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Thermal fracture of ceramic thermal barrier coatings under high heat flux with time-dependent behavior.
Materials Science and Engineering A299 (2001) 296 – 304 www.elsevier.com/locate/msea
Thermal fracture of ceramic thermal barrier coatings under high heat flux with time-dependent behavior. Part 1. Experimental results Brian D. Choules a, Klod Kokini a,*, Thomas A. Taylor b a
School of Mechanical Engineering, Purdue Uni6ersity, 1288 Mechanical Engineering Building, West Lafayette, IN 47907 -1288, USA b Praxair Surface Technologies, Inc., 1500 Polco Street, Indianapolis, IN 46224, USA Received 18 October 1999; received in revised form 14 April 2000
Abstract The objective of this research was to study the thermal fracture of ceramic thermal barrier coatings under high heat flux laser heating and investigate the effect of time-dependent behavior of the ceramic. Continuously plasma-sprayed zirconia coatings with thicknesses varying from 0.26 to 1.5 mm were heated with a CO2 laser to maximum surface temperatures varying from 700 to 1700°C. Temperature differences from 600 to 1300°C across the ceramic coating were applied. High heat flux heating was found to cause changes in the material, leading to a denser microstructure and relaxation of compressive stresses. These changes lead to the development of surface and interface cracks during ambient air cooling following laser heating. Increasing the coating thickness was found to decrease the number of surface cracks developed and increase the distance between cracks. Surface cracks were found to extend further into thin coatings than thick coatings. Increasing the maximum surface temperature was found to increase the surface crack length. The thinnest and thickest coatings developed fewer and shorter interface cracks than coatings with intermediate thicknesses. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermal barrier coatings; Interface crack; High heat flux; Stress relaxation; Surface crack
1. Introduction Thermal barrier coatings (TBCs) have been developed for various applications for more than 30 years. A thermal barrier coating is a thin layer or coating of insulating material that is bonded to a substrate for protection from high-temperature environments. The coating is usually ceramic while the substrate is typically metal. The TBC allows for increased operating temperatures, increased efficiency, improved component durability, simplified designs, improved fuel economy, and sometimes the substitution of less-expensive metallic substrates. The TBC is applied by first depositing a thin bond coat layer (typically 120 mm MCrAlY (M=Co, Ni, Cr)) onto a metallic substrate. A low thermal conductivity ceramic layer is then deposited on * Corresponding author. Tel.: +1-765-4945727; fax: + 1-7654940539. E-mail address: [email protected] (K. Kokini).
top of the bond coat. One common material used as a TBC is plasma-sprayed yttria (Y2O3)-stabilized zirconia (ZrO2). TBCs are currently being used or considered for engine applications such as aerospace (space planes), aircraft, marine automobiles, nuclear fusion reactors and heavy-duty utilities (i.e. diesel trucks, electric power generation, etc.). The durability requirements of TBCs for these applications are increasing rapidly. For example, reusable rocket-powered vehicles and air-breathing space transportation vehicles encounter extremely high heat loads in their engines and air frames [1]. Also, the blades in the high-pressure fuel turbopump on the space shuttle main engines undergo very large thermal transients on start-up of engines [2]. In addition to these references, other workers have also indicated the need for coatings to survive temperatures of 1400– 1500°C [3,4]. As engines run closer and closer to temperatures where TBCs are considered unstable, the possibility of unanticipated overloads increases. While
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5093(00)01393-9
B.D. Choules et al. / Materials Science and Engineering A299 (2001) 296–304
the thermal load provided by the laser experiment developed in this study may be considered to be more severe than the requirements of the current applications, it represents an upper bound of the loading that can be applied on such coatings and may model the overload scenario. The results found in this study are necessary for the design of the next generation of engines that will be required to sustain higher temperatures and gradients than those in today’s applications. Development of ceramic TBCs for high heat flux environments requires a fundamental understanding of the fracture mechanisms in order to prevent the spalling of the coating from the substrate. The first such studies were performed by Takeuchi and Kokini [5,6] on graded thermal barrier coatings using lower heat fluxes. Only a few studies have been performed to investigate the failure of TBCs under high heat flux heating [7–10]. Thus, a CO2 laser was used to study the effect of time-dependent behavior of the material on surface, and interface cracking of thermal barrier coatings under high heat flux heating. In particular, the effect of surface temperature and TBC thickness on the failure mechanisms under high heat flux heating were investigated. The focus of this study was to investigate the failure mechanisms that occur during the application of a simple high heat flux cycle. Thermal fatigue, by application of multiple high heat flux cycles, was not investigated in this study.
2. Test specimen The test specimens were beam shaped and consisted of a 7 wt.% yttria-stabilized zirconia coating bonded to a steel substrate by two intermetallic bond coatings, as shown in Fig. 1. The steel substrate was 12.7 mm tall, 31.75 mm long and 3 mm wide. The zirconia coating (LZ-26) had a relative density of 87% (measured using ASTM-B328 corrected for water temperature [11]). A chemical analysis of the starting ceramic powder indi-
Fig. 1. Specimen schematic diagram.
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Fig. 2. Continuous plasma-sprayed coating thicknesses studied.
cated 7.06 wt.% Y2O3, 1.73 wt.% HfO2, and the balance ZrOd2. Ceramic coatings with thicknesses (lc) of 0.26, 0.57, 0.75, 1.04 and 1.5 mm were manufactured for research and are shown in Fig. 2. Both bond coatings, designated as LCO-22 and LCO-35, have a nominal composition of 39Co–32Ni–21Cr–8Al–0.5Y However, the as-coated LCO-22 and LCO-35 bond coatings without heat-treatment have relative densities of 91.4 and 90.1% [12]. The composition of these coatings was selected on the basis of oxidation resistance and bonding strength with metal and ceramic. Both LCO-22 and LCO-35 layers were sprayed to 0.08 mm. A 50× 100× 12.7 mm ANSI 1020 steel plate was prepared for coating by grit blasting the surface with 60 mesh Al2O3 grit at 40 psig1 to increase the mechanical bonding between the steel substrate and bond coat. The bond coatings and zirconia coating were then plasma sprayed on one at a time. The average temperature of the substrate during plasma spraying was measured as 100°C. The test specimens were cut out by a CNC-controlled abrasive waterjet in the shape of beams.2 The external coating surface was kept in the as-sprayed condition, leaving a surface roughness of : 13 mm RA. The cross-sectional face of each specimen was metallographically polished to better observe crack formation during subsequent laser heating and to eliminate any flaws induced during water jet cutting. The ceramic surface was painted with Pyromark® Series 2500 silicon-based paint. Three coats were applied with a paint brush for a total thickness not exceeding 40 mm. The paint was allowed to dry for 18 1 The specimens were manufactured by Praxair Surface Technologies, Indianapolis, IN. 2 The waterjet cutting was performed by Luick Quality Gage & Tool, Inc., Muncie, IN.
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3.2. Temperature measurement The substrate temperature was measured using a Type K thermocouple that was spot welded to the bottom of the steel substrate. The surface temperature was measured using an Ircon3 (Niles, IL, USA) infrared pyrometer (Modline 7000). This pyrometer was able to measure surface temperatures between 500 and 2500°C by measuring the intensity of the radiation emitted from the target surface. The intensity of the radiation emitted at a single wavelength depends on the temperature and surface characteristics of the material. The pyrometer used in the present study measures radiation in the wavelength range of 4.8–5.2 mm. The known surface emissivity of the Pyromark® paint allowed the pyrometer to measure the correct surface temperature.
3.3. In situ and post-experiment crack obser6ation
Fig. 3. Experimental set-up.
h. The painted surface was cured by heating at 250°C for 2 h in a furnace. During curing and the subsequent cooling, the furnace doors were kept shut to eliminate the possibility of cracks forming due to sudden air cooling. The specimens were checked with an optical microscope to insure no cracking was induced during the paint curing procedure. The paint provided for more accurate surface temperature measurements using infrared pyrometry.
3. Experimental set-up To investigate the failure mechanisms of ceramic thermal barrier coatings subjected to heating by a high heat flux, the experimental set-up shown in Fig. 3 was used.
3.1. High heat flux laser heating A 1.5 kW CO2 laser manufactured by Convergent Energy was used to heat the surface of the ceramic coating. Characteristic of a CO2 laser, the laser light is of 10.6 mm wavelength. At this wavelength, the absorptivity of the zirconia coating is essentially 1, thus the laser light is absorbed at the surface of the coating [13]. The heat flux distribution was determined to be Gaussian with a diameter of 12 mm, which contains 86.6% of the power. Each specimen was laser heated for 4 s and allowed to cool by ambient air.
A Questar telescopic microscope was used to optically observe and record the cracking in the coating during the experiment. The microscope had 100× magnification. A CCD camera was used to digitize the image and a Super VHS VCR recorded the images at a rate of 32 frames per second. After each specimen was heated with the laser, the cracks were identified and measured with a Zeiss microscope.
4. Results and discussion Continuously sprayed coating thicknesses of 0.26, 0.57, 0.75, 1.0, and 1.5 mm were laser heated only once, for 4 s, and allowed to cool in ambient air. Approximately four specimens of each thickness were heated in 100°C maximum surface temperature increments between 700 and 1700°C. The surface temperature versus time for typical specimens of each thickness is shown in Fig. 4. The substrate temperatures during the heating remained under 150°C. Two different cracking modes were found to occur upon cooling after laser heating: surface cracks, and surface/interface cracks. A surface crack was perpendicular to the surface, and a surface/ interface crack included a surface crack as well as an interface crack that was located at or above the interface between the zirconia coating and bond coating. The surface cracks were all found to occur within the zone of the specimen heated by the laser. Interface cracks were always found directly below surface cracks. However, the surface cracks did not necessarily extend through the coating to the interface. A typical surface crack in a 1.0 mm thick coating is shown in Fig. 5. A typical surface/interface crack in a 0.75 mm thick coating is shown in Fig. 6. 3
Ircon, Inc., 7301 North Caldwell Ave., Niles, Illinois 60714.
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A schematic diagram of the cracking behavior as a function of thickness and temperature is shown in Fig. 7. Typically coatings heated below 1000°C did not develop any cracks. In the maximum surface temperature range of 1000 – 1400°C, the coatings were found to develop surface cracks. The thinnest coating (lc = 0.26 mm) developed five to seven multiple surface cracks spaced close together (d =0.70 90.24 mm). The thicker coating with lc =0.57 mm also exhibited multiple cracking, typically three to five cracks spaced 1.1 90.3 mm
Fig. 6. Surface and interface crack in a lc =0.75 mm thick ceramic layer specimen after surface heating with a laser to a maximum temperature of 1540°C.
Fig. 4. Measured surface temperature versus time for coatings of different thickness.
apart. The three thickest coatings (lc = 0.75, 1.0, and 1.5 mm) developed usually three surface cracks spaced 1.4290.37, 1.79 0.46 and 1.990.43 mm apart, respectively. These results clearly showed that, in this surface temperature range, increasing the coating thickness resulted in an increase in surface crack spacing and a decrease in the number of surface cracks. Coatings subjected to maximum temperatures in the range of 1400–1700°C developed surface cracking and, depending on their thickness, interface cracking. The thin coatings (lc = 0.26 and 0.57 mm) did not readily develop interface cracks, but the number of surface cracks was larger. The 0.75 and 1.0 mm coatings both developed large interface cracks, while the 1.5 mm coating developed only small interface cracks. For example, with Tmax between 1450 and 1550°C, the average interface crack length in the 0.75, 1.0 and 1.5 mm coatings was 1.36, 0.79, and 0.46 mm, respectively.
