Ag�ero, Alina C.
| 1. | A method of producing a reactive element modifiedaluminide diffusion coating on a metal substrate, the method comprising the steps of: providing a metal substrate having a composition such that a reactive element modifiedaluminide diffusion coating can be formed thereon and fixing said metal substrate within a coating apparatus; depositing a metal coating on said metal substrate by a physical vapor deposition process to form a coated substrate, the metal coating including at least two elements, one element being aluminum and the other element being at least one reactive element; and heat treating the coated metal substrate in an inert gas or a reducing gas atmosphere within an effective temperature range for an effective period of time to cause interdiffusion between components of the metal substrate and the deposited metal coating. |
| 2. | The method according to claim 1 including the step of forming a plasma within said coating apparatus adjacent and impressing a negative potential on said metal substrate effective to ion plate said metal coating. |
| 3. | The method according to claim 2 wherein said reactive element is selected from the group consisting of yttrium, scandium, hafnium, cerium, calcium, lanthanum, silicon, zirconium, thorium, samarium and rhenium. |
| 4. | The method according to claim 3 wherein the metal substrate is a metal selected from the group consisting of iron, nickel and cobalt based alloys. |
| 5. | The method according to claim 2 wherein said metal coating is produced by ion plating of thermally evaporated metal from a source comprising aluminum and at least one of said reactive elements. |
| 6. | The method according to claim 5 wherein said reactive elements comprise from about 0.01% to about 20% by weight of said metal coating. |
| 7. | The method according to claim 6 wherein said reactive elements comprise from about 1% to about 10% by weight of said metal coating. |
| 8. | The method according to claim 2 wherein said metal coating is produced by codepositing aluminum from a first source and said at least one of said reactive element emitted from a second source. |
| 9. | The method according to claim 8 wherein said reactive elements comprise from about 0.01% to about 20% by weight of said metal coating. |
| 10. | The method according to claim 4 wherein the effective temperature range is from about 500 °C to about 1200°C. |
| 11. | The method according to claim 4 wherein the effective period of time is from about 1 hour to about 24 hours. |
| 12. | The method according to claim 4 wherein the metal coating is deposited to an effective to produce a reactive element modifiedaluminide diffusion coating with a thickness in the range of about 20 micrometers to about 150 micrometers. |
| 13. | The method according to claim 12 wherein the reactive element is yttrium in the range from about 1% to about 10% by weight. |
| 14. | A method of producing a modifiedaluminide diffusion coating on a metal substrate, the method comprising the steps of: providing a metal substrate having a composition such that a modifiedaluminide diffusion coating can be formed thereon and fixing the metal substrate within a coating apparatus; depositing a metal coating on said metal substrate by a physical vapor deposition process to form a coated metal substrate, said metal coating including at least two elements, one element being aluminum and the other element being at least one element selected from the group consisting of ruthenium, rhodium, osmium, palladium, niobium, platinum and iridium; and heat treating said coated metal substrate in an inert gas or a reducing gas atmosphere within an effective temperature range and for an effective period of time to cause interdiffusion between elements of the metal substrate and the deposited metal coating. |
| 15. | The method according to claim 14 including the step of forming a plasma adjacent to said metal substrate and impressing a negative potential on said metal substrate effective to ion plate said metal coating. |
| 16. | A method of producing a reactive element modifiedaluminide diffusion coating on a metal substrate, the method comprising the steps of: providing a metal substrate having a composition such that a modifiedaluminide diffusion coating can be formed thereon and fixing the metal substrate within a coating apparatus, depositing a metal coating on said metal substrate by ion plating to form a coated metal substrate, said metal coating including aluminum and from about 0.01% to about 20% by weight of yttrium; and heat treating said coated metal substrate in an inert gas or reductive gas atmosphere within an effective temperature range and for an effective period of time to promote interdiffusion between elements of the metal substrate and the deposited metal coating. |
| 17. | The method according to claim 16 wherein the metal substrate is a metal selected from the group consisting of iron, nickel, and cobalt based alloys. |
| 18. | The method according to claim 17 wherein said metal coating is produced by ion plating of thermally evaporated metal from a source comprising aluminum and yttrium. |
| 19. | The method according to claim 16 wherein said reactive elements comprise from about 1 % to about 10% by weight of said metal coating. |
| 20. | The method according to claim 17 wherein said metal coating is produced by codepositing aluminum from a first source and said at least one of said reactive element emitted from a second source. |
