Patent Application
20080152814
This invention relates to the deposition of a platinum-group-containing layer on a surface and, more particularly, to deposition using a water-base platinum-group-containing paint that is applied by spraying.
A platinum-group-containing layer may be applied to the surfaces of a substrate to improve the corrosion resistance and oxidation resistance of the substrate. The platinum-group precious metals do not react to any substantial degree with oxygen, exhaust gas, or most other common oxidants and corrodants. The result is that the substrate with the platinum-group-containing layer thereon is at least partially inert in such environments. Platinum-group metals are expensive, but the improvement in properties produced by even a thin platinum-group-containing layer often justifies the cost.
One approach to applying a platinum-group-containing layer is electrodeposition. The substrate to be protected is made the cathode of an electrochemical cell in which the electrolyte is a platinum-group-containing salt. The application of an electrical current results in the deposition of the platinum-group metal onto the substrate. Electrodeposition is slow and can be difficult to apply in practice, such as in conditions where the substrate is irregularly shaped or the platinum-group layer is to be deposited only in selected areas of the surface of the substrate.
An alternative approach that has been considered is to apply the platinum-group metal from a platinum-group-containing paint. The proposed approaches are not environmentally acceptable, because they release volatile organic components during one or more of the processing steps.
There is a need for an improved approach to the deposition of a platinum-group-containing layer on a substrate. Such an approach must be compatible with other processing steps and be environmentally acceptable. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for depositing a platinum-group-containing layer overlying a substrate. The layer is deposited from a water-base platinum-group-containing paint that contains no volatile organic compounds whose release would be detrimental to the environment. Organic solvents often leave residues or produce volatile organic compounds when heated, and the present approach avoids these detrimental features. The deposition is preferably performed by spraying, which produces a uniform deposited layer of controllably small thickness. The spraying approach is fast and economical. The deposition of a platinum-group-containing layer on a complexly shaped article, and the masking of areas that are not to be coated, are readily accomplished.
In accordance with the invention, a method for depositing a platinum-group-containing layer on a substrate comprises the steps of furnishing the substrate, preparing a water-base paint comprising metallic platinum-group powder, water, and a binder, and depositing the water-base paint overlying the substrate to form a platinum-group-containing layer. Liquid water is the carrier for the remaining metallic and binder components, and no liquid organic carrier is used.
As used herein, the “platinum group” of elements includes platinum, palladium, rhodium, iridium, ruthenium, and osmium. A “platinum-group-containing layer” may include any one of these elements of the platinum group, or a mixture of elements of the platinum group.
Preferably, the substrate is a nickel-base superalloy substrate. There is no limitation on the types of substrates and articles that may be provided with the platinum-containing layer, but some examples of interest are a turbine blade, a turbine vane, and a combustor splash plate.
In one preferred application, the water-based paint is prepared by mixing together from about 25 to about 40 percent by weight metallic platinum-group powder, from about 10 to about 30 percent by weight binder, balance water. The preferred binder is methyl cellulose.
The water-base paint may be deposited by any operable technique. However, a preferred approach is spraying the water-base paint overlying the substrate. Spraying, as with an air or airless sprayer, is fast, and can produce a uniform layer on the substrate, both before and after a subsequent diffusion heat treatment. The application of the water-base paint desirably produces the platinum-group-containing layer in a thickness of from about 0.0001 to about 0.0003 inches of high-purity platinum group metal after a heat treatment of 1950° F. for 4 hours. The platinum-group-containing layer desirably includes at least about 96 percent, most preferably at least about 99 percent, platinum-group metal (or mixture of platinum-group metals) by weight exclusive of the water and the binder. After a heat treatment of 1950° F. for 4 hours, there is no water and no binder in the platinum-group-containing layer. It is preferred that there be less than about 100 parts per million (ppm), more preferably less than about 25 ppm, total of the elements sulfur and carbon.
The platinum-group-containing layer may be left unmodified on the substrate. More typically, however, the platinum-group-containing layer is heated to interdiffuse the platinum-group-containing layer into the underlying layer, which may be the substrate or a previously deposited underlayer.
One application of the present approach is to transform the platinum-group-containing layer into an alloy layer, preferably with aluminum. Various platinum-group/aluminum compositions provide excellent corrosion and oxidation protection for the underlying substrate. In this approach, there may be an additional step of depositing an aluminum-containing layer in contact with the platinum-group-containing layer. It is preferred to heat the aluminum-containing layer and the platinum-group-containing layer to interdiffuse the aluminum-containing layer and the platinum-group-containing layer. The heating of the aluminum-containing layer and the platinum-group-containing layer to accomplish interdiffusion may be performed concurrently with the deposition of the aluminum-containing layer, or after the deposition of the aluminum-containing layer is complete, or both.
