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Microstructure and oxidation behavior of Si–MoSi2 functionally graded coating on Mo substrate
Ceramics International 43 (2017) 6250–6256
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Microstructure and oxidation behavior of Si–MoSi2 functionally graded coating on Mo substrate
MARK
⁎
Yingyi Zhanga, , Yungang Lib, Chenguang Baia a b
College of Material Science and Engineering, Chongqing University, Chongqing 400030, PR China College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063009, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Molybdenum MoSi2 Siliconizing Oxidation Functionally graded coating
The Si–MoSi2 functionally graded coating on Mo substrate consisting of a Si–MoSi2 layer (2.5 µm), a MoSi2 layer (18 µm) and a Mo–Mo5Si3–Mo3Si layer (2–4 µm) was prepared by a liquid phase siliconizing method. The siliconized coating has a dense layered structure and no micro-cracks and holes. The Si element mainly enriches on the surface with the highest content of about 50 wt%, and inhibits the formation of Mo5Si3 and volatile MoO3 and improves the high-temperature oxidation resistance of the coating. The mass gain of Si-MoSi2 coating is only 0.17 wt% after oxidized at 1600 ℃ for 70 h. The Si–MoSi2 functionally graded coating exhibits better high temperature oxidation resistance than pure MoSi2 coating.
1. Introduction Molybdenum (Mo) with a high melting point of 2617 °C, good electrical and thermal conductivity and mechanical properties, is widely applied in electronics, metallurgy, nuclear industry, aerospace [1,2], and used as a emitter of space thermionic reactor, missile nozzle, satellite rocket boosters, aero engine blades and high-temperature electrodes [3,4]. However, a poor oxidation resistance of molybdenum seriously restricts its applications at high temperatures. This oxidation behavior has been ascribed to the formation of several volatile oxide species such as MoO3(g), MoO2(g), etc [5,6]. Therefore, the use of this metal at high temperatures necessitates the application of a suitable oxidation resistant coating. Such a coating aims at forming a self-healing protective oxide layer on the surface at high temperatures, prevents the direct contact of atmospheric oxygen with the coated substrate [7]. At present, several techniques such as electroless deposition [8], chemical vapor deposition (CVD) [9,10] and fused salt electrolysis [11] have adopted for applying silicide coating on Mo and Mo-based composites. Typically, a silicide coating on Mo is constituted of molybdenum disilicide (MoSi2) phase, which has long been known as an attractive coating material for protecting Mo and Mo-based alloys in an oxidative atmosphere at high temperatures [12]. MoSi2 phase has a high melting point (2020 ℃), low density(6.24 g/cm3) and excellent high-temperature oxidation resistance due to the formation of an adherent and continuous SiO2 film on the surface, which protects the substrate material from further oxidation [13,14]. However, due to the mismatch of coefficient of thermal expansion (CTE) between MoSi2
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coating (8.5×10−6/K) and Mo substrate (5.8×10−6/K), cracks will be formed along the grain boundaries of MoSi2 coating perpendicular to the Mo substrate during cooling down from the service temperature to room temperature, leads to the coating failure [15]. Moreover, the silicon can be easily diffused into the Mo substrate at the intermediate temperatures, results in a continuously loss of silicon content and the formation of low oxidation resistant sub-silicides (i.e., Mo5Si3 and Mo3Si) at the coating surface [16]. However, using liquid-phase siliconizing method to prepare Si– MoSi2 functionally graded coating on Mo substrate has not been reported. This method has many advantages, such as having a faster diffusion rate of silicon, high silicon content, good coating quality, simple operation process, etc [17–19]. A large amount of researches have been focused on siliconizing or aluminizing coatings on titanium alloys substrates. The researches show that the liquid-phase siliconizing method can remarkably improve the high oxidation resistance of the TiAl-based alloy [20–22], high silicon content has a marked effect on the growth rate of coating, using the melt with a higher silicon content results in formation of the coating with almost a double thickness of traditional coating [23]. In this study, the Si–MoSi2 functionally graded coating on Mo substrate was prepared by a liquid phase siliconizing method, which was expected to eliminate the thermal expansion mismatch between coating and Mo substrate, and to protect Mo substrate at high temperature in air for long-term application. Moreover, the microstructure and oxidation behavior of the Si–MoSi2 functionally graded coating were also investigated.
Corresponding author. Tel.: +86 18580628595. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2017.02.024 Received 22 November 2016; Received in revised form 16 January 2017; Accepted 5 February 2017 Available online 07 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 43 (2017) 6250–6256
Y. Zhang et al.
Fig. 1. The block polysilicon (a) XRD pattern of Mo substrate (b).
