Trans. Nonferrous Met. Soc. China 25(2015) 2587-2593
Effect of Al-deposition on erosion resistance of plasma sprayed thermal barrier coating
Xiao-feng ZHANG1,2, Ke-song ZHOU1,2, Shu-juan DONG3, Wei XU2, Jin-bing SONG2, Min LIU2
1. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China;
2. Institute of New Materials, Guangzhou Research Institute of Non-ferrous Metals, Guangzhou 510650, China;
3. IRTES-LERMPS Laboratory, 
de Technologie de Belfort-Montbéliard, Belfort Cedex 90010, France
Received 31 October 2014; accepted 28 December 2014
Abstract: A columnar Al film was firstly deposited on the top of 7%Y2O3-stabilized zirconia (7YSZ) ceramic coating in thermal barrier coating (TBC) system by magnetron sputtering. A vacuum treatment was then carried out at 700 °C for 1 h and 900 °C for 5 h to improve the erosion resistance of Al-deposited TBC. A α-Al2O3 layer was in situ synthesized on the top of 7YSZ coating via vacuum heat treatment. The microstructure evolution of Al-deposited TBC illustrated that a loose surface-layer and a dense sub-layer formed on the top of 7YSZ coating after vacuum treatment. The phase structures of the as-sprayed TBC and the Al-deposited TBC after vacuum heat treatment were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) assisted with focused ion beam (FIB). Particulate erosion resistances of the as-sprayed TBC and treated TBC were compared at room temperature. In addition, erosion mechanism and schematic diagram were proposed. The results show that the Al-deposited TBC after vacuum heat treatment has better particulate erosion resistance than the as-sprayed one.
Key words: thermal barrier coating; 7%Y2O3-stabilized zirconia (7YSZ); erosion resistance; in situ synthesis; α-Al2O3
1 Introduction
Thermal barrier coatings (TBCs) have been widely used in the high-temperature components of aircraft and industrial gas-turbine engine [1,2]. The structure of TBCs generally consists of a thermally insulating ceramic top coating and an intermediate oxidation resistant metallic bond coating. The top ceramic coating is typically made of yttria-stabilized zirconia for low thermal conductivity and high thermal-expansion coefficient. The bond coating is typically made of MCrAlY (M: Ni, Co and Ni+Co) for its good oxidation resistance [3]. The failure mechanism of TBCs is very complex, including oxidation of bond coating, corrosion of ceramic coating, etc. Especially, particulate erosion is also an important degradation aspect [4,5]. Particulate erosion of TBCs is generally caused by the action of sliding or impact of solids, liquids or a combination of these elements [6,7].
In the last decades, the erosion failure mechanism of TBCs attracted many researchers, such as MARASHALL and EVANS [8], CHEN et al [9], and WELLMANA et al [10]. It is well recognized that erosion rate of TBCs is affected by many factors which could be broadly classified into three types: impingement, particle and materials variables. The impingement variables mainly consist of particle velocity, particle concentration and angle of incidence. The particle variables include particle shape, size and hardness, etc. Materials variables involve target material properties, such as hardness and porosity [11,12]. The majority of the derived relationship for the erosion rate (v) is of the Eq. (1) given by JANOS et al [5].
v=1010a Ht-b (1)
where v is the erosion rate, Ht is the Vickers microhardness of the target material (TBC), a and b are two constants (>0). So, the higher the TBCs’ microhardness is, the lower the erosion rate is.
Up to now, a lot of methods were proposed to improve the erosion resistance of TBCs, such as, laser glazing and remelting of ceramic coating [13,14], doping of rare earth material like gadolinia [15]. In this work, an innovative approach was presented. Firstly, Al was deposited on the surface of 7YSZ coating by direct current circular magnetron sputtering. And then vacuum heat treatment (at 700 °C for 1 h and 900 °C for 5 h) was carried out in order to in situ synthesize a α-Al2O3 overlay through the reaction of Al and ZrO2 on the top of 7YSZ coating. The surface microhardness and porosity of the treated TBCs were investigated. It is expected to improve the particulate erosion resistance of TBC.
