Thermal spray method utilizing in-transit powder particle temperatures below their melting point

DETAILED DESCRIPTION This invention advantageously uses an internal burner capable of flame spraying nearly all the high melting point materials previously only sprayed using devices operating with oxygen contents greater than that contained in ordinary compressed air.
Needless to say, large operating economics are realized where expensive pure oxygen is not required and simplicity and reliability of the operation are greatly enhanced by eliminating forced cooling water flow for such burners.
This invention is directed to a thermal spray method in which a fuel and an oxidant are continuously combusted at elevated pressure within a restricting volume of a combustion chamber (or by other thermal source) to produce a sonic or supersonic flow of hot gases from an extended nozzle to produce and direct a supersonic jet of the hot gases toward a workpiece surface to be coated.
Powdered material is fed to the stream to be heated by the stream and projected at high velocity onto the workpiece surface.
The improvement lies in feeding the powdered material into the extended nozzle, well down stream of the throat and after expansion of the hot gases thereby limiting the step of heating of the powdered material by the jet stream to that of raising the temperature of the particles to a temperature lower than the melting point of the material, maintaining the in-transit temperature of the particles to the workpiece below the melting point and providing sufficient velocity to the particles striking the workpiece to achieve an impact energy capable of releasing additional heat upon impact to fuse the material to the workpiece surface to form a dense coating thereon.
The thermal spray method may utilize a plasma torch operating at high pressure to produce the hot jet stream issuing from the extended length nozzle bore or an internal burner.
The powder or like particles may be preheated in a separate container from the source of the flame spray such as by inductive heating or a flame exterior of a ceramic container for the powder so long as the powder particles do not fuse, and with the flame temperature limited to prevent fusing of the powder particles prematurely in the ceramic container or other preheating support