International Journal of Fatigue

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1 International Journal of Fatigue 53 (2013) Contents lists available at SciVerse ScienceDirect International Journal of Fatigue journal homepage: Adhesion strength of ceramic top coat in thermal barrier coatings subjected to thermal cycles: Effects of thermal cycle testing method and environment Masakazu Okazaki a,, Satoshi Yamagishi a, Yasuhiro Yamazaki b, Kazuhiro Ogawa c, Hiroyuki Waki d, Masayuki Arai e a Department of Mechanical Engineering, Nagaoka University of Technology, Kamitomiokamachi, Nagaoka-shi, Niigata , Japan b Department of Mechanical Engineering, Niigata Institute of Technology, Japan c Fracture and Reliability Research Institute, Tohoku University, Japan d Department of Mechanical Engineering, Iwate University, Japan e Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, Japan article info abstract Article history: Received 7 May 2011 Received in revised form 22 February 2012 Accepted 24 February 2012 Available online 14 March 2012 Keywords: Thermal Barrier Coatings (TBCs) Thermal cycle Isothermal exposure Elastic modulus Adhesion strength This paper deals with the adhesion strength of ceramic top coat in thermal barrier coatings (TBCs) subjected to thermal cycles under several different test conditions. Here the TBC specimens consisting of 8% yttria stabilized zirconia, CoNiCrAlY alloy bond coat and Ni-base superalloy were prepared by plasma spraying. The isothermal exposure and the thermal cycles were applied to the TBC specimens by several conditions at high temperatures. A series of the test results clearly demonstrated that the adhesion strength of the top coat was significantly changed by the application of thermal cycles and by the isothermal exposure. It was also found that the thermal fatigue damage might be evolved depending on of the testing method by which the thermal cycles are applied. Some background of these findings were discussed, based on the measurements of elastic modulus, tensile strength, and thermal conductivity of the ceramic top coat, as well as both the thermally grown oxide at the top coat/bond coat interface and the residual stress in the TBC specimens. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Thermally insulating ceramic coatings, known as thermal barrier coatings (TBCs) are essential to improve the performance and efficiency of advanced gas turbines in service at extremely high temperatures [1 6]. The key role of TBCs is, of course, to protect the metal substrate from high temperature oxidation and environmental attack [1,2]. In general the TBC system consists of at least three layers; ceramic top coat, bond coat and metal substrate. The most critical issue limiting durability of TBCs is spallation of the ceramic top coat. Once this type of damage is realized, hot section components made of superalloy substrate may be overheated, resulting in complete failure. Adhesion strength is a major parameter characterizing the resistance of the ceramic top coat against spallation [7,8], where the strength has been often evaluated according to the ASTM standard [7]. In general the adhesion strength of top coat is significantly changed during service. (i) Thermal stress, which is promoted by the mismatches in thermal expansion coefficient and thermal conductivities between the metal substrate, bond coat and ceramic top coat, and (ii) the influence of Corresponding author. address: okazaki@mech.nagaokaut.ac.jp (M. Okazaki). environment, i.e. formation of thermally grown oxides (TGOs) at the bond coat/top coat interface, are essential factors [9,10,12 14]. The interaction between both factors is also important in some cases [12 14]. In the actual TBCs in service the thermal cycle failure is often a critical issue to be concerned, to which all of the above factors commit. Nevertheless, the basic understanding of failure mechanisms and their interaction still is still on the way of research. It is an objective of this work to get basic understanding on effects of thermal cycle testing method on the thermal fatigue damage of TBCs, through the measurements of both the mechanical and physical properties of top coat and the remaining adhesion strength. 2. Experimental procedures 2.1. Preparation of specimens The TBC specimens consisting of three layers; Ni-base superalloy, bond coat and top coat, were prepared in the present work. Here, an 8 wt.% yttria partially stabilized zirconia (YSZ), METCO 204NS, and a CoNiCrAlY alloy, AMDRY9951, were selected as the top and bond coat materials, respectively. The chemical compositions of the powders used are listed up in Table 1. The substrate /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.

