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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St, New York, N.Y GT-435 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of Its Divisions or Sections, or printed in its publications. Discussion is printed only it the paper is published in an ASME Journal. Authorization to photocopy material for internal or personal use under circumstance not falling within the fair use provisions of the Copyright Act is granted by ASME to libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of $0.30 per page is pald directly to the CCC, 27 Congress Street, Salem MA Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department Copyright by ASME All Rights Reserved Printed in U.S.A. Engine Test Experience with HVOF WC-Co Coated Fan Blade Dampers Lars Pejryd and Jan Wigren Volvo Aero Corporation Trollhattan, Sweden ,0) ABSTRACT Tungsten carbide thermal spray coatings are important to the aerospace industry for reducing wear on jet engine components, fanblade mid-span dampers being one example. However, the fatigue life of a component is often reduced when a coating is applied and for some cases the coating can fail due to spallation and cracking. Coating failures can result in decreased engine performance and costly maintenance. To provide insight and possible explanations for the reduced service life of coated midspan dampers, identify the best coating and application processes for future use, and to develop methods for improving coating performance, a comprehensive experimental research program was conducted. The program involved coating performance in jet engine tests, coating crack resistance in bending, low cycle fatigue properties of the coating and substrate, and microstructures for a wide range of coating compositions and application processes. Eleven coatings Were ranked according to their performance relative to the other coatings in each evaluation category. Five of the coatings were selected for engine test runs. Results from the engine test runs for more than 800h (AMTcycle) were compared to bend and low cycle fatigue evaluations and to meassured residual stresses. Strong correlation between engine performance and the residual stresses in the coatingsubstrate system was found. Results from the program were used for selecting a suitable coating system for final in-service use. vibration). Figure 1 is a photograph of mid-span dampers on a fan blade. These contact points are subjected to impact wear. To reduce this wear tungsten carbide thermal spray coatings are applied. While coatings have controlled the wear of mid-span dampers, coating failures have been observed due to cracking and spallation resulting from cyclic fatigue and impact. The result is a loss of coating performance or cracks propagating through the mid-span damper resulting in loss of the damper and in some cases complete engine failure. 1 INTRODUCTION Mid-span dampers are often used to control vibration of fan and compressor blades in jet engines. A mid-span damper provides a contact point between blades to constrain lateral and torsional motion which could result in flutter (damaging blade Figure 1. Photograph of mid-span dampers on a fan blade. Presented at the International Gas Turbine and Aeroengine Congress & Exhibition Birmingham, UK June 10-13,1996 Downloaded From: on 12/26/2017 Terms of Use:

2 Residual stresses have in many racps been identified as, or suspected to be a contributing factor of shortened service life for thermal spray coatings [1], [2]. Residual stresses in thermal spray coatings have been linked to bending strength, fatigue life, bond strength and microstructures [3], [4], [5]. The magnitude of the residual stress and whether it is tensile or compressive can have a significant influence on coating performance. A compressive residual stress zone near the surface of a material, can hinder surface crack initiation and slow down crack propagation [6], [2]. Fatigue behavior of thermally sprayed parts can be considered as a combination of coating and substrate performance [7]. The fatigue characteristics of a thermally sprayed part can in some situations be linked to the residual stresses in the coating and substrate. In some cases, a coating can have a high degree of crack resistance due to compressive residual stresses in the coating. When the coating is in compression, the substrate will likely have a tensile residual stress at the coating/substrate interface which could result in accelerated substrate cracking once a crack has propagated through the coating. Possible mechanisms for cracks initiating in the substrate have been reported by Hwang et. al. [8] and by Rakitsky et. al. [9]. Hwang et. at [8] contend that cracks do not propagate from the coating into the substrate, instead substrate cracks initiate at nonspecific locations. They [8] then conclude that the fatigue life of a part should be based on the substrate properties if coating cracking is acceptable. In order to get a better understanding of these questions an experimental program adding to the engine testing a range of other tests, including residual stress measurements, was identified. 