Evaluation of Simple Aluminide and Platinum Modified Aluminide Coatings on High Pressure Turbine Blades after Factory Engine Testing

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-379 The Society shall not be responsible for stateents or opinions advanced in papers or in discussion at eetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Joual. Papers are available fro ASME for fifteen onths after the eeting. Printed in USA. Copyright 1991 by ASME Evaluation of Siple Aluinide and Platinu Modified Aluinide Coatings on High Pressure Turbine Blades after Factory Engine Testing JEFFREY A. CONNER, DAVID A. MOORE and ROGER D. WUSTMAN Engineering Materials Technology Laboratories GE Aircraft Engines Cincinnati, OH ABSTRACT This paper presents results fro recent factory engine testing of siple aluinide coatings produced using pack ceentation processes and platinu odified aluinide coatings produced using both pack ceentation and cheical vapor deposition processes. These coatings were evaluated on DS nickel base superalloy high pressure turbine blades in a coercial high bypass turbofan engine. Operating conditions were such that turbine inlet air contained up to 3 ppb of sodiu. Details of the factory engine testing, coating selection and application, and environental protection provided by the coatings are highlighted. Future testing plans are also presented. INTRODUCTION The superior environental resistance of platinu odified aluinide coatings versus conventional aluinide coatings on nickel base superalloys is well docuented for a range of oxidizing and corrosive environents. Initial work by Bungardt et al. (1972) and extensive follow-up work by nuerous investigators has shown considerable iproveent in both oxidation and high teperature hot corrosion resistance of aluinide coatings on any substrates with the addition of platinu. Lehnert and Meinhardt (1972) reported a 4X iproveent in oxidation resistance and a 2.3X iproveent in high teperature hot corrosion resistance of aluinide coatings with the addition of platinu. Mach 1. oxidation testing at 275 F (1135 C) and 215 F (1177 C) and hot corrosion testing at 17 F (927 C) (5 pp sea salt added to the fuel) of platinu aluinide coatings by GE on a range of nickel base superalloys used for gas turbine engine coponents has shown coating life iproveents ranging fro 2X to 5X versus siple aluinides depending on substrate cheistry, coating thickness, coating coposition and test condition. As gas turbine design has becoe ore coplex, correlation of laboratory testing with actual service conditions has becoe increasingly ore difficult. Today's high pressure turbine coponents cycle through varying teperature profiles, experience severe theral and echanical stresses and operate in a range of environental conditions. A single coponent can be alteately exposed to oxidizing and corrosive environents and often ay have specific locations operating in both regies. Variations in operating characteristics between different engine types, changes that occur over the life of a coponent, and varied operating locations further increase the range of factors associated with "protective coating life". Service evaluations of environentally protective coatings in aircraft engines have been initiated by any (Malik, 198; Johnson and Richards, 1982; Betteridge and Wing, 1986; Cocking et al., 1988) to include all of the factors influencing protective coatings. However, for sufficient inforation to be gained fro service evaluations, coated hardware usually reains in service for ties exceeding three years preventing tiely introduction of successful coatings. A third testing option, factory engine testing, has great potential. FACTORY ENGINE TESTING GE Aircraft Engines is successfully using the approach of factory engine testing to overcoe any of the inherent shortcoings of laboratory testing and still provide tiely results for evaluation of environentally resistant coatings on actual engine hardware. Coated coponents are cycled through noral flight engine segents ('C' cycles) representing aircraft taxi, take-off, clib, cruise and thrust reverse during landing. Tie spent under cruise conditions is iniized since cruise Presented at the Inteational Gas Turbine and Aeroengine Congress and Exposition Orlando, FL June 3-6, 1991 Downloaded Fro: on 11/11/218 Ters of Use:

