Patent Application
20070098912
The present invention relates to coating substrates, such as gas turbine engine components, with a coating having high strength and hardness and, more particularly, to methods of forming a graded coating on the substrate.
Turbine engines are used as the primary power source for aircraft, in the forms of jet engines and turboprop engines, as auxiliary power sources for driving air compressors, hydraulic pumps, on aircraft, and as stationary power supplies, such as backup electrical generators for hospitals and the like. The same basic power generation principles apply for all these types of turbine engines. Compressed air is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge on the turbine blades mounted on a rotationally mounted turbine disk or wheel.
The force of the impinging gas causes the turbine disk to spin at high speeds and to produce power. Some engines use this power to turn a propeller, electrical generator, or other devices, while jet propulsion engines use this power to draw more air into the engine. When the high velocity combustion gas is passed out of the aft end of a jet turbine, forward thrust is created. Thus, during operation of the turbine engine, these components are subjected to stress loadings and high heat (often in excess of 2000° F.). The high stress and heat can cause erosion, oxidation, corrosion, thermal fatigue cracks and/or foreign object damage in the component, resulting in unacceptably high rates of degradation.
To protect the components from the above-mentioned environments, graded coatings may be included thereon. Conventionally, graded coatings have been deposited onto the component using one of various thermal spraying techniques, for example, low pressure plasma spraying, high velocity oxygen fuel thermal spraying, arc-plasma spraying, and electric wire arc spraying; however, these techniques suffer from certain drawbacks, which have made them expensive and limited their use. For example, because the coating material and component are both heated during the deposition process, the component may be more prone to distortion. Additionally, a thermal spraying apparatus typically needs to be stopped and re-started each time the composition of the coating changes. As a result, the coating may be deposited layer by layer and physical differences between each of the layers may promote cracking. Also, spraying mixtures of powders to obtain a gradual transition from one layer to the next is relatively difficult to achieve due to the different temperatures (spray conditions) required for depositing each powder. Moreover, thermal spraying is generally costly and time-consuming to implement, especially when depositing more than one layer.
Hence, there is a need for a spraying method that is capable of efficiently and cost-effectively producing a wear and oxidation-resistant coating that has high strength or hardness. Preferably, the method is capable of producing a graded coating that experiences little to no cracking. There is also a need for a spraying method by which graded coatings can be uniformly and thoroughly applied at temperatures that will not distort the coating component.
The present invention provides a method of forming a graded coating on a surface of a substrate. The method comprises the step of cold gas-dynamic spraying powder mixtures on the substrate surface to form the graded coating thereon.
In another exemplary embodiment, the method comprises cold gas-dynamic spraying a first powder mixture on the substrate surface, the first powder mixture formulated to form a first portion of the graded coating and comprising a first component and a second component. Additionally, the method includes gradually decreasing the first component and increasing the second component to form additional powder mixtures, while cold gas-dynamic spraying the additional powder mixtures over the portion of the graded coating to form a second portion of the graded coating.
In still another exemplary embodiment, the method comprises the step of cold gas-dynamic spraying a first powder mixture on the substrate surface to form a first portion of the graded coating. The method also includes decreasing a first component of the first powder mixture and increasing a second component of the first powder mixture to form additional powder mixtures, while cold gas-dynamic spraying the additional powder mixtures on the substrate surface over the first layer to form a second portion of the graded coating. Additionally, the method includes repeating the step of decreasing and cold-gas spraying the additional powder mixtures over the substrate to form other portions of the graded coating until a top portion having a predetermined composition is formed.
Other independent features and advantages of the preferred method will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
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.
Turning now to
Preferably, a cold spraying system, such as the system depicted diagrammatically in
The controller 302 controls system parameters, such as the amount of carrier gas and the pressure of the gas in the system 300, and is coupled to the powder feeds 304 and carrier gas supply 306. The powder feeds 304 are configured to contain different component powders that will be mixed to form a desired powder mixture. The carrier gas supply 306 contains a conventionally used carrier gas such as air, helium or nitrogen for carrying the powder mixture through the system 300.
Each powder feed 304 is coupled to a heater 308 that is configured to warm the carrier gas and component powders to a suitable temperature. The mixing chamber 310 is configured to receive and mix the component powders to form the desired powder mixture. The nozzle 312 is coupled to the mixing chamber 310 the nozzle accelerates the powder to the required high velocities for cold spray. In one exemplary embodiment, the nozzle 312 is a Laval nozzle.
