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A study on the thermal cyclic behavior of thermal barrier coatings with different MCrAlY roughness
Vacuum 137 (2017) 72e80
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A study on the thermal cyclic behavior of thermal barrier coatings with different MCrAlY roughness Kang Yuan a, *, Yueguang Yu a, Jian-Feng Wen b, c a
Beijing General Research Institute of Mining and Metallurgy, Beijing 100160, PR China School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, PR China c Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2016 Received in revised form 12 December 2016 Accepted 16 December 2016 Available online 18 December 2016
Thermal cycling behavior of thermal barrier coatings (TBCs) with different MCrAlY (M for Ni and Co) interface roughness was studied under two thermal cycling simulation conditions, namely thermal shock with short heating time and high cooling rate, and thermal-ageing fatigue with long heating time but low cooling rate. MCrAlY powders with various sizes were used to obtain different bond coat interface roughness for the TBCs. The MCrAlY bond coat (BC) was sprayed by HVOF, APS, or the combination of them, and the top coats (TCs) with different thickness (300 or 1000 mm) were made. It was found that while the coarser MCrAlY powder gave higher BC roughness, the APS spraying provided more microroughness. The increased BC roughness was found to improve the thermal cycling resistance of the TBCs under both testing conditions. The failure mechanism of the TBCs with various MCrAlY roughness was investigated by analyzing microstructures and via finite-elemental modeling. © 2016 Elsevier Ltd. All rights reserved.
Keywords: MCrAlY TBC Roughness Thermal shock Thermal-ageing fatigue
1. Introduction The increased demands of energy supply and the continuously worsen situation of global greenhouse effect have urged the gas turbine industries to make more efforts to enhance gas turbine efficiency and reduce emissions in power generations. Researchers in the material field have made great contributions in improving the material properties and working lifetime in gas turbines. As a great invention, thermal barrier coatings (TBCs) have been widely used to support a temperature increase for metallic components in service and to improve the gas turbine efficiency [1,2]. TBCs are usually composed of a 7e8 wt% yttria-stabilized zirconia (YSZ) top coat (TC) providing thermal insulation and a MCrAlY (M for Ni/Co) bond coat (BC) providing adhesion for the ceramic TC and metallic substrate. MCrAlY BC also provides the protection for the metallic component against oxidation and corrosion by forming a thermally-grown oxide (TGO) layer. The importance of the investigation on the failure mechanism of such material system has been strengthened for many decades. The failure of the TBCs under a thermal cycling load can be affected by such aspects as material properties (like thermal expansion, E modulus), microstructures
* Corresponding author. E-mail address: [email protected] (K. Yuan). http://dx.doi.org/10.1016/j.vacuum.2016.12.033 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
and environment factors, TGO growth, sintering of the ceramic, creep/plastic deformation of the materials and BC interface roughness [3e5]. The roughness of the interface between the MCrAlY BC and the ceramic has been found to play a significant role on the thermal mechanical properties of TBCs [4e10]. Finite-element modeling (FEM) work has shown that stress distribution are remarkably influenced by the interface roughness [6,11e13]. The BC interface roughness can be modified via several ways like varying the spray processing parameters and changing powder size. A standard thermal cycling test of TBCs is usually performed by heating at a certain temperature followed by a quick cooling to a low temperature [14e18]. In a laboratory test, thermal cycles with a short hold time such as some minutes and 1 h at the maximum temperature are often used, while the thermal cycles related to the operation of gas turbine especially land-based ones have a much longer hold time at the high temperature. That could bring some misunderstanding of the failure mechanism of real TBC components in gas turbines by doing the conventional thermal cycling tests. To provide understanding on how the heating cycle time influences the TBC failure with different MCrAlY interface roughness, two tests were designed in this research. One test was thermal shock which contains cycles of 5 min heating at 1100 C followed by water cooling, while the other was thermal-ageing fatigue containing
K. Yuan et al. / Vacuum 137 (2017) 72e80
cycles of 24-h heating followed by air cooling. In the two tests, two different coating spraying processes (HVOF, APS) were studied, and the effect of MCrAlY interface roughness on the coating failure behavior was discussed.
