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Performance of laser-glazed zirconia thermal barrier coatings in cyclic oxidation and corrosion burner rig tests
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METALLURGICAL AND PROTECTIVE COATINGS
PERFORMANCE OF LASER-GLAZED ZIRCONIA THERMAL BARRIER COATINGS IN CYCLIC OXIDATION AND CORROSION B U R N E R R I G TESTS* I. ZAPLATYNSKY National Aeronautics and Space Admin&tration Lew& Research Center, Cleveland, 0H44135 (U.S.A.)
(Received March 22, 1982; accepted April 5, 1982)
The performance of laser-glazed zirconia (containing 8 wt.% Y 2 0 3 ) thermal barrier coatings was evaluated in cyclic oxidation and cyclic corrosion tests. Plasma-sprayed zirconia coatings of two thicknesses (0.02 and 0.04 cm) were partially melted with a CO2 laser. The power density of the focused laser beam was varied from 35 to 75 W m m -2, while the scanning speed was about 80 cm m i n - 1. In cyclic oxidation tests, the specimens were heated in a burner rig for 6 min and cooled for 3 min. The results obtained indicated that the laser-treated samples had the same life as the untreated samples. However, in corrosion tests, in which the burner rig flame contained 100 p p m N a fuel equivalent, the laser-treated samples exhibited a nearly fourfold life improvement over that of the reference samples. In both tests, the lives of the samples varied inversely with the thickness of the laser-melted layer of zirconia.
1. INTRODUCTION The development and the potential benefits of ceramic thermal barrier coatings (TBCs) for gas turbine high temperature components have been well documented in the technical literature. Some of the key early work from the 1950s to the early 1970s was conducted at NASA Lewis Research Center 1-6. In the mid-1970s Stecura and Liebert 7 developed a successful zirconia-based TBC. This work stimulated further development and testing of TBCs for potential aircraft and non-aircraft applications 1°. The duplex TBCs identified by NASA consist of an inner layer of oxidationresistant plasma-sprayed N i - C r - A 1 - Y bond coat and an outer plasma-sprayed yttria-stabilized zirconia layer. The bond coat is about 0.01 cm thick while the ceramic layer can vary in thickness from 0.01 to 0.04 cm or more. Although TBCs performed well in the clean combustion gases resulting from burning Jet A fuel or natural gas, preliminary studies 11 ~5 revealed that their durability was in general reduced when they were tested in combustion gases * Paper presented at the International Conference on Metallurgical Coatings and Process Technology, San Diego, CA, U.S.A., April 5 8, 1982. 0040-6090/82/0000-0000/$02.75
© ElsevierSequoia/Printed in The Netherlands
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containing gaseous impurities such as sodium, vanadium and sulfur. The failures were observed to occur in the ceramic layer in a manner similar to TBC failures in clean fuel tests 13' 16. The cracks initiated in the ceramic layer parallel and close to the ceramic-bond coat interface. This is in agreement with the findings of Levine I ~ who measured the tensile strength of (ZrOz-12Y203)/(Ni-Cr-AI-Y) TBCs and determined that this was the weakest part of the coating system. The failures of TBCs tested in combustion gases doped with sodium and vanadium were correlated by Miller 18 in terms of dew points 19, melting points of the condensates and the temperature distribution within the coatings. Palko et al. 15 observed that liquid sodium sulfate was absorbed into the open pores of the ceramic coating and speculated that the difference in thermal expansion between the coating and the solidified salt was the cause of spallation. To alleviate the problems caused by dirty fuels, two approaches were undertaken. The first approach 13 was to develop new TBC systefns, e.g. calcium silicate, that would resist corrosive environments. The second approach is represented by the present work. The idea behind this approach is to improve the performance of zirconia-based TBCs through partial densification of the ceramic layer by treatment with a laser. Densification of the surface of a ceramic coating was expected to reduce the coating penetration by salts contained in the combustion gases, and their attack on the bond coat. To test this idea, two separate experiments were performed. One experiment was a cyclic oxidation test of laser-glazed TBCs in a Mach 0.3 burner rig fired with a Jet A fuel to determine the effect of glazing on coating performance in a clean environment. The other experiment was a cyclic corrosion test in a similar burner rig fired with Jet A fuel, but with the combustion gases doped with sodium to determine whether glazing reduced penetration of sodium sulfate into the ceramic coat. The laser glazing approach is not unique in the sense that the laser surface fusion technique has been used by others 2°-22 to "segment" ceramic coatings and ceramic turbine shrouds to increase strain tolerance. 2.
EXPERIMENTAL PROCEDURE
2.1. Materials
The composition of the Ni-Cr-A1-Zr powder used to form the bond coat is listed in Table I. The same table shows the composition of the yttria-stabilized zirconia powder. The particle size of both powders was between 200 and 325 mesh. Waspaloy tubing (of diameter 1.27 cm and wall thickness 0.124 cm) was used as the substrate material for the cyclic oxidation tests. Its nominal composition is shown in Table I, fourth column. Hollow MAR-M 509 erosion bars, cast to the configuration described by Hodge et al. 13, were used as the substrate for the cyclic corrosion test. The composition of the alloy is shown in Table I, fifth column. 2.2. Coating applications
The Waspaloy tubing was cut into pieces 18 cm long. Each piece was provided with fittings to permit internal cooling during plasma spraying. The procedure described in ref. 8 was used in sample preparation and plasma spraying. The coatings were manually sprayed in air to nominal thicknesses of 0.012 cm for the bond coat, and 0.020 and 0.040 cm for the ceramic coat. The thickness of the bond
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ZIRCONIA
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COATINGS
TABLE I CHEMICAL COMPOSITION OF MATERIALS USED IN THIS INVESTIGATION
Element
AI B C Ca Co Cr Fe Hf Mg Mn Mo Ni S Si Ta Ti V W Y Zr
Content (wt.%) Bond coat a
Zirconia
14 ---
0.011 ND b -0.078 ND ND 0.03 2.10 0.014 ND ND ND ND 0.05 ND 0.054 ND ND 6.28 Balance
-14 -----Balance -0.08 -----0.1
Waspaloy ~
1.4 0.006 0.07 -13.5 -2.0 max --0.75 max 4.3 Balance ---3.0 ---0.009
M A R - M 509
ND b 0.004 0.56 ND Balance 23.26 0.32 ND ND ND ND 10.95 0.011 < 0.10 3.66 0.30 ND 6.50 ND 0.32
a Nominal compositions. b Not determined. c o a t v a r i e d f r o m t h e n o m i n a l v a l u e b y a b o u t ___20~o in e x t r e m e cases a n d t h a t o f t h e c e r a m i c c o a t b y ___15~o. A f t e r p l a s m a s p r a y i n g , e a c h t u b e was c u t to l e n g t h s of a b o u t 9 cm, a p p r o p r i a t e for b u r n e r rig testing. T h e h o l l o w e r o s i o n b a r s w e r e p l a s m a s p r a y e d in t h e s a m e m a n n e r as t h e o x i d a t i o n s a m p l e s b u t w i t h o u t i n t e r n a l air cooling. The nominal thicknesses of the bond coat and the ceramic coat were the s a m e as before. F o r a n y single e r o s i o n bar, t h e b o n d c o a t t h i c k n e s s v a r i e d b y as m u c h as _+ 40~o a n d t h a t o f t h e c e r a m i c c o a t b y _+ 25~o f r o m t h e n o m i n a l values. T h e s e s i g n i f i c a n t v a r i a t i o n s in t h i c k n e s s w e r e d u e to the difficulty o f m a n u a l l y spraying and of measuring a body without cylindrical symmetry. 2. 3.
