PLATINUM ALUMINIDE CVD COATING METHOD

Abstract: A method of improving the oxidation resistance of a platinum modified aluminide coating formed on a nickel base superalloy substrate, comprises providing a platinum layer on the substrate, heating the substrate in a coating retort to a temperature of at least about 1000 DEG C, forming external of the coating retort a high purity coating gas comprising hydrogen and aluminum trichloride, and introducing the coating gas into the coating retort to contact the heated substrate to form an outwardly grown, single phase [(Ni,Pt)Al] platinum modified aluminide coating having reduced concentration of at least one of a substrate substitutional alloying element and a surface active tramp element selected from at least one of B, P, and S in said coating as compared to a platinum modified aluminide coating formed at a lower temperature.

Description: PLATINUM ALUMINIDE CVD COATING METHOD FIELD OF THE INVENTION The present invention relates to an outwardly grown, single phase (Ni,Pt)Al platinum modified aluminide coating produced by chemical vapor deposition (CVD) under selected conditions effective to substantially reduce the concentration of substrate substitutional alloying elements, such as W, Mo, etc., and surface active tramp elements, such as B, P, and S, in the coating and thereby improve coating high temperature oxidation resistance. The outward growth coating method can be used in the repair of engine run components, such as turbine blades. BACKGROUND OF THE INVENTION At temperatures greater than about 1000°C (1832°F) , high temperature oxidation is the most important form of environmental attack observed with aluminide diffusion coatings. High temperature oxidation is a chemical reaction whose rate controlling process for an aluminide coating is the diffusion of aluminum through a product (oxide) layer. Diffusion is a thermally activated process, and consequently, the diffusion coefficients are exponential functions of temperature. Since the oxidation of aluminide coatings is a diffusion controlled reaction and diffusion coefficients are exponential functions of temperature, the oxidation rate is also an exponential function of temperature. At low temperatures where diffusion coefficients are relatively small, the growth rate of a protective scale on any aluminide coating is also small. Thus, adequate oxidation resistance should be provided by any state of the art aluminide coatings, such as: chromium aluminide, aluminide or two phase [PtAl 2 + (Ni,Pt)Al] platinum aluminide, all inward grown coatings made by pack cementation. However, at high temperatures where the diffusion coefficients and consequently the oxidation rate increase rapidly with increasin temperature, only coatings which form high purity alumina (A1 2 0 3 ) scales are likely to provide adequate resistance to environmental degradation. This point is clearly illustrated by the cyclic oxidation test results from three state of the art aluminide coatings (designated MDC-51, MDC-351 and LDC-2E by the Assignee, Howmet Corporation) on IN-100 substrates provided in Figure 1. The observed variations in life of coatings with the same initial thickness is the result of differences in the growth rate and/or adherence of the protective oxide scale. Specifically, the dissolution of substitutional substrate alloying elements into the aluminide coating and consequently the alumina scale can result in doping effects which can produce significant increases in the growth rate of the oxide scale. In addition, the presence of surface active tramp substrate impurities (S, P etc.) in the aluminide coating can have a detrimental effect on the adherence of the protective oxide scale. Thus, the inward grown chromium modified and simple aluminides (which contain all the elements in the substrate) exhibit poor resistance to high temperature oxidation. The presence of platinum in nickel aluminide has been concluded to provide a number of thermodynamic and kinetic effects which promote the formation of a slow growing, high purity protective alumina scale. Consequently, the high temperature oxidation resistance of LDC-2E coating is about five times better than the other coatings, see Figure 1, in spite of the fact it is an inward grown coating. In recent years, several limitations of the industry standard, two phase [PtAl 2 + (Ni,Pt)Al], inward grown platinum aluminide coatings have been identified. First, the two phase coatings have metastable phase assemblages and thicknesses, as demonstrated in engine tests at both General Electric and Rolls-Royce. Second, the two phase coatings are sensitive to TMF (thermal mechanical fatigue) cracking in engine service, and the growth of these coatings in service only makes this problem worse. Third, the thick, inward grown platinum aluminides exhibit rumpling during both cyclic oxidation and engine testing. This phenomenon can have undesirable consequences when platinum aluminide coatings are used as the bond coat in thermal barrier coating systems. Fourth, the two phase platinum aluminide coatings are hard and brittle, and this can result in chipping problems during post coat handling and assembly operations. Many of the problems encountered with the previous industry standard platinum aluminides can be attributed to the two phase, inward grown structure and can be overcome by using outwardly grown, single phase platinum aluminde coatings as described, for example, in the Conner et al. technical articles entitled "Evaluation of Simple Aluminide and Platinum Modified Aluminide Coatings on