2. Experimentals Inconel 718 (52.5Ni, 19Cr, 0.5Al, 3.05Mo, 1.03Ti, 5.25Nb þ Ta, 0.08C, balanced by Fe, wt%) was used as the substrate material for TBCs. Three MCrAlY powders (BC1, BC2 and BC3 in Table 1) were used for making the BCs by high-velocity oxy-fuel (HVOF) or atmospheric plasma spray (APS) process. The powder of BC1 and BC2 was made via vacuum atomization followed by a process of cleaning and sieving; powder BC3 was a commercial product namely Amdry 365-2. A commercial YSZ powder (Metco 234A) was used for the TC spray via APS. The composition and the size of the powders are listed in Table 1. Spraying parameters in the HVOF and APS processes are given in Table 2. The TBC samples, as listed in Table 3, were divided into two groups: thin TBCs (#1-#5, TC thickness ~300 mm) and thick TBCs (#6-#8, TC thickness ~1 mm). In the thin TBC group, the BCs of #1 and #2 were sprayed by HVOF (#1 used a coarser powder than #2) while the BCs of #4 and #5 were sprayed by APS (#5 used a coarser powder than #4); the BC of #3 was made by combining HVOF and APS to form an inner low-oxygen HVOF layer and an outer, rougher APS layer. In the thick TBC group, #8 used coarser BC powder (BC3) than #6 and #7 (BC2), and #7 had a thicker BC than #6. The typical BC microstructures made by HVOF, APS and combination of them are shown in Fig. 1. Such design of the samples was aimed to investigate the influence of coating thickness, BC interface roughness, and spraying process on the TBC life. Thermal shock and thermal aging fatigue tests were carried out on the TBC samples. The thermal shock cycling contained heating at 1100 C for 5 min in a furnace followed by water cooling to about
Table 1 Nominal composition and powder size of MCrAlY (BC1, BC2, BC3) and YSZ. Materials
Composition, wt%
Powder size, um
BC1 BC2 BC3 YSZ
Ni-23Co-17Cr-12Al-0.5Y
(75 þ 50) (75 þ 38) (109 þ 53) (90 þ 17)
Ni-22Co-25Cr-7Al-0.5Y ZrO2, 6-8Y2O3/HfO2
73
room temperature in 10 s; while the thermal-ageing fatigue test was carried out through cycles containing an ageing process at 1100 C for 24 h followed by forced-air cooling to about 100 C in 10 min. During the thermal cycles, the surface of the cooled samples was photographed to record the coating spallation. The TBC samples were tested till failure and the coating life was defined as the number of cycles that caused 20% area spallation of the ceramic TCs. To better study the failure machanism of TBCs in thermalageing fatigue condition, samples subjected to a certain number of thermal cycles were removed from the furnace and crosssections were then prepared for observation of cracking behavior of the samples before failure. Microstructure study was done in a scanning-electron microscope (SEM). In addition, a quick finite element analysis was performed using the codes ABAQUS to investigate the effect of the BC interface roughness on the stress distributions in TBCs, in which the materials (TC and BC) were simply set to be elastic and rigid. Two cosine interface contours were simulated with wavelength/amplitude of 60/30 mm (W60A30) and 100/50 mm (W100A50). The material constants employed in the FE analysis are listed in Table 4 [4,19] and the temperature change of 1000 C was used.
3. Results and discussion 3.1. Thermal shock Fig. 1 shows the typical morphology of MCrAlY coatings made by HVOF (Fig. 1a), HVOF þ APS (Fig. 1b) and APS (Fig. 1c). Relatively smoother TC-BC interface was observed in HVOF MCrAlY, while interface pegs (indicated by arrows in Fig. 1b and c) were typically observed in APS MCrAlY. The interface pegs could provide microroughness for APS coatings [20]. The “micro-roughness”, however, was not mathematically counted in the measured roughness “Ra”, namely average interface roughness. The Ra of the BCs was measured by image analyzing method. As shown in Fig. 2, the usage of a coarser powder size essentially increased the MCrAlY interface roughness. The spraying process, HVOF and APS, did not cause much difference of the Ra; however, if the “micro-roughness” is taken into accounted, the APS BCs would have higher roughness values. In the thermal shock test, the ceramic TCs were spalled starting from the sample edge and moving towards the center typically during the cooling process. The life of the TBC samples (20% TC
Table 2 Spraying parameters. Coat
Spray process Spray parameters Fuel (kerosene) flow rate 24 L/h, carrier gas (N2) flow rate 7 L/min, O2 flow rate 900 L/h, spray distance 360 mm, powder feed rate ~75 g/min. Ar/H2 flow rate 50/10 L/min, plasma power 46 kW, spray distance 100 mm, powder feed rate ~35 g/min. Ar/H2 flow rate: 38/13 L/min, plasma power: 47 kW, spray distance: 100 mm, powder feed rate: ~25 g/min.
MCrAlY BC HVOF APS YSZ TC APS
Table 3 TBC samples. No. 1# 2# 3# 4# 5# 6# 7# 8#
Thin TBC
Thick TBC
MCrAlY material
BC spray
BC thickness, um
YSZ spray
TC thickness, um
BC1 BC2 BC2 BC2 BC1 BC2 BC2 BC3
HVOF HVOF HVOF þ APS APS APS APS APS APS
150 150 150 150 150 150 350 150
APS APS APS APS APS APS APS APS
300 300 300 300 300 1000 1000 1000
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K. Yuan et al. / Vacuum 137 (2017) 72e80
Fig. 1. Cross sections of MCrAlY bond coats using diffent spray processes. (a) HVOF-sprayed (2#), (b) HVOF þ APS (3#), (c) APS-sprayed (5#). The arrows mark some MCrAlY interface pegs typically formed by APS spray.
Table 4 Material parameters used for FEM simulation. Component
Young's Modulus, GPa
Poisson's Ratio
Coefficient of Expansion, 106 1/ C
TC BC
53 225
0.25 0.3
10.2 14
K. Yuan et al. / Vacuum 137 (2017) 72e80
Fig. 2. The average roughness Ra of the MCrAlY interfaces measured by image analyzing approach (cutting-off length 0.8 mm). The powder size used for the MCrAlYs is labeled (in um). The error bars are standard deviation by measuring different positions at the MCrAlY interfaces.