| 21. | The method according to claim 19 wherein the effective temperature range is from about 500 °C to about 1200°C. |
| 22. | The method according to claim 21 wherein the effective period of time is from about 1 hour to about 24 hours. |
| 23. | The method according to claim 22 wherein the metal coating is deposited to an effective thickness to produce a reactive element modifiedaluminide diffusion coating in the range of from about 20 micrometers to about 150 micrometers. |
| 24. | A coated product prepared in accordance with the process of claim 23 wherein the metal coating comprises about 1.5% by weight of yttrium, said reactive element modifiedaluminide diffusion coating being characterized by high resistance to cyclic oxidation and high corrosion resistance in molten carbonate at 650°C. |
| 25. | A coated product comprising a metal or alloy substrate on which a modifiedaluminide diffusion coating can be formed thereon having a mixed metal coating of yttriumaluminum deposited thereon by ion vapor plating, the yttrium being present in the range of about 0.01% by weight to about 20% by weight. |
| 26. | The method according to claim 6 wherein the reactive elements are yttrium and silicon. |
| 27. | The method according to claim 6 wherein the reactive elements are yttrium and hafnium. |
| 28. | The method according to claim 6 wherein the reactive elements are silicon and hafnium. |
DIFFUSION COATINGS
FIELD OF THE INVENTION
The present invention relates to a method of producing modified - aluminide diffusion coatings on metal substrates.
BACKGROUND OF THE INVENTION In many industries the need for protective coatings for metal parts is crucial to maintain the mechanical integrity of the part due to constant exposure of the metal to high temperatures, thermal cycling, hot corrosive and/or oxidizing environments. The aerospace industry is one example where accelerated aging in engine components occurs due to engine parts being cycled between temperatures in excess of 1000°C during operation and ambient temperature when not in use. The high temperatures and corrosive environments characteristic of operating gas turbines results in rapid corrosion and/or oxidation rates of unprotected metal parts and ultimately failure of the component and the turbine. Similarly, metallic materials used in moderate to high temperature corrosive environments such as coal gasification systems, furnace fixtures, heating elements, heat exchangers, components for automotive and fossil energy applications as well as for nuclear reactors, chemical processing equipment and molten carbonate fuel cells are prone to rapid degradation and failure if unprotected. Two methods of forming protective coatings on metal components for these types of applications are well known. In the first method a mixed metal overlay coating is deposited onto a metal substrate. The mixed metal deposited on the substrate is from the family of coatings generally referred to as MCrAIY overlay coatings, where M comprises cobalt, iron or nickel (or mixtures thereof), Al is aluminum, Cr is chromium and Y is yttrium. These coatings are applied to the metal substrate and act as protective coatings in and of themselves as no significant diffusion coating per se is formed between the substrate and MCrAIY overlay coat. Interdiffusion is avoided if possible
because aluminum diffusion into the substrate may be detrimental as it can lead to spading of the overlay coating.
Methods currently used to deposit MCrAIY overlay coatings include electron beam vapor deposition, plasma spraying and other physical vapor deposition techniques. These are line of sight techniques so that components with complex geometries can be only partially coated or otherwise require a complex set-up for rotation of the component during coating. For example, United States Patent No. 4,910,092 issued to Olsen discloses deposition of MCrAIY coatings by plasma spraying. The MCrAIY coatings themselves are very expensive as are most of the deposition techniques.
The second method of forming a protective coating on a metal substrate involves alteration of the substrate outer layer by interaction with selected elements resulting in the formation of so-called diffusion coatings. The most widely used diffusion coatings are aluminides, which include several intermetallic phases comprising aluminum and other elements from the substrate. This family of coatings can be applied by two groups of methods. The first group includes a process during which the deposition and diffusion steps occur concurrently and include pack cementation, high temperature chemical vapour deposition (CVD) and hot dipping. The second group of methods for growing the diffusion coatings comprise two stages; an initial deposition of an aluminum coating by slurry-fusion, low temperature CVD, ion vapour deposition (IVD), evaporation, electrodeposition or electrophoresis and a subsequent heat treatment to induce diffusion.
Of all the above-mentioned methods of producing diffusion coatings, only pack cementation, high temperature CVD and hot dipping are employed for industrial scale production. In the pack cementation process, the part to be treated is packed in a bed containing a source of aluminum and an activator, and then heated to between 700-1100 °C for several hours, whereby aluminum is transported to the metal substrate and diffuses into the surface thereof.