Laser glazing
A C O 2 laser w a s u s e d in this i n v e s t i g a t i o n to m e l t t h e s u r f a c e o f t h e c e r a m i c c o a t i n g s . T h e laser b e a m w a s f o c u s e d w i t h a Z n S e lens (focal d i s t a n c e , 30.5 cm) to c o v e r a c i r c u l a r a r e a a p p r o x i m a t e l y 0.11 c m in d i a m e t e r . P r e l i m i n a r y e x p e r i m e n t a t i o n h a d s h o w n t h a t t h e laser b e a m w i t h a p o w e r d e n s i t y o f 50 W m m - 2 c o u l d m e l t p l a s m a - s p r a y e d z i r c o n i a to a d e p t h o f 0.01 c m at a s c a n n i n g s p e e d o f a b o u t 8 0 c m m i n -1. B u r n e r rig s a m p l e s w e r e g l a z e d in a n a p p a r a t u s t h a t a l l o w e d c o n t r o l l e d r o t a t i o n of the s p e c i m e n a b o u t its axis a n d c o n t r o l l e d t r a n s l a t i o n a l o n g t h e s a m e axis. By a d j u s t i n g t h e s p e e d s of r o t a t i o n a n d t r a n s l a t i o n , t h e c y l i n d r i c a l surface of t h e s a m p l e c o u l d be s c a n n e d in a spiral fashion. A l l o w a n c e w a s m a d e for 50~o o v e r l a p of the scans. F i g u r e 1 s h o w s the surface of a z i r c o n i a c o a t i n g laser g l a z e d w i t h a 50 W m m - 2 b e a m . T h e s m a l l a r e a o n the l e f t - h a n d side s h o w s the as-
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plasma-sprayed condition. The glazed surface is smooth and shiny. Thus glazing offers an aerodynamic benefit. Furthermore the laser-glazed surface is segmented. Figure 2 shows the surface of a glazed zirconia coat on the flat portion of a corrosion sample at higher magnification. In evidence are the striations and the segmentation or mud-flat cracking produced by the laser melting. The average diameter of the segments is about 0.05 cm. The width of the bands represents one-half of the laser beam diameter. Because of lack of cylindrical symmetry, the scanning speed for the corrosion samples was not constant. The average speed was about 85 cm min-1. Laser beams with power densities of 35, 50 and 75 W mm 2 were used.
Fig. 1. Laser-glazedzirconiacoating0.020cm thick (power density, 50 W mm 2). Fig. 2. Laser-glazedzirconiacoating0.020cm thick (powerdensity, 35 W mm- 2).
2.4. Burner rig testing The Mach 0.3 burner rig used in these tests is shown in Fig. 3 and has been described in ref. 23. Eight specimens were placed in a holder which was rotated at 630 rev m i n - 1 in front of the burner nozzle. The same type of burner was used for cyclic oxidation and cyclic corrosion tests. In the cyclic oxidation test, the samples were heated for 6 min and then cooled with a stream of compressed air for 3 min. The burner rig was fired at a fuel-to-air ratio of about 0.049 using Jet A fuel. The test temperature was 1050+ 20 °C as determined with a calibrated (for emissivity and window absorption) disappearing-filament optical pyrometer focused on the center of the sample surfaces facing the nozzle of the burner. About 3 min in the flame were required for the hot zone of the samples to reach test temperature. At the end of the 3 min of forced air cooling, the temperature was about 80 °C. All of the TBC failures occurred or apparently started on the back surfaces of the samples where the temperature was determined to be about 100 °C higher 24. In this test, the samples were visually examined and the temperature monitored at least once every day. Samples were tested until TBC failure (cracking and/or spallation) was observed. Figure 4 illustrates typical samples after test. All coatings were run in triplicate. After test, the specimens were photographed and mounted in epoxy for metallographic examination. The corrosion test was run at a substrate temperature of 843 °C. The level of sodium, introduced to the combustor as an aqueous solution of NaC1, was 100 ppm in the fuel equivalent. The sulfur content of the Jet A fuel was approximately 0.05 wt.~o. The operation and calibration of the equipment was as described in ref. 13.
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Fig. 3. Mach 0.3 burner rig and specimen assembly. Fig. 4. Laser-glazed samples after a cyclic oxidation test at 1050°C (zirconia coat thickness, 0.040 cm; power density, 50 W mm 2; cycle, 6 min heating, 3 min cooling): sample A, life, 2800 cycles; sample B, life, 2500 cycles.
The samples were exposed to thermal cycles consisting of 55 min at temperature followed by 5 min out of the flame with the internal and external cooling air on. The samples were examined for failure after ten cycle intervals. Testing was stopped when the TBC showed evidence of spallation. The reference non-glazed samples were run in duplicate and the laser-glazed samples in triplicate. After the test, representative samples were photographed, plasma sprayed with copper (to facilitate electron probe analysis), mounted in epoxy and sectioned very slowly through the hot zone with a diamond wheel. No liquid was used during cutting and polishing in order to retain all of the sodium sulfate that penetrated the zirconia coating during test. These cross sections were subjected to electron microprobe analysis and metallographic examination. Figure 5 illustrates typical corrosion samples after test. 3. RESULTS AND DISCUSSION 3.1. Cyclic oxidation test The definition of failure in these types of experiments is rather arbitrary. In this investigation, the appearance of a crack or spallation of a small piece of ceramic coating was the criterion for removal of the sample from test as illustrated in Figs. 4 and 5. In other investigations 21 a failure was considered to have occurred when the ceramic coat spalled from 50~o or more of the test zone. Therefore, it is difficult to compare the results obtained in two different investigations. In this investigation, the failures occurred on the back surface of the samples, where the temperature was about 100 °C higher than the test temperature of 1050 °C. Figure 6 summarizes the results obtained in the cyclic oxidation test. Two sets of data are shown. One set pertains to a group of samples in which the thickness of the ceramic coating was 0.020 cm and the other set represents samples with ceramic coatings 0.040 cm thick. In the first set, the group of samples glazed with the 50 W m m - 2 laser beam did not perform as well as the group glazed with the 35 W m m -2 beam. Also in the case of 0.040cm thick ceramic coatings the power density of the laser beam, and
I. ZAPLATYNSKY
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Fig. 5. Samples after a cycliccorrosion test at 843 'C (zirconia coat thickness, 0.040cm; fuel, Jet A doped with 100 ppm Na; cycle,55 min heating, 5 min cooling): sample A (non-glazed),life, 122 cycles; sample B (glazed with 50 W mm 2 laser beam), life, 355 cycles; sample C (glazed with 35 W mm- 2 laser beam), life, 366 cycles. Fig. 6. Life of zirconia (ZrO2-Swt.°/,Y203) TBCs in a burner rig cyclic oxidation test at 1050°C (Ni-C> AI Y thickness, 0.012 cm; zirconia thicknesses, 0.020 and 0.040cm; cycle,6 min heating, 3 min cooling). consequently the thickness of the glazed layer, had a small but negative effect on the performance of the TBC. There is a slight improvement in T B C life with increasing thickness of the ceramic coat. Expressing these observations in statistical terms it can be said that, within the 90~o confidence level, laser power density has a negative effect and that, within the 95~o confidence level, the thickness of Z r O 2 coating has a positive effect on the life of TBCs. The interaction term was found to be positive and significant at the 90~o confidence level. This means that at lower thickness the negative effect of the power density is more pronounced. Examination of Fig. 7, which shows the microstructure of a representative sample, reveals that the m o d e of failure was cracking in the ceramic coat, near and parallel to the interface between the b o n d coat and the ceramic coat. This type of failure has been observed in all investigations dealing with cyclic oxidation tests of TBCs. Also, the a m o u n t of porosity in the plasma-sprayed ceramic coat and the complete densification of the laser-glazed surface layer can be observed. The b o n d coat seems to be partly oxidized. Levine 25 performed cyclic oxidation tests on similarly prepared samples which were not laser glazed and obtained similar results (within 95'Uo confidence level). Therefore, it appears that glazing did not negatively affect the performance of the TBC. In refs. 20-22 it is claimed that a "segmented" ceramic structure, formed by precracking the ceramic in a direction perpendicular to the plane of the coating as a result of laser glazing, m a y significantly increase its strain tolerance. I m p r o v e d T B C performance, as a result of laser-induced segmentation, was not observed in the present study. 3.2. Cyclic corrosion test
The experimental results obtained in the cyclic corrosion test are summarized in graphic form in Fig. 8. There are two sets of data. One set represents data obtained with ceramic coatings 0.020 cm thick and the other set pertains to coatings 0.040 cm thick. It is evident that the glazed TBCs performed significantly better than the reference non-glazed TBCs. At the 50 W m m - z power density level the improve-
281
L A S E R - G L A Z E D ZIRCONIA COATINGS
36O m 3OO ,.02.240
c~180 ~120 z
60
355 321 O.0~O 0.(~O O.~O 0. 040 0.940 0.040 ZrO2 COATTHICKNESS, cm
0i ~
cm
O
50 75 0 35 LASER POWER DENSITY,wlmm2
50
Fig. 7. Microstructure of cyclic oxidation sample after 2800 cycles at 1050°C (zirconia coat thickness, 0.040 cm; power density, 50 W mm- 2). Fig. 8. Life of zirconia (ZrO2-8wt.%Y203) TBCs in a burner rig cyclic corrosion test at 843 °C (Ni-CrA1-Y thickness, 0.012 cm; zirconia thicknesses, 0.020 and 0.040 cm; fuel, Jet A doped with 100 ppm Na; cycle, 55 min heating, 5 rain cooling).
Fig. 9. Microstructure near leading edge of a cyclic corrosion sample after 408 cycles at 843 °C (zirconia coat thickness, 0.040cm; power density, 50 W mm- 2). ment in life is nearly fourfold. Similar i m p r o v e m e n t can be observed when the thickness of the ceramic coat is doubled. As can be seen in Fig. 5 the location of the failures is random. Figure 9 shows the microstructure of the T B C near the leading edge. The samples failed in the usual manner, i.e. cracking occurred in the ceramic coat near and parallel to the interface between the b o n d coat and the ceramic coat. O n several occasions an unusual type of failure was observed, namely separation of the b o n d coat from the substrate. The degree of attack of the b o n d coat appears to be insignificant. Further examination of this microstructure reveals the loss of thickness of the ceramic coating as the glazed layer is nearly gone. Since yttriastabilized zirconia does not react with sodium sulfate 26 this loss m a y be attributed to erosion. Spallation was p r o b a b l y not responsible for this loss since the surface remains smooth. Figures 10 and 11 show electron m i c r o p r o b e traverses of nonglazed and glazed TBCs respectively. Although determinations for several elements were made, only the traces for sodium, sulfur and oxygen are shown. The non-glazed sample (Fig. 10) which failed in 66 cycles exhibits a significant degree of b o n d coat
282
I. ZAPLATYNSKY
ZIRCONIA
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DIbTANCE FROM THE SURFACE, 10-2 cm
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5
Fig. 10. Electron microprobe traverse of non-glazed corrosion sample after 66 cyclesat 843 °C (zirconia thickness, 0.040 cm): - - , sulfur; • , sodium; - - -, oxygen. Fig. 11. Electron microprobe traverse of laser-glazed corrosion sample after 315 cycles at 843~'C (zirconia thickness,0.040 cm; power density, 35 W mm 2): , sulfur, • , sodium; - - -, oxygen. o x i d a t i o n a n d / o r sulfidation a l o n g grain b o u n d a r i e s as revealed by the oxygen a n d sulfur traces. It appears that every pore in the ceramic c o n t a i n s s o d i u m sulfate. It can be seen that the glazed sample (Fig. ! 1) is considerably less affected despite the fact that its exposure to the corrosive e n v i r o n m e n t was 5 times longer. The oxygen a n d sulfur traces in the b o n d coat indicate less oxidation a n d / o r sulfidation along the grain boundaries. Also the ceramic coat shows less p e r m e a t i o n by s o d i u m sulfate. I n b o t h cases there is a c o n c e n t r a t i o n of s o d i u m sulfate at the interface between the ceramic coat a n d the b o n d coat. The experimental d a t a o b t a i n e d in this investigation provide strong evidence that laser glazing of zirconia-based TBCs will i m p r o v e their life in a corrosive e n v i r o n m e n t as a result of reduced permeability of the surface. 4. CONCLUSIONS As a result of this preliminary study of the effect of laser glazing o n the life of zirconia-based TBCs in cyclic o x i d a t i o n a n d cyclic corrosion tests, the following conclusions have been drawn. (1) The laser-glazed zirconia-based TBCs show at least a fourfold improvem e n t in life over n o n - g l a z e d TBCs in cyclic corrosion tests.
LASER-GLAZED ZIRCONIA COATINGS
283
(2) Laser glazing of zirconia-based TBCs to give an aerodynamically smooth surface has no apparent effect on their life in cyclic oxidation tests. Glazed and nonglazed TBCs endured similar numbers of cycles in a burner rig fired with Jet A fuel. (3) The concept of increasing strain tolerance of TBCs by "segmenting" the ceramic layer through exposure to a high intensity heat source such as a laser was not borne out by this study. REFERENCES
4 5 6 7 8 9 l0 ll
12 13
14 15 16 17 18
L. Y. Schafer, Jr., Comparison of theoretically and experimentally determined effects of oxide coatings supplied by fuel additives on uncooled turbine blade temperature during transient turbojetengine operation, NACA Research Memo. R M E53A19, 1953 (National Advisory Committee for Aeronautics). E. R. Bartoo and Y. L. Clure, Experimental investigation of air-cooled turbine blades in turbojet engine XIII endurance evaluation of several protective coatings applied to turbine blades of nonstrategic steels, NACA Research Memo. RM E53E18, 1953 (National Advisory Committee for Aeronautics). L. N. Hjelm and B. R. Bornhorst, Development of improved ceramic coatings to increase the life of XLR 99 thrust chamber, Research Airplane Committee Rep. on Conf. on the Progress of the X-15 Project, in NASA Tech. Memo. TM X-59072, 1961, pp. 227-253. N. M. Nijpjes, ZrO z coatings on nimonic alloys. In F. Benesovsky (ed.), Proc. 6th Plansee Semin. on High Temperature Materials, Reutte, Springer, New York, 1969, pp. 481-499. S. J. Grisaffe, Coatings and protection. In C. Sims and W. C. Hagel (eds.), The Superalloys, Wiley, New York, 1972, Chap. 12, pp. 341-370. A. N. Curren, S. J. Grisaffe and K. C. Wycoff, Hydrogen plasma tests of some insulating coating systems for the nuclear rocket thrust chamber, NASA Tech. Memo. TM X-2461, 1972. C. H. Liebert and S. Stecura, Ceramic thermal protective coating withstands hostile environment of rotating turbine blades, NASA Teeh. Brief, B75-10290, 1975. S. Stecura, Two-layer thermal barrier coating for turbine airfoils furnace and burner rig test results, NASA Tech. Memo. TM X-3425, 1976. S. Stecura and C. H. Liebert, Thermal barrier coating system, U.S. Patent 4,055,705, October 25, 1977. S. R. Levine, R. A. Miller and P. H. Hodge, Thermal barrier coatings for heat engine components, S A M P E Q., 12 (1) (1980) 20 26. R. Y. Bratton, S. C. Singhal and S. Y. Lee, Ceramic gas turbine components research and development, part 2 evaluation of ZrO 2 Y2Oa/NiCrAIY thermal barrier coatings exposed to stimulated gas turbine environments, Final Rep., June 1978 (Electric Power Research Institute Contract RP421-1 ). S. R. Levine and Y. S. Clark, Thermal barrier coatings a near term, high payofftechnology, NASA Tech. Memo. TM X-73586, 1977. P. E. Hodge, S. Stecura, M. A. Gedwill, I. Zaplatynsky and S. R. Levine, Thermal barrier coatings : burner rig hot corrosion test results, NASA Tech. Memo. TM-79005, 1978 (Department of Energy-NASA 2593-78/3). P. E. Hodge, R. A. Miller and M. A. Gedwill, Evaluation of hot corrosion behavior of thermal barrier coatings, NASA Tech. Memo. TM-81520, 1980 (Department of Energy-NASA 2593-16). Y. E. Palko, K. L. Luthra and D. W. McKee, Evaluation of performance of thermal barrier coatings under simulated industrial/utility gas turbine conditions, Final Rep., 1978 (General Electric Co.). S. Stecura, Effects of compositional changes on the performance of a thermal barrier coating system, NASA Tech. Memo. TM-78976, 1978. S. R. Levine, Adhesive/cohesive strength of ZrOz-12w/oY203/NiCrAIY thermal barrier coating, NASA Tech. Memo. TM-73792, 1978. R. A. Miller, Analysis of the response of a thermal barrier coating to sodium- and vanadium-doped combustion gases, NASA Tech. Memo. TM-79205, 1979 (Department of Energy-NASA 2593-79/7).
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19 F . J . Kohl, G. J. Santaro, C. A. Stearns, G. C. Fryburg and D. E. Rosner, Theoretical and experimental studies of the deposition of Na2SO 4 from seeded combustion gases, J. Electrochem. Soc., 126(6)(1979) 1054-1061. 20 W.O. Gaffin and D. E. Webb, JT8D and JT9D jet engine performance improvement program-task I: feasibility analysis, NASA Contract. Rep. CR-159449, 1979 (PWA Internal Rep. 5518-38 (Pratt and Whitney Aircraft Group)). 21 I.E. Sumner and D. Ruckle, Development of improved-durability plasma sprayed ceramic coatings for gas turbine engines, Proc. American Institute of Aeronautics and Astronautics-Society of Automotive Engineers-American Society of Mechanical Engineers 16th Joint Propulsion Conf., Hartford, CT, June 30-July 2. 1980, Paper AIAA-80-1193. 22 R.C. Bill, Plasma-sprayed zirconia gas path seal technology, a state-of-the-art review, NASA Tech. Memo. TM-79273, 1979 (A VRADCOM Teeh. Rep. 79-47 (Aviation Readiness and Development Command)). 23 H.R. Gray and W. A. Sanders, Effect of thermal cycling in a Mach 0.3 burner rig on properties and structure of direction solidified 7/7' 8 eutectic, NASA Tech. Memo. T M )(-3271, 1975. 24 M . A . Gedwill, Burner rig evaluation of thermal barrier coating systems for nickel-base alloys, NASA Tech. Memo. TM-81684, February 1981 (Department of Energy-NASA 2593-26). 25 S.R. Levine, personal communication, 1982. 26 I. Zaplatynsky, Reactions of yttria-stabilized zirconia with oxides and sulfates of various elements, NASA Tech. Memo. TM-79005, 1978 (Department of Energy-NASA 2593-78/3).
METALLURGICAL AND PROTECTIVE COATINGS
PERFORMANCE OF LASER-GLAZED ZIRCONIA THERMAL BARRIER COATINGS IN CYCLIC OXIDATION AND CORROSION B U R N E R R I G TESTS* I. ZAPLATYNSKY National Aeronautics and Space Admin&tration Lew& Research Center, Cleveland, 0H44135 (U.S.A.)
