Abstract:
This invention pertains to the repair of parts comprising metals, and surfaces and coatings of these parts using reactive metals coating processes. Processes such as chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma spray, and reactive coating (boronizing, carburizing, nitridizing, carbonitridizing, etc.) are known for producing durable coatings or surfaces on metal parts, and the present invention provides a means to spot-repair these coatings or surfaces without excessive buildup of repair material on undamaged areas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from provisional application serial 60/226,295, filed Aug. 21, 2000, entitled “REPAIR OF COATINGS AND SURFACES USING REACTIVE METALS COATING PROCESSES” which is incorporated herein, in its entirety, by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention pertains to the repair of parts comprising metals, and surfaces and coatings of said parts using reactive metals coating processes. Coating and surface repair fall under U.S. Patent Class 427 (COATING PROCESSES), Subclass 140 (Processes directed to the restoration or repair of coatings or surfaces of objects). Surface treatments via reactive metal coating processes fall under U.S. Patent Class 148 (METAL TREATMENT), Class Definition C ( . . . processes of reactive coating of metal wherein an externally supplied carburizing or nitriding agent is combined with the metal substrate to produce a carburized or nitridized or carbonitrided coating thereon or a uniformly carburized, nitrided, or carbonitrided metal alloy containing a metal element from said substrate) and Class Definition D ( . . . processes of reactive coating of metal wherein an externally supplied agent combines with the metal substrate to produce a coating thereon which contains at least one element from said metal substrate). This invention is applicable in maintenance and restoration of parts in many industries including, but not limited to, aviation and space industries. 
     Coatings and Surface Treatments 
     Various processes are well-known for providing coatings or modified surfaces on metals to protect them from effects such as wear, erosion, and corrosion. Such processes include chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma spray, and reactive coating (boronizing, carburizing, nitridizing, carbonitridizing, etc.). For instance, U.S. Pat. No. 5,272,014 (Leyendecker) teaches a wear-resistant CVD coating for substrates such as forming or cutting tools. U.S. Pat. No. 5,656,364 (Rickerby) and U.S. Pat. No. 5,702,829 (Paidassi) teach multiple-layer erosion-resistant PVD coatings for substrates such as gas turbine engine compressor or turbine blades. U.S. Pat. No. 4,850,794 (Reynolds, Jr.) teaches solution-bath and gas nitriding to enhance the wear-resistance of steam turbine components. U.S. Pat. No. 4,588,450 (Purohit) teaches nitriding of nickel-based super alloys including inconel to improve their creep strength, fatigue strength, and resistance to oxidation. U.S. Pat. No. 6,129,988 (Vance , et al.) teaches gas nitriding of metallic bond coatings for thermal barrier coating systems. Nitriding of metallic bond coatings enhances oxidation resistance thereby prolonging the adherence of ceramic thermal barrier coatings applied thereon. CVD, PVD and plasma spray processes generally involve deposition of additional material on the surface of a substrate. Reactive coating processes generally involve incorporation or dispersion of additional chemical constituents into the existing lattice structure of a metal substrate. 
     Functionally Gradient Surfaces 
     Reactive coating processes are known for producing treated surfaces with chemical compositions that vary as a function of depth, also known as functionally gradient surfaces. For instance, surfaces produced via nitriding consist of a hard nitride layer above a nitrogen-containing diffusion zone, with nitrogen content gradually decreasing deeper into the substrate material. Richter discusses a plasma nitriding process for producing functionally gradient surfaces on stainless steel and aluminum alloys (“Nitriding of Stainless Steel and Aluminum Alloys by Plasma Immersion Ion Implantation”, Surface and Coatings Technology, Vol. 128-129, 2000, pp. 21-27). U.S. Pat. No. 4,762,756 (Bergmann) teaches a plasma nitriding process that is enhanced using arc discharge, whereby functionally gradient surfaces are produced on metals including stainless steel and titanium. Meletis discusses an enhanced plasma nitriding process for producing functionally gradient surfaces on titanium (“Characteristics of DLC Films and Duplex Plasma Nitriding/DLC Coating Treatments”, Surface and Coatings Technology, Vol. 73, 1995, pp. 39-45). This (enhanced nitriding process is also taught in expired U.S. Pat. No. 4,460,415 (Korhonen, issued Jul. 17, 1984) and U.S. Pat. No. 5,334,264 (Meletis, issued Aug. 2, 1994). U.S. Pat. No. 4,568,396 (Vardiman) teaches a carburizing method via carbon ion implantation wherein carbon content of the treated surface varies as a function of depth. PVD and CVD processes are better-known for producing coatings of uniform composition as a function of depth (monolayers), but can also be adapted to produce functionally gradient surfaces. For example, U.S. Pat. No. 5,989,397 (Laube) teaches a method and apparatus for producing deposited surfaces with depth-varying compositions of titanium, carbon, and nitrogen. 
