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Influence of heat treatment on thermal cyclic fatigue of TBC systems
Surface & Coatings Technology 379 (2019) 125050
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of heat treatment on thermal cyclic fatigue of TBC systems Jianhong He , Timothy Sharobem
T
∗
Oerlikon Metco, Westbury, NY, 11590, USA
ARTICLE INFO
ABSTRACT
Keywords: TBC coating MCrAlY bond coat Vacuum heat treatment TGO
Vacuum heat treatment of engine components applied with thermal barrier coatings (TBC) is a common industrial process. However, there has been no systematic investigation explaining this practice. In this work, influences of the heat treatments under different degrees of vacuum (as sprayed, heat treated in atmosphere, 0.5 mbar and 6.67 × 10-4 mbar) on thermal cyclic fatigue lifetime of a TBC system was evaluated. Significant differences in the furnace cycle lifetime at 1135 °C of TBCs heat treated under different degrees of vacuum were found. The TBC samples treated at 6.67 × 10-4 mbar exhibited four times longer than other treatment configurations. Examinations of the thermally grown oxide (TGO) layer and EDS analysis suggest that TGOs with different characteristics formed under the different degrees of vacuum are attributed to the differences in thermal cyclic fatigue lifetime.
1. Introduction In thermal barrier coating (TBC) systems, MCrAlY- type bond coats are applied between the component surface and a low thermal conductivity ceramic top. The bond coat provides an oxygen diffusion barrier for the component by forming a continuous, dense, and slowly grown α-Al2O3 layer [1–4]. However, other oxides, such as chromia (Cr, Al)2O3, spinel Ni(Cr, Al)2O4 and nickel oxide NiO can also form during thermal exposure [3–6], and the morphologies of these oxides, which are considered detrimental as they have been shown to accelerate thermal cyclic fatigue, have been characterized in detail [5,6]. Heat treatment of TBCs in a vacuum has been shown to significantly increase thermal cycle fatigue lifetime [7], Libert and Stepka suggested that heat treatment in a vacuum environment is necessary for a number of different critical turbine parts in both commercial and military aero engines based on industrial durability tests [8]. Studies have also indicated that pre-oxidation and pre-annealing of the bond coat layer prior to the deposition of the ceramic top coat can significantly affect TBC performance due to changes in the characteristics of the thermal grown oxide (TGO) layer [9–14]. During heat treatment in vacuum or under Ar with ultra low oxygen partial pressure (< 10-5 mbar), oxide films on the splat surfaces in metal coatings broke down and shrank to particulate oxides [15–17]. In the absence of vacuum heat treatment, the splashed particles of YSZ on the thermally sprayed MCrAlY bond coat surface were weakly bonded to the underlying bulk coating, leading to the formation of mixed oxides and contributing to the TBC failure. During vacuum heat treatment, the small granular splashed
∗
particles were immersed into the bulk coating resulting from the element diffusion on the interface between splashed particles and underlying bulk coating. Thus the formation of mixed oxides was effectively restrained due to the healing of the splashed particle/underlying bulk coating interface [18]. Heat treatment in ultra low oxygen environment also suppressed the formation of mixed oxides and reduced growth rate of alumina oxides [17,19]. However, a relationship between the postspray heat treatment (heat treatment applied after an entire TBC coating is deposited) and thermal cyclic fatigue resistance of TBC has not been well investigated. The present study will provide an understanding of the influence of the post-spray heat treatment on TBC lifetime by comparing the effects of the three post-spray heat treatments. 2. Materials and experiments 2.1. Chemical compositions of powders and synthesis of coatings CoNiCrAlY (Diamalloy 4700, Oerlikon Metco) powder was used as the bond coat material, and the nominal chemical composition is shown in Table 1. A 300 μm thick bond coat with the surface roughness of 6.2 μm was sprayed using HVOF (DJ-2600) on a Hastelloy-X substrate, and then a 300 μm thick top coat of 7YSZ (7 wt % Y2O3 partially stabilized ZrO2, Amdry 204NS-1, Oerlikon Metco) was plasma sprayed on the bond coat. The Hastelloy-X substrate buttons have a dimension of 1.0 inch in diameter, 0.25 inch in thickness. After spraying, the specimens were
Corresponding author. E-mail address: [email protected] (J. He).
https://doi.org/10.1016/j.surfcoat.2019.125050 Received 12 March 2019; Received in revised form 26 August 2019; Accepted 8 September 2019 Available online 05 October 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 379 (2019) 125050
J. He and T. Sharobem
and propagate, and at the second stage, horizontal cracking (delamination) occurs. The present study combined a simple tensile test-ASTM C633 tensile test with FCT to evaluate the influence of FCT on initiation and propagation of cracks.
Table 1 Nominal chemical compositions of Diamalloy 4700 powder (wt. %). Co
Cr
Ni
Al
Y
Others
Bal.
20.0–22.0
31.0–33.0
7.0–9.0
0.35–0.65
< 0.4
2.4. SEM examinations of cross sections and measurement of microhardness
Table 2 Influence of post heat treatment on FCT lifetime and hardness. Heat treatment
HV300
Average FCT cycle
As sprayed (no heat treatment) 1013.25 mbar (in atmosphere) 0.5 mbar 6.67 × 10-4 mbar
462 433 428 430
60 68 117 328
Samples were mounted, cut, ground, polished, and then examined with a Hitachi (Hitachi, Ibaraki, Japan) S-3400 N scanning electron microscope (SEM) with a Bruker (Billerica, MA) X-flash 5030 energy dispersive spectroscopy (EDS). Micro-hardness on sample cross sections before FCT were measured following ASTM E384 Rev. 17 using a load of 300 g. 3. Results and discussions 3.1. Influence of the post heat treatment on FCT lifetime and hardness Table 2 provides FCT lifetime of the TBC samples at 1135 °C at different post-spray heat treatments. A significant difference was observed in the FCT lifetime at these conditions. Microhardness (HV300) values of all heat treated samples were slightly lower than the as–sprayed coating, which is thought to be attributed to internal stress relief caused by heat treatment. Therefore the decrease in microhardness was only related to heat treating parameters (heating rate, dwell temperature and time, and cooling rate) which were the same for all samples treated at three vacuum levels. Heat treatment in atmosphere has no meaningful impact on FCT lifetime, which implies that the FCT lifetime of as sprayed TBC cannot be improved by internal stress relief, microstructural homogenization, recrystallization and grain growth which are usually achieved by a common heat treatment. The heat treatment at 0.5 mbar almost doubled FCT lifetime, and the heat treatment at 6.67 × 10-4 mbar had 5.5 times FCT lifetime as compared to an as sprayed TBC.
Fig. 1. Microstructure of as-sprayed TBC system.
3.2. Evolution of TGO in the samples heat treated at 6.67 × 10-4 mbar
divided and heat treated under three different degrees of vacuum, 1013.25 mbar (ambient pressure), 0.5 mbar (low vacuum) and 6.67 × 10-4 mbar (high vacuum) at 1080 °C for 4 h, respectively.
The heat treatment under vacuum at 6.67 × 10-4 mbar and the evolution of its TGO during FCT was examined more thoroughly and will be used as the benchmark in this study. Fig. 1 shows the TBC microstructure prior to heat treatment (as-sprayed). The bond coat was dense with porosity less than 1 vol %, and no unmelted particles were observed. It has been previously reported that there were very thin oxide films on the splat surfaces of Amdry 9951 (Amdry 9951 has the same nominal chemical composition as Diamalloy 4700 used in the present study, and the only difference between is particle size. Amdry 9951 is slightly smaller. A powder with smaller particle size may experience more oxidation during spray), and the oxide films broke down and shrank to particulate oxides during heat treatments in vacuum or argon with ultra low oxygen partial pressure (< 10-5 mbar) [15–17]. Fig. 2 shows microstructure and TGO of the sample heat treated under high vacuum (6.67 × 10-4 mbar) at 1080 °C for 4 h. At low magnification, the microstructure shown in Fig. 2 (a) is almost the same as the as sprayed coating shown in Fig. 1. Internal oxide chains indicated by arrows in Fig. 2 (b) were alumina according to mapping in Fig. 2 (C). The internal alumina particulate oxide chains indicated by the arrows were also observed for Amdry 9951 bond coats treated in vacuum or argon with ultra low oxygen partial pressure (< 10-5 mbar) [15–17]. After vacuum heat treatment, a thin (average 0.33 μm) dense, alumina layer with uniform thickness was observed at the interface (Fig. 2(b)). There was no evidence for non-alumina oxides. As Ni, Cr and Co are heavier than Al, mixed oxides would appear brighter than
2.2. Thermal cyclic fatigue test Furnace cycle testing (FCT) was carried out with a CM Furnace 1712BL (Bloomfield, NJ) rapid temperature cycling furnace. A thermal cycle consisted of a 10 min ramp-up to 1135 °C (2075 °F), a 50 min dwell, and a 10 min forced air quenching. FCT samples were inspected every 20 cycle, and a sample was considered as failure if there were 20% coating spallation. The reported lifetime of TBCs is an average lifetime of three buttons. During FCT, additional samples were removed after 24, 96 and 200 cycles and were subjected to tensile test (ASTM C633, described below) and underwent metallographic analysis to understand the formation and evolution of TGO layer. 2.3. Tensile tests The stresses in a TBC system are closely associated to properties of the TGO, namely, its thickness, continuity, density, structure, and chemical composition. In a mechanical test, the applied load interacts with the stresses risen by TGO. A bending device with an acoustic emission (AE) sensor and a strain gauge was used to exam crack nucleation and propagation and evaluate delamination or spallation at interface caused by TGO formation and growth [20]. In general, there are two stages of cracking. At the first stage, vertical cracks nucleate
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Surface & Coatings Technology 379 (2019) 125050
J. He and T. Sharobem
Fig. 2. Microstructure and TGO in the sample of as heat treated at 6.67 × 10-4 mbar.