2. Experimental procedure 2.1. Materials The substrates were cut into 50 mm×20 mm×2 mm from a block of pure molybdenum substrate with a purity of more than 99.95 wt%. These specimens were hand-polished up to 2 mm by using 2000 grit SiC paper, then ultrasonically cleaned in alcohol and dried at 373 K for 20 min. The block polysilicon with a purity of 7 N was used as a source of siliconizing in this work (shown in Fig. 1(a)). The XRD pattern and crystallographic pole figures of Mo substrate are shown in Fig. 1(b) and Fig. 2, respectively. It can be seen that molybdenum substrate has strong preferred orientation on the (110), (200) and (211) crystal faces. Fig. 3. Experimental installation drawing. (1) Electrical furnace; (2) Polysilicon melt; (3) and (4) Thermocouple; (5) Molybdenum wire; (6) Alumina ceramic tube; (7) Graphite sleeve; (8) Corundum crucible; (9) Mo substrate.
2.2. Coating preparation 2.3. Oxidation test The Si–MoSi2 functionally graded coating on Mo substrate was prepared by siliconizing method. The experiment instrument is shown in Fig. 3. As shown in Fig. 3, the polysilicon was placed in an Al2O3 crucible, then smelted inside a vertical alumina tube furnace. The Si– MoSi2 functionally graded coating was prepared as follows. Firstly, pure Mo substrates were inserted into the molten silicon melt by means of Mo wire (φ2 mm×1000 mm). Secondly, the siliconizing process was carried out at a reaction temperature of 1460 °C for 20 min in Ar atmosphere. Finally, the Si–MoSi2 functionally graded coating was obtained by siliconizing method. Sample:
Mo
POLE FIGURE:
# Mo 110
Sample:
To investigate the oxidation resistance of the Si–MoSi2 functionally graded coating, the isothermal oxidation tests of the coated samples were carried out at 1473–1873 K in an electrical furnace in air atmosphere. During isothermal oxidation test, these coated samples were weighed by electronic balance with a sensitivity of ± 0.1 mg in order to determine the extent of the reactions. After the oxidation, the oxidized samples were taken out from the furnace, then cooled to room temperature. For comparison, some pure MoSi2 samples were also Mo
POLE FIGURE:
# Mo
Sample:
200
Fig. 2. The crystallographic pole figures of (110), (200) and (211) crystal faces.
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Mo
POLE FIGURE:
# Mo 211
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Y. Zhang et al.
Therefore, the intermediate layer can be identified as columnar crystals of MoSi2. The average diameter of columnar MoSi2 grains measured using image analyzer is approximately 7.5 µm. The transitional layer (2–4 µm) is observed between MoSi2 coating and Mo substrate, which is composed of Mo and low silicon silicides (i.e., Mo5Si3 and Mo3Si) (spot 4 & Fig. 4b).
oxidized under the same conditions. Weight change percentages (ΔW %) of the samples were calculated by the following equation:
ΔW % =
m t−mo × 100% mo
(1)
where mt and mo represent time t and initial weights of the samples, respectively.
3.2. Phase composition of the coating
2.4. Characterization
The surface XRD pattern of Si–MoSi2 functionally graded coating was obtained at a siliconizing temperature of 1460 °C for 20 min, as shown in Fig. 5(a). The XRD analysis result shows that the coating surface is composed of Si and MoSi2, which is due to the Si diffused through the pure molybdenum substrate. Furthermore, MoSi2 crystals have a strong preferred orientation on the (110), (200) and (211) crystal faces. Therefore, the MoSi2 columnar crystals successfully grow on (110), (200) and (211) crystal faces of molybdenum substrate. The cross-section elements distribution of Mo and Si is shown in Fig. 5(b). It can be seen that the siliconized coating is about 30 µm, the distribution of Si and Mo elements shows a three stages gradient change regulation. For the surface layer, the surface silicon content is very high and the highest silicon content is about 50 wt%. Therefore, the surface layer is a silicon-rich layer, which is attributed that the silicon exhibits excellent wetting with MoSi2 at temperatures above its melting point (1410 ℃), and it can easily infiltrate into the MoSi2 matrix and fill in the pores by capillary action. In addition, the silicon content decreases sharply with increasing diffusion depth, which means that the surface layer is very thin (2.5 µm). For the intermediate layer, the thickness of silicide coating is about 18 µm, which is thicker than that of the surface layer. It can be seen that the silicon content is almost no change with increasing diffusion depth, the silicon content is about 30–34 wt% and Si to Mo atomic ratio is about 2:1. Therefore, the intermediate layer is composed of MoSi2. For the transitional layer (9 µm), the silicon content decreases sharply with increasing diffusion depth, and the Si to Mo atomic ratio decreases from 2:1 to 0. Therefore, the transitional layer is a low silicon content layer, which should be consist of Mo and low silicon silicides (i.e., Mo5Si3 and Mo3Si).