(Received March 22, 1982; accepted April 5, 1982)
The performance of laser-glazed zirconia (containing 8 wt.% Y 2 0 3 ) thermal barrier coatings was evaluated in cyclic oxidation and cyclic corrosion tests. Plasma-sprayed zirconia coatings of two thicknesses (0.02 and 0.04 cm) were partially melted with a CO2 laser. The power density of the focused laser beam was varied from 35 to 75 W m m -2, while the scanning speed was about 80 cm m i n - 1. In cyclic oxidation tests, the specimens were heated in a burner rig for 6 min and cooled for 3 min. The results obtained indicated that the laser-treated samples had the same life as the untreated samples. However, in corrosion tests, in which the burner rig flame contained 100 p p m N a fuel equivalent, the laser-treated samples exhibited a nearly fourfold life improvement over that of the reference samples. In both tests, the lives of the samples varied inversely with the thickness of the laser-melted layer of zirconia.
1. INTRODUCTION The development and the potential benefits of ceramic thermal barrier coatings (TBCs) for gas turbine high temperature components have been well documented in the technical literature. Some of the key early work from the 1950s to the early 1970s was conducted at NASA Lewis Research Center 1-6. In the mid-1970s Stecura and Liebert 7 developed a successful zirconia-based TBC. This work stimulated further development and testing of TBCs for potential aircraft and non-aircraft applications 1°. The duplex TBCs identified by NASA consist of an inner layer of oxidationresistant plasma-sprayed N i - C r - A 1 - Y bond coat and an outer plasma-sprayed yttria-stabilized zirconia layer. The bond coat is about 0.01 cm thick while the ceramic layer can vary in thickness from 0.01 to 0.04 cm or more. Although TBCs performed well in the clean combustion gases resulting from burning Jet A fuel or natural gas, preliminary studies 11 ~5 revealed that their durability was in general reduced when they were tested in combustion gases * Paper presented at the International Conference on Metallurgical Coatings and Process Technology, San Diego, CA, U.S.A., April 5 8, 1982. 0040-6090/82/0000-0000/$02.75
© ElsevierSequoia/Printed in The Netherlands
276
I. ZAPLATYNSKY
containing gaseous impurities such as sodium, vanadium and sulfur. The failures were observed to occur in the ceramic layer in a manner similar to TBC failures in clean fuel tests 13' 16. The cracks initiated in the ceramic layer parallel and close to the ceramic-bond coat interface. This is in agreement with the findings of Levine I ~ who measured the tensile strength of (ZrOz-12Y203)/(Ni-Cr-AI-Y) TBCs and determined that this was the weakest part of the coating system. The failures of TBCs tested in combustion gases doped with sodium and vanadium were correlated by Miller 18 in terms of dew points 19, melting points of the condensates and the temperature distribution within the coatings. Palko et al. 15 observed that liquid sodium sulfate was absorbed into the open pores of the ceramic coating and speculated that the difference in thermal expansion between the coating and the solidified salt was the cause of spallation. To alleviate the problems caused by dirty fuels, two approaches were undertaken. The first approach 13 was to develop new TBC systefns, e.g. calcium silicate, that would resist corrosive environments. The second approach is represented by the present work. The idea behind this approach is to improve the performance of zirconia-based TBCs through partial densification of the ceramic layer by treatment with a laser. Densification of the surface of a ceramic coating was expected to reduce the coating penetration by salts contained in the combustion gases, and their attack on the bond coat. To test this idea, two separate experiments were performed. One experiment was a cyclic oxidation test of laser-glazed TBCs in a Mach 0.3 burner rig fired with a Jet A fuel to determine the effect of glazing on coating performance in a clean environment. The other experiment was a cyclic corrosion test in a similar burner rig fired with Jet A fuel, but with the combustion gases doped with sodium to determine whether glazing reduced penetration of sodium sulfate into the ceramic coat. The laser glazing approach is not unique in the sense that the laser surface fusion technique has been used by others 2°-22 to "segment" ceramic coatings and ceramic turbine shrouds to increase strain tolerance. 2.
EXPERIMENTAL PROCEDURE
2.1. Materials
The composition of the Ni-Cr-A1-Zr powder used to form the bond coat is listed in Table I. The same table shows the composition of the yttria-stabilized zirconia powder. The particle size of both powders was between 200 and 325 mesh. Waspaloy tubing (of diameter 1.27 cm and wall thickness 0.124 cm) was used as the substrate material for the cyclic oxidation tests. Its nominal composition is shown in Table I, fourth column. Hollow MAR-M 509 erosion bars, cast to the configuration described by Hodge et al. 13, were used as the substrate for the cyclic corrosion test. The composition of the alloy is shown in Table I, fifth column. 2.2. Coating applications
The Waspaloy tubing was cut into pieces 18 cm long. Each piece was provided with fittings to permit internal cooling during plasma spraying. The procedure described in ref. 8 was used in sample preparation and plasma spraying. The coatings were manually sprayed in air to nominal thicknesses of 0.012 cm for the bond coat, and 0.020 and 0.040 cm for the ceramic coat. The thickness of the bond
LASER-GLAZED
ZIRCONIA
277
COATINGS
TABLE I CHEMICAL COMPOSITION OF MATERIALS USED IN THIS INVESTIGATION
Element
AI B C Ca Co Cr Fe Hf Mg Mn Mo Ni S Si Ta Ti V W Y Zr
Content (wt.%) Bond coat a
Zirconia
14 ---
0.011 ND b -0.078 ND ND 0.03 2.10 0.014 ND ND ND ND 0.05 ND 0.054 ND ND 6.28 Balance
-14 -----Balance -0.08 -----0.1
Waspaloy ~
1.4 0.006 0.07 -13.5 -2.0 max --0.75 max 4.3 Balance ---3.0 ---0.009
M A R - M 509
ND b 0.004 0.56 ND Balance 23.26 0.32 ND ND ND ND 10.95 0.011 < 0.10 3.66 0.30 ND 6.50 ND 0.32
a Nominal compositions. b Not determined. c o a t v a r i e d f r o m t h e n o m i n a l v a l u e b y a b o u t ___20~o in e x t r e m e cases a n d t h a t o f t h e c e r a m i c c o a t b y ___15~o. A f t e r p l a s m a s p r a y i n g , e a c h t u b e was c u t to l e n g t h s of a b o u t 9 cm, a p p r o p r i a t e for b u r n e r rig testing. T h e h o l l o w e r o s i o n b a r s w e r e p l a s m a s p r a y e d in t h e s a m e m a n n e r as t h e o x i d a t i o n s a m p l e s b u t w i t h o u t i n t e r n a l air cooling. The nominal thicknesses of the bond coat and the ceramic coat were the s a m e as before. F o r a n y single e r o s i o n bar, t h e b o n d c o a t t h i c k n e s s v a r i e d b y as m u c h as _+ 40~o a n d t h a t o f t h e c e r a m i c c o a t b y _+ 25~o f r o m t h e n o m i n a l values. T h e s e s i g n i f i c a n t v a r i a t i o n s in t h i c k n e s s w e r e d u e to the difficulty o f m a n u a l l y spraying and of measuring a body without cylindrical symmetry. 2. 3.