alumina on a back scattered electronic (BSE) image and can be identified even without an assistance of elemental distribution map. The microstructure of the cross-section following a tensile test for the sample heat treated under high vacuum is shown in Fig. 3. The fracture surface examination indicates that tensile fracture primarily happened in top coat (see Fig. 3(b), only very small portion facture happened at interface, indicated by the arrow), indicating cohesion strength of the top coat was smaller than the adhesion strength of the interface. Thus, the interface strength of the alumina TGO, freshly formed during heat treatment at 6.67 × 10-4 mbar, was higher than the top coat cohesion strength. However, cross-section examination shows that fracture happened in the top coat, but very close to the interface, as shown in Fig. 3(c). Fig. 4 shows the microstructure of the tensile test sample treated in high vacuum and 24 FCT-cycles. The continuous alumina TGO increased to an average thickness of 3.7 μm, and the interfaces between TGO/bond coat, TGO/top coat were weakened. Cracks by the applied tensile load initiated and propagated at the interfaces between the TGO/bond coat, the TGO/top coat, and inside the TGO, as indicated by
arrows in Fig. 4 (b), although failure primarily happened in top coat. From the contrast of oxides, it is feasible to identify alumina and non-alumina on SEM on the back scatter image [3–6]. After 24 thermal cycles, non-alumina oxide clusters, indicated by the arrows in Fig. 4 (a), were formed above the alumina layer although non-alumina oxides were not detected in samples treated in high vacuum. However, the cracks were not necessarily associated with these discontinued nonalumina oxide clusters, but were primarily related to the continuous alumina layer (Fig. 4 (b)), or occurred inside the top coat. As cycling increased, the thickness of the continuous alumina TGO layer increased, while the non-alumina clusters remained the same, as observed in Fig. 5. Fig. 6 shows the cross section of the sample following failure from thermal cycling alone. The location of the dominant failure switched from within the top coat to the interfaces, specifically the interfaces of TGO/top coat, TGO/bond coat and the TGO itself. The non-alumina clusters did not grow noticeably, and the sample failure was due to the growth of the continuous alumina TGO. Non-alumina oxides formed on the fracture surface of the continuous alumina layer (bright thin film covered fracture surfaces of
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Surface & Coatings Technology 379 (2019) 125050
J. He and T. Sharobem
Fig. 3. Cross-section and fracture surface of as heat treated at 6.67 × 10-4 mbar.
Fig. 4. Cross-section of the samples after 24 FCT cycles and tensile test.
(ten data points per image). The results, plotted in Fig. 7, showed that the continuous alumina TGO layer grew very fast at the beginning of thermal cycling, then grew steadily. This indicates that the migration rates of aluminum significantly decrease when the continuous alumina TGO layer reached a thickness of about three to four microns. At the same time, Co, Cr and Ni were almost unable to penetrate the continuous alumina layer to promote non-alumina cluster growth. 3.3. Evolution of TGO in the samples heat treated under low vacuum (0.5 mbar) Compared to the samples heat treated under high vacuum, the continuous alumina TGO layer formed in the sample as treated at low vacuum, 0.5 mbar, was much thicker, and the thickness of alumina layer was not uniform, as shown in Fig. 8 (b). It should be noted that a scale bar of 10 μm was used in Fig. 8 (b), while a 5 μm one was used in order to display thinner TGO in Fig. 2 (b). Moreover, the non-alumina cluster was also observed in the as treated sample, see Fig. 9. Non-alumina oxides were observed in the samples treated at 0.5 mbar, shown in Fig. 9. Fig. 10 shows the cross-section of the failed
Fig. 5. Cross-section of samples after 96 cycles.
TGO, see Fig. 6(b)), indicate that the cracks did not form immediately prior to the sample failure. The average thickness of the continuous alumina TGO layer was measured using five continuous SEM images
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Surface & Coatings Technology 379 (2019) 125050
J. He and T. Sharobem
Fig. 6. Cross-section of the failed sample by thermal cycle alone.
FCT sample for the heat treatment at 0.5 mbar. The alumina TGO layer was not uniform in thickness. The thickest segment of the continuous alumina TGO layer exceeded 20 μm. Cracks also propagated across nonalumina clusters, shown in Fig. 10 (b). 3.4. Evolution of TGO in the samples heat treated at atmosphere Nearly two layers of the oxides are shown in the sample heat treated at atmosphere. The non-alumina oxides formed a continuous thick layer with many ridges and overlapped on the alumina TGO layer which was the bottom thin layer with dark contrast, see Fig. 11. Elemental mapping shows the non-alumina oxides were comprised of Al, Co, Cr, and Ni, as shown in Fig. 12. There is a relationship between Gibbs free energy of oxide formation and oxygen pressure. At 1080 °C, several Al2O3, Cr2O3, CrO2, CoO and NiO can form in air. NiO cannot form when oxygen pressure is reduced to 0.1 mbar (20.95 % × 0.5 mbar, assuming oxygen and nitrogen were
Fig. 7. Thickness of continuous alumina layer and thermal cycle number.
Fig. 8. Microstructure and TGO in the sample of as heat treated at 0.5 mbar.
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Surface & Coatings Technology 379 (2019) 125050
J. He and T. Sharobem
Fig. 9. Non-alumina cluster (indicated by an arrow) formed in the sample as heat treated at 0.5 mbar, the arrow points an area containing high O, Al, Cr, Co and Ni.
Fig. 10. Cross-section of the failed samples treated at 0.5 mbar by FCT alone.
samples were treated at 6.67 × 10-4 mbar, and mixed oxides were present in the samples heat treated at 0.5 mbar and were well developed in the samples heat treated in air. Other researchers also found
pumped out proportionally); and both NiO and CoO cannot form when oxygen pressure is reduced to 1.4 × 10-4 mbar (20.95% × 6.67 × 104 mbar) [21]. Therefore, non-alumina oxides were suppressed when
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Surface & Coatings Technology 379 (2019) 125050
J. He and T. Sharobem
Fig. 11. TGO configuration in the sample as heat treated at atmosphere.
other segments reached or even exceeded alumina TGO thickness in the samples treated in high vacuum. The samples heat treated in high vacuum had a thin (∼0.33 μm), continuous and dense alumina layer, and there were no non-alumina oxides. At beginning of thermal cycling, the alumina TGO grew quickly to about 3–4 μm, then steadily and uniformly grew to 7 to 8 μm to lead to a final spallation of TBC due to risen stresses from grown TGO. Non-alumina oxides were formed during the beginning of thermal cycling, but stopped growing due to limited migrations of Ni, Co and Cr through a thick and dense alumina layer. Therefore, non-alumina oxides were not critical to final TBC lifetime although Ni, Co and Cr could penetrate through damaged alumina TGO at the final stage of TBC life.
that heat treatment in argon with ultra low oxygen partial pressure (< 10-5 mbar) reduced growth rate of oxide and suppressed the formation of mixed oxides in Amdry 9951 coating [17,19]. The cross-section of failed sample is shown in Fig. 13. The thickness of TGO in the some segments of the failed sample of as the heat treated in atmosphere was only 2 to 3 μm (after an average of 68 FCT cycles), and the average thickness of alumina TGO in the failed sample of as heat treated in atmosphere was much smaller than the failed samples heat treated under high vacuum, shown in Fig. 7. In other words, the samples heat treated in atmosphere failed prematurely compared to the critical TGO thickness [4], and it is thought to be attributed to its nonuniformity in thickness of TGO and well developed non-alumina oxides. As sprayed samples were directly exposed to air at high temperature during thermal cycling (equivalent to heat treatment in atmosphere) and had the same TGO configurations as the samples heat treated in atmosphere. There were two factors causing the premature failure of the samples heat treated in the atmosphere and for the as sprayed samples. First, the samples heat treated in air had well developed non-alumina oxides overlapped on alumina layer. These non-alumina oxides featured large ridges with areas greater than 20 μm. In other words, oxides in the samples treated in atmosphere exceeded the critical TGO thickness of TBC spallation before FCT test was started, resulting in a very short TBC lifetime. Secondly, alumina oxides formed in the samples treated in the atmosphere during heat treatment and thermal cycling were also not uniform in thickness. It was found that the alumina TGO on the crosssection of the failed samples was only 2 to 3 μm in some segments of the coating which was significantly thinner than the layer formed in the samples treated at high vacuum (7–8 μm), however, alumina TGO in
4. Conclusions Based on the thermal cycling fatigue tests and analyses of thermal grown oxides of the samples heat treated in atmosphere, at 0.5 mbar and 6.67 × 10 -4 mbar, the following conclusions have been obtained. The post-spray heat treatment in vacuum affects characteristics of TGO and significantly improved TBC thermal cycling lifetime. The post-spray heat treatment of TBC coating in vacuum generated a very thin (∼0.33 μm), continuous and alumina layer with uniform thickness at interface between top coat and bond coat, which leads to a steady and uniform growth of alumina TGO thereafter while TBC is exposed to a high temperature. Post-spray heat treatment at reduced vacuum levels or without postspray heat treatment leads to the formation of a non-uniform (in thickness) alumina layer and well developed non-alumina oxides to promote premature TBC coating failure.
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Surface & Coatings Technology 379 (2019) 125050
J. He and T. Sharobem
Fig. 12. Elemental distribution in TGO of the sample as heat treated at atmosphere.