Scanning electron microscopy (SEM, S3400-N) was used to study the microstructural details and the surface morphology of the coating. Chemical composition analysis of the coating was carried out by using an energy dispersive spectroscopy (EDS) unit attached to SEM. The cross-section element distribution of the coating was tested by glow discharge spectrum (GDS). X-ray diffraction (XRD) method was used to determine the phase composition of the coating. In order to better explain the oxidation mechanism of Si–MoSi2 functionally graded coating, the Gibbs free energy changes (ΔGθ) with temperature were calculated by FactSage 6.2, and the weight changes of samples were also investigated. 3. Results and discussion 3.1. Microstructure of the coating The SEM images of the coating and the corresponding EDS patterns are shown in Fig. 4. The EDS results are shown in Table 1. The SEM results that the silicide coating is about 30 µm thickness and has a dense layered structure and no micro-cracks and holes, which is the typical characteristic of liquid phase siliconizing coatings. The SEMEDS results show that the coating surface has a high silicon content with the Si concentration of 47.41 wt%, the surface coating is composed of Si (spot 1 & Fig. 4a) and MoSi2 (spot 2 & Fig. 4a). The intermediate layer (18 µm) exhibits a typical columnar microstructure perpendicular to the Mo substrate with the Si concentration of 36.67 wt %, and the Si to Mo atomic ratio is about 2:1 (spot 3 & Fig. 4b).
Fig. 4. SEM images of surface (a) and cross-section (b) and the corresponding EDS patterns (c).
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Table 1 SEM-EDS analysis results. Microstructure
Surface
Intermediate layer transitional layer Substrate
Testing Spots
Fig. Fig. Fig. Fig. Fig. Fig.
2(a)-Spot 1 2(a)-Spot 2 2(a)-Area 3 2(b)-Spot 4 2(b)-Spot 5 2(b)-Spot 6
Weight (%)
Atom (%)
Si
Mo
Si
Mo
99.18 36.45 47.41 36.67 1.69 –
0.82 63.55 52.59 63.33 98.31 100
99.18 66.28 75.55 66.49 5.56 –
0.82 33.72 24.45 33.51 96.44 100
Phase composition
Grain size (μm)
Si MoSi2 Si+MoSi2 MoSi2 Mo5Si3+Mo3Si Mo
– 5–10 – 20 3–4 –
Fig. 5. Surface XRD pattern (a) and cross-section element distribution (b) of the coating obtained at 1460 °C for 20 min.
3.3. High temperature oxidation behavior
MoO3 as indicated by reactions (1) and (2). If the oxygen is insufficient during the oxidation, the oxidation behavior of MoSi2 coating becomes complicated, and the selective oxidation of silicon occurs as given by reactions (3) and (4). The reaction taking priority depends on the oxidation conditions and could be determined by the thermodynamic calculation. Reaction (3) is preferred in an oxygen-lean environment as shown in Fig. 9.
The variation of Gibbs free energies of some reactions related to the oxidation of Si–MoSi2 coating with oxidation temperature is shown in Fig. 6. When the Si–MoSi2 coating is exposed to sufficient oxygen at high temperatures, its oxidation behavior is very complex, molybdenum disilicide and silicon will be oxidized to stable SiO2 and volatile
Si + O2(g) = SiO2 ΔG θ = − 858.944 + 0.1754T
(1)
2/7MoSi2 + O2(g) = 2/7MoO3(g) + 4/7SiO2 ΔG θ = − 547.918 + 0.1146T (2)
5/7MoSi2 + O2(g) = 1/7Mo5Si3(g) + SiO2 ΔG θ = − 808.260 + 0.1672T (3)
Fig. 6. Variation of Gibbs free energies of some reactions related to the oxidation of MoSi2 coating with oxidation temperature.
Fig. 7. XRD patterns of coating surfaces exposed to the atmosphere for 2 h at different temperatures.
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Fig. 8. Surface microstructures of the oxidized coating at different temperatures in air atmosphere for 2 h. (a) 1200 ℃, (b) 1400 ℃, (c) 1600 ℃, and (d) EDS patterns.