High Pressure Turbine Blades after Factory Engine testing", Proc. AMSE Int. Conf. of Gas Turbines and Aero Engine Congress June 3-6, 1991 and June 1-4, 1992. For example, the outwardly grown, single phase coating microstructure on directionally solidified (DS) Hf-bearing nickel base superalloy substrates was relatively unchanged after factory engine service in contrast to the microstructure of the previous industry standard two phase coatings. Further, the growth of a CVD single phase platinum aluminide coating was relatively insignificant compared to two phase coatings during factory engine service. Moreover, the "high temperature low activity" outward grown platinum aluminde coatings were observed to be more ductile than inward grown "low temperature high activity" platinum aluminide coatings. These technical articles indicate only that the outwardly grown, single phase platinum aluminide coatings involved in the factory engine testing program were made by CVD aluminizing the substrate with a prediffused, electroplated platinum layer thereon. SUMMARY OF THE INVENTION The present invention provides a method of improving the oxidation resistance of a platinum modified aluminide coating on a nickel base superalloy substrate wherein the platinum modified aluminide coating is produced under CVD conditions effective to form an outwardly grown, single phase coating and concurrently reduce the concentration of one or more substrate substitutional alloying elements, such as , Mo, etc., and/or one or more surface active tramp elements, such as B, P, and S, in the coating to improve coating high temperature oxidation resistance. The present invention relates to the discovery that certain CVD conditions associated with making outwardly grown, single phase platinum modified aluminide coatings on nickel base superalloy substrates achieve a purification effect on the as-formed coating from the standpoint of substantially reducing the concentration of one or more substrate substitutional alloying elements, such as W, Mo, etc., and/or one or more surface active tramp elements, such as B, P, and S, in the coating as compared to a platinum modified aluminide coating formed at a lower temperature. As noted hereabove, the substitutional impurities (particularly substrate refractory alloying elements) can produce doping effects which result in significant increases in the growth rate of the protective scale, and the surface active tramp elements can adversely effect the adherence of the protective scale. Reductions of these elements in the coating improve coating high temperature oxidation resistance. In particular, the present invention results from the discovery that exposure of a nickel base superalloy substrate at an elevated substrate coating temperature of at least about 1832°F (1000°C) in a coating retort to a high purity coating gas comprising a mixture of hydrogen and aluminum trichloride gases results in a significant decrease in the concentrations of substrate substitutional alloying elements and the surface active tramp elements that could be present in the platinum modified nickel aluminide additive layer. In generating the coating gas mixture of hydrogen and aluminum trichloride, a relatively low temperature coating gas generator residing external of the high temperature coating retort is used in practice of the invention and involves passing high purity hydrogen chloride and hydrogen carrier gases over a high purity source of aluminum at a relatively low temperature, such as not exceeding about 600°F (316°C) . Preferably, in practicing the invention, a high purity (at least 99.999 weight % Al) source of aluminum is employed in the coating gas generator. High purity hydrogen typically comprises less than 30 ppb (parts per billion) impurities and high purity hydrogen chloride typically comprises less than 25 ppm (parts per million) impurities. The present invention also involves repair of an engine-run component by overcoating with an outwardly grown Pt bearing aluminide coating of generally uniform thickness, as well as the repaired component, wherein the engine-run component has an existing aluminide coating at a region and an uncoated, fresh metallic repair material at another region. A layer of Pt is deposited on the regions and the Pt coated regions are aluminized under chemical vapor deposition conditions to outwardly grow a generally uniform thickness Pt bearing aluminide coating at the regions. The region having the existing aluminide coating typically is an airfoil region of a turbine blade and the repair material is at a tip region of the turbine blade. DESCRIPTION OF THE DRAWINGS Figure 1 is a graph depicting comparative cyclic oxidation resistance of various conventional aluminide coatings on IN 100 nickel base superalloy substrates. Figure 2A is a photomicrograph of IN-738 nickel base superalloy foil CVD coated pursuant to an embodiment of the invention. Figure 2B is a graph of the B, P, S, and W concentrations in the thickness direction of the coating shown in Figure 2A. Figures 3A and 3B are photomicrographs of an inwardly grown, two phase platinum modified aluminide coating designated LDC-2E as coated on IN-738 nickel base superalloy substrate (Fig. 3A) and as coated and solution heat treated (Fig. 3B) . Figures 4A and 4B are photomicrographs of an inwardly grown, two phase platinum modified aluminide coating designated MDC-150 as coated on IN-738 nickel base superalloy substrate (Fig. 4A) and as coated and solution heat treated (Fig. 4B) . Figures 5A and 5B are photomicrographs of an outwardly grown, single