4.1. Time-Dependent beha6ior of plasma-sprayed zirconia under laser heating
Fig. 5. Scanning electron microsopy micrograph of polished cross-section showing surface crack in a lc = 1.0 mm thick ceramic layer specimen after surface heating with a laser to a maximum temperature of 1050°C.
Choules et al. showed that, without the presence of stress relaxation and sintering, the stresses incurred during laser heating and subsequent cooling were not large enough to cause fracture in the coating [14,15]. It was determined that the time-dependent behavior of the coating led to the fracture of the coating. This behavior was first observed by Takeuchi and Kokini [5] in lower heat flux, longer-term experiments. Subsequent research showed that using a ceramic such as mullite, with significantly reduced stress relaxation, resulted in reduction in cracking [16]. The stress relaxation behavior is related to a stress-enhanced ceramic sintering phe-
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Fig. 7. Schematic diagram showing effect of thickness and temperature on cracking behavior of ceramic coatings under laser heating.
nomenon, as has been observed in past experiments by other workers [17 – 19]. The relative boundary sliding of plasma-sprayed splats and grains, and the stress redistribution around the splats and microcracks are important mechanisms for ceramic coating shrinkage and stress relaxation. As a result of stress relaxation, the compressive stresses during heating decrease. Then upon cooling, tension results in the now compacted coating as it tries to shrink to a shorter length than its original one. This tension leads to surface crack propagation and possibly the initiation and propagation of an interface crack between the bond coat and ceramic.
4.2. Effect of surface temperature and coating thickness on surface cracking beha6ior The surface and interface cracks resulting from the application of the laser heat flux were measured using an optical microscope at 200 × magnification. A schematic showing the crack length and spacing definitions is shown in Fig. 8. The measured center surface crack length (asc) and side surface crack length (ass), normalized with respect to coating thickness (lc), as a function of maximum surface temperature during laser heating are presented in Figs. 9 and 10, respectively. The surface crack lengths for each temperature were averaged for approximately four specimens, each in approximately 100°C temperature intervals. The variability in the data may be attributed to the highly microcracked and porous nature of plasma-sprayed zirconia. Even with the considerable variability in the measured data, two trends are noticeable in Figs. 9 and 10: (1) the surface crack lengths increase with temperature; (2) the normalized surface crack length increases as coating thickness decreases. Therefore, surface cracks penetrate further into thin coatings than thicker ones.
The thin 0.26 mm coatings developed between four and nine surface cracks. Micrographs of two 0.26 mm specimens are shown in Figs. 11 and 12. Fig. 11 shows that, prior to heating, surface cracks were not present, while Fig. 12 shows a specimen heated to 1500°C that developed multiple cracks spaced approximately 0.5– 0.6 mm apart. As shown in Figs. 9 and 10, surface cracks begin to appear at 1000°C and their length increases sharply with temperature. Beyond 1400°C, the majority of the cracks extend all the way through the ceramic layer. The 0.57 mm thick coatings heated above approximately 1050°C developed three to five surface cracks within the laser-heated zone. Surface crack lengths increased with temperature, however not nearly as sharply as the 0.26 mm specimens (Figs. 9 and 10). As shown in Fig. 7 (configuration A), the 0.75 and 1.0 mm coatings developed three surface cracks with
Fig. 8. Schematic diagram showing crack length and spacing definitions.
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Fig. 11. Scanning electron microsopy micrograph of an unheated lc =0.26 mm specimen.
Fig. 9. Experimental averaged asc/Ic versus maximum surface temperature for different coating thicknesses (lc).
one at the center of the specimen and two off to the side of the center of the specimen. Surface cracks began to form around 1050°C and their length increased with temperature (Figs. 9 and 10). At 1200°C, the average ass/lc value was approximately 0.27 and 0.33 for the
0.75 and 1.0 mm coatings, respectively. At 1600°C, the average ass/lc value was approximately 0.41 and 0.65 for the 0.75 and 1.0 mm coatings, respectively. The 1.5 mm coatings developed three surface cracks similar to the 0.75 and 1.0 mm coating. Surface cracks began appearing at 1000°C and their length increased with temperature (Figs. 9 and 10). However, contrary to the thinner coatings, the crack length ratio ass/lc was only 0.22 near 1200°C and 0.37 near 1600°C. Thus, the ratio of surface crack length to coating thickness was smaller for the thickest coatings at higher temperatures.
4.3. Effect of surface temperature and coating thickness on interface cracking beha6ior The center (aic) and side (ais) interface crack lengths versus the maximum surface temperature for the different thicknesses studied are shown in Figs. 13 and 14, respectively.
4.3.1. Coating thicknesses of 0.26 and 0.57 mm The 0.26 mm coatings did not develop any interface cracks following laser heating, while only three out of 24 (12.5%) of the 0.57 mm specimens heated above 1400°C developed interface cracks. These interface cracks developed at 1400, 1525, and 1650°C.
Fig. 10. Experimental averaged ass/lc versus maximum surface temperature for different coating thicknesses (lc).
Fig. 12. Scanning electron microsopy micrograph of a lc = 0.26 mm specimen showing multiple surface cracks.
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Of the 12 specimens that developed interface cracks, seven (58.3%) were in the center surface/interface crack configuration (B2), while the remaining five (41.7%) were in the side surface/interface crack configuration (B1). The center interface cracks in configuration (B2) developed for maximum surface temperatures ranging from 1300 to 1650°C, while the side interface cracks in configuration (B1) developed for maximum surface temperatures ranging from 1450 to 1700°C. Thus, slightly higher surface temperatures were needed to induce the side/interface crack configuration. As shown in Figs. 13 and 14, the center interface cracks (aic) began appearing at 1300°C, while the side interface cracks (ais) begin appearing at 1500°C.
Fig. 13. Experimental center interface crack length (aic) versus temperature for different coating thicknesses (lc).