3/5MoSi2 + O2(g) = 1/5Mo3Si(g) + SiO2 ΔG θ = − 800.038 + 0.1729T
at the reacting interface, the volatile MoO3(g) will be produced through reactions (2), (5), and (6). Fig. 7 shows the surface XRD patterns of the coated samples after oxidized at different temperatures for 2 h. It can be concluded that when the oxidation temperature is 1200 ℃, the reactions (1), (3), and (4) are the predominant reactions, whereas the reactions (2), (5), and (6) are negative. The oxidation products are mainly consist of SiO2 and small amounts of Mo5Si3. When the oxidation temperatures range from 1400 ℃ to 1600 ℃, the MoSi2 and Mo5Si3 phases are further oxidized to SiO2 and volatile MoO3, and the volatile MoO3 completely evaporates from the coating surface. Therefore, the oxidation products are mainly consist of SiO2. Since the oxide scale is very thin, the peaks of MoSi2 beneath the oxide scale appear in the XRD patterns. After the isothermal oxidation, the surface microstructures of the oxidized coating are shown in Fig. 8. It can be seen that the coating surface is very rough with small amount pores when the temperature is 1200 ℃ (Fig. 8(a)), the oxygen will be delivered through these pores and oxidize MoSi2 phase with a lower oxygen partial pressure. Therefore, the oxidation reaction preferable at this condition becomes Mo5Si3 and SiO2 formations in reaction (3). On the contrary, when the oxidation temperatures are 1400 ℃ and 1600 ℃, the samples are
(4) The further oxidation of Mo5Si3 in the oxidation atmosphere can be expressed by reactions (5) and (6). Reaction (5) is preferred in an excessive oxygen environment, while the preferential oxidation of silicon occurs in an oxygen-lean environment as indicated by reaction (6). 2/21Mo5Si3 + O2(g) = 10/21MoO3(g) + 2/7SiO2 ΔG θ = − 374.356 + 0.0795T
(5) 2/11Mo5Si3 + O2(g) = 6/11MoO3(g) + 2/11SiO2 ΔG θ = − 318.717 + 0.0616T
(6) It is apparent that the oxidation reaction of MoSi2 strongly depends on the oxygen partial pressure at the reacting interface from the thermodynamic consideration. It can be seen that the reactions (1), (3) and (4) were the predominant reactions, whereas the reactions (2), (5), and (6) were negative at the given experimental temperature range. Therefore, the initial oxidation products are mainly composed of Mo5Si3 and SiO2. When the partial pressure of oxygen was high enough
Fig. 9. The oxidation curves of Mo substrate (a), pure MoSi2, and siliconized sample (b).
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Mo–MoSi2 functionally graded coating on Mo substrate shows an excellent oxidation resistance at high temperatures.
Table 2 Element composition of the coating after oxidized at different temperatures for 2 h. Oxidizing temperature (℃)
1200 1400 1600
Testing positions
Spot 1 Spot 2 Area 3 Area 4
Atom (%)
Phase composition
Si
Mo
O
36.29 24.21 20.86 24.91
9.26 1.48 0.87 –
66.54 72.20 78.27 75.09
4. Conclusions Si-MoSi2 functionally graded coating on Mo substrate consisting of a surface layer (Si–MoSi2 layer), an intermediate layer (MoSi2 layer) and a transition layer (Mo, Mo5Si3, and Mo3Si layer) was prepared by a liquid phase siliconizing technology. The coating has a dense layered structure and no micro-cracks and holes. The Si element mainly enriches on the surface at the highest content of about 50 wt%, and inhibits the formation of Mo5Si3 and volatile MoO3 and improves the high temperature oxidation resistance of MoSi2 coating. The mass gain of Si–MoSi2 coating is only 0.17 wt% after oxidized at 1600 ℃ for 70 h. Si–MoSi2 functionally graded coating exhibits better high temperature oxidation resistance than pure MoSi2 coating, which can effectively protect Mo substrate for long-term application above 1600 ℃.
