# Improving Pressure-Velocity Limit of Mechanical
## 2 Materials and Methods  
Hot filament chemical vapor deposition (HFCVD) was used to perform the diamond deposition due to its versatility, simplicity, and low cost of implementation, maintenance, and operation, while providing better control of the growth parameters [16].  
The commercially available graphite-loaded silicon carbide seal rings (GSiC, Ningbo FLK Technology Co., Ltd., Ningbo, China) with 57 mm external diameter and 43 mm inner diameter sealing face were selected as a substrate for diamond deposition. Before depositing, the surface of the GSiC seal ring was ground uniformly with diamond grinding paste (particle size is about 0.1 \(\upmu\)m) for 10 min to improve the nucleation density. After cleaning the surface with acetone and methanol, the GSiC seal rings were transferred into a deposition chamber. Several tantalum filaments 0.35 mm in diameter and 250 mm in length were uniformly distributed above the GSiC substrate with a distance of 10 mm between filament and substance and heated to over 2000 \({}^{\circ}\)C during the whole deposition process. The surface temperature of GSiC substrate was kept at 850 \({}^{\circ}\)C, controlled by cooling water. A gas mixture ratio of H\({}_{2}\):CH\({}_{4}\) = 100:7 at around 25 Torr was adopted as nucleation ambient for 0.5 h. Then a PCD coating was grown for another 5.5 h with a gas mixture ratio of H\({}_{2}\):CH\({}_{4}\) = 100:6 at around 20 Torr. After deposition, the surfaces were polished to reduce surface roughness Ra to 0.05 \(\upmu\)m for application and performance evaluation.  
The evolution of phase and morphologies on the substrate surface during the diamond growth process was characterized and measured using a scanning electron microscope (SEM, Model EVO18, ZEISS, Jena, Germany), X-ray diffractometer (XRD, D8 Advance, BRUKER, Karlsruhe, Germany) and confocal microscope Raman spectrometer (inVia Reflex, RENISHAW, Gloucester, UK) with 532 nm laser excitation.  
The PV limit evaluation of the mechanical seal was performed on the Tribometer designed as in Figure 1 with loading and rotary devices. The upper dynamic ring rotates against the stationary one surrounded by circulating water at room temperature. The load, friction torque and temperature rise of the seal faces were measured by load sensor, torque sensor and thermoscope respectively.  
The evaluating procedure was that the seal was started up at 60 N load and accelerated to 1000 rpm. Then, the load was immediately increased to 0.2 kN. After establishing a basepoint condition, the seal was tested with load level and rotate speed increasing until the torque or temperature increased rapidly and the rotating suddenly paused with the rapid and complete failure of sealing. At this moment, the PV limits of each combination were recorded.  
The PCD coatings were tested against SSiC, which was a typical hard material, and graphite, which was a typical soft material. For a comparison, the substrate GSiC against SSiC and graphite were tested under the same conditions. All the SSiC seal rings and graphite seal rings were commercially available and supplied by Ningbo FLK Technology Co., Ltd. Each test run provided information on PV performance. Each material combination provided a PV profile, including a PV limit above which stable conditions could not be sustained. In addition, information was obtained on friction coefficients and temperature variation.  
After PV limit evaluation, the damaged scenes of different seal faces were observed by optical microscope (OM, DM2500 M, LEICA, Wetzlar, Germany).  
Figure 1: Schematic and photograph of the PV evaluating experimental rig.