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Microstructure design and mechanical properties of thermal barrier coatings with layered top and bond coats
Surface & Coatings Technology 205 (2010) 1229–1235
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Microstructure design and mechanical properties of thermal barrier coatings with layered top and bond coats Sang-Won Myoung a, Jae-Hyun Kim a, Woo-Ram Lee a, Yeon-Gil Jung a,⁎, Kee-Sung Lee b, Ungyu Paik c,⁎ a b c
School of Nano and Advanced Materials Engineering, Changwon National University, #9 Sarim-dong, Changwon, Kyungnam 641-773, Republic of Korea School of Mechanical Engineering, Kookmin University, 861-1 Chongnungdong, Songbukgu, Seoul 136-702, Republic of Korea Department of Energy Engineering, Hanyang University, #17 Haengdang-dong, Sungdon-gu, Seoul 133-791, Republic of Korea
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Available online 20 August 2010 Keywords: Thermal barrier coating Microstructure Mechanical property Layered structure Indentation
a b s t r a c t The microstructure of layered thermal barrier coatings (TBCs) with three coating layers in the bond and top coats, respectively, prepared using a specialized coating system (TriplexPro™-200), was controlled and its mechanical properties were investigated, which were then compared with the common TBCs with a single layer in each coat. The bond and top coats were coated with 100 and 200 μm for each feedstock, resulting in 300 and 600 μm thicknesses in the bond and top coats, respectively. The microstructure of the top coat could be controlled by changing the feedstock and using a multiple hopper system—dense/intermediate/porous layers from surface to interface or reverse microstructure. In the case of the bond coat, a compositional gradient was achieved. The adhesive strength values of the top coats were strongly dependent on the microstructure, whereas the values for the bond coat were similar. The hardness and toughness values gradually changed from surface to interface, indicating that the mechanical properties corresponded well with the microstructure of the TBCs. The indentation stress–strain curves of both TBCs with the layered structure were located between the curves for TBCs with the single structure of relatively dense and porous microstructures. Damage on the surface and subsurface was strongly affected by the microstructure of the top coat, showing a similar trend with the stress–strain behavior. This evidence allowed us to propose an efficient coating in protecting the substrate from mechanical environments. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The microstructure of thermal barrier coating (TBC) systems is determined by the feedstock powder as well as the spraying conditions. In particular, the size, shape, and density of the powder are critical to the microstructure and are the most important parameters for the quality of TBCs [1–3]. Thermal properties and failure mechanisms of TBCs are also closely related to their microstructure [5–7]. Therefore, the control of microstructure in TBCs including a bond coat is proposed as a new strategy for advanced coatings, using a specialized coating system (TriplexPro™-200) [8–10]. In this coating system, a higher particle velocity and temperature can be produced, compared with conventional APS system [8,10]. The particle velocity in the TriplexPro™-200 can be produced up to 560 m/s, which is nearly HVOF quality with the particle velocity of about 700 m/s. For this reason, TBC by the TriplexPro™-200 has coarse grain sections. In
⁎ Corresponding authors. Jung is to be contacted at Tel.: + 82 55 213 3712; fax: + 82 55 262 6486. Paik, Tel.: + 82 2 2220 0508; fax: + 82 2 2281 0502. E-mail addresses: [email protected] (Y.-G. Jung), [email protected] (U. Paik). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.063
addition, it is reported that the TriplexPro™-200 shows a significant improvement in powder feeding rate and deposition efficiency, compared with conventional APS system. Powder feeding rates up to 150 g/min are possible in the TriplexPro™-200 using three powder feeders (hoppers) and three plasma arcs. Therefore, the different processing parameter of the TriplexPro™-200, compared with the conventional APS system, may affect the thermal durability of TBC, and microstructural design through the control of coating parameter should be achieved to enhance the thermal durability. Numerous factors, besides the thermomechanical properties, have to be considered in practical applications of TBCs, such as erosion and wear resistance. There is, therefore, a need to improve the adhesive strength, damage resistance, and mechanical characteristics, which are essential to improving the reliability and lifetime performance of the APS–TBC system. In the present study, to improve the mechanical properties and to enhance the damage resistance, the microstructures in both the top and bond coats were designed using the specialized coating system. The effects of the microstructure design on the mechanical properties and damage resistance were investigated in the newly developed TBCs showing a layered structure (hereinafter layered TBCs). These results were compared with the common TBC system of a single structure in each coat.
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2. Experimental procedure
Table 2 Coating parameters for preparing bond and top coats.