SiO2+MoSi2+Mo5Si3 SiO2 SiO2 SiO2
completely coated by a continuous and dense film without micro-cracks and holes (Fig. 8(a) and (b)). In order to confirm the phase constitution of the oxidized sample, the EDS analysis is performed, and the EDS analysis results are shown in Table 2. It can be seen that the surface element compositions of the spot 1 and 2 are 9.26Mo–36.29Si– 66.54 O (at%), 1.48Mo–24.21Si–72.2 O (at%), respectively. The dark gray phase can be identified as SiO2, and the light gray phase can be identified as the mixtures of SiO2, MoSi2, and Mo5Si3. The surface element composition of the area 3 and 4 are 0.87Mo–20.86Si–78.27 O (at%), 24.91Si–75.09 O (at%), respectively. The smooth glassy film can be identified as SiO2, which prevents the inward diffusion of oxygen and the further oxidation of MoSi2. The oxidation process of Mo, MoSi2 and Mo–MoSi2 materials in air atmosphere was characterized by thermogravimetric analysis (TGA) system at the temperatures from room temperature to 1873 K. Fig. 9(a) shows the thermogravimetric curve of pure Mo substrate. It can be seen that the oxidation process of pure Mo was divided to three stages. During the initial stage (stage A), there is a slightly mass loss with the increasing temperature from 100 ℃ to 650 ℃, this is due to the contents of solid state molybdenum oxides are lower than the contents of the volatile intermediates (MoO2(g), MoO3(g), Mo3O6(g), Mo6O9(g), Mo4O12(g), Mo5O15(g)). For the stage B, there is a sharply mass gain with the increasing temperature from 650 ℃ to 840 ℃, this is due to the contents of solid state molybdenum oxides sharply increase and are much higher than the contents of the volatile intermediates. For the stage C, there is a severe mass loss with a lot of white smoke when the temperature is greater than 840 ℃, this is due to the contents of volatile MoO2(g) and MoO3(g) sharply increase and eventually lead to oxidation failure of the pure Mo [24,25]. In addition, the Si–MoSi2 coating was used to protect Mo substrate at high temperature in air for long-term application. For comparison, some pure MoSi2 samples were also oxidized under the same conditions. Fig. 9(b) shows the thermogravimetric curves of pure MoSi2 sample and siliconized sample at 1600 ℃ for 120 h in air atmosphere. For the pure MoSi2 sample, a rapid mass loss phenomenon is observed during the initial oxidation stage (Stage A), and then the mass loss becomes slow at the stage B. This is due to the higher oxygen partial pressure at the interface between oxide film and MoSi2 accelerates further formation of volatile MoO3. The volatilization of MoO3 prevents a dense film from forming, which provides a pathway for oxygen diffusion [26]. Therefore, it is thought that the loose oxidation film acts as a path for simultaneous oxidation of Mo and Si. The formation of the MoO3 whiskers causes volume expansion and leads to the oxidation film separating from the Mo substrate. However, for the Si-MoSi2 functionally graded coating, a relatively rapid mass gain phenomenon can be observed during the initial oxidation stage (Stage A), and then the weight of sample became stable (Stage B and C). The mass gain of siliconized sample is only 0.17 wt% after oxidized at 1600 ℃ for 70 h, which results from a continuous and dense of protective SiO2 scale forming on the coating surface. Because the silicon is oxidized to SiO2 easier than MoSi2, the oxidation of silicon effectively reduces the oxygen partial pressure of coating surface. Therefore, the reactions (2), (5), and (6) are prevented effectively. To some extent, the dense and continuous SiO2 film prevents Mo5Si3 from further oxidation. The
Acknowledgments This work was partially supported by the Special Fund for Basic Scientific Research in Colleges and Universities of the Central Business (0903005203413) and the National Natural Science Foundation Young Investigator Grant Program (51604049). References [1] F. Monteverde, The addition of SiC particles into a MoSi2-doped ZrB2 matrix: effects on densification, microstructure and thermo-physical properties, Chem. Phys. 113 (2009) 626–633. [2] S. Tang, J. Deng, S. Wang, W. Liu, K. Yang, Ablation behaviors of ultra-high temperature ceramic composites, Mater. Sci. Eng. A. 465 (2007) 1–7. [3] R.H. Zee, Z. Xiao, B.A. Chin, J. Liu, Processing of single crystals for high temperature applications, J. Mater. Process. Tech. 113 (2001) 75–80. [4] A.K. Vasudévan, J.J. Petrovic, A comparative overview of molybdenum disilicide composites, Mater. Sci. Eng. A 155 (1992) 1–17. [5] X.A. Fei, Y.R. Niu, H. Jia, L.P. Huang, X.B. Zheng, Oxidation behavior of Al2O3 reinforced MoSi2 composite coatings fabricated by vacuum plasma spraying, Ceram. Int. 36 (2010) 2235–2239. [6] L. Ingemarsson, K. Hellström, S. Canovic, T. Jonsson, M. Halvarsson, L.G. Johansson, J.E. Svensson, Oxidation behavior of a Mo(Si, Al)2 composite at 900–1600 °C in dry air, J. Mater. Sci. 48 (2013) 1511–1523. [7] M.Z. Alam, B. Venkataraman, B. Sarma, D.K. Das, MoSi2 coating on Mo substrate for short-term oxidation protection in air, J. Alloy. Compd. 487 (2009) 335–340. [8] M. Shoeib, M.A. Maamoun, Electroless deposition and structure of Ni–MoSi2 composite coatings, Tribotest 9 (2002) 49–56. [9] J.K. Yoon, J.K. Lee, J.Y. Byun, G.H. Kim, Y.H. Paik, Jae-Soo Kim, Effect of ammonia nitridation on the microstructure of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates, Surf. Coat. Technol. 160 (2002) 29–37. [10] J.K. Yoon, G.H. Kim, J.Y. Byun, J.K. Lee, Y.H. Paik, J.S. Kim, Formation of MoSi2– Si3N4 composite coating by reactive diffusion of Si on Mo substrate pretreated by ammonia nitridation, Scr. Mater. 47 (2002) 249–253. [11] R.O. Suzuki, M. Ishikawa, K. Ono, MoSi2 coating on molybdenum using molten salt, J. Alloy. Compd. 306 (2000) 285–291. [12] J.K. Yoon, G.H. Kim, J.Y. Byun, J.K. Lee, J.S. Kim, Formation of crack-free MoSi2/ α-Si3N4 composite coating on Mo substrate by ammonia nitridation of Mo5Si3 layer followed by chemical vapor deposition of Si, Surf. Coat. Tech. 165 (2003) 81–89. [13] H.A. Zhang, S.Y. Gu, Preparation and oxidation behavior of MoSi2–CrSi2–Si3N4 composite coating on Mo substrate, Int. J. Refract. Met. H. 41 (2013) 128–132. [14] Z.D. Liu, S.X. Hou, D.Y. Liu, L.P. Zhao, B. Li, J.J. Liu, An experimental study on synthesizing submicron MoSi2-based coatings using electrothermal explosion ultrahigh speed spraying method, Surf. Coat. Technol. 202 (2008) 2917–2921. [15] J.K. Yoon, G.H. Kim, J.H. Han, I.J. Shon, J.M. Doh, K.T. Hong, Low-temperature cyclic oxidation behavior of MoSi2/Si3N4 nanocomposite coating formed on Mo substrate at 773 K, Surf. Coat. Tech. 200 (2005) 2537–2546. [16] Y.T. Zhu, M. Stan, S.D. Conzone, D.P. Butt, Thermal oxidation kinetics of MoSi2based powders, J. Am. Ceram. Soc. 82 (1999) 2785–2790. [17] Y.Y. Zhang, Y.H. Qi, Y.G. Li, Z.S. Zou, The characters of Mo–MoSi2 functionally graded coating, High. Temp. Mat. PR-Isr. 33 (2014) 239–244. [18] Y.Y. Zhang, Y.G. Li, Y.H. Qi, X.F. Shi, Microstructure and phase transformation behavior of Mo–MoSi2 gradient material, RARE Metals. 34 (11) (2015) 808–813. [19] Y.Y. Zhang, Y.G. Li, X.F. Shi, Y.H. Qi, Effect of liquid-phase siliconizing process on silicon diffusion behavior in Mo matrix, HIGH TEMP MAT PR-Isr. 33 (2014) 1–6. [20] M. Goral, L. Swadzba, G. Moskal, M. Hetmanczyk, T. Tetsui, Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method, Intermetallics 17 (2009) 965–967.
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Microstructure and oxidation behavior of Si–MoSi2 functionally graded coating on Mo substrate
MARK
⁎
Yingyi Zhanga, , Yungang Lib, Chenguang Baia a b
College of Material Science and Engineering, Chongqing University, Chongqing 400030, PR China College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063009, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Molybdenum MoSi2 Siliconizing Oxidation Functionally graded coating
The Si–MoSi2 functionally graded coating on Mo substrate consisting of a Si–MoSi2 layer (2.5 µm), a MoSi2 layer (18 µm) and a Mo–Mo5Si3–Mo3Si layer (2–4 µm) was prepared by a liquid phase siliconizing method. The siliconized coating has a dense layered structure and no micro-cracks and holes. The Si element mainly enriches on the surface with the highest content of about 50 wt%, and inhibits the formation of Mo5Si3 and volatile MoO3 and improves the high-temperature oxidation resistance of the coating. The mass gain of Si-MoSi2 coating is only 0.17 wt% after oxidized at 1600 ℃ for 70 h. The Si–MoSi2 functionally graded coating exhibits better high temperature oxidation resistance than pure MoSi2 coating.