75
thin TBC group, the longest TBC life occurred in #5 whose BC was sprayed by APS and with a rougher BC interface. A fact that a rougher BC interface gave a longer TBC life was also observed in HVOF MCrAlYs (#1 > #2). APS BCs gave longer life than HVOF BCs. The smoother BC interface and the unmelted particles on HVOF overlay coatings could increase the overall stresses in the TC and therefore caused earlier failure according to M. Gupta's study [10]. Using the same MCrAlY powder, the combination of HVOF and APS for making BC (#3) seemed to provide higher thermal shock resistance than that using pure HVOF (#2) or APS (#4) process. W. Nowak's research indicated that the APS-flashcoat exhibited more “micro roughness” on the BC interface which resulted in a more homogeneously-distributed microcracking and a delayed cracking linking [20]. For the thick TBCs (#6-#8), increasing BC interface roughness was also benefit to improve TBC life (#8 > #6, #7). The typical fracture morphology of the TBCs is presented in Fig. 4 (HVOF BC) and Fig. 5 (APS BC). The cracks in the TC away from the BC interface caused a “white” fracture (failure inside of TC), while the cracks along BC interface caused a “black” fracture (failure along TGO). In #1 (Fig. 4a) a mixture of “white” and “black” fracture was observed while in #2 mainly “white” fracture was found (Fig. 4b). The position of the cracks was strongly related to the stress distribution in the TBCs. In the whole, the results suggest that the increased MCrAlY interface roughness could transfer the failure from the TC to the BC interface or even inside of BC, i.e. from “white” failure to “black” failure. The TBCs with coarser MCrAlY powder (#5) showed increased amount of through-BC cracks in MCrAlY part, as seen in Fig. 5b. Since the cracks were vertical, it indicated that the BC sustained a tensile stress parallel with the BC interface during cooling process. The magnified details in Fig. 6 revealed that the cracks preferred to initiate from the MCrAlY splats' boundaries at the BC interface, particularly at the boundaries of large unmelted particles. Along the through-BC cracks, oxidation of the MCrAlY was accelerated. 3.2. Thermal-ageing fatigue
Fig. 3. TBC life in the thermal shock test as a function of BC interface roughness. Two specimens were tested for each TBC sample.
spallation) as a function of BC interface roughness is presented in Fig. 3. It was not surprising to find that thin TBCs (TC ~ 300 mm thick) had longer life than thick ones (TC ~ 1 mm thick) [8]. In the
In the thermal-ageing fatigue condition, failure of the TBCs occurred by delamination of the whole TC layer rather than breaking off in pieces observed in the thermal shock test. The overview of the fractured interfaces on BC side and TC side are given in Fig. 7. The thin TBCs (#1-#5) presented grey color, indicating a fracture along/near MCrAlY interfaces, while the white color of the interfaces in the thick TBCs (6#-8#) indicated a fracture
Fig. 4. Cross sections of the TBCs failed under thermal shock condition comparing two HVOF-sprayed bond coats: (a) 1# (using powder BC1), (b) 2# (using powder BC2). “w-f” for “white-fracture”, “b-f” for “black-fracture”.
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K. Yuan et al. / Vacuum 137 (2017) 72e80
Fig. 5. Cross sections of the TBCs failed under thermal shock condition comparing two APS-sprayed bond coats: (a) 4# (using powder BC2), (b) 5# (using powder BC1). “w-f” for “white-fracture”, “b-f” for “black-fracture”.
Fig. 6. The morphology of BC cracking in (a) HVOF sprayed 1# (106 cycles) and (b) APS sprayed 5# (215 cycles) in thermal shock condition.
K. Yuan et al. / Vacuum 137 (2017) 72e80
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Fig. 7. The overview of the fractured surfaces of the samples after thermal ageing-fatigue failure showing TC side and BC sides.
inside of TC away from the MCrAlY interface. The typical morphology of the cross section of the thin TBCs is shown in Fig. 8a, displaying a “mixture” failure; different from that, a “white” failure was found in the thick TBCs (Fig. 9b). In the samples from #1 to #7 alumina TGO was formed at the BC interface, while in sample #8 (Fig. 8c) the break-down oxidation of MCrAlY occurred by forming large oxide mixtures. The formation of such oxide mixtures was quite local as seen in Fig. 7. Our previous work has demonstrated that the same MCrAlY, tested at 1100 C/1 h for 600 cycles/hours,
showed no break-down oxidation [18], indicating that such destructive oxidation was not just a simple chemical failure. Actually, the usage of the coarse powder (BC3) created many largesize unmelted particles in the BC. The break-down oxidation could be due to the oxidation of those particles since the weak particle boundaries blocked the diffusion support of metallic elements from the underling BC and substrate. The coating life under the thermal-ageing failure condition is shown in Fig. 9. Sample #3 with HVOF þ APS sprayed BC showed
Fig. 8. Typical morphology of (a) “black” dominant fracture (1#), (b) “white” fracture (6#), and (c) break-down oxidation of MCrAlY (8#) under thermal-ageing fatigue condition.