4.3.2. Coating thickness of 0.75 mm Interface cracks in the 0.75 mm coatings occurred in two configurations (Fig. 7): 1. side surface/interface cracking, two side surface crack formed with an interface crack directly below each side surface crack (indicated as B1 in Fig. 7); and 2. center surface/interface cracking, three surface cracks formed with an interface crack directly below the center surface crack (indicated as B2 in Fig. 7).
Fig. 14. Experimental side interface crack length (ais) versus temperature for different coating thicknesses (lc).
4.3.3. Coating thickness of 1.0 mm The 1.0 mm coatings were also found to develop interface cracks in the same two configurations (B1 and B2) as those for the 0.75 mm thick coating. However, only three of the 11 (27.3%) specimens had the center surface/interface crack configuration (B2), while the remaining eight (72.7%) were in the side surface/interface cracking configuration (B1). Similar to the 0.75 mm coating, interface cracks began appearing when the maximum surface temperature was about 1300°C (Figs. 13 and 14). Fig. 15 presents the side interface crack length (ais) versus the side surface crack length (ass) for different temperatures. A surface crack length equal to at least 0.4 mm was required to initiate an interface crack in the 1.0 mm coating.
Fig. 15. Experimental side interface crack length (ais) versus side surface crack length (ass) for different maximum surface temperature intervals of a lc =1.0 mm specimen.
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4.3.4. Coating thickness of 1.5 mm The 1.5 mm coatings also developed interface cracks in the two configurations (Fig. 7) similar to the 0.75 and 1.0 mm coating, with the exception of configuration (B1*) where three surface cracks were present instead of two. The most important result with this thickness was that only five of the 14 (35.7%) specimens (lc =1.5 mm) heated above 1400°C developed interface cracks. This represents a significantly decreased amount of interface cracking when compared with the 0.75 and 1.0 mm thick coatings. For specimens heated above 1400°C, 12 out of 15 (80%) 0.75 mm thick specimens and ten out of 11 (90.9%) 1.0 mm thick specimens developed interface cracks. Unlike the 0.75 and 1.0 mm coatings, interface cracks did not appear in the 1.5 mm thick coatings until 1500°C (Figs. 13 and 14). Fig. 16 presents the center interface crack length (aic) versus the surface crack length (ass) for different maximum surface temperatures. No interface cracks were developed unless a 0.85 mm long surface crack was present. At 1650°C, the average surface crack length in the 1.5 mm coating was 0.6 mm, indicating that, because the surface cracks did not penetrate deep enough into the 1.5 mm coatings, a small amount of interface cracking resulted. The experimental average interface crack lengths (ai) versus the coating thickness (lc) for different maximum surface temperatures are presented in Fig. 17. From this figure, it is clear that coatings subjected to increasingly larger surface temperatures result in longer interface cracks. More interestingly, however, the thinnest (0.26 and 0.57 mm) and thickest (1.5 mm) coatings result in less interface cracking than 0.75 and 1.0 mm thick coatings. This behavior is caused by the fact that
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Fig. 17. Experimental averaged interface crack length (ai) versus coating thickness (lc).
the shorter surface cracks formed in the 1.5 mm coatings decrease the propensity for the interface cracks to propagate. The results of analytical models describing this behavior are presented elsewhere [15].
5. Conclusions Under constant surface temperature and high heat flux thermal loading, the effect of coating thickness on the thermal fracture behavior of ceramic coatings was investigated. Coating thickness was found to influence the number, spacing and length of surface cracks. Increasing the coating thickness was found to decrease the number of surface cracks and increase the distance between them. Also, as the coating thickness increased, surface cracks extended a shorter distance into the coating layer. Higher maximum surface temperatures were found to cause longer surface cracks independent of coating thickness. It was also found that, as the coating thickness increased and the surface cracks became shorter, the development of interface cracking increased, reached a maximum and started decreasing for the thickest coatings. These results indicate that the morphology of surface cracks in such coatings plays a significant role in the response of interface cracks to a thermal load generated by a high heat flux. Thus, thicker thermal barrier coatings could be made more resistant to interface crack propagation.
Acknowledgements Fig. 16. Experimental center interface crack length (aic) versus side surface crack length (ass) for different maximum surface temperature intervals (lc = 1.5 mm).
The authors thank the National Science Foundation and Praxair Surface Technologies, Inc.
