METALLIC COATING OF IMPROVED LIFE

Abstract: A method for improving oxidation resistance life of a metallic coating deposited on a metallic alloy including boron treats the alloy to reduce the boron prior to depositing the coating. The article provided by using a diffusion coating method has a diffusion zone between the coating and the alloy substrate characterized by the substantial absence of boride needles traversing the diffusion zone from the coating into the alloy.

Description: This invention relates to metallic coatings on a metallic surface and, more particularly, to a method for improving the oxidation resistance life of such coatings and to the resulting article. The application of certain protective metallic coatings to alloy surfaces, particularly of the nickel base or cobalt base type are described in such U.S. Patents as US-A-3,540,878, 3,598,638 (forms of which are sometimes referred to as CODEP coating) and 3,976,436 (representative of those types of coatings sometimes referred to as the MCrAl class of coatings). In addition, use of fluoride ions for cleaning or treatment of metallic surfaces or materials is described in U.S-A- 4,098,450 and 4,249,963. The development of advanced gas turbine engines has led to the design of certain hot section parts intended to operate under increasingly more strenuous environmental conditions, for example conditions of oxidation. It is common practice in the art to improve the oxidation resistance of the surfaces of such parts through the application of metallic coatings, for example of the type identified above. The result can be improved operating life of the coated part, which can be very expensive to replace and costly to repair. It is a principal object of the present invention to provide a method for improving the oxidation resistance life of a metallic coating applied to a metallic substrate. Another object is to provide a method for improving the oxidation resistance life of high temperature operating metallic coatings applied to surfaces of nickel base or cobalt base superalloy articles. Still another object is to provide a metal coated alloy article of improved oxidation resistance. These and other objects and advantages will be more fully understood from the following detailed description, the drawing and the specific examples. Briefly, the present invention provides a method for improving the oxidation resistance life of the combination of a metallic coating deposited on a metallic portion surface which includes the element boron in its composition. The method comprises the steps of treating the surface portion to reduce its boron content up to a depth of about 0.127 mm (0.005") to provide a treated surface. Thereafter, a metallic coating is deposited on the treated surface. In one form, such treatment comprises exposing the surface to gaseous fluoride ions which will react with the boron in the surface to form a gaseous boron compound which thereafter is emitted from the surface. In a more specfic form, the method of the present invention provides improving the oxidation resistance life of the combination of a metallic coating deposited on an article surface which includes a repaired portion. For example, such a repaired portion comprises the article alloy itself, which includes the element boron, and a metallic repair material, typically in a recess or crack in the article, the repair material being different in composition from that of the article alloy. The repair material is bonded to the article alloy. The method comprises treating the repaired portion to reduce the boron content of the repair material thereby providing a treated surface, and then depositing the metallic coating on the treated surface. The coated article of the present invention which comprises an alloy surface based on Ni and/or Co and which also includes B, has a diffusion zone characterized by the significantly reduced amount of chromium boride needles, traversing the diffusion zone from the coating into the alloy surface. Figure 1 is an enlarged, sectional, diagrammatic view of a fragment of a metallic material including a repaired portion; Figure 2 is a diagrammatic presentation of a photomicrograph of 1000 magnifications of a coated specimen not treated according to the present invention; Figure 3 is a diagrammatic presentation of a photomicrograph at 1000 magnifications of a coated specimen which has been treated according to the present invention. Because of the complexity in the design and difficulty in the manufacture of high temperature operating gas turbine engine parts, particularly those which rotate in a high temperature, highly oxidizing atmosphere, generally it is less expensive to repair the part than to replace it. As a result, there has developed a relatively broad body of technology relating to the repair of such parts or articles. One method is identified in the above mentioned US-A- 4,098,450. Other repair methods involving metallic powders or power mixtures, useful in such method, are described in US-A-4,381,994 In the evaluation of repair technology and the repair of gas turbine engine high temperature articles of the type manufactured from nickel base or cobalt base superalloys, it was observed that the above identified aluminide type coating, sometimes referred to as CODEP coating and more fully described in US-A- 3,540,878 and 3,598,638, deteriorated under oxidizing conditions significantly more rapidly in some cases than in others. Such deterioration was more prevalent when such coating was applied over a repaired portion of a nickel base or cobalt base superalloy article which had been repaired using a material of composition different than the superalloy. Such a combination of metallic materials and coatings are shown in Figure 1 of the drawing. In that Figure, an alloy article 10 includes a repaired portion shown generally at 12 comprising a