Laser glazing
A C O 2 laser w a s u s e d in this i n v e s t i g a t i o n to m e l t t h e s u r f a c e o f t h e c e r a m i c c o a t i n g s . T h e laser b e a m w a s f o c u s e d w i t h a Z n S e lens (focal d i s t a n c e , 30.5 cm) to c o v e r a c i r c u l a r a r e a a p p r o x i m a t e l y 0.11 c m in d i a m e t e r . P r e l i m i n a r y e x p e r i m e n t a t i o n h a d s h o w n t h a t t h e laser b e a m w i t h a p o w e r d e n s i t y o f 50 W m m - 2 c o u l d m e l t p l a s m a - s p r a y e d z i r c o n i a to a d e p t h o f 0.01 c m at a s c a n n i n g s p e e d o f a b o u t 8 0 c m m i n -1. B u r n e r rig s a m p l e s w e r e g l a z e d in a n a p p a r a t u s t h a t a l l o w e d c o n t r o l l e d r o t a t i o n of the s p e c i m e n a b o u t its axis a n d c o n t r o l l e d t r a n s l a t i o n a l o n g t h e s a m e axis. By a d j u s t i n g t h e s p e e d s of r o t a t i o n a n d t r a n s l a t i o n , t h e c y l i n d r i c a l surface of t h e s a m p l e c o u l d be s c a n n e d in a spiral fashion. A l l o w a n c e w a s m a d e for 50~o o v e r l a p of the scans. F i g u r e 1 s h o w s the surface of a z i r c o n i a c o a t i n g laser g l a z e d w i t h a 50 W m m - 2 b e a m . T h e s m a l l a r e a o n the l e f t - h a n d side s h o w s the as-
278
I. ZAPLATYNSKY
plasma-sprayed condition. The glazed surface is smooth and shiny. Thus glazing offers an aerodynamic benefit. Furthermore the laser-glazed surface is segmented. Figure 2 shows the surface of a glazed zirconia coat on the flat portion of a corrosion sample at higher magnification. In evidence are the striations and the segmentation or mud-flat cracking produced by the laser melting. The average diameter of the segments is about 0.05 cm. The width of the bands represents one-half of the laser beam diameter. Because of lack of cylindrical symmetry, the scanning speed for the corrosion samples was not constant. The average speed was about 85 cm min-1. Laser beams with power densities of 35, 50 and 75 W mm 2 were used.
Fig. 1. Laser-glazedzirconiacoating0.020cm thick (power density, 50 W mm 2). Fig. 2. Laser-glazedzirconiacoating0.020cm thick (powerdensity, 35 W mm- 2).
2.4. Burner rig testing The Mach 0.3 burner rig used in these tests is shown in Fig. 3 and has been described in ref. 23. Eight specimens were placed in a holder which was rotated at 630 rev m i n - 1 in front of the burner nozzle. The same type of burner was used for cyclic oxidation and cyclic corrosion tests. In the cyclic oxidation test, the samples were heated for 6 min and then cooled with a stream of compressed air for 3 min. The burner rig was fired at a fuel-to-air ratio of about 0.049 using Jet A fuel. The test temperature was 1050+ 20 °C as determined with a calibrated (for emissivity and window absorption) disappearing-filament optical pyrometer focused on the center of the sample surfaces facing the nozzle of the burner. About 3 min in the flame were required for the hot zone of the samples to reach test temperature. At the end of the 3 min of forced air cooling, the temperature was about 80 °C. All of the TBC failures occurred or apparently started on the back surfaces of the samples where the temperature was determined to be about 100 °C higher 24. In this test, the samples were visually examined and the temperature monitored at least once every day. Samples were tested until TBC failure (cracking and/or spallation) was observed. Figure 4 illustrates typical samples after test. All coatings were run in triplicate. After test, the specimens were photographed and mounted in epoxy for metallographic examination. The corrosion test was run at a substrate temperature of 843 °C. The level of sodium, introduced to the combustor as an aqueous solution of NaC1, was 100 ppm in the fuel equivalent. The sulfur content of the Jet A fuel was approximately 0.05 wt.~o. The operation and calibration of the equipment was as described in ref. 13.
LASER-GLAZED ZIRCONIA COATINGS
279
Fig. 3. Mach 0.3 burner rig and specimen assembly. Fig. 4. Laser-glazed samples after a cyclic oxidation test at 1050°C (zirconia coat thickness, 0.040 cm; power density, 50 W mm 2; cycle, 6 min heating, 3 min cooling): sample A, life, 2800 cycles; sample B, life, 2500 cycles.