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References [1] Kumakawa, A., Niino, M., Kiyoto, S., Nagata, S., Thermal fatigue of functionally gradient materials under high heat fluxes, Proceedings of the International Symposium on Functionally Gradient Materials, American Ceramic Society, San Francisco, CA, vol. 34, 1992, pp. 213–220. [2] J.A. Nesbitt, Surface and Coatings Technology 43/44 (1990) 458 – 469. [3] S. Arnagasa, K. Shimomura, M. Kadowaki, K. Takeishi, H. Kawai, S. Aoki, K. Aoyama, Journal of Engineering for Gas Turbines and Power 116 (1994) 597–604. [4] M. Borom, C.A. Johnson, L.A. Peluso, Surface and Coatings Technology 86-87 (1996) 116–126. [5] Y.R. Takeuchi, K. Kokini, Journal of Gas Turbines and Power 116 (1994) 266 – 271. [6] K. Kokini, Y.R. Takeuchi, Materials Science and Engineering A188 (1994) 301 –309. [7] D. Zhu, R.A. Miller, Surface and Coatings Technology 94 – 95 (1997) 94 – 101. [8] Kawasaki, A., Watanabe, R. Thermal shock fracture mechanism of metal/ceramic functionally gradient materials, Proceedings of Thermal Shock and Thermal Fatigue of Advanced Ceramics, Kluwer Academic Publishers, Netherlands, NATO ASI Series vol. 241, 1993, pp. 509–520. [9] Takahashi, H., Ishikawa, T., Okugawa, D., Hashida, T. Laser and plasma-arc thermal shock/fatigue fracture evaluation procedure for functionally gradient materials, Proceedings of Thermal
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Shock and Thermal Fatigue of Advanced Ceramics, Kluwer Academic Publishers, Dordrecht, NATO ASI Series vol. 241, 1993, pp. 543 – 554. C.Y. Jian, H. Toshiyuki, H. Takahashi, Composites Engineering 5 (7) (1995) 879 – 889. ASTM Standard, ASTM B328-96, Standard Test Method for Density, Oil Content, and Interconnected Porosity of Sintered Metal Structural Parts and Oil-Impregnated Bearings. Hull, F.C., Estimating alloy densities, Metal Progress, November (1969) 139 – 140. T. Makino, T. Kunitomo, Transactions of the Japan Society of Mechanical Engineers 50 (452) (1984) 1045 – 1052. Choules, B.D., Thermal fracture mechanisms in thermal barrier coatings under laser heating, Ph.D. Dissertation, Purdue University, West Lafayette, IN, 1998. Choules, B.D., Kokini, K., Interface thermal fracture of ceramic coatings in a high heat flux environment, Proceedings of Thermal Stresses, Rochester Institute of Technology, 1997. K. Kokini, B.D. Choules, Y.R. Takeuchi, Surface and Coatings Technology 82 (1996) 77 – 82. Thum, G., Schneider, G.A., Aldinger, F., Stress relaxation and ageing in thermal barrier coatings, Proceedings of Thermal Stresses, Rochester Institute of Technology, 1997. Novak, R.C., Process, Properties, and Environmental Response of Plasma Sprayed Thermal Barrier coatings, Proc. Thermal Barrier Coating Workshop, NASA Conference Publication, Cleveland, OH, vol. 3312, 1995, pp. 153 – 165. D.M. Nissley, Journal of Thermal Spray Technology 6 (1) (1997) 91 – 98.
Thermal fracture of ceramic thermal barrier coatings under high heat flux with time-dependent behavior. Part 1. Experimental results Brian D. Choules a, Klod Kokini a,*, Thomas A. Taylor b a
School of Mechanical Engineering, Purdue Uni6ersity, 1288 Mechanical Engineering Building, West Lafayette, IN 47907 -1288, USA b Praxair Surface Technologies, Inc., 1500 Polco Street, Indianapolis, IN 46224, USA Received 18 October 1999; received in revised form 14 April 2000
Abstract The objective of this research was to study the thermal fracture of ceramic thermal barrier coatings under high heat flux laser heating and investigate the effect of time-dependent behavior of the ceramic. Continuously plasma-sprayed zirconia coatings with thicknesses varying from 0.26 to 1.5 mm were heated with a CO2 laser to maximum surface temperatures varying from 700 to 1700°C. Temperature differences from 600 to 1300°C across the ceramic coating were applied. High heat flux heating was found to cause changes in the material, leading to a denser microstructure and relaxation of compressive stresses. These changes lead to the development of surface and interface cracks during ambient air cooling following laser heating. Increasing the coating thickness was found to decrease the number of surface cracks developed and increase the distance between cracks. Surface cracks were found to extend further into thin coatings than thick coatings. Increasing the maximum surface temperature was found to increase the surface crack length. The thinnest and thickest coatings developed fewer and shorter interface cracks than coatings with intermediate thicknesses. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermal barrier coatings; Interface crack; High heat flux; Stress relaxation; Surface crack
1. Introduction Thermal barrier coatings (TBCs) have been developed for various applications for more than 30 years. A thermal barrier coating is a thin layer or coating of insulating material that is bonded to a substrate for protection from high-temperature environments. The coating is usually ceramic while the substrate is typically metal. The TBC allows for increased operating temperatures, increased efficiency, improved component durability, simplified designs, improved fuel economy, and sometimes the substitution of less-expensive metallic substrates. The TBC is applied by first depositing a thin bond coat layer (typically 120 mm MCrAlY (M=Co, Ni, Cr)) onto a metallic substrate. A low thermal conductivity ceramic layer is then deposited on * Corresponding author. Tel.: +1-765-4945727; fax: + 1-7654940539. E-mail address: [email protected] (K. Kokini).
top of the bond coat. One common material used as a TBC is plasma-sprayed yttria (Y2O3)-stabilized zirconia (ZrO2). TBCs are currently being used or considered for engine applications such as aerospace (space planes), aircraft, marine automobiles, nuclear fusion reactors and heavy-duty utilities (i.e. diesel trucks, electric power generation, etc.). The durability requirements of TBCs for these applications are increasing rapidly. For example, reusable rocket-powered vehicles and air-breathing space transportation vehicles encounter extremely high heat loads in their engines and air frames [1]. Also, the blades in the high-pressure fuel turbopump on the space shuttle main engines undergo very large thermal transients on start-up of engines [2]. In addition to these references, other workers have also indicated the need for coatings to survive temperatures of 1400– 1500°C [3,4]. As engines run closer and closer to temperatures where TBCs are considered unstable, the possibility of unanticipated overloads increases. While
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5093(00)01393-9
B.D. Choules et al. / Materials Science and Engineering A299 (2001) 296–304
the thermal load provided by the laser experiment developed in this study may be considered to be more severe than the requirements of the current applications, it represents an upper bound of the loading that can be applied on such coatings and may model the overload scenario. The results found in this study are necessary for the design of the next generation of engines that will be required to sustain higher temperatures and gradients than those in today’s applications. Development of ceramic TBCs for high heat flux environments requires a fundamental understanding of the fracture mechanisms in order to prevent the spalling of the coating from the substrate. The first such studies were performed by Takeuchi and Kokini [5,6] on graded thermal barrier coatings using lower heat fluxes. Only a few studies have been performed to investigate the failure of TBCs under high heat flux heating [7–10]. Thus, a CO2 laser was used to study the effect of time-dependent behavior of the material on surface, and interface cracking of thermal barrier coatings under high heat flux heating. In particular, the effect of surface temperature and TBC thickness on the failure mechanisms under high heat flux heating were investigated. The focus of this study was to investigate the failure mechanisms that occur during the application of a simple high heat flux cycle. Thermal fatigue, by application of multiple high heat flux cycles, was not investigated in this study.