The platinum-group-containing layer may be deposited directly upon the substrate. It may instead be deposited upon a previously applied layer or structure. In one application, an MCrAlY underlayer is first applied to the substrate. The platinum-group-containing layer is deposited overlying and contacting the MCrAlY underlayer, and may be interdiffused with the MCrAlY underlayer. In a similar application, a NiAl-base underlayer (which may include other alloying elements such as Cr and Zr) is applied to the substrate. The platinum-group-containing layer is deposited overlying and contacting the NiAl-base underlayer, and may be interdiffused with the NiAl-base underlayer.
In a preferred approach, a method for depositing a platinum-group-containing layer on a substrate comprises the steps of furnishing the substrate, preparing a water-base paint comprising metallic platinum-group powder, water, and a binder, spraying the water-base paint overlying the substrate to form a platinum-group-containing layer, and thereafter heating the platinum-group-containing layer to interdiffuse the platinum-group-containing layer and the substrate. Other compatible processing, such as that described herein, may be employed in conjunction with this approach.
The present approach provides important advances in the art of platinum-group-containing layers and coatings by using a water-base platinum-group-containing paint that does not produce volatile organic compounds during application or subsequent processing. The water-base platinum-group-containing paint may be applied to the substrate by spraying, which produces a uniform and thin deposited layer that is suitable for interdiffusion and is suitable for further processing into a platinum-group/aluminide layer. Spray application is fast and economical, and may be used for irregularly shaped substrate workpieces and in situations where only a portion of the surface is to be coated and another portion is masked.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
The substrate 50 is typically processed to remove any pre-existing scale, excessive oxide or corrosion, dirt, or other contamination in the areas to be painted. The cleaning procedure is appropriate to the nature of the contamination. One effective technique is to use fine grit blasting on the substrate 50, which also prepares the surface to receive the subsequently deposited water-base paint.
Optionally, an underlayer 52 may be applied, numeral 22 of
A water-base paint comprising metallic platinum-group powder, water, and a binder is prepared, numeral 24 of
The platinum-group powder, water, and binder are mixed together in an operable ratio. Most preferably, the platinum-group powder is present as 25 to 40 percent by weight of the total mixture, the methyl cellulose is present as 10 to 30 percent by weight of the total mixture, and the water is the balance. The amount of water may be adjusted as desired to control the fluidity and consistency of the mixture as necessary for the selected application technique. This mixture is first manually mixed by hand or by machine stirring, and then is further mixed and compounded by mixing in a ball mill without media for at least 1 hour. The resulting mixture performs satisfactorily as a paint, with the platinum-group particles remaining in suspension for an extended period of time. If the paint is not used immediately after mixing and compounding, it may be used at a later time after mechanical re-stirring.
The water-base paint is deposited, step 26, overlying a surface 53 of the substrate 50 (or underlayer 52 if present) to form a platinum-group-containing layer 54, as shown in
The water-base paint may be applied by any operable approach. Examples include air or airless spraying, dipping, spinning, and brushing. The viscosity consistency of the water-base paint may be adjusted by adding a higher proportion of water.
The greatest advantages of the water-base paint are realized when it is sprayed onto the substrate 50 or the underlayer 52. The particular formulation of the water-base paint allows it to be sprayed through fine nozzles, and it may be thinned as needed with the addition of more water. Spraying deposits a thin liquid layer that dries as the water evaporates. The sprayed paint layer, and thence the platinum-group-containing layer 54, may be made highly uniform using automated spraying and, with skill, even using manual spraying. Fine details of the substrate article may be uniformly painted, for example by using an airbrush. Areas of the surface that are not to be painted may be masked to prevent any deposition of the water-based paint.