2 Experimental
2.1 Sample preparation
The superalloy K4169 sheet with sizes of d25.4 mm×5 mm was used as the substrate material. The material used for bond coating was commercial NiCoCrAlYTa powders. Agglomerated and sintered ZrO2-7% Y2O3 (7YSZ) powders as top coating materials were provided by H. C. Starck. The composition of these materials is presented in Table 1. Prior to the deposition of bond coating, the substrates were degreased and cleaned with petrol and ethanol, followed by grit blasting with alumina under 0.2 MPa. Both the bond and ceramic coatings were prepared by atmospheric plasma spray (APS, MF-P1000, GTV, Germany). The thicknesses of bond and top ceramic coatings were about 100 and 250 μm, respectively. Then, Al film with a thickness of 15 μm was deposited on the surface of 7YSZ coating by direct current circular magnetron sputtering (J-1250, Jingzhou Industrial Coating, China). Before deposition of Al film, the TBC surface was polished by diamond paste with a size of 2.5 μm (D50) for 30 min. For the magnetron sputtering, Al target (99.99%) was used and the direct current, voltage and pressure were set at 3 A, 150 V and 5×10-3 Pa, respectively. Then, Al-deposited TBC samples were put in a closed vacuum tube (about 2×10-3 Pa) for heat treatment at 700 °C for 1 h and continuously treated at 900 °C for 5 h.
Table 1 Information of powder and substrate material
2.2 Particulate erosion tests
Particulate erosion tests were performed on the erosion experiment equipment which was self-modified from the sand-blasting machine, as schematically illustrated in Fig. 1. Compressed air (0.2 MPa) was used for the power, which sprayed angular Al2O3 particles (about 100 μm) on the surface of TBC samples. And the erosion angle and distance were 90° and 20 cm, respectively. The erosion resistances of as-sprayed and treated TBCs were compared through the eroded mass loss, which was obtained by weighing the sample per 15 s during erosion test using a precision electronic balance with 1×10-4g.
Fig. 1 Schematic diagram of sand-blasting erosion tester
2.3 Characterization of TBC
The cross-sectional microstructures of as-sprayed and Al-deposited TBCs after vacuum heat treatment were characterized by field emission-scanning electron microscope (FE-SEM, Nova-Nono430, FEI, Holland). Phase structures of both TBCs were analyzed by X-ray diffraction (XRD, D8-Advance, Bruker) with a step of 0.02°. Moreover, phase structures of the 7YSZ coatings with a α-Al2O3 overlay were studied by transmission electron microscope (TEM, JEM2100F, JEOL, Japan) assisted with focused ion beam (FIB, SMI3050MS2, SII, Japan). The surface microhardness of both TBCs before and after polishing was measured based on 10 values by Vickers indentation test (MH-5, Everone, China) with a load of 300 g for 15 s. Besides, the cross-sectional microstructures of as-sprayed and treated TBCs after erosion test were characterized by FE-SEM.
3 Results and discussion
3.1 Microstructure and phase analysis of as-sprayed and Al-deposited TBC
Figure 2 shows the surface and cross-sectional morphologies of the as-sprayed TBC. It can be found that the surface of as-sprayed TBC exhibits a lot of vertical cracks, which is a typical characterization of plasma-sprayed ceramic coatings (Fig. 2(a)). From the cross-sectional image of the as-sprayed TBC, it can be seen that the thicknesses of bond coating and ceramic coating are about 100 and 250 μm, respectively (Fig. 2(b)).
Fig. 2 SEM images of as-sprayed TBC
Fig. 3 SEM images of columnar Al film deposited by magnetron sputtering on ceramic coating before vacuum treatment
Figure 3 shows the SEM images of the Al-sputtered TBC before vacuum treatment. A columnar Al film (about 15 μm in thickness) forms on the surface of TBC. After vacuum treatment at 700 °C for 1 h and continuously at 900 °C for 5 h, a mixed surface morphology can be observed with loose and dense layers, as shown in Fig. 4(a). The cross-sectional image (Fig. 4(b)) confirms the presence of loose surface-layer on the top of TBC. The magnified image of marked area in Fig. 4(b) presents that a dense sub-layer forms at the interface between loose layer and ceramic coating, as shown in Fig. 4(c).
Fig. 4 SEM images of Al-deposited TBC after vacuum treatment at 700 °C for 1 h and continuously at 900 °C for 5 h
Phase transformations of the as-sprayed and Al-deposited TBCs after vacuum heat treatment were analyzed by XRD, as shown in Fig. 5. The as-sprayed TBC is mainly composed of C-ZrO2 and t′-ZrO2 phases (Fig. 5(a)), while the Al-deposited and vacuum-treated TBCs consist of α-Al2O3, Al3Zr and ZrO2 (Fig. 5(b)). The α-Al2O3 and Al3Zr phases result from in situ synthesis of Al and ZrO2 at high temperatures [16,17].
