Thermal spray processes are used to apply corrosion-resistant and wear-resistant coatings to a wide variety of substrates and articles. Four types of thermal processes are used for different types of materials and applications: flame spraying, plasma spraying, detonation spraying, and electric arc spraying. Of these processes, electric arc spraying is preferred in many applications for its high deposition rates and economical operation.
The development of coating applications using the electric arc spray process to produce hard, wear-resistant coatings using cored wires which contain powdered metal or ceramic/metal components in a metal sheath has been initiated over the past several years. In the spraying process, usually carried out in a surrounding air atmosphere with compressed air as the atomizing gas, alloys and/or composites are formed between the sheath and core materials and are deposited on the substrate to form hard, wear-resistant coatings. Core powder materials can be selected from a variety of elements and compounds including combinations such as tungsten carbide-cobalt, nickel-chromium-boron, nickel-iron, boron carbide-iron-molybdenum, and chromium-boron-silicon. Likewise, sheath materials can be selected from a range of alloys comprising iron, nickel, and other elements. The use of cored wires allows the application of powdered components by the efficient and economical arc spray process; powdered components otherwise must be applied by the flame, detonation, or plasma spray methods.
The production of cored wires for arc spraying is discussed in a paper by H. Drzeniek et al in Proceedings, 10th International Thermal Spraying Conference, Essen, W. Germany, May 2-6, 1983, at p.136. Various cross-section types of iron wire filled with nickel powder are disclosed and the effect of spray parameters on coating properties are described.
U.S. Pat. No. 4,741,974 discloses a composite wire for use in arc gun spraying formed of an alloy sheath comprising iron, nickel, or cobalt, and a core comprising boron-containing powder of boron, boron carbide, and/or a ferromolybdenum alloy powder. Coatings are formed with such wire using a standard arc spray gun using air at 60 psia for atomizing and 40 psia for air cap.
An article by H.-D. Steffens et al in Advances in Thermal Spraying, Proceedings of the 11th International Thermal Spraying Conference, Montreal, Sep. 8-12, 1986, at p.457 discloses the arc spraying of cored wire using a methane-air mixture to atomize the molten metal. The use of methane in this mixture reduces problems with burnoff (oxidation) of the powder material when air alone is used as the propelling gas. The article cites earlier work in which argon and hydrogen-nitrogen mixtures were used in an attempt to eliminate burnoff problems associated with the use of air as the propelling gas. It is stated that this approach was relatively ineffectual, and that the trend has been towards arc spraying using low pressure chambers or chambers filled with inert gases.
V. I. Pokhmurskii et al in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 22, No. 6, Nov.-Dec. 1986 at p.11 discuss the arc spray coating of powder-filled wires in which aluminum powder is added to reduce the oxidation losses of iron and titanium which occur when air is used as the propelling gas.
Arc spraying of cored wires containing powder comprising carbon, manganese, silicon, chromium, and titanium carbide is disclosed in U.S. Pat. No. 4,810,850 wherein the particle size of the core powder is controlled at between 20-300 microns. The control of the powder particle size is described as important to prevent burnoff of the particles, a condition which decreases the efficiency of the arc spray process and lowers the amount of alloying elements in the coating.
S. J. Harris et al in an article in Surface Engineering, Proceedings of the 2nd International Conference, Stratford-upon-Avon, UK, Jun. 16-18, 1987 at p.447 describe the arc spraying of a low carbon steel wire filled with a tungsten carbide-cobalt powder and a nickel wire filled with nickel-boron and high carbon ferro-chrome powder. The arc spray gun is operated using compressed air as the atomizing gas.
The arc spraying of cored wires using air-methane mixtures for atomization is further described by H. Drzeniek and H.-D. Steffans in a paper in Proceedings of the National Thermal Spray Conference, Orlando, Fla., Sep. 14-17, 1987 at p.33. It is stated that since the atomizing gas is preferably air, the interaction of metal with air and the oxidation reactions therebetween are significant. The addition of methane to the atomization air reduces metal oxidation and thereby improves the coating properties compared with the use of air alone.
An overview of thermal spray coating methods and a description of electric arc spray coating using cored wires containing tungsten carbide-cobalt and nickel-chromium-boron powders are given R. C. Cobb et al in an article in Welding and Metal Fabrication, Vol.56, No.5, July 1988 at p.226. Air and inert gas are disclosed as potential atomizing gases, although air is preferred for actual applications.
The use of rare earth elements in cored wires used for electric arc spray coating applications is disclosed by H.-D. Steffans et al in Proceedings of the National Thermal Spray Conference, Oct. 24-27, 1988, Cincinnati, Ohio at p.325. It is pointed out that electric arc spraying of cored wires using air causes oxidation of particles during flight and after impact with the substrate, leading to reduction of adhesion of the coating. The use of unidentified rare earth alloys as powders in a low carbon steel sheath containing iron powder, when the cored wire is arc sprayed with air, gives increasing tensile and compressive strength of the coatings at increasing levels of the rare earth alloys up to 0.9 wt % in the core powder.
H.-D. Steffans et al in Proceedings of the National Thermal Spray Conference, Oct, 24-27, 1988, Cincinnati, Ohio at p.325. disclose the arc spraying of cored wires made from low carbon steel sheaths containing ferrochrome or chrome carbide powders to which carbides or borides are added in varying amounts. Spraying in an air atmosphere causes a loss of carbon and/or boron, but the authors teach that the content of carbon and boron in the coating can be controlled and oxidation reduced by adding additional carbon and/or deoxidizers such as phosphorous to the core filler material.
The background art discloses the desirability of electric arc spray coating using cored wires, and also discloses that the use of air as the preferred atomizing gas can cause oxidation of metal components during spraying and thus reduce the overall effectiveness of the coating process. Methods to reduce such oxidation losses have been disclosed, specifically the addition of deoxidizers to the core powder and the addition of methane to the atomizing air. Control of powder particle size is also a potential approach to control metal oxidation. The earlier attempt to use argon and hydrogen-nitrogen mixtures to reduce metal oxidation when arc spraying in a surrounding air atmosphere has been noted in the background art; this attempt was relatively ineffective, and led to the use of low pressure chambers or chambers filled with inert gases in which arc spraying is carried out.
Electric arc spraying for the application of wear-resistant coatings has important economic and operational advantages over other thermal spray coating methods, and therefore is expected to find increasing use in the future. Accordingly, there is a need for improving the effectiveness of electric arc spraying of cored wires for the application of high-quality, wear-resistant coatings. The invention described in the following disclosure and claims is directed towards such an improvement.