Fig. 13. Cross-section of the failed sample of as heat treated in atmosphere after FCT failure.
Acknowledgement
References
Oerlikon Metco Metallurgical lab have done microstructural analyses, and authors also thank Oerlikon Metco management approval for the publication of this paper.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of heat treatment on thermal cyclic fatigue of TBC systems Jianhong He , Timothy Sharobem
T
∗
Oerlikon Metco, Westbury, NY, 11590, USA
ARTICLE INFO
ABSTRACT
Keywords: TBC coating MCrAlY bond coat Vacuum heat treatment TGO
Vacuum heat treatment of engine components applied with thermal barrier coatings (TBC) is a common industrial process. However, there has been no systematic investigation explaining this practice. In this work, influences of the heat treatments under different degrees of vacuum (as sprayed, heat treated in atmosphere, 0.5 mbar and 6.67 × 10-4 mbar) on thermal cyclic fatigue lifetime of a TBC system was evaluated. Significant differences in the furnace cycle lifetime at 1135 °C of TBCs heat treated under different degrees of vacuum were found. The TBC samples treated at 6.67 × 10-4 mbar exhibited four times longer than other treatment configurations. Examinations of the thermally grown oxide (TGO) layer and EDS analysis suggest that TGOs with different characteristics formed under the different degrees of vacuum are attributed to the differences in thermal cyclic fatigue lifetime.
1. Introduction In thermal barrier coating (TBC) systems, MCrAlY- type bond coats are applied between the component surface and a low thermal conductivity ceramic top. The bond coat provides an oxygen diffusion barrier for the component by forming a continuous, dense, and slowly grown α-Al2O3 layer [1–4]. However, other oxides, such as chromia (Cr, Al)2O3, spinel Ni(Cr, Al)2O4 and nickel oxide NiO can also form during thermal exposure [3–6], and the morphologies of these oxides, which are considered detrimental as they have been shown to accelerate thermal cyclic fatigue, have been characterized in detail [5,6]. Heat treatment of TBCs in a vacuum has been shown to significantly increase thermal cycle fatigue lifetime [7], Libert and Stepka suggested that heat treatment in a vacuum environment is necessary for a number of different critical turbine parts in both commercial and military aero engines based on industrial durability tests [8]. Studies have also indicated that pre-oxidation and pre-annealing of the bond coat layer prior to the deposition of the ceramic top coat can significantly affect TBC performance due to changes in the characteristics of the thermal grown oxide (TGO) layer [9–14]. During heat treatment in vacuum or under Ar with ultra low oxygen partial pressure (< 10-5 mbar), oxide films on the splat surfaces in metal coatings broke down and shrank to particulate oxides [15–17]. In the absence of vacuum heat treatment, the splashed particles of YSZ on the thermally sprayed MCrAlY bond coat surface were weakly bonded to the underlying bulk coating, leading to the formation of mixed oxides and contributing to the TBC failure. During vacuum heat treatment, the small granular splashed
∗
particles were immersed into the bulk coating resulting from the element diffusion on the interface between splashed particles and underlying bulk coating. Thus the formation of mixed oxides was effectively restrained due to the healing of the splashed particle/underlying bulk coating interface [18]. Heat treatment in ultra low oxygen environment also suppressed the formation of mixed oxides and reduced growth rate of alumina oxides [17,19]. However, a relationship between the postspray heat treatment (heat treatment applied after an entire TBC coating is deposited) and thermal cyclic fatigue resistance of TBC has not been well investigated. The present study will provide an understanding of the influence of the post-spray heat treatment on TBC lifetime by comparing the effects of the three post-spray heat treatments. 2. Materials and experiments 2.1. Chemical compositions of powders and synthesis of coatings CoNiCrAlY (Diamalloy 4700, Oerlikon Metco) powder was used as the bond coat material, and the nominal chemical composition is shown in Table 1. A 300 μm thick bond coat with the surface roughness of 6.2 μm was sprayed using HVOF (DJ-2600) on a Hastelloy-X substrate, and then a 300 μm thick top coat of 7YSZ (7 wt % Y2O3 partially stabilized ZrO2, Amdry 204NS-1, Oerlikon Metco) was plasma sprayed on the bond coat. The Hastelloy-X substrate buttons have a dimension of 1.0 inch in diameter, 0.25 inch in thickness. After spraying, the specimens were
Corresponding author. E-mail address: [email protected] (J. He).
https://doi.org/10.1016/j.surfcoat.2019.125050 Received 12 March 2019; Received in revised form 26 August 2019; Accepted 8 September 2019 Available online 05 October 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 379 (2019) 125050
J. He and T. Sharobem
and propagate, and at the second stage, horizontal cracking (delamination) occurs. The present study combined a simple tensile test-ASTM C633 tensile test with FCT to evaluate the influence of FCT on initiation and propagation of cracks.