2.1. Preparation of TBCs A nickel-based alloy (Nimonic 263, a nominal composition of Ni– 20Cr–20Co–5.9Mo–0.5Al–2.1Ti–0.4Mn–0.3Si–0.06C, in wt.%, ThyssenKrupp VDM, Germany) was used as a substrate. Two types of feedstock powder with different compositions and particle size distributions were used to coat a bond coat on the substrate—METCO 46 1NS (hereinafter 461 NS; nominal composition of Ni–17.5Cr–5.5Al– 2.5Co–0.5Y2O3 in wt.% and particle size of 22–150 μm, Sultzer Metco, Switzerland) and AMDRY 9625 (hereinafter 9625; nominal composition of Ni–22Cr–10Al–1.0Y2O3 in wt.% and particle size of 45–75 μm, Sultzer Metco, Switzerland) [11]. Two sets of bond coat were air plasma-sprayed with a total thickness of d = 300 ± 30 μm, as shown in Table 1. The bond coats of TBC-1 and TBC-2 were prepared with 461 NS and 9625, and while those of TBC-3 and TBC-4 consisted of three layers with a thickness of d = 100 ± 10 μm, respectively—upper layer of 461 NS, middle layer of composite with 9625 and 461 NS (mixing ratio of 50:50 in vol.%), and base layer of 9625. To prepare the bond coats of TBC-3 and TBC-4, a multiple hopper system was used. First, the base layer was plasma-sprayed onto the substrate with d = 100 ± 10 μm. Second, the feeding rate of 9625 for the middle layer was reduced to half and the feed of 461 NS was started with a half amount, again making d = 100 ± 10 μm. Third, the feed of 9625 was stopped and the feed of 461 NS was increased to the full amount. Four sets of top coat with a thickness of d = 600–650 μm were deposited onto the bond coat using two types of feedstock powder with different particle size distributions—METCO 204 C-NS (hereinafter 204 C-NS; nominal composition of ZrO2–8.0Y2O3 in wt.% and particle size of 45–125 μm, Sultzer Metco, Switzerland), METCO 204 NS (hereinafter 204 NS; Sultzer Metco, Switzerland) [11]. The top coats were also designed in the same sequence with the bond coat, as shown in Table 1. Detailed deposition and spray parameters, carried out using a specialized gun (TriplexPro™-200, Sulzer Metco Holding AG, Switzerland), are shown in Table 2 with modifications to the manufacturer's specifications. 2.2. Characterization The TBC samples with the single and layered structures were sectioned and polished to observe the cross-sectional microstructure, including the thickness of each TBC prepared, using a scanning electron microscope (SEM, JEOL Model JSM-5610, Japan). The preparation of the sample followed the specimens' preparing procedure [12]. Two-dimensional image analysis was conducted on the sectional planes, using Image-Pro Plus software (Media Cybernetics, USA), and the microstructures of each TBC were classified by the horizontal, vertical, and spherical defects [4]. The hardness, H, and toughness, T, values were determined using a microindenter (HM114, Mitutoyo Corp., Japan) for loads of 10 and 50 N, respectively, with a Vickers tip. The size of the hardness impression and the crack length produced were measured using SEM. Each value of H and T was determined from equations proposed by Lawn [13]. The adhesive strength of each as-prepared TBC was measured according to the
Parameters
Bond coat
Top coat
Feedstock species Feed rate (g/min) Powder carrier gas (ℓ/min) Gun to work distance (mm) Current/voltage (A/V) Gun moving speed (mm/s) Primary and secondary gas (Ar/He) Step distance (mm)
461 NS and 9625 90 3.5 180 450/90 500 30/20 8
204 C-NS and 204 NS 100 3 150 540/99 500 45/5 5
ASTM standard [14]. The indentation stress–strain behavior of each TBC was investigated by indentations on the top surfaces with tungsten carbide (WC) spherical indenters with a radius of r = 2.32 mm (J & L Industrial Supply Co., MI, USA), using an Instron testing machine (Model 5566, Instron Corp., Norwood, MA, USA) [15– 17]. The values of H and E for the WC indenter used in this study were 19 and 614 GPa, respectively [16]. For examination of the subsurface damage dependence on microstructure, the surfaces of each TBC were indented at a load of P = 100 N, using the microindenter with a Vickers tip, and then they were polished down to the center of the damage site and finished off using 1 μm diamond paste. For both indentation tests, the top surface was slightly polished with 1 μm diamond paste. All experiments were performed in air at room temperature. 3. Results and discussion 3.1. Microstructure Cross-sectional micrographs of each TBC at low magnification are shown in Fig. 1, which were designed with different feedstock powders in both the top and bond coats, as indicated in Table 1. The microstructure of the top coat prepared with 204 C-NS (Fig. 1(A)) was relatively porous, compared with that with 204 NS (Fig. 1(B)), because of the particle size distribution of the feedstock. The layered top coats were well deposited, with the designed concept indicated in Table 1, showing the dense, intermediate, and porous microstructures from upper to base in Fig. 1(C) and the reverse trend in Fig. 1(D). More detailed microstructures of TBCs with the layered structure are shown in Fig. 2. The dense layer with 204 NS (upper layer in Fig. 2(A) and base layer in Fig. 2(B)) contained smaller and relatively uniform “splat” boundaries/cracks, whereas the porous layers with 204 C-NS (base layer in Fig. 2(A) and upper layer in Fig. 2(B)) indicated thicker “splat” boundaries/cracks, global pores, and unmelted particles. By moving to the porous layer, relatively large defects such as global pores and unmelted particles were increased. Even though it was difficult to distinguish the boundary between each layer, the microstructural evolution could be defined by highly magnified microstructures, shown in the left side of Fig. 2. The interface between the bond and top coats showed a sound condition without any cracking or delamination.
Table 1 Feedstock powders and coating conditions for TBC systems. TBC-1
TBC-2
TBC-3
TBC-4
Coating thickness
Top coat
204 C-NS
204 NS
461 NS, 9625
461 NS, 9625
204 C-NS: upper 204 C-NS/204 NS: middle 204 NS: base 461 NS: upper 461 NS/9625: middle 9625: base
600 μm (each coat: 200 μm)
Bond coat
204 NS: upper 204 NS/204 C-NS: middle 204 C-NS: base 461 NS: upper 461 NS/9625: middle 9625: base
300 μm (each coat: 100 μm)
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Fig. 1. Cross-sectional microstructures of TBCs with single and layered structures: (A) TBC-1, (B) TBC-2, (C) TBC-3, and (D) TBC-4.
The microstructural analysis revealed different porosities with the feedstock powder used, as shown in Table 3. The pores constituted a large portion of the total porosity obtained in the image analysis, especially in the case of the top coat prepared with 204 C-NS (TBC-1). The porosities in the top coats of the single structure (TBC-1 and TBC-2) were 19.6% and 10.2%, respectively, and those of the layered structure (TBC-3 and TBC-4) changed from 8.2% to 18.9%. In the layered TBCs, the pore fraction in the total porosity and the pore size were proportional to the change in the porosity. The horizontal and vertical defects (boundary or cracks) did not extend to 100 μm, indicating that the horizontal defects were longer than the vertical defects in all samples observed. Usually, the commercial APS coating shows a porosity of 15– 25% [5,6]. It is important that the porosity can be tailored from 8% to 19% by changing the feedstock in the specialized coating system. The microstructure of commercial APS coatings has been studied extensively and the failure mechanism has been well documented in the literature, including those for graded coatings [18–20]. The coating reliability can be greatly improved through the development of an appropriate bond coat, which can provide strain isolation for the ceramic coating and protect the substrate from oxidation. Thus, the stress arising from thermal expansion mismatch can be minimized and interfacial strength can be maintained. The strain isolation
provided by a layered bond coat is especially beneficial for coating edges that exist due to component geometry, or due to throughthickness cracking resulting from ceramic sintering and creep at high temperatures. Even though the graded coatings could provide interface stability in the APS coatings, the preparation cost limits the application. However, in this work, the specialized coating system using a multihopper could prepare the layered or semigraded coatings by changing the feedstock powder, showing continuous changes in microstructure and composition in the top and bond coats, respectively. The microstructure prepared could satisfy the continuously increasing functional and structural demands on TBCs. 3.2. Mechanical properties The H and T values measured on sectional planes for each region of the layered TBCs and post-indentation micrographs are shown in Fig. 3. The H values in the TBCs are not as easily and clearly defined as those in dense material because the “splat” boundaries/cracks and pores obscure the impression measurements. The H values of each layer with 204 C-NS, composite of 204 C-NS/204 NS, and 204 NS were determined to be 3.2 ± 0.1 (mean ± standard deviation), 4.0 ± 0.2, and 4.4 ± 0.2 GPa, respectively, while the boundary regions, such as the
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Fig. 2. Highly magnified microstructures of layered TBCs: (A) TBC-3 and (B) TBC-4. Each figure on the left side indicates upper, middle, base, and interface of top and bond coats, respectively.
interface of 204 C-NS and 204 C-NS/204 NS and that of 204 C-NS/204 NS and 204 NS, showed intermediate values of 3.9 ± 0.2 and 4.1 ± 0.4 GPa, respectively. Even though the H value of the 204 C-NS layer was slightly lower than the reported nominal value of 3.9 GPa [21], the H values gradually increased on moving to the dense layer with 204 NS. The results were in good agreement with the microstructures shown in Figs. 1 and 2. The T values for each region of the layered TBCs, such as the porous, intermediate, and dense microstructures, were determined to be 0.3 ± 0.1, 0.5 ± 0.2, and 1.1 ± 0.3 MPa m0.5, respectively. The T values obtained in this study, except for the 204 C-NS layer, were higher than that of the top coat prepared with commercial powder, reported as a nominal value of 0.3 MPa m0.5 [21]. The post-
indentation micrographs at a peak load of P = 50 N are shown in Fig. 4, as evidence of toughness variation. Indentation was conducted on the sectional surface. The cracks were well developed and propagated in the case of 204 C-NS. The T value on the section is particularly important because the primary location of delamination failure in APS-TBCs is at the interface of the top and bond coats. Because the TBC failure prepared by APS is largely controlled by local or short-crack toughness, the indentation toughness is the most relevant toughness in the context of TBC failure, although the ultimate delamination failure of APS-TBCs is a long-crack phenomenon [21– 23]. In all samples tested, the toughness values of the surfaces were higher than those of the cross-sectionals. This was caused by the deposited microstructure, including horizontal defects such as splat
Table 3 Microstructural analysis of TBCs with single and layered structures in the top coat. Microstructural parameters
Total porosity from image analyzer (%) Porosity fraction of horizontal cracks (%) Porosity fraction of vertical cracks (%) Porosity fraction of pores (%) Length of horizontal cracks (μm) Length of vertical cracks (μm) Mean size of pores (μm) a
Layered structurea
Single structure TBC-1 (204 C-NS)
TBC-2 (204 NS)
204 NS
204 C-NS/204 NS
204 C-NS
19.6 1.4 1.1 17.1 24.7 (3.6–89.5) 13.2 (1.2–53.9) 21.7
10.2 2.2 1.5 6.5 23.5 (5.0–90.8) 18.3 (3.6–56.8) 15.4
8.2 2.0 1.6 4.6 14.5 (3.0–58.4) 11.4 (2.4–32.0) 9.3
12.2 0.8 0.8 10.6 17.5 (3.6–71.8) 8.0 (2.8–23.6) 15.9
18.9 0.7 0.6 17.6 20.6 (7.8–42.2) 13.3 (3.6–42.8) 26.0
Mean values of each corresponding layer in TBC-3 and TBC-4.
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Fig. 3. Hardness and toughness values as a function of the indentation position for layered TBCs. Indentation was conducted on the sectional surface. The sites indented at P = 10 N are indicated at (A)–(C) marked on the plot in the upper panel: (A) 204 C-NS, (B) 204 C-NS/204 NS, and (C) 204 NS.
boundaries/cracks. Crack formation and propagation behaved anisotropically because of the intrinsic microstructure of APS coatings, and the cracks propagated easily through the splat boundaries [24]. The adhesive strength values and the fracture morphologies of each TBC that were carried out are shown in Fig. 5. TBC-2 showed the highest strength value (14.1 ± 0.4 MPa) because of the relatively dense microstructure, compared with those of TBC-1 and commercial TBC prepared using the 9 MB system (7.9 ± 0.6 and 7.9 ± 1.0 MPa, respectively). The adhesive strength values of TBC-3 and TBC-4 were determined to be 7.4 ± 0.1 and 10.5 ± 0.5 MPa, respectively. The values of the bond coats prepared using the TriplexPro-200 were found to be 73.9 ± 0.2 MPa for the single structure (TBC-1 and TBC-2) and to be 84.3 ± 1.1 MPa for the layered structure (TBC-3 and TBC-4), which is the intermediate and similar values, respectively, compared with those of the commercial APS system (nominal value of 66 MPa) and the HVOF system (nominal value of 83 MPa). The adhesive strength values of the bond and top coats were strongly dependent on microstructure, and the fracture origins were the relatively porous layer for the top coat and the interface of bond coat and substrate for the bond coat. 3.3. Indentation stress-strain behavior The indentation stress–strain curves by Hertzian indentations are shown in Fig. 6 for the prepared TBCs. The curves showed a shift away to higher stress for TBC-2, while strain tolerance was seen for TBC-1. When the bond coat of TBC-1 and TBC-2 was changed with 9625, the curves moved to higher stress regions, indicating that the TBC system with 9625 is less strain tolerance. Therefore, the layered bond coat system, which consists of plasma-sprayed 461 NS, mixed powder (461 NS and 9625) and 9625 layers, was tried for minimizing thermal stresses and providing oxidation resistance, based on the stress–strain behavior and previous report [20]. The layered bond coat system
possesses lower interfacial stresses, better strain isolation, and oxidation resistance in TBC system, which will lead to improved coating performance and durability. The stress–strain curves for the layered TBCs (TBC-3 and TBC-4) were located between the two TBCs with the single structure, and not affected much by a change in the microstructure, showing a slightly lower curve in the case of TBC-3. The difference in the curves of TBC-3 and TBC-4 is due to the microstructural design—in TBC-3, the relatively porous base layer of 204 C-NS reacts to the compressive stress, while, in TBC-4, the compressive stress applied to the upper layer is constrained by the relatively harder base layer. These results indicate that feedstock control is essential to the microstructure design, and thus indentation behavior. Other evidence for the effect of the microstructure design of top coat on the damage resistance could be observed in the Vickers indentation tests, as shown in Fig. 7. As the microsize pores or cracks reduced in the microstructure of top surface, the indentation size was smaller. In the subsurface observations, the damage depth and zone were narrower and smaller for TBC-2 (Fig. 7(B)), respectively. The contact damage for the layered TBC-3 corresponded well to the indentation stress–strain curves already mentioned in Fig. 6, showing a little dip and bigger damages compared with TBC-2, but narrower and smaller ones rather than TBC-1. The damage diameters indicated by arrows between surface and subsurface micrographs were 424, 206, and 211 μm for TBC-1, TBC-2, and TBC-3, respectively. The damage modes in TBC-3 followed the H values of each layer and showed a gradual transition on passing from the upper to the base layers. The upper layer with a relatively dense microstructure could work as a protection layer against impacts. However, the base layer with a relatively porous microstructure enhances any damage in the top coat, showing a bigger damage zone. In TBC-4, the harder base layer could protect the bond coat and the substrate from any contact environment. However, the dense microstructure at the interface
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Fig. 6. Indentation stress–strain curves for the TBC systems with single and layered structures. The lines are empirical data fits. Dot lines indicate the TBC-1 and TBC-2 with single structure of 9625 in bond coat.
4. Conclusions
Fig. 4. Micrographs after measuring toughness at a peak load of P = 50 N: (A) 204 C-NS and (B) TBC 204 NS. Indentation was conducted on the sectional surface.
between the top and bond coats leads to a higher degree of stiffness, resulting in low thermomechanical stability because of a high residual stress and a low strain tolerance. Therefore, TBC-3 with a relatively porous microstructure on the base layer in the layered TBCs will be advantageous from the viewpoint of stress concentration and wear resistance, compared with TBC-4 and both TBCs of TBC-1 and TBC-2 with the single structure.
The microstructure of TBCs was well controlled by changing the feedstock powder in the specialized coating system, TriplexPro-200 system, showing the dense, intermediate, and porous microstructures in the top coat. The porosities in the top coats of the TBCs with the single structure were 19.6% and 10.2% for 204 C-NS and 204 NS, respectively, whereas those with the layered structure changed from 8.2% to 18.9%, indicating that the porosity could be tailored from about 8% to 19% in the coating system. The hardness and toughness values gradually increased on moving to the dense layer of 204 NS. The adhesive strength values were determined to be 7.9 and 14.1 MPa for TBCs with the single structure of 204 C-NS and 204 NS, respectively, and 7.4 and 10.5 MPa in the TBCs with the layered structure of 204 CNS and 204 NS in the base layer, respectively, showing a microstructural effect. The values for the bond coats with the single and layered structures prepared using the TriplexPro-200 were 73.9 and 84.3 MPa, respectively, which is the intermediate and similar values, compared with those prepared using the commercial APS and HVOF systems. The indentation stress–strain curves were more affected by the microstructure of the top coat and the composition of the bond coat in the TBCs with the single structure, whereas the curves of the layered TBCs were located between TBCs with the single structure, indicating the minor effect of the microstructure of the top coat. This evidence allowed us to control the mechanical properties of the TBC and to propose an efficient coating in protecting the substrate from mechanical environments using the specialized coating system. Acknowledgements This work was supported by the Power Generation & Electricity Delivery (R-2007-1-003-02 and 2009T100200025) and Human Resources Development (2007-P-EP-HM-E-02-0000) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grants funded by the Korea Government Ministry of Knowledge Economy. References
Fig. 5. Adhesive strength values of the TBC systems with single and layered structures: (A) TBC-1, (B) TBC-2, (C) TBC-3, and (D) TBC-4.
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Fig. 7. Top and sectional micrographs after Vickers indentation tests at a peak load of P = 100 N. The surface and subsurface damages are shown in (A) TBC-1, (B) TBC-2, and (C) TBC-3. Damage diameters are indicated by arrows between surface and subsurface micrographs.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Microstructure design and mechanical properties of thermal barrier coatings with layered top and bond coats Sang-Won Myoung a, Jae-Hyun Kim a, Woo-Ram Lee a, Yeon-Gil Jung a,⁎, Kee-Sung Lee b, Ungyu Paik c,⁎ a b c
School of Nano and Advanced Materials Engineering, Changwon National University, #9 Sarim-dong, Changwon, Kyungnam 641-773, Republic of Korea School of Mechanical Engineering, Kookmin University, 861-1 Chongnungdong, Songbukgu, Seoul 136-702, Republic of Korea Department of Energy Engineering, Hanyang University, #17 Haengdang-dong, Sungdon-gu, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Available online 20 August 2010 Keywords: Thermal barrier coating Microstructure Mechanical property Layered structure Indentation
a b s t r a c t The microstructure of layered thermal barrier coatings (TBCs) with three coating layers in the bond and top coats, respectively, prepared using a specialized coating system (TriplexPro™-200), was controlled and its mechanical properties were investigated, which were then compared with the common TBCs with a single layer in each coat. The bond and top coats were coated with 100 and 200 μm for each feedstock, resulting in 300 and 600 μm thicknesses in the bond and top coats, respectively. The microstructure of the top coat could be controlled by changing the feedstock and using a multiple hopper system—dense/intermediate/porous layers from surface to interface or reverse microstructure. In the case of the bond coat, a compositional gradient was achieved. The adhesive strength values of the top coats were strongly dependent on the microstructure, whereas the values for the bond coat were similar. The hardness and toughness values gradually changed from surface to interface, indicating that the mechanical properties corresponded well with the microstructure of the TBCs. The indentation stress–strain curves of both TBCs with the layered structure were located between the curves for TBCs with the single structure of relatively dense and porous microstructures. Damage on the surface and subsurface was strongly affected by the microstructure of the top coat, showing a similar trend with the stress–strain behavior. This evidence allowed us to propose an efficient coating in protecting the substrate from mechanical environments. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The microstructure of thermal barrier coating (TBC) systems is determined by the feedstock powder as well as the spraying conditions. In particular, the size, shape, and density of the powder are critical to the microstructure and are the most important parameters for the quality of TBCs [1–3]. Thermal properties and failure mechanisms of TBCs are also closely related to their microstructure [5–7]. Therefore, the control of microstructure in TBCs including a bond coat is proposed as a new strategy for advanced coatings, using a specialized coating system (TriplexPro™-200) [8–10]. In this coating system, a higher particle velocity and temperature can be produced, compared with conventional APS system [8,10]. The particle velocity in the TriplexPro™-200 can be produced up to 560 m/s, which is nearly HVOF quality with the particle velocity of about 700 m/s. For this reason, TBC by the TriplexPro™-200 has coarse grain sections. In
⁎ Corresponding authors. Jung is to be contacted at Tel.: + 82 55 213 3712; fax: + 82 55 262 6486. Paik, Tel.: + 82 2 2220 0508; fax: + 82 2 2281 0502. E-mail addresses: [email protected] (Y.-G. Jung), [email protected] (U. Paik). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.063
addition, it is reported that the TriplexPro™-200 shows a significant improvement in powder feeding rate and deposition efficiency, compared with conventional APS system. Powder feeding rates up to 150 g/min are possible in the TriplexPro™-200 using three powder feeders (hoppers) and three plasma arcs. Therefore, the different processing parameter of the TriplexPro™-200, compared with the conventional APS system, may affect the thermal durability of TBC, and microstructural design through the control of coating parameter should be achieved to enhance the thermal durability. Numerous factors, besides the thermomechanical properties, have to be considered in practical applications of TBCs, such as erosion and wear resistance. There is, therefore, a need to improve the adhesive strength, damage resistance, and mechanical characteristics, which are essential to improving the reliability and lifetime performance of the APS–TBC system. In the present study, to improve the mechanical properties and to enhance the damage resistance, the microstructures in both the top and bond coats were designed using the specialized coating system. The effects of the microstructure design on the mechanical properties and damage resistance were investigated in the newly developed TBCs showing a layered structure (hereinafter layered TBCs). These results were compared with the common TBC system of a single structure in each coat.
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2. Experimental procedure
Table 2 Coating parameters for preparing bond and top coats.
2.1. Preparation of TBCs A nickel-based alloy (Nimonic 263, a nominal composition of Ni– 20Cr–20Co–5.9Mo–0.5Al–2.1Ti–0.4Mn–0.3Si–0.06C, in wt.%, ThyssenKrupp VDM, Germany) was used as a substrate. Two types of feedstock powder with different compositions and particle size distributions were used to coat a bond coat on the substrate—METCO 46 1NS (hereinafter 461 NS; nominal composition of Ni–17.5Cr–5.5Al– 2.5Co–0.5Y2O3 in wt.% and particle size of 22–150 μm, Sultzer Metco, Switzerland) and AMDRY 9625 (hereinafter 9625; nominal composition of Ni–22Cr–10Al–1.0Y2O3 in wt.% and particle size of 45–75 μm, Sultzer Metco, Switzerland) [11]. Two sets of bond coat were air plasma-sprayed with a total thickness of d = 300 ± 30 μm, as shown in Table 1. The bond coats of TBC-1 and TBC-2 were prepared with 461 NS and 9625, and while those of TBC-3 and TBC-4 consisted of three layers with a thickness of d = 100 ± 10 μm, respectively—upper layer of 461 NS, middle layer of composite with 9625 and 461 NS (mixing ratio of 50:50 in vol.%), and base layer of 9625. To prepare the bond coats of TBC-3 and TBC-4, a multiple hopper system was used. First, the base layer was plasma-sprayed onto the substrate with d = 100 ± 10 μm. Second, the feeding rate of 9625 for the middle layer was reduced to half and the feed of 461 NS was started with a half amount, again making d = 100 ± 10 μm. Third, the feed of 9625 was stopped and the feed of 461 NS was increased to the full amount. Four sets of top coat with a thickness of d = 600–650 μm were deposited onto the bond coat using two types of feedstock powder with different particle size distributions—METCO 204 C-NS (hereinafter 204 C-NS; nominal composition of ZrO2–8.0Y2O3 in wt.% and particle size of 45–125 μm, Sultzer Metco, Switzerland), METCO 204 NS (hereinafter 204 NS; Sultzer Metco, Switzerland) [11]. The top coats were also designed in the same sequence with the bond coat, as shown in Table 1. Detailed deposition and spray parameters, carried out using a specialized gun (TriplexPro™-200, Sulzer Metco Holding AG, Switzerland), are shown in Table 2 with modifications to the manufacturer's specifications. 2.2. Characterization The TBC samples with the single and layered structures were sectioned and polished to observe the cross-sectional microstructure, including the thickness of each TBC prepared, using a scanning electron microscope (SEM, JEOL Model JSM-5610, Japan). The preparation of the sample followed the specimens' preparing procedure [12]. Two-dimensional image analysis was conducted on the sectional planes, using Image-Pro Plus software (Media Cybernetics, USA), and the microstructures of each TBC were classified by the horizontal, vertical, and spherical defects [4]. The hardness, H, and toughness, T, values were determined using a microindenter (HM114, Mitutoyo Corp., Japan) for loads of 10 and 50 N, respectively, with a Vickers tip. The size of the hardness impression and the crack length produced were measured using SEM. Each value of H and T was determined from equations proposed by Lawn [13]. The adhesive strength of each as-prepared TBC was measured according to the
Parameters
Bond coat
Top coat
Feedstock species Feed rate (g/min) Powder carrier gas (ℓ/min) Gun to work distance (mm) Current/voltage (A/V) Gun moving speed (mm/s) Primary and secondary gas (Ar/He) Step distance (mm)
461 NS and 9625 90 3.5 180 450/90 500 30/20 8
204 C-NS and 204 NS 100 3 150 540/99 500 45/5 5
ASTM standard [14]. The indentation stress–strain behavior of each TBC was investigated by indentations on the top surfaces with tungsten carbide (WC) spherical indenters with a radius of r = 2.32 mm (J & L Industrial Supply Co., MI, USA), using an Instron testing machine (Model 5566, Instron Corp., Norwood, MA, USA) [15– 17]. The values of H and E for the WC indenter used in this study were 19 and 614 GPa, respectively [16]. For examination of the subsurface damage dependence on microstructure, the surfaces of each TBC were indented at a load of P = 100 N, using the microindenter with a Vickers tip, and then they were polished down to the center of the damage site and finished off using 1 μm diamond paste. For both indentation tests, the top surface was slightly polished with 1 μm diamond paste. All experiments were performed in air at room temperature. 3. Results and discussion 3.1. Microstructure Cross-sectional micrographs of each TBC at low magnification are shown in Fig. 1, which were designed with different feedstock powders in both the top and bond coats, as indicated in Table 1. The microstructure of the top coat prepared with 204 C-NS (Fig. 1(A)) was relatively porous, compared with that with 204 NS (Fig. 1(B)), because of the particle size distribution of the feedstock. The layered top coats were well deposited, with the designed concept indicated in Table 1, showing the dense, intermediate, and porous microstructures from upper to base in Fig. 1(C) and the reverse trend in Fig. 1(D). More detailed microstructures of TBCs with the layered structure are shown in Fig. 2. The dense layer with 204 NS (upper layer in Fig. 2(A) and base layer in Fig. 2(B)) contained smaller and relatively uniform “splat” boundaries/cracks, whereas the porous layers with 204 C-NS (base layer in Fig. 2(A) and upper layer in Fig. 2(B)) indicated thicker “splat” boundaries/cracks, global pores, and unmelted particles. By moving to the porous layer, relatively large defects such as global pores and unmelted particles were increased. Even though it was difficult to distinguish the boundary between each layer, the microstructural evolution could be defined by highly magnified microstructures, shown in the left side of Fig. 2. The interface between the bond and top coats showed a sound condition without any cracking or delamination.
Table 1 Feedstock powders and coating conditions for TBC systems. TBC-1
TBC-2
TBC-3
TBC-4
Coating thickness
Top coat
204 C-NS
204 NS
461 NS, 9625
461 NS, 9625
204 C-NS: upper 204 C-NS/204 NS: middle 204 NS: base 461 NS: upper 461 NS/9625: middle 9625: base
600 μm (each coat: 200 μm)
Bond coat
204 NS: upper 204 NS/204 C-NS: middle 204 C-NS: base 461 NS: upper 461 NS/9625: middle 9625: base
300 μm (each coat: 100 μm)
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Fig. 1. Cross-sectional microstructures of TBCs with single and layered structures: (A) TBC-1, (B) TBC-2, (C) TBC-3, and (D) TBC-4.