phase platinum modified aluminide coating of the invention designated MDC-150L as coated on IN-738 nickel base superalloy substrate (Fig. 5A) and as coated and solution heat treated (Fig. 5B) . Figure 6 is a bar graph of the 1100°C cyclic oxidation resistance of the as coated and heat treated IN 738 substrates of Figures 3B, 4B, and 5B. Figure 7A is a backscatter electron image of a repaired coated blade tip region of an engine-run turbine blade pursuant to another embodiment of the invention and Figures 7B through 7D are X-ray maps for Al, Ni, and Pt of the coating on the repaired tip region. Figures 8A is a backscatter electron image of a repaired overcoated transition region of an engine run turbine blade between the blade tip and airfoil regions pursuant to another embodiment of the invention and Figures 8B through 8D are X-ray maps for Al, Ni, and Pt of the overcoating on the repaired transition region. DETAILED DESCRIPTION The purification effects of the aforementioned CVD process for aluminizing were discovered in a series of coating experiments with 3 mil thick nickel foil substrates. The foil substrates were CVD coated in a manner to form 1) a simple aluminide single phase coating, 2) - a hafnium modified outwardly grown, single phase aluminide coating and 3) a hafnium plus silicon modified CVD outwardly grown, two phase aluminide coating. The coating parameters for each coating formed are set forth herebelow: The simple aluminide coating 1) was formed at a substrate temperature of 1080°C using coating gas mixture comprising 9 volume % aluminum trichloride and 91 volume % hydrogen at a flow rate of 300 standard cubic feet per minute (scfm) . The coating gas mixture was generated by passing hydrogen and hydrogen chloride in mixture of hydrogen/13 volume % HCl over a 99.999 % pure source of aluminum at 290°C. The hafnium modified aluminide coating 2) was formed at a substrate temperature of 1080°C using coating gas mixture comprising 9 volume % aluminum trichloride, 89 volume % hydrogen and 2 volume % hafnium chloride at a flow rate of 300 scfm. The coating gas mixture was generated by passing hydrogen and hydrogen chloride in mixture of hydrogen/13 volume % HCl over a 99.999 % pure source of aluminum at 290°C. A mixture of Ar/11 volume % HCl was flowed in an external chloride generator through a 99.99 % pure hafnium bed at 430°C to form hafnium chloride. The mixtures were introduced concurrently to the coating retort. The hafnium plus silicon modified aluminide coating 3) was formed at a substrate temperature of 1080°C using coating gas mixture comprising 9 volume % aluminum trichloride plus silicon tetrachloride, 89 volume % hydrogen, and 2 volume % hafnium chloride at a flow rate of 300 scfm. The aluminum based coating gas mixture was generated by passing hydrogen and hydrogen chloride in mixture of hydrogen/13 volume % HCl over a 99.999 % pure source of aluminum and 99.9999 % pure Si in an external chloride cogenerator at 290°C. A mixture of Ar/ll volume % HCl was flowed in a second external chloride generator through a hafnium bed at 430°C to form hafnium chloride. The mixtures were introduced concurrently to the coating retort. Inductively Coupled Plasma spectroscopy (ICP) was used to chemically analyze the starting foil substrate and the foil substrate with the simple CVD aluminide coating 1) , the hafnium modified CVD aluminide coating 2) , and the hafnium plus silicon modified CVD aluminide coating 3) . The results are presented in Table 1, and indicate the CVD aluminizing process can have a significant impact upon the concentration of both the substitutional impurities (e.g. Cr and Fe) and the surface active impurity (e.g. S) in the aluminide coating. Based upon thermodynamic calculations, the removal of the substitutional impurities, Fe and Cr, involved chlorination of the metals an subsequent evaporation of the metal chlorides. The surface activ impurity, sulfur, was removed by reaction with the hydrogen carrie gas to form H 2 S gas. HCl involved in the purification was produce by hydrogen reduction of metal halides in the coating gas on th sample surface. Since the CVD process appeared to be capable of reducing th concentrations of both substrate (foil) substitutional impurit elements (which can cause doping effects) and surface activ impurities (which can affect scale adherence) , the next step in th evaluation of the purification effects of the CVD aluminizin process was to examine the aluminide coating compositional change produced in a nickel base superalloy substrate of the type commonl used in the manufacture of turbine blades and vanes for gas turbin engines during the growth of a platinum modified aluminide coating. In this series of experiments, IN-738 substrates (both foil 0.008 inch thick and 1/8 inch thick tabs suitable for oxidatio testing from the same alloy heat) were coated using two differen (a CVD and a pack cementation) aluminide coating processes fo comparison purposes. These Pt aluminide coating processe produced: a) a coating designated LDC-2E that is an inward grow coating made by pack cementation, b) a coating designated MDC-150 that is an inward growth coating made by CVD outside the inventio and c) a coating of the invention designated MDC-150L that is a outward grown coating made by CVD pursuant to the invention. The LDC-2E aluminide coating a) was formed by firs electroplating the substrate with a 9-11 milligrams/centimete squared platinum layer, diffusing the Pt by heat treatment, an pack aluminizing the electroplated/diffused substrate at substrate temperature