recess or crevice such as a crack 14 in article 10, a metallic repair material 16 bonded in recess 12 and a metallic coating 18 deposited over the repaired portion 12. During the evaluation of the present invention as it relates to the type of metallic combination shown in Figure 1, it was recognized that the oxidation life of a metallic coating, such as one which includes the element of aluminum (as in an aluminide coating) could be improved by at least two times and in some cases ten times through the depletion of the element boron from surface of the repaired portion prior to application of the metallic coating. Because the type of alloy generally referred to as superalloys or the repair alloy or both includes the element chromium, boron in the surface frequently is in the form of chromium boride phases. The present invention relates to treating the surface portion of the alloy; therefore, reactions are surface phenomena, affecting material within 0.127 mm (0.005") of the surface, and generally within about 0.05 mm (0.002") of the surface. Reducing of such boride phases before application of a metallic coating is significantly beneficial for at least two reasons : first, removing such stable precipitates from the surface reduces the number of crack initiation sites, promoting good oxide adherence during thermal cycling; second, it appears to promote the formation of a more effective, continuous diffusion zone. It was observed that this treatment allowed the aluminum oxide protective film to regenerate itself at elevated temperatures, for example in the range of 1121-1149°C (2050-2100°F). During the evaluation of the present invention, studies were conducted to more fully understand the effect of surface related phenomena. One such study involved a gas turbine engine airfoil made of a cobalt base superalloy sometimes referred to as WI-52 as the structural or base alloy. The nominal composition, by weight, of WI-52 alloy is 21% Cr, 11% W, 2% Nb, 2% Fe, 0.45% C with the balance essentially Co and incidental impurities. Such an airfoil material was prepared using a repair sequence developed for such an alloy: the surface was grit blasted with aluminum oxide media and chemically treated to remove a diffused aluminide coating, after which it was exposed to fluoride ions and vacuum cleaned. With the base material thus prepared, a cobalt base repair alloy identified as SA-1 alloy was applied. The nominal composition of SA-1 alloy is, by weight, 28% Cr, 4.5% W, 10 % Ni, 1 % Al, 1.5 % Ti, 1.5 % Ta, 1 % B, 0.3% Si, 0.15% Zr, with the balance Co and incidental impurities. The SA-1 alloy was applied to random surface areas of the airfoil, after which the specimen was processed through the brazing/diffusion cycle developed for SA-1 alloy: brazing in the range of 1177-1232°C (2150-2250°F) for about one-half hour followed by diffusion in the range of 1093-1177°C (2000-2150°F) for 8-15 hours. The brazed areas on the WI 52 base alloy were benched with a carbide cutter to remove the tantalum/titanium rich surface region, and the airfoil was then sectioned into multiple pieces for further evaluation and for the establishment of baseline samples. Some of the pieces were exposed to a fluoride ion cycle prior to the application of an aluminide coating. Such a cycle involved exposing the samples to an atmosphere of fluoride ions in a manner described in the above incorporated U.S-A- 4,249,963 and 4,098,450. In this example the temperature of exposure was about (1750°F) 954°C, in the range of 927-982°C (1700-1800°F, for about 1-2 hours. The fluoride ions were from hydrogen fluoride gas in a gaseous mixture at a concentration of 5-15 volume percent, with the balance hydrogen gas. An aluminide-type coating, sometimes referred to as CODEP coating and more fully described in US-A- 3,540,878 was applied to specimens which had been exposed to the fluoride ion atmosphere, as well as those which had not been so exposed. Involved in such coating application is a diffusion treatment in the temperature range of 1038-1066°C (1900-1950°F), which creates a diffusion zone between the coating and the substrate on which the coating was applied, in this case the SA-1 alloy. This was accomplished to evaluate the interaction and surface phenomena associated with such procedures. Micrographic studies of portions of such specimens, as they relate to the present invention, are summarized in the diagrammatic presentations of Figs. 2 and 3. Such views are fragments of sections taken through the specimens processed as above and observed at 1000 magnifications. With reference to Figs. 2 and 3, portion 16 is the repair alloy in the form of the above described SA-1 alloy deposited on a WI-52 alloy substrate (not shown). Coating 18 was the CODEP aluminide diffusion coating described above. Involved in the CODEP coating process is a diffusion step which, as it relates to the present invention, generated a diffusion zone which included a chromium boride phase 20 and a tungsten rich phase 22 as a result of those elements being present in the SA-1 repair alloy. Figure 2 represents the results of processing of the specimen without exposure of the surface of the SA-1 repair alloy to fluoride ions, according to the present invention, prior to application of the CODEP coating. The presentation of Figure 3 represents a specimen which was exposed to fluoride ion treatment, according to the present invention, prior to CODEP coating. Comparison of Figures 2 and 3 clearly shows that use of fluoride ion exposure prior to coating, according to the present invention, significantly reduces the capability of the chromium boride phase to generate or precipitate "needles" such as those shown at 24 and 26 in Figure 2, traversing the diffusion region from the CODEP coating into the SA-1 repair alloy. Such needles are believed to constitute crack initiation sites and a path for oxygen to