A method has been devised for high-volume, high-velocity surface deposition of protective metallic coatings on otherwise vulnerable surfaces. Thermal spraying is used whereby the material to be deposited is heated to the melting point by passing through a flame. Rather than the usual method of deposition from the jet formed from the combustion products, this innovation uses non-steady combustion (i.e. high- frequency, periodic, confined bursts), which generates not only higher temperatures and heat transfer rates, but exceedingly high impingement velocities an order of magnitude higher than conventional thermal systems. Higher impingement rates make for better adhesion. The high heat transfer rates developed here allow the deposition material to be introduced, not as an expensive powder with high surface-area-to-volume, but in convenient rod form, which is also easier and simpler to feed into the system. The nonsteady, resonant combustion process is self-aspirating and requires no external actuation or control and no high-pressure supply of fuel or air.
The innovation has been demonstrated using a commercially available resonant combustor shown in the figure. Fuel is naturally aspirated from the tank through the lower Tygon tube and into the pulsejet. Air for starting is ported through the upper Tygon tube line. Once operation commences, this air is no longer needed as additional air is naturally aspirated through the inlet. A spark plug on the device is needed for starting, but the process carries on automatically as the operational device is resonant and reignites itself with each 220-Hz pulse.
Through a small access port in the side of the device, the aluminum rod material is deposited into the combustion chamber. Thrust production from the combustor results creates a periodic, high-speed jet, which is emitted from the tailpipe, downstream of the combustion chamber. The material to be deposited melts in the combustion chamber, and then is carried downstream and ejected from the tailpipe at high speed where it impinges and solidifies on the target surface. The residence times in the combustion chamber for the material to be deposited are low. By avoiding overheating, problems like further chemical changes (like oxidation) are avoided.
This sort of device is mechanically simple and has potential to be used to create a mobile, high-volume spray unit. The only external power required would be needed to control and actuate the feeding of the coating material into the device.
This work was done by Daniel E. Paxson of Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Innovative Partnerships Office
Attn: Steve Fedor
Mail Stop 4–8
21000 Brookpark Road
Refer to LEW-18221-1.