of 1875°F (1074°C) using a pack compositio comprising 33% by weight Al alloy and balance alumina. The MDC-150 aluminide coating b) was formed by firs electroplating the substrate with a 9-11 milligrams/centimete squared platinum layer, diffusing the Pt by heat treatment, an PCIYUS95/14629 - 9 - then CVD aluminizing the electroplated/diffused substrate at a substrate temperature of 1850°F (1010°C) using.coating gas mixture comprising 9 volume % aluminum subchloride and 91 volume % hydrogen at a flow rate of 300 scfm. The coating gas mixture was generated by passing hydrogen and hydrogen chloride in mixture of hydrogen/13 volume % HCl over a 99.999 % pure source of aluminum at 290°C and then passing the mixture through a high temperature generator to form a mixture of aluminum subchlorides. The MDC-150L aluminide coating c) was formed by first electroplating the substrate with a 9-11 milligrams/centimeter squared platinum layer and then CVD aluminizing the electroplated substrate at a substrate temperature of 1080°C using coating gas mixture comprising 9 volume % aluminum trichloride and 91 volume % hydrogen at a flow rate of 300 scfm. The coating gas mixture was generated by passing high purity hydrogen (e.g. less than 30 ppb impurities) and high purity hydrogen chloride (e.g. less than 25 ppm impurities) in mixture of hydrogen/13 volume % HCl over a high purity 99.999 % pure source of aluminum at 290°C. No predif usion heat treatment for the platinum layer was used. The HCl which produced the purification of the substrate and coating was produced by the hydrogen reduction of aluminum trichloride on the sample surface. The foils were subsequently chemically analyzed using the following techniques: a) carbon and sulfur concentrations were measured with a Leco CS444LS analyzer, and b) major metal (P, w, B etc.) concentrations were established with the Perkin and Elmer 5000 Atomic Absorption Spectrometer. The results of the chemical analysis which distinguish the outwardly grown, single phase coating of the invention from the inwardly grown coatings produced by the other processes are presented in Table 2. Specifically, the CVD process of the invention removed both substitutional and surface active impurities from the nickel aluminide as it grew. It should be noted, platinum dissolved in nickel aluminide can inhibit the outward diffusion of impurities from the substrate into the coating, and this effect should help maintain the high purity o the nickel aluminide during testing or engine service. This coul explain why the life of outwardly grown coatings improves as th concentration of Pt in the additive layer increases. The purification effect of the CVD aluminizing process wa also demonstrated by the composition profiles of Figure 2B acros the MDC-150L coated foil shown in Figure 2A. As can be seen, th concentrations of S,P,B and W decrease continuously from a maximu in the diffusion zone to minimum values (significantly below thos in the alloy) at the external surface. Concentration gradients o this type are typical of gas-solid reactions where solute element are being removed from the external surface of the solid b chemical reaction with the gas phase. It should be noted that th structure and composition gradients were symmetric about a plan through the midthickness of the foil, so that only one half of th profile is shown in Figure 2. Also, the concentration profile were smoothed for clarity, and the X-ray counts for each elemen were normalized to the maximum value in the sample so all the dat could be shown on one graph. Superalloys contain different concentrations of a variety o substitutional solutes, and the removal of these elements durin the CVD process involves the chlorination of the metal an subsequent evaporation of the metal chlorides. Since the diffusio coefficients of various alloying elements in nickel aluminid differ, the free energies of formation and the boiling points o the various metal chlorides differ and much of this data is no available in the scientific literature. It would be difficult t predict which elements will be removed during the CVD coating of a alloy of interest. The available thermodynamic data o substitutional alloying elements commonly used in superalloys ar summarized in Table 3. In addition, the valance of th substitutional impurities in their most stable oxide, and th excess charge each element produces when dissolved in A1 2 0 3 ar provided in Table 3. - li ¬ lt is known however, that charge neutrality must be maintained in oxide crystals. Hence, when a cation with a charge greater than or less than 3+ is dissolved in alumina, the point defect concentration must change to compensate for the excess charge. The refractory elements generally have a more positive charge than aluminum in oxides, see Table 3, and so the substitution of a refractory element cation for Al 3+ results in an excess positive charge on the cation site in aluminum oxide. Alumina is a p-type semiconductor; therefore the excess positive charge produced by cations with valences greater than 3+ is balanced by the elimination of electron holes and/or the creation of aluminum vacancies. Any increase in the aluminum vacancy concentration over the equilibrium value results in an increase in the cation diffusion rate, and consequently, the growth rate of the alumina scale. This phenomenon is commonly known as a doping effect, and it can have a significant effect of the oxidation resistance of an aluminide coating. TABLE 1 CHEMICAL ANALYSIS OF Ni FOIL SAMPLES CVD CVD CVD ELEMENT Ni FOIL ALUMINIZE Al-Hf Al+Hf+SI Cr 0.17% 0.07% 0.06% Fe 0.31% 0.01% 0.14% 0.16% S 0.008% 0.0017% 0.0006% 0.001% Note to Table 1: The substitutional solutes in the nickel aluminide coating were removed by chlorination of the metals and subsequent evaporation of the metal chloride, in accordance with the following reactions. Cr(s) + 3/2 Cl 2 = CrCl 3 (g) Fe(s) + 3/2 Cl 2 = FeCl 3 (g) The surface active tramp impurity, S, was removed by reaction wit the hydrogen carrier gas during the CVD cycle to form hydroge sulfide gas, as shown in the following equation. S + H 2 = H 2 S(g) TABLE 2 CHEMICAL ANALYSIS OF IN-738 SAMPLES Pt LDC-2E MDC-150 MDC-150L ELEMENT PLATED COATED COATED COATED B 0.012% 0.013% 0.015% 0.005% P 0.045% 0.044% 0.039% 0.014% S 6ppm 7ppm 8ppm 5ppm W 2.39% 2.3% 0.57% 0.13% Note to Table 2 : The substitutional solute in the nickel aluminide coating wa removed by chlorination of the metal and subsequent evaporation o the metal chloride, in accordance with the following reaction. W(s) + 5/2 Cl 2 = WCl 5 (g) Some of the surface active tramp impurities were removed b chlorination and subsequent evaporation of the chlorides and th sulfur was taken out by reaction with the hydrogen carrier gas, a shown in the following expressions. P(s) + 3/2 Cl 2 = PCl 3 (g) B(s) f + 3/2 Cl 2 = BCl 3 (g) S(s) + H 2 = H 2 S(g) TABLE 3 ELEMENTS WHICH CAN BE CHLORINATED DURING THE CVD CYCLE G° EXCESS 1500K " VOLATILIZATION VALENCE IN CHARGE ELEMENT HALIDE (KCAL/MOLE) POINT OXIDE IN Al-O, Ti TiCl 4 -69.1 τ B = 136°C 4+ 1+ Ta TaCl 5 -49.7 T B = 233°C 5+ 2+ Cr CrCl 3 -34.8 τ s = 947°C 3+ 0 W WC1 S -6.8 T B = 276°C 6+ 3+ Mo MoCl s -4.1 T B = 267°C 6+ 3+ Re ReCl 3 T B = 827°C 7+ 4+ Cb CbCl s T B = 243°C 5+ 2+ Fe FeCl 3 T B = 319°C 3+ 0 V VC1 4 T B = 164°C 5+ 2+ EXAMPLES OF CYCLIC OXIDATION TESTING OF PLATINUM ALUMINIDES The evaluation of the aforementioned coatings for cyclic oxidation testing involved first examining coating structure and thickness to be tested. The following paragraphs summarize the results of the metallographic and electron microprobe analysis of these coatings in both the as coated and the as coated and solution heat treated condition, see Figures 3, 4 and 5 for LDC-2E aluminide coating a) , MDC-150 aluminide coating b) , and MDC-150L aluminide coating c) the outwardly grown, single phase coating of the invention, respectively. COATING STRUCTURE AND THICKNESS In the as coated condition, both the LDC-2E coating a) and MDC-150 coating b) exhibited a two phase outer layer (see Figures 3A and 4A) on top of an (Ni,Pt)Al layer and finally a thi diffusion zone. This structure is typical of the present industr standard platinum aluminides. In contrast, the MDC-150L coating c) of the invention exhibited a single phase (Ni,Pt)Al additive layer, and about one third of the thickness was diffusion zone, see Figur 5A. A well developed diffusion zone is characteristic of a growt process dominated by the outward transport of nickel. In the as coated condition, the coating thicknesses were as follows: LDC-2 coating a) = 1.96 mils, MDC-150 coating b) = 2.11 mils and MDC-150 coating c) = 3.13 mils. After solution heat treatment at 2050 °F for 120 minutes, bot the LDC-2E b) and MDC-150 b) two phase coatings exhibite significant changes in structure, composition and thickness (Figures 3B, 4B) , while the MDC-150L coating c) was relativel unaffected (Figure 5B) . Specifically, the coatings were all single phase following the heat treatment, the average Pt and Al concentrations (see Figures 3 and 4) decreased significantly, an the thicknesses increased about 52% and 67% for LDC-2E coating a) and MDC-150 coating b) , respectively. In comparison, the changes in the MDC-150L coating c) after the thermal cycle (see Figure 5B) were insignificant. It should be noted that the observe metamorphoses of the two phase coatings a) and b) during the solution heat treatment is similar to the changes encountere during engine testing. CYCLIC OXIDATION TEST RESULTS The IN-738 test tabs with the three different Pt aluminid coatings a) , b) and c) were used for cyclic oxidation testing a 1100°C. A one hour cycle included: fifty minutes at temperatur and ten minutes cooling to room temperature. Samples were weighe to the nearest 0.1 milligram (mg) before the test and after eac fifty cycle test interval. Failure was defined as zero crossover,- that is, when the sample weight after a test interval equaled th initial weight. For each coating, three samples were tested t failure, and then the cycles to failure were averaged. The average number of cycles to failure was subsequently normalized to account for differences in coating thickness. The results of the testing are summarized in Figure 6. A comparison of the normalized test results for the three platinum modified aluminide coatings shows that the MDC-150L coating c) (i.e. the outward grown CVD coating of the invention) had about 40% longer life than LDC-2E coating a) and about a 35% life improvement compared to MDC-150 coating b) . The superior oxidation resistance of the MDC-150L coating c) compared to the two inward grown, two phase coatings a) and b) results from the purification produced by CVD aluminizing at high temperature (1080°C) in a mixture of hydrogen, hydrogen chloride and aluminum trichloride gases. That is, the higher purity MDC-150L coating c) formed a slower growing more adherent protective alumina scale, and consequently, exhibited greater oxidation resistance. The CVD purified outward grown, single phase [(Ni,Pt)Al] platinum modified aluminide coating of the invention represents a significant advancement in platinum modified aluminide coating technology. Specifically, the CVD method of the invention substantially reduces the concentration of substrate substitutional alloying elements, such as W, Mo, etc., and surface active tramp elements, such as B, P, and S, in the coating. Consequently, the coating of the invention exhibits better resistance to high temperature oxidation than a standard two phase coating. In practicing the present invention, the substrate temperature in the coating retort is at least about 1000°C, and the coating gas generated at low temperature outside the coating retort comprises hydrogen and aluminum trichloride. The aluminum trichloride typically does not exceed about 10 volume % of the coating gas, and preferably is in the range of 4 to 6 volume % of the coating gas. The coating flow rate typically is within the range of 200 to 400 scfm. Moreover, the invention is preferably practiced by CVD aluminizing the nickel base superalloy coated with the platinum layer directly without a prediffusion heat treatment of th platinum into the substrate. In the past, nickel based superalloy high pressure blades hav been traditionally repaired in the following manner: (l) stri engine run coating from the component, (2) repair the component primarily by removing the damaged blade tip section and applying new tip either by welding or brazing, and (3) recoating the entir component. This type of repair (termed a "full repair") has been in us for a number of years, especially on high pressure blades. Unti very recently, the coatings used on these components have been simple aluminide coating applied by pack cementation techniques. One of the most significant limitations of this conventiona repair technique is that when the engine run coating is stripped, a substantial amount of wall thickness of the blade base materia is also removed. Therefore, the number of repairs of this type t which the blade can be subjected has traditionally been limited t one for the life of the blade. Because of this limitation a repair technique termed a "min repair" was developed. In this case the repair steps are a follows: (1) remove the damaged tip section while leaving th coating on the balance of the blade intact, (2) apply a new tip t the blade, and (3) apply a simple aluminide coating to the ne blade tip by a slurry coating application (so called "Sermetal J") . A blade "repaired in this manner is limited in the sense tha the un-repaired portion of the blade is returned to engine servic without improvement of the existing engine run coating which is o the majority of the blade. Some coating processes have used a pac cementation aluminide coating to recoat the entire blade, but th growth nature of such coatings ("inward") has led to coatings of significantly different thickness on the blade with the overcoate area being substantially thicker than the coating on the newl applied tip area. In addition, over the past few years the use of simple aluminide coatings on these high pressure blades have been replaced by platinum modified aluminide coatings at the OEM stage. Therefore, during the repair cycle, a number of the end users (airlines, etc.) would like to have a platinum modified aluminide coating applied to older model blades which had originally been simple aluminide coated. In order to address the above limitations, the present invention also provides an overcoating method of applying a platinum modified aluminide coating to engine run blades which does not require the stripping of the engine run coating and thus is achieved without loss of base material wall thickness. The present invention in one embodiment involves practicing the aforementioned outwardly grown coating method to repair an engine- run component, such as a turbine blade, that includes a preexisting, engine-run aluminide coating over some blade regions (e.g. airfoil region) but no aluminide coating at other blade regions, such as, for example, the blade tip region where fresh blade tip material is applied by, for example, welding, brazing and the like, to the uncoated (stripped) tip region. The invention can be practiced to grow (overcoat) a relatively uniform thickness Pt aluminide coating on the different regions of such an engine-run component in a one step process. In order to produce the platinum modified aluminide coating or overcoat on an engine run component, the invention electroplates (or otherwise deposits) platinum on the engine-run component (e.g. turbine blade) . The electroplated Pt unexpectedly can be deposited without spallation of the as platinum plated or subsequently aluminized coating. The Pt electroplate can be deposited in an amount of 9-10 grams/cm 2 on the blade. The CVD aluminizing process employed to overcoat the platinum plated blade preferably is of a completely "outward" nature as described hereabove for producing the MDC-150L Pt aluminide coating, as opposed to the "inward" or "intermediate" nature of conventional pack cementation or out of contact pack typ techniques (such as "above the pack") . The use of such completely outward CVD aluminizing technique is self limiting i nature in the sense that areas of the previously aluminide coate repaired blade do not gain much additional coating thickness whil areas of the previously aluminide coated repaired blade which ar significantly lacking in coating are returned to the require coating thickness. For purposes of illustration, an engine-run turbine airfoi (turbine blade or vane) which has had a standard tip repai comprises an uncoated tip material (e.g. IN 625) welded onto blade tip region previously stripped of coating material. The blad has an existing engine-run simple aluminide coating at, fo example, the airfoil region. The blade (substrate) can compris Rene 80 alloy for example. A platinum modified aluminide coatin of relatively uniform thickness which incorporates the existin engine run simple aluminide coating can be produced by firs platinum plating the entire gas path surface area of the airfoil ( to 10 grams/cm 2 Pt layer) , then aluminizing that area with a CV outward growth process of the type described to form the MDC-150 coating hereabove. In the regions where there is engine ru aluminide coating, the diffusion distance of nickel required fo coating growth is large (e.g. 0.0025 to 0.004 inches), and so, th growth rate is very low compared to the uncoated tip region. Thus, at these already coated regions, two processes occur: (l) diffusio of platinum into the engine run aluminide coating and (2) slo growth of the coating. The coating process in the tip and wel areas follows a normal growth sequence,- that is, a very rapi growth rate initially which gradually decreases as the coatin thickness (nickel diffusion distance) increases. The result is platinum modified aluminide coating produced over the entire ga path area of the airfoil which is fairly uniform in thickness (e.g. between 0.003 to 0.0045 inches thickness) and which incorporate the engine run coating in its structure. Thus, the natural rat controlling process of an outward growth aluminide coating (diffusion of nickel through the coating) limits coating growth in the previously coated areas enough that the coating thickness on the tip and weld areas can become nearly equal to that on the majority of the airfoil where the existing engine run alumnide coating was present. With this overcoating process of this invention, gas turbine engine users can obtain the improved resistance to environmental degradation provided by a platinum modified aluminide coating without performing the one full repair which is allowed before the overall condition of the airfoil requires extensive rework. Metallographic examination of cross-sections through over coated, engine run high pressure turbine hardware illustrates this repair process of the invention. Cross-sections of samples overcoated in the manner described hereabove were examined on an electron microscope, and the analysis included: photomicrographs and X-ray maps for the major coating constituents. This type of data was obtained in two areas of the airfoils: (1) the coated tip and (2) the transition between the coated welded tip and the overcoated airfoil. In the following paragraphs a brief discussion of the results will be provided. The results of the metallographic and microprobe analysis from the coated replacement blade tip are presented in Figures 7A, 7B, 7C and 7D. Examination of this information indicates the coating on the tip has a structure and element distribution typical of the aforementioned MDC-150L coating. Specifically, the additive layer is single phase (Ni,Pt)Al with a fairly uniform distribution of elements. The as coated thickness is about 80 microns, of which about 65% is additive layer and 35% is diffusion zone (indicative of outward coating growth) . The most dramatic example of the diffusion distance limiting coating growth enough in overcoated areas to permit the aluminide coating on the tip to attain a similar thickness as that on formed on the existing aluminide coated airfoil regions can be found in the transition region between the new tip and the overcoate airfoil. Figures 8A, 8B, 8C and 8D contain metallographic an microprobe results for a typical transition region from the coate tip weld into the overcoated airfoil region. As can be seen, th overall coating thickness in the weld and overcoated areas ar nearly equal . The results clearly indicate that the natural rat controlling process involved with the outward growth aluminid coating can be utilized to produce an aluminide overcoating o nearly equal thickness in the repaired tip having no existin coating and the airfoil region having an existing engine ru aluminide coating thereon. It should be noted however, the maximum thickness which th repair process of the invention can produce on polycrystallin alloys is in the range of about 70 to 90 microns. Thus, if th thickness of the engine run aluminide coating on the airfoil regio is greater than about 70 microns, the process will not produce thick enough coating on the repaired tip area (sans coating) t equal that on the airfoil region. For example, for an overcoate airfoil where the engine run aluminide coating was approximatel 130 microns thick as received from the overhaul shop, following th overcoat process, the coating in the airfoil region was about 14 microns thick, which represents only a 10% increase in thicknes compared to the as received condition. Hence, use of the overcoa process of this invention should include an evaluation of th thickness of the existing engine run coating on the component to b repaired, and then, some means should be employed to reduce th thickness of the existing engine run coating which exceed about 7 microns to 90 microns, such means comprising a partial strip of th coating for example. Although the invention has been described with respect t certain specific embodiments thereof, those skilled in the art wil recognize that these embodiments were offered for the purpose o illustration rather than limitation and that the invention is no limited thereto but rather only as set forth in the appended claims.

Claims: CLAIMS 1. A method of improving the oxidation resistance of a platinum modified aluminide coating formed on a nickel base superalloy substrate, comprising: providing a platinum layer on the substrate, heating the substrate in a coating retort to a temperature of at least about 1000°C, forming external of the coating retort a coating gas comprising hydrogen and aluminum trichloride, and introducing the coating gas into the coating retort to contact the heated substrate to form an outwardly grown, single phase (Ni,Pt)Al platinum modified aluminide coating having reduced concentration of at least one of a substrate substitutional alloying element and a surface active tramp element selected from at least one of B, P, and S in said coating as compared to a platinum modified aluminide coating formed at a lower temperature. 2. The method of Claim 1 wherein the substrate is coated with the platinum layer and then contacted with the coating gas without a prediffusion of platinum into the substrate. 3. The method of Claim l wherein the coating gas is formed by passing high purity hydrogen and hydrogen chloride gases over a high purity source of aluminum. 4. The method of Claim 3 wherein the high purity aluminum source is 99.999 weight % Al or higher, 5. The method of Claim 3 wherein the coating gas is generated at a temperature not exceeding 300°C. 6. The method of Claim l wherein the substrate includes a refractory metal alloying element whose concentration in the coating is reduced by chlorination of the refractory metal and evaporation of the refractory metal chloride, at said substrate temperature. 7. The method of Claim 1 wherein the substrate includes a surface active tramp element selected from one of B and P whose concentration in the coating is reduced by chlorination of the tramp element and evaporation of the tramp element chloride at said substrate temperature. 8. The method of Claim 1 wherein the substrate includes sulfur as a surface active tramp element, the concentration of sulfur in the coating being reduced by reaction of the sulfur with hydrogen in the coating gas to form hydrogen sulfide. 9. A method of repairing an engine-run component having an existing aluminide coating at a region and an uncoated, metallic repair material at another region, comprising depositing a layer of Pt on the regions and aluminizing the Pt coated regions under chemical vapor deposition conditions to outwardly grow a generally uniform thickness Pt bearing aluminide coating at said regions . 10. The method of Claim 9 wherein the region having the existing aluminide coating is an airfoil region of a turbine blade and the repair material is at a tip region of the turbine blade. 11. The method of Claim 9 wherein the existing aluminide coating is not greater than about 70 microns. 12. The method of Claim 9 wherein the Pt coated regions are aluminized at a temperature of at least about 1000 degrees C. 13. A repaired engine-run component having a generally uniform thickness Pt bearing aluminide coating grown in-situ outwardly through a preexisting engine-run aluminide coating at a region and grown outwardly on an uncoated, metallic repair material at another region of said component. 1 . The method of Claim 13 wherein the region having the existing aluminide coating is an airfoil region of a turbine blade and the repair material is at a tip region of the turbine blade. 15. The method of Claim 13 wherein the existing aluminide coating is not greater than about 70 microns.

Inventors: BASTA WILLIAM C, PUNOLA DAVID C, WARNES BRUCE M, BASTA, WILLIAM, C., PUNOLA, DAVID, C., WARNES, BRUCE, M.
Applicants: HOWMET CORP, HOWMET CORPORATION
Application year: 1995
Main classification: C23C16
IPC codes: C23C10, C23C16, F01D5