1. Introduction Molybdenum (Mo) with a high melting point of 2617 °C, good electrical and thermal conductivity and mechanical properties, is widely applied in electronics, metallurgy, nuclear industry, aerospace [1,2], and used as a emitter of space thermionic reactor, missile nozzle, satellite rocket boosters, aero engine blades and high-temperature electrodes [3,4]. However, a poor oxidation resistance of molybdenum seriously restricts its applications at high temperatures. This oxidation behavior has been ascribed to the formation of several volatile oxide species such as MoO3(g), MoO2(g), etc [5,6]. Therefore, the use of this metal at high temperatures necessitates the application of a suitable oxidation resistant coating. Such a coating aims at forming a self-healing protective oxide layer on the surface at high temperatures, prevents the direct contact of atmospheric oxygen with the coated substrate [7]. At present, several techniques such as electroless deposition [8], chemical vapor deposition (CVD) [9,10] and fused salt electrolysis [11] have adopted for applying silicide coating on Mo and Mo-based composites. Typically, a silicide coating on Mo is constituted of molybdenum disilicide (MoSi2) phase, which has long been known as an attractive coating material for protecting Mo and Mo-based alloys in an oxidative atmosphere at high temperatures [12]. MoSi2 phase has a high melting point (2020 ℃), low density(6.24 g/cm3) and excellent high-temperature oxidation resistance due to the formation of an adherent and continuous SiO2 film on the surface, which protects the substrate material from further oxidation [13,14]. However, due to the mismatch of coefficient of thermal expansion (CTE) between MoSi2
⁎
coating (8.5×10−6/K) and Mo substrate (5.8×10−6/K), cracks will be formed along the grain boundaries of MoSi2 coating perpendicular to the Mo substrate during cooling down from the service temperature to room temperature, leads to the coating failure [15]. Moreover, the silicon can be easily diffused into the Mo substrate at the intermediate temperatures, results in a continuously loss of silicon content and the formation of low oxidation resistant sub-silicides (i.e., Mo5Si3 and Mo3Si) at the coating surface [16]. However, using liquid-phase siliconizing method to prepare Si– MoSi2 functionally graded coating on Mo substrate has not been reported. This method has many advantages, such as having a faster diffusion rate of silicon, high silicon content, good coating quality, simple operation process, etc [17–19]. A large amount of researches have been focused on siliconizing or aluminizing coatings on titanium alloys substrates. The researches show that the liquid-phase siliconizing method can remarkably improve the high oxidation resistance of the TiAl-based alloy [20–22], high silicon content has a marked effect on the growth rate of coating, using the melt with a higher silicon content results in formation of the coating with almost a double thickness of traditional coating [23]. In this study, the Si–MoSi2 functionally graded coating on Mo substrate was prepared by a liquid phase siliconizing method, which was expected to eliminate the thermal expansion mismatch between coating and Mo substrate, and to protect Mo substrate at high temperature in air for long-term application. Moreover, the microstructure and oxidation behavior of the Si–MoSi2 functionally graded coating were also investigated.
Corresponding author. Tel.: +86 18580628595. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2017.02.024 Received 22 November 2016; Received in revised form 16 January 2017; Accepted 5 February 2017 Available online 07 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. The block polysilicon (a) XRD pattern of Mo substrate (b).
2. Experimental procedure 2.1. Materials The substrates were cut into 50 mm×20 mm×2 mm from a block of pure molybdenum substrate with a purity of more than 99.95 wt%. These specimens were hand-polished up to 2 mm by using 2000 grit SiC paper, then ultrasonically cleaned in alcohol and dried at 373 K for 20 min. The block polysilicon with a purity of 7 N was used as a source of siliconizing in this work (shown in Fig. 1(a)). The XRD pattern and crystallographic pole figures of Mo substrate are shown in Fig. 1(b) and Fig. 2, respectively. It can be seen that molybdenum substrate has strong preferred orientation on the (110), (200) and (211) crystal faces. Fig. 3. Experimental installation drawing. (1) Electrical furnace; (2) Polysilicon melt; (3) and (4) Thermocouple; (5) Molybdenum wire; (6) Alumina ceramic tube; (7) Graphite sleeve; (8) Corundum crucible; (9) Mo substrate.