2. Test specimen The test specimens were beam shaped and consisted of a 7 wt.% yttria-stabilized zirconia coating bonded to a steel substrate by two intermetallic bond coatings, as shown in Fig. 1. The steel substrate was 12.7 mm tall, 31.75 mm long and 3 mm wide. The zirconia coating (LZ-26) had a relative density of 87% (measured using ASTM-B328 corrected for water temperature [11]). A chemical analysis of the starting ceramic powder indi-
Fig. 1. Specimen schematic diagram.
297
Fig. 2. Continuous plasma-sprayed coating thicknesses studied.
cated 7.06 wt.% Y2O3, 1.73 wt.% HfO2, and the balance ZrOd2. Ceramic coatings with thicknesses (lc) of 0.26, 0.57, 0.75, 1.04 and 1.5 mm were manufactured for research and are shown in Fig. 2. Both bond coatings, designated as LCO-22 and LCO-35, have a nominal composition of 39Co–32Ni–21Cr–8Al–0.5Y However, the as-coated LCO-22 and LCO-35 bond coatings without heat-treatment have relative densities of 91.4 and 90.1% [12]. The composition of these coatings was selected on the basis of oxidation resistance and bonding strength with metal and ceramic. Both LCO-22 and LCO-35 layers were sprayed to 0.08 mm. A 50× 100× 12.7 mm ANSI 1020 steel plate was prepared for coating by grit blasting the surface with 60 mesh Al2O3 grit at 40 psig1 to increase the mechanical bonding between the steel substrate and bond coat. The bond coatings and zirconia coating were then plasma sprayed on one at a time. The average temperature of the substrate during plasma spraying was measured as 100°C. The test specimens were cut out by a CNC-controlled abrasive waterjet in the shape of beams.2 The external coating surface was kept in the as-sprayed condition, leaving a surface roughness of : 13 mm RA. The cross-sectional face of each specimen was metallographically polished to better observe crack formation during subsequent laser heating and to eliminate any flaws induced during water jet cutting. The ceramic surface was painted with Pyromark® Series 2500 silicon-based paint. Three coats were applied with a paint brush for a total thickness not exceeding 40 mm. The paint was allowed to dry for 18 1 The specimens were manufactured by Praxair Surface Technologies, Indianapolis, IN. 2 The waterjet cutting was performed by Luick Quality Gage & Tool, Inc., Muncie, IN.
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3.2. Temperature measurement The substrate temperature was measured using a Type K thermocouple that was spot welded to the bottom of the steel substrate. The surface temperature was measured using an Ircon3 (Niles, IL, USA) infrared pyrometer (Modline 7000). This pyrometer was able to measure surface temperatures between 500 and 2500°C by measuring the intensity of the radiation emitted from the target surface. The intensity of the radiation emitted at a single wavelength depends on the temperature and surface characteristics of the material. The pyrometer used in the present study measures radiation in the wavelength range of 4.8–5.2 mm. The known surface emissivity of the Pyromark® paint allowed the pyrometer to measure the correct surface temperature.
3.3. In situ and post-experiment crack obser6ation
Fig. 3. Experimental set-up.
h. The painted surface was cured by heating at 250°C for 2 h in a furnace. During curing and the subsequent cooling, the furnace doors were kept shut to eliminate the possibility of cracks forming due to sudden air cooling. The specimens were checked with an optical microscope to insure no cracking was induced during the paint curing procedure. The paint provided for more accurate surface temperature measurements using infrared pyrometry.
3. Experimental set-up To investigate the failure mechanisms of ceramic thermal barrier coatings subjected to heating by a high heat flux, the experimental set-up shown in Fig. 3 was used.
3.1. High heat flux laser heating A 1.5 kW CO2 laser manufactured by Convergent Energy was used to heat the surface of the ceramic coating. Characteristic of a CO2 laser, the laser light is of 10.6 mm wavelength. At this wavelength, the absorptivity of the zirconia coating is essentially 1, thus the laser light is absorbed at the surface of the coating [13]. The heat flux distribution was determined to be Gaussian with a diameter of 12 mm, which contains 86.6% of the power. Each specimen was laser heated for 4 s and allowed to cool by ambient air.
A Questar telescopic microscope was used to optically observe and record the cracking in the coating during the experiment. The microscope had 100× magnification. A CCD camera was used to digitize the image and a Super VHS VCR recorded the images at a rate of 32 frames per second. After each specimen was heated with the laser, the cracks were identified and measured with a Zeiss microscope.
4. Results and discussion Continuously sprayed coating thicknesses of 0.26, 0.57, 0.75, 1.0, and 1.5 mm were laser heated only once, for 4 s, and allowed to cool in ambient air. Approximately four specimens of each thickness were heated in 100°C maximum surface temperature increments between 700 and 1700°C. The surface temperature versus time for typical specimens of each thickness is shown in Fig. 4. The substrate temperatures during the heating remained under 150°C. Two different cracking modes were found to occur upon cooling after laser heating: surface cracks, and surface/interface cracks. A surface crack was perpendicular to the surface, and a surface/ interface crack included a surface crack as well as an interface crack that was located at or above the interface between the zirconia coating and bond coating. The surface cracks were all found to occur within the zone of the specimen heated by the laser. Interface cracks were always found directly below surface cracks. However, the surface cracks did not necessarily extend through the coating to the interface. A typical surface crack in a 1.0 mm thick coating is shown in Fig. 5. A typical surface/interface crack in a 0.75 mm thick coating is shown in Fig. 6. 3
Ircon, Inc., 7301 North Caldwell Ave., Niles, Illinois 60714.
