Diamond Advances Seal Face Performance

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.

Smooth Diamond Faces

The U.S. Department of Energy's Argonne National Laboratory (Argonne) is internationally known for synthesizing thin, smooth diamond materials with controllable properties which are now available commercially as ultrananocrystalline diamond (UNCD). Ultrananocrystalline diamond has grain-sizes measured in nanometers and not micrometers. As a result of its fine grain structure, ultrananocrystalline diamond can be deposited on SiC with a surface roughness of 2 to 5 nm Ra, well below the surface finish requirements of contacting mechanical seals.

Researchers tend to think of synthetic diamond as a family of materials similar to the different types of carbides and carbon-based materials available today for seals. The technology developed at Argonne allows for different types of diamond materials with different surface roughness and other engineering specifications (see Figure 2). By accurately controlling the surface roughness, these diamond materials can be made smooth enough to run against counterfaces of much softer carbon or conventional uncoated SiC, enabling a wide choice of design options for seal manufacturers. Controllable surface roughness also allows for two diamond faces to be paired against each other without the ringing or seizing that can occur when the same hard materials are paired together.

Conformality and Flatness

Vapor-deposited diamond, including UNCD, is applied to SiC faces as a thin film with thicknesses ranging from 2 to 20 µm. The diamond's thickness is routinely applied with uniformity within ± 0.3 µm (11.6 micro inches). Due to these small variations in thickness, the diamond face retains the flatness of the original within one light band of helium. Ultrananocrystalline diamond is also suitable for non-contacting seal face designs in which the seal face is precisely machined with spiral grooves or other patterns that give the face three-dimensional features. UNCD is applied to an as-finished, non-contacting SiC ring with the diamond film conforming to the size and shape of the grooves.

Wear and Friction

Diamond overcomes two of the major challenges of seal face materials. First, a seal face must be fabricated to micrometer or sub-micrometer precision, enabling the seal design to properly maintain the face lubrication film. Second, the seal face must maintain the required surface quality and geometry even in poorly lubricating conditions (e.g., pumping near a fluid's vapor pressure or intermittent dry running), during exposure to abrasive solids in the media and to highly corrosive environments. Diamond-faced seals have been shown to have significantly longer useful life in poor lubricating environments such as hot water and during extremely abrasive pumping applications. Diamond is an ideal engineering material due to its extreme chemical resistance and unsurpassed hardness.

The ideal face material reduces both the heat generation that can result in thermal distortions of the faces and secondary seal failures. The low coefficient of friction (µ = 0.04) between diamond and SiC results in less heat generated when face lubrication is interrupted, minimizing seal failures during start-up and improving seal life during dry operation. It is because of the energy saving benefits of low friction materials that the U.S. Department of Energy originally funded the development of ultrananocrystalline diamond at Argonne.

Applications

It is important to remember that a seal face is one of many elements within a properly operating mechanical seal and all the major elements should be considered when diagnosing problem pumps. Diamond, however, enables pumps to function with higher reliability in demanding applications. These new diamond products enable hard face materials to be used while only softer carbon materials are used today due to the requirements of dry running. High performance single seals are now available that are practical alternatives to the higher costs and maintenance of dual seals and associated systems.

As a result of these and other benefits, diamond-faced seals are finding their way into a wide range of applications such as pumping light hydrocarbons, boiler feeds, water recirculation systems, deionized (DI) water systems, refinery and material processing applications, pulp and paper processing, and pharmaceutical production.

The added cost of a conventional ANSI pump seal with diamond is equivalent to other face upgrade options. If an application could benefit from increased hardness and reduced friction, particularly if it is susceptible to dry running, consider installing diamond-faced seals during the next maintenance cycle to experience the benefits first-hand.

Pumps & Systems, May 2009