Fig. 5 XRD patterns of as-sprayed (a) and Al-deposited TBCs after vacuum treatment at 700 °C for 1 h and continuously at 900 °C for 5 h (b)
According to Refs. [18,19], the 
of Reaction (2) is less than 0 when the temperature is lower than 3232 K.
13Al+3ZrO2→2α-Al2O3+3Al3Zr (2)
The treatment temperatures (700 °C for 1 h and 900 °C for 5 h) in this work meet the reaction condition. It is thus reasonable that the top layer of vacuum-treated TBC consists of α-Al2O3 and Al3Zr as well as ZrO2. The formation of α-Al2O3 on the top of 7YSZ coating is expected to improve the erosion resistance of TBC. Besides, the simultaneous formed Al3Zr phase is an intermetallic compound with good fracture toughness, which could also contribute to the improvement in the erosion resistance. In addition, it is recognized that Al3Zr phase has high melting temperature, good physical and chemical stability [20].
3.2 Comparison of erosion resistance
Figure 6 shows the room temperature erosion results of the as-sprayed and vacuum-treated TBCs. Before the erosion time of 60 s, the eroded mass loss of the vacuum-treated TBC is higher than that of the as-sprayed one due to the presence of loose surface-layer on the top of 7YSZ coating. The measured surface microhardness of porous loose layer (HV0.3 459, see Table 2) is lower than that of the as-sprayed TBC (HV0.3 568). It is thus reasonable that the erosion rate of the treated TBC is lower than that of the as-sprayed one based on Eq. (1). Subsequently, there is a steady range between 60 and 90 s, due to the presence of a dense sub-layer at the interface between loose surface-layer and ceramic coating. The dense sub-layer has a higher surface microhardness (HV0.3 1167) than the polished as-sprayed TBC (HV0.3 621). Here, the surface microhardness of the dense sub-layer was measured after polishing by diamond paste (D50, 2.5 μm) for 10 min to get rid of the loose surface-layer. However, the mass loss of treated TBC begins to decrease obviously after the erosion time of 90 s, similar to that of the as-sprayed TBC. This is because the in-situ formed dense sub-layer is eroded by Al2O3 particles. The microstructure of the underside coating is the same as that of the as-sprayed sample.
Fig. 6 Comparison of erosion resistance between as-sprayed TBC and treated TBC
Table 2 Surface microhardness of as-sprayed and treated TBCs before and after polishing
Figure 7 shows the cross-sectional images of as-sprayed and treated TBC samples after the erosion time of 180 s. It can be seen that the 7YSZ ceramic coating in the as-sprayed TBC sample is nearly fully eroded (Fig. 7(a)). However, there are still some ceramic coatings with a thickness of about 150 μm rest in the treated TBC sample after erosion for 180 s (Fig. 7(b)). Therefore, the Al-deposited TBC after vacuum treat treatment has better particulate erosion resistance than the as-sprayed one.
3.3 Erosion mechanism in treated TBC
The microstructural evolution of Al-deposited TBC via vacuum heat treatment is schematically presented in Fig. 8. Prior to vacuum treatment, columnar Al film is observed on porous 7YSZ ceramic coating, in which typical laminar structure can be seen with some pores and cracks (see Fig. 8(a)). During the vacuum treatment process, the Al film will translate into liquid state when heating temperature is above melting point. Then, a part of Al will infiltrate into porous ceramic coating along surface crack by capillary force [21]. The remaining molten Al is retained on the surface of ceramic coating, where the underside Al will in situ react with ZrO2 directly, forming a dense sub-layer, and the upside Al will form a loose surface-layer with O2 from environment (see Fig. 8(b)). The compositions of the loose surface-layer and dense sub-layer are presented in Fig. 9. It can be found that the loose surface-layer consists of Al and O elements. While the dense sub-layer comprises mixed Al, Zr and O, whose thickness is about 4 μm. Moreover, the surface of vacuum-treated ceramic coating was analyzed by TEM, as shown in Fig. 10. Combining with the results in Fig. 10(b), it is indicated that the crack in 7YSZ ceramic coating near the interface between the dense sub-layer and the ceramic coating is filled with α-Al2O3 phase. The filled α-Al2O3 would lead to a decrease in porosity in top ceramic coating.
