Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U. S. Standard screen size (149 microns) and about 2 microns. The carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air.
The material alternatively may be fed into a heating zone in the form of a rod or wire such as described in U.S. Pat. No. 3,148,818 (Charlop). In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, such as a combustion flame, where it is melted or at least heat-softened and atomized, usually by blast gas, and thence propelled in finely divided form onto the surface to be coated. Especially high quality coatings of thermal spray materials may be produced by spraying at very high velocity. Plasma spraying has proven successful with high velocity in many respects but in certain cases, especially with carbides, it is not as good as combustion, apparently due to overheating and/or to poor particle entrainment which must be effected by feeding powder laterally into the high velocity plasma stream.
U.S. Pat. No. 2,714,563 (Poorman et al) discloses a detonation gun for blasting powdered material in a series of detonations to produce coatings such as carbides. Since the detonation pulses are very harmful to the ears the apparatus must be operated by remote control in an isolated room, and also the process is quite complex. Therefore this method has been expensive and commercially limited in availability. Also it has not lent itself to full control of spray pattern and efficient target efficiency. However, the detonation process has demonstrated the desirability of spraying at very high velocity. High density and tenacity of coatings are achieved by high impact of the powder particles, and the short dwell time in the heating zone minimizes oxidation at the high spray temperatures.
A rocket type of powder spray gun can produce excellent coatings and is typified in U.S. Pat. No. 4,416,421 (Browning). This type of gun has an internal combustion chamber with a high pressure combustion effluent directed through an annular opening into the constricted throat of a long nozzle chamber. Powder is fed axially within the annular opening into the nozzle chamber to be heated and propelled by the combustion effluent. In practice the gun must be water cooled and a long nozzle is particularly susceptible to powder buildup. Also, ignition in an internal chamber requires special technique; for example a hydrogen pilot flame is used. There are safety concerns with an enclosed high pressure combustion chamber. A long nozzle is not geometrically suitable for spraying on inside diameters or other such remote areas, and is somewhat restricted with respect to varying and selecting the size of the spray stream. Best results have been effected commercially in such a rocket gun with hydrogen for the combustion gas which must be used at high flow rates, causing the process to be quite expensive.
Short-nozzle spray devices are disclosed for high velocity spraying in French Patent Nos. 1,041,056 and U.S. Pat. No. 2,317,173 (Bleakley). Powder is fed axially into a melting chamber within an annular flow of combustion gas. An annular air flow is injected coaxially outside of the combustion gas flow, along the wall of the chamber. The spray stream with the heated powder issues from the open end of the combustion chamber. There are not sufficient details taught in the Bleakley and French patents for one to attain truly high velocity powder spraying, and apparently no significant commercial use has been made of these devices, despite the references being 45 and 35 years old respectively.
The Bleakley and French short-nozzle devices superficially have a nozzle construction similar to commercial wire spray guns of the type disclosed in the aforementioned U.S. Pat. No. 3,148,818. However, wire guns function quite differently, with the combustion flame melting the wire tip and the air atomizing the molten material from the tip and propelling the droplets. Wire guns generally have been used to spray only at moderate velocity.