Nanogate (Tribological Coating for Automobiles)

Environmental requirements demand that car combustion motor exhaust gases are thoroughly reburned in exhaust gas recirculation systems instead of being simply emitted into the atmosphere via the exhaust pipe. Condensate deposits from the exhaust gases of the combustion chambers are tar-like sooty residues that can agglutinate on valves in exhaust gas recirculation systems, resulting in larger force to lift from seat to open the valves, a subsequent loss of function, and the need for replacement.

The German company Nanogate developed a ceramic surface coating system for the valves in such systems that prevents agglutination of condensate deposits and that can successfully be applied in series production, resulting in exhaust recycling systems with improved performance. The metal valve has the valve plate and/or the valve seat provided with a ceramic coating with a layer thickness between 10 and 1,000 nanometers (nm), especially effective are coatings between 100 and 220 nm thick. The coating materials comprise zirconium oxide (ZrO2) for general applications, and oxides of metals of the third, fourth and/or fifth main and auxiliary groups of the periodic system of chemical elements for specific high-temperature applications.

The valves have to be operative in the temperature range between 400 and 450 degrees Celsius (between 750 and 850 degrees Farenheit), in some cases (for specific motors) between 700 and 800 degrees Celsius (between 1300 and 1400 degrees Fahrenheit). The partial pre-cooling of exhaust gases in more recent motors can lead to enhanced buildup of condensate in the valve.

The coating of the metallic valve decreases the force to lift from seat if condensate deposits contaminate the valve disk and/or seat, and prevents or decreases condensate attachment. Because of the high operating temperatures, the coating material is ceramics. Nanogate coatings between 100 and 220 nm reduce the force to lift from seat of valve disks compared to uncoated valve disks by up to 30 percent. The coating is especially suited for double-disk system valves. Therefore an additional motor to open the valves can be dimensioned smaller or might be not necessary at all.

Closed and continuous coatings are of uttermost importance. Coatings thicker than 1,000 nm exhibit cracks [Page 486]that allow the condensate to establish direct contact with the metal or oxidized metal surface of the valve disk, resulting in strong adhesion of the valve disk to the sealing surface of the valve disk.

For effective reduction of the force to lift from seat, the coating is applied to at least one sealing surface of the valve. The coatings can be applied via physical or chemical vapor deposition in vacuum, via painting with metallic alcoholate solutions and subsequent drying at elevated temperature, or via spray coating methods. To reduce the amount of coating material and to make the process more economically, only the sealing surfaces are coated. The valve has increased lifetime and better closing properties if the valve disk is made of a harder material than the valve seat, for example, if steel is used for the valve disk and aluminum for the valve seat.

The Nanogate coating technology is not limited to metallic valve systems in exhaust gas recirculation systems, but can also be applied in all valves that have to be high temperature resistant and run the risk to become agglutinated. The coating can be applied on any metallic surface that comes into contact with exhaust gases, especially in exhaust pipes, exhaust gas coolers, pistons, compressor blades, or throttles.

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