     Enhanced Plasma Nitriding 
     A review of enhanced nitriding processes is presented by Czerwiec et al (“Low-pressure, high-density plasma nitriding: mechanisms, technology and results”, Surface and Coatings Technology, Vol. 108-109, 1998, pp. 182-190). These processes can be classified under the following four categories: Thermionically assisted d.c. triode (TAT); plasma immersion ion implantation (PIII) or plasma source ion implantation (PSII); electron cyclotron resonance (ECR) systems; and thermionic arc discharge (TAD). A version of the TAT enhanced plasma nitriding method and apparatus presented by Meletis in U.S. 
     Pat. No. 5,334,264 is previously taught by expired U.S. Pat. No. 4,460,415 (Korhonen), and also by earlier references including Matthews and Teer (“Characteristics of a Thermionically Assisted Triode Ion-Plating System”, Thin Solid Films, Vol. 80, 1981, pp. 41-48), Korhonen and Sirvio (“A New Low Pressure Plasma Nitriding Method”, Thin Solid Films, Vol. 96, 1982, pp. 103-108), Korhonen et al (“Plasma Nitriding and Ion Plating With an Intensified Glow Discharge”, Thin Solid Films, Vol. 107, 1983, pp. 387-394), Fancey and Matthews (“Some Fundamental Aspects of Glow Discharges in Plasma-Assisted Processes”, Surface and Coatings Technology, Vol. 33, 1987, pp. 17-29), Ahmed (“Ion Plating Technology, Develoments and Applications”, John Wiley and Sons, New York, 1987, pp. 68-70), Fancy and Matthews (“Process Effects in Ion Plating”, Vacuum, Vol. 41, No. 7-9, 1990, pp. 2196-2200), and Leyland et al (“Enhanced Plasma Nitriding at Low Pressures: A Comparative Study of D. C. and R. F. Techniques”, Surface and Coatings Technology, Vol. 41, 1990, pp. 295-304. Furthermore, Molarius et al teaches that the process of U.S. Pat. No. 4,460,415 (Korhonen) can be used to treat titanium (“Ion Nitriding of Steel and Titanium at Low Pressures”, 4th Int. Congress on Heat Treatment of Materials. Jun. 3-7, 1985. Berlin (West), Proceedings, Vol I, p. 625-643. Härterei-Technische Mitteilungen 4(1986)6, 391-398.). These references establish prior art that pre-dates the filing of the Meletis Patent by 2 to 10 years. None of these references is cited in the Meletis Patent. U.S. Pat. No. 5,334,264 therefore teaches very little that was not previously taught by prior art. 
     Performance of Functionally Gradient Surfaces 
     Functionally gradient surfaces are known to have superior wear and erosion properties compared to monolayer coatings. Voevodin presents results of scratch tests for multiple-layer titanium, titanium carbide, and diamond-like carbon (DLC) surfaces prepared using the process of U.S. Pat. No. 5,989,397 (“Design of a Ti/TiC/DLC Functionally Gradient Coating Based on Studies of Structural Transitions in Ti-C Thin Films”, Thin Solid Films, Vol. 298, 1997, pp. 107-115). Meletis presents results of wear tests for functionally gradient, nitrided titanium surfaces (“Characteristics of DLC Films and Duplex Plasma Nitriding/DLC Coating Treatments”, Surface and Coatings Technology, Vol. 73, 1995, pp. 39-45). Gachon presents results of erosion tests for functionally gradient, multiple-layer tungsten carbide coatings (“Study of Sand Particle Erosion of Magnetron Sputtered Multilayer Coatings”, Wear, Vol. 233-235, 1999, pp. 263-274). Gupta presents results showing that PVD multilayer titanium nitride coatings have superior erosion resistance compared to titanium nitride monolayer coatings on turbine engine compressor blades (“Protective Coatings in the Gas Turbine Engine”, Surface and Coatings Technology, Vol. 68/69, 1994, pp. 1-9). Because of their superior performance, functionally graded surfaces are preferred over monolayer coatings. In general, thicker coatings or surface treatments (monolayer or functionally gradient) tend to provide better wear and erosion protection. 
     Surface Treatments and Fatigue Strength 
     Coating or surface treatment thickness determines not only wear and erosion resistance, but can also affect fatigue strength of the substrate. For instance, previous attempts to plasma nitride titanium and titanium alloys have most often produced surfaces with increased wear resistance, but often reductions in substrate fatigue strength. Morita presents a list of references dating from 1964 to 1996 for which this is true (“Factors Controlling the Fatigue Strength of Nitrided Titanium”, Fatigue &amp; Fracture of Engineering Materials &amp; Structures, Vol. 20, No. 1, 1997, pp. 85-92). Morita also shows the relationship between substrate fatigue strength, substrate grain size, and surface treatment depth (case depth) for nitrided titanium. Morita gas nitrided samples at temperatures from 620 degrees C. to 1200 degrees C. to achieve a range of case depths and grain sizes. Results show that for equivalent grain sizes, the fatigue strength of nitrided titanium with a case depth of 40 micrometers is greater than the fatigue strength of the untreated substrate. When the case depth is increased to 100 micrometers (same grain size), fatigue strength of the nitrided material is significantly decreased compared to the untreated substrate. These results apply over a wide range of grain sizes. The diffusion zone of the nitrided surface appears to help suppress crack propagation in the substrate, but only to a limited degree. The tendency of the 40 micrometer depth case to fracture and initiate substrate crack growth tends to be countered by decreased tendency for slip and dislocations in the diffusion zone. Under a similar level of substrate strain the 100 micrometer case is more likely to fracture, and the diffusion zone is unable to counter the increased tendency for crack growth. Morita&#39;s results also indicate that long nitriding times at high temperatures tend to degrade fatigue strength via excessive case thickness and excessive grain growth (e.g., material annealing). 
     Degradation of fatigue strength due to thick coatings on turbine engine compressor blades is mentioned by Friedrich (“Improving Turbine Engine Compressor Performance Retention Through Airfoil Coatings”, NASA Lewis Research Center Aircraft Engine Diagnostics, Document ID 19810022661 N (81N31203), January 1981, pp. 109-117) and in U.S. Pat. No. 4,761,346 (Naik). There appears to be a correlation between thick coatings and degradation in fatigue strength. Thicker coatings tend to provide better wear and erosion protection but often at the expense of fatigue strength. These factors must be considered carefully for coatings and surface treatments, particularly in applications where superior fatigue strength is important. 
     Surface Damage and Repair 
     Despite improved protection of the substrate, monolayer coatings or functionally gradient surfaces will eventually wear, erode, or corrode in-service and the underlying metal substrate can be exposed. In general, damage to coated or treated surfaces is not uniform, and consists of local damage sites surrounded by areas where the coating or surface treatment is intact. This is particularly true in cases where the surface has experienced impact or micro-chipping damage due to erosive service conditions. For instance, Gupta shows localized damage to a titanium nitride coated turbine engine compressor blade (“Protective Coatings in the Gas Turbine Engine”, Surface and Coatings Technology, Vol. 68/69. 1994, pp. 1-9). Once damaged, coated or treated parts must be restored or repaired to reestablish the original level of protection provided to the substrate. 
     Damaged areas of some coatings can be cleaned of loose debris and the surface spot-repaired or re-coated. For instance, U.S. Pat. No. 5,958,511 teaches a process for spot-repairing conversion coatings such as Alodine (Henkel Surface Technologies, Madison Heights, Mich.—formerly Parker-Amchem). U.S. Pat. No. 3,248,251 (Allen) describes aluminum-filled inorganic phosphate overlay coatings that are used to protect components in turbomachinery. A commercial version of this coating manufactured by Sermatech International Inc. (Limerick, Pa.) is reportedly spot-repairable. U.S. Pat. No 6,042,880 (Rigney) teaches repair and spot-repair of metallic bond coats used under thermal barrier coatings (TBCs) on turbine blades, wherein the TBC is completely removed to expose the bond coat, then the bond coat spot-repaired. Rigney emphasizes that complete removal of the TBC and bond coat, and simultaneous unintentional removal of substrate is detrimental to blade fatigue life. 