The samples were exposed to thermal cycles consisting of 55 min at temperature followed by 5 min out of the flame with the internal and external cooling air on. The samples were examined for failure after ten cycle intervals. Testing was stopped when the TBC showed evidence of spallation. The reference non-glazed samples were run in duplicate and the laser-glazed samples in triplicate. After the test, representative samples were photographed, plasma sprayed with copper (to facilitate electron probe analysis), mounted in epoxy and sectioned very slowly through the hot zone with a diamond wheel. No liquid was used during cutting and polishing in order to retain all of the sodium sulfate that penetrated the zirconia coating during test. These cross sections were subjected to electron microprobe analysis and metallographic examination. Figure 5 illustrates typical corrosion samples after test. 3. RESULTS AND DISCUSSION 3.1. Cyclic oxidation test The definition of failure in these types of experiments is rather arbitrary. In this investigation, the appearance of a crack or spallation of a small piece of ceramic coating was the criterion for removal of the sample from test as illustrated in Figs. 4 and 5. In other investigations 21 a failure was considered to have occurred when the ceramic coat spalled from 50~o or more of the test zone. Therefore, it is difficult to compare the results obtained in two different investigations. In this investigation, the failures occurred on the back surface of the samples, where the temperature was about 100 °C higher than the test temperature of 1050 °C. Figure 6 summarizes the results obtained in the cyclic oxidation test. Two sets of data are shown. One set pertains to a group of samples in which the thickness of the ceramic coating was 0.020 cm and the other set represents samples with ceramic coatings 0.040 cm thick. In the first set, the group of samples glazed with the 50 W m m - 2 laser beam did not perform as well as the group glazed with the 35 W m m -2 beam. Also in the case of 0.040cm thick ceramic coatings the power density of the laser beam, and
I. ZAPLATYNSKY
280
30OO
270O
29~0 2800 250O ;0O
2000
l~
10O0 0.020 35
0.0~0 0.040 ZrO2 COATTHICKNESS, crn 50 35 LASER POWERDENSITY,wlmm2
1i
0.040 50
Fig. 5. Samples after a cycliccorrosion test at 843 'C (zirconia coat thickness, 0.040cm; fuel, Jet A doped with 100 ppm Na; cycle,55 min heating, 5 min cooling): sample A (non-glazed),life, 122 cycles; sample B (glazed with 50 W mm 2 laser beam), life, 355 cycles; sample C (glazed with 35 W mm- 2 laser beam), life, 366 cycles. Fig. 6. Life of zirconia (ZrO2-Swt.°/,Y203) TBCs in a burner rig cyclic oxidation test at 1050°C (Ni-C> AI Y thickness, 0.012 cm; zirconia thicknesses, 0.020 and 0.040cm; cycle,6 min heating, 3 min cooling). consequently the thickness of the glazed layer, had a small but negative effect on the performance of the TBC. There is a slight improvement in T B C life with increasing thickness of the ceramic coat. Expressing these observations in statistical terms it can be said that, within the 90~o confidence level, laser power density has a negative effect and that, within the 95~o confidence level, the thickness of Z r O 2 coating has a positive effect on the life of TBCs. The interaction term was found to be positive and significant at the 90~o confidence level. This means that at lower thickness the negative effect of the power density is more pronounced. Examination of Fig. 7, which shows the microstructure of a representative sample, reveals that the m o d e of failure was cracking in the ceramic coat, near and parallel to the interface between the b o n d coat and the ceramic coat. This type of failure has been observed in all investigations dealing with cyclic oxidation tests of TBCs. Also, the a m o u n t of porosity in the plasma-sprayed ceramic coat and the complete densification of the laser-glazed surface layer can be observed. The b o n d coat seems to be partly oxidized. Levine 25 performed cyclic oxidation tests on similarly prepared samples which were not laser glazed and obtained similar results (within 95'Uo confidence level). Therefore, it appears that glazing did not negatively affect the performance of the TBC. In refs. 20-22 it is claimed that a "segmented" ceramic structure, formed by precracking the ceramic in a direction perpendicular to the plane of the coating as a result of laser glazing, m a y significantly increase its strain tolerance. I m p r o v e d T B C performance, as a result of laser-induced segmentation, was not observed in the present study. 3.2. Cyclic corrosion test
The experimental results obtained in the cyclic corrosion test are summarized in graphic form in Fig. 8. There are two sets of data. One set represents data obtained with ceramic coatings 0.020 cm thick and the other set pertains to coatings 0.040 cm thick. It is evident that the glazed TBCs performed significantly better than the reference non-glazed TBCs. At the 50 W m m - z power density level the improve-
281
L A S E R - G L A Z E D ZIRCONIA COATINGS
36O m 3OO ,.02.240
c~180 ~120 z
60
355 321 O.0~O 0.(~O O.~O 0. 040 0.940 0.040 ZrO2 COATTHICKNESS, cm
0i ~
cm
O
50 75 0 35 LASER POWER DENSITY,wlmm2
50
Fig. 7. Microstructure of cyclic oxidation sample after 2800 cycles at 1050°C (zirconia coat thickness, 0.040 cm; power density, 50 W mm- 2). Fig. 8. Life of zirconia (ZrO2-8wt.%Y203) TBCs in a burner rig cyclic corrosion test at 843 °C (Ni-CrA1-Y thickness, 0.012 cm; zirconia thicknesses, 0.020 and 0.040 cm; fuel, Jet A doped with 100 ppm Na; cycle, 55 min heating, 5 rain cooling).
Fig. 9. Microstructure near leading edge of a cyclic corrosion sample after 408 cycles at 843 °C (zirconia coat thickness, 0.040cm; power density, 50 W mm- 2). ment in life is nearly fourfold. Similar i m p r o v e m e n t can be observed when the thickness of the ceramic coat is doubled. As can be seen in Fig. 5 the location of the failures is random. Figure 9 shows the microstructure of the T B C near the leading edge. The samples failed in the usual manner, i.e. cracking occurred in the ceramic coat near and parallel to the interface between the b o n d coat and the ceramic coat. O n several occasions an unusual type of failure was observed, namely separation of the b o n d coat from the substrate. The degree of attack of the b o n d coat appears to be insignificant. Further examination of this microstructure reveals the loss of thickness of the ceramic coating as the glazed layer is nearly gone. Since yttriastabilized zirconia does not react with sodium sulfate 26 this loss m a y be attributed to erosion. Spallation was p r o b a b l y not responsible for this loss since the surface remains smooth. Figures 10 and 11 show electron m i c r o p r o b e traverses of nonglazed and glazed TBCs respectively. Although determinations for several elements were made, only the traces for sodium, sulfur and oxygen are shown. The non-glazed sample (Fig. 10) which failed in 66 cycles exhibits a significant degree of b o n d coat
282
I. ZAPLATYNSKY
ZIRCONIA
• BOND .h SUBS[RATE • COATING ZIRCONIA COATING
~,
A
j
",v'xj.,.t t "x I a ~
,'qx.
[1 ,~ \/1 ]
v
"1" BOND COATING ,] SUBSTRATE I
f~
I
"-'M
!,
1
V
I,i I
tl
I
~b;
It] "@ ," I
O
I
1
I
2
I
3
~+. . . . . . . . -t
DIbTANCE FROM THE SURFACE, 10-2 cm
4
5
r
J
1 2 3 4 DISTANCE FROM THE SURFACE, 10-2 cm
5
Fig. 10. Electron microprobe traverse of non-glazed corrosion sample after 66 cyclesat 843 °C (zirconia thickness, 0.040 cm): - - , sulfur; • , sodium; - - -, oxygen. Fig. 11. Electron microprobe traverse of laser-glazed corrosion sample after 315 cycles at 843~'C (zirconia thickness,0.040 cm; power density, 35 W mm 2): , sulfur, • , sodium; - - -, oxygen. o x i d a t i o n a n d / o r sulfidation a l o n g grain b o u n d a r i e s as revealed by the oxygen a n d sulfur traces. It appears that every pore in the ceramic c o n t a i n s s o d i u m sulfate. It can be seen that the glazed sample (Fig. ! 1) is considerably less affected despite the fact that its exposure to the corrosive e n v i r o n m e n t was 5 times longer. The oxygen a n d sulfur traces in the b o n d coat indicate less oxidation a n d / o r sulfidation along the grain boundaries. Also the ceramic coat shows less p e r m e a t i o n by s o d i u m sulfate. I n b o t h cases there is a c o n c e n t r a t i o n of s o d i u m sulfate at the interface between the ceramic coat a n d the b o n d coat. The experimental d a t a o b t a i n e d in this investigation provide strong evidence that laser glazing of zirconia-based TBCs will i m p r o v e their life in a corrosive e n v i r o n m e n t as a result of reduced permeability of the surface. 4. CONCLUSIONS As a result of this preliminary study of the effect of laser glazing o n the life of zirconia-based TBCs in cyclic o x i d a t i o n a n d cyclic corrosion tests, the following conclusions have been drawn. (1) The laser-glazed zirconia-based TBCs show at least a fourfold improvem e n t in life over n o n - g l a z e d TBCs in cyclic corrosion tests.