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K. Yuan et al. / Vacuum 137 (2017) 72e80
particularly high in the TC zone above BC interface. Furthermore, the fracturing crack (width 2 mm), as marked by arrows, passed mainly by such zone, demonstrating the high concentration of stresses there. FEM modeling work by other researchers has suggested that the high tensile stresses in TC above the BC interface were generated during thermal cycling processes, which could promote the cracking of TC [22e24]. 3.3. Further discussion
Fig. 9. TBC life in the thermal ageing-fatigue test as function of BC interface roughness. Two specimens were tested for each TBC sample.
the longest life, which could be due to a good combination of oxidation resistance, provided by inner HVOF MCrAlY layer, and BC interface roughness, provided by an outer APS MCrAlY layer [21]. The result also shows that the change in MCrAlY interface roughness (#1 vs. #2, or #4 vs. #5) seemed not to have a big influence on the coating life for the thin TBC series. But in the thick TBCs, increasing the MCrAlY roughness greatly improved the coating life (#8>#6). Such life improvement was quite significant since the life of thick TBC (#8) was comparable with the thin APS coating series (#4/#5); while in the thermal shock test (Fig. 2), the lifetime of 8# was only 20% of that of the thin coatings (#5). In addition, increasing the BC thickness could improve the thick-coating life, as the life of #7 (BC ~ 350 mm) was much longer than that of 6# (BC ~ 150 mm). A thicker BC could be beneficial for TBC life due to improved adhesion with the ceramic TC or by providing better oxidation resistance (as Al receiver). Interrupting test was performed by removing samples from the furnace at different testing cycles before failure. The cross section of a sample (#4) after the interrupting test is given in Fig. 10. Horizontal cracks were observed in the TC which was still attached on the MCrAlY BC. The cracks were counted at different depths through the TC, and it was found that the crack density was
As discussed above, increasing MCrAlY interface roughness by using coarser powders was beneficial to enhancing the TBC life in both thermal shock and thermal-ageing fatigue processes. A FEM model, simply considering thermal mismatch between TC and BC and ignoring BC creeping, was applied to study the stress distribution in TBC with two different MCrAlY interface roughness; the stress Smax was an integration of S11 and S22 components. The result (Fig. 11) shows that zones of larger stress were formed in both the TC and BC parts for the coating with the rougher interface. The highest tensile stress was generated at the BC interface valley in the BC part, but the stress values in the two coatings were not much different, indicating that increasing the interface roughness did not significantly increase thermal stress in the vicinity of the interface. Taking into account of the microstructure of TC, the extended stress zone with the rougher interface could embrace more splat cracks, as shown in Fig. 11. The more parallel cracks are driven to propagate, the more elastic energy in TC can be released. That could be an explanation for the longer TBC life of the coating with higher MCrAlY interface roughness. On the other hand, however, spraying coarse MCrAlY powder makes it more difficult to fully melting the particles, which resulted in weaker BC splat boundaries. Therefore, to further improve the lifetime of TBCs by increasing BC interface roughness using coarser powders, the undesirable effect by the unmelted particles like isolation oxidation and cracking at the weak boundaries need to be avoided. Possible approaches are, for instance, powder metallurgy, modified spraying processing, and coating heat treatment. 4. Conclusion The thermal cycling behavior of TBCs with different coating thickness (TC thickness as 300 mm or 1000 mm), coating spray
Fig. 10. The section morphology of an interrupted testing sample (4#). The arrows mark the fracturing cracks whose width larger than 2 mm.
K. Yuan et al. / Vacuum 137 (2017) 72e80
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Fig. 11. FEM modeling on the stress distribution in TBCs with BC interface having different wavelengths/amplitudes: (a) 60/30 mm and (b) 100/50 mm. Smax is an integration of S11 and S22.
process (HVOF or APS), and MCrAlY BC interface roughness was studied at 1100 C under thermal shock (~5 min heating and ~10 s cooling) and thermal-ageing fatigue (~24 h heating and ~10 min cooling) conditions. The remarks can be summarized as followings: 1) It was evident that larger BC interface roughness provided higher resistance against thermal cycling for the TBCs. The improvement of the coating life by increasing the roughness was particularly significant for thick TBC (TC thickness as 1000 mm) under the thermal-aging fatigue process. 2) As APS-sprayed BCs had higher “micro-roughness” than HVOF BCs, TBCs with APS BCs showed longer life in both thermal shock and thermal-ageing fatigue processes. Applying an APS flashcoat on HVOF inner coating displayed an improved coating performance. Particularly under the thermal-ageing fatigue condition, the BC made in such way gave the longest TBC life in the studied samples. 3) Failure of the TBCs tends to occur inside the BC due to the weak boundaries of unmelted MCrAlY particles when coarser MCrAlY powders were used. That urges more investigations on strengthening the splat boundaries for further enhancing the coating lifetime.
Acknowledgment The authors would like to greatly thank the financial and technical support from Beijing General Research Institute of Mining and Metallurgy (BGRIMM) via the international cooperation project (Grant No. 2015DFA51530). The manufacturing of BC3 MCrAlY powder, coating spray and the experimental tests were carried out in BGRIMM. The co-author Jian-Feng Wen, who carried out the FEM modeling work, would like to acknowledge the financial support provided by China Postdoctoral Science Foundation (Grant No. 2015M581543) and Shanghai Sailing Program (Grant No. 15YF1402900).