A disadvantage of the aluminide diffusion coatings is that the mechanical properties, thickness and uniformity of the coating are a strong
function of the chemical reactions and/or solid solution formation occurring between the elements of the substrate and aluminum coating, which can vary significantly depending on the metal or metal alloy comprising the substrate. Aluminide diffusion coatings are also prone to fracturing or cracking in the thickness range generally considered optimum for oxidation/corrosion resistance due to brittleness of the coating. Because the aluminide diffusion coating is an integral part of the substrate surface, these flaws will generally propagate from the diffusion coating into the bulk of the substrate thereby resulting in failure. Thinner aluminide diffusion coatings may exhibit increased ductility and hence increased fracture resistance, but this is at the expense of reduced oxidation resistance of the coatings. These types of problems are not as significant with MCrAIY coatings because interdiffusion between the overlay coating with the substrate material is a second order effect which does not generally impact on the protective function of the overlay coat. Platinum-aluminide diffusion coatings are currently available as disclosed in United Kingdom Patent No. 1 ,210,026 and United States Patent No. 4,501 ,776. This type of coating is produced by a process involving first electroplating platinum onto the metal substrate, followed by heat treatment to induce interdiffusion, followed by a pack cementation process or high temperature CVD to produce the corresponding aluminide. A significant improvement in the performance of diffusion aluminides was observed when platinum was incorporated into the aluminide coating. However, as much as 10 μm of platinum must be electroplated so that the coatings produced in this way are quite expensive. In addition, the waste solutions resulting from the electroplating step represents a significant environmental hazard.
Chemical vapour deposition (CVD) has also been used for producing enhanced aluminide diffusion coatings on more complex geometries using forced flow of metal species. The protective properties of aluminide diffusion coatings can be enhanced by addition of, for example, platinum or yttrium, and such metal species are present initially in the form of gaseous organometallic precursors which chemically decompose to the elemental metal. United States Patent No. 5,292,594 discloses the use of CVD to deposit such
metals from their organometallic precursors followed by deposition of an aluminum coating and then heat treatment.
As mentioned above, a key component in both MCrAIY overlay coats or some modified aluminide coatings is the presence of a reactive element. The most common reactive elements for the modified aluminide diffusion coatings are yttrium (Y), cerium (Ce) and hafnium (Hf), with yttrium also being used in the MCrAIY overlay coatings. United States Patent Nos. 4,835,011 , 5,000,782 and 4,910,092 issued to Olsen et al. disclose aluminizing MCrAIY coatings containing silicon to form protective aluminide coatings. The presence of the reactive elements is essential to providing enhanced aluminum oxide scale adherence under cyclic oxidation conditions and while the mechanism of enhancement is not fully understood, it is believed to be due to a combination of several factors including mitigation of transient oxidation; improved scale plasticity; and modification of scale structure and moφhology.
As discussed above, these reactive elements may be incorporated into aluminide coatings using CVD wherein the reactive metal is introduced as an organometallic precursor.
In view of the expense of producing MCrAIY overlay coatings and the limitations associated with depositing coatings onto substrates having complex geometries, there has been a real need for a method of producing aluminide diffusion coatings on metal substrates containing a reactive element which avoids the limitations found in known aluminide diffusion coatings.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of producing reactive element modified-aluminide diffusion coatings on metal substrates which is simpler and more economical as compared to prior art methods and provides equivalent or superior oxidation/corrosion protection. It is also an object of the present invention to provide a method of producing reactive element modified-aluminide diffusion coatings on metal substrates which exhibit oxidation and/or corrosion resistance at elevated
temperatures superior to aluminide diffusion coatings produced using prior art methods.
In accordance with these objects, the present invention provides a method of producing a reactive element modified-aluminide diffusion coating on a metal substrate. The method comprises the steps of providing a metal substrate having a composition such that a reactive element modified-aluminide diffusion coating can be formed thereon and fixing the metal substrate within a coating apparatus. A metal coating is deposited on the metal substrate by a physical vapor deposition process to form a coated substrate. The metal coating includes at least two elements. One element is aluminum and the other element is at least one reactive element. The method comprises heat treating the coated metal substrate in an inert gas or a reducing gas atmosphere within an effective temperature range for an effective period of time to cause interdiffusion between components of the metal substrate and the deposited metal coating.
