Advances in diamond are bringing the well-known properties of the world's hardest material to mechanical shaft seals. Diamond has long been sought after as a seal face material because of its unsurpassed hardness, high thermal conductivity, chemical inertness and low friction. Due to the processing limitations of synthetic diamond, however, early applications were relegated to abrasives and tooling that do not require the same high quality surface finishes and tolerances as those found in mechanical shaft seals.
In the last decade, profound advances have been made in the development of economical, large-scale diamond synthesis processes. These processes have led to an ability to produce smooth diamond that meets the surface requirements of seal faces. Along with the capability of manufacturing a large number of seal faces simultaneously, these improvements have made diamond-surfaced seals commercially viable.
The use of carbon coatings on seal faces is not new. For years, a form of softer amorphous carbon known as diamond-like-carbons (DLCs) has been providing scuff resistance to the faces of gas compressor seals. DLCs adequately reduce scuffing during the brief contact that occurs during start-ups and shutdowns of compressors but have failed to demonstrate sufficient improvement over ceramic material options such as silicon carbide (SiC) and tungsten carbide (WC) in contact sealing applications.
John Crane Inc., Huhnseal AB, and Burgmann Industries GmbH & Co. KG are marketing products incorporating significantly harder crystalline diamond face materials in universal ANSI pump cartridge seals, as well as in several other seal designs, where the extreme properties of diamond are meeting the demanding requirements of customers' applications. These new products are used when SiC and WC are not performing adequately, increase seal life by reducing face temperatures during intermittent dry operation and reduce wear that results from abrasives and poorly lubricating conditions.
Structure of Diamond Materials
Two forms of carbon are of interest to users of mechanical seals-graphite and diamond. Materials that integrate these allotropes of carbon are typically characterized by two key parameters: crystallinity and the percentage of diamond relative to graphite. DLCs, which is the material used on gas compressor faces, are not crystalline (i.e., the grains are disordered) and vary in composition with about 10 to 80 percent of the carbon bonded as it is in diamond. Accordingly, the range of hardness values in DLCs is also wide. The typical hardness is similar or below that of SiC, but the literature has reported some values exceeding a-SiC.
In the last couple decades, researchers have developed new diamond materials that can be grown directly onto seal faces such as SiC, and provide significant improvement to SiC's properties. These synthetic diamond technologies are 100 percent crystalline and nearly all of the carbon in these materials is diamond. Whereas a diamond in an engagement ring consists of a single crystal, these materials are polycrystalline (i.e., they consist of many small crystals chemically bonded to each other) and consist of diamond grains that are about 10 to 20 µm (1 µm is about 40 micro inches) in size. These new materials are much harder than SiC and DLCs and are nearly as hard as natural diamond. They are also, due to the crystalline nature, more chemically resistant and have higher thermal conductivities than other seal materials.
Unfortunately, these synthetic diamond materials suffer from a few drawbacks-the surface roughness is far too high, and they exhibit high friction and excessive counter-face wear. Conventional diamond coatings are rough as deposited with a surface roughness of 1 µm Ra or more. It is cost prohibitive to finish these rough diamond surfaces to meet the requirements of seal faces, and early attempts to use these materials for sealing applications failed.
How Diamond Is Applied to Seal Faces
The diamond seal products now available from John Crane Inc., EagleBurgmann, Huhnseal and those supplied to seal manufacturers by Advanced Diamond Technologies, Inc. are fabricated by growing a polycrystalline diamond film onto the face of a conventional finished SiC ring. It is then placed into a chamber where the pressure, gas composition and temperature are accurately controlled. A carbon bearing gas such as methane is introduced into the chamber and, under the right combination of processing conditions, diamond crystals grow on the SiC. The process occurs under vacuum at temperatures around 800-deg C (1,472-deg F).
The diamond is not precipitating out from the vapor phase but grows up from the surface of the SiC. As these small diamond crystals grow, they coalesce together and form a continuous diamond surface. Specific processing conditions determine the diamond's properties. The relatively high temperature of the process results in a significant chemical interaction and subsequent bonding of the diamond onto the SiC. Excellent bonding is critical to ensure the diamond adheres well. Tests have shown that the bond between the SiC and the diamond can be stronger than the strength of the SiC itself.