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Polycrystalline Diamond Characterisations for High End Technologies (3)

2022/1/11 15:53:41 TKD CO., LTD Reading 1 Times

 

2.3 Mechanical and tribological characterisations of PCDs

Polished surfaces of the PCD samples were seen under a ZEISS Supra 35 (Germany) field emission scanning electron microscope (FESEM) with EDAX attachment. Roughness of the PCD surfaces were evaluated by a contact profilometer (Talysurf PGI 2000S, Taylor Hobson). The Nano hardness and Young’s modulus were measured by nanoindentation technique applied to the plan section of the CVD grown polycrystalline diamond polished and nucleation side surfaces. A typical load of 1000 mN was used for the nanoindentation experiments. A standard nano indenter (Fischerscope H100XYP, Fischer, Switzerland) was used for this purpose. The Nano indenter machine was operated according to the DIN 50359-1 standard. It offered a load range of 0.4–1000 mN. It was equipped with a Berkovich indenter. The indenter possessed a tip radius of 150 nm. The machine was used in ambient laboratory conditions (23 ± 4°C, 70 ± 5% relative humidity). The depth sensing resolution of the machine was 1 nm. The force-sensing resolution of the machine was 0.2 μN. The machine was calibrated with nanoindentation based independent evaluation of H ≈ 4.14 ± 0.1 GPa and E ≈ 84.6 ± 3.5 GPa of a BK7 Glass (Schott, Germany). This material was provided by the supplier as a standard reference block. A ball-on-disc tribometer assembly (NANOVEA) was used for highly accurate and repeatable wear friction testing in rotational modes. The counter-face was a Si3N4 ball which was attached to a load arm and the load arm was directly in contact with experimental PCD disc surface. 3 mm diameter Si3N4 ball was in circular motion with 5 N applied normal load against PCD surface using commercially available machine oil lubricant in ambient laboratory condition. The speed of the ball was 0.04 m/s. Optical microscope (Olympus BX 51, country) study was done after each tribological experiment to know the radius of worn-out silicon nitride ball.

 

3. Results and discussion

3.1 Physical characterisations—DND seed particle size, PCD crystallinity, porosity, growth defects and thermal stability

Detonation nanodiamond (DND) suspension in dimethyl sulfoxide (DMSO) by 0.5 wt% is used as seeding slurry for CVD growth of polycrystalline diamond material [118]. Nucleation density determines the faster grain coalescence phenomenon during chemical vapour deposition. It has been observed that DND seeds are much superior in enhancing nucleation of diamond than conventional micron size grits for seeding diamond nuclei on the foreign substrates [119]. The particle size of seeds is thus important in determining nucleation efficiency. One commercially available DND in DMSO was used for seeding. Before using to make seeding suspension by mixing it with 3 parts of methanol, it was essential to know the exact particle size distribution of such suspensions. Dynamic light scattering (DLS), is a non-invasive, well-established technique for measuring the size and size distribution of molecules and particles. The Brownian motion of particles or molecules in suspension causes laser light to be scattered at different intensities. Analysis of these intensity fluctuations yields the velocity of the Brownian motion and hence the particle size using the Stokes-Einstein relationship. The DND seeding suspension was procured from a commercial supplier and it was being used from time to time over a period of 1 year. When the particle size was calculated from DLS technique using Malvern make Zetasizer, it was found that the original suspension has become agglomerated considerably. The fresh stock of commercial suspension came with 25–30 nm DND particle size on its label but to our surprise after one long year in the shelf, the suspension only has 7% with original size of 25.56 nm whereas, 93% of the DND particles agglomerated to give an average size of 123.4 nm, as shown in the following Figure 1a. But such agglomeration does not appear to hinder its seeding efficiency.

1.png

Figure 1.

(a) DND-DMSO seeding suspension particle size distribution; (b) cross-sectional SEM image of freestanding PCD; and (c) corresponding XRD signals from as-grown and nucleation side (NS)

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