In this aspect of the invention the physical vapour deposition process is an ion vapour plating process whereby a negative potential is impressed on the metal substrate and a plasma is formed in the vicinity of the substrate. The reactive element is selected from the group consisting of yttrium, scandium, calcium, hafnium, cerium, lanthanum, silicon, zirconium, thorium, samarium and rhenium.
In another aspect of the invention there is provided a method of producing modified-aluminide diffusion coatings on a metal substrate. The method comprises the steps of providing a metal substrate having a composition such that a modified-aluminide diffusion coating can be formed thereon and fixing the metal substrate within a coating apparatus. A metal coating is deposited on the metal substrate by a physical vapor deposition process to form a coated metal substrate. The metal coating includes at least two elements. One element is aluminum and the other element is at least one element selected from the group consisting of ruthenium, rhodium, osmium, palladium, niobium, platinum and iridium. The method comprises heat treating the coated metal substrate in an inert gas or a reducing gas atmosphere within
an effective temperature range and for an effective period of time to cause interdiffusion between elements of the metal substrate and the deposited metal coating.
The present invention provides a method of producing a reactive element modified-aluminide diffusion coating on a metal substrate. The method comprises the steps of providing a metal substrate having a composition such that a modified-aluminide diffusion coating can be formed thereon and fixing the metal substrate within a coating apparatus. A metal coating on the metal substrate by a physical vapor deposition process to form a coated metal substrate. The metal coating includes aluminum and from about 0.01 % to about
20% by weight of yttrium. The method comprises heat treating the coated metal substrate in an inert gas atmosphere within an effective temperature range and for an effective period of time to promote interdiffusion between elements of the metal substrate and the deposited metal coating. In another aspect of the invention, a coated product comprising a metal or alloy substrate on which a modified-aluminide diffusion coating can be formed thereon having a mixed metal coating of yttrium-aluminum deposited thereon, the yttrium being present in the range of about 0.01% by weight to about 20% by weight.
BRIEF DESCRIPTION OF THE DRAWINGS The method of producing modified-aluminide diffusion coatings on metal substrates according to the present invention will now be described, by way of example only, with reference being made to the accompanying drawings, in which:
Figure 1 is a diagrammatic representation of an ion plating process used in the method of the present invention for producing modified- aluminide diffusion coatings on metal substrates;
Figure 2 shows cross sectional optical micrographs of samples after 200 hours of cyclic oxidation testing (50 minutes at 1050°C, followed by
10 minutes at room temperature per cycle) for, (a) an uncoated IN738 substrate; (b) an IN738 substrate having an aluminide coating formed thereon
by depositing 25 to 28 μm aluminum by ion plating followed by heat treatment for 1 hour at 700°C and 3 hours at 1080°C; and (c) an IN738 substrate having a 1.5% yttrium-aluminide diffusion coating of the same initial thickness as in Figure 2(b) deposited by ion plating on IN738 and followed by heat treatment for 1 hour at 700°C and 3 hours at 1080°C in accordance with the present invention; and
Figure 3 shows a cross sectional optical micrograph of a 304 stainless steel sample coated with a yttrium-aluminide diffusion coating formed by depositing 20 μm of 1.5% yttrium-aluminum and heat treating for 5 hours at 850°C according to the present invention, the micrograph was taken after 500 hours of immersion in a 62 mole% Li 2 C0 3 -38 mole % K 2 C0 3 mixture at 650°C.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to the industrial scale production of modified-aluminide diffusion coatings by physical vapour deposition of a metal coating comprising aluminum and another element. Aluminum and the other element are concurrently or simultaneously deposited so that the metal coating is a mixed metal coating referred to as a "modified-aluminum coating". The coated substrate is then heat treated to produce a "modified-aluminide diffusion coating". As will be more fully described hereinafter, in the term "modified- aluminide diffusion coating" as used herein "modified" refers to modification of standard aluminide coatings to include an element, the presence of which ultimately results in an enhancement, over straight aluminide diffusion coatings, of the protective qualities of the aluminide coating under corrosive and/or oxidative environments. The substrates are metals such as iron, nickel and cobalt based alloys.
