Cylinder bore coating modified for wider automotive market

To date, lightweight engine blocks use cylinder liners, but there is now a practical alternative in twin-wire arc spraying, an extremely cost-effective technology for coating cylinder bore surfaces of combustion engines. In cooperation with Daimler AG and other partners, Gebr. Heller Maschinenfabrik GmbH in Nürtingen, Germany, is taking on the task of integrating the process into volume engine production.

 Daimler AG is using the newly developed arc spraying technology for coating cylinder bore surfaces and aluminum crankcases of automobile engines under the name of NANOSLIDE®.

The technology uses a twin-wire arc spraying process to continuously melt iron/carbon wires and spray them onto the cylinder surfaces of the lightweight aluminum crankcase with the help of a nitrogen gas flow. Compared to other thermal coating processes, the technology is considered extremely cost-effective and technologically superior--a reliable, repeatable, highly controllable economical process that vastly improves the cylinder bore surfaces, reducing wear and promoting long-life. Spray particle velocity 60-80 m/s and spray particle temperature up to 2000 °C.

All thermal spray processes rely on the same principle:

feed stock material is melted by heating  

particles are atomized and speed-up to a high velocity by a gas

particles will deform and freeze on the cold surface

The coating is formed when millions of particles are deposited on top of each other. These particles are bended by the substrate by either mechanical or metallurgical bending. A common feature of all thermal spray coatings is their lamellar grain structure with pores, unmelted particles, and oxides resulting from the rapid solidification of small globules, flattened from striking a cold surface at high velocities.

Advantages of Twin Wire Arc Spray process include:

high material deposit

wires as the feed stock material (easier to handle than powder)

‘simple process’ as only a few components are necessary

coating without process control possible

no extra cooling of torch necessary;

However, the coating process requires optimal calibration of quality-determining parameters such as current, voltage, wire feed and process gas flow.

Advantages of the spray coating for combustion engines:

weight and space advantages over liners

favorable frictional conditions between the liner, the piston and the ring

fuel  consumption advantages for diesel and gas engines

a very high wear resistance in-service

manufacturability in a reliable process and at favorable costs

Due to their thermally coated cylinder liners, NANOSLIDE®-designed engines are superior in terms of displacement and torque, especially when compared with similar aspirated engines incorporating conventional lining technology. Daimler AG started development of the coating technology in 1998 with small, series production of AMG motors, including the 6.3L V8. Since 2006, Daimler has treated more than 80.000 motors.

From prototype to series production - HELLER automates CBC

 To provide a practical, automated process for higher production applications, Daimler AG has entered a partnership with Gebr. Heller Maschinenfabrik GmbH in Nürtingen.  HELLER is to provide automation of processes and equipment, and improved processes stability, developing and demonstrating the process in its CBC Technology Center at its global headquarters in Nurtingen.

With its global range of systems and services to support wider application of the process, HELLER in co-operation with Daimler and other interested parties will automate all steps of the process and develop it into a reliable application for the global markets under the name HELLER CBC (CylinderBoreCoating).

With its CBC Technology Center HELLER can:

coat trial parts for future coating cells

enhance all process steps and equipment for future requirements

adapt the process to different  crankcases

produce a constant surface quality with guaranteed properties

The CBC machining process at HELLER includes fine boring, roughening, coating, rough honing, finish honing to expose pores, finish machining—all accomplished in the HELLER MC20 machining center. Total cycle time, including part load/unload,  for an 8-cylnder engine 5 to 6 minutes. The engine blocks may be direct-loaded into the machining center or mounted on a swiveling exchanger.

A key contribution of HELLER to the process is to fine-bore the arc-sprayed cylinder coating to impart a final finish. Previously this was accomplished with honing, but the HELLER fine boring process takes about half the time and assures a much more consistent finish and form to the cylinder due to the machining  center spindle axial orientation to the bore.  Tooling cost is also much less compared to honing.

Measuring the coating assures positive results

The applied coating is measured with a Jenoptik IPS 100 bore inspection sensor system which provides in a rapid, single pass automatic surface inspection of bores from 75-110 mm diameter, instantly providing a 360-degree panoramic view.

The sensor permits detention of surface defects such as blow holes, scratches, porosities and more on bores from 75 mm to 110 mm diameter, to any depth. Utilizing state of the art CMOS imaging technology, the IPS100 continuously scans the bore surface at high speed, creating a viewable image of the bore interior.

In operation, a 360-degree circular optic is advanced into the bore and generates an image of the surface. Circumferential lines in the bore are seen as rings in the image. As the sensor advances into the bore, circular scanning allows for the generation of an undistorted image of the inner surface.

Although the technology has only been used for exclusive low-volume series until now, its application in medium-volume production already provides significant competitive advantages compared to existing cylinder lining technologies. The technology complies with the production rules and criteria of the automotive industry.