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A schematic diagram of the cracking behavior as a function of thickness and temperature is shown in Fig. 7. Typically coatings heated below 1000°C did not develop any cracks. In the maximum surface temperature range of 1000 – 1400°C, the coatings were found to develop surface cracks. The thinnest coating (lc = 0.26 mm) developed five to seven multiple surface cracks spaced close together (d =0.70 90.24 mm). The thicker coating with lc =0.57 mm also exhibited multiple cracking, typically three to five cracks spaced 1.1 90.3 mm
Fig. 6. Surface and interface crack in a lc =0.75 mm thick ceramic layer specimen after surface heating with a laser to a maximum temperature of 1540°C.
Fig. 4. Measured surface temperature versus time for coatings of different thickness.
apart. The three thickest coatings (lc = 0.75, 1.0, and 1.5 mm) developed usually three surface cracks spaced 1.4290.37, 1.79 0.46 and 1.990.43 mm apart, respectively. These results clearly showed that, in this surface temperature range, increasing the coating thickness resulted in an increase in surface crack spacing and a decrease in the number of surface cracks. Coatings subjected to maximum temperatures in the range of 1400–1700°C developed surface cracking and, depending on their thickness, interface cracking. The thin coatings (lc = 0.26 and 0.57 mm) did not readily develop interface cracks, but the number of surface cracks was larger. The 0.75 and 1.0 mm coatings both developed large interface cracks, while the 1.5 mm coating developed only small interface cracks. For example, with Tmax between 1450 and 1550°C, the average interface crack length in the 0.75, 1.0 and 1.5 mm coatings was 1.36, 0.79, and 0.46 mm, respectively.
4.1. Time-Dependent beha6ior of plasma-sprayed zirconia under laser heating
Fig. 5. Scanning electron microsopy micrograph of polished cross-section showing surface crack in a lc = 1.0 mm thick ceramic layer specimen after surface heating with a laser to a maximum temperature of 1050°C.
Choules et al. showed that, without the presence of stress relaxation and sintering, the stresses incurred during laser heating and subsequent cooling were not large enough to cause fracture in the coating [14,15]. It was determined that the time-dependent behavior of the coating led to the fracture of the coating. This behavior was first observed by Takeuchi and Kokini [5] in lower heat flux, longer-term experiments. Subsequent research showed that using a ceramic such as mullite, with significantly reduced stress relaxation, resulted in reduction in cracking [16]. The stress relaxation behavior is related to a stress-enhanced ceramic sintering phe-
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Fig. 7. Schematic diagram showing effect of thickness and temperature on cracking behavior of ceramic coatings under laser heating.
nomenon, as has been observed in past experiments by other workers [17 – 19]. The relative boundary sliding of plasma-sprayed splats and grains, and the stress redistribution around the splats and microcracks are important mechanisms for ceramic coating shrinkage and stress relaxation. As a result of stress relaxation, the compressive stresses during heating decrease. Then upon cooling, tension results in the now compacted coating as it tries to shrink to a shorter length than its original one. This tension leads to surface crack propagation and possibly the initiation and propagation of an interface crack between the bond coat and ceramic.
4.2. Effect of surface temperature and coating thickness on surface cracking beha6ior The surface and interface cracks resulting from the application of the laser heat flux were measured using an optical microscope at 200 × magnification. A schematic showing the crack length and spacing definitions is shown in Fig. 8. The measured center surface crack length (asc) and side surface crack length (ass), normalized with respect to coating thickness (lc), as a function of maximum surface temperature during laser heating are presented in Figs. 9 and 10, respectively. The surface crack lengths for each temperature were averaged for approximately four specimens, each in approximately 100°C temperature intervals. The variability in the data may be attributed to the highly microcracked and porous nature of plasma-sprayed zirconia. Even with the considerable variability in the measured data, two trends are noticeable in Figs. 9 and 10: (1) the surface crack lengths increase with temperature; (2) the normalized surface crack length increases as coating thickness decreases. Therefore, surface cracks penetrate further into thin coatings than thicker ones.
The thin 0.26 mm coatings developed between four and nine surface cracks. Micrographs of two 0.26 mm specimens are shown in Figs. 11 and 12. Fig. 11 shows that, prior to heating, surface cracks were not present, while Fig. 12 shows a specimen heated to 1500°C that developed multiple cracks spaced approximately 0.5– 0.6 mm apart. As shown in Figs. 9 and 10, surface cracks begin to appear at 1000°C and their length increases sharply with temperature. Beyond 1400°C, the majority of the cracks extend all the way through the ceramic layer. The 0.57 mm thick coatings heated above approximately 1050°C developed three to five surface cracks within the laser-heated zone. Surface crack lengths increased with temperature, however not nearly as sharply as the 0.26 mm specimens (Figs. 9 and 10). As shown in Fig. 7 (configuration A), the 0.75 and 1.0 mm coatings developed three surface cracks with
Fig. 8. Schematic diagram showing crack length and spacing definitions.
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Fig. 11. Scanning electron microsopy micrograph of an unheated lc =0.26 mm specimen.
Fig. 9. Experimental averaged asc/Ic versus maximum surface temperature for different coating thicknesses (lc).
one at the center of the specimen and two off to the side of the center of the specimen. Surface cracks began to form around 1050°C and their length increased with temperature (Figs. 9 and 10). At 1200°C, the average ass/lc value was approximately 0.27 and 0.33 for the
0.75 and 1.0 mm coatings, respectively. At 1600°C, the average ass/lc value was approximately 0.41 and 0.65 for the 0.75 and 1.0 mm coatings, respectively. The 1.5 mm coatings developed three surface cracks similar to the 0.75 and 1.0 mm coating. Surface cracks began appearing at 1000°C and their length increased with temperature (Figs. 9 and 10). However, contrary to the thinner coatings, the crack length ratio ass/lc was only 0.22 near 1200°C and 0.37 near 1600°C. Thus, the ratio of surface crack length to coating thickness was smaller for the thickest coatings at higher temperatures.
