**Figure 9.**

*The optical images of single crystal diamond indenters (a) before the hardness testing and (b) after the testing on CFPCD material, showing the hardness of the CFPCD material is higher than single crystal diamond (120 GPa). Arrow in* **Figure 4b** *indicates the damage on the indenter tip [19].*

repeated on 4 different specimens and all 4 new indenters made of single crystal diamond were broken in the same pattern as shown in **Figure 9(b)**. The study found that even when the load was increased from 1 N to 9.8 N, the Vickers indentation on the polished CFPCD sample was too small to be accurately measured. As a good comparison, standard diamond indenters remain in good shape after commercial PDC sample testing. The comparison results showed that the hardness of the CFPCD sample exceeded the Vickers hardness limit of single crystal diamond (120 GPa) and was by far the hardest material in the world. The hardness of commercial PDC materials is only about 64GPa.

The ultra-high hardness of microcrystalline diamond materials can be attributed to nanostructural defects such as stacked nanoplate layers, stacked faults, and twin microstructures caused by high-pressure hardening. A schematic diagram of the mechanism of microstructural change of pressure increase is given, as shown in **Figure 10**.

The fracture toughness of MPD samples has been characterized and calculated by the following equation [21].

$$K\_{\rm tc} = \xi \left( E / H\_{\rm v} \right)^{1/2} \left( P / c^{3/2} \right) \left( M P a \,\mathbf{m}^{1/2} \right), \tag{2}$$

where *ξ* is the calibration constant of 0.0166 (±0.004), *E* is Yong's modulus (GPa) (in the experiment Young's modulus, 1050 GPa, is used for diamond), *P* is the loading force (*N*), and *c* is the length of the crack.

The *K*IC of MPD prepared at 14 GPa and 1900°C is 18.7 MPa m1/2, the highest in the world of diamond materials. This is 3.7 to 5.5 times higher than a single crystal diamond. Interestingly, microcracks are mainly generated on diamond microcrystallines and terminated at the grain boundary (Y-zone). The Y-region of the nanostructure is composed of nanocrystallines, turbo graphite, and amorphous carbon, which can significantly prevent further propagation of cracks, thereby greatly improving the fracture toughness of the prepared sample. For 16-GPa CFPCD materials, it is difficult to measure indentation fracture toughness due to the previously-mentioned indenter damage. However, other ways are explored to assess the fracture toughness of materials and will report in the future.

**Figure 10.** *Schematic deformation micro-mechanism of UHPHT superstrong diamond.*

*Applications of Polycrystalline Diamond (PCD) Materials in Oil and Gas Industry DOI: http://dx.doi.org/10.5772/intechopen.107355*

#### **4.3 Wear resistance**

The cutting performance of a sample is the most widely used method to evaluate the cutting performance on a turned granite log by cutting it on a CNC lathe. Granite has high hardness and abrasion resistance, as well as low thermal conductivity. The cutting parameters of the granite log turning test are as follows: cutting speed (*Vc*) of 100 m/min, depth of cutting (*Ap*) of 0.5 mm, and feed rate (*f*) of 0.4 mm/rpm. CFPCD and commercial PDC samples are processed into cylindrical cutting tools with a diameter of 11 mm and a height of 6 mm. *G* ratio is the ratio of rock loss volume to wear flat volume, which is used to evaluate the wear resistance of the material to granite. The reference HPHT cutter is also carefully selected from the best PDC cutters currently used in drilling hard formations. The wear resistance of the CFPCD and commercial PDC samples is quantified using the abrasion wear ratio, *G*, and the wear rate or ratio is calculated using the following equation:

$$G = \frac{V1}{V2} \tag{3}$$

Where V1 is the volume loss from granite, (mm3 ); V2 is the volume loss from cutting tools (mm3 ). The higher *G* means the higher wear resistance of the material.

**Figure 11** shows an optical image of the cutting edge of a CFCD sample and a commercial PDC sample after cutting the granite by length of 1260 m. It can be clearly seen that the wear area of the commercial PDC sample is uneven and significantly larger than the wear area of the CFPCD sample, especially when the turning length reaches 1260 m. The study found that the average wear ratio of CFPCD samples is more than four times that of commercial PDC samples, making it the best diamond material currently used in the industry. Previously, it took a decade for PDC tools to improve their wear resistance by 30 to 50 percent. This breakthrough represents a 50-year technological leap in the development of PDC cutting machine technology. As the cutting length increases, the CFPCD sample is at a stable level because the diamond blocks do not fall off during turning granite testing. The extraordinary wear

#### **Figure 11.**

*Wear flats development of (a1-a3) UHPHT 16-GPa synthesized CFPCD, and (b1-b3) reference cutter [14].*

resistance of CFPCD materials is directly related to their ultra-high hardness, which is due to the fact that it is catalyst-free.