As used herein, the phrase "reactive element modified-aluminum metal coating" means a metal coating comprising aluminum and at least one suitable reactive element and possibly more. As used herein the phrase "reactive element modified-aluminide diffusion coating" refers to an aluminide diffusion coating which is formed when the reactive element modified-aluminum metal coating is heat treated to sufficiently a high temperature to induce
interdiffusion between the metal coating and the substrate. It is to be understood that the diffusion coating produced in this way will contain the reactive element, aluminum and elements from the substrate. Reactive element modified-aluminide diffusion coatings formed in this way provide enhanced protection in corrosive and high temperature environments such as encountered in ground-and aero-based gas turbines, chemical and fossil energy industries, molten carbonate fuel cells and the like.
The formation of the reactive element modified-aluminide diffusion coatings forming the subject invention is a two step tandem process, involving first the deposition of a reactive element modified-aluminum metal coating onto a substrate, followed by a heat treatment step within an effective temperature range and for an effective period of time to induce interdiffusion. The deposition process may either be the deposition of a reactive element modified-aluminum coating from a single source comprising both the reactive element and aluminum, or alternatively the two elements of the coating may be deposited concurrently onto the substrate from separate sources. Thus, the first step comprises the codeposition of aluminum and at least one reactive element. As used herein the term "reactive element" is meant to cover those reactive elements known to those skilled in the art comprising yttrium, scandium, hafnium, cerium, lanthanum, silicon, zirconium, rhenium, calcium, samarium and thorium.
The reactive element modified-aluminum metal coating is deposited in an inert gas environment. When the metal coating is deposited from a single source, the reactive element concentration in the reactive element-aluminum metal source is chosen so that the deposited metal coating contains the reactive element preferably in the range of from about 0.01 to about 20% by weight, with the balance being aluminum. A more preferred range is from about 1 to about 10% by weight of reactive element, with the balance being aluminum. Yttrium is the most preferred reactive element when a single reactive element is present in the reactive element-aluminum coating, while other preferred reactive element-aluminum combinations include hafnium- aluminum and silicon-aluminum.
The reactive element modified-aluminum metal coating deposited prior to heat treatment may contain more than one reactive element. For example, tertiary coatings may also be produced so long as the aluminum and reactive elements are uniformly distributed through the metal mixture when a single source is used. Preferred mixtures include yttrium-hafnium-aluminum, yttrium-silicon-aluminum and hafnium-silicon-aluminum. The total sum of the reactive elements is in the range from about 0.01 to about 20% by weight and preferably from about 1 to about 10% by weight.
If individual sources are used for each element, the deposition parameters for each element are chosen to ensure the resulting reactive element-aluminum coating contains the reactive element in the range from 0.01 to 20%. The source(s) may be wires, ingots, powders, targets and the like.
The preferred process for depositing the reactive element modified-aluminum metal coatings on the substrate includes the use of ion plating. A review of the theory and operation of ion plating is found in Nadir
A.G. Ahmed, Ion Plating: Optimum Surface Performance And Material Conservation, Thin Solid Films, 241, 179-187, (1994). This method of deposition will be known to those skilled in the art. will Several advantages of ion plating are 1) because the entire substrate is biased negatively, the positively charged metal species are attracted to the entire exposed surface so that irregularly shaped substrates can be provided with a relatively uniform coating; and 2) because the substrate is negatively biased, it is constantly bombarded by positively charged species which provides enhanced adhesion and control of the density of the deposited coatings. The basic ion plating system comprises a steel vacuum chamber, a pumping system, a parts holder or rack for fixturing the substrate in the vacuum chamber, a high voltage power supply which can be used to apply a negative potential to the parts holder or rack, and an evaporation source. Referring to Figure 1 , the ion plating technique is carried out in a plating system shown generally at 10 comprising a vacuum chamber 12 with the metal substrate or workpiece 14 to be coated fixtured to a rack 16 located above a resistively heated evaporation crucible 18 containing a reactive element
modified-aluminum source (not shown). Substrate 14 is biased with a negative potential by an external power supply 20. A gas inlet 22 supplies vacuum chamber 12 with inert or reductive gas(es). There are several commercial plating systems available and a McDonnel Douglas Ivadizer™ was used to deposit the coating in the examples described hereinafter.
While evaporation of the metallic sources using resistively heated crucibles 18 was used in conjunction with ion plating employed to produce the coatings in the EXAMPLES 1 and 2 described below, it will be understood by those skilled in the art that there are other alternative techniques or devices which could be used to produce the metal vapors to be deposited on the substrate. Some examples are flash evaporators, electron beam guns, R.F. inductively heated sources, arc evaporators, ion beam evaporators and the like.
