Automotive Applications Division Features

Improved Performance in Car Engines through Plasma Sprayed Coatings
by Keith Harrison, Sulzer Metco (UK) Ltd

Introduction

The use of lightweight materials has become more prevalent as car manufacturers strive to reduce vehicle weight in order to improve performance, lower fuel and oil consumption and to reduce emissions.

Most manufacturers are beginning to replace cast iron engine blocks with lightweight and low-cost aluminium-silicon crankcases. The cylinder bores of these aluminium alloy blocks must have wear resistant surfaces and usually have cast iron liners because of their good operating characteristics. These liners need to have a specific wall thickness, which results in a relatively large web width between the individual cylinder bores and increases the dimensions and weight of the engine.

Therefore, Sulzer Metco has developed a novel coating solution to obviate the need for liners. By using a rotating plasma torch, the RotaPlasma® 500, it is possible to apply a thin, plasma sprayed, coating to the bore of an aluminium alloy crankcase from a short distance and within a short time frame. For example, a typical cylinder bore of 80mm diameter can be coated within 60 seconds and, thanks to the rotating spray gun, the engine block does not have to be rotated any more. Thus, by removing the need for liners, the engine length can be significantly reduced and weight savings of more than 1kg per engine can be achieved. At least one European car manufacturer has gained experience with using this system in series car production since October 2000.

Coatings with Good Lubrication

Various materials can be used to produce the coating, especially iron-carbon alloys with various chemical compositions. As the coatings are generally applied using the APS method (Atmospheric Plasma Spray), the coating attains a certain degree of oxidation that has positive effects, since the correct type of oxides are formed if the oxygen content is correctly optimised.

The oxides FeO (Wustite) and Fe3O4 (Magnatite) are to be seen as solid lubricants that improve the tribological properties of the coating and it’s machinability.

It is also possible to apply non-corrosive coatings of iron-based alloys by adding chrome and molybdenum. These coatings are resistant to sulphuric and formic acid when used in internal combustion engines. Adding fine particles of tribologically functional ceramics can reinforce the materials, which increases the compression strength and abrasion resistance and reduces the tendency to scuffing. These MMC coatings have already been tested and are especially suited for high performance petrol or diesel engines.

The adhesive strength of plasma sprayed coatings is typically 40 – 50 MPa when applied to AlSi alloys with 7 to 10% Si. This is achieved by activating the cylinder wall surface with alumina grit propelled from a rotating blast nozzle. The micro-hardness is selected to ensure that the required compressive strength is obtained (minimum HV0.3 = 350) whilst maintaining good machinability (maximum HV0.3 = 650) Under boundary lubrication conditions, these coating materials have low wear rate compared to cast iron liners with flake graphite and a low coefficient of friction.

After the cylinder wall has been coated, a special parallel honing process is used to create a surface topography with numerous small recesses, which serve as oil reservoirs. This leads to an improvement in hydrodynamic lubrication and reduces piston ring stress. Compared to the traditional cast iron process, the specially honed coatings result in 20 – 30% lower friction. Thanks to the good tribological characteristics of the plasma coating, the life cycle of the engine is lengthened, while emissions decrease as a result of the reduction in fuel and oil consumption.

The surface topography is dependent on the residual porosity of the coating, typically 1 –3 volume percent, being finely stochastically distributed. This provides the favourable tribological properties. The micropores of the coating with a diameter of several microns fill up with lubricating oil, and the fine distribution of the pores ensures the safe lubrication of the process, achieving an oil consumption lower than with cast iron. The honing process, therefore, is critical to the success of the plasma sprayed coating and various techniques have been developed, depending on the coating composition.

Honing is the preferred machining technique for plasma coatings because it offers the following advantages:

• Coating damage (fissures, breaks) and the closure of pores (important as oil pockets) can be prevented
• A very large range of coating types can be used for series production. Due to the fact that honing is a self-sharpening process, very hard ceramic or metal/ceramic (MMC) coatings can be machined with a very long service tool life. Such coatings would otherwise quickly blunt geometrically defined cutters, even if diamond cutters were used.

The maximum coating thickness depends on the heat expansion coefficient and the mechanical properties of the coating. If the heat expansion varies too much in the base material of if the elasticity or ductility is too low, a coating that is too thick may not adhere correctly due to internal stress. On the other hand, a residual coating thickness of 100 to 180µm must remain after honing in order to cover any casting pores in the base material.

Thus, the aim is to keep the necessary honing layer as close to the minimum thickness as possible but without going below it. There must also be sufficient coating thickness to allow correction of the contour deviations that occur due to the coating. The thickness allowance for a tested, stabilised, coating process is between 100 and 150µm in the bore diameter.

Excellent Price - Performance Ratio

During the coating process, care must be taken that heat transfer to the block is kept to an absolute minimum, otherwise it can result in metallurgical changes of the crankcase microstructure. Furthermore, the typical cylinder diameters of 70 – 100mm necessitate a short spraying distance, and the application spectrum has to be very wide because of the diversity of the AlSi casting processes. In comparison to other techniques, plasma spraying fulfils these requirements better. It distinguishes itself not only through high flexibility in the casting process with which the engine blocks can be manufactured, but also with its excellent price performance ratio.

In comparison with other coating processes for cylinder bores, such as - nickel dispersion coatings, HVOF and electric arc wire spraying, plasma is less expensive and more efficient.

Meeting the high-quality requirement

The cylinder bore; piston ring and lubricant interact as a tribological system in the combustion engine. The main challenge of an industrial solution for the coating of cylinder bores is not only to develop a suitable material and process but also to integrate that process into a cost effective, high volume, fully automated production, coating system.

This coating machine must be capable of meeting the stringent quality requirements of the engine manufacturer. The coating system not only controls the coating process but also the surface preparation, cleaning, cooling and transportation of the engine blocks through the system. The focal point of the coating system is the SM F210 plasma torch that achieves a coating efficiency of about 80% when spraying metallic powder. This high efficiency and high powder feed rate facilitate short coating times and low consumption of powder material.

Great Potential

Rising fuel prices, as well as the growing necessity to handle natural resources with care are influencing the developments in the automotive industry. Thanks to the coating technology of Sulzer Metco, automobile manufactures are now able to reduce fuel consumption and emissions of their engines significantly and cost-effectively. The wide range of readily available spray materials enables a tailored coating solution for every engine application.

Acknowledgement

The author wishes to thank his colleagues, Gerard Barbezat and Ralph Herber, for their help with this article.

For more information, please contact Keith Harrison on 0115 928 5279 or e-mail keith.harrison@sulzer.com or visit www.sulzermetco.com.

As a global leader in surface engineering solutions and services using thermal spray and other advanced coating and process technologies, Sulzer Metco offers a complete portfolio of surface technology solutions, products, and services, including: advanced thermal spray equipment and materials.

• system integration design and support
• customer support services as field service technical support and training
• specialized coating services using state-of-the-art technologies
• turbine component manufacturing
• PVD coating services and equipment.

With more than 1,500 professionals around the world, Sulzer Metco serves and supports companies in aerospace, power generation, automotive and specialty markets to control surface processes for a wide variety of applications on a global basis.

The rotating inner plasma-spray burner of the RotaPlasma 500 system coats the cylinder bore of the stationary housing from a short distance with a period of 60 seconds. Thanks to the rotating system, the engine block does not have to be turned any more.

The coating system adapts itself to an integral concept for the manufacture of engine blocks. Automation, control and the employment of the plasma burner SM 210 result in excellent quality.