LASER-GLAZED ZIRCONIA COATINGS
283
(2) Laser glazing of zirconia-based TBCs to give an aerodynamically smooth surface has no apparent effect on their life in cyclic oxidation tests. Glazed and nonglazed TBCs endured similar numbers of cycles in a burner rig fired with Jet A fuel. (3) The concept of increasing strain tolerance of TBCs by "segmenting" the ceramic layer through exposure to a high intensity heat source such as a laser was not borne out by this study. REFERENCES
4 5 6 7 8 9 l0 ll
12 13
14 15 16 17 18
L. Y. Schafer, Jr., Comparison of theoretically and experimentally determined effects of oxide coatings supplied by fuel additives on uncooled turbine blade temperature during transient turbojetengine operation, NACA Research Memo. R M E53A19, 1953 (National Advisory Committee for Aeronautics). E. R. Bartoo and Y. L. Clure, Experimental investigation of air-cooled turbine blades in turbojet engine XIII endurance evaluation of several protective coatings applied to turbine blades of nonstrategic steels, NACA Research Memo. RM E53E18, 1953 (National Advisory Committee for Aeronautics). L. N. Hjelm and B. R. Bornhorst, Development of improved ceramic coatings to increase the life of XLR 99 thrust chamber, Research Airplane Committee Rep. on Conf. on the Progress of the X-15 Project, in NASA Tech. Memo. TM X-59072, 1961, pp. 227-253. N. M. Nijpjes, ZrO z coatings on nimonic alloys. In F. Benesovsky (ed.), Proc. 6th Plansee Semin. on High Temperature Materials, Reutte, Springer, New York, 1969, pp. 481-499. S. J. Grisaffe, Coatings and protection. In C. Sims and W. C. Hagel (eds.), The Superalloys, Wiley, New York, 1972, Chap. 12, pp. 341-370. A. N. Curren, S. J. Grisaffe and K. C. Wycoff, Hydrogen plasma tests of some insulating coating systems for the nuclear rocket thrust chamber, NASA Tech. Memo. TM X-2461, 1972. C. H. Liebert and S. Stecura, Ceramic thermal protective coating withstands hostile environment of rotating turbine blades, NASA Teeh. Brief, B75-10290, 1975. S. Stecura, Two-layer thermal barrier coating for turbine airfoils furnace and burner rig test results, NASA Tech. Memo. TM X-3425, 1976. S. Stecura and C. H. Liebert, Thermal barrier coating system, U.S. Patent 4,055,705, October 25, 1977. S. R. Levine, R. A. Miller and P. H. Hodge, Thermal barrier coatings for heat engine components, S A M P E Q., 12 (1) (1980) 20 26. R. Y. Bratton, S. C. Singhal and S. Y. Lee, Ceramic gas turbine components research and development, part 2 evaluation of ZrO 2 Y2Oa/NiCrAIY thermal barrier coatings exposed to stimulated gas turbine environments, Final Rep., June 1978 (Electric Power Research Institute Contract RP421-1 ). S. R. Levine and Y. S. Clark, Thermal barrier coatings a near term, high payofftechnology, NASA Tech. Memo. TM X-73586, 1977. P. E. Hodge, S. Stecura, M. A. Gedwill, I. Zaplatynsky and S. R. Levine, Thermal barrier coatings : burner rig hot corrosion test results, NASA Tech. Memo. TM-79005, 1978 (Department of Energy-NASA 2593-78/3). P. E. Hodge, R. A. Miller and M. A. Gedwill, Evaluation of hot corrosion behavior of thermal barrier coatings, NASA Tech. Memo. TM-81520, 1980 (Department of Energy-NASA 2593-16). Y. E. Palko, K. L. Luthra and D. W. McKee, Evaluation of performance of thermal barrier coatings under simulated industrial/utility gas turbine conditions, Final Rep., 1978 (General Electric Co.). S. Stecura, Effects of compositional changes on the performance of a thermal barrier coating system, NASA Tech. Memo. TM-78976, 1978. S. R. Levine, Adhesive/cohesive strength of ZrOz-12w/oY203/NiCrAIY thermal barrier coating, NASA Tech. Memo. TM-73792, 1978. R. A. Miller, Analysis of the response of a thermal barrier coating to sodium- and vanadium-doped combustion gases, NASA Tech. Memo. TM-79205, 1979 (Department of Energy-NASA 2593-79/7).
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I. ZAPLATYNSKY
19 F . J . Kohl, G. J. Santaro, C. A. Stearns, G. C. Fryburg and D. E. Rosner, Theoretical and experimental studies of the deposition of Na2SO 4 from seeded combustion gases, J. Electrochem. Soc., 126(6)(1979) 1054-1061. 20 W.O. Gaffin and D. E. Webb, JT8D and JT9D jet engine performance improvement program-task I: feasibility analysis, NASA Contract. Rep. CR-159449, 1979 (PWA Internal Rep. 5518-38 (Pratt and Whitney Aircraft Group)). 21 I.E. Sumner and D. Ruckle, Development of improved-durability plasma sprayed ceramic coatings for gas turbine engines, Proc. American Institute of Aeronautics and Astronautics-Society of Automotive Engineers-American Society of Mechanical Engineers 16th Joint Propulsion Conf., Hartford, CT, June 30-July 2. 1980, Paper AIAA-80-1193. 22 R.C. Bill, Plasma-sprayed zirconia gas path seal technology, a state-of-the-art review, NASA Tech. Memo. TM-79273, 1979 (A VRADCOM Teeh. Rep. 79-47 (Aviation Readiness and Development Command)). 23 H.R. Gray and W. A. Sanders, Effect of thermal cycling in a Mach 0.3 burner rig on properties and structure of direction solidified 7/7' 8 eutectic, NASA Tech. Memo. T M )(-3271, 1975. 24 M . A . Gedwill, Burner rig evaluation of thermal barrier coating systems for nickel-base alloys, NASA Tech. Memo. TM-81684, February 1981 (Department of Energy-NASA 2593-26). 25 S.R. Levine, personal communication, 1982. 26 I. Zaplatynsky, Reactions of yttria-stabilized zirconia with oxides and sulfates of various elements, NASA Tech. Memo. TM-79005, 1978 (Department of Energy-NASA 2593-78/3).