In operation, the metal substrate 14 on which the reactive element-aluminide diffusion coating is formed may be cleaned by sand blasting, followed by vapour degreasing in organic solvents such as trichloroethylene, isopropanol and other suitable solvents. The reverse of this cleaning process may also be used. Substrate 14 is then fixed in vacuum chamber 12 by being attached to the electrically conductive rack 16 so that electrical contact is made between the substrate and rack. Vacuum chamber 12 is pumped down to the desired pressure and the inert or reductive is gas admitted to the vacuum chamber. When a single source is used, the reactive element modified- aluminum metal source material is thermally evaporated from crucibles 18 into a plasma 30 formed between the grounded crucibles and the negatively biased rack 16. A fraction of the thermally evaporated neutral atomic constituents 32 of the source material is ionized and the resulting positively charged species
34 are accelerated toward metal substrate 14 and plated thereon.
The sources used in the examples described hereinafter comprised wires produced using the Ohno continuous casting process. Details of this process are disclosed in A. Ohno, "Continuous Casting Of Single Crystal Ingots"; J. Metals, Vol. 38, 14 (1986). The wires must have a fairly uniform composition along their length and diameter. Other methods of production of the sources may include for example powder metallurgical techniques, casting
or any other conventional wire manufacturing process, carried out under an inert or reductive gas atmosphere to avoid oxidation of the reactive element. A relatively uniform distribution of the reactive element throughout the aluminum/reactive element source material is required to ensure the resulting coatings exhibit increased corrosion and or oxidation resistance.
After the reactive element modified-aluminum coating has been deposited the coated substrate is heat-treated under an inert gas atmosphere to form a reactive element modified-aluminide diffusion coating containing aluminum, the corresponding reactive element(s) and other elements from the substrate with a thin protective top layer of Al 2 0 3 .
Heat treatment of the coated samples is carried out under an inert or reductive atmospheric, vacuum or sub-atmospheric pressures in the presence of either inert or reductive gases to avoid oxidation during interdiffusion and also to reduce losses of aluminum by evaporation. The heat treatment temperatures must are selected to be effective to induce interdiffusion with the substrate and to form the protective intermetallic phases.
The deposition and heat treatment parameters are chosen to promote formation of about 20 μm to about 150 μm thick diffusion coating. The heat treatment comprises subjecting the coated substrates to one or more temperatures for prolonged periods of time, and usually within the temperature range of from about 500-1200°C for about 1-24 hours. The temperature and duration of the heat treatment used depends on the substrate composition and the end application for the substrate. Those skilled in the art will appreciate that because the formation of diffusion coatings is caused by interdiffusion between the deposited metal coating and the substrate, the present invention is directed to producing reactive element-aluminide coatings on those metals and alloys comprising elements which are capable of reacting to form suitable aluminides on which a coherent, protective AI 2 O 3 scale can be formed, and provided that the selected metals or alloys are stable at the processing temperatures employed throughout the coating process.
The method of the present invention has been described with particular emphasis on the aforementioned reactive elements. The inventors
contemplate that the present method is also applicable for producing modified- aluminide coatings when other elements are incorporated that increase the oxidation and/or corrosion resistance of the aluminide coatings, even though such elements are not recognized as falling in the category of "reactive elements". Such elements include ruthenium, rhodium, iridium, osmium, palladium, niobium and platinum. As mentioned above, the platinum-aluminide coatings that have been produced using electrodeposition provide good protection but are expensive to produce. As discussed above, aluminide coatings incorporating reactive elements, or those different elements which advantageously modify aluminide coatings, are more generally referred to as
"modified-aluminide coatings", of which "reactive element modified-aluminide coatings" are a subset.
While evaporation of the metals combined with ion plating deposition has been disclosed herein as being the preferred process for depositing the reactive element modified-aluminum coatings, it will be appreciated by those skilled in the art that other combinations could be used to deposit the coatings. For example, where line of sight deposition is sufficient, ion plating need not be used and other physical vapour deposition methods may be used including electron beam evaporation, atomic sputtering and plasma spraying to mention a few. It is to be noted, however, that in those applications where it is desirable to coat most of the substrate, ion plating is preferred because it is not inherently limited to line-of-sight deposition of the metal coating.