Fig. 7 Cross-sectional images of as-sprayed (a) and treated (b) TBCs after erosion time of 180 s
Fig. 8 Schematic diagram of Al-deposited TBC before (a) and after (b) vacuum heat treatment showing microstructure evolution
Fig. 9 SEM image of cross-section of treated TBC including loose surface-layer and dense sub-layer (a) and elemental line scanning (b)
These results agree well with the microstructure and phase analysis results mentioned above. By means of vacuum treatment, a dense sub-layer comprised α-Al2O3 and Al3Zr formed between Al film and 7YSZ coating in Al-deposited TBC system due to the short diffusion path between Al and ZrO2. And a loose surface-layer only consisted of α-Al2O3 formed from the remaining Al reacting with O2 from environment. Although the loose surface-layer has a relatively high erosion rate due to the lower surface microhardness than the as- sprayed TBC, the dense layer has good erosion properties due to its high surface microhardness (HV0.3 1167 vs HV0.3 621 for 7YSZ in the as-sprayed TBC, see Table 2). Therefore, the Al-deposited TBC after vacuum heat treatment has better particulate erosion resistance than the as-sprayed one.
Fig. 10 Bright field TEM image of cross-section of vacuum-treated TBC showing pore in 7YSZ ceramic coating near in-situ synthesized layer (a) and HR-TEM image and SAEDP (b) of marked yellow area in (a) showing interplanar spacing of crystal plane of 0.25 nm
4 Conclusions
1) Columnar Al film was deposited on the TBC surface by direct current circular magnetron sputtering. The Al-deposited TBC was then vacuum-treated at 700 °C for 1 h and continuously at 900 °C for 5 h. The microstructure, phase composition and particulate erosion resistance of the Al-deposited and vacuum- treated TBCs were investigated and compared with those of the as-sprayed TBC system.
2) By means of vacuum heat treatment, the columnar Al film of the Al-deposited TBC evolved into a loose surface-layer and a dense sub-layer with a thickness of 4 μm. The dense sub-layer mainly consisted of α-Al2O3 and Al3Zr which were in situ synthesized from the reaction of Al and ZrO2 during the vacuum treatment process, and the loose surface-layer only comprised α-Al2O3 due to the reaction of the reminder molten Al with O2.
3) A 7YSZ top layer with less porosity was obtained because of the pore filling by α-Al2O3 phases. The Al-deposited TBC after vacuum heat treatment thus exhibits better particulate erosion resistance than that of the as-sprayed one due to higher surface microhardness of the formed dense sub-layer.
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镀铝对7YSZ热障涂层冲刷性能的影响
张小锋1, 2，周克崧1, 2，董淑娟3，徐 伟2，宋进兵2，刘 敏2
1. 华南理工大学 材料科学与工程学院，广州 510640；
2. 广州有色金属研究院 新材料研究所，广州 510650；
3. IRTES-LERMPS Laboratory, de Technologie de Belfort-Montbéliard, Belfort Cedex 90010, France
摘 要：采用磁控溅射在大气等离子喷涂的ZrO2-7%Y2O3(7YSZ)热障涂层表面制备一层厚度约15 μm铝膜，对镀有铝膜的热障涂层样品在700、900 °C分别保温1 h和5 h进行真空热处理。采用场发射-扫描电子显微镜对经真空热处理前后镀铝热障涂层的微观结构进行表征。采用X射线衍射和透射电子显微镜对经真空热处理前后镀铝热障涂层的物相变化进行分析。对等离子喷涂的7YSZ原始热障涂层和镀铝真空热处理后的热障涂层进行室温粒子冲刷性能对比。结果表明：镀铝热障涂层经真空热处理后在涂层表面出现疏松表层和致密底层，其成分主要为α-Al2O3，其中致密底层来源于Al膜与ZrO2的高温原位反应且该层有较高的显微硬度。此外，冲刷实验后发现镀铝真空热处理后热障涂层的冲刷性能比无镀铝热障涂层有较大的提高。
关键词：热障涂层；ZrO2-7%Y2O3 (7YSZ)；抗冲刷性；原位反应；α-Al2O3
(Edited by Xiang-qun LI)
Foundation item: Project (2012CB625100) supported by the National Basic Research Program of China; Project (2012AA03A512) supported by the National High-tech Research and Development Program of China
Corresponding author: Ke-song ZHOU; Tel: +86-20-37238263; Fax: +86-20-37238531; E-mail: kszhou2004@163.com
DOI: 10.1016/S1003-6326(15)63879-1