     Other more durable coatings including some produced via CVD, PVD, or plasma spray processes are not typically spot-repaired. Usual practice for these coatings is to completely remove all old surface materials, thereby helping to ensure the integrity of the replacement coatings. For instance, U.S. Pat. No. 5,368,444 (Anderson) discusses the strip and re-coat of copper-nickel-indium anti-fretting and anti-wear coatings commonly employed on compressor and (turbine blade dovetails. U.S. Pat. No. 5,813,118 (Roedl) and U.S. Pat. No. 6,049,978 (Arnold) describe grit blast and chemical stripping for turbine engine airfoils. U.S. Pat. No. 5,421,517 (Knudson) teaches a waterjet removal process for gas turbine engine components and also aircraft exterior surfaces. U.S. Pat. No. 6,036,995 (Kircher) teaches removal of the surface layer of a metallic coating by first applying a slurry of aluminum in an inorganic binder to the surface of a part coated with the coating, then heating the coated part to melt the aluminum which flows inward into the surface and reacts with the surface to form a brittle aluminide layer, and finally removing the layer via chemical or physical means. Coating removal processes such as these can be effective, but tend to be slow, equipment-intensive, or labor-intensive for removing durable coatings and are therefore expensive. A means to easily remove and/or spot-repair coatings such as these is needed in the art. 
     Another aspect of coating or surface treatment repair is addressing significant wear or damage that extends into the substrate material. In some applications, infrequent but severe damage events can occur that will breach protective coatings and penetrate deeply into the substrate. For instance, Gravett presents data from a field inspection campaign of foreign object damaged turbine engine compressor blades (“The Foreign Object Damage Project of the PRDA V HCF Materials and Life Methods Program”, 4th National Turbine Engine High Cycle Fatigue Conference, Monterey, Calif., USA, Feb. 9, 1999). Data presented shows that the depth of foreign object damage to compressor blades can range from 0.02 inches to 0.5 inches, with an average depth of 0.06 inches. This average damage depth is much greater than a typical protective coating or treated surface. 
     Surface damage to such depths is unacceptable for some applications, but is acceptable for others. In the case of cutting tools, significant erosion or wear of the tool will cause parts machined by the tool to be out of tolerance and therefore unacceptable. However, in the case of turbine engines, significant wear and erosion on in-service compressor blades is commonplace. Gupta presents data showing local compressor airfoil erosion can be on the order of 10 percent of the original airfoil chord (“Protective Coatings in the Gas Turbine Engine”, Surface and Coatings Technology, Vol. 68/69, 1994, pp. 1-9). Schwind presents similar, but more detailed information regarding blade erosion (“Blade Erosion Effects on Aircraft-Engine Compressor Performance”, Department of Energy Report DOE/CS/50095-T2, 1982) In fact, special procedures have been developed to classify and repair such damage to turbine engine blades. U.S. Pat. No. 5,625,958 (DeCoursey) teaches a method to determine the service life remaining in a blade after erosion has occurred. U.S. Pat. No. 5,197,191 (Dunkman) teaches a method and apparatus to repair gouged out and damaged leading and trailing edges of gas turbine engine blades by cutting away a curved section including the damaged area and forming a blend radius along the repaired edge. Clearly, it would be advantageous to coat or surface treat parts such as turbine engine airfoils to improve their erosion resistance and durability, yet retain the ability to repair the parts as is common in the art. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention discloses and teaches restoration of durable coatings or surface treatments on metal substrates and how to overcome deficiencies of the prior art. 
     Various embodiments of this invention disclose and teach the following methods of how to: 
     Restore damaged durable coatings or surface treatments on metal substrates. 
     Restore damaged CVD, PVD, plasma spray, and reactive coatings or surface treatments on metal substrates. 
