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A scientific study was undertaken to evaluate the performance, under dry running conditions, of two types of diamond coatings applied to silicon carbide faces of mechanical seals. The coatings studied were polycrystalline diamond (PCD) and plasma-assisted chemical vapor deposition (PA-CVD). Both methods produced coating thicknesses on the order of10 microns (pm), with the former being three times the cost of the latter. A seal face dry running condition can occur when the flow of process fluid is interrupted, causing a reduction of the fluid that would otherwise serve as a lubricant between the contacting seal faces. Without this lubricating effect, the friction-induced temperature increases, allowing the process fluid to evaporate and causing the contacting faces to dry run.
It was widely believed that diamondcoated seal faces would be the solution to solving long-term wear-free operation, even when dry running. One such coating is claimed to be immune to dry running and is said to safely operate at temperatures higher than the effective maximum operating temperature of the installed Oseals. Skeptical of these claims, the author collaborated with the Materials Engineering Research Institute (MERI), based at the UK's Sheffield Hallam University, to test assemblies with both surface coatings under dry running conditions.1,2
Coatings test 1: Polycrystalline diamond. A diamond can be one single, continuous crystal, or it can comprise many smaller crystals (i.e., a poly-crystal). Large, clear, transparent, single-crystal diamonds are typically used in gemstones. PCD refers specifically to diamond particles that have been sintered together into a coherent structure using a chemo-mechanical binder and high-pressure, high-temperature conditions. These conditions are similar to those used in single-crystal diamond synthesis. The structure and conditions are unsuitable for gems; they are used for industrial applications, such as cutting tools and rotary seal surfaces.
PCD is often described by the average size (or grain size) of the crystals it comprises. Grain sizes range from nanometers (nm) to hundreds of pm, usually referred to as "nanocrystalline" and "microcrystalline" diamond, respectively. These coatings have the extreme hardness of diamonds-on the order of 10,000 Hv- and are usually produced through chemical vapor deposition at or above 800°C. The various production techniques developed over recent years differ by the grain size that is produced.
Several names have been given to these coatings,...