2 34 M. Okazaki et al. / International Journal of Fatigue 53 (2013) Table 1 Chemical compositions of the powders used (wt.%). (a) Top coating powder ZrO 2 Y 2 O 3 HfO 2 MgO SiO 2 TiO 2 CaO < (b) Bond coating powder Co Ni Cr Al Y Fig. 1. Geometry of specimens used. (a) Adhesion strength, (b) residual stress, and (c) thermal conductivity. material employed was a polycrystalline Ni-base superalloy, Mar M247. The geometries of TBC specimens used are illustrated in Fig. 1. These TBC specimens were fabricated as follows: after spraying the bond coat alloy by 100 lm in thickness on the substrate, the YSZ top coat was overlay coated by 500 lm in thickness. These processes were managed and performed by atmospheric plasma spraying at Plasma Giken Co. Ltd., Japan. The details are presented in Refs. [11,18]. Moreover, the free-standing top coat specimens were also prepared to measure their some basic properties: elastic stiffness, tensile strength and thermal conductivity. Here, for the former two properties, the specimens were extracted by removing the substrate material from the TBC specimen by chemical solution technique. The outline of the measurement method will be given in each subsection in next chapter Thermal cycle and isothermal exposure tests Either thermal cycles or isothermal exposure was applied to the TBC specimen in air, according to the test conditions summarized in Table 2. Thermal cycles were applied, following the two different type of test methods; hereinafter, denoted by METHOD-I and -II in this work, respectively. In the METHOD-I one single furnace was used to cyclically heat-up and cool-down the TBC specimen under such a cycle frequency low enough that the temperature in the TBC specimens might be changed in steady state without significant temperature gradient [17] (Fig. 2a). The heating and cooling rates employed in this work were approximately C/s and C/s, respectively. The control of test temperature was conducted via R type thermocouples which represented the temperature within the electric furnace. At the same time the TBC specimen temperature was monitored by the thermocouples welded on the substrate, to ensure insignificant temperature difference between the furnace and specimen. In the METHOD-II thermal cycle test, on the other hand, two furnaces isolated with each other; higher and lower furnaces, were used. The temperature of the two furnaces was kept constant so that they corresponded to higher and lower temperatures in the thermal cycle test. At the same time, the specimen temperature was continuously monitored by the thermocouples, as well. During the test the TBC specimen traveled periodically between the two furnaces, through a mechanical driving system. Here, the traveling time was so short (i.e., within 30 s) that the temperature gradient in the specimens was significant during the traveling period, as will be given in Section 3.2. The heating and cooling rates in the MEHOD-II were approximately 1.5 C/s and 0.67 C/s in average, respectively. The thermal cycle test conditions are summarized in Table 2. The isothermal exposure test was also carried out in air, by means of an electric furnace. After applying either thermal cycles or isothermal exposure, the residual adhesion strength of the ceramic top coat was evaluated, according to the ASTM standard, C633 [7], as a representative measure to assess the damages. The residual stress in the top coat was also measured by the stress relief method. Here the stress was evaluated from the measurement of relief strain between before/ after the removal of metallic substrate and bond coat. 3. Results and discussion 3.1. Isothermal exposure test Elastic stiffness of the ceramic top coat was measured by applying an external tensile load to the free-standing mm rectangular plate specimen, where the tab plates were adhered on the both sides of the specimen parts for clamping (Fig. 3a). Stress strain relation was monitored via a load cell and a strain gauge directly adhered at the center of specimen gauge section (see Fig. 3a), from which slope the elastic modulus was determined. Fig. 3b reveals some typical stress strain curves. The elastic modulus evaluated is summarized in Fig. 4 as a function of isothermal Table 2 Test program of thermal cycle and isothermal exposure tests. Type of test Environment Temperature/temperature range Time/number of cycle High temperature (isothermal) exposure In air 800 C, 900 C, 1000 C, 1100 C 0, 100, 300, 1000 h METHOD-I thermal cycle (uniform heating and cooling) In air (in vac.) C, C, C, C 0, 10, 100, 1000 (cycles) METHOD-II thermal cycle (non-uniform heating and cooling) In air C 0, ,1000 (cycles)

3 M. Okazaki et al. / International Journal of Fatigue 53 (2013) Fig. 2. Test equipments used for the thermal cycle tests. (a) METHOD-I. (b) METHOD-II. Fig. 4. Change in elastic stiffness with high temperature exposure. Fig. 3. Tensile test of the self-standing top coat specimen. (a) Grip tab and strain gauges adhered on the specimen surface. An appearance after the tensile test, (b) typical stress strain curves measured. exposure time and temperature. It is found that the modulus significantly increased when the specimens were exposed at high temperature for long time. For example, the elastic modulus after the exposure at 1000 C increased from an initial value of about 20 GPa to about 35 GPa after 1000 h. When the exposure time was long enough, the modulus seemed to be almost saturated. These increases might be resulted from progress of sintering of the top coat during isothermal exposures. As a matter of fact, a decrease in porosity fraction with isothermal exposure has been confirmed in the Phase II collaboration activities in the Society of Materials Science, Japan [11]. Fig. 5 expresses the change in tensile strength of the top coat as a function of isothermal exposure time, where the measurements were carried out following the same method as that for the Young s modulus measurement. It is found from Fig. 5 that the tensile strength also dramatically increased and then converged into a saturated value. It is worthy to note that the increasing behavior in tensile strength is very similar to that in elastic modulus, suggesting that these two changes were caused by the same mechanism(s). One of mechanisms may be a progress of sintering. The thermal conductivity of the top coat, k tc, was also measured applying the laser heat flux rig test method employed elsewhere [12,13,18]. In this work the value of k tc was approximately