The following non-limiting examples and the optical micrograph photocopies of Figures 2 and 3 will further illustrate the present method and representative coating formed using the method disclosed herein.
EXAMPLE I
A yttrium-aluminum wire containing 1.5% by weight yttrium and having a diameter of 1.6 mm was produced using the Ohno method. The yttrium was uniformly distributed throughout the aluminum. The wire was fed to three resistively heated crucibles. IN738 coupons were employed as
substrates. The yttrium-aluminum metal wire was thermally evaporated and deposited by ion plating onto the IN738 substrates to a thickness of about 25 to 35 μm. The wires were fed at a rate of 55 cm/min and a current of 550 amperes was applied to the crucibles to effect evaporation. After initial evacuation, the chamber was back-filled with argon and a 250 volt, 0.1 ampere bias was applied to the metal substrates being coated. The deposition time was 20 minutes and the pressure in the chamber during deposition was 1.4x10 '2 torr. The coated substrates were then heat treated in argon for 1 hour at 700°C, followed by 3 hours at 1080°C under argon at a pressure slightly higher than atmospheric pressure for both temperatures. The resulting substrate with the yttrium modified-aluminide diffusion coating was tested for resistance to cyclic oxidation, where each cycle corresponded to 10-15 minutes at room temperature and 45-50 minutes at 1050°C. The coated substrates were subjected to 800 cycles and showed no signs of substrate penetration. Figure 2 shows cross sectional optical micrographs of three different samples after 200 hours of cyclic oxidation testing (50 minutes at 1050°C, followed by 10 minutes at room temperature) for, (a) an uncoated IN738 substrate; (b) an aluminide coating (absent a reactive element) deposited on IN738 by ion plating followed by heat treatment for 1 hour and (c) the yttrium modified-aluminide diffusion coating of EXAMPLE I. Degradation of the uncoated substrate is clearly visible while the yttrium modified-aluminide coating of Figure 2(c) does not show any significant degradation.
EXAMPLE II Coupons of 304 stainless steel were coated with 20-22 μm of yttrium-aluminum following the same procedure as described in EXAMPLE I. The samples were heat treated under argon for 5 hours at 850°C to produce a yttrium modified-aluminide coating. The corrosion kinetics were determined using electrochemical methods while immersed in a mixture of 62 mole % Li 2 CO 3 -38 mole % I^CO j at 650°C under an inert atmosphere. To measure the instantaneous corrosion rates, a potentiometer interconnected with working, counter and reference electrodes was employed with the working electrode
being the coated coupon. Continuous current methods were used to determine the corrosion rate using interception and polarization resistance (PR) techniques. The weight loss as a function of time and the corrosion kinetics were determined using Faraday's law. From the kinetic results it was estimated that the total weight loss for a coated sample was approximately 0.1mg/mm 2 after 20,000 hours, whereas an uncoated sample exhibited a weight loss of about 2.5 mg/mm 2 .
Referring to the optical micrograph of Figure 3 it is shown that there were no signs of degradation or penetration after 500 hours immersion in molten carbonate at 650°C.
As previously discussed, in view of the fact that the reactive element modified-aluminide diffusion coating is formed by interdiffusion between the deposited metal coating and the metal substrate, it will be appreciated that the present method may be used with metals or metal alloy substrates comprising elements which are capable of reacting to form suitable aluminide coatings, on which relatively uniform and contiguous aluminum oxide layers or scales can form. Suitable metal or metal alloy substrates include, for example, alloys typically used in the aerospace industry which are predominantly iron, nickel, and cobalt based superalloys. The method for producing reactive element modified-aluminide diffusion coatings disclosed herein is highly advantageous in comparison to the known methods for producing the same general type of coatings. The present invention provides for the first time a straightforward and economical process for the industrial scale production of reactive element modified-aluminide coatings. The present process does not produce waste by-products as for example pack cementation or electroplating, and so it is not problematic with respect to the environment. Further, the coatings produced by the present method disclosed herein exhibit performance equivalent to or better than platinum-aluminide coatings, but at a much lower cost. While the method of producing reactive element modified- aluminide diffusion coatings on metal substrates according to the present invention has been described and illustrated with respect to the preferred and
alternative embodiments, those skilled in the art will appreciate that variations of this method may be made without departing from the scope of the invention disclosed herein.
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