2 conditions do little to degrade high pressure turbine coatings. Degradation by oxidation proceeds at very slow rates at cruise teperatures and salts deposited during taxi and take-off which lead to hot corrosion are evaporated prior to cruise. Factory engine testing also reoves the requireent to generate revenue allowing nuerous cycles to be perfored in uch shorter tie periods. Metallurgical evaluation of various coated surfaces after testing is used to docuent coating perforance under engine conditions. GE's factory engine testing uses the latest generation of advanced coercial and ilitary engines as test vehicles. Both high pressure and low pressure blades and vanes are tested using various alloys and coatings. The results discussed in this paper are derived fro testing of Stage I high pressure turbine blades ade fro an advanced DS nickel base superalloy (containing hafniu) in a coercial high bypass turbofan engine. Testing consisted of 1464 'C' cycles (See Figure 1) at a location where turbine inlet air contained up to 3 ppb of sodiu. Aluinide A was applied by pack ceentation at 1975 F (8 C) (high teperature low activity aluiniding). Coating thickness was controlled by liiting the available aluinu in the pack. Thickness build-up occurred at the leading and trailing edges (typical for HTLA pack ceentation processes). Aluinu oxide (Al23) particles fro the pack ix were entrapped in the additive layer of the coating during the coating process by the outward diffusion of nickel fro the substrate. Aluihide B was also applied by pack ceentation at 1975 F (8 C). Coating thickness was controlled by liiting the aount of pack powder adjacent to the part. The resulting coating structure was a ixed inward/outward structure. The pack concentration (i.e. aluinu content) for Aluinide B was six tie greater than the concentration used for Aluinide A. Thickness build-up occurred at the leading and trailing edges. Platinu Ahiinide Coating TAKE OFF aii REVERSE APPIlAE11!.E MIN. 7.E M.41. I TON START/ RESFART Ii I z b Tie - MEnute; Figure 1. Factory Engine Test 'C Cycle COATING SELECTION Four aluinide coatings were selected for evaluation via factory engine testing: two variations of siple aluinide coatings deposited by pack ceentation currently used by GE and platinu odified aluinide coatings deposited by pack ceentation and cheical vapor deposition. Qualification of coatings prior to engine installation consisted of etallurgical evaluation of Stage I HPT blades for coating thickness, distribution, coposition and icrostructure. The coated blades were inspected to the stringent requireents placed on hardware for flight engines. The thickness range for all four coatings was ils with a target noinal thickness of 2. ils. Platinu aluinide coating was applied to turbine blades using pack ceentation and cheical vapor deposition aluiniding processes. Platinu was deposited to a thickness of ils by electroplating and diffused into the substrate prior to aluiniding. The resultant icrostructure for the pack ceentation version was a two phase (PtAl 2 in a 13 NiAl atrix) outer layer, a iddle zone of 13 NiAI and a diffusion zone. The CVD PtAI coating consisted of an additive layer coprised of Ni(Pt)AI and a diffusion zone. Additive layer to diffusion zone ratio for the pack PtAI coating was 6:1 indicating an inward coating structure; the ratio for the CVD PtAl coating was 1.6:1 indicating an outward coating structure. Typical coating icrostructures are shown in Figure 2. Copositions for each of the coatings were deterined by icroprobe wavelength analysis. Coposition easureents were initiated at the surface of the coating and were taken in five icron intervals through the coating into the substrate. A five icron spot size was used for all analysis. Coating copositions prior to engine operation are presented in Figure 3. ENGINE TEST RESULTS Following factory engine testing for 1464 'C' cycles, the Stage I HPT blades were reoved fro the engine for evaluation. Visual review of the blades revealed heavy surface deposits on all blades. Areas of localized coating distress were seen on soe blades along leading edges and on the concave side at the tip and at id-chord above the 5% span. Representative blades with each of the four coatings were selected for destructive evaluation (See Figure 4). 2 Downloaded Fro: on 11/11/218 Ters of Use:

3 ALUMINIDE A ---o-- AI ^ 14 Aluinide A Distance Fro Surface (icrons) ALUMINIDE B ---a-- AI 9 -f Distance Fro Surface {icrons} PACK PLATINUM ALUMINIDE Distance Fro Surface (icrons) CVD PLATINUM ALUMINIDE 22 ils r..,,:..', ter Ni(Pt)A1 Diffusion Zone 9 a. 2 a--- Al T PtI Substrate CVD Platinu Aluinide Distance Fro Surface (icrons) Figure 2. Typical Coating Microstructure Prior to Engine Testing Figure 3. Coating Copositions Prior to Engine Testing 3 Downloaded Fro: on 11/11/218 Ters of Use:

4 Aluinide A Pack Platinu Aluinide Aluinide B CVD Platinu Aluinide Figure 4. Representative Blade of Each Coating After Engine Testing Downloaded Fro: on 11/11/218 Ters of Use: 4

5 Coplete coating penetration resulting in substrate attack was found on the blades coated with Aluinide A and Aluinide B in localized areas at and above the 5% span on the concave side. Maxiu observed substrate attack was on the order of 4.7 ils. The ajority of the coating on the upper half of the concave side of the blades was copletely transfored to y' (Ni 3 AI) indicating substantial loss of aluinu. This depleted coating would provide liited additional environental protection. Coating below the 5% span on the concave side of the blades and the ajority of the coating on the convex side was still protective. Platinu in' Coatings No coating penetration was found on the blades coated with platinu aluinide coating. Regions of transfored coating (13-NiAI -3 y') were found along the leading edge at the 5% span and at the tip on the concave side of both blades (regions of substrate attack on the blades coated with siple aluinides). The pack PtAI coating exhibited a single phase Ni(Pt)AI additive layer after engine exposure. Figure 5 shows the observed coating perforance of each of the coatings as a function of location on the blades. Figure 6 & 7 show the coposition of each of the coatings after engine testing at the concave leading edge and the convex trailing edge near the platfor, respectively. DISCUSSION Microstructural exaination after engine testing confired the superior perforance of the platinu odified aluinide coatings versus the siple aluinide coatings for these engine test conditions. The identified attack echanis was cyclic oxidation/hot corrosion with oxidation being the priary contributor. Hotter areas on the blades like the leading edge apparently experienced degradation solely by oxidation. Coparison of the coposition profiles for each coating along the leading edge revealed substantial aluinu reoval fro the coatings. Surface aluinu contents were reduced to weight percent or less in the siple aluinide coatings. Surface aluinu contents for the platinu aluinide coatings were around 17 weight percent. As expected, coposition profiles fro the convex trailing edge near the platfor (area of inial coating attack) were soewhat different. Surface aluinu contents of the siple aluinide coatings were close to original values. Surface aluinu contents of the platinu aluinide coatings were around 18 weight percent. The higher aluinu contents reaining in the platinu odified coatings in areas of severe environental attack result fro the use of platinu odification and the presence of hafniu in the substrate (Schaeffer, et. al., 1989). The final coposition profiles of the two platinu aluinide coatings are rearkably siilar considering the large variance in the initial copositions indicating a tendency toward equilibriu for these test conditions. The initial variation in coposition for the pack ceentation platinu aluinide coating versus the CVD coating can be directly attributed to: (1) tie at teperature during coating foration and (2) aluinu activity during coating foration. The pack coating was an inward coating with a two phase (PtAI2 in a 13 NiAl atrix) icrostructure fored using higher aluinu activity, shorter coating tie, and lower coating teperature copared to the CVD coating. The CVD coating was an outward coating with a single phase Ni(Pt)AI icrostructure. During exposure to engine teperatures, nickel diffusion fro the substrate into the coating (Goward and Boone, 1971) as well as aluinu reoval fro the coating to for an oxide scale causes the coating icrostructure to change fro the as-coated structure to a Ni(Pt)AI structure. CONCLUSIONS 1 The platinu odified aluinide coatings clearly outperfored the siple aluinide coatings in the cyclic oxidation / hot corrosion environent encountered in this testing. 2. The two versions of platinu aluinide coating evaluated in this testing perfored alost identically. A larger aount of transfored coating [13 Ni(Pt)AI ) y'] was found along the leading edge of the CVD coating but this difference is relatively sall in ters of total surface area coated. 3. Relative ranking of the two siple aluinide coatings is not possible since both coatings were copletely penetrated during this testing. 4. Copositional analysis of all four coatings after engine exposure indicates that the platinu odified coatings retained higher aluinu contents at the surface in areas where environental attack was the ost severe. FUTURE FACTORY ENGINE TESTING Additional factory engine testing is scheduled for three different high bypass turbofan engines. Coating evaluations have been expanded to include different substrates, new coating suppliers, new processes and new coatings. This round of testing will focus on the effects of aluiniding processes and copositional variations on coating perforance, as well as ranking coating systes based on environental protection provided during engine operation. Coating systes will include siple aluinides and two versions of precious etal odified aluinides. Gas phase aluiniding processes will be highlighted based on the potential for coating inteal surfaces with these ethods as well as to further evaluate clais that Downloaded Fro: on 11/11/218 Ters of Use:

6 1 1 ' - v O. O Pack Platinu Aluinide Protective Coating Depleted Coating Base Metal Attack Figure 5. Perforance of Coatings as a Function of Location on Concave Airfoil Downloaded Fro: on 11/11/218 Ters of Use: 6

7 ALUMINIDE A s-- AI C a` C ALUMINIDE A At Distance Fro Surface (icrons) Distance Fro Surface (icrons) ALUMINIDE B ALUMINIDE B -- Al ^W AI 8 a Distance Fro Surface (icrons) o r I Distance Fro Surface (icrons) PACK PLATINUM ALUMINIDE PACK PLATINUM ALUMINIDE a $ Pt --- a 4 Pt a U 2 ^' 1D Distance Fro Surface (icrons) Distance Fro Surface (icrons) CVD PLATINUM ALUMINIDE -- ^ P1t CVD PLATINUM ALUMINIDE - Au Pt C, 2 so ' Distance Fro Surface {icrons) Distance Fro Surface (icrons) Figure 6. Coating Copositions After Engine Testing Figure 7. Coating Copositions After Engine Testing - Concave Leading Edge - Convex Trailing Edge Near the Platfor 7 Downloaded Fro: on 11/11/218 Ters of Use:

8 platinu aluinide coatings fored by gas phase aluiniding are superior to pack ceentation versions (Shankar, 1985; Sith and Boone, 199). REFERENCES Bettridge, D.F. and Wing, R.G., 1986, "Buer Rig and Engine Test Experience of Platinu Aluinide Coatings", presented at the 31st ASME Inteational Gas Turbine Conference and Exhibit, Dusseldorf, Gerany. Bungardt, K., Lehnert, G., and Meinhardt, H., 1972a, "Protective Diffusion Layer on Nickel and/or Cobalt- Based Alloys", U.S. Patent #3,677,789. Cocking, J.L., Richards, P.G., and Johnston, G.R., 1988, "Coparative Durability of Six Coating Systes on First-Stage Gas Turbine Blades In The Engines Of A Long-Range Maritie Patrol Aircraft", Surface and Coatings Technology, Volue 36, pp Goward, G.W. and Boone, D.H., 1971, "Mechaniss of Foration of Diffusion Aluinide Coatings on Nickel- Base Superalloys", Oxidation of Metals, Vol. 3, pp Johnson, G.R., and Richards, P.G., 1982, "The Relative Durabilities of Conventional and Platinu-Modified Aluinide Coatings in an Operational Gas Turbine Engine," Proceedings of the Syposiu on Corrosion Ia Fossil Fuel Systes, ECS, Detroit, MI. Lehnert, G. and Meinhardt, H.W., 1972, "A New Protective Coating For Nickel Alloys", Electrodeposition Surface Treatent, Vol. 1, pp Malik, M., 198, "Perforance Evaluation Of High Teperature Coatings in Transport Aircraft Gas Turbines", Final Report - European Concerted Action COST-5, Materials For Gas Turbines Round II, D-11. Schaeffer, J., Ki, G.M., Meier, G.H. and Pettit, F.S., 199, "The Effects of Precious Metals on the Oxidation and Hot Corrosion of Coatings, The Role of Active i i n Behaviour of High Teperature Metals and Alloys, ed., Lang, E., pp Shankar, S., 1985, "Methods of Foring a Protective Diffusion Layer on Nickel, Cobalt and Iron Base Alloys", U.S. Patent 4,51,776. Sith, J.S. and Boone, D.H., 199, "Platinu Modified Aluinides-Present Status", presented at the Gas Turbine and Aeroengine Congress and Exposition, Brussels, Belgiu. Downloaded Fro: on 11/11/218 Ters of Use: