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Effects of substrate material and TBC structure on the cyclic oxidation resistance of TBC systems
Surface & Coatings Technology 258 (2014) 49–61
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Effects of substrate material and TBC structure on the cyclic oxidation resistance of TBC systems Xiaolong Li a, Xiao Huang a,⁎, Qi Yang b, Zhaolin Tang c a b c
Carleton University, Department of Mechanical and Aerospace, 1125 Colonel By Dr, Ottawa, Ontario K1S 5B6, Canada Aerospace Portfolio, National Research Council, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada Northwest Mettech, 467 Mountain Hwy, North Vancouver, British Columbia V7J 2L3, Canada
a r t i c l e
i n f o
Article history: Received 19 May 2014 Accepted in revised form 3 October 2014 Available online 13 October 2014 Keywords: Plasma spray Thermal barrier coatings Yttria-stabilized zirconia (YSZ) Cyclic oxidation IN738LC CMSX-4
a b s t r a c t The effects of different substrates (CMSX-4 and IN738LC) on cyclic oxidation behavior of two thermal barrier coatings (TBCs) were investigated in this study. Cyclic oxidation test was conducted at a peak temperature of 1080 °C with 30 min hold time. The TBC systems tested consisted of two nickel-base superalloy substrates (CMSX-4 and IN738LC), a platinum aluminide bond coat and two types of 8YSZ top coat (vertical cracks (VC) and columnar structure). Samples with CMSX-4 substrate showed greater cyclic oxidation lifetimes than that with IN738LC, and VC YSZ on CMSX-4 offered longer cyclic life than those with columnar structure. After the oxidation test, the samples were analyzed using a scanning electron microscope (SEM) and an X-Ray diffractometer. The failure location of all TBC systems examined was found to be in the vicinity of TGO, consistent with past research of similar coating systems. Alumina and spinels (CS) were detected on all spalled surfaces although the amount of each phase differs from sample to sample. Ta-rich oxide was found on spalled coating surfaces with CMSX-4 substrate while islands of YSZ on the exposed TGO surface of samples with IN738LC substrate. It is believed that alloy composition, TGO rumpling and stresses induced by TGO growth and coefficient of thermal expansion (CTE) mismatch between different layers all contributed to the reduced cycles to failure of TBC systems with IN738LC substrate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Modern gas turbines operate at high temperature and constantly increasing of inlet temperature is required to improve turbine efficiency [1,2]. The hot sections of gas turbines are subjected to not only high temperature [3], but also severe oxidizing and corrosive environments [4]. Therefore, the efficiency of gas turbine engine is highly depended on the thermal and mechanical properties of materials used for the hot section components such as turbine blades [5]. Nickel-base superalloys are widely used for gas turbine blades due to their high temperature resistance and high strength at elevated temperature. These superalloys can work for thousands of hours even at a high temperature of 1100 °C [6]. With the application of thermal barrier coatings (TBCs) on gas turbine blades and vanes, metal surface temperature can be reduced by 100 °C to 300 °C; the increased temperature capability of the coated components has led to a significant increase in operating temperature, hence turbine efficiency [7,8]. State-of-the-art TBC system usually consists of a metallic bond coat and an yttria-stabilized zirconia (YSZ) ceramic top coat. The low thermal conductivities of YSZ and the superior oxidation/corrosion resistance of ⁎ Corresponding author at: Mechanical and Aerospace Engineering Carleton University 1125 Colonel By Drive Ottawa, ON K1S 5B6.
http://dx.doi.org/10.1016/j.surfcoat.2014.10.004 0257-8972/© 2014 Elsevier B.V. All rights reserved.
metallic bond coat are beneficial in protecting the underlying superalloy substrate from the environmental degradation. There are two typical types of bond coats: traditional MCrAlY (M = Ni or/and Co) overlay coatings and platinum-modified aluminide ((Pt,Ni)Al) diffusion coatings. (Pt, Ni)Al coatings exhibit better protection against high temperature oxidation damage than MCrAlY coatings due to their ability to form a slowgrowing alumina scale [9]. The addition of platinum to the aluminide coating improves the oxidation resistance by promoting selective oxidation of aluminum and enhances coating stability and scale adhesion in return [10]. The YSZ ceramic top coats are usually deposited by air plasma spraying (APS) method or electron beam physical vapor deposition (EBPVD) technique. APS method is more commonly used for gas turbine stationary components due to its low cost, high deposition rate and large range of applicable coating compositions [11]. The YSZ coatings deposited by APS exhibit high thermal resistance because of the inherent laminar structure [12]. However, the presence of porosity and micro-cracks and the lack of strain tolerant microstructure features in the traditional APS coatings lead to coating spallation during exposure to high temperature, particularly under cyclic conditions [13]. On the contrary, EB-PVD deposited YSZ coatings have shown superior strain and thermo-shock tolerant behavior due to the columnar microstructures. They also exhibit better resistance to spallation, lower surface roughness, better thickness
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Table 1 Summarized properties of APS, ASPS and EB-PVD coatings [17]. Properties
APS
ASPS
EB-PVD
Bond strength (MPa) Thermal conductivity (W/m·K)
20–40 0.9–1.5
50–82 1–2
65–75 1.7–2
Table 2 Chemical composition of the substrate materials.
IN738LC CMSX-4
Ni
Cr
Co
Mo
W
Ta
Nb
Al
Ti
Re
C
Bal Bal
16 6.5
8.5 9
1.7 0.6
2.6 6
1.7 6.5
0.9 –
3.4 5.6
3.4 1
3
0.11 –
coated with EB-PVD YSZ reached 1340 cycles (at 1150 °C for 1 h followed by 10 min cooling), longer than that observed of same coating system applied to N5 (907 cycles) which has higher Cr, Al and Mo but lower Co, W and Ti than CMSX-4 [20]. Changing the bond coat from Pt-modified aluminide to Pt-diffusion coating (γ + γ′) reversed this trend [20]. Co and Ti content in superalloys was believed to be detrimental to coating performance in general [24,25] while Re was not [21]. On the contrary, IN100, a polycrystalline superalloy with higher Co and Ti than CMSX-4, had longer cyclic life than CMSX-4 with the same coatings applied [23]. In this study, the objectives are to investigate the effects of substrate composition (CMSX-4 and IN738LC) and top coat microstructure (ASPS VC and columnar) on cyclic lifetimes of four TBC systems. 2. Experiments
uniformity and superior adherence to the substrate over those produced by plasma spraying [14]. However, the equipment cost of EB-PVD systems is much higher and the deposition rate is relatively low [15,16]. Axial suspension plasma spray (ASPS), as an emerging technology, has been used to produce durable TBCs with much lower operating and capital costs when being compared to EB-PVD [17]. ASPS coatings have similar properties to EB-PVD coatings, but exhibit much higher bond strength and slightly higher thermal conductivity than APS coatings (Table 1). In suspension plasma spraying fine particles (usually in nm scale) suspended in a liquid carrier are injected into the plasma stream, instead of direct powder injection; in the case of ASPS, the suspension liquid is injected axially, providing more acceleration to the suspension [17]. ASPS deposition process, through control of the process parameters, can produce coatings with columnar structure or dense coatings with vertical cracks (VCs). For example, while columnar structured coatings are deposited with a longer stand-off distance (SOD), dense coatings with vertical cracks for better strain tolerance can be produced with a shorter SOD [18]. Similar to the columnar structure, VC structure reduce the tensile stress buildup in the YSZ top coat upon heating and compressive stress upon cooling [19]. Therefore, VC and columnar coatings usually have longer oxidation life than lamellar APS coatings. The oxidation behaviors of TBCs are also influenced by the use of different substrate materials. The effects of substrate on cyclic oxidation life of a TBC system have been reported by many researchers [20–23]. In a recent study of EB-PVD coating systems with IN 100 and CMSX-4 as substrate materials, the cyclic life tested at 1100 °C (50 min duration) was found to be higher in the system with polycrystalline IN 100 coated with EB-PVE NiCrAlY (overlay) and YSZ [23]. The reduced cyclic life of TBC system with CMSX-4 substrate was attributed to the diffusion of Ta, W and Re to TGO (all were found on the failure surface). However using a Pt-modified diffusion aluminide bond coat, the cyclic life of CMSX-4
2.1. Substrate materials Two substrate materials, CMSX-4 and IN738LC, were used in this study. Their chemical compositions are listed in Table 2. IN738LC, a precipitation strengthened nickel-based superalloy, is widely used for the hot section components of aero-engines and gas turbines [30] due to its microstructure stability, excellent strength and oxidation/corrosion resistance at elevated temperature [29]. The twophase microstructure of as-received IN738LC substrate is shown in Fig. 1(a). CMSX-4 is a second generation nickel-based single crystal superalloy; as shown in Fig. 1(b), like IN738LC it contains cubical intermetallic γ′ (Ni3Al) phase coherently embedded in γ nickel solid solution matrix [30]. CMSX-4 exhibits improved strength and creep resistance at high temperature because of the large volume fraction of γ′ strengthening phase (due to elevated Al), lack of grain boundaries and increased refractory element contents [31]. All of these characteristics make CMSX-4 a widely used material for blade applications in land-base gas turbines [32]. Adding refractory elements tantalum, tungsten and rhenium improves mechanical properties at high temperature; but the Cr content must be reduced in order to maintain microstructure stability during service. The Cr/Al ratio in IN738LC is 4.7 [33], higher than that in CMSX-4 (1.25) [34]. Therefore, CMSX-4 substrate is usually coated with platinum aluminide to protect it against high temperature oxidation [35]. 2.2. Coating processes (Pt,Ni)Al bond was applied by Chromalloy Inc. using standard process involving Pt plating, pack cementation and diffusion heat treatment. The columnar and VC 8YSZ coatings were applied using the Axial III™ plasma torch with a modified injector for suspension
Fig. 1. γ/γ′ structure in (a) IN738LC and (b) CMSX-4.
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
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Fig. 2. ASPS deposition process [17].
atomization and the NanoFeed™ suspension feeder at Northwest Mettech Corp. (North Vancouver, Canada). The Axial III™ injects the atomized suspension feedstock axially to the direction of spray into the central core of the plasma. Axial injection overcomes the injection difficulties that arise when attempting to penetrate the plasma radially with fine particles or droplets. The NanoFeed is designed to feed submicron suspensions using mass flow control of both suspension and atomizing gas to provide uniform atomizing dynamics at the injector. An overview of the ASPS process is shown in Fig. 2. The powder used was customized ZrO2-8wt.%Y2O3 (8YSZ) with submicron size. After suspending the fine powders in the ethanol liquid, the suspension feedstock was fed by Mettech Nanofeed 350 Suspension feeder to the injector with stable flow rate and injection pressure for the axial injection plasma spraying performed by Axial III™ Torch.
Table 3 Sample description and identification. Material Composition IN738LC with PtAl + columnar 8YSZ
Appearance before cyclic oxidation test
The as-coated samples were heat-treated in a vacuum furnace at 1080 °C for 4 h. The reason for employing this treatment was based on our previous work which has found that vacuum heat treatment had the effect of increasing the cyclic oxidation life of TBCs [36]. The specimens are identified as Cyc#1, Cyc#2, Cyc#3 and Cyc#4, as shown in Table 3. Each type consists of two samples: one for observation under as-received condition and another for cyclic oxidation test. The samples were disk-shaped with a diameter of about 19 mm and a thickness of 5 mm. The top YSZ coating had an average thickness of 200 μm. 2.3. Sample preparation and analysis before oxidation One sample from each group was cross-sectioned, mounted and polished. Microscopic evaluation of top surfaces and crosssections of the samples were performed with Philips XL30S FEG scanning electron microscopes (SEMs) equipped with Phoenix energy dispersive X-ray spectroscopy (EDS). The X-ray diffraction (XRD) analysis was performed using Bruker AXS D8 Discover diffractometer.
Sample IDs
2.4. Oxidation tests Cyc#1
IN738LC with PtAl + VC 8YSZ
Cyc#2
CMSX-4 with PtAl + columnar 8YSZ
Cyc#3
The cyclic oxidation test was conducted in air using a CM furnace (Rapid Temp Furnace supplied by Bloom Field, N.J., USA). The duration of each cycle was 60 min and each cycle consists of heating from room temperature to 1080 °C (about 10 min) and holding for 30 min and then fan-cooling at room temperature for 20 min (temperature measurement showed that the samples reached 275–300 °C after cooling). Visual inspections were performed after every 100 cycles as such the reported cyclic lifetime represents the time when failure was observed not the actual cycles to failure. Samples were withdrawn from test once the failure criterion (N 30% spallation of top coat) was reached. 3. Results
CMSX-4 with PtAl + VC 8YSZ
Cyc#4
3.1. Microstructure and composition of as-coated samples The surface morphologies of the pseudo columnar structured (columnar will be used in the text for simplicity) and VC YSZ coatings before oxidation tests are shown in Fig. 3. The columnar YSZ coating
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(a)
(b)
(c)
(d)
Fig. 3. Surface morphologies of the columnar (a and b) and VC coatings (c and d).
has a “broccoli-like” feature with a few long surface cracks and visible surface pores (Fig. 3(a) and (b)) while VC coating has smoother and denser surface but numerous shorter, connected cracks (Fig. 3(c) and (d)). XRD analysis was performed on the top surfaces of samples Cyc#1 to 4 and an example of the results is shown in Fig. 4 since all four samples display the same diffraction peaks. Traces of (Ni, Pt)Al (β phase from the bond coat) and γ′-Ni3Al (strengthening phase in
IN738LC and CMSX 4) phases are present in the samples, in addition to ZrO2, a main phase in YSZ. The cross-sectional microstructures of the columnar and VC TBC are shown in Fig. 5. Similar to that seen on the surface, the columnar coating has a rougher surface with more pores towards the top. And for the VC coating most of the vertical cracks extend to about half of the YSZ total coating thickness. In the original test plan, a conventional plasma sprayed YSZ top coat was also included.
Fig. 4. X-ray diffraction pattern from the top surface before cyclic test (all samples have identical diffraction patterns).
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
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Fig. 5. Cross-sections of columnar structured (a) and VC (b) TBC coatings.
Fig. 6. TBC coatings on IN738 (a) and CMSX-4 (b) substrate.
whereas a three-layer bond coat is observed on CMSX-4 substrate. The chemical compositions of different bond coat layers and substrate (close to and far away from bond coat) were measured by EDS analysis and summarized in Table 4. From the table it is obvious that the bond coat layer one on CMSX-4 contained more Pt and Al, while much
However, the PS YSZ sprayed on PtAl bond coat immediately spalled off after spraying. As such this set of samples could not be included in this study. Morphologies of PtAl bond coat and IN738LC/CMSX-4 substrate are shown in Fig. 6. A two-layered bond coat exits on the IN738LC substrate
Table 4 Measured compositions (by EDS) of different layers in PtAl bond coat (wt.%).
PtAl + IN738LC
PtAl + CMSX-4
wt.%
Pt
Al
Ni
Cr
Co
Ta
Ti
W
Si
O
Layer 1 Layer 2 Area 1 Area 2 Layer 1 Layer 2 Layer 3 Area 1 Area 2
13.25 – – – 24.33 10.12 – – –
17.05 7.24 4.02 2.95 20.79 20.88 12.85 6.27 5.69
55.16 37.11 60.21 61.71 42.04 53.69 45.79 62.70 63.36
6.48 24.34 18.06 16.44 2.63 3.05 8.40 7.07 6.67
6.21 8.26 8.18 9.11 5.35 7.12 9.89 9.85 10.61
0.30 9.57 2.01 2.04 0.68 2.17 8.76 5.84 5.66
1.55 6.03 3.78 3.72 – – 1.21 1.05 –
– 7.45 – 3.90 2.17 2.96 13.09 7.22 6.67
– – – 0.12 – – – – –
– – – – 2.02 – – – –
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Fig. 7. Cyclic oxidation life (in terms of number of cycles before 30% top coat spallation was found).
more Cr and refractory elements Ti, Ta and W were detected in the bond coat layer 1 on IN738LC. The difference in Pt concentrations was resulted due to the separation of bond coat layer into layer 1 (Pt rich) and layer 2 (Pt lean) on the surface of CMSX-4 substrate (Fig. 6(b)). Elements from the base metals, such as Ni, Cr, Co, Ta, Ti and W, were found in layer 2 on IN738LC (Fig. 6(a)) and layer 3 on CMSX-4 (Fig. 6(b)) due to diffusion. The EDS analysis also revealed that there were more Ta and W (less Cr) in layer 3 of CMSX-4 while more Cr and Ti (and some W) were found in bond coat layer 2 on IN738LC, illustrating the influence of base metal composition on coating microstructure. 3.2. Cyclic oxidation test result The cyclic oxidation lives of different samples are represented in Fig. 7. All samples failed within 1000 cycles. Cyc#1 and Cyc#2 (both with IN738LC substrate) spalled after 365 and 300 cycles, respectively; Whereas samples with CMSX-4 substrate, Cyc#3 and Cyc#4, experienced 500 and 1000 cycles, respectively, before failure. Examples of periodical visual inspections are shown in Fig. 8. 3.3. Microstructure analysis of cyclic oxidation tested samples 3.3.1. Samples with PtAl bond coat and columnar YSZ top coat During cyclic oxidation test, sample Cyc#1 (IN738LC with PtAl + columnar YSZ) failed after 365 cycles (this failure was found ahead of the regular inspection interval by coincidence) while sample Cyc#3 (CMSX4 with PtAl + columnar YSZ) was found to exceed the spallation limit
(a)
(b)
after 500 cycles. SEM/EDS analysis of sample Cyc#1 reveals both alumina and NiAl2O4 on the backside of spalled YSZ coating and bond coat surface, as shown in Fig. 9(a)–(c). From the cross sectional view of the microstructure, it is clear that YSZ has completely separated from the TGO and also vertical cracks in YSZ have progressed to the interface between YSZ and TGO (Fig. 9(d) and (e)). The formation of vertical cracks in ceramic layer has been observed in similar systems with EB-PVD top coat and it was attributed to the need to release stored elastic energy induced by TGO growth [37], heating-cooling cycles and possibly TGO rumpling [39]. Sintering which has the effect of increasing elastic modulus accelerates the tendency for vertical cracking. Also to be noted are patches of YSZ (ZrO2 from EDS analysis) on the exposed bond coat surface (Fig. 9(c)); this suggests cracking within YSZ top coat. EDS analysis results measured from sample Cyc#1 are summarized in Table 5. XRD analysis was carried out on both the backside of the spalled YSZ and the top surface of exposed bond coating surface; the diffraction spectra are given in Figs. 10 and 11. These further confirm that spinel (NiAl2O4) and alumina (Al2O3) are attached to the spalled YSZ. EDS analysis on the backside of spalled YSZ of sample Cyc#3 and cross-section suggested that NiAl2O4 and Al2O3 also formed during the cyclic oxidation test (Fig. 12(a), (b) and (c)). Some Ta-rich particles also formed within TGO (Fig. 12(a)). XRD analysis detected the presence of NiAl2O4, NiCr2O4 and trace Cr2O3 on the backside of YSZ and the surface of exposed bond coat (Figs. 13 and 14). There is an increased amount of mixed oxide formation on sample Cyc#3, as compared to Cyc#1, as a result of the longer exposure time than that for Cyc#1 (500 cycles vs. 365 cycles). In general, as the oxidation exposure time increases, Al becomes depleted and chromium and nickel oxide start to form leading to the reactions of NiO + Cr2O3 → NiCr2O4 and NiO + Al2O3 → NiAl2O4. Another microstructural change occurred to Cyc#3 after thermal cycling was the development of vertical cracks (Fig. 12(d)) in the top coat. This also occurred to sample Cyc#1, as noted earlier. With the through vertical cracks developing, oxygen can readily reach the TGO and accelerate Al depletion and the growth of mixed oxide(s). As such, spinels were found on both Cyc#1 and Cyc#3 after top coat spallation. EDS results from sample Cyc#3 are summarized in Table 6.
(c)
Fig. 8. Cyc#4 (CMSX-4 with VC TC) in the (a) as-coated, (b) after 900 cycles and (c) after 1000 cycles.
3.3.2. Samples with PtAl bond coat and VC YSZ top coat During cyclic oxidation test, sample Cyc#2 (IN738LC with PtAl + VC YSZ) failed after 300 cycles while Cyc#4 (CMSX-4 with PtAl + VC YSZ)
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
(a) Backside of spalled YSZ
55
(b) Backside of spalled YSZ
(c) Exposed bond coat surface
(d) Cross section
(e) Cross section
Fig. 9. Microstructures of sample Cyc#1 (IN738LC with PtAl + columnar YSZ) after oxidation test.
completed 1000 cycles before spallation was observed. SEM/EDS analysis of sample Cyc#2 revealed large coverage of Al2O3 on both the backside of spalled YSZ and exposed bond coat surface, whereas a small amount of NiAl2O4 was observed on the backside of spalled YSZ
Table 5 EDS analysis results of Cyc#1. Elements Zr NiAl2O4 (YSZ backside) Al2O3 (YSZ backside) ZrO2 (YSZ backside) NiAl2O4 (Bond coat/substrate surface)
wt.% at.% wt.% at.% wt.% at.% wt.% at.%
– – – – 22.93 6.4 – –
Y
Al
Ni
Co
O
Cr
Ti
– – – – 2.65 0.76 – –
26.01 22.73 55.05 42.07 32.85 31 35.16 34.41
23.98 9.63 – – – – 19.68 8.85
5.65 2.26 – – – – 4.33 1.94
44.36 65.38 44.95 57.93 37.63 59.9 29.63 48.89
– – – – 3.95 1.93 6.04 3.06
– – – – – – 5.17 2.85
(Fig. 15). XRD detected also a very small amount of NiAl2O4 on the backside of YSZ and bond coat/substrate surface, in addition to Al2O3 and YSZ (ZrO2) as shown in Figs. 16 and 17. Similar phases were also observed in Fig. 9(c) where cracking within YSZ top coat was suspected. EDS results of Cyc#2 are summarized in Table 7. For Cyc#4 sample, larger amounts of alumina and spinels were found on both the backside of spalled YSZ and on top of the exposed bond coat (Figs. 18, 19 and 20). In particular, NiAl2O4 was observed on the backside of spalled YSZ, exposed bond coat surface and crosssection (Fig. 18). Although no indication of YSZ on the exposed bond coat surface, some Ta-rich particles formed on top of TGO (Fig. 18(c)). The isolated area of metallic bond coat on the same figure implies that the interfacial bond strength between TGO and bond coat has been weakened after 1000 cycles. The microstructure on the cross-section of Cyc#4, Fig. 18(d), had similar features as that of Cyc#3 with columnar top coat (Fig. 12(c)); yet sample Cyc#4 with VC top coat experienced more cycles prior to failure (1000 cycles vs. 500 cycles for Cyc#3). Although more tests are
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Fig. 10. X-ray diffraction pattern from the backside of spalled YSZ (sample Cyc#1, IN738LC with PtAl + columnar YSZ).
needed to confirm this, the VC seems to display more strain tolerance than the columnar YSZ. EDS analysis results of Cyc#4 are shown in Table 8. 4. Discussion 4.1. Elemental effect In this study, the cyclic oxidation behavior of TBC systems with IN738LC and CMSX-4 substrates was studied. Samples Cyc#1 and Cyc#2, with IN738LC substrate, suffered from early coating spallation, after 365 and 300 cycles, respectively. Samples Cyc#3 and Cyc#4 with CMSX-4 substrate exhibited longer cyclic oxidation lives than the ones with IN738LC substrates, reaching 500 and 1000 cycles, respectively. Comparing the compositions of the two alloys, IN738 has higher Ti and Cr but lower W, Ta and Re than CMSX-4. Also more Ti, Ta and W were found in the as-coated bond coat surface of IN738LC (Table 4). As Ta, Co and Ti in superalloys were believed to be detrimental to
coating performance in general [23–25], the increased Ti content in IN738LC and elevated Ti and Ta in its bond coat may have played a role in reduced cyclic lifetime. Additionally, higher Pt in the bond coat on CMSX-4 was also measured (Table 4). This could also played a role in ensuring better TGO adhesion [38] thus extending the cyclic life of TBC on CMSX-4. However, based on the microstructural analysis carried out in this study on the spalled surfaces, there is no microstructural evidence so far to suggest that the elevated Ti or Ta content in IN738LC or its bond coat played a role in early coating spallation. Instead, Ta-containing particles were observed on failed coatings with CMSX-4 substrate. A simplified approach to relate cyclic oxidation life and coatingsubstrate system is to compare the interfacial fracture toughness Gc to driving force G accumulated during cycling (due to TGO thickening, mismatch in CTE) [21]. As shown in Fig. 21, the substrate composition and continuous thermal exposure (elemental diffusion and phase changes) affect the interfacial strength or fracture toughness Gc. Once G reaches Gc, spallation occurs. Harmful elements (Ti, Ta and others)
Fig. 11. X-ray diffraction pattern from the exposed bond coat surface (sample Cyc#1, IN738LC with PtAl + columnar YSZ).
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
(a) Backside of spalled YSZ
(b) Backside of spalled YSZ
(c) Cross section
(d) Cross section
57
Fig. 12. Microstructures of sample Cyc#3 (CMSX-4 with PtAl + columnar YSZ) after oxidation test.
diffuse TGO/YSZ interface of IN738LC TBC system could reduce Gc and lead to earlier failure. 4.2. Effects of accumulated strain and TGO rumpling Thermal cycling in oxidizing environment, in particular, causes growth or linkage of small internal cracks/delamination at either top coat (TC)/TGO or TGO/bond coat (BC) interface. In TBC systems
with EB-PVD TC, spallation and/or cracking of the TGO often leads to ultimate failure [26]. There are two commonly accepted TBC failure mechanisms [39]. The first one is based on the formation and growth of cracks within defective sites in or near TGO in order to release the accumulated strain energy in the nondeformable TGO during cooling; CTE mismatch between different layers, TGO thickening and temperature/cooling rate all contribute to the increase of stored energy in TGO. The second mechanism is
Fig. 13. X-ray diffraction pattern from the backside of spalled YSZ (sample Cyc#3, CMSX-4 with PtAl + columnar YSZ).
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Fig. 14. X-ray diffraction pattern from bond coat surface (sample Cyc#3, CMSX-4 with PtAl + columnar YSZ).
Table 6 EDS results of Cyc#3.
NiAl2O4 (YSZ backside) Al2O3 (YSZ backside)
Elements
Al
Ni
Co
O
wt.% at.% wt.% at.%
36.38 30.39 55.82 42.83
14.89 5.72 – –
4.65 1.78 – –
44.08 62.11 44.18 57.17
based on bond coat creep and the resulted TGO rumpling/ ratcheting/undulation [40] and cracking at TGO extremities leading to eventual spallation of TBC (TGO is considered deformable). This is often observed in a TBC system with Pt-modified diffusion aluminide. And the wavelength of the rumpling was found to be consistent with the grain size (of β-(Ni,Pt)Al) [41]. Rumpling of TGO is only associated with a coated system [41], i.e., with the presence of a substrate. To reduce the TGO rumpling, a bond coat with higher creep resistance is desirable [26]. Failure within the ceramic TC may also occur [27]; this happens when sintering changes the elastic modulus of the usually porous ceramic layer.
(a) Exposed bond coat surface
TBC samples with CMSX-4 substrate experienced longer cyclic life than those with IN738LC substrate in general (Fig. 7). Based on the above cited failure mechanisms, it is possible to suggest that longer cyclic life of coating systems with CMSX-4 substrate may arise due to (1) less CTE mismatch between layers (reduced strain energy) and (2) better creep resistance of diffusional bond coat on CMSX-4 (less TGO rumpling). Revisiting the reported coefficients of thermal expansion (CTEs) in the temperature range of interest, it is realized that CMSX-4 has a lower CTE than IN738LC (15.6 vs. 17 × 10− 6/k) [42] whereas the CTE of TGO is lower than both substrates and similar to YSZ [43]. Assuming a CTE value of 15.1 × 10−6/K for diffusional aluminide coating (NiAl) [44], the CTE mismatch is much less in TBC with CMSX-4, hence potentially lower stored strain energy for crack formation [39] or reduced driving force for interfacial delamination [21]. Furthermore, in diffusional coatings TGO rumpling is often observed [20]. As rumpling is affected by creep deformation of the bond coat, if the bond coat on CMSX-4 has higher creep resistance, it could have experienced less TGO rumpling and led to longer cyclic life. Although there is no direct study of the creep rate of (Ni,Pt)Al with various compositions, CMSX-4 has shown superior creep resistance than IN738 [45]. It is possible that the creep property of diffusional bond coat follows the same trend as the substrate alloy.
(b) Backside of spalled YSZ
Fig. 15. Microstructures of sample Cyc#2 (IN738LC with PtAl + VC YSZ) after oxidation test.
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
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Fig. 16. X-ray diffraction pattern from the backside of spalled YSZ (Cyc#2, IN738LC with PtAl + VC YSZ).
Fig. 17. X-ray diffraction pattern from the exposed bond coat surface (Cyc#2, IN738LC with PtAl + VC YSZ).
4.3. Oxide formations on the spalled surfaces and crack path Coating systems with shorter life are often found to fail at TGO/bond coat interface while failure within TGO is associated with better performance [21]. In this study, Al2O3 and spinels are detected on all failed surfaces, in addition to other phases. Both samples Cyc#3 and Cyc#4 have more spinel phases (due to longer exposure time before final failure than Cyc#1 and Cyc#2) and also occasional Ta-rich oxide near alumina
Table 7 EDS results of Cyc#2.
Al2O3 (YSZ backside) ZrO2 (YSZ backside) NiAl2O4 + Al2O3 (YSZ backside)
Elements
Zr
Y
Al
Ni
Cr
O
Si
wt.% at.% wt.% at.% wt.% at.%
– – 56.92 21.43 – –
– – 5.93 2.29 – –
59.51 46.56 2.92 3.71 49.62 39.25
– – – – 3.08 1.12
– – – – 3.75 1.54
40.49 53.44 33.23 71.34 43.55 58.09
– – 1 1.23 – –
(Ta diffusion from substrate to TGO). As shown in Figs. 12 and 18, it is apparent that the failure locations on Cyc#3 and Cyc#4 are within or along TGO and spinel as both are prominent on the spalled bond coat and back side of YSZ. Ta-rich particles are present on failed surfaces of both samples (Figs. 12 and 18). Indications of TGO/bond coating separation were also found on Cyc#4. On the exposed surface of the spalled samples Cyc#1 and Cyc#2, in addition to Al2O3 and a small amount of NiAl2O4, some ZrO2 was found (Figs. 9 and 15). This suggests that cracking in the YSZ occurred on samples with IN738 substrate. As TGO rumpling was found to also induce crack formation in YSZ [46], more TGO rumpling may have occurred on samples Cyc#1 and Cyc#2. Coating spallation on these two samples occurred primarily along or within TGO as well as within YSZ. 5. Conclusion The cyclic oxidation resistance of TBC systems with two different YSZ top coats (columnar and vertically cracked) and substrates (IN738LC and CMSX-4) was examined in this study. TBC systems with
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(a) Backside of spalled YSZ
(c) Exposed bond coat
(b) Backside of spalled YSZ
(d) Cross section
Fig. 18. Microstructures of sample Cyc#4 (CMSX-4 with PtAl + VC YSZ) after cyclic oxidation test for 1000 cycles.
IN738LC substrate and both types of top coats exhibited shorter cyclic oxidation life than that with CMSX-4 substrate. In addition to substrate composition effect, higher CTE mismatch and increased TGO rumpling may have contributed to the reduced cyclic oxidation lifetimes of TBC systems with IN738LC substrate. The effect of different types of YSZ top coat (columnar vs. VC) on cyclic oxidation life was not obvious for TBC systems with IN738LC substrate but VC TBC seems to experience much longer cyclic life when applied to CMSX-4.
Acknowledgment The financial support to Ms. Xiaolong Li is provided by the Natural Science and Engineering Research Council of Canada. We thank National Research Council of Canada for allowing us the access to the equipment needed for this research. The authors would like to thank Ms. O. Lupandina for the experimental advice. And lastly, we sincerely thank Mrs. F. Lingnau and Mr. Sam at Chromalloy American Corporation
Fig. 19. X-ray diffraction pattern from the backside of spalled YSZ (Cyc#4, CMSX-4 with PtAl + VC YSZ).
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
61
Fig. 20. X-ray diffraction pattern from the exposed bond coat surface (Cyc#4, CMSX-4 with PtAl + VC YSZ).
Table 8 EDS analysis results of Cyc#4. Elements Zr Al2O3 (YSZ backside) ZrO2 (YSZ backside) NiAl2O4 (YSZ backside) NiAl2O4 (Bond coat/ substrate surface)
wt.% at.% wt.% at.% wt.% at.% wt.% at.%
– – 66.73 27.07 – – – –
Y
Al
Ni
Cr O
Si
Co
– – 3.72 1.66 – – – –
55.4 42.76 1.45 2.14 35.04 30.4 32.65 28.95
– – – – 19.26 7.68 27.25 11.1
– – – – – – – –
1.48 1.1 2.49 3.53 – – – –
– – – – 4.62 1.83 – –
43.12 56.14 25.61 63.6 41.08 60.09 40.1 59.96
Fig. 21. Variation of driving force G and interfacial fracture toughness Gc during thermal cycling [21].
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effects of substrate material and TBC structure on the cyclic oxidation resistance of TBC systems Xiaolong Li a, Xiao Huang a,⁎, Qi Yang b, Zhaolin Tang c a b c
Carleton University, Department of Mechanical and Aerospace, 1125 Colonel By Dr, Ottawa, Ontario K1S 5B6, Canada Aerospace Portfolio, National Research Council, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada Northwest Mettech, 467 Mountain Hwy, North Vancouver, British Columbia V7J 2L3, Canada
a r t i c l e
i n f o
Article history: Received 19 May 2014 Accepted in revised form 3 October 2014 Available online 13 October 2014 Keywords: Plasma spray Thermal barrier coatings Yttria-stabilized zirconia (YSZ) Cyclic oxidation IN738LC CMSX-4
a b s t r a c t The effects of different substrates (CMSX-4 and IN738LC) on cyclic oxidation behavior of two thermal barrier coatings (TBCs) were investigated in this study. Cyclic oxidation test was conducted at a peak temperature of 1080 °C with 30 min hold time. The TBC systems tested consisted of two nickel-base superalloy substrates (CMSX-4 and IN738LC), a platinum aluminide bond coat and two types of 8YSZ top coat (vertical cracks (VC) and columnar structure). Samples with CMSX-4 substrate showed greater cyclic oxidation lifetimes than that with IN738LC, and VC YSZ on CMSX-4 offered longer cyclic life than those with columnar structure. After the oxidation test, the samples were analyzed using a scanning electron microscope (SEM) and an X-Ray diffractometer. The failure location of all TBC systems examined was found to be in the vicinity of TGO, consistent with past research of similar coating systems. Alumina and spinels (CS) were detected on all spalled surfaces although the amount of each phase differs from sample to sample. Ta-rich oxide was found on spalled coating surfaces with CMSX-4 substrate while islands of YSZ on the exposed TGO surface of samples with IN738LC substrate. It is believed that alloy composition, TGO rumpling and stresses induced by TGO growth and coefficient of thermal expansion (CTE) mismatch between different layers all contributed to the reduced cycles to failure of TBC systems with IN738LC substrate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Modern gas turbines operate at high temperature and constantly increasing of inlet temperature is required to improve turbine efficiency [1,2]. The hot sections of gas turbines are subjected to not only high temperature [3], but also severe oxidizing and corrosive environments [4]. Therefore, the efficiency of gas turbine engine is highly depended on the thermal and mechanical properties of materials used for the hot section components such as turbine blades [5]. Nickel-base superalloys are widely used for gas turbine blades due to their high temperature resistance and high strength at elevated temperature. These superalloys can work for thousands of hours even at a high temperature of 1100 °C [6]. With the application of thermal barrier coatings (TBCs) on gas turbine blades and vanes, metal surface temperature can be reduced by 100 °C to 300 °C; the increased temperature capability of the coated components has led to a significant increase in operating temperature, hence turbine efficiency [7,8]. State-of-the-art TBC system usually consists of a metallic bond coat and an yttria-stabilized zirconia (YSZ) ceramic top coat. The low thermal conductivities of YSZ and the superior oxidation/corrosion resistance of ⁎ Corresponding author at: Mechanical and Aerospace Engineering Carleton University 1125 Colonel By Drive Ottawa, ON K1S 5B6.
http://dx.doi.org/10.1016/j.surfcoat.2014.10.004 0257-8972/© 2014 Elsevier B.V. All rights reserved.
metallic bond coat are beneficial in protecting the underlying superalloy substrate from the environmental degradation. There are two typical types of bond coats: traditional MCrAlY (M = Ni or/and Co) overlay coatings and platinum-modified aluminide ((Pt,Ni)Al) diffusion coatings. (Pt, Ni)Al coatings exhibit better protection against high temperature oxidation damage than MCrAlY coatings due to their ability to form a slowgrowing alumina scale [9]. The addition of platinum to the aluminide coating improves the oxidation resistance by promoting selective oxidation of aluminum and enhances coating stability and scale adhesion in return [10]. The YSZ ceramic top coats are usually deposited by air plasma spraying (APS) method or electron beam physical vapor deposition (EBPVD) technique. APS method is more commonly used for gas turbine stationary components due to its low cost, high deposition rate and large range of applicable coating compositions [11]. The YSZ coatings deposited by APS exhibit high thermal resistance because of the inherent laminar structure [12]. However, the presence of porosity and micro-cracks and the lack of strain tolerant microstructure features in the traditional APS coatings lead to coating spallation during exposure to high temperature, particularly under cyclic conditions [13]. On the contrary, EB-PVD deposited YSZ coatings have shown superior strain and thermo-shock tolerant behavior due to the columnar microstructures. They also exhibit better resistance to spallation, lower surface roughness, better thickness
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Table 1 Summarized properties of APS, ASPS and EB-PVD coatings [17]. Properties
APS
ASPS
EB-PVD
Bond strength (MPa) Thermal conductivity (W/m·K)
20–40 0.9–1.5
50–82 1–2
65–75 1.7–2
Table 2 Chemical composition of the substrate materials.
IN738LC CMSX-4
Ni
Cr
Co
Mo
W
Ta
Nb
Al
Ti
Re
C
Bal Bal
16 6.5
8.5 9
1.7 0.6
2.6 6
1.7 6.5
0.9 –
3.4 5.6
3.4 1
3
0.11 –
coated with EB-PVD YSZ reached 1340 cycles (at 1150 °C for 1 h followed by 10 min cooling), longer than that observed of same coating system applied to N5 (907 cycles) which has higher Cr, Al and Mo but lower Co, W and Ti than CMSX-4 [20]. Changing the bond coat from Pt-modified aluminide to Pt-diffusion coating (γ + γ′) reversed this trend [20]. Co and Ti content in superalloys was believed to be detrimental to coating performance in general [24,25] while Re was not [21]. On the contrary, IN100, a polycrystalline superalloy with higher Co and Ti than CMSX-4, had longer cyclic life than CMSX-4 with the same coatings applied [23]. In this study, the objectives are to investigate the effects of substrate composition (CMSX-4 and IN738LC) and top coat microstructure (ASPS VC and columnar) on cyclic lifetimes of four TBC systems. 2. Experiments
uniformity and superior adherence to the substrate over those produced by plasma spraying [14]. However, the equipment cost of EB-PVD systems is much higher and the deposition rate is relatively low [15,16]. Axial suspension plasma spray (ASPS), as an emerging technology, has been used to produce durable TBCs with much lower operating and capital costs when being compared to EB-PVD [17]. ASPS coatings have similar properties to EB-PVD coatings, but exhibit much higher bond strength and slightly higher thermal conductivity than APS coatings (Table 1). In suspension plasma spraying fine particles (usually in nm scale) suspended in a liquid carrier are injected into the plasma stream, instead of direct powder injection; in the case of ASPS, the suspension liquid is injected axially, providing more acceleration to the suspension [17]. ASPS deposition process, through control of the process parameters, can produce coatings with columnar structure or dense coatings with vertical cracks (VCs). For example, while columnar structured coatings are deposited with a longer stand-off distance (SOD), dense coatings with vertical cracks for better strain tolerance can be produced with a shorter SOD [18]. Similar to the columnar structure, VC structure reduce the tensile stress buildup in the YSZ top coat upon heating and compressive stress upon cooling [19]. Therefore, VC and columnar coatings usually have longer oxidation life than lamellar APS coatings. The oxidation behaviors of TBCs are also influenced by the use of different substrate materials. The effects of substrate on cyclic oxidation life of a TBC system have been reported by many researchers [20–23]. In a recent study of EB-PVD coating systems with IN 100 and CMSX-4 as substrate materials, the cyclic life tested at 1100 °C (50 min duration) was found to be higher in the system with polycrystalline IN 100 coated with EB-PVE NiCrAlY (overlay) and YSZ [23]. The reduced cyclic life of TBC system with CMSX-4 substrate was attributed to the diffusion of Ta, W and Re to TGO (all were found on the failure surface). However using a Pt-modified diffusion aluminide bond coat, the cyclic life of CMSX-4
2.1. Substrate materials Two substrate materials, CMSX-4 and IN738LC, were used in this study. Their chemical compositions are listed in Table 2. IN738LC, a precipitation strengthened nickel-based superalloy, is widely used for the hot section components of aero-engines and gas turbines [30] due to its microstructure stability, excellent strength and oxidation/corrosion resistance at elevated temperature [29]. The twophase microstructure of as-received IN738LC substrate is shown in Fig. 1(a). CMSX-4 is a second generation nickel-based single crystal superalloy; as shown in Fig. 1(b), like IN738LC it contains cubical intermetallic γ′ (Ni3Al) phase coherently embedded in γ nickel solid solution matrix [30]. CMSX-4 exhibits improved strength and creep resistance at high temperature because of the large volume fraction of γ′ strengthening phase (due to elevated Al), lack of grain boundaries and increased refractory element contents [31]. All of these characteristics make CMSX-4 a widely used material for blade applications in land-base gas turbines [32]. Adding refractory elements tantalum, tungsten and rhenium improves mechanical properties at high temperature; but the Cr content must be reduced in order to maintain microstructure stability during service. The Cr/Al ratio in IN738LC is 4.7 [33], higher than that in CMSX-4 (1.25) [34]. Therefore, CMSX-4 substrate is usually coated with platinum aluminide to protect it against high temperature oxidation [35]. 2.2. Coating processes (Pt,Ni)Al bond was applied by Chromalloy Inc. using standard process involving Pt plating, pack cementation and diffusion heat treatment. The columnar and VC 8YSZ coatings were applied using the Axial III™ plasma torch with a modified injector for suspension
Fig. 1. γ/γ′ structure in (a) IN738LC and (b) CMSX-4.
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
51
Fig. 2. ASPS deposition process [17].
atomization and the NanoFeed™ suspension feeder at Northwest Mettech Corp. (North Vancouver, Canada). The Axial III™ injects the atomized suspension feedstock axially to the direction of spray into the central core of the plasma. Axial injection overcomes the injection difficulties that arise when attempting to penetrate the plasma radially with fine particles or droplets. The NanoFeed is designed to feed submicron suspensions using mass flow control of both suspension and atomizing gas to provide uniform atomizing dynamics at the injector. An overview of the ASPS process is shown in Fig. 2. The powder used was customized ZrO2-8wt.%Y2O3 (8YSZ) with submicron size. After suspending the fine powders in the ethanol liquid, the suspension feedstock was fed by Mettech Nanofeed 350 Suspension feeder to the injector with stable flow rate and injection pressure for the axial injection plasma spraying performed by Axial III™ Torch.
Table 3 Sample description and identification. Material Composition IN738LC with PtAl + columnar 8YSZ
Appearance before cyclic oxidation test
The as-coated samples were heat-treated in a vacuum furnace at 1080 °C for 4 h. The reason for employing this treatment was based on our previous work which has found that vacuum heat treatment had the effect of increasing the cyclic oxidation life of TBCs [36]. The specimens are identified as Cyc#1, Cyc#2, Cyc#3 and Cyc#4, as shown in Table 3. Each type consists of two samples: one for observation under as-received condition and another for cyclic oxidation test. The samples were disk-shaped with a diameter of about 19 mm and a thickness of 5 mm. The top YSZ coating had an average thickness of 200 μm. 2.3. Sample preparation and analysis before oxidation One sample from each group was cross-sectioned, mounted and polished. Microscopic evaluation of top surfaces and crosssections of the samples were performed with Philips XL30S FEG scanning electron microscopes (SEMs) equipped with Phoenix energy dispersive X-ray spectroscopy (EDS). The X-ray diffraction (XRD) analysis was performed using Bruker AXS D8 Discover diffractometer.
Sample IDs
2.4. Oxidation tests Cyc#1
IN738LC with PtAl + VC 8YSZ
Cyc#2
CMSX-4 with PtAl + columnar 8YSZ
Cyc#3
The cyclic oxidation test was conducted in air using a CM furnace (Rapid Temp Furnace supplied by Bloom Field, N.J., USA). The duration of each cycle was 60 min and each cycle consists of heating from room temperature to 1080 °C (about 10 min) and holding for 30 min and then fan-cooling at room temperature for 20 min (temperature measurement showed that the samples reached 275–300 °C after cooling). Visual inspections were performed after every 100 cycles as such the reported cyclic lifetime represents the time when failure was observed not the actual cycles to failure. Samples were withdrawn from test once the failure criterion (N 30% spallation of top coat) was reached. 3. Results
CMSX-4 with PtAl + VC 8YSZ
Cyc#4
3.1. Microstructure and composition of as-coated samples The surface morphologies of the pseudo columnar structured (columnar will be used in the text for simplicity) and VC YSZ coatings before oxidation tests are shown in Fig. 3. The columnar YSZ coating
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(a)
(b)
(c)
(d)
Fig. 3. Surface morphologies of the columnar (a and b) and VC coatings (c and d).
has a “broccoli-like” feature with a few long surface cracks and visible surface pores (Fig. 3(a) and (b)) while VC coating has smoother and denser surface but numerous shorter, connected cracks (Fig. 3(c) and (d)). XRD analysis was performed on the top surfaces of samples Cyc#1 to 4 and an example of the results is shown in Fig. 4 since all four samples display the same diffraction peaks. Traces of (Ni, Pt)Al (β phase from the bond coat) and γ′-Ni3Al (strengthening phase in
IN738LC and CMSX 4) phases are present in the samples, in addition to ZrO2, a main phase in YSZ. The cross-sectional microstructures of the columnar and VC TBC are shown in Fig. 5. Similar to that seen on the surface, the columnar coating has a rougher surface with more pores towards the top. And for the VC coating most of the vertical cracks extend to about half of the YSZ total coating thickness. In the original test plan, a conventional plasma sprayed YSZ top coat was also included.
Fig. 4. X-ray diffraction pattern from the top surface before cyclic test (all samples have identical diffraction patterns).
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
53
Fig. 5. Cross-sections of columnar structured (a) and VC (b) TBC coatings.
Fig. 6. TBC coatings on IN738 (a) and CMSX-4 (b) substrate.
whereas a three-layer bond coat is observed on CMSX-4 substrate. The chemical compositions of different bond coat layers and substrate (close to and far away from bond coat) were measured by EDS analysis and summarized in Table 4. From the table it is obvious that the bond coat layer one on CMSX-4 contained more Pt and Al, while much
However, the PS YSZ sprayed on PtAl bond coat immediately spalled off after spraying. As such this set of samples could not be included in this study. Morphologies of PtAl bond coat and IN738LC/CMSX-4 substrate are shown in Fig. 6. A two-layered bond coat exits on the IN738LC substrate
Table 4 Measured compositions (by EDS) of different layers in PtAl bond coat (wt.%).
PtAl + IN738LC
PtAl + CMSX-4
wt.%
Pt
Al
Ni
Cr
Co
Ta
Ti
W
Si
O
Layer 1 Layer 2 Area 1 Area 2 Layer 1 Layer 2 Layer 3 Area 1 Area 2
13.25 – – – 24.33 10.12 – – –
17.05 7.24 4.02 2.95 20.79 20.88 12.85 6.27 5.69
55.16 37.11 60.21 61.71 42.04 53.69 45.79 62.70 63.36
6.48 24.34 18.06 16.44 2.63 3.05 8.40 7.07 6.67
6.21 8.26 8.18 9.11 5.35 7.12 9.89 9.85 10.61
0.30 9.57 2.01 2.04 0.68 2.17 8.76 5.84 5.66
1.55 6.03 3.78 3.72 – – 1.21 1.05 –
– 7.45 – 3.90 2.17 2.96 13.09 7.22 6.67
– – – 0.12 – – – – –
– – – – 2.02 – – – –
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X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
Fig. 7. Cyclic oxidation life (in terms of number of cycles before 30% top coat spallation was found).
more Cr and refractory elements Ti, Ta and W were detected in the bond coat layer 1 on IN738LC. The difference in Pt concentrations was resulted due to the separation of bond coat layer into layer 1 (Pt rich) and layer 2 (Pt lean) on the surface of CMSX-4 substrate (Fig. 6(b)). Elements from the base metals, such as Ni, Cr, Co, Ta, Ti and W, were found in layer 2 on IN738LC (Fig. 6(a)) and layer 3 on CMSX-4 (Fig. 6(b)) due to diffusion. The EDS analysis also revealed that there were more Ta and W (less Cr) in layer 3 of CMSX-4 while more Cr and Ti (and some W) were found in bond coat layer 2 on IN738LC, illustrating the influence of base metal composition on coating microstructure. 3.2. Cyclic oxidation test result The cyclic oxidation lives of different samples are represented in Fig. 7. All samples failed within 1000 cycles. Cyc#1 and Cyc#2 (both with IN738LC substrate) spalled after 365 and 300 cycles, respectively; Whereas samples with CMSX-4 substrate, Cyc#3 and Cyc#4, experienced 500 and 1000 cycles, respectively, before failure. Examples of periodical visual inspections are shown in Fig. 8. 3.3. Microstructure analysis of cyclic oxidation tested samples 3.3.1. Samples with PtAl bond coat and columnar YSZ top coat During cyclic oxidation test, sample Cyc#1 (IN738LC with PtAl + columnar YSZ) failed after 365 cycles (this failure was found ahead of the regular inspection interval by coincidence) while sample Cyc#3 (CMSX4 with PtAl + columnar YSZ) was found to exceed the spallation limit
(a)
(b)
after 500 cycles. SEM/EDS analysis of sample Cyc#1 reveals both alumina and NiAl2O4 on the backside of spalled YSZ coating and bond coat surface, as shown in Fig. 9(a)–(c). From the cross sectional view of the microstructure, it is clear that YSZ has completely separated from the TGO and also vertical cracks in YSZ have progressed to the interface between YSZ and TGO (Fig. 9(d) and (e)). The formation of vertical cracks in ceramic layer has been observed in similar systems with EB-PVD top coat and it was attributed to the need to release stored elastic energy induced by TGO growth [37], heating-cooling cycles and possibly TGO rumpling [39]. Sintering which has the effect of increasing elastic modulus accelerates the tendency for vertical cracking. Also to be noted are patches of YSZ (ZrO2 from EDS analysis) on the exposed bond coat surface (Fig. 9(c)); this suggests cracking within YSZ top coat. EDS analysis results measured from sample Cyc#1 are summarized in Table 5. XRD analysis was carried out on both the backside of the spalled YSZ and the top surface of exposed bond coating surface; the diffraction spectra are given in Figs. 10 and 11. These further confirm that spinel (NiAl2O4) and alumina (Al2O3) are attached to the spalled YSZ. EDS analysis on the backside of spalled YSZ of sample Cyc#3 and cross-section suggested that NiAl2O4 and Al2O3 also formed during the cyclic oxidation test (Fig. 12(a), (b) and (c)). Some Ta-rich particles also formed within TGO (Fig. 12(a)). XRD analysis detected the presence of NiAl2O4, NiCr2O4 and trace Cr2O3 on the backside of YSZ and the surface of exposed bond coat (Figs. 13 and 14). There is an increased amount of mixed oxide formation on sample Cyc#3, as compared to Cyc#1, as a result of the longer exposure time than that for Cyc#1 (500 cycles vs. 365 cycles). In general, as the oxidation exposure time increases, Al becomes depleted and chromium and nickel oxide start to form leading to the reactions of NiO + Cr2O3 → NiCr2O4 and NiO + Al2O3 → NiAl2O4. Another microstructural change occurred to Cyc#3 after thermal cycling was the development of vertical cracks (Fig. 12(d)) in the top coat. This also occurred to sample Cyc#1, as noted earlier. With the through vertical cracks developing, oxygen can readily reach the TGO and accelerate Al depletion and the growth of mixed oxide(s). As such, spinels were found on both Cyc#1 and Cyc#3 after top coat spallation. EDS results from sample Cyc#3 are summarized in Table 6.
(c)
Fig. 8. Cyc#4 (CMSX-4 with VC TC) in the (a) as-coated, (b) after 900 cycles and (c) after 1000 cycles.
3.3.2. Samples with PtAl bond coat and VC YSZ top coat During cyclic oxidation test, sample Cyc#2 (IN738LC with PtAl + VC YSZ) failed after 300 cycles while Cyc#4 (CMSX-4 with PtAl + VC YSZ)
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
(a) Backside of spalled YSZ
55
(b) Backside of spalled YSZ
(c) Exposed bond coat surface
(d) Cross section
(e) Cross section
Fig. 9. Microstructures of sample Cyc#1 (IN738LC with PtAl + columnar YSZ) after oxidation test.
completed 1000 cycles before spallation was observed. SEM/EDS analysis of sample Cyc#2 revealed large coverage of Al2O3 on both the backside of spalled YSZ and exposed bond coat surface, whereas a small amount of NiAl2O4 was observed on the backside of spalled YSZ
Table 5 EDS analysis results of Cyc#1. Elements Zr NiAl2O4 (YSZ backside) Al2O3 (YSZ backside) ZrO2 (YSZ backside) NiAl2O4 (Bond coat/substrate surface)
wt.% at.% wt.% at.% wt.% at.% wt.% at.%
– – – – 22.93 6.4 – –
Y
Al
Ni
Co
O
Cr
Ti
– – – – 2.65 0.76 – –
26.01 22.73 55.05 42.07 32.85 31 35.16 34.41
23.98 9.63 – – – – 19.68 8.85
5.65 2.26 – – – – 4.33 1.94
44.36 65.38 44.95 57.93 37.63 59.9 29.63 48.89
– – – – 3.95 1.93 6.04 3.06
– – – – – – 5.17 2.85
(Fig. 15). XRD detected also a very small amount of NiAl2O4 on the backside of YSZ and bond coat/substrate surface, in addition to Al2O3 and YSZ (ZrO2) as shown in Figs. 16 and 17. Similar phases were also observed in Fig. 9(c) where cracking within YSZ top coat was suspected. EDS results of Cyc#2 are summarized in Table 7. For Cyc#4 sample, larger amounts of alumina and spinels were found on both the backside of spalled YSZ and on top of the exposed bond coat (Figs. 18, 19 and 20). In particular, NiAl2O4 was observed on the backside of spalled YSZ, exposed bond coat surface and crosssection (Fig. 18). Although no indication of YSZ on the exposed bond coat surface, some Ta-rich particles formed on top of TGO (Fig. 18(c)). The isolated area of metallic bond coat on the same figure implies that the interfacial bond strength between TGO and bond coat has been weakened after 1000 cycles. The microstructure on the cross-section of Cyc#4, Fig. 18(d), had similar features as that of Cyc#3 with columnar top coat (Fig. 12(c)); yet sample Cyc#4 with VC top coat experienced more cycles prior to failure (1000 cycles vs. 500 cycles for Cyc#3). Although more tests are
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X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
Fig. 10. X-ray diffraction pattern from the backside of spalled YSZ (sample Cyc#1, IN738LC with PtAl + columnar YSZ).
needed to confirm this, the VC seems to display more strain tolerance than the columnar YSZ. EDS analysis results of Cyc#4 are shown in Table 8. 4. Discussion 4.1. Elemental effect In this study, the cyclic oxidation behavior of TBC systems with IN738LC and CMSX-4 substrates was studied. Samples Cyc#1 and Cyc#2, with IN738LC substrate, suffered from early coating spallation, after 365 and 300 cycles, respectively. Samples Cyc#3 and Cyc#4 with CMSX-4 substrate exhibited longer cyclic oxidation lives than the ones with IN738LC substrates, reaching 500 and 1000 cycles, respectively. Comparing the compositions of the two alloys, IN738 has higher Ti and Cr but lower W, Ta and Re than CMSX-4. Also more Ti, Ta and W were found in the as-coated bond coat surface of IN738LC (Table 4). As Ta, Co and Ti in superalloys were believed to be detrimental to
coating performance in general [23–25], the increased Ti content in IN738LC and elevated Ti and Ta in its bond coat may have played a role in reduced cyclic lifetime. Additionally, higher Pt in the bond coat on CMSX-4 was also measured (Table 4). This could also played a role in ensuring better TGO adhesion [38] thus extending the cyclic life of TBC on CMSX-4. However, based on the microstructural analysis carried out in this study on the spalled surfaces, there is no microstructural evidence so far to suggest that the elevated Ti or Ta content in IN738LC or its bond coat played a role in early coating spallation. Instead, Ta-containing particles were observed on failed coatings with CMSX-4 substrate. A simplified approach to relate cyclic oxidation life and coatingsubstrate system is to compare the interfacial fracture toughness Gc to driving force G accumulated during cycling (due to TGO thickening, mismatch in CTE) [21]. As shown in Fig. 21, the substrate composition and continuous thermal exposure (elemental diffusion and phase changes) affect the interfacial strength or fracture toughness Gc. Once G reaches Gc, spallation occurs. Harmful elements (Ti, Ta and others)
Fig. 11. X-ray diffraction pattern from the exposed bond coat surface (sample Cyc#1, IN738LC with PtAl + columnar YSZ).
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
(a) Backside of spalled YSZ
(b) Backside of spalled YSZ
(c) Cross section
(d) Cross section
57
Fig. 12. Microstructures of sample Cyc#3 (CMSX-4 with PtAl + columnar YSZ) after oxidation test.
diffuse TGO/YSZ interface of IN738LC TBC system could reduce Gc and lead to earlier failure. 4.2. Effects of accumulated strain and TGO rumpling Thermal cycling in oxidizing environment, in particular, causes growth or linkage of small internal cracks/delamination at either top coat (TC)/TGO or TGO/bond coat (BC) interface. In TBC systems
with EB-PVD TC, spallation and/or cracking of the TGO often leads to ultimate failure [26]. There are two commonly accepted TBC failure mechanisms [39]. The first one is based on the formation and growth of cracks within defective sites in or near TGO in order to release the accumulated strain energy in the nondeformable TGO during cooling; CTE mismatch between different layers, TGO thickening and temperature/cooling rate all contribute to the increase of stored energy in TGO. The second mechanism is
Fig. 13. X-ray diffraction pattern from the backside of spalled YSZ (sample Cyc#3, CMSX-4 with PtAl + columnar YSZ).
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X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
Fig. 14. X-ray diffraction pattern from bond coat surface (sample Cyc#3, CMSX-4 with PtAl + columnar YSZ).
Table 6 EDS results of Cyc#3.
NiAl2O4 (YSZ backside) Al2O3 (YSZ backside)
Elements
Al
Ni
Co
O
wt.% at.% wt.% at.%
36.38 30.39 55.82 42.83
14.89 5.72 – –
4.65 1.78 – –
44.08 62.11 44.18 57.17
based on bond coat creep and the resulted TGO rumpling/ ratcheting/undulation [40] and cracking at TGO extremities leading to eventual spallation of TBC (TGO is considered deformable). This is often observed in a TBC system with Pt-modified diffusion aluminide. And the wavelength of the rumpling was found to be consistent with the grain size (of β-(Ni,Pt)Al) [41]. Rumpling of TGO is only associated with a coated system [41], i.e., with the presence of a substrate. To reduce the TGO rumpling, a bond coat with higher creep resistance is desirable [26]. Failure within the ceramic TC may also occur [27]; this happens when sintering changes the elastic modulus of the usually porous ceramic layer.
(a) Exposed bond coat surface
TBC samples with CMSX-4 substrate experienced longer cyclic life than those with IN738LC substrate in general (Fig. 7). Based on the above cited failure mechanisms, it is possible to suggest that longer cyclic life of coating systems with CMSX-4 substrate may arise due to (1) less CTE mismatch between layers (reduced strain energy) and (2) better creep resistance of diffusional bond coat on CMSX-4 (less TGO rumpling). Revisiting the reported coefficients of thermal expansion (CTEs) in the temperature range of interest, it is realized that CMSX-4 has a lower CTE than IN738LC (15.6 vs. 17 × 10− 6/k) [42] whereas the CTE of TGO is lower than both substrates and similar to YSZ [43]. Assuming a CTE value of 15.1 × 10−6/K for diffusional aluminide coating (NiAl) [44], the CTE mismatch is much less in TBC with CMSX-4, hence potentially lower stored strain energy for crack formation [39] or reduced driving force for interfacial delamination [21]. Furthermore, in diffusional coatings TGO rumpling is often observed [20]. As rumpling is affected by creep deformation of the bond coat, if the bond coat on CMSX-4 has higher creep resistance, it could have experienced less TGO rumpling and led to longer cyclic life. Although there is no direct study of the creep rate of (Ni,Pt)Al with various compositions, CMSX-4 has shown superior creep resistance than IN738 [45]. It is possible that the creep property of diffusional bond coat follows the same trend as the substrate alloy.
(b) Backside of spalled YSZ
Fig. 15. Microstructures of sample Cyc#2 (IN738LC with PtAl + VC YSZ) after oxidation test.
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
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Fig. 16. X-ray diffraction pattern from the backside of spalled YSZ (Cyc#2, IN738LC with PtAl + VC YSZ).
Fig. 17. X-ray diffraction pattern from the exposed bond coat surface (Cyc#2, IN738LC with PtAl + VC YSZ).
4.3. Oxide formations on the spalled surfaces and crack path Coating systems with shorter life are often found to fail at TGO/bond coat interface while failure within TGO is associated with better performance [21]. In this study, Al2O3 and spinels are detected on all failed surfaces, in addition to other phases. Both samples Cyc#3 and Cyc#4 have more spinel phases (due to longer exposure time before final failure than Cyc#1 and Cyc#2) and also occasional Ta-rich oxide near alumina
Table 7 EDS results of Cyc#2.
Al2O3 (YSZ backside) ZrO2 (YSZ backside) NiAl2O4 + Al2O3 (YSZ backside)
Elements
Zr
Y
Al
Ni
Cr
O
Si
wt.% at.% wt.% at.% wt.% at.%
– – 56.92 21.43 – –
– – 5.93 2.29 – –
59.51 46.56 2.92 3.71 49.62 39.25
– – – – 3.08 1.12
– – – – 3.75 1.54
40.49 53.44 33.23 71.34 43.55 58.09
– – 1 1.23 – –
(Ta diffusion from substrate to TGO). As shown in Figs. 12 and 18, it is apparent that the failure locations on Cyc#3 and Cyc#4 are within or along TGO and spinel as both are prominent on the spalled bond coat and back side of YSZ. Ta-rich particles are present on failed surfaces of both samples (Figs. 12 and 18). Indications of TGO/bond coating separation were also found on Cyc#4. On the exposed surface of the spalled samples Cyc#1 and Cyc#2, in addition to Al2O3 and a small amount of NiAl2O4, some ZrO2 was found (Figs. 9 and 15). This suggests that cracking in the YSZ occurred on samples with IN738 substrate. As TGO rumpling was found to also induce crack formation in YSZ [46], more TGO rumpling may have occurred on samples Cyc#1 and Cyc#2. Coating spallation on these two samples occurred primarily along or within TGO as well as within YSZ. 5. Conclusion The cyclic oxidation resistance of TBC systems with two different YSZ top coats (columnar and vertically cracked) and substrates (IN738LC and CMSX-4) was examined in this study. TBC systems with
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(a) Backside of spalled YSZ
(c) Exposed bond coat
(b) Backside of spalled YSZ
(d) Cross section
Fig. 18. Microstructures of sample Cyc#4 (CMSX-4 with PtAl + VC YSZ) after cyclic oxidation test for 1000 cycles.
IN738LC substrate and both types of top coats exhibited shorter cyclic oxidation life than that with CMSX-4 substrate. In addition to substrate composition effect, higher CTE mismatch and increased TGO rumpling may have contributed to the reduced cyclic oxidation lifetimes of TBC systems with IN738LC substrate. The effect of different types of YSZ top coat (columnar vs. VC) on cyclic oxidation life was not obvious for TBC systems with IN738LC substrate but VC TBC seems to experience much longer cyclic life when applied to CMSX-4.
Acknowledgment The financial support to Ms. Xiaolong Li is provided by the Natural Science and Engineering Research Council of Canada. We thank National Research Council of Canada for allowing us the access to the equipment needed for this research. The authors would like to thank Ms. O. Lupandina for the experimental advice. And lastly, we sincerely thank Mrs. F. Lingnau and Mr. Sam at Chromalloy American Corporation
Fig. 19. X-ray diffraction pattern from the backside of spalled YSZ (Cyc#4, CMSX-4 with PtAl + VC YSZ).
X. Li et al. / Surface & Coatings Technology 258 (2014) 49–61
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Fig. 20. X-ray diffraction pattern from the exposed bond coat surface (Cyc#4, CMSX-4 with PtAl + VC YSZ).
Table 8 EDS analysis results of Cyc#4. Elements Zr Al2O3 (YSZ backside) ZrO2 (YSZ backside) NiAl2O4 (YSZ backside) NiAl2O4 (Bond coat/ substrate surface)
wt.% at.% wt.% at.% wt.% at.% wt.% at.%
– – 66.73 27.07 – – – –
Y
Al
Ni
Cr O
Si
Co
– – 3.72 1.66 – – – –
55.4 42.76 1.45 2.14 35.04 30.4 32.65 28.95
– – – – 19.26 7.68 27.25 11.1
– – – – – – – –
1.48 1.1 2.49 3.53 – – – –
– – – – 4.62 1.83 – –
43.12 56.14 25.61 63.6 41.08 60.09 40.1 59.96
Fig. 21. Variation of driving force G and interfacial fracture toughness Gc during thermal cycling [21].
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