Mechanical Face Seals And CVD Hard Coatings

In this ‘insights from industry’ interview, Dr Yuri Zhuk, Technical Director at Hardide Coatings Ltd., talks to AZoM regarding the importance of seal reliability, especially in adverse conditions.

GT: Could you please provide a brief overview of Hardide and some of the new coatings that Hardide is producing?

YZ: Hardide Coatings Ltd, an advanced surface coating company, is a leading innovator in the development and production of metal surface coatings based on Tungsten Carbide. The company is currently producing the Hardide range of chemical vapour deposition (CVD) tungsten carbide / tungsten hard coatings, which have exciting and wide-ranging applications in industries including oil and gas, valves, pumps, aerospace and advanced engineering..

GT: What are the primary applications of these coatings?

YZ: Applications where wear, erosion, corrosion or chemically aggressive media cause component and tool or equipment downtime, failure or loss of performance. The coatings are hard, tough, flexible, ductile and impact resistant and can coat internal surfaces and complex geometries so can be used in many applications where traditional hard coatings are not suitable.

An important application of these CVD coatings is their use on mechanical seals for heavy-duty rotating equipment.

Rotating equipment, including turbines, compressors and pumps, use mechanical face seals to prevent leakage, whilst also excluding any abrasive media and lubricating the mechanical assemblies. The importance of seal reliability cannot be overstated when looking at overall equipment reliability, because seal failure results in the damage of costly equipment, and would also cause higher downtime costs.

GT: Which industries benefit from these coatings?

YZ: Primarily the oil, gas and chemical processing industries will benefit from Hardide’s CVD coatings for mechanical face seals. However, any industry that employs heavy-duty mechanical face seals will benefit, especially if these are employed in adverse conditions.. In particular, these coatings are used to make the ‘primary’ sealing surface durable and capable of resisting three-body abrasion, erosion and corrosion.

 GT: What materials are traditionally used on mechanical seals and what are the limitations of these?

YZ: Solid hard materials, such as ceramic, silicon carbide, or sintered tungsten carbide are frequently used. However, these materials have several significant limitations and disadvantages, which are listed below:

    They are brittle
    They are expensive and difficult to produce (especially if there are finer features included in the seal design)
    The thermal expansion properties of these materials can differ significantly from steel, leading to jamming with a change in temperature.

HVOF, plasma spray coatings (such as WC/Co), DLC and other PVD coatings are already used on mechanical seals. However, these are often not effective for heavy-duty seal applications. For example, WC/Co HVOF and plasma spray coatings contain hard WC grains in a soft cobalt binder. These coatings can become very rough in abrasive/corrosive environments due to the selective leaching or abrasion of the binder.

GT: How do the Hardide coatings address these common problems?

YZ: An even wear pattern is ensured by the uniform structure of Hardide coatings, so an optimum finish can last for longer without hard asperities forming. Both tungsten carbide and tungsten have excellent resistance to chemically aggressive/corrosive media, particularly acids. The coating might be further alloyed with small amounts of Fluorine or Fluorocarbons (as disclosed in Hardide patents), which can further enhance its mechanical properties.

An alternative approach is to manufacture the mechanical seal components out of steel and then coat with a hard coating. Using steel is beneficial for the following reasons:

    It is tough
    It is inexpensive
    It can lead to improved and larger ranges of seal design
    It is relatively easy to machine

GT: How does using CVD tungsten carbide coatings effect the lifespan of machinery?

YZ: It has been shown that the coatings can triple the operational life of critical parts in abrasive conditions. Mechanical face seals display similar, long-lasting results.

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