4 36 M. Okazaki et al. / International Journal of Fatigue 53 (2013) Fig. 5. Change in tensile strength with high temperature exposure. Fig. 7. Change in residual stress built-up in the ceramic top coat with high temperature exposure. Fig. 6. Change in thermal conductivity with high temperature exposure. calculated from the steady state temperature gradient in the TBC system by k tc ¼ k bm ðt i T b Þ=t bm ðt s T i Þ=t tc where k bm is thermal conductivities of the base metal, and t tc and t bm are thickness of the top coat and that of base metal including bond coat, respectively. T i, T b and T s are temperatures at the bond/top coat interface, at the bottom of substrate and at the surface of top coat, respectively: those were measured by thermocouples mounted inside or outside of the specimen (see Fig. 1c). The changes in thermal conductivity are summarized in Fig. 6. As shown here, whereas the thermal conductivity rapidly increased at the beginning of exposure, the increasing rates almost converged after a prolonged exposure, depending on the exposing temperature. These behaviors must be also corresponding to progress of sintering. The residual stress, r r, built-up in the ceramic top coat was estimated from the relief strain, e relief,by r r ¼ E tc e relief where E tc and e relief are elastic modulus of top coat (measured in Fig. 4), and a released strain on the substrate being removed from ð1þ ð2þ Fig. 8. Change of the remaining adhesion strength of ceramic top coat with high temperature exposure. the TBC specimen, that is measured via strain gauge. In Eq. (2) the stiffness of substrate is approximated to be high enough, compared with that of top coat. The measurement result is summarized in Fig. 7. The residual stress in the as-sprayed top coat was almost zero or slightly tensile. After being exposed at high temperature, it varied into compression, which was corresponding with the typical results reported elsewhere [14]. The change of the adhesion strength of top coat with isothermal exposure time and temperatures is given in Fig. 8, where the measurements were on the basis of the ASTM standards [7]. It is found from this figure that the adhesion strength was slightly increased with the increase of exposure time. This trend is roughly similar to the results by other research works [14 16]. The final rupture occurred almost within the top coat near the interface in all the specimens, as shown in Fig. 9. The formation of thermally grown oxide (TGO) was pronounced at the bond/top coat interface, as indicated by Fig. 10. However, it was not directly conjunctive with the final rupture area in the adhesion test. Comparing Fig. 9 with Fig. 5, it is reasonably interpreted that the increase in adhesion strength would be attributed to the increase of top coat strength itself.

5 M. Okazaki et al. / International Journal of Fatigue 53 (2013) Fig. 9. Cross sections of the specimens exposed to isothermal aging after the adhesion test. Top coat Mixed oxide Top coat Alumina Alumina Bond coat Bond coat (a) 300 hours at 900 (b) 1000 hours at 1000 Fig. 10. Thermally grown oxide (TGO) formed at the bon/top coat interface, after the high temperature exposure test. Fig. 11. Temperature history at representative local areas in the TBC specimen during the METHOD-II thermal cycle test Thermal cycle test It is important to know previously how the temperature gradient is inside of the TBC specimen and whether there are any significant differences between the METHOD-I and -II testing methods. Fig. 11 depicts the temperature history at the two representative areas inside of the TBC specimen subjected to the thermal cycles by the METHOD-II in air. Here the temperature at the top/bond coat interface and that at the metal substrate were measured by the thermo-couples mounted at inside the specimen and on the specimen surface, respectively. The measurement for the former part was realized as follows: after the thermocouples were welded at the interface through a small artificially drilled hole across the top coat, the hole was filled again by a zirconia paste to minimize possible influences of the hole; see an illustration in Fig. 11. For the latter measurement location the thermocouples were welded directly near the bond coat/substrate interface on the specimen surface. It is seen from Fig. 11 that the temperature was not uniform inside of the specimen, especially during the traveling period of the specimen between the two furnaces. This must be due to a difference in thermal conductivity between the top coat and substrate. Thus, thermal stress is generated not only from the thermal expansion coefficient mismatch between the YSZ top coat and metal substrate but also from the temperature gradient in

