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 conventional 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.
A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. Other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion thermal spray gun, a 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. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas.
One form of powder for thermal spraying is composite powder such as disclosed in U.S. Pat. No. 3,617,358 (Dittrich). This patent teaches the use of the spray drying process for making the composites, involving the spraying of a slurry of very fine powdered constituents with a binder to form droplets, and drying the droplets into a powder. There may be only a single constituent, or multiple constituents may be incorporated, for example in a cermet powder of a metal and a non-metal.
Other composite forms are known for thermal spraying, for example metal cladding of a ceramic core as disclosed in U.S. Pat. No. 4,291,089 (Adamovic). According to this patent a clad powder such as nickel alloy clad bentonite is useful for producing thermal sprayed abradable seal coatings for gas turbine engines. Cladding of metal core particles with finer particles of ceramic is taught in U.S. Pat. No. 3,655,425 (Longo and Patel) for similar purpose.
The metal in a composite may have any of a variety of roles, such as to provide a binding function for a non-metal in a coating, or to increase ductility in an otherwise ceramic coating. A further function of the metal may be to provide a melting phase in the thermal spray process so as to carry and bond the non-metal to the coating. This is particularly a requirement for spraying non-metals which are substantially non-meltable, including the bentonite of the above-mentioned patent. Generally, however, conventional composite powders with a high proportion of a non-meltable constituent are difficult to spray and have relatively low deposit efficiency, and some clad powders tend to be costly and difficult to manufacture with consistency. Clad powders are inherently limited in available range of metal to non-metal.