2.2. Coating preparation 2.3. Oxidation test The Si–MoSi2 functionally graded coating on Mo substrate was prepared by siliconizing method. The experiment instrument is shown in Fig. 3. As shown in Fig. 3, the polysilicon was placed in an Al2O3 crucible, then smelted inside a vertical alumina tube furnace. The Si– MoSi2 functionally graded coating was prepared as follows. Firstly, pure Mo substrates were inserted into the molten silicon melt by means of Mo wire (φ2 mm×1000 mm). Secondly, the siliconizing process was carried out at a reaction temperature of 1460 °C for 20 min in Ar atmosphere. Finally, the Si–MoSi2 functionally graded coating was obtained by siliconizing method. Sample:
Mo
POLE FIGURE:
# Mo 110
Sample:
To investigate the oxidation resistance of the Si–MoSi2 functionally graded coating, the isothermal oxidation tests of the coated samples were carried out at 1473–1873 K in an electrical furnace in air atmosphere. During isothermal oxidation test, these coated samples were weighed by electronic balance with a sensitivity of ± 0.1 mg in order to determine the extent of the reactions. After the oxidation, the oxidized samples were taken out from the furnace, then cooled to room temperature. For comparison, some pure MoSi2 samples were also Mo
POLE FIGURE:
# Mo
Sample:
200
Fig. 2. The crystallographic pole figures of (110), (200) and (211) crystal faces.
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Mo
POLE FIGURE:
# Mo 211
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Therefore, the intermediate layer can be identified as columnar crystals of MoSi2. The average diameter of columnar MoSi2 grains measured using image analyzer is approximately 7.5 µm. The transitional layer (2–4 µm) is observed between MoSi2 coating and Mo substrate, which is composed of Mo and low silicon silicides (i.e., Mo5Si3 and Mo3Si) (spot 4 & Fig. 4b).
oxidized under the same conditions. Weight change percentages (ΔW %) of the samples were calculated by the following equation:
ΔW % =
m t−mo × 100% mo
(1)
where mt and mo represent time t and initial weights of the samples, respectively.
3.2. Phase composition of the coating
2.4. Characterization
The surface XRD pattern of Si–MoSi2 functionally graded coating was obtained at a siliconizing temperature of 1460 °C for 20 min, as shown in Fig. 5(a). The XRD analysis result shows that the coating surface is composed of Si and MoSi2, which is due to the Si diffused through the pure molybdenum substrate. Furthermore, MoSi2 crystals have a strong preferred orientation on the (110), (200) and (211) crystal faces. Therefore, the MoSi2 columnar crystals successfully grow on (110), (200) and (211) crystal faces of molybdenum substrate. The cross-section elements distribution of Mo and Si is shown in Fig. 5(b). It can be seen that the siliconized coating is about 30 µm, the distribution of Si and Mo elements shows a three stages gradient change regulation. For the surface layer, the surface silicon content is very high and the highest silicon content is about 50 wt%. Therefore, the surface layer is a silicon-rich layer, which is attributed that the silicon exhibits excellent wetting with MoSi2 at temperatures above its melting point (1410 ℃), and it can easily infiltrate into the MoSi2 matrix and fill in the pores by capillary action. In addition, the silicon content decreases sharply with increasing diffusion depth, which means that the surface layer is very thin (2.5 µm). For the intermediate layer, the thickness of silicide coating is about 18 µm, which is thicker than that of the surface layer. It can be seen that the silicon content is almost no change with increasing diffusion depth, the silicon content is about 30–34 wt% and Si to Mo atomic ratio is about 2:1. Therefore, the intermediate layer is composed of MoSi2. For the transitional layer (9 µm), the silicon content decreases sharply with increasing diffusion depth, and the Si to Mo atomic ratio decreases from 2:1 to 0. Therefore, the transitional layer is a low silicon content layer, which should be consist of Mo and low silicon silicides (i.e., Mo5Si3 and Mo3Si).
Scanning electron microscopy (SEM, S3400-N) was used to study the microstructural details and the surface morphology of the coating. Chemical composition analysis of the coating was carried out by using an energy dispersive spectroscopy (EDS) unit attached to SEM. The cross-section element distribution of the coating was tested by glow discharge spectrum (GDS). X-ray diffraction (XRD) method was used to determine the phase composition of the coating. In order to better explain the oxidation mechanism of Si–MoSi2 functionally graded coating, the Gibbs free energy changes (ΔGθ) with temperature were calculated by FactSage 6.2, and the weight changes of samples were also investigated. 3. Results and discussion 3.1. Microstructure of the coating The SEM images of the coating and the corresponding EDS patterns are shown in Fig. 4. The EDS results are shown in Table 1. The SEM results that the silicide coating is about 30 µm thickness and has a dense layered structure and no micro-cracks and holes, which is the typical characteristic of liquid phase siliconizing coatings. The SEMEDS results show that the coating surface has a high silicon content with the Si concentration of 47.41 wt%, the surface coating is composed of Si (spot 1 & Fig. 4a) and MoSi2 (spot 2 & Fig. 