The use of coatings on piston rings has become necessary for optimum time between overhaul (TBO) on larger marine engines. There is a continuing trend to increase the power output of all engines. Higher power output means higher heat load and higher mechanical load on piston rings and cylinder liners. This in turn has lead to the development of new piston ring coatings with excellent wear properties under increasingly challenging running conditions.

There are two main categories of functional piston ring coatings that are currently being used in large-bore engines: running-in coatings and/or wear-resistant coatings.

Figure 1 – Running-in coatings and wear-resistant coatings applied on a top piston ring.

Figure 2 – Typical time scale for radial wear of a large bore top piston ring with controlled pressure release grooves.

Running-in coatings

The soft running-in coating is applied to the ring’s running face in order for the ring to seal properly within the first few hours of operation (during shop-testing or after overhaul for marine engines). Another function of the running-in coating is to gently finish the running surface on the cylinder liner during the first few hundred hours after installation of a new liner. Daros offers a range of alternative running-in coatings. The running-in coating is applied directly to the base material or in addition to a wear-resistant coating.

Wear-resistant coatings on running face

Cermets represent an important family of thermally sprayed wear-resistant coatings. Cermets are applied to the running face of the ring for highly loaded applications and consist of a mixture of metallic and ceramic compounds. The Cermet coatings prolong the lifetime of the ring package significantly, as shown in Figure 1. Daros offers a variety of different Cermet coatings. All Cermets are produced in-house with state-of-the-art plasma-spraying equipment. The coatings are always thoroughly tested in Daros’ laboratory, ensuring world-leading performance.

Figure 3: Principle of plasma thermal spraying.

Wear-resistant coating on ring plane

In order to make the bottom ring plane more resistant to wear, hard chromium is sometimes applied on the ring plane as well as the piston groove, in order to increase the TBO. Hard chromium is a well-tested technology providing excellent resistance to ring groove wear.

Daros Piston Rings P.O. Box 138, SE-435 23 Mölnlycke, Sweden. Phone +46 31 33 84 000