7cycle Chrome Oxide Densification



Figure 9. Chemical Thermal Barrier Coatings.

Advanced thermal barrier coatings, as alternatives to thermal spray coatings, have been and are currently being explored. It is believed that the plasma spray coatings are limited; and that residual stresses which are induced during the coating thermal spray processing cannot be completely eliminated. However, the residual stresses can be altered through chemical treatment. In addition, complete alternatives to plasma spray have been investigated. These coatings have in some cases been referred to as Drain-Cast Coating or Chemical-Bonded Coating. They are generally applied by non-thermal means at room temperature, dried and cured at elevated temperatures. A detailed discussion by one source on chrome oxide impregnated coatings is provided in reference 2. A summary of some key chemical coatings with visually-aided descriptions are included in Figure 9. Surface photographs of chemical composite coatings are included in Figure 10 showing the coating (a) before, and (b) after the thermal rig tests.

The small bore engine rig which was used for more advanced levels of material, supplier, and design assessment was composed of an 80 mm (3.150 inch) bore engine. The engine used for this program is shown in Figure 11. The original piston bowl shape for this engine was significantly changed to look more like, the target program's engine

(a) Chemical Composite Coating Before Test - 10OX

(b) Chemical Composite Coating After Test - 100X

Figure 10. Surface Photographs of Chemical Composite Coating.

Figure 11. Cross Section of Small Bore Test Engine.

configuration. This design change also resulted in a significant loss in combustion efficiency and an increase in operating gas and component temperatures which more closely resemble the technology-targeted engine. For example, the exhaust gas temperatures increased from 1100 degrees F to 1400 degrees F, liner temperatures increased from 330 degrees to 615 degrees F, and the oil sump temperature increased from 210 degrees to 300 degrees F. The original aluminum piston was changed from a one-piece to a two-piece design as illustrated in Figure 4. The piston cross-head material was changed to iron and titanium alloy, as appropriate, while the skirt remained aluminum. Thermal barrier coatings are currently being applied and assessed in order to establish a correlation with the bench rig results and to aid in selecting the best coating for further proof testing on the large AVL-SCE.

Major ceramic coating companies, ceramic manufacturers, universities, research organizations, and other organizations including government installations have been contacted for their support. Among these are:

• APS Materials

• Plasma Technics

• Turbine Components

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