     Restore damaged functionally gradient coatings or surface treatments on metal substrates. 
     Spot-repair damaged durable coatings or surface treatments on metal substrates. 
     Restore protective surfaces on the damaged areas of substrates without excessive buildup of repair material on undamaged areas. 
     Spot-repair durable coatings or surface treatments while allowing smoothing and blending of local part damage to acceptable conditions or dimensions prior to conducting the surface repair. 
     Spot-repair durable coatings or surface treatments to restore the protective surface over weld repair areas on metal substrates. 
     Reduce the difficulty of removing protective top-coats from metal substrates as part of surface repairs. 
     Reduce the difficulty of removing protective top-coats from metal substrates in conjunction with spot-repair of coatings or surface treatments that lie between the top-coats and the substrates. 
     Other objects, features, and advantages of the present invention will become apparent upon reading the following detailed description of the preferred embodiment of the invention when taken in conjunction with the drawings and the appended claims. 
     All articles deriving from the methods disclosed in this invention are within the scope of this invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 represents a portion of a functionally gradient surface with damaged and undamaged areas. 
     FIG. 2 represents a time vs. treatment depth curve for reactive metals processes. 
     FIG. 3A represents a portion of a functionally gradient surface with damaged and undamaged areas. 
     FIG. 3B represents the surface of FIG. 3A after repair of the present invention. 
     FIG. 4 represents a repair treatment curve of the present invention compared to a reactive metals process used to form the original surface. 
     FIG. 5 represents the present invention as applied to airfoil portions of gas turbine engine compressor blades and vanes. 
     FIG. 6 represents the present invention as applied to stem areas of gas turbine engine compressor variable vanes. 
     FIG. 7 represents the present invention as applied to dovetail areas of gas turbine engine compressor blades. 
     FIG. 8 represents the present invention as applied to airfoil portions of gas turbine engine turbine blades and vanes. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Surface Repair Process 
     FIG. 1 represents a portion of a functionally gradient surface. Substrate atoms  1  comprise substrate  5 . Substrate atoms  1  may comprise a single metal or an alloy of several elements. Substrate atoms  1  and interstitial atoms  3  comprise gradient layer  7  and hard surface layer  9 . Interstitial atoms  3  may comprise a single element or multiple elements. Some interstitial atoms  3  in gradient layer  7  and surface layer  9  may be chemically combined with substrate atoms  1  to form compounds of the elements present. Surface layer  9  consists primarily of such compounds. Portions of gradient layer  7  and hard surface layer  9  are missing in damaged area  11 . Gradient layer  7  and hard surface layer  9  are intact in undamaged area  13 . 
     FIG. 2 represents a time vs. treatment depth curve  15  for a typical reactive metals process that could include but is not limited to boronizing, carburizing, nitridizing and carbonitridizing. In such processes, depth of treatment is dependant upon volume diffusion of interstitial atoms though the lattice of substrate atoms. Depth of treatment is proportional to the square root of time. Therefore, beginning treatment rate  17  is substantially higher than final treatment rate  19 . Diffusion of interstitial atoms slows as surface treatment depth increases, (hereby decreasing treatment rate as time progresses. 
     Referring to FIGS. 1 and 2, the outcome of applying the process of FIG. 2 to the damaged surface of FIG. 1 is as follows: The missing surface layer and thinned gradient layer in damaged area  11  present less of a diffusion barrier to additional treatment. Damaged area  11  therefore initially experiences a much higher rate of interstitial atom diffusion than undamaged area  13 , e.g., initial treatment rate  17 . As the process continues treatment rate slows to final rate  19 , and surface treatment depth in the damaged area increases to nearly the same depth as in the original undamaged area—refer to FIG.  3 . This results in repaired hard surface  21  and repaired gradient layer  23 . Note that damaged area  11  is not built-up to replenish the missing material, and that undamaged area  13  receives little additional treatment. The preferred reactive metals process for the present invention is enhanced plasma nitriding as taught in expired U.S. Pat. No. 4,460,415 (Korhonen). The use of this and other reactive metals processes to create such spot-repairs is not known in the prior art. 
