' TkHArab Conference on the Peaceful Uses of Atomic Energy, Damascus 9-15 Dec. 1996 AAEA Charaterization of Failure Mechanisms of Duplex and Graded Thermal Barrier Coatings Exposed to Thermal Shocks
EG0000321 A.F.WAHEED andH.M.SOUMAN Materials Division, Department of Metallurg, Nuclear Research Centre Atomic Energy Authority, Cairo, Egypt.
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IN738LC 4 l ^yk-ij—j-j lj_j^>_L jjjc 4—J jlj -> 11 OJI—o .1 i n 11 j I t "i •% I J« r ^Jj J—Sj ij ;5_»jli 40 (j j ^ •»"i 11 j>o j O^J L^J'J- *.j4 750 ^ 'j J.- ^ •••"•" SjJl_i JiLi. sLki_J | I . 1 ft (, I 1, I I 4.. J ft^ ^,,^ ] j L ** J ) , V 1 /I -S. V I . *" I 1 |^ A y * •%. r II I I 1 ^ . 1 Q i. \ \ \ j C, \ } + I ] 1 J^^. \ b^^XsvoJ I ^^^A_^^^!^^^J I L^^^,^^^^^^^^ ^ I | \ \ i \ t f j fi • i ^. J 11 \ \ j ^ ** t | ^ ftwi *J _^ f i | . ** i "v \ | I i i... 11 o Inconel 716 5J Abstract Thermal- barrier coatings (TBC's), which have insulating properties and hot-corrosion resistant, have increasing utilization in the hot sections of gas turbine engines. All such coatings are applied by plasma spraying. The major benefit of theTBC is the reduction of heat transferred into air-cooled components that they provide. The beginning of failure in thermal shock tests of (ZrOo 7%Y?O3)/ (Ni-22%Co-17%Cr-12.5%Al-0.6%Y) duplex and graded coating systems on Inconel 617 and IN 738LC has been characterized. The used burner rig provided a temperature fluctuation of the substrate between 265 and 765 °C, with 40 and 80 sec heating and cooling periods, respectively. The appearance of the first bright spot on the coating surface during heating was considered as the beginning of failure. Stresses due to thermal expansion mismatch on cooling are the probable cause of coating failure. Introduction Gas turbines are used to generate electric power since 1939. Using combined cycle in generating electriciry is a new system. Combined cycle is merely a steam turbine and a gas turbin combine, in this system the critical upper treatment of the cycle increase and this increases the process efficiency significally. The efficiency of combined cycle power plants to more than 50%. The gas stream must reach about 1400 °C or more and turbine components life of 100,000 hours must be attained. To meet these requirments of high temperature for gas turbines, super alloys are used in hot sections of the gas turbine and very high temperature coating materials (as Thermal Barrier Coatings TBC's ) must be applied to the stationary parts of turbine to increase the surface stability and improve resistance agianst oxidation and corrosion. r A The ceramic thermal barrier coatings (TBC's) are recognized to hold considerable promise for a variety of applications involving high heat flux and/or moderately high temperature environments [1]. The active development of ceramic TBC's is currently undertaken by aircraft and gas turbine industries [2,3]- The use of TBC's on high pressure turbine components can improve gas turbine efficiency through reduction of cooling airflow. However, the risk involved in reducing cooling airflow requires highly reliable thermal barrier coatings [4]. These coatings systems generally consist of a plasma- sprayed oxidation/corrosion resistant metallic bond coat of the MCrAIY type (M = Ni and/or Co) and either a single composition ceramic over coat, or a graded structure ceramic over coat to minimize the thermal expansion mismatch between ceramic and metallic substrate. To obtain a high performance TBC, the bond coat layer should idealy have a thermal expansion coefficient, close to that of the substrate and the ceramic part should have a low thermal conductivity, to serve as an effective heat flux barrier [5]. The graded TBC can be utilized to reduce the thermal expasion coefficient mismatch with the substrate and extend the life of the TBC. The coating properties required for turbine engine applications are obtained by the proper selection of coating materials [6] and coating process parameters [7]. The plasma spray process gives an increase of thermal protection and an improvement in the thermal stress resistance. The coating characteristics, especially its porosity, are largely groverned by the extent of particale melting and its velosity to be coated [ 8, 9 ]. The aim of the present work is to characterize the beginning of failure of a(ZrO2)/( Ni-22% Co- 17% Cr- 12.5% Al-0.6 Y ) as duplex and gradded coating systems on Inconel 716 and IN 738LC in burner rig tests. The test conditions are 40 sec. heating up to 750°C substrate temperature, followed by 80 sec. air cooling.substrate temperature at the end of the cooling period was 250°C. Experimental The applied ceramic material was agglomerated and sintered -45 +10 mm particle size powders of partially stabilized zirconia ZrO2/Y2C>3 (93/7). The bond metal Ni-22%Co-17% Cr-17%Cr-12.5%Al-0.6%Y was also supplied in form of -45 +22.5 mm particle size powder.Two plasma sprayed coatings structures were tested, the first was a duplex coating consisting of bond coat and ceramic over coat, and the second was a graded coating consisting of bond coat /bond- ceramic graded zone/ceramic coat. Both thermal barrier coatings were deposited by Vacuum Plasma Spraying (VPS) machine from Plasma-Technik AG, Switzerland. The selected spraying parameters were 55 kw and 720 A. The carrier gas was argon with 1.7 SLPM, and plasma gas was 20 SLPM Ar, 8 SLPM H2 , 25 SLPM He. The plasma spraying was conducted at 100 mbar, with gun to workpiece distance of 250 mm. In the case of duplex thermal barrier, the coating consists of 50-70mm bond coat of Ni-Co-Cr-Al-Y and 270-290 u.m of ceramic coat. This bond coat material was chosen to provide a good protection against oxidation to the substrate alloy. These