Improving Mechanical Seal Performance with Diamond


Written by:
Charles F. West, Advanced Diamond Technologies, Inc.

The growing body of diamond performance data is casting doubt on past assumptions about the ideal face materials for dry-running and poorly lubricated applications. A new family of diamond-enhanced silicon carbide (SiC) seal faces has resulted in increased reliability and mean time between repairs (MTBR) of mechanical seals in a wide range of demanding applications. Demonstrated in field use, the increased performance of these new diamond sealing components is the result of the outstanding mechanical, thermal and chemical properties of diamond.

 

Diamond's ability to reduce the amount of frictional heat generated at the seal face enables the seal to survive poorly lubricated or intermittent dry-running conditions. The low friction and reduced heat generated on the seal face calls into question the common wisdom that a hard face matched with a carbon counterface is the most appropriate face pair combination to address dry-running and poorly lubricated applications.

Diamond-faced seals have been shown to run dry for longer periods of time, without reaching critical temperatures, than uncoated silicon carbide running against carbon. Diamond faced seal rings are being installed into customer applications by several major seal manufacturers for situations where face lubrication is an issue, including hot-water and light hydrocarbons. Diamond seal rings have also shown increased seal life in traditional hard-on-hard face sealing applications such as heavily loaded slurries found in mining, paper processing and drilling operations.

How is a Diamond Seal Manufactured?

Diamond-faced seal rings are made by growing the diamond from a carbon-bearing gas source such as methane (CH4) in the presence of hydrogen (H2) at high temperatures, approximately 1,475 deg F (800 deg C) onto the surface of a silicon carbide ring. The diamond forms a dense, continuous surface on the SiC with surface finishes and properties that can be tailored for a variety of sealing applications. The surface finish of the diamond when applied to a silicon carbide ring can be accurately controlled to produce a range of finishes-from a polished surface with a roughness of <1 micro-inch (25 nanometers) Ra to matte finishes of 10 micro-inches (300 nanometers) and above. The diamond deposition process enables the production of diamond-faced rings that can be used in combination with a wide range of counterfaces according to the application's needs. Diamond-faced rings are being successfully run in the field against carbon faces, silicon carbide and against other diamond faces.

Another important feature of the diamond coating process is its ability to closely replicate the underlying SiC surface and maintain the ring's original flatness. After deposition, diamond coated rings maintain the flatness of the silicon carbide ring to deviations well below 2 light-bands of helium (approximately 600 nanometers).

Diamond Adhesion on SiC

Due to major technological advances, coatings now serve successfully in critical and demanding tribological applications including components in race car engines, bearing surfaces in artificial heart valves and jet engine blades. The mechanical seal industry has recognized the advanced performance of coatings and modified both the API 682 and ISO 21049:2004 (6.1.6.2.5) standards to now state that "wear resistant materials such as silicon carbide or tungsten carbide may be enhanced by applying a coating." To ensure that engineered surfaces (coatings) provide the desired performance and reliability benefits in tribological applications, it is critical that coatings tenaciously adhere to the underlying substrate material. To ensure that diamond seals meet this requirement, a modified version of the ASTM C 1624-05 adhesion test (Standard Test Method for Adhesion Strength and Mechanical Failure Modes of Ceramic Coatings by Quantitative Single Point Scratch Testing) was developed that focused on the specific demands of evaluating diamond coatings on silicon carbide.

Figure 1 shows the adhesion test of a diamond-faced O-ring stationary seat. This adhesion standard is routinely applied to hard coatings where exceptional adhesion eliminates the possible use of simpler methods. These simpler methods typically measure the force required to pull the coating off the surface and represent the results as the force per unit area at which the coating separates. These methods are limited by both the strength of the attachment point used to pull on the film and the intrinsic strengths of the underlying materials (in this case silicon carbide). When extremely well bonded coatings are required, ASTM C 1624-05 is one of the most reliable methods of quantifying adhesion properties.

Adhesion testing of diamond (T30) seal face

Figure 1. Adhesion testing of diamond (T30) seal face

The ASTM C 1624-05 procedure results in a numeric value for coating adhesion, represented by the critical load required to damage the film. Dragging a diamond indenter over the coated surface while simultaneously increasing the load on the indenter normal to the surface creates the damage. To enable the testing of diamond coatings, the diamond indenter was replaced with a silicon nitride (Si3N4) sphere whose radius was 0.03125 inches (approximately 790 micrometers). This modified procedure enabled the ball to be rotated and exchanged so that a fresh, consistent surface could be presented against the diamond coating for each test.

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