Ford Developing Thermally Sprayed Nano-Coating for Cylinders to Reduce Friction, Support Lighterweight Construction

Ford Research Centre Aachen (Germany) is developing a thermally sprayed nano-coating using a Plasma Transferred Wire Arc (PTWA) process that could replace the heavier cast iron liners that provide the necessary wear resistance of cylinder bores in aluminum block engines. Dr. Clemens Verpoort of the Aachen center, the thin, wear-resistant coating reduces weight and improves friction performance while delivering equal durability and reliability to the product.

Fuel consumption can be reduced by utilizing lightweight construction as well as by decreasing internal friction losses in the drive train. Modern engine blocks are partly made of cast iron or aluminum material whereas for the later hypo-eutectic AlSi-alloys dominate. Due to the low hardness, surfaces made of these alloys cannot be used as a friction partner for the piston rings. Cast iron liners are often inserted into the engine block to provide a wear-resistant surface for the piston rings. This work describes how cast iron liners can be replaced by thin, nanocrystalline iron based coatings in order to decrease friction losses as well as reduce the engine weight. “Thermal Spraying of Nano-Crystalline Coatings for Al-Cylinder Bores”

The process for coating cylinder bores consists of four steps. First is machining the cylinder bore to a certain diameter and then machining the surface so the coating is able to adhere. Next steps are the thermal spraying and the honing of the coated bore to receive a surface that meets tribological requirements. Four thermal spraying systems are currently either commercially available or under research. One system uses powder feedstock while three of them use wire as feedstock:

· Rotary powder plasma process (in serial production);

· Rotating twin wire arc system (in serial production);

· High velocity oxygen fuel system (under research); and

· Plasma Transferred Wire Arc system (in serial production).

Verpoort said that the best coatings and most consistent results have been seen with the Plasma Transferred Wire Arc technology, and therefore it was utilized within the framework of the Ford study.

Honed cylinder coating made from one of the materials (SUNA) under test.

Ford coated test liners and two Ford Zetec inline 1.4-liter engines with a reference material of 0.1% carbon-steel and an iron-based flux-cored wire feedstock called SUNA. Friction tests were carried out with a stripped down engine. The test can be carried out with or without the cylinder head. For the engine including the cylinder head, the friction with the reference material was 6.8% below the values measured for the standard engine with liners made from grey cast iron. Without the cylinder head, the friction was 14.1% lower compared to the standard engine.

Ford has not tested the SUNA-coated engine yet, but calculated a 10% decrease in friction for the engine including the cylinder head.

The decrease in friction is produced in two ways, said Matthew Zaluzec, manager of the Materials Science & Nanotechnology Department for Ford Research and Advanced Engineering. The first is from the mechanical aspects of honing and sizing the piston and cylinder bore enabled because of the coating, and second is the tribological properties of the lining itself.

The coating study is only one of a larger set of nanomaterials research initiatives spanning applications from paint to engine blocks (and not including the work on electrochemistry and catalysts). Ford has called out vehicle weight reduction as a key part of its strategy to improve fuel economy by 40% by 2020 to meet the new CAFE standards. The company’s goal is reduce vehicle weight from 250 to 750 pounds—depending on the model—between 2012 and 2020 without compromising safety.

A Ford study called “Atoms to Engines” looked at the structure of cast aluminum alloys at near atomic levels. From this work, a detailed analysis of the structure/property/process relationship of the aluminum alloy engine blocks has led to reduced engine weight and, in turn, increased fuel efficiency.

Many thought our aluminum engine technology was mature and fully optimized. Not until we looked at every aspect of the materials and manufacturing process were we able to pull out another 10 percent in structural performance out of our engines, which directly translates into weight and fuel economy savings year over year. It’s nano at the working level.