2. EXPERIMENTAL PROGRAM The research program consisted of the testing of coating behaviour in jet engine performance as well as in-depth evaluations of coatings including the following tests and property evaluations: low cycle fatigue, bending crack resistance, residual stresses, impact and abrasive wear, bond strength, porosity, hardness, and microstructures. Five of the coatings were evaluated both in engine test and for material properties while the other six coatings were evaluated on material properties only, Table 1. Wear resistance was found to be acceptable for all but the most porous coatings. However, results from the fatigue, bending, and residual stress evaluations varied significantly among the coatings. This paper concentrates on the behaviour in engine tests and the correlation of this to properties such as residual stress, crack resistance in bending, and low cycle fatigue since the characteristics represented by these evaluations support a linkage with in-service observations and jet engine performance evaluations. The coating materials selected for this study were tungsten carbide thermal spray coatings with cobalt, nickel, or Table 1. Tungsten carbide coating system designations. Coating Application Process Composition HVOF Plasma ABCDEF G H 1- (WC-Co) ox o x o o o x 2- (WC-Ni) x x 3- (WC-Co/C0 x o = coating tested in engine and material properties evaluation, x = material properties evaluation only. cobalt/chrome binders. The exact coating compositions and application processes are proprietary and can not be reported in detail. The general coating composition and application processes are listed in Table 1. The HVOF processes are labeled A to G and the plasma spray process is labeled H. A coating composition of 1 is for WC-Co, 2 is for WC-Ni, and 3 is for WC- Co/Cr. As an example, a coating system designation of A-1 indicates the application process is HVOF process "A" with a coating composition of WC-Co. The substrate for all evaluations was Ti-6A1-4V. 2.1 Engine Test The engine test was performed as a "rainbow" test with five of the coatings as shown in Table 1 (marked: o) in the "starting" fan. The engine was run at Sea Level Static conditions repeating a cycle of aproicimately 30 minutes according to Fig. 2. The test was performed in three different test periods, Tests 1-3 (Table 2) at 260, 294 and 280h respectively, with intermediate inspection of the fan blades. TIME Figure 2. "Typical" test cycle, total time approximately 30 minutes. The inspection after each test period consisted of optical examination and Eddy Current (EC) inspection. Fan blades with EC indication or spallation were removed and examined metallographically. Missing fan blades were then replaced by 2 Downloaded From: on 12/26/2017 Terms of Use:

3 new blades. The choice of coating type on the new blades was based on the results from the preceeding tests. The number of blades of each type used in the different engine test periods is shown in Table 2. Table 2. Number of blades of each coating type used per engine test. Test time/h CENTRIFUGAL FORCE 4 mm CRACKS A-1 C-1 E-1 F-1 G-I Test Test Test Other Evaluations Apart from the engine test the coatings were also examined with respect to: -Crack initiation resistance in bending. -Crack initiation resistance in fatigue loading. -LCF life of precracked coating systems. -Through thickness residual stress measurements. -Microstrucnue. The results from these investigations were reported by Pejryd et. al. [10]. A short description of the methods used is however given here. A three point bend test according to Fig. 3 was used for determining coating crack resistance under a bending load. The test was designed to simulate the bending strains that the midspan dampers experience in service due to centrifugal forces. The coating is applied to one edge of a test specimen having the same width as a mid-span damper. A central load is applied to the noncoated top surface and coating cracks are observed on the coated edge. After bending the samples were checked for cracks using a standardised fluorescent penetrant inspection (FPI) method under UV-light. FAN BLADE/ MID-SPAN DAMPER THREE-POINT BEND SPECIMEN CENTRAL LOAD 4 mm coating was obtained. The load step used was corresponding to a step in strain of 0.1%. The cracks were detected with FPI. The "precraciced" systems was then subjected to an "ordinary" LCF test. Microstructural evaluations were performed on as sprayed non-tested specimens of each coating type using cold mounting techniques and standard metallographic procedures. Additional microstructural evaluations on fracture surfaces in bending and fatigue were also performed. Through-thickness residual stress distribution in each coating and substrate system was measured using the modified layer removal method [11]. 3. RESULTS AND DISCUSSION 31 Ermine Test Results from the engine test are shown in Table 3. The first engine test period was 260h. The results show : Coating A-1, one blade out of six shows on one of the dampers a small spallation ( <5% of the surface). Coating C-1, two blades out of six show spallation on one damper each (10 and 5% respectively). Coating E-1, one out of seven was intact, all others show spallation (5-10%) or EC indications or both. Coating F-1, two blades out of six show spallation on one damper each (10 and <5% respectively). Coating G-1, three blades out of 15 show spallation and two show EC indications. Additionaly the E-1 coating showed high porosity levels and was rejected from further testing. The other coatings all continued but to a varying degree. All six A-1 coated fan blades continued to the second test, five out of six C-1 coated fan blades continued, five out of six F-1 coated fan blades continued, and 13 out of blades continued. Table 3. Number of blades with failure indications (causing rejection) from each engine test divided by the number of tested blades. A-1 C-1 E-1 F-1 G-1 Test 1 0/6 1/6 6/7 1/6 2/15 Test 2 0/6 3/6 1/6 15/21 Test 3 0/20 0/10 CENTER SUPPORTS Figure 1 Illustration of 3-point bend evaluation. In the LCF crack initiation test the samples were first loaded with a stepwise increase in load until cracking of the For the second test period (294h) one new blade each of C- I and F-1 and 8 new 0-1 blades were added. The results were: The A-1 coated blades show no further damage compared to what was detected after test period 1. The C-1 blades show EC indications on three and one show slight spallation. 3 Downloaded From: on 12/26/2017 Terms of Use:

4 The F-1 coated blades show EC indications on one blade and spallation on one. The G-1 show EC indications on 8 blades and spallation on three. For the third engine test period (280h), only two coatings were selected, A-1 and F-1. For the A-1 coated blades, five were in the engine for the third time, the other 25 were in this test run for the first time. The F-1 coated blades had a similar story, five blades for the third time and five new blades. No further damage as compared to engine test no. 2 was detected for the "old" A-1 coated blades. For the 25 new blades only one slight spallation (<5%) was detected. The F-1 coating one "long time" blade show a new slight spallation while the spallation on one of the blades from test no. 2 had not grown. The five new F-1 coated blades were all intact after this test. are parallell to the coating/substrate interface (Fig. 5) while for the other coating systems the cracks are perpendicular to the Substratei Figure 5. Cracks parallel to the coating/substrate interface appering in the coating system F-1.. substrate surface (Fig. 4). The reason for this is high residual compressive stresses in the coating as discussed below. The behavior in the engine test was used to rank the coating systems with respect to each other. Table 4 gives the ranking of engine performance, bending and LCF. a) EC signal Figure 4. Typicall EC indication and microstructure of the coresponding crack for a rejected fan blade. Coaling system G-I. The damages seen in the coatings were spallations and EC indications connected to cracks. In most cases the cracks were perpendicular to the coating/substrate interface and penetrating through the coating and into the substrate (Fig. 4.). This is of course detrimental to the life of the damper. The F-1 system on the other hand showed cracks that where parallell to the substrate surface, Fig.5. A summary of the results is given in Table 3, where the number of fan blades with failure indications divided by the number of tested in each test run is shown. Two of the coating systems show a very good behaviour with long life in the engine test Only minor damages was seen. The A-1 system show in two cases a slight spoliation, none however resulting in rejection of the blades. It is interesting to note that the cracks in F-1 coatings 3.2 Bend and LCF Tests The three-point bend cracks show a high degree of resemblance with the cracks on the blades as shown in Figures 3 & 4. A ranking was made for each coating system based on the spacing and lengths (as they appear on the coating surface) of cracks where a coating with the lowest number of relatively short cracks would receive a ranking of 1. From the LCF test two different results, were obtained, 1) strain levels for initiating cracks after 1(Y00 cycles, and 2) LCF life after initiated cracks in the coating. Table 4 lists the three point bend rankings for all coating systems together with the ranking in LCF performance and in engine performance. Table 4. Ranking of the coating system behavior in engine, bendin LCF tests and residual comuressive SYSTEM ENGINE BEND LCF INIT. LCF RES. STRESS A B C D E F I 8 1 G H A D E Downloaded From: on 12/26/2017 Terms of Use:

5 3.3 Residual Stress Distributions Details of the stress situation in the different coaling/substrate systems is given in Pejryd et al. [10]. Figure 6 shows the typical through-thickness residual stress distributions in the longitudinal direction, cr, for two of the coating systems, E-3 and F-1. The result for the E-3 coating system is tensile residual stresses in the coaling and compressive residual stresses in the substrate near the interface. The result for the F-1 coating system is compressive residual stresses in the coating and tensile residual stresses in the substrate. 400 I am7inc %I:010710TE INTERFACE HVOF E4 IIVOF F-1 TAM (MUSE L. Celina meo Dahr 0311W/CII DISTANCE FROM SURFACE, wan Figure 6. Typical through-thickness residual stress distribution for coatings E-3 and F Correlations Results from the engine tests suggest that coating systems with compressive residual stresses in the coating have extended lives over coatings with tensile residual stresses. However, the coating system with the highest magnitude of compressive residual stresses in the coaling (-822 MPa), F-1, show spallation failure (Fig. 7.) and cracking parallel to the coating/substrate interface (Fig. 5) after engine testing. An explanation for the spallation failures of this coating is believed to be the high level of compressive residual stress in the coating that may have superceeded the ultimate compressive strength of the thermal spray tungsten carbide material. Therefore, a coating with a lower magnitude of compressive residual stress (approximately 250 Mpa lower) would be less susceptible to spallation since the stress would likely be further away from the ultimate compressive stress limit Other coatings with lower magnitudes of compressive stress, such as A-1, had a carisfactory engine performance. Coatings with tensile residual stresses, such as E-1, experienced coating cracking and spallation failures that rendered their performance unsatisfactory. This coaling was also verry porous, thus giving caracteristics more like a plasma sprayed coating rather than a HVOF coating. The coatings with compressive residual stresses also had the best bend rankings while coatings with tensile residual stresses had reduced crack resistance. A comparison of the average coating residual stresses to the bend rankings for each coating system as well as LCF performance is given in Table 4. These results suggest that compressive residual stresses in the coating will greatly enhance coating crack resistance in bending. The result from the bend test also supports the conclusion that coating compressive stresses are essential for satisfactory mid-span damper performance as long as the sum of the inservice stress and the residual stress is below the level which induces spallation. This is especially true since the cracks in the bend test in most cases show a high degree of resemblance with the cracks in engine tests. The fatigue life of precracked systems however behaves in quite the opposite way, to the bend tests as discussed by Penyd et. al. [10]. In this case substrates with compressive residual stresses can have a significantly improved fatigue life over substrates with tensile residual stresses. This is an interesting result since coating systems with compressive residual stresses in the substrate will likely have tensile residual stresses in the coating which results in reduced crack resistance for the coating. Figure 8 summarizes the relation 4C0 REDUCED REDUCED SUBSTRATE MACK RESISTANCE F * 71GUE LIFE \ IMPROVED SUBSTRATE FATIGUE LIFE WISTATI MAR mono IMPROVED a wnne ti,...crack RESISTANCE Cestart/Santrutt Week:, DC: Abl: nal Para DISTANCE FROM SURFACE, ma Figure 7. Spallations in F-1 coated blades after engine test. Figure 8. Relation of through-thickness residual stresses to cracking resistance of the coaling and fatigue properties of the substrate. 5 Downloaded From: on 12/26/2017 Terms of Use:

6 of residual stresses to coating crack resistance and substrate fatigue life. Based primarily on the engine tests but also on the bend results, fatigue life and residual stress evaluations, the primary coating system selected for mid-span damper production is A-1. The A-1 coating system was selected berause it has a level of compressive residual stress in the coating that does not induce spallation and has had the best jet engine performance evaluations. While the substrate fatigue life of this coating system may be lower than that of other candidate coating systems, the level of coating compressive residual stress is large enough to suppress coating crack initiation which could lead to crack propagation into the substrate. The possibility of reduced substrate fatigue life has been considered for production mid-span damper life evaluations. 5. SUMMARY AND CONCLUSIONS An HVOF coating for the mid-span dampers of fan blades has been developed as a result of an experimental research program at Volvo Aero Corporation, Sweden. The program included evaluation of engine performance, microstructural evaluations, hardness, wear resistance, LCF, bend testing and residual stress measurements. Residual stresses were evaluated at The University of Tulsa, USA. The new coating exhibited resistance to crack initiation by having compressive residual stresses. Extensive compressive residual stresses, however lead to spallation problems in the engine, whereas tensile stresses ultimately end up in damper failure. Thus the residual stress condition must be carefully optimised for each application in order to take the full advantage of the possibilities of the coating system. ACKNOWLEDGMENTS The authors acknowledge the cooperation with Daniel Greying, Edmund Rybicld and John Shadley, all from the University of Tulsa on the subject of residual stresses and Robert Keiser of Allied Signal Engines in Phoenix, Arizona for his advice and input to this project. Coatings", pp of Thermal Spray: International Advances in Coatings Technology, Ed CC. Berndt, ASM huanational, Materials Park, OIL USA. (1002). [4] Tobe, S., ICodarna, S., Misawa, H., and Ishikawa, K, 'Rolling Fatigue Behavior of Plasma Sprayed Coatings on Aluminum Alloy", pp of Thermal Spray Research and Applications, Ed. T.F. Bemedd, ASM International, Materials Park OIL USA, (1991). [5] Greying, DJ, Shadley, JR., and Rybicki, ES 'Effects of Coating Thickness and Residual Stresses on the Bond Surnoh of ASTM C Thermal Spray Coating Test Specimens", To be published in The Journal of Thermal Spray Technology, ASM hibernation& Materials Park, OIL (1994). [6] Sharp, P.K, Clayton, J.Q., and Clark G., 'The Fatigue Resistance of Peened Aluminum Alloy-Repair and Re-Treatment of a Component Surface", Fatigue Frac:. Engng Mater. Strict., 17[3] (1994). Hashem, A.M., and Aly, LH., "High-Cycle Fatigue Life of Coated Low- Carbon Steer', Fatigue, 16, (1994). Hwang, J.U., Ogawa, T., and Tokaji, K. "Fatigue Strength and Fracture Mechanisms of Ceramic-Sprayed Steel in Air and a COITOSIVC Environment", Fatigue Fratt Engng Mater Strum, 17[7] (1994). Ralcitslcy, AA, De Los Rios, ER., and Miller, KJ., 'Fatigue Resistance of a Medium Carbon Steel with a Wear Resistant Thermal Spray Coating", Fatigue Fract. Engng Mater Sinai., 17[5] (1994). Pejryd, L. Wigren, J., Greying, DJ., Rybidd, ES, and Shadley, " Residual Stresses as a Factor in the Selection of Tungsten Carbide Coatings for a Jet Engine Application." 3 Thermal Spray, Vol 4, (1995). Greying, DJ., Rybicki, RE., and Shadley, JR_ Though-Thickness Residual Stress Evaluations for Some Thermal Spray Coatings of Industrial Importance Using a Modified Layer Removal Method" To be published in The Journal of Thermal Spray Technology, ASM hittmational, Materials Park. OIL (1994). REFERENCES [1] Gudge, M., Rickerby, D.S., ICingswell, KT., and Scott, T "Residual Stress in Plasma Metallic and Ceramic Coatings", pp of Thermal Spray Research and Applications, Ed. T.F. Bemedd, ASM International, Materials Park, OH. USA. (1001). [2] Morishita, T.. Whitfield, R.W., Kuramode, E., and Tanabe, S., "Coatings with Compressive Stress", pp of Thermal Spray: International Advances in Coatings Technology, Ed. CC. Berndt, ASM International, Materials Park, OH, USA, (1992). [3] Kuroda, S., Fukushima, T., and!wham, S., "Significance of the Quenching Stress in the Cohesion and Adhesion of Thermally Sprayed 6 Downloaded From: on 12/26/2017 Terms of Use: