Coating Materials

metal o-rings in various coatings
Resilient Metal Seals

Coating Materials

Coated metal seals from Eurosealings have proved their reliability worldwide under numerous extreme operating conditions. Our seals excel in situations where traditional seals fall short.

A proper coating can greatly enhance the sealing effectiveness of O-rings, C-rings, and U-rings. Before we take a deep dive into coating materials, let’s have a look at the mechanics of coating and why it’s needed.

Metal Seal Coating

On any ring material, the achieved leakage rate can significantly be improved with a suitable coating. When a sealing ring is compressed, the coating material fills the microscopic irregularities on the counter-sealing surface, creating a more effective seal.

metal o-ring without coating vs with coating

The decision to apply a coating – as well as the specification of its thickness – depends on the target leakage rate and the surface finish of the groove and flange.

Choosing the Right Coating for the Ring Material

It’s crucial to match the coating with the pressing load of the ring to ensure reliable tiling.

In general Nickel can be coated on SS321, but it is less common.

It all depends on the application, temperatures, and the required leakage rate.

For example, a hard nickel coating is typically applied to Inconel and is used in high-temperature environments.

It can be utilized in combustion engines operating at high temperatures.

Do you always need coating on a metal seal?

Although coating can greatly improve leakage rates, resilient metal rings without a coating are also common.

If you’re dealing with a very viscous medium, it’s possible to forgo a coating and still achieve the required leakage rates. Or, if you have precision-machined housing, a bare ring might also provide a sufficient seal on its own.

Yet, should scratches arise from assembly or disassembly, applying a coating can be a suitable countermeasure, improving the performance of the seal.

The necessity of a coating largely depends on the nature of the contained medium. For high-viscosity substances, the likelihood of the material breaching the sealing surface can be rather low. In contrast, gaseous mediums or vacuum are far more challenging due to their propensity to escape through small imperfections.

When you are consulting reported leakage rates, bear in mind that we can offer a leakage test with helium in our lab. Helium leakage test is a standard test for seals.  Due to the small molecular, Helium  is offering a rigorous assessment of the seal’s performance.

Coating Materials

The following table gives an overview of the standard coating materials for O-rings, C-rings, and U-rings. Other materials are available on request.


Max. Temperature


200 °C


290 °C


430 °C (650 °C*)


930 °C


930 °C


1200 °C

* only for non-oxidizing media

Let’s take a closer look at every coating material, evaluate its characteristics and uses, and determine when it’s best to opt for each one.


Tin is a soft metal known for its malleability and ductility, which allows it to conform easily under less pressure. This makes it particularly useful for applications where a high leakage performance is required

With a maximum temperature tolerance of 200°C, tin coatings are, for example, well-suited for metal c-rings in laser applications, where maintaining an accurate vacuum is crucial.

Tin is the coating of choice in applications that demand precision seals at lower operating temperatures.


Polytetrafluoroethylene is known for its outstanding chemical inertness. With a maximum service temperature of 290°C, PTFE coatings are ideal for sealing rings in processing equipment where chemical inertness is essential. PTFE does not react with the vast majority of industrial chemicals.

PTFE is non-galvanic and is applied through a spraying process that forms a non-conductive and non-stick layer on the surface of sealing components.


Silver covers the widest range of applications and is considered the standard coating material.

With a substantial temperature threshold of 430°C, extendable to 650°C in non-oxidizing conditions, silver’s thermal conductivity and resistance to galling make it highly suitable for environments that fluctuate in temperature.

Because of these qualities, silver is often the default coating for metal rings that require a balance between thermal resilience and hardness, making it a reliable option for a broad spectrum of sealing challenges.


Copper is primarily used for its excellent thermal properties, withstanding temperatures up to 930°C. However, copper is less commonly used as a metal ring seal coating due to its tendency to react with other substances.

It is occasionally chosen for specialized high-temperature applications where alternatives like nickel are too hard, but its reactive nature limits its use.


Gold has exceptional corrosion resistance and biocompatibility, handling temperatures up to 930°C. It is often reserved for critical applications as in aerospace where both high reliability and resistance to aggressive chemicals are non-negotiable.

Its high cost typically confines its use to these high-value applications where performance cannot be compromised.


Nickel is a very hard coating material suitable for high-temperature environments up to 1200°C.

It is selected for its strength under high pressure and its chemical inertness, making it ideal for challenging industrial applications that involve extreme temperatures and corrosive substances. Its chemical inertness is particularly valuable in situations where silver is not compatible and vice versa.

Nickel’s resilience makes it the preferred choice for seals in high-pressure systems and for components that must retain their shape and effectiveness in demanding conditions.