Original Date: 04/26/1999
Revision Date: 01/18/2007
Best Practice : Thermal Spray Coating and Forming Processes
Thermal spray coating and forming is a process where a coating thickness of 0.001 to over 0.750 inch is applied to a surface. In addition, this process can layer dissimilar coating materials so that their desired properties work together, such as in functional gradient coatings. A typical example is the coating of alumina on tungsten and molybdenum. Thermal spray coating and forming is applicable to many metallic and non-metallic substrates. The process may also be a suitable alternative to electro-plating and organic paints, especially if portability, high deposition rate, or environmental issues are important.
Marshall Space Flight Center (MSFC) uses three thermal spray coating and forming processes: Vacuum Plasma Spray can apply exotic metals in thick layers, but is costly and is limited to vacuum chamber operations. In an inert environment, plasma is generated by ionizing gas via an internally conducted arc and accelerates the coating material through the plasma flame to the substrate.
High-Velocity Oxyfuel Spray can apply many material types and is cost effective. This method has no coating thickness restrictions nor noise considerations. The supersonic gas velocity from a combustion process propels the powder. The powder then melts as it passes through the flame and is deposited on the workpiece surface. The intense kinetic energy results in a dense, well-adhered coating.
Wire Arc Spray is the most cost-effective method for material applications. To apply the coating, this method melts two advancing wires through an electrical arc, then introduces a high velocity gas that propels the coating toward the substrate. Another strength of this method is the rapid prototyping (RP) of parts by coating a foam mandrel and subsequently washing out the foam, leaving a working prototype that can be tested and assembled if desired.
The main differences among MSFC’s thermal spray coating and forming processes are layering thickness, operation/equipment cost, and type of base material being coated. Many techniques can remove these coatings, including abrasion and machining as these methods do not produce molecular bonds. MSFC’s website at http://map1.msfc.nasa.gov offers additional information on these processes as well as collaboration opportunities.
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