# Improving Pressure-Velocity Limit of Mechanical
## 3 Results and Discussion  
### Preparing of PCD Coating  
The morphologies of the GSiC substrate surface before diamond deposition, diamond nucleation and growth were observed by the scanning electron microscope (SEM). According to the SEM images of the original substrate (Figure 2a), the scratches caused by grinding were detected. The depressions with the size of 1-2 \(\upmu\)m, caused by the relatively soft graphite, which distributed in the silicon carbide ceramics, were clearly visible. After a 0.5 h nucleation process (Figure 2b), very tiny particles which were newly generated diamond seed crystals with the size of 50-300 nm were observed on the surface of the substrate, especially around the graphite depressions. During the deposition process, the tiny particles gradually spread over the substrate surface and continued to grow. After 4 h deposition (Figure 2c), it can be seen the PCD coating was continuous and very dense with the diamond grain size of 0.5-2 \(\upmu\)m. Ultimately, the diamond grew to around 4 \(\upmu\)m with different orientations after a 6 h deposition (Figure 2d). From the cross-sectional image of the PCD-coated seal ring (Figure 5c), it can be seen that the thickness of PCD coating was about 12.3 \(\upmu\)m, and the diamond grown outwards in a randomized fashion.  
The evolution of the seal ring face in the HFCVD process was simultaneously confirmed by the Raman spectrum and X-ray diffraction (XRD) results. According to the Raman spectrum results (Figure 3), two characteristic peaks of SiC at 785 and 965 cm\({}^{-1}\)[17; 18], and a low peak indicated graphite sp\({}^{2}\) were detected on the GSiC substrate. In contrast, the Raman spectrum recorded from a complete PCD coating shows the characteristic diamond sp\({}^{3}\) peak at 1335 cm\({}^{-1}\), a broad amorphous carbon peak centered at 1520 cm\({}^{-1}\) and a tiny nano-diamond peak at 1130 cm\({}^{-1}\)[19; 20; 21; 22; 23]. It illustrates that there are fewer internal stresses and a few diamond-like or graphite regions in the coating. From the initial to final state, the intensity of SiC peaks decreased and the intensity of diamond peaks increased. The full  
Figure 2: Initial surface morphology of hot filament chemical vapor deposition (HFCVD) diamond coated graphite-loaded silicon carbide (GSiC) seals ring deposited for (**a**) 0 h (GSiC substrate), (**b**) 0.5 h (after nucleation), (**c**) 4 h and (**d**) 6 h.  
width at half maximum (FWHM) of diamond sp\({}^{3}\) peak in the final PCD coating was 5 cm\({}^{-1}\) which was very small, meaning low crystalline defects and anisotropic stress distribution.  
X-ray diffraction results associated with the PCD coating at the critical growth points are shown in Figure 4. It can be seen the GSiC substrate is hexagonal. The presence of diamond (111) and (311) peaks confirmed that the PCD coatings were not textured. The XRD results show the good crystallinity of finial PCD coating, which ensures the unique performance of the diamond itself.  
Figure 4: X-ray diffraction (XRD) results of the HFCVD diamond coated GSiC seals ring deposited for 0 h (GSiC substrate), 0.5 h (after nucleation), 4 h and 6 h.  
Figure 3: Raman spectrum of the HFCVD diamond coated GSiC seals ring deposited for 0 h (GSiC substrate), 0.5 h (after nucleation), 4 h and 6 h.  
However, large grains of PCD coating with good crystallization may result in rough surfaces, which were not conducive to mechanical seal applications. According to Figure 5b, after polishing, the edges and corners of diamond particle in PCD coating were cut off and ground flush.