The new coating also was developed to help meet requirements for new production engines planned for 2018-2020, which call for less cylinder-bore friction and the use of lower-viscosity oils and alternative fuels. It provides the superior performance of a more costly premium inlaid top ring.

Typically high-output engines under 3.5 liters are susceptible to low-speed pre-ignition or “mega-knock.” HVOF coatings are the only family of coatings that currently can survive high levels of low-speed pre-ignition in GTDI engines without damaging the top piston ring.

Tested against the MSC385 currently used in several North American production engines, MSC312 improves wear by up to 25% as well as providing improved scuff resistance. The thickness of the new coating also can be adjusted to meet engine durability requirements.

Chromium nitride (CrN) coatings are common in the piston-ring industry when applied by physical vapor deposition (PVD). CrN generically provides excellent wear resistance and low friction between piston ring and cylinder wall. It also resists scuffing at the ring-to-wall interface.

PVD processes deposit thin layers of material and broadly use three steps:

• Vaporization of the deposited material from a solid source assisted by high temperature vacuum or gaseous plasma.
• Transportation of the vapor in vacuum or partial vacuum to the substrate surface.
• Condensation onto the substrate to generate thin films.

By contrast, thermal spraying uses coatings formed from melted or semi-melted droplets. The deposited material—in the form of powder, wire or rod—is fed into a flame produced by a spray gun. The material melts, and the formed droplets are accelerated towards the substrate. The thermal and kinetic energy of the flame can be produced either with burning mixtures of fuel gas and oxygen, or by using an electrical power source.

There are a number of thermal spray methods, differentiated by the energy source:

• plasma spray methods (atmospheric plasma APS, vacuum plasma VPS, and low pressure plasma LPPS);
• combustion flame spray methods (flame spray);
• high velocity oxy/air-fuel methods (HVOF/HVAF);
• electrical arc methods (wire arc);
• detonation method (D-Gun); and
• cold gas methods (CGS).

Mahle’s new coating is the first thermally applied piston ring coating to use chromium nitride in its formulation. The addition of molybdenum-chrome provides enhancements to pure CrN.

High velocity processes—especially HVOF (High velocity oxy-fuel) spraying—are the preferred methods for producing coating with low porosity and high adhesion, noted a team from Finland’s VTT in a 2011 open access paper on HVOF technology in the journal Coatings.

In the past, top-ring coatings were applied using conventional plasma methods. In downsized GTDI engines conventional top-ring plasma rings are not sufficiently robust to survive high levels of low speed pre-ignition activity associated with low-speed, high-boost conditions found with GTDI engines.

Conventional plasma methods are used for ring production at Mahle’s St. Johns, Michigan plant and at Mahle’s facilities in Europe, South America and Aguascalientes, Mexico.

Resources

• Maria Oksa, Erja Turunen, Tomi Suhonen, Tommi Varis and Simo-Pekka Hannula (2011) “Optimization and Characterization of High Velocity Oxy-fuel Sprayed Coatings: Techniques, Materials, and Applications” Coatings 1, 17-52; doi: 10.3390/coatings1010017