Patent Application
20080308425
The present invention generally relates to magnetic steel that is used in the construction of aircraft and industrial components including solenoids and electric motors. More particularly, the present invention relates to wear and corrosion resistant coatings for magnetic steels to prevent damage during use of the components that may occur due to friction or corrosion as a result of operation and the operating environments.
Magnetic steel is used to manufacture electric motors and gas turbine engine components for aircraft and industrial applications. As just one example, solenoid actuated valves are also manufactured from magnetic steel. A solenoid valve includes a valve assembly that is coupled to a linear electromechanical solenoid. The assembly functions as the interface between electronic controller and pneumatic or hydraulic systems, and allows an electrical input to control pneumatic or hydraulic flow. Consequently, the solenoid-actuated valve, hereinafter referred to as a solenoid, is frequently used for controlling flow of fluids in turbine engines and aircraft pneumatic and hydraulic systems.
Because solenoids include moveable mechanical components and are tightly disposed in high pressure conduits though which contaminated and elevated temperature gases may flow, they are subject to wear and corrosion. Particularly, in order to optimize magnetic force, moving magnetic steel components often have small gaps between the moving magnetic pieces. Accordingly, the moving pieces may experience frictional wear and corrosion. Wear and corrosion of magnetic steel will inhibit proper motion of devices made therefrom.
A variety of coatings are used to enhance the wear and corrosion behavior for a solenoids, electric motor components, and other articles manufactured from magnetic steel that may experience friction due to relative motion between the articles and their adjacent components and corrosion due to environmental and control fluid composition during use. Electroplating is just one of many common methods for forming protective coatings on magnetic steel components. However, protective electroplated coatings developed for magnetic steels have limited field service due to inherent coating porosity and defects and subsequent penetration of a corroding electrolyte.
It is therefore desirable to provide improved coatings that function to both prevent corrosion and improve wear resistance to thereby increase the functional life of magnetic steel components. In addition, it is desirable to provide methods for forming such coatings. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
A method is provided for manufacturing a magnetic steel component. An electroless nickel plating is formed on a substrate that includes magnetic steel. A thermal cycle is thereafter performed at a temperature that is sufficiently high to sinter the electroless nickel plating and thereby form a densified plating on the substrate. According to one embodiment, the thermal cycle includes a solid state diffusion sintering process wherein the substrate and the densified plating are heated to a temperature of at least about 1300° F. (about 704° C.) but.below the melting temperature of the electroless nickel plating. According to another embodiment, the thermal cycle includes a transient liquid phase sintering process wherein the substrate and the densified plating are heated at least to the melting temperature of the electroless plating.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
The present invention provides the advantages of using plating methods to form a metal layer as a corrosion resistance coating for magnetic steel components. The plating methods may be used to form a series of coatings in order to optimize the coating formulations and to concentrate particular metals in various portions of the plated coating.
According to an exemplary embodiment, a thin metal strike 12 is formed on the substrate 10 in order to provide a surface that has little to no oxide. The magnetic steel in the substrate 10 is highly susceptible to formation of metal oxides such as iron III oxide. Oxide formation tends to reduce adhesion of overlying coatings to the substrate 10, and further tends to reduce diffusion of metals between the substrate 10 and any overlying coatings during subsequent thermal processing. Consequently, the substrate 10 is coated with the strike 12, which is a metal coating applied using a deposition process that removes oxides of the magnetic steel and replaces the oxides with a thin metal layer. The metal layer may also form oxides, although they are less difficult to remove during the application of a thicker top coat compared to the thicker oxides commonly formed on the magnetic steel that is part of the substrate 10. Exemplary metal strike materials include copper and nickel. According to one embodiment, the metal strike 12 is formed by an electrolytic plating process until the strike material reaches a thickness of about 0.0001 to 0.0005 inches (about 2.54 to 12.7 micrometers).
As depicted in
After forming at least the electroless nickel plating 14, a thermal cycle is performed. The plating 14 as originally formed has microscopic pores and defects, and includes an amorphous mixture of nickel and phosphorous. The plating process results in residual stress throughout the plating 14. The defects and the internal stresses within the plating are reduced by inter-atomic diffusion that is induced by heating the coating 14 at a sufficient temperature and for a sufficient time. The resulting coating has improved corrosion resistance.
Furthermore, performing a thermal cycle produces a metallurgical bond between the plating 14 and the substrate 10. Conventional coatings may spall off during service as they are mechanically bonded. According to the present invention, a thermal cycle is performed to prevent the plating 14 from spalling.
The time and temperature for the thermal cycle vary according to the electroless nickel plating thickness and composition. Furthermore, the thermal cycle temperature should be commensurate with an annealing temperature for the magnetic steel in the substrate 10. The thermal cycle produces a densified electroless nickel plating 16 as depicted in
Two exemplary methods for forming the densified electroless nickel plating 16 are solid state diffusion sintering and transient liquid phase sintering. Either thermal process will effectively reduce the coating porosity by closing and sealing the pores. As a result, the corrosion resistance properties of the electroless nickel plating are improved. Solid state diffusion sintering is driven by the differential composition between the magnetic steel substrate 10 and the overlying plating 14. The differential is enhanced by the residual stress within the plating 14 caused by the mismatched grains of nickel and nickel phosphorus, and the gaps that the mismatched grains produce. Transient liquid phase sintering is performed at a higher temperature than solid state diffusion sintering in order to at least partially melt the eutectic composition in the plating 14 and thereby substantially eliminate the porosity within the plating 14 by capillary action.
Either of the solid state diffusion sintering process and the transient liquid phase sintering process is performed in a vacuum or an inert gas environment. An exemplary solid state diffusion process is performed by heating the substrate 10 with the electroless nickel plating 14 formed thereon to a temperature ranging between about 1300 and about 1600° F. (between about 704 and about 870° C.). As previously discussed, the thermal cycle temperature should also be commensurate with an annealing temperature for the substrate 10. The elevated temperature is maintained for a period ranging between about 1 minute and about 4 hours, depending on the thickness and composition of the electroless nickel plating 14 and the mechanism that is required to fuse the densified plating 16 to the substrate 10.
An exemplary transient liquid phase sintering process is performed at a higher temperature than the temperature for the solid state diffusion process. The thermal cycle is performed at a temperature that at least partially melts the electroless nickel plating material during transient liquid phase sintering. Capillary action causes the pores in the plating 14 to close. Densification of the coating occurs rapidly as a result of the partial melting of the electroless nickel plating 14. Also, increased diffusion of atoms between the substrate 10 and the plating 14, and the nickel strike 12 if included, is a result of performing a transient liquid phase sintering process instead of a solid state diffusion sintering process.
The densified electroless nickel plating 16, may also be formed by applying an additional coating over the electroless nickel plating 14 in
For many applications, particularly for components that experience occasional friction or contact with particles in flowing air or with other components, a wear resistant coating may be formed over the densified electroless nickel plating and, when included, over other overlying coatings such as an overlying metal strike 18.
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