Table 1 Nominal chemical compositions of Diamalloy 4700 powder (wt. %). Co
Cr
Ni
Al
Y
Others
Bal.
20.0–22.0
31.0–33.0
7.0–9.0
0.35–0.65
< 0.4
2.4. SEM examinations of cross sections and measurement of microhardness
Table 2 Influence of post heat treatment on FCT lifetime and hardness. Heat treatment
HV300
Average FCT cycle
As sprayed (no heat treatment) 1013.25 mbar (in atmosphere) 0.5 mbar 6.67 × 10-4 mbar
462 433 428 430
60 68 117 328
Samples were mounted, cut, ground, polished, and then examined with a Hitachi (Hitachi, Ibaraki, Japan) S-3400 N scanning electron microscope (SEM) with a Bruker (Billerica, MA) X-flash 5030 energy dispersive spectroscopy (EDS). Micro-hardness on sample cross sections before FCT were measured following ASTM E384 Rev. 17 using a load of 300 g. 3. Results and discussions 3.1. Influence of the post heat treatment on FCT lifetime and hardness Table 2 provides FCT lifetime of the TBC samples at 1135 °C at different post-spray heat treatments. A significant difference was observed in the FCT lifetime at these conditions. Microhardness (HV300) values of all heat treated samples were slightly lower than the as–sprayed coating, which is thought to be attributed to internal stress relief caused by heat treatment. Therefore the decrease in microhardness was only related to heat treating parameters (heating rate, dwell temperature and time, and cooling rate) which were the same for all samples treated at three vacuum levels. Heat treatment in atmosphere has no meaningful impact on FCT lifetime, which implies that the FCT lifetime of as sprayed TBC cannot be improved by internal stress relief, microstructural homogenization, recrystallization and grain growth which are usually achieved by a common heat treatment. The heat treatment at 0.5 mbar almost doubled FCT lifetime, and the heat treatment at 6.67 × 10-4 mbar had 5.5 times FCT lifetime as compared to an as sprayed TBC.
Fig. 1. Microstructure of as-sprayed TBC system.
3.2. Evolution of TGO in the samples heat treated at 6.67 × 10-4 mbar
divided and heat treated under three different degrees of vacuum, 1013.25 mbar (ambient pressure), 0.5 mbar (low vacuum) and 6.67 × 10-4 mbar (high vacuum) at 1080 °C for 4 h, respectively.
The heat treatment under vacuum at 6.67 × 10-4 mbar and the evolution of its TGO during FCT was examined more thoroughly and will be used as the benchmark in this study. Fig. 1 shows the TBC microstructure prior to heat treatment (as-sprayed). The bond coat was dense with porosity less than 1 vol %, and no unmelted particles were observed. It has been previously reported that there were very thin oxide films on the splat surfaces of Amdry 9951 (Amdry 9951 has the same nominal chemical composition as Diamalloy 4700 used in the present study, and the only difference between is particle size. Amdry 9951 is slightly smaller. A powder with smaller particle size may experience more oxidation during spray), and the oxide films broke down and shrank to particulate oxides during heat treatments in vacuum or argon with ultra low oxygen partial pressure (< 10-5 mbar) [15–17]. Fig. 2 shows microstructure and TGO of the sample heat treated under high vacuum (6.67 × 10-4 mbar) at 1080 °C for 4 h. At low magnification, the microstructure shown in Fig. 2 (a) is almost the same as the as sprayed coating shown in Fig. 1. Internal oxide chains indicated by arrows in Fig. 2 (b) were alumina according to mapping in Fig. 2 (C). The internal alumina particulate oxide chains indicated by the arrows were also observed for Amdry 9951 bond coats treated in vacuum or argon with ultra low oxygen partial pressure (< 10-5 mbar) [15–17]. After vacuum heat treatment, a thin (average 0.33 μm) dense, alumina layer with uniform thickness was observed at the interface (Fig. 2(b)). There was no evidence for non-alumina oxides. As Ni, Cr and Co are heavier than Al, mixed oxides would appear brighter than
2.2. Thermal cyclic fatigue test Furnace cycle testing (FCT) was carried out with a CM Furnace 1712BL (Bloomfield, NJ) rapid temperature cycling furnace. A thermal cycle consisted of a 10 min ramp-up to 1135 °C (2075 °F), a 50 min dwell, and a 10 min forced air quenching. FCT samples were inspected every 20 cycle, and a sample was considered as failure if there were 20% coating spallation. The reported lifetime of TBCs is an average lifetime of three buttons. During FCT, additional samples were removed after 24, 96 and 200 cycles and were subjected to tensile test (ASTM C633, described below) and underwent metallographic analysis to understand the formation and evolution of TGO layer. 2.3. Tensile tests The stresses in a TBC system are closely associated to properties of the TGO, namely, its thickness, continuity, density, structure, and chemical composition. In a mechanical test, the applied load interacts with the stresses risen by TGO. A bending device with an acoustic emission (AE) sensor and a strain gauge was used to exam crack nucleation and propagation and evaluate delamination or spallation at interface caused by TGO formation and growth [20]. In general, there are two stages of cracking. At the first stage, vertical cracks nucleate
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Fig. 2. Microstructure and TGO in the sample of as heat treated at 6.67 × 10-4 mbar.