The microstructural analysis revealed different porosities with the feedstock powder used, as shown in Table 3. The pores constituted a large portion of the total porosity obtained in the image analysis, especially in the case of the top coat prepared with 204 C-NS (TBC-1). The porosities in the top coats of the single structure (TBC-1 and TBC-2) were 19.6% and 10.2%, respectively, and those of the layered structure (TBC-3 and TBC-4) changed from 8.2% to 18.9%. In the layered TBCs, the pore fraction in the total porosity and the pore size were proportional to the change in the porosity. The horizontal and vertical defects (boundary or cracks) did not extend to 100 μm, indicating that the horizontal defects were longer than the vertical defects in all samples observed. Usually, the commercial APS coating shows a porosity of 15– 25% [5,6]. It is important that the porosity can be tailored from 8% to 19% by changing the feedstock in the specialized coating system. The microstructure of commercial APS coatings has been studied extensively and the failure mechanism has been well documented in the literature, including those for graded coatings [18–20]. The coating reliability can be greatly improved through the development of an appropriate bond coat, which can provide strain isolation for the ceramic coating and protect the substrate from oxidation. Thus, the stress arising from thermal expansion mismatch can be minimized and interfacial strength can be maintained. The strain isolation
provided by a layered bond coat is especially beneficial for coating edges that exist due to component geometry, or due to throughthickness cracking resulting from ceramic sintering and creep at high temperatures. Even though the graded coatings could provide interface stability in the APS coatings, the preparation cost limits the application. However, in this work, the specialized coating system using a multihopper could prepare the layered or semigraded coatings by changing the feedstock powder, showing continuous changes in microstructure and composition in the top and bond coats, respectively. The microstructure prepared could satisfy the continuously increasing functional and structural demands on TBCs. 3.2. Mechanical properties The H and T values measured on sectional planes for each region of the layered TBCs and post-indentation micrographs are shown in Fig. 3. The H values in the TBCs are not as easily and clearly defined as those in dense material because the “splat” boundaries/cracks and pores obscure the impression measurements. The H values of each layer with 204 C-NS, composite of 204 C-NS/204 NS, and 204 NS were determined to be 3.2 ± 0.1 (mean ± standard deviation), 4.0 ± 0.2, and 4.4 ± 0.2 GPa, respectively, while the boundary regions, such as the
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Fig. 2. Highly magnified microstructures of layered TBCs: (A) TBC-3 and (B) TBC-4. Each figure on the left side indicates upper, middle, base, and interface of top and bond coats, respectively.
interface of 204 C-NS and 204 C-NS/204 NS and that of 204 C-NS/204 NS and 204 NS, showed intermediate values of 3.9 ± 0.2 and 4.1 ± 0.4 GPa, respectively. Even though the H value of the 204 C-NS layer was slightly lower than the reported nominal value of 3.9 GPa [21], the H values gradually increased on moving to the dense layer with 204 NS. The results were in good agreement with the microstructures shown in Figs. 1 and 2. The T values for each region of the layered TBCs, such as the porous, intermediate, and dense microstructures, were determined to be 0.3 ± 0.1, 0.5 ± 0.2, and 1.1 ± 0.3 MPa m0.5, respectively. The T values obtained in this study, except for the 204 C-NS layer, were higher than that of the top coat prepared with commercial powder, reported as a nominal value of 0.3 MPa m0.5 [21]. The post-
indentation micrographs at a peak load of P = 50 N are shown in Fig. 4, as evidence of toughness variation. Indentation was conducted on the sectional surface. The cracks were well developed and propagated in the case of 204 C-NS. The T value on the section is particularly important because the primary location of delamination failure in APS-TBCs is at the interface of the top and bond coats. Because the TBC failure prepared by APS is largely controlled by local or short-crack toughness, the indentation toughness is the most relevant toughness in the context of TBC failure, although the ultimate delamination failure of APS-TBCs is a long-crack phenomenon [21– 23]. In all samples tested, the toughness values of the surfaces were higher than those of the cross-sectionals. This was caused by the deposited microstructure, including horizontal defects such as splat
Table 3 Microstructural analysis of TBCs with single and layered structures in the top coat. Microstructural parameters
Total porosity from image analyzer (%) Porosity fraction of horizontal cracks (%) Porosity fraction of vertical cracks (%) Porosity fraction of pores (%) Length of horizontal cracks (μm) Length of vertical cracks (μm) Mean size of pores (μm) a
Layered structurea
Single structure TBC-1 (204 C-NS)
TBC-2 (204 NS)
204 NS
204 C-NS/204 NS
204 C-NS
19.6 1.4 1.1 17.1 24.7 (3.6–89.5) 13.2 (1.2–53.9) 21.7
10.2 2.2 1.5 6.5 23.5 (5.0–90.8) 18.3 (3.6–56.8) 15.4
8.2 2.0 1.6 4.6 14.5 (3.0–58.4) 11.4 (2.4–32.0) 9.3
12.2 0.8 0.8 10.6 17.5 (3.6–71.8) 8.0 (2.8–23.6) 15.9
18.9 0.7 0.6 17.6 20.6 (7.8–42.2) 13.3 (3.6–42.8) 26.0
Mean values of each corresponding layer in TBC-3 and TBC-4.
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Fig. 3. Hardness and toughness values as a function of the indentation position for layered TBCs. Indentation was conducted on the sectional surface. The sites indented at P = 10 N are indicated at (A)–(C) marked on the plot in the upper panel: (A) 204 C-NS, (B) 204 C-NS/204 NS, and (C) 204 NS.