     Repair Process Optimization 
     Minimal treatment of undamaged areas is extremely important from a surface repair standpoint. This means that damaged areas can effectively be “spot-repaired” without excessive build up in undamaged areas. This avoids problems associated with excessive surface treatment depth such as reduced fatigue strength. In fact, the repair treatment can be purposely made less efficient to ensure no additional treatment in undamaged areas. Refer to FIG.  4 . Treatment curve  15  from FIG. 2 is shown along with repair curve  25 . Original treatment time  27  establishes original treatment depth  29 . Repair curve  25  is selected to produce a slightly decreased depth of treatment than original treatment curve  15  for an equivalent treatment time. For example, in plasma nitriding this can be accomplished using process changes that include, but are not limited to higher vacuum chamber pressures and lower treatment voltages. Repair treatment time  31  can be selected to be slightly longer than original treatment time  27 . This produces repair treatment depth  33  that is nearly the same as original treatment depth  29 . This repair optimization ensures that depth of treatment for damaged areas is nearly the same as the original treatment depth. However no additional treatment of undamaged areas occurs since final repair treatment depth  33  is less than treatment depth  29  on the undamaged areas. Even if repair treatment time  27  were made significantly longer, the repair curve would not yield additional treatment in undamaged areas. The optimum repair process is defined as that which produces maximum treatment in damaged areas, minimum treatment in undamaged areas, all in minimum time. 
     EXAMPLE 1 
     Repair of Turbine Engine Blade Airfoils 
     One example application of the present invention is repair of durable surfaces for turbine engine airfoils. For instance, turbine engine compressor blades and vanes suffer from a multitude of degradation mechanisms including erosion, corrosion, impact damage, fretting wear and fretting fatigue. Erosion of airfoil portions of blades and vanes is common. Refer to FIG.  5 . Untreated compressor airfoil  35  has damage  37  on the airfoil leading edge. Damage  37  could be due to erosion or foreign object damage (FOD). Standard industry practice for maintaining and repairing uncoated compressor blades involves smoothing and blending minor damage, then returning the blades to service. U.S. Pat. No. 5,197,191 (Dunkman) describes this process. The smoothing and blending produces results represented by smoothed area  39  on airfoil  35 . 
     This process is supplemented using the present invention as follows: A durable functionally gradient surface is applied to airfoil  35  prior to placing it in service. Processes including, but not limited to boronizing, carburizing, nitridizing and carbonitridizing could be used. The functionally gradient surface increases the service life of the blade, but it eventually receives damage and must be repaired. Damage  37  on the airfoil is smoothed and blended per industry standard practice as represented by smoothed area  39 , then airfoil  35  undergoes the repair process of the present invention to restore the functionally gradient surface only in the damaged and smoothed areas. As can readily be seen, the repair process of the present invention is compatible with and enhances established industry practices for airfoil repair and use. 
     EXAMPLE 2 
     Repair of Turbine Engine Variable Vane Stem Areas 
     The present invention can also be used to repair the stem areas of variable stator vanes in gas turbine engine compressors. Refer to FIG.  6 . Variable stator vane  40  includes stem areas  41  and airfoil areas  42 . Also shown are bushing  43  and a portion of the engine casing  44 . Stem areas  41  act as rotating bearing surfaces for vane  40  during engine operation and therefore are subject to sliding wear. Stem areas  41  can be, and are preferably repaired simultaneously with airfoil areas  42  using the process described in Example 1. If airfoil areas  42  have experienced wear (erosion) and stem areas  41  have not, the present invention ensures repair of airfoil areas  42  whereas stem areas  41  receive little or no additional treatment. 
     EXAMPLE 3 
     Repair of Turbine Engine Blade Dovetail Areas 
     In Example 1, the focus was repair of the airfoil portion of a turbine engine blade. Dovetail areas of blades could also receive treatment as part of airfoil repair using the present invention. It is important to consider potential impacts of the present invention on repairing dovetail areas. This ensures the present invention does not conflict with existing operational or repair considerations. 
     Copper-nickel-indium and other soft anti-fretting and anti-wear coatings are commonly employed on compressor and turbine blade dovetails in the prior art. U.S. Pat. No. 5,368,444 (Anderson) describes the use of such coatings. Referring to FIG. 7, blade  45  has anti-fretting or anti-wear coating  47  applied to dovetail areas  49 . Coating  47  is often stripped and reapplied as part of blade repair, most often when excessive wear of coating  47  occurs. Dovetail areas  49  can be isolated from the repair process of the present invention by using masking and/or substrate holders that prevent reactive metals treatment of these areas. However, if coating  47  is excessively worn and must be stripped, the present invention is useful for expediting the stripping process. 