6 38 M. Okazaki et al. / International Journal of Fatigue 53 (2013) Fig. 12. Remaining adhesion strength of the ceramic top. (a) After thermal cycles by METHOD-I (associating with almost uniformly heating and cooling), (b) after thermal cycles by METHOD-II (associating with non-uniformly heating and cooling). specimen. In the METHOD-I, on the other hand, the temperature gradient inside of the TBC specimen was negligible, because the heating and cooling rates were pretty slow (Table 2). The adhesion strengths of the ceramic top coat were summarized in Fig. 12. It is found from Fig. 12a that the adhesion strength by the METHOD-I increased at least up to 300 cycles. This trend is similar to the test results shown in Fig. 8. Here, the final decohesion of the ceramic top coat occurred within the top coat somewhat apart from the interface, as shown in Fig. 13a. It is also interesting in Fig. 12a that there were no significant effects of the thermal cycle test environment; compare the results between in air and vacuum. Note again that a major factor to produce thermal stress is thermal expansion mismatch between the top coat and substrate in the METHOD-I, since the TBC specimen was heated and cooled with negligible temperature gradient. In the case of the METHOD-II thermal cycle test, the remaining adhesion strength was monotonically decreased with the number of thermal cycles (see Fig. 12b). This trend was in contrast with that in Fig. 12a. As indicated by Fig. 11, the temperature gradient inside the specimen was significant in the METHOD-II, especially during the traveling period. In the other words, in the METHOD-II the thermal stress is generated, not only by the thermal expansion coefficient mismatch between YSZ top coat and metal substrate, but also by the temperature gradient in specimen. It is worthy to note from the comparison between Fig. 12a and b that not only the adhesion strength itself but also their changing behavior were not always similar. In addition, the change of residual stress with thermal cycles was also different between the M- ETHOD-I and -II, as shown in Fig. 14. Thus, it is natural to postulate that the temperature gradient induced thermal stress might change the internal stress state, resulting in a change of Fig. 13. Cross sections after adhesion test of the specimen subjected to thermal cycles. (a) After thermal cycles by METHOD-I, (b) after thermal cycles by METHOD-II.

7 M. Okazaki et al. / International Journal of Fatigue 53 (2013) was found in the remaining adhesion strength as well as the residual stress, those were strongly dependent on the testing method to give the thermal cycles. It was suggested that the mechanical factor(s) to induce thermal stress might play more major role in the above differences and little contribution from environmental influence in this work. Acknowledgements Some of the present tests have been carried out as a part of collaborative research organized by Sub-committee on Superalloys and Coatings, The Society of Material Science, Japan. The authors are expressing their gratitude to the Sub-committee member for their fruitful discussions. One of authors, M. Okazaki, is expressing his gratitude to a financial support to a part of this project by the Grain-in-Aid for Scientific Research by JSPS (No ). References Fig. 14. Change of residual stress with thermal cycles depending on the testing methods. thermal fatigue damage. Here a possible additional phenomenon must be an acceleration of microcracking inside the top coat. Between the METHOD-I and -II there was little difference in the final rupture site in the adhesion test; that was inside of top coat apart enough from the prior bond/top coat interface where the formation of TGO layer was significant (Fig. 13). This means that the TGO was not responsible directly for the difference in remaining adhesion strength between the METHOD-I and -II. Thus, it is reasonable to consider that a mechanical role to produce thermal stress played more essentially in the differences seen in Figs It should be noted that the lower temperature in the present thermal cycle test was 400 C, which is relatively close to the ductile brittle transition temperature (DBTT) of MCrAlY bond coat alloys; from 300 to 500 C in many cases [19,20]. This kind of mechanical properties of bond coat can also commit to the differences shown in Figs , since thermal cycle damage seems to be evolved during cooling at temperatures below the DBTT [19]. When this is the case, the difference between METHOD-I and -II may get more pronounced with the decrease of minimum temperature in thermal cycle test. No matter which factors are more responsible for, it is important to keep in mind that the thermal cycle failure life should be dependent on the testing method. Now an advanced project has begun to more quantitatively explore the backgrounds of dependence of testing method, via numerical stress analysis. 4. Conclusion This paper dealt with the adhesion strength of ceramic top coat in thermal barrier coatings (TBCs) subjected to thermal cycles under several different test conditions and testing systems, comparing with that exposed to isothermal aging at high temperature. 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