alumina on a back scattered electronic (BSE) image and can be identified even without an assistance of elemental distribution map. The microstructure of the cross-section following a tensile test for the sample heat treated under high vacuum is shown in Fig. 3. The fracture surface examination indicates that tensile fracture primarily happened in top coat (see Fig. 3(b), only very small portion facture happened at interface, indicated by the arrow), indicating cohesion strength of the top coat was smaller than the adhesion strength of the interface. Thus, the interface strength of the alumina TGO, freshly formed during heat treatment at 6.67 × 10-4 mbar, was higher than the top coat cohesion strength. However, cross-section examination shows that fracture happened in the top coat, but very close to the interface, as shown in Fig. 3(c). Fig. 4 shows the microstructure of the tensile test sample treated in high vacuum and 24 FCT-cycles. The continuous alumina TGO increased to an average thickness of 3.7 μm, and the interfaces between TGO/bond coat, TGO/top coat were weakened. Cracks by the applied tensile load initiated and propagated at the interfaces between the TGO/bond coat, the TGO/top coat, and inside the TGO, as indicated by
arrows in Fig. 4 (b), although failure primarily happened in top coat. From the contrast of oxides, it is feasible to identify alumina and non-alumina on SEM on the back scatter image [3–6]. After 24 thermal cycles, non-alumina oxide clusters, indicated by the arrows in Fig. 4 (a), were formed above the alumina layer although non-alumina oxides were not detected in samples treated in high vacuum. However, the cracks were not necessarily associated with these discontinued nonalumina oxide clusters, but were primarily related to the continuous alumina layer (Fig. 4 (b)), or occurred inside the top coat. As cycling increased, the thickness of the continuous alumina TGO layer increased, while the non-alumina clusters remained the same, as observed in Fig. 5. Fig. 6 shows the cross section of the sample following failure from thermal cycling alone. The location of the dominant failure switched from within the top coat to the interfaces, specifically the interfaces of TGO/top coat, TGO/bond coat and the TGO itself. The non-alumina clusters did not grow noticeably, and the sample failure was due to the growth of the continuous alumina TGO. Non-alumina oxides formed on the fracture surface of the continuous alumina layer (bright thin film covered fracture surfaces of
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Fig. 3. Cross-section and fracture surface of as heat treated at 6.67 × 10-4 mbar.
Fig. 4. Cross-section of the samples after 24 FCT cycles and tensile test.
(ten data points per image). The results, plotted in Fig. 7, showed that the continuous alumina TGO layer grew very fast at the beginning of thermal cycling, then grew steadily. This indicates that the migration rates of aluminum significantly decrease when the continuous alumina TGO layer reached a thickness of about three to four microns. At the same time, Co, Cr and Ni were almost unable to penetrate the continuous alumina layer to promote non-alumina cluster growth. 3.3. Evolution of TGO in the samples heat treated under low vacuum (0.5 mbar) Compared to the samples heat treated under high vacuum, the continuous alumina TGO layer formed in the sample as treated at low vacuum, 0.5 mbar, was much thicker, and the thickness of alumina layer was not uniform, as shown in Fig. 8 (b). It should be noted that a scale bar of 10 μm was used in Fig. 8 (b), while a 5 μm one was used in order to display thinner TGO in Fig. 2 (b). Moreover, the non-alumina cluster was also observed in the as treated sample, see Fig. 9. Non-alumina oxides were observed in the samples treated at 0.5 mbar, shown in Fig. 9. Fig. 10 shows the cross-section of the failed
Fig. 5. Cross-section of samples after 96 cycles.
TGO, see Fig. 6(b)), indicate that the cracks did not form immediately prior to the sample failure. The average thickness of the continuous alumina TGO layer was measured using five continuous SEM images
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Fig. 6. Cross-section of the failed sample by thermal cycle alone.
FCT sample for the heat treatment at 0.5 mbar. The alumina TGO layer was not uniform in thickness. The thickest segment of the continuous alumina TGO layer exceeded 20 μm. Cracks also propagated across nonalumina clusters, shown in Fig. 10 (b). 3.4. Evolution of TGO in the samples heat treated at atmosphere Nearly two layers of the oxides are shown in the sample heat treated at atmosphere. The non-alumina oxides formed a continuous thick layer with many ridges and overlapped on the alumina TGO layer which was the bottom thin layer with dark contrast, see Fig. 11. Elemental mapping shows the non-alumina oxides were comprised of Al, Co, Cr, and Ni, as shown in Fig. 12. There is a relationship between Gibbs free energy of oxide formation and oxygen pressure. At 1080 °C, several Al2O3, Cr2O3, CrO2, CoO and NiO can form in air. NiO cannot form when oxygen pressure is reduced to 0.1 mbar (20.95 % × 0.5 mbar, assuming oxygen and nitrogen were
Fig. 7. Thickness of continuous alumina layer and thermal cycle number.