It will be appreciated that in other embodiments of the system 300, the mixing chamber 310 may not be included. In one exemplary embodiment, the powder feeds 304 and nozzle 312 are directly coupled to each other. For example, the powder feeds 304 may be situated upstream of the nozzle 312 and distributed equidistantly around an axis of the nozzle 312. In another exemplary embodiment, the powder feeds 304 are disposed such that they feed the powders directly upstream of the nozzle 312. Alternatively, the powder feeds 304 may be disposed such that they feed the powders into larger tube upstream of the nozzle 312 that is positioned on the same axis as the nozzle 312
Returning to
Next, initial parameters are selected, step 206. As mentioned briefly above, the initial parameters may include the powder materials to be used, and apparatus parameters, such as the carrier gas pressure, the powder feed rate, and the heater temperature. It will be appreciated that the selection of each parameter is dependent upon the particular powder materials that will be deposited onto the substrate surface 102, and the powder material selection depends on the desired composition of the graded coating. It will be appreciated that the desired composition may include one or more constituents. For example, the composition may be an aluminum alloy or an aluminum-based alloy that also include copper, tungsten carbide, cobalt, silicon, silicon carbide, any other constituent, and any combination thereof. In any case, one or more of the constituents may make up a component of the powder mixture, and each component is individually placed into a powder feed 304.
After the powder materials are selected, initial parameters are then inputted into the cold spray system controller 302. For example, in processes in which aluminum alloys are used, the carrier gas pressure may be between about 100 psi and about 150 psi, the powder feed rate may be between about 3 g/sec and 5 g/sec, and the heater temperature may be between about 200° C. and about 500° C. Other parameters may also be selected, such as, for example, a nozzle 312 to substrate surface 102 distance or a raster rate (i.e. the rate at which the nozzle 312 moves across the substrate surface 102). Examples of suitable values include a distance of between about 5 mm and 10 mm, and a raster rate of between about 5 mm/sec. and 15 mm/sec.
Next, the system 300 is used in a cold-gas dynamic spraying process to form a graded coating 104 on the substrate 100, step 208. In one exemplary embodiment, the first section 106 of the graded coating 104 is deposited onto the substrate 100. In this regard, suitable amounts of the components used to make up the first section 106 are flowed from each appropriate powder feed 304 to the mixing chamber 310. In one exemplary embodiment, portions of the carrier gas are flowed at velocities that are each sufficient to carry respective predetermined portions of the components to the mixing chamber 310. It will be appreciated that each component may have a different density, thus the controller 302 is preferably appropriately pre-programmed to adjust the pressure of the carrier gas flow to each powder feed 304. Specifically, the pressure is adjusted to suitably carry the desired amount of component powder from the powder feed 304 to the mixing chamber 310. For example, in one exemplary embodiment, a first component has a higher density than a second component and a portion of the carrier gas flows through the first component at a first pressure, while another portion of the carrier gas flows through the second component at a second pressure that is greater than the first pressure.
After the appropriate amounts of components are dispensed in the mixing chamber 310, they are mixed to form a powder mixture that is suitable for forming the first section 106. The powder mixture is subsequently flowed to the nozzle 312 and deposited onto the substrate surface 102. As briefly mentioned above, the nozzle 312 may have any one of numerous configurations. Typically, the nozzle 312 will have a convergent/divergent shape, thereby including a high pressure region at its smallest diameter and a low pressure region at a larger diameter. It will be appreciated that the powder mixture may enter the nozzle 312 in any suitable portion thereof. In some embodiments, the powder mixture enters upstream of the nozzle 312. In another example, the powder mixture is flowed into a non-illustrated feed tube disposed upstream of the nozzle 312 and on the axis of the nozzle 312.
After the first section 106 is deposited onto the substrate surface 102, the second section 108 is then deposited thereover. As mentioned above, the second section 108 has a graded composition that gradually changes from the first composition to the second composition. In one exemplary embodiment, the controller 302 is pre-programmed to gradually change the ratio between the components in the powder feeds 304 while simultaneously causing the components to be deposited over the substrate surface 102. Thus, for example, in an embodiment in which the first composition includes a first component amount that is greater than a second component amount and the second composition includes a first component amount that is less than a second component amount, the first component amount is gradually decreased while the second component amount is gradually increased until the desired ratio is achieved between the two to form the second composition.
After the graded coating 104 is formed on the substrate surface 102, post-coating processes are performed, step 210. For example, the substrate 100 having the graded coating 104 thereon may be heat-treated to increase bonding between the component powders.
A method has now been provided for forming a graded coating that does not distort the substrate 100. Additionally, the method does not require the use of an apparatus that needs to be stopped and re-started each time the composition of the graded coating changes. Moreover, the method is generally inexpensive, efficient, and yields high quality graded coatings.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