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
A study on the thermal cyclic behavior of thermal barrier coatings with different MCrAlY roughness Kang Yuan a, *, Yueguang Yu a, Jian-Feng Wen b, c a
Beijing General Research Institute of Mining and Metallurgy, Beijing 100160, PR China School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, PR China c Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2016 Received in revised form 12 December 2016 Accepted 16 December 2016 Available online 18 December 2016
Thermal cycling behavior of thermal barrier coatings (TBCs) with different MCrAlY (M for Ni and Co) interface roughness was studied under two thermal cycling simulation conditions, namely thermal shock with short heating time and high cooling rate, and thermal-ageing fatigue with long heating time but low cooling rate. MCrAlY powders with various sizes were used to obtain different bond coat interface roughness for the TBCs. The MCrAlY bond coat (BC) was sprayed by HVOF, APS, or the combination of them, and the top coats (TCs) with different thickness (300 or 1000 mm) were made. It was found that while the coarser MCrAlY powder gave higher BC roughness, the APS spraying provided more microroughness. The increased BC roughness was found to improve the thermal cycling resistance of the TBCs under both testing conditions. The failure mechanism of the TBCs with various MCrAlY roughness was investigated by analyzing microstructures and via finite-elemental modeling. © 2016 Elsevier Ltd. All rights reserved.
Keywords: MCrAlY TBC Roughness Thermal shock Thermal-ageing fatigue
1. Introduction The increased demands of energy supply and the continuously worsen situation of global greenhouse effect have urged the gas turbine industries to make more efforts to enhance gas turbine efficiency and reduce emissions in power generations. Researchers in the material field have made great contributions in improving the material properties and working lifetime in gas turbines. As a great invention, thermal barrier coatings (TBCs) have been widely used to support a temperature increase for metallic components in service and to improve the gas turbine efficiency [1,2]. TBCs are usually composed of a 7e8 wt% yttria-stabilized zirconia (YSZ) top coat (TC) providing thermal insulation and a MCrAlY (M for Ni/Co) bond coat (BC) providing adhesion for the ceramic TC and metallic substrate. MCrAlY BC also provides the protection for the metallic component against oxidation and corrosion by forming a thermally-grown oxide (TGO) layer. The importance of the investigation on the failure mechanism of such material system has been strengthened for many decades. The failure of the TBCs under a thermal cycling load can be affected by such aspects as material properties (like thermal expansion, E modulus), microstructures
* Corresponding author. E-mail address: [email protected] (K. Yuan). http://dx.doi.org/10.1016/j.vacuum.2016.12.033 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
and environment factors, TGO growth, sintering of the ceramic, creep/plastic deformation of the materials and BC interface roughness [3e5]. The roughness of the interface between the MCrAlY BC and the ceramic has been found to play a significant role on the thermal mechanical properties of TBCs [4e10]. Finite-element modeling (FEM) work has shown that stress distribution are remarkably influenced by the interface roughness [6,11e13]. The BC interface roughness can be modified via several ways like varying the spray processing parameters and changing powder size. A standard thermal cycling test of TBCs is usually performed by heating at a certain temperature followed by a quick cooling to a low temperature [14e18]. In a laboratory test, thermal cycles with a short hold time such as some minutes and 1 h at the maximum temperature are often used, while the thermal cycles related to the operation of gas turbine especially land-based ones have a much longer hold time at the high temperature. That could bring some misunderstanding of the failure mechanism of real TBC components in gas turbines by doing the conventional thermal cycling tests. To provide understanding on how the heating cycle time influences the TBC failure with different MCrAlY interface roughness, two tests were designed in this research. One test was thermal shock which contains cycles of 5 min heating at 1100 C followed by water cooling, while the other was thermal-ageing fatigue containing
K. Yuan et al. / Vacuum 137 (2017) 72e80
cycles of 24-h heating followed by air cooling. In the two tests, two different coating spraying processes (HVOF, APS) were studied, and the effect of MCrAlY interface roughness on the coating failure behavior was discussed.