Another advantage of spraying is that the thickness of the as-deposited layer may be carefully controlled. Because the platinum-group metals are expensive, the thickness of the platinum-group-containing layer 34 is made no thicker than necessary to accomplish the required protective function. For most applications, the platinum-group-containing layer 54 is preferably from 0.0001 to 0.0003 inches in thickness, after drying and subsequent heat treatment. If the layer 54 is thicker, an undesirable gold-colored PtAl2 phase is formed, and platinum-group metal is wasted. If the layer 54 is thinner, there is a risk of pinholes through the layer 54 and unpainted areas. Normally, a single coat of the water-based paint is sufficient and is preferred. Such a single coat is preferably accomplished by a single pass of the paint sprayer. However, if desired, two or more coats may be applied to build up the thickness of the platinum-group-containing layer 54, allowing each coat to dry before the next coat is applied. If a thicker platinum-group-containing layer is desired, it is preferred to create that thicker platinum-group-containing layer using multiple thin coats rather than a single thick coat. Desirably, the final platinum-group-containing layer 54 includes at least 96 percent, more preferably at least 99 percent, platinum-group metal (or mixture of platinum group metals) by weight exclusive of the water and the binder. It is preferred that there be less than 100 parts per million (ppm), more preferably less than 25 ppm, total of the elements sulfur and carbon. Sulfur and carbon, when present in larger amounts, may contribute to the degradation of the coating during service.
After the water-based paint is deposited upon the substrate 50, the water therein quickly evaporates so that the paint dries to the touch, resulting in the platinum-group containing layer 54 on the substrate 50. The platinum-group-containing layer 54 may be used in this as-deposited-and-dried state. More typically, however, it is further processed.
Optionally, the platinum-group-containing layer 54 is heated, to interdiffuse the platinum-group-containing layer 54 and the substrate 50 (or underlayer 52, if present), step 28. Atoms of the platinum-group metal(s) diffuse into the substrate 50 (or the underlayer 52, if present), and atoms of the substrate 50 (or the underlayer 52, if present) diffuse into the platinum-group-containing layer 54. This interdiffusion serves to densify the platinum-group-containing layer 54, to bind the platinum-group-containing layer 54 more strongly to the substrate 50 (or the underlayer 52, if present), and to create a surface compositional gradient. Any operable heat treatment 28 may be used. In a preferred approach, the heating 28 is at 1950° F. for 4 hours in vacuum. The result is that the platinum-group-containing layer 54 becomes highly adherent and highly corrosion-resistant. The interdiffused substrate 50, underlayer 52 if present, and platinum-group containing layer 54 may be used in this state, or may be further processed as described next.
In yet a further optional processing, an aluminum-containing layer 56 is deposited in contact with the platinum-group-containing layer, numeral 30. The aluminum-containing layer 56 may be pure aluminum or an aluminum alloy. The aluminum-containing layer 56 may be deposited by any operable technique. Examples of operable deposition techniques include chemical vapor deposition, vapor phase aluminiding, or physical vapor deposition. The thickness of the aluminum-containing layer 56 is typically approximately the same as the initial thickness of the platinum-group-containing layer 54 prior to interdiffusion, or slightly thicker. In the preferred embodiment, the aluminum-containing layer 56 has about the same thickness as the platinum-group-containing layer 54.
Optionally but preferably, the substrate 50, the underlayer 52 if present, the platinum-group-containing layer 54, and the aluminum-containing layer 56 are then heated, step 32, to interdiffuse the atoms of the aluminum-containing layer 56, the platinum-group-containing layer 54, the underlayer 52 if present, and the substrate 50. Any operable heat treatment 32 may be used. In a preferred approach, the heating 32 is at 1975° F. for 6 hours in a flow of 150 standard cubic feet per hour of argon. The result is an interdiffused layer 58, as illustrated in
The present invention has been reduced to practice using the processing defined by steps 20, 24, 26, 28, 30, and 32, using the preferred values discussed above. The substrate 50 was Rene™ N5 alloy, having a nominal composition in weight percent of about 7.5 percent cobalt, about 7.0 percent chromium, about 1.5 percent molybdenum, about 5 percent tungsten, about 3 percent rhenium, about 6.5 percent tantalum, about 6.2 percent aluminum, about 0.15 percent hafnium, about 0.05 percent carbon, about 0.004 percent boron, about 0.01 percent yttrium, balance nickel and minor elements. The as-deposited platinum-group-containing layer 54 was pure platinum about 3-4 micrometers thick deposited by spraying. The aluminum-containing layer 56 was pure aluminum deposited by vapor phase aluminiding. The vapor phase aluminiding was accomplished at elevated temperature, so that the resulting structure was like that shown in
The chemical composition of the specimen was measured as a function of distance from the surface of the specimen.
In some instances, it is preferred to apply the platinum-containing layer 54 and optionally the aluminum-containing layer 56 overlying only a portion of the substrate 50, while leaving other portions bare. As shown in
Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.