boundaries/cracks. Crack formation and propagation behaved anisotropically because of the intrinsic microstructure of APS coatings, and the cracks propagated easily through the splat boundaries [24]. The adhesive strength values and the fracture morphologies of each TBC that were carried out are shown in Fig. 5. TBC-2 showed the highest strength value (14.1 ± 0.4 MPa) because of the relatively dense microstructure, compared with those of TBC-1 and commercial TBC prepared using the 9 MB system (7.9 ± 0.6 and 7.9 ± 1.0 MPa, respectively). The adhesive strength values of TBC-3 and TBC-4 were determined to be 7.4 ± 0.1 and 10.5 ± 0.5 MPa, respectively. The values of the bond coats prepared using the TriplexPro-200 were found to be 73.9 ± 0.2 MPa for the single structure (TBC-1 and TBC-2) and to be 84.3 ± 1.1 MPa for the layered structure (TBC-3 and TBC-4), which is the intermediate and similar values, respectively, compared with those of the commercial APS system (nominal value of 66 MPa) and the HVOF system (nominal value of 83 MPa). The adhesive strength values of the bond and top coats were strongly dependent on microstructure, and the fracture origins were the relatively porous layer for the top coat and the interface of bond coat and substrate for the bond coat. 3.3. Indentation stress-strain behavior The indentation stress–strain curves by Hertzian indentations are shown in Fig. 6 for the prepared TBCs. The curves showed a shift away to higher stress for TBC-2, while strain tolerance was seen for TBC-1. When the bond coat of TBC-1 and TBC-2 was changed with 9625, the curves moved to higher stress regions, indicating that the TBC system with 9625 is less strain tolerance. Therefore, the layered bond coat system, which consists of plasma-sprayed 461 NS, mixed powder (461 NS and 9625) and 9625 layers, was tried for minimizing thermal stresses and providing oxidation resistance, based on the stress–strain behavior and previous report [20]. The layered bond coat system
possesses lower interfacial stresses, better strain isolation, and oxidation resistance in TBC system, which will lead to improved coating performance and durability. The stress–strain curves for the layered TBCs (TBC-3 and TBC-4) were located between the two TBCs with the single structure, and not affected much by a change in the microstructure, showing a slightly lower curve in the case of TBC-3. The difference in the curves of TBC-3 and TBC-4 is due to the microstructural design—in TBC-3, the relatively porous base layer of 204 C-NS reacts to the compressive stress, while, in TBC-4, the compressive stress applied to the upper layer is constrained by the relatively harder base layer. These results indicate that feedstock control is essential to the microstructure design, and thus indentation behavior. Other evidence for the effect of the microstructure design of top coat on the damage resistance could be observed in the Vickers indentation tests, as shown in Fig. 7. As the microsize pores or cracks reduced in the microstructure of top surface, the indentation size was smaller. In the subsurface observations, the damage depth and zone were narrower and smaller for TBC-2 (Fig. 7(B)), respectively. The contact damage for the layered TBC-3 corresponded well to the indentation stress–strain curves already mentioned in Fig. 6, showing a little dip and bigger damages compared with TBC-2, but narrower and smaller ones rather than TBC-1. The damage diameters indicated by arrows between surface and subsurface micrographs were 424, 206, and 211 μm for TBC-1, TBC-2, and TBC-3, respectively. The damage modes in TBC-3 followed the H values of each layer and showed a gradual transition on passing from the upper to the base layers. The upper layer with a relatively dense microstructure could work as a protection layer against impacts. However, the base layer with a relatively porous microstructure enhances any damage in the top coat, showing a bigger damage zone. In TBC-4, the harder base layer could protect the bond coat and the substrate from any contact environment. However, the dense microstructure at the interface
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Fig. 6. Indentation stress–strain curves for the TBC systems with single and layered structures. The lines are empirical data fits. Dot lines indicate the TBC-1 and TBC-2 with single structure of 9625 in bond coat.
4. Conclusions
Fig. 4. Micrographs after measuring toughness at a peak load of P = 50 N: (A) 204 C-NS and (B) TBC 204 NS. Indentation was conducted on the sectional surface.
between the top and bond coats leads to a higher degree of stiffness, resulting in low thermomechanical stability because of a high residual stress and a low strain tolerance. Therefore, TBC-3 with a relatively porous microstructure on the base layer in the layered TBCs will be advantageous from the viewpoint of stress concentration and wear resistance, compared with TBC-4 and both TBCs of TBC-1 and TBC-2 with the single structure.
The microstructure of TBCs was well controlled by changing the feedstock powder in the specialized coating system, TriplexPro-200 system, showing the dense, intermediate, and porous microstructures in the top coat. The porosities in the top coats of the TBCs with the single structure were 19.6% and 10.2% for 204 C-NS and 204 NS, respectively, whereas those with the layered structure changed from 8.2% to 18.9%, indicating that the porosity could be tailored from about 8% to 19% in the coating system. The hardness and toughness values gradually increased on moving to the dense layer of 204 NS. The adhesive strength values were determined to be 7.9 and 14.1 MPa for TBCs with the single structure of 204 C-NS and 204 NS, respectively, and 7.4 and 10.5 MPa in the TBCs with the layered structure of 204 CNS and 204 NS in the base layer, respectively, showing a microstructural effect. The values for the bond coats with the single and layered structures prepared using the TriplexPro-200 were 73.9 and 84.3 MPa, respectively, which is the intermediate and similar values, compared with those prepared using the commercial APS and HVOF systems. The indentation stress–strain curves were more affected by the microstructure of the top coat and the composition of the bond coat in the TBCs with the single structure, whereas the curves of the layered TBCs were located between TBCs with the single structure, indicating the minor effect of the microstructure of the top coat. This evidence allowed us to control the mechanical properties of the TBC and to propose an efficient coating in protecting the substrate from mechanical environments using the specialized coating system. Acknowledgements This work was supported by the Power Generation & Electricity Delivery (R-2007-1-003-02 and 2009T100200025) and Human Resources Development (2007-P-EP-HM-E-02-0000) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grants funded by the Korea Government Ministry of Knowledge Economy. References
Fig. 5. Adhesive strength values of the TBC systems with single and layered structures: (A) TBC-1, (B) TBC-2, (C) TBC-3, and (D) TBC-4.
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Fig. 7. Top and sectional micrographs after Vickers indentation tests at a peak load of P = 100 N. The surface and subsurface damages are shown in (A) TBC-1, (B) TBC-2, and (C) TBC-3. Damage diameters are indicated by arrows between surface and subsurface micrographs.
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