2. Experimentals Inconel 718 (52.5Ni, 19Cr, 0.5Al, 3.05Mo, 1.03Ti, 5.25Nb þ Ta, 0.08C, balanced by Fe, wt%) was used as the substrate material for TBCs. Three MCrAlY powders (BC1, BC2 and BC3 in Table 1) were used for making the BCs by high-velocity oxy-fuel (HVOF) or atmospheric plasma spray (APS) process. The powder of BC1 and BC2 was made via vacuum atomization followed by a process of cleaning and sieving; powder BC3 was a commercial product namely Amdry 365-2. A commercial YSZ powder (Metco 234A) was used for the TC spray via APS. The composition and the size of the powders are listed in Table 1. Spraying parameters in the HVOF and APS processes are given in Table 2. The TBC samples, as listed in Table 3, were divided into two groups: thin TBCs (#1-#5, TC thickness ~300 mm) and thick TBCs (#6-#8, TC thickness ~1 mm). In the thin TBC group, the BCs of #1 and #2 were sprayed by HVOF (#1 used a coarser powder than #2) while the BCs of #4 and #5 were sprayed by APS (#5 used a coarser powder than #4); the BC of #3 was made by combining HVOF and APS to form an inner low-oxygen HVOF layer and an outer, rougher APS layer. In the thick TBC group, #8 used coarser BC powder (BC3) than #6 and #7 (BC2), and #7 had a thicker BC than #6. The typical BC microstructures made by HVOF, APS and combination of them are shown in Fig. 1. Such design of the samples was aimed to investigate the influence of coating thickness, BC interface roughness, and spraying process on the TBC life. Thermal shock and thermal aging fatigue tests were carried out on the TBC samples. The thermal shock cycling contained heating at 1100 C for 5 min in a furnace followed by water cooling to about
Table 1 Nominal composition and powder size of MCrAlY (BC1, BC2, BC3) and YSZ. Materials
Composition, wt%
Powder size, um
BC1 BC2 BC3 YSZ
Ni-23Co-17Cr-12Al-0.5Y
(75 þ 50) (75 þ 38) (109 þ 53) (90 þ 17)
Ni-22Co-25Cr-7Al-0.5Y ZrO2, 6-8Y2O3/HfO2
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room temperature in 10 s; while the thermal-ageing fatigue test was carried out through cycles containing an ageing process at 1100 C for 24 h followed by forced-air cooling to about 100 C in 10 min. During the thermal cycles, the surface of the cooled samples was photographed to record the coating spallation. The TBC samples were tested till failure and the coating life was defined as the number of cycles that caused 20% area spallation of the ceramic TCs. To better study the failure machanism of TBCs in thermalageing fatigue condition, samples subjected to a certain number of thermal cycles were removed from the furnace and crosssections were then prepared for observation of cracking behavior of the samples before failure. Microstructure study was done in a scanning-electron microscope (SEM). In addition, a quick finite element analysis was performed using the codes ABAQUS to investigate the effect of the BC interface roughness on the stress distributions in TBCs, in which the materials (TC and BC) were simply set to be elastic and rigid. Two cosine interface contours were simulated with wavelength/amplitude of 60/30 mm (W60A30) and 100/50 mm (W100A50). The material constants employed in the FE analysis are listed in Table 4 [4,19] and the temperature change of 1000 C was used.
3. Results and discussion 3.1. Thermal shock Fig. 1 shows the typical morphology of MCrAlY coatings made by HVOF (Fig. 1a), HVOF þ APS (Fig. 1b) and APS (Fig. 1c). Relatively smoother TC-BC interface was observed in HVOF MCrAlY, while interface pegs (indicated by arrows in Fig. 1b and c) were typically observed in APS MCrAlY. The interface pegs could provide microroughness for APS coatings [20]. The “micro-roughness”, however, was not mathematically counted in the measured roughness “Ra”, namely average interface roughness. The Ra of the BCs was measured by image analyzing method. As shown in Fig. 2, the usage of a coarser powder size essentially increased the MCrAlY interface roughness. The spraying process, HVOF and APS, did not cause much difference of the Ra; however, if the “micro-roughness” is taken into accounted, the APS BCs would have higher roughness values. In the thermal shock test, the ceramic TCs were spalled starting from the sample edge and moving towards the center typically during the cooling process. The life of the TBC samples (20% TC
Table 2 Spraying parameters. Coat
Spray process Spray parameters Fuel (kerosene) flow rate 24 L/h, carrier gas (N2) flow rate 7 L/min, O2 flow rate 900 L/h, spray distance 360 mm, powder feed rate ~75 g/min. Ar/H2 flow rate 50/10 L/min, plasma power 46 kW, spray distance 100 mm, powder feed rate ~35 g/min. Ar/H2 flow rate: 38/13 L/min, plasma power: 47 kW, spray distance: 100 mm, powder feed rate: ~25 g/min.
MCrAlY BC HVOF APS YSZ TC APS
Table 3 TBC samples. No. 1# 2# 3# 4# 5# 6# 7# 8#
Thin TBC
Thick TBC
MCrAlY material
BC spray
BC thickness, um
YSZ spray
TC thickness, um
BC1 BC2 BC2 BC2 BC1 BC2 BC2 BC3
HVOF HVOF HVOF þ APS APS APS APS APS APS
150 150 150 150 150 150 350 150
APS APS APS APS APS APS APS APS
300 300 300 300 300 1000 1000 1000
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Fig. 1. Cross sections of MCrAlY bond coats using diffent spray processes. (a) HVOF-sprayed (2#), (b) HVOF þ APS (3#), (c) APS-sprayed (5#). The arrows mark some MCrAlY interface pegs typically formed by APS spray.
Table 4 Material parameters used for FEM simulation. Component
Young's Modulus, GPa
Poisson's Ratio
Coefficient of Expansion, 106 1/ C
TC BC
53 225
0.25 0.3
10.2 14
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Fig. 2. The average roughness Ra of the MCrAlY interfaces measured by image analyzing approach (cutting-off length 0.8 mm). The powder size used for the MCrAlYs is labeled (in um). The error bars are standard deviation by measuring different positions at the MCrAlY interfaces.