coatings, deposited on IN 738LC and Inconel 617 alloy specimens in the form of. discs with 45 vm diameter and about 3 mm thickness, were subjected to thermal cycles in a burner rig with heating time of 40 sec and natural cooling time 80 sec . The surface temperature was measured by a pyrometer, and it was about 1200-1250°C.The Substrate lemperature was measured by a thermocouple and it was about 765°C at the end of the heating period and about 265°C after the cooling period. The thermal shock test apparatus and thermal cycle of substrate are shown in Figs. (I) and (2), respectively. The specimens were visually inspected every 25 cycles, and no cracks or spall ing were observed. The appearance of bright spots during heating period was considered as the beginning of failure. Results and Discussion The average life to the beginning of failure of thermal barrier coatings is shown in Table (1). Substrate Duplex Graded Alloy Thickness No. of Cycles Thickness No. of Cycles um to failure (am to failure Inconel 617 345 520 467 810 IN738LC 300 935 462 1500 The typical microstructure of tested thermal barrier coatings are shown in Fig.(3), where the porosity of the ceramic part of the coating is obvious. Porous materials are known to have lower elastic moduli than dense materials [5]. Hence, the porous ceramic part of the thermal barrier is subjected to a lower stress for a constant strain would be the case of dense ceramic [10-12]. The good behavior of the highly porous coating (= 20%) is believed to be due to porosity and the tendency for cracks to be diverted or stopped by the included pores. Specimens tested by thermal shock were cut and examined by optical microscope and SEM. Figures (4) and (5) show the microstructures of duplex and graded coatings, respectively. Both figures indicate the microstructure at mid-point, where the flame strikes the test specimens, and at its edge. The volume expansion in the ceramic part and formation of some cracks are obvious in figs (4) and (5). Pyrometer Burner Figure (1): Thermal shock test apparatus Time Heating period Cooling period Figure (2) : Substrate temperature during thermal cycle The effect of bond coat on the beginning of failure of TBC's was studied. Figs. (6-11) show bond coat of duplex TBC's after thermal shock test and ED AX analysis for some selected points, for both edge and middle zones of the specimens. In Fig. (6), a SEM micrograph for edge zone is presented, in which microcracks can be observed through bond coat. Fig. (7) gives the ED AX analysis for points 1, 2 and 3 shown in Fig. (6). It can be seen that there is an increase of Co and Al elements at points 2 and 3. Fig. (8) shows the bond coat deposited on IN 738LC for the middle zone, and fig. (9) shows the EDAX analysis for points 1 and 2 in fig. (8). The observed fine-cracks may be due to stress generated during thermal cycling, and the formation of brittle phases which resulted from the diffusional transport of elements within the coating, either by Al depletion or formation of aluminide compounds [13,14]. Figs.(10) and (11) show the SEM and EDAX anof bond coat formed on Inconel 617 for the middle zone. It can be seen from Table (1) that the graded coatings were always superior to the duplex coatings, provided that the oxidation limit of the graded layers was not exceeded. This shows the beneficial effect of the graded structure in preventing crack propagation and subsequent spalling. However, some microcracks can be observed in the.outer layer of ceramic, specially in case of Inconel 617 substrate. These cracks may be due to phase transformation of ZrO2- On heating and cooling, monoclinic ZrC>2 undergoes transformation associated with volume change between 4% and 6% [8 , 15). This destabilization reaction can therefore lead to formation of cracks in the coating. Figure (3): Microstructure of thermal barrier coating \rr Hdge X200 M'ddlo X100 Figure (4) : Microstructure of duplex TBC on Incone! 617 after thermal shock test Edgn X20G Middle XtOO Figure (5) : Microstructure of grade TBC on IN 738 LC after thermal shock test \rr Substrate Point (3) Rond Coat .7% Y2O3 ^3C3 SS Figure (6) : SEM for TBC on IN 738LC Point(3) Figure (7) : EDAX analysis for photo 6 kint (1) Bond Coat oint (2) ZrO2.7% Y2O3 Figure (8) : SEM for TBC on IN 738LC for middle zone 2 3 4 5 6 7 8 9ton. 2 3 4 5 6 7 8 9.m Point (i) Point (2) Figure (9) : ED AX analysis for photo (8) Point (1) Point (3) Bond Coat Figure (10) : SHM for TBC on Inconel 617 for middle zone ool 2 3 t, S 3 7 8 9io.-i-j •QQI 2 3 c 5 6 7 6 ftp faint 13) HI)AX analysis for photo (10) Conclusions 1. In the test conditions the main cause of.failure is the stresses due to thermal expansion mismatch encountered during thermal cycling. 2. The life to the begining of failure in case of graded TBC's is approximately double that in case of duplex TBC's . 3. In case of IN 738 LC the time to failure is more than that in case of Inconel-617 for both systems of TBC's. 4. The EDAX analysis showed that the interdiffusion of elements in bond coat may lead to the formation of hard phases.These hard phases with the effect of thermal stresses during heating and cooling cycles are considered of the main causes for bond coat failure. Acknowledgement The authers wish to thank the staff of IAW-KFA Julich, for providing the facilities to do this work, and to express deep thanks to IAEA for the financial suppert of the fellowship. References [1] R.A.Miller and C.C.Brendt, Thin Solid Film, 119, (1984), 195. [2] S.C.Singal and R.I.Bratton, J.Eng. for Power, 102 (4), (1980), 770. [3] A.R .Nicol, S.Keller and G. Wuest, in "Surface Modification and Coatings", R . D. Sisson Jr. (ed.), American Society of Metals, Ohio, (1986). 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