Patent Application
20210156041
Metallic coatings are applied to substrates to protect the substrate. For instance, the metallic coatings can provide corrosion resistance, high temperature oxidation resistance, protection from mechanical damage (e.g., scratching) or other protections for the substrate. Metallic coatings, and in particular refractory metal coatings that comprise high entropy alloys (HEAs) are typically applied to substrates in a very limited variety of ways, including ball milling and sintering, arc induction plasma melting, plasma spray, or laser cladding. However, it is difficult to apply coatings to non-line-of-sight surfaces, such as internal cooling passages of a component, with these methods. Some metallic coatings can be applied to non-line-of-sight surfaces by electroplating. However, electroplating requires the preparation of a solution containing metallic cations dissolved in a carrier liquid. An electrical current is applied to the solution to deposit the metallic cations into a substrate. Preparing such an aqueous solution for electrodeposition of HEA is difficult and expensive, and leads to poor coating structure and low current efficiencies due to the high rate of hydrogen reduction and evolution at the substrate cathode, which makes electrodepositon of refractory HEA from such an aqueous solution difficult and impractical.
A method of depositing a metallic coating onto a substrate according to an example of this disclosure includes mixing metallic salts of one or more elements with a solvent to form a mixture, heating the mixture to form a liquid, such that constituents of the mixture are in a mobile ionic state, and electroplating the metallic salts onto a substrate from the liquid.
In a further example of the foregoing, the mixture has a eutectic point that is lower than about 100° C.
In a further example of any of the foregoing, the heating is to a temperature at or near the eutectic point.
In a further example of any of the foregoing, the heating is to a temperature that is between the eutectic point and a temperature 10° C. above the eutectic point.
In a further example of any of the foregoing, the eutectic point is between about 10° C. and 100° C.
In a further example of any of the foregoing, one or more elements include one or more refractory metals.
In a further example of any of the foregoing, one or more elements include at least one of zirconium, niobium, titanium, tantalum, molybdenum, tungsten, rhenium, and hafnium, and combinations thereof.
In a further example of any of the foregoing, one or more elements include at least one metal selected from the group of zirconium, niobium, titanium, tantalum, molybdenum, tungsten, rhenium, and hafnium.
In a further example of any of the foregoing, one or more elements include at least one of zirconium, niobium, titanium, tantalum, molybdenum, tungsten, rhenium, and hafnium.
In a further example of any of the foregoing, the substrate is a component of a gas turbine engine.
In a further example of any of the foregoing, the substrate includes at least one non-line-of-sight surface. The electroplating is on the at least one non-line-of-sight-surfaces.
In a further example of any of the foregoing, the liquid is free from water.
In a further example of any of the foregoing, the solvent is selected from the group of ionic liquids and deep eutectic solvents.
In a further example of any of the foregoing, the solvent is a deep eutectic solvent, and incudes at least one of choline chloride with urea, ethylene glycol, glycerol, and malonic acid.
A solution for electroplating according to an example of this disclosure includes a mixture of metallic salts of one or more refractory metal elements and a solvent, wherein the mixture has a eutectic point below about 100° C.
In a further example of the foregoing, the eutectic point is between about 10° C. and 100° C.
In a further example of any of the foregoing, the refractory metal elements include at least one of zirconium, niobium, titanium, tantalum, molybdenum, tungsten, rhenium, and hafnium, and combinations thereof.
In a further example of any of the foregoing, the refractory metal elements include at least one metal selected from the group of zirconium, niobium, titanium, tantalum, molybdenum, tungsten, rhenium, and hafnium.
In a further example of any of the foregoing, the mixture is free from water.
In a further example of any of the foregoing, the solvent is selected from the group of ionic liquids and deep eutectic solvents.
The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.
Metallic coatings can be applied to substrates in order to protect the substrates from high temperatures, corrosion, mechanical damage, or other conditions. In a particular example, metallic coatings are applied to substrates, such as gas turbine engine components, to provide high temperature resistance for the substrates.
Metallic coatings can be alloys of multiple metals. One example type of metallic coating is a high entropy alloy (HEA). HEAs typically include four or more metallic elements in relatively high proportions, e.g., the four or more metallic elements each comprise higher than trace amounts of the HEA. In a particular example, one or more of the elements is a refractory metal. A refractory metal HEA coating example comprises at least one of niobium, molybdenum, tantalum, tungsten, rhenium, and combinations thereof. Another refractory HEA coating example comprises at least one of zirconium, niobium, titanium, tantalum, hafnium, and combinations thereof. And yet another example of a refractory metals HEA coating comprises at least one of niobium, molybdenum, tantalum, tungsten, rhenium, zirconium, hafnium, titanium, and combinations thereof.
Electroplating is generally known in the art. Essentially, electroplating includes dissolving metallic cations in a carrier liquid to form a solution. The solution is then applied to the substrate, and an electrical current is applied to the solution and substrate. The electrical current causes the metallic cations to deposit onto the substrate, forming a coating on the substrate. Electroplating can be used to apply coatings to non-line-of-sight surfaces, such as internal cooling passages of a component. For instance, the component can be submerged in the solution, which infiltrates the component to the non-line-of-sight surfaces. The metallic cations can then be electroplated onto the component.
In step 104, the mixture is heated to a temperature at or near its eutectic point to melt it (e.g., form a liquid). For example, the mixture is heated to a temperature between the eutectic point and about 10° C. above the eutectic point. In some examples, the eutectic point of the salt mixture is below 100° C. In a more particular example, the eutectic point is between about 10° C. and 100° C. Because the eutectic point is on the order of room temperature, the mixture can be heated easily and inexpensively by any known means. The heating causes the constituent chemical species exist in the liquid in a mobile ionic state (e.g., a liquid that includes ionic assemblies of the metallic salts (cations) and anions).
In some examples, the liquid mixture is free from water. For instance, the metal salts and solvents from step 102 are selected so that the liquid in step 104 is free from water.
In step 106, the metal salts (such as refractory metal cations, in one example) in the liquid mixture are electroplated onto the substrate 10 as generally discussed above and known in the art to form the coating 12 on the substrate 10. The electroplating can be any known type of electroplating, including pulsed electroplating and potentiostatic electroplating. In potentiostatic electroplating, the electrical current remains constant through the process. In pulsed electroplating, the electrical current is periodically raised and lowered between high and low values, and the polarity can also be momentarily reversed, to control the composition and thickness t of the resulting coating 12.
Refractory metals in particular were previously difficult to electroplate because the refractory metals have very high melting points. Creating a low eutectic point liquid with constituents including the desired refractory metals as discussed above greatly reduces the heating required to melt the mixture and form the liquid for electroplating the desired metallic composition onto the substrate 10 of choice, as discussed above. Therefore, the above-described method allows for the relatively simple and inexpensive electrodeposition of metallic coatings such as HEAs containing one or more refractory metals onto substrates, including on non-line-of-sign surfaces.
Use of the above described method overcomes the barriers involved with electroplating of refractory metals via aqueous electrolyte baths. The reduction half-reaction for refractory metal cations generally involves very high potential as compared to other metals like nickel, zinc, etc. This means that if the carrier liquid includes water, the reaction system will tend towards the electrolysis of water, which reaction has a much lower potential than the electroplating reaction for other metals. This in turn produces hydrogen, which is detrimental to the properties of the substrate metal (e.g. hydrogen embrittlement). In addition, the electroplating efficiency (e.g., amount of metallic coating deposited on the substrate over time) is low. To counteract this, very high voltages would be required to be applied to create the electrical current to drive the electroplating. However, in some examples, the above-described method includes a liquid including refractory metal that is free from water and can be used for electrodeposition. Accordingly, by using the above described method with a water-free liquid, electroplating of refractory metals and metal alloys becomes possible.
Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.
In some examples, plasma deposition of refractory metal alloys would create different surface morphologies and potentially compositions compared to electrodeposited coatings via the method described in the present invention.
The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can be determined by studying the following claims.