75
thin TBC group, the longest TBC life occurred in #5 whose BC was sprayed by APS and with a rougher BC interface. A fact that a rougher BC interface gave a longer TBC life was also observed in HVOF MCrAlYs (#1 > #2). APS BCs gave longer life than HVOF BCs. The smoother BC interface and the unmelted particles on HVOF overlay coatings could increase the overall stresses in the TC and therefore caused earlier failure according to M. Gupta's study [10]. Using the same MCrAlY powder, the combination of HVOF and APS for making BC (#3) seemed to provide higher thermal shock resistance than that using pure HVOF (#2) or APS (#4) process. W. Nowak's research indicated that the APS-flashcoat exhibited more “micro roughness” on the BC interface which resulted in a more homogeneously-distributed microcracking and a delayed cracking linking [20]. For the thick TBCs (#6-#8), increasing BC interface roughness was also benefit to improve TBC life (#8 > #6, #7). The typical fracture morphology of the TBCs is presented in Fig. 4 (HVOF BC) and Fig. 5 (APS BC). The cracks in the TC away from the BC interface caused a “white” fracture (failure inside of TC), while the cracks along BC interface caused a “black” fracture (failure along TGO). In #1 (Fig. 4a) a mixture of “white” and “black” fracture was observed while in #2 mainly “white” fracture was found (Fig. 4b). The position of the cracks was strongly related to the stress distribution in the TBCs. In the whole, the results suggest that the increased MCrAlY interface roughness could transfer the failure from the TC to the BC interface or even inside of BC, i.e. from “white” failure to “black” failure. The TBCs with coarser MCrAlY powder (#5) showed increased amount of through-BC cracks in MCrAlY part, as seen in Fig. 5b. Since the cracks were vertical, it indicated that the BC sustained a tensile stress parallel with the BC interface during cooling process. The magnified details in Fig. 6 revealed that the cracks preferred to initiate from the MCrAlY splats' boundaries at the BC interface, particularly at the boundaries of large unmelted particles. Along the through-BC cracks, oxidation of the MCrAlY was accelerated. 3.2. Thermal-ageing fatigue
Fig. 3. TBC life in the thermal shock test as a function of BC interface roughness. Two specimens were tested for each TBC sample.
spallation) as a function of BC interface roughness is presented in Fig. 3. It was not surprising to find that thin TBCs (TC ~ 300 mm thick) had longer life than thick ones (TC ~ 1 mm thick) [8]. In the
In the thermal-ageing fatigue condition, failure of the TBCs occurred by delamination of the whole TC layer rather than breaking off in pieces observed in the thermal shock test. The overview of the fractured interfaces on BC side and TC side are given in Fig. 7. The thin TBCs (#1-#5) presented grey color, indicating a fracture along/near MCrAlY interfaces, while the white color of the interfaces in the thick TBCs (6#-8#) indicated a fracture
Fig. 4. Cross sections of the TBCs failed under thermal shock condition comparing two HVOF-sprayed bond coats: (a) 1# (using powder BC1), (b) 2# (using powder BC2). “w-f” for “white-fracture”, “b-f” for “black-fracture”.
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Fig. 5. Cross sections of the TBCs failed under thermal shock condition comparing two APS-sprayed bond coats: (a) 4# (using powder BC2), (b) 5# (using powder BC1). “w-f” for “white-fracture”, “b-f” for “black-fracture”.
Fig. 6. The morphology of BC cracking in (a) HVOF sprayed 1# (106 cycles) and (b) APS sprayed 5# (215 cycles) in thermal shock condition.
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Fig. 7. The overview of the fractured surfaces of the samples after thermal ageing-fatigue failure showing TC side and BC sides.
inside of TC away from the MCrAlY interface. The typical morphology of the cross section of the thin TBCs is shown in Fig. 8a, displaying a “mixture” failure; different from that, a “white” failure was found in the thick TBCs (Fig. 9b). In the samples from #1 to #7 alumina TGO was formed at the BC interface, while in sample #8 (Fig. 8c) the break-down oxidation of MCrAlY occurred by forming large oxide mixtures. The formation of such oxide mixtures was quite local as seen in Fig. 7. Our previous work has demonstrated that the same MCrAlY, tested at 1100 C/1 h for 600 cycles/hours,
showed no break-down oxidation [18], indicating that such destructive oxidation was not just a simple chemical failure. Actually, the usage of the coarse powder (BC3) created many largesize unmelted particles in the BC. The break-down oxidation could be due to the oxidation of those particles since the weak particle boundaries blocked the diffusion support of metallic elements from the underling BC and substrate. The coating life under the thermal-ageing failure condition is shown in Fig. 9. Sample #3 with HVOF þ APS sprayed BC showed
Fig. 8. Typical morphology of (a) “black” dominant fracture (1#), (b) “white” fracture (6#), and (c) break-down oxidation of MCrAlY (8#) under thermal-ageing fatigue condition.