Fig. 8. Microstructure and TGO in the sample of as heat treated at 0.5 mbar.
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Fig. 9. Non-alumina cluster (indicated by an arrow) formed in the sample as heat treated at 0.5 mbar, the arrow points an area containing high O, Al, Cr, Co and Ni.
Fig. 10. Cross-section of the failed samples treated at 0.5 mbar by FCT alone.
samples were treated at 6.67 × 10-4 mbar, and mixed oxides were present in the samples heat treated at 0.5 mbar and were well developed in the samples heat treated in air. Other researchers also found
pumped out proportionally); and both NiO and CoO cannot form when oxygen pressure is reduced to 1.4 × 10-4 mbar (20.95% × 6.67 × 104 mbar) [21]. Therefore, non-alumina oxides were suppressed when
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Fig. 11. TGO configuration in the sample as heat treated at atmosphere.
other segments reached or even exceeded alumina TGO thickness in the samples treated in high vacuum. The samples heat treated in high vacuum had a thin (∼0.33 μm), continuous and dense alumina layer, and there were no non-alumina oxides. At beginning of thermal cycling, the alumina TGO grew quickly to about 3–4 μm, then steadily and uniformly grew to 7 to 8 μm to lead to a final spallation of TBC due to risen stresses from grown TGO. Non-alumina oxides were formed during the beginning of thermal cycling, but stopped growing due to limited migrations of Ni, Co and Cr through a thick and dense alumina layer. Therefore, non-alumina oxides were not critical to final TBC lifetime although Ni, Co and Cr could penetrate through damaged alumina TGO at the final stage of TBC life.
that heat treatment in argon with ultra low oxygen partial pressure (< 10-5 mbar) reduced growth rate of oxide and suppressed the formation of mixed oxides in Amdry 9951 coating [17,19]. The cross-section of failed sample is shown in Fig. 13. The thickness of TGO in the some segments of the failed sample of as the heat treated in atmosphere was only 2 to 3 μm (after an average of 68 FCT cycles), and the average thickness of alumina TGO in the failed sample of as heat treated in atmosphere was much smaller than the failed samples heat treated under high vacuum, shown in Fig. 7. In other words, the samples heat treated in atmosphere failed prematurely compared to the critical TGO thickness [4], and it is thought to be attributed to its nonuniformity in thickness of TGO and well developed non-alumina oxides. As sprayed samples were directly exposed to air at high temperature during thermal cycling (equivalent to heat treatment in atmosphere) and had the same TGO configurations as the samples heat treated in atmosphere. There were two factors causing the premature failure of the samples heat treated in the atmosphere and for the as sprayed samples. First, the samples heat treated in air had well developed non-alumina oxides overlapped on alumina layer. These non-alumina oxides featured large ridges with areas greater than 20 μm. In other words, oxides in the samples treated in atmosphere exceeded the critical TGO thickness of TBC spallation before FCT test was started, resulting in a very short TBC lifetime. Secondly, alumina oxides formed in the samples treated in the atmosphere during heat treatment and thermal cycling were also not uniform in thickness. It was found that the alumina TGO on the crosssection of the failed samples was only 2 to 3 μm in some segments of the coating which was significantly thinner than the layer formed in the samples treated at high vacuum (7–8 μm), however, alumina TGO in
4. Conclusions Based on the thermal cycling fatigue tests and analyses of thermal grown oxides of the samples heat treated in atmosphere, at 0.5 mbar and 6.67 × 10 -4 mbar, the following conclusions have been obtained. The post-spray heat treatment in vacuum affects characteristics of TGO and significantly improved TBC thermal cycling lifetime. The post-spray heat treatment of TBC coating in vacuum generated a very thin (∼0.33 μm), continuous and alumina layer with uniform thickness at interface between top coat and bond coat, which leads to a steady and uniform growth of alumina TGO thereafter while TBC is exposed to a high temperature. Post-spray heat treatment at reduced vacuum levels or without postspray heat treatment leads to the formation of a non-uniform (in thickness) alumina layer and well developed non-alumina oxides to promote premature TBC coating failure.
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Fig. 12. Elemental distribution in TGO of the sample as heat treated at atmosphere.
Fig. 13. Cross-section of the failed sample of as heat treated in atmosphere after FCT failure.
Acknowledgement
References
Oerlikon Metco Metallurgical lab have done microstructural analyses, and authors also thank Oerlikon Metco management approval for the publication of this paper.
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