# Improving Pressure-Velocity Limit of Mechanical
## 3 Results and Discussion
### Evaluation of PV Limit  
The PV limit as a maximum serviceability parameter in the PCD/SSiC combination of a mechanical seal was tested on the Tribometer described above. Meanwhile, the evolution of PV values with applied load and rotational speed was recorded as shown in Figure 6. In the early stage, the PCD/SSiC combination was started up at 60 N load and accelerated to 1000 rpm. At this point, the load was immediately increased to 0.2 kN for a transient adaptive operation. As the load increased to 1 kN, full sealing was achieved for a PV product of approximately 2.32 MPa\(\cdot\)m/s. In the subsequent stages, the applied load at the rate of 1 kN, and the rotational speed at the rate of 500 or 1000 rpm, increased alternately according to the running state of the Tribometer. Every change in rotational speed will result in an instant increase in the PV value, while the PV increases slowly with the load. Ultimately, the mechanical seal gets stuck suddenly during loading from 4 kN to 5 kN at 4500 rpm. The PV limit of prepared PCD/SSiC combination is 42.31 MPa\(\cdot\)m/s. Figure 5: The scanning electron microscope (SEM) images of (**a**) initial surface, (**b**) polished surface and (**c**) cross-sectional morphology of the polycrystalline diamond (PCD) coating. A similar process was used for the other three combinations. The comparison results showed that the PV limits of PCD/graphite combination, GSiC/SSiC combination and GSiC/graphite combination were 29.97, 23.61 and 20.75 MPa\(\cdot\)m/s respectively as in Figure 7. It can be seen that the PV limit of mechanical seal rings is significantly improved comparing the uncoated ones with mating with the same material face. Figure 6: Evolution of PV values with applied load and rotational speed of PCD/SSiC combination. High load and high speed were responsible for excessive heat at the contact provoking the raising of the circulating water temperature. The relationship between the frictional heat generated \(\Delta\)T (temperature rise) and the PV value of the four-seal face material combination is shown in Figure 8. The frictional heat \(\Delta\)T is as a function of increasing PV for the combinations examined. The two hard-face combinations with SSiC produced less frictional heat, for a given load and PV, when compared to that produced by the other hard-to-soft face combinations with graphite. Graphite as a soft material and could not withstand strong shocks by a high load, resulting in micro-abrasion to accelerate wear and improve temperature. This shows the value that could be gained from the hard-to-hard face comparing seals for ultra-high load and VP value applications. The friction curves corresponding to the variation of the PV value are given in Figure 9. In the initial stage, the friction coefficient of all combinations is higher than 0.2. After some fluctuation with self-polishing (micro-abrasion), resulting in a further accommodation of the contact mating surfaces, low friction values less than 0.04 were obtained. When the operating conditions become severe enough to improve the PV value above 5 MPa\(\cdot\)m/s, the friction coefficient diminished very quickly. The trend for hard-to-hard face comparing seals was identified as that reducing the friction coefficient with increasing PV values, which can be seen for the PCD/SSiC combination and GSiC/SSiC combination. In contrast, for hard-to-soft face comparing seals, such as the PCD/graphite combination and GSiC/graphite combinations, the friction coefficient decreases in the initial stage with increasing PV value, then increases moderately in the later stage with increasing load. The reason may be that excessive loads accelerate the wear and tear of graphite because of the low strength of graphite. Broken debris falls into the gap between two seal faces leading to increasing friction. Meanwhile, resulting in the relatively low strength, the graphite seal ring is bound to be damaged early against a high load. Figure 7: Comparison of PV values and PV limit values of four mating combinations.