain during UHPHT. Extensive twins and stacked faults in the particles were observed. This is due to the high-pressure work hardening mechanism [13], which can greatly improve the strength of UHPHT CFPCD materials. Typically, CFPCD materials consist of a diamond skeleton consisting of micron-sized particles and an isolated Y-zone. Each micron-sized particle has a substructure of stacked nanoplates, while the Y-zone consists of NPDs embedded in turbo graphite and amorphous carbon. This unique micro-nested structure stems from the plastic deformation of the diamond particles and the mutual transformation of the diamond produced during the high temperature and high-pressure process.
