Thermal spray coating is not a new process. There are different techniques utilized that depend on the objective function of the coating, the environment to which the coated piece will be subjected, and the coating material used. In any application, quality is ultimately measured by how well the coating material adheres to the sprayed surface. This, in turn, is controlled by the velocity at which the coating material impinges on the substrate, the size of the molten coating particles, and the degree to which the coating material is prevented from chemically reacting while in a molten state.
A method has been devised for highvolume, high-velocity surface deposition of protective metallic coatings on otherwise vulnerable surfaces. The method is a form of thermal spraying whereby the material to be deposited is heated to the melting point by passing through hot products resulting from combustion (aka flame spraying). In such systems, the molten material is normally transported to the deposition surface by the jet formed from the combustion products. In typical steady systems, combustion occurs at high pressure, and high velocity is achieved by expanding (accelerating) the products and coating material through a nozzle. This means that fuel and oxidizer must be supplied at high pressure, necessitating bulky storage tanks or expensive pumps.
The method described here utilizes non-steady combustion processes (i.e., high frequency, periodic, resonant, confined volume) that generate high temperatures, high heat transfer rates (e.g., more molten material), atomizing acoustic waves, and very high impingement velocities in a simple device with only one moving part. Equally important, the non-steady combustion process is self-actuating. It is a resonant process requiring no external actuation or control, and no high-pressure supply of fuel or air. Hence, the system is potentially simpler than conventional thermal spraying systems. Furthermore, the high heat transfer rates developed allow the deposition material to be introduced not as an expensive powder with high surface-area-to-volume ratio, but in convenient rod form, which is also easier and simpler to feed into the system.
The system utilizes a low-cost combustion system to heat the material. The particular combustion technique naturally generates periodic high-velocity flows, which greatly enhance adhesion and heat transfer. Furthermore, the residence times in the combustion device are low and will therefore result in contact with the deposition surface before significant reaction has occurred. The combustion device is mechanically simple and could potentially be used to create a mobile, high-volume flame spray unit. The combustion device is self-aspirating, requiring no external air or fuel supply energy. The only external power required would be 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.