4.3. Effect of surface temperature and coating thickness on interface cracking beha6ior The center (aic) and side (ais) interface crack lengths versus the maximum surface temperature for the different thicknesses studied are shown in Figs. 13 and 14, respectively.
4.3.1. Coating thicknesses of 0.26 and 0.57 mm The 0.26 mm coatings did not develop any interface cracks following laser heating, while only three out of 24 (12.5%) of the 0.57 mm specimens heated above 1400°C developed interface cracks. These interface cracks developed at 1400, 1525, and 1650°C.
Fig. 10. Experimental averaged ass/lc versus maximum surface temperature for different coating thicknesses (lc).
Fig. 12. Scanning electron microsopy micrograph of a lc = 0.26 mm specimen showing multiple surface cracks.
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Of the 12 specimens that developed interface cracks, seven (58.3%) were in the center surface/interface crack configuration (B2), while the remaining five (41.7%) were in the side surface/interface crack configuration (B1). The center interface cracks in configuration (B2) developed for maximum surface temperatures ranging from 1300 to 1650°C, while the side interface cracks in configuration (B1) developed for maximum surface temperatures ranging from 1450 to 1700°C. Thus, slightly higher surface temperatures were needed to induce the side/interface crack configuration. As shown in Figs. 13 and 14, the center interface cracks (aic) began appearing at 1300°C, while the side interface cracks (ais) begin appearing at 1500°C.
Fig. 13. Experimental center interface crack length (aic) versus temperature for different coating thicknesses (lc).
4.3.2. Coating thickness of 0.75 mm Interface cracks in the 0.75 mm coatings occurred in two configurations (Fig. 7): 1. side surface/interface cracking, two side surface crack formed with an interface crack directly below each side surface crack (indicated as B1 in Fig. 7); and 2. center surface/interface cracking, three surface cracks formed with an interface crack directly below the center surface crack (indicated as B2 in Fig. 7).
Fig. 14. Experimental side interface crack length (ais) versus temperature for different coating thicknesses (lc).
4.3.3. Coating thickness of 1.0 mm The 1.0 mm coatings were also found to develop interface cracks in the same two configurations (B1 and B2) as those for the 0.75 mm thick coating. However, only three of the 11 (27.3%) specimens had the center surface/interface crack configuration (B2), while the remaining eight (72.7%) were in the side surface/interface cracking configuration (B1). Similar to the 0.75 mm coating, interface cracks began appearing when the maximum surface temperature was about 1300°C (Figs. 13 and 14). Fig. 15 presents the side interface crack length (ais) versus the side surface crack length (ass) for different temperatures. A surface crack length equal to at least 0.4 mm was required to initiate an interface crack in the 1.0 mm coating.
Fig. 15. Experimental side interface crack length (ais) versus side surface crack length (ass) for different maximum surface temperature intervals of a lc =1.0 mm specimen.
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4.3.4. Coating thickness of 1.5 mm The 1.5 mm coatings also developed interface cracks in the two configurations (Fig. 7) similar to the 0.75 and 1.0 mm coating, with the exception of configuration (B1*) where three surface cracks were present instead of two. The most important result with this thickness was that only five of the 14 (35.7%) specimens (lc =1.5 mm) heated above 1400°C developed interface cracks. This represents a significantly decreased amount of interface cracking when compared with the 0.75 and 1.0 mm thick coatings. For specimens heated above 1400°C, 12 out of 15 (80%) 0.75 mm thick specimens and ten out of 11 (90.9%) 1.0 mm thick specimens developed interface cracks. Unlike the 0.75 and 1.0 mm coatings, interface cracks did not appear in the 1.5 mm thick coatings until 1500°C (Figs. 13 and 14). Fig. 16 presents the center interface crack length (aic) versus the surface crack length (ass) for different maximum surface temperatures. No interface cracks were developed unless a 0.85 mm long surface crack was present. At 1650°C, the average surface crack length in the 1.5 mm coating was 0.6 mm, indicating that, because the surface cracks did not penetrate deep enough into the 1.5 mm coatings, a small amount of interface cracking resulted. The experimental average interface crack lengths (ai) versus the coating thickness (lc) for different maximum surface temperatures are presented in Fig. 17. From this figure, it is clear that coatings subjected to increasingly larger surface temperatures result in longer interface cracks. More interestingly, however, the thinnest (0.26 and 0.57 mm) and thickest (1.5 mm) coatings result in less interface cracking than 0.75 and 1.0 mm thick coatings. This behavior is caused by the fact that
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Fig. 17. Experimental averaged interface crack length (ai) versus coating thickness (lc).
the shorter surface cracks formed in the 1.5 mm coatings decrease the propensity for the interface cracks to propagate. The results of analytical models describing this behavior are presented elsewhere [15].
5. Conclusions Under constant surface temperature and high heat flux thermal loading, the effect of coating thickness on the thermal fracture behavior of ceramic coatings was investigated. Coating thickness was found to influence the number, spacing and length of surface cracks. Increasing the coating thickness was found to decrease the number of surface cracks and increase the distance between them. Also, as the coating thickness increased, surface cracks extended a shorter distance into the coating layer. Higher maximum surface temperatures were found to cause longer surface cracks independent of coating thickness. It was also found that, as the coating thickness increased and the surface cracks became shorter, the development of interface cracking increased, reached a maximum and started decreasing for the thickest coatings. These results indicate that the morphology of surface cracks in such coatings plays a significant role in the response of interface cracks to a thermal load generated by a high heat flux. Thus, thicker thermal barrier coatings could be made more resistant to interface crack propagation.
Acknowledgements Fig. 16. Experimental center interface crack length (aic) versus side surface crack length (ass) for different maximum surface temperature intervals (lc = 1.5 mm).
The authors thank the National Science Foundation and Praxair Surface Technologies, Inc.
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