     The present invention is used to supplement repair of dovetail areas  49  as follows: Durable functionally graded surface  51  is applied to blade  45 , including dovetail areas  49 , prior to applying coating  47 . Coating  47  is then applied over functionally gradient surface  51  in the dovetail area, and the blade is placed into service. If coating  47  experiences excessive wear and must be stripped, functionally gradient surface  51  on the dovetail substrate makes these areas more resistant to erosion damage from the stripping process (e.g., grit blasting). Undesired minor dovetail area damage to surface  51  incurred while stripping coating  47  is then repaired using the present invention, preferably in conjunction with repair of airfoil portion  53  of the blade per Example 1. Coating  47  is then reapplied and blade  45  is returned to service. 
     EXAMPLE 4 
     Repair of Turbine Engine Blades With Thermal Barrier Coatings 
     Thermal barrier coatings are often used on turbine blades to protect the underlying metal substrate, and are also commonly stripped using processes including grit blasting as part of repair procedures. U.S. Pat. No. 4,576,874 (Spengler, et al) and U.S. Pat. No. 5,813,118 (Roedl, et al) describe thermal barrier coatings commonly employed. Referring to FIG. 8, thermal barrier coating  53  is applied to airfoil  55  of turbine blade  57 . A metallic bond coat  54  is often applied between thermal barrier coating  53  and airfoil  55 . Bond coat  54  is compositionally tailored to grow an adherent, predominately aluminum oxide scale to inhibit oxidation of the blade  57  and provide a satisfactory bonding surface for thermal barrier coating  53 . Dense overcoat  56  is also sometimes applied over thermal barrier coating  53 . Note that cooling holes  63  may be present in airfoil  55 . Coatings  53  and  56  are often stripped and reapplied as part of blade repair, most often upon excessive spalling of these coatings. Usual repair practice in the existing art is to strip coatings  53  and  56  using chemical and mechanical means while attempting to leave bond coat  54  intact. If bond coat  54  is damaged it too, must be stripped. U.S. Pat. No. 5,972,424 (Draghi, et al) discusses these repair procedures. Weld repairs of airfoil  55  can also be made as described in U.S. Pat. No. 5,686,001 (Wrabel, et al). 
     The present invention makes the stripping process more efficient, and is used to supplement repair of airfoil  55  as follows: Durable functionally graded surface  59  is applied to airfoil  55 , including fir tree portion  61 , prior to applying coatings  54 ,  53  or  56 . Coatings  54 ,  53  and if necessary  56  are then applied over Functionally gradient surface  59  on airfoil  55  and the blade is placed into service. When coatings  53  and  56  experience excessive wear and must be stripped, functionally gradient surface  59  on the airfoil substrate makes these areas more resistant to erosion (e.g., grit blasting). Undesired minor damage to surface  59  incurred while stripping coatings  54 ,  53  and  56  is then repaired using the present invention, preferably in conjunction with repair of fir tree portion  61  of the blade per Example 1. The present invention will also restore a functionally gradient surface over weld repair areas. The present invention does not clog cooling holes  63  as can occur with other coating processes. Coatings  54 ,  53  and if necessary  56  are then reapplied and blade  57  is returned to service. Note that bond coat  54  and dense overcoat  56  may be omitted without departing from the present invention. 
     Alternatives 
     It should be understood that the present invention is not restricted to repairing surfaces originally produced using reactive metals coating processes. Functionally gradient surfaces produced using CVD, PVD, plasma spraying and other processes can also be repaired. The repair process of the present invention can also be used to repair such surfaces, providing the elements present and their concentrations by depth are similar to those expected for the repair process. 
     CONCLUSION 
     Therefore it may be seen that the present invention includes many advantages, most notably the ability to spot-repair durable functionally gradient surfaces. 
     While this invention has been described in specific detail with reference to the disclosed embodiments, it will be understood that many variations and modifications may be effected within the spirit and scope of the invention as described in the appended claims.