# Improving Pressure-Velocity Limit of Mechanical
## 1 Introduction  
Mechanical seals, which are readily available for sealing liquid and gaseous media, minimize liquid or gas leakage between the rotating shaft and stationary housing by maintaining a very small axial gap [1, 2]. In these devices, the opposing faces of two annular rings, which form a rotating-stationary combination, contact together to obtain a sealing action. Because of their special structure and unique functions, mechanical seals, as a critical part of centrifuges, reactors, pumps, mixers, compressors and other similar devices, are used in a wide range of industrial applications, such as the metallurgy, mechanical, petrochemical, pharmacy, automobile, aerospace and nuclear industries. In practical applications, mechanical seals have to meet high demands regarding working pressure, rotation speed, chemical resistance, thermal resistance and extremely harsh operating environments. Especially, in heavy duty industrial applications, for example boiler feedwater, reactor cooling, oil and gas well drilling, mechanical seals require service at high speeds under elevated pressures [1]. For the reliability, longevity, emission control and safety of mechanical seals, and in order to meet increasingly harsh service requirements, the performance of the paired dynamic and stationaryannular rings of mechanical seals plays a major role. Meanwhile, the performance of the mechanical seal is critically dependent on the seal face materials. Many different materials and combinations of mechanical seal have been successfully tested and are routinely used in the industry today [3,4]. Among all the comparing seal materials, sintered silicon carbide (SSiC) is commonly used due to its abrasion resistance, corrosion resistance, high hardness and thermal shock resistance. In order to offer the low frictional benefits of graphite within the mechanical strength of silicon carbide, graphite-loaded silicon carbide (GSiC) was designed and used [3]. Normally, the hard-to-soft face comparing seals, such as GSiC against graphite, is used due to their low cost and favorable self-lubricating properties. In particular, the hard-to-hard face comparing seals, such as GSiC against SSiC, are used in certain circumstances, such as high-viscosity fluid, sealing media with abrasive particles, loading or processes exceeding the mechanical properties of graphite. However, the useful lifetimes and serviceability limits of the mechanical seal are still limited by the failures due to unacceptable deformations and abrasive wear caused by severe or uneven loading, blistering, deficiencies, dry or near-dry running (especially at the startup) [5]. Therefore, an upper limit on the severity of the mechanical seal application is imposed by the material properties [6]. It is important to find the most appropriate materials to maximize the operating lives of these components, especially with sealing fluids under harsh operating conditions. In the case of crude oil applications, which is typical for high pressures, high speed and abrasive particle conditions, their mechanical seals required extremely hard and strong face materials. Diamond is well-known for its unmatched hardness and offers low friction, high wear protection, and excellent chemical and thermal resistance. Thus, a mechanical seal face designed in combination with a polycrystalline diamond (PCD) coating could offer an outstanding abrasion resistance and, at the same time, a considerably improved performance under poor lubrication conditions like in high-viscosity fluids and even in dry-running condition due to the low friction coefficient. Numerous studies have confirmed that the diamond coatings considerably lower the friction and enhance the tribological performance in mechanical face seals [2,4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. However, there are few researches on the upper limit of mechanical seals with PCD coating application under conditions where the combination of high pressure and high surface speeds create high levels of pressure-velocity (PV).