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particularly high in the TC zone above BC interface. Furthermore, the fracturing crack (width 2 mm), as marked by arrows, passed mainly by such zone, demonstrating the high concentration of stresses there. FEM modeling work by other researchers has suggested that the high tensile stresses in TC above the BC interface were generated during thermal cycling processes, which could promote the cracking of TC [22e24]. 3.3. Further discussion
Fig. 9. TBC life in the thermal ageing-fatigue test as function of BC interface roughness. Two specimens were tested for each TBC sample.
the longest life, which could be due to a good combination of oxidation resistance, provided by inner HVOF MCrAlY layer, and BC interface roughness, provided by an outer APS MCrAlY layer [21]. The result also shows that the change in MCrAlY interface roughness (#1 vs. #2, or #4 vs. #5) seemed not to have a big influence on the coating life for the thin TBC series. But in the thick TBCs, increasing the MCrAlY roughness greatly improved the coating life (#8>#6). Such life improvement was quite significant since the life of thick TBC (#8) was comparable with the thin APS coating series (#4/#5); while in the thermal shock test (Fig. 2), the lifetime of 8# was only 20% of that of the thin coatings (#5). In addition, increasing the BC thickness could improve the thick-coating life, as the life of #7 (BC ~ 350 mm) was much longer than that of 6# (BC ~ 150 mm). A thicker BC could be beneficial for TBC life due to improved adhesion with the ceramic TC or by providing better oxidation resistance (as Al receiver). Interrupting test was performed by removing samples from the furnace at different testing cycles before failure. The cross section of a sample (#4) after the interrupting test is given in Fig. 10. Horizontal cracks were observed in the TC which was still attached on the MCrAlY BC. The cracks were counted at different depths through the TC, and it was found that the crack density was
As discussed above, increasing MCrAlY interface roughness by using coarser powders was beneficial to enhancing the TBC life in both thermal shock and thermal-ageing fatigue processes. A FEM model, simply considering thermal mismatch between TC and BC and ignoring BC creeping, was applied to study the stress distribution in TBC with two different MCrAlY interface roughness; the stress Smax was an integration of S11 and S22 components. The result (Fig. 11) shows that zones of larger stress were formed in both the TC and BC parts for the coating with the rougher interface. The highest tensile stress was generated at the BC interface valley in the BC part, but the stress values in the two coatings were not much different, indicating that increasing the interface roughness did not significantly increase thermal stress in the vicinity of the interface. Taking into account of the microstructure of TC, the extended stress zone with the rougher interface could embrace more splat cracks, as shown in Fig. 11. The more parallel cracks are driven to propagate, the more elastic energy in TC can be released. That could be an explanation for the longer TBC life of the coating with higher MCrAlY interface roughness. On the other hand, however, spraying coarse MCrAlY powder makes it more difficult to fully melting the particles, which resulted in weaker BC splat boundaries. Therefore, to further improve the lifetime of TBCs by increasing BC interface roughness using coarser powders, the undesirable effect by the unmelted particles like isolation oxidation and cracking at the weak boundaries need to be avoided. Possible approaches are, for instance, powder metallurgy, modified spraying processing, and coating heat treatment. 4. Conclusion The thermal cycling behavior of TBCs with different coating thickness (TC thickness as 300 mm or 1000 mm), coating spray
Fig. 10. The section morphology of an interrupted testing sample (4#). The arrows mark the fracturing cracks whose width larger than 2 mm.
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Fig. 11. FEM modeling on the stress distribution in TBCs with BC interface having different wavelengths/amplitudes: (a) 60/30 mm and (b) 100/50 mm. Smax is an integration of S11 and S22.
process (HVOF or APS), and MCrAlY BC interface roughness was studied at 1100 C under thermal shock (~5 min heating and ~10 s cooling) and thermal-ageing fatigue (~24 h heating and ~10 min cooling) conditions. The remarks can be summarized as followings: 1) It was evident that larger BC interface roughness provided higher resistance against thermal cycling for the TBCs. The improvement of the coating life by increasing the roughness was particularly significant for thick TBC (TC thickness as 1000 mm) under the thermal-aging fatigue process. 2) As APS-sprayed BCs had higher “micro-roughness” than HVOF BCs, TBCs with APS BCs showed longer life in both thermal shock and thermal-ageing fatigue processes. Applying an APS flashcoat on HVOF inner coating displayed an improved coating performance. Particularly under the thermal-ageing fatigue condition, the BC made in such way gave the longest TBC life in the studied samples. 3) Failure of the TBCs tends to occur inside the BC due to the weak boundaries of unmelted MCrAlY particles when coarser MCrAlY powders were used. That urges more investigations on strengthening the splat boundaries for further enhancing the coating lifetime.
Acknowledgment The authors would like to greatly thank the financial and technical support from Beijing General Research Institute of Mining and Metallurgy (BGRIMM) via the international cooperation project (Grant No. 2015DFA51530). The manufacturing of BC3 MCrAlY powder, coating spray and the experimental tests were carried out in BGRIMM. The co-author Jian-Feng Wen, who carried out the FEM modeling work, would like to acknowledge the financial support provided by China Postdoctoral Science Foundation (Grant No. 2015M581543) and Shanghai Sailing Program (Grant No. 15YF1402900).
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