penetrate from the CODEP coating into the SA-1 repair alloy, thereby promoting oxidation failure. As can be seen from Figure 3, representative of results of the present invention in which an average of at least about 50% of the needles are eliminated, there is generated a more effective, continuous chromium boride phase 20 adjacent a tungsten rich phase 22 in the diffusion zone between the CODEP coating and the SA-1 repair alloy. It was observed that this allowed an aluminum oxide protective film from the CODEP coating to regenerate itself at elevated temperatures for example, in the range of 1093-1149°C (2000-2100°F), indicating a more significant reduction in traversing needles. As was mentioned above, the present invention provides improvement in coating life of at least two times. In the case of the use of CODEP coating over SA-1 repair alloy, the multiplier was significantly greater, for example up to 10 times improvement after exposure in the range of 1093-1149°C (2000-2100°F). In this evaluation, it was observed that the general coating thickness and composition was substantially the same with or without the fluoride ion treatment: no meaningful changes were made to the compositions in the near surface region (up to about 0.127 mm (0.005")), except for the above described depletion of boron to inhibit the formation of the chromium boride needles described above and shown in Figure 2. The coating thickness and aluminum content were essentially unaltered by the additional processing. A slight reduction (for example less than two weight percent) in the chromium content was noted, presumably because of the formation of a chromium oxide film during processing. The present invention, through the reduction of boron within up to about 0.127 mm (0.005") of a surface to be coated, removes crack initiation sites which are particularly significant during thermal cycling. Once a substrate is exposed in this manner, oxygen can diffuse relatively rapidly along exposed grain boundaries. Formation of internal cobalt and chromium oxides can then accelerate failure of the aluminide type coating.

Claims: A method for improving the oxidation resistance life of the combination of a metallic surface portion which includes a nickel base or cobalt base repair alloy including the element boron in its composition, predominantly in the form of chromium boride, and a metallic coating, including aluminum, deposited in said metallic surface position, characterized in that it comprises : treating the surface portion to reduce the boron content of the surface portion up to a depth of about 0.127 mm (0.005") to provide a treated surface, said treatment of the surface portion including exposing the surface portion to gaseous fluoride ions with which the boron will react to form a gaseous compound ; and then, deposing the metallic coating on the treated surface. The method of claim 1 in which : The gaseous fluoride ions are from a hydrogen fluoride gas in a gaseous mixture ; the concentration of the hydrogen fluoride gas in the mixture is in the range of 5-15 volume percent, with the balance hydrogen gas ; and the treatment is conducted at 927-982°C (1700-1800°F) for 1-2 hours. The method of claim 1 in which the coating is of the diffusion aluminide type. The method of claim 1 wherein said combination of a metallic coating deposited on a metallic surface portion is part of an article and wherein said metallic surface/portion comprises at least first and second metallic materials different in composition one from the other, said said first metallic material being a nickel base or cobalt base superalloy and said second metallic material being said repair alloy which includes the element boron and which is metallurgically bonded to the first metallic material. The method of claim 4 wherein the metallic surface portion includes a repaired portion of said article, the repaired portion comprising an article alloy and said metallic repair alloy, said repair material being bonded in a recess in the article alloy. The method of claim 5 including the step of diffusing the deposited coating with the article surface to provide a diffusion zone therebetween. A coated article of improved oxidation resistance comprising : An alloy surface based on an element selected from the group consisting of Ni and Co and including the element B predominantly in the form of chromium boride ; a metallic coating diffused with the alloy surface providing a diffusion zone therebetween ; characterized in that said article has been treated with the method of claim 1 resulting in said diffusion zone having an average amount of chromium boride needles traversing the diffusion zone from the coating into the alloy of the surface which is at least 50 % less than the amount existing without the treatment. A repaired article having a repaired portion of improved oxidation resistance, the repaired portion comprising a superalloy structural alloy based on an element selected from the group consisting of Ni and Co, a recess in the structural alloy with a repair alloy therein, the repair alloy comprising the elements B, Cr and W, and a metallic coating diffused with the structural alloy and the repair alloy, characterized in that said article has been treated according to the process of claim 1 resulting in the repaired portion comprising a diffusion zone, between the metallic coating and the repair alloy, having a substantially continuous chromium boride phase in which the average amount of boride needles traversing the diffusion zone is at least 50 % less than the amount existing without the treatment. The article of claim 8 wherein there is a tungsten rich phase between the substantially continuous chromium boride phase and the repair alloy.

Inventors: RASCH, LYLE TIMOTHY
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1988
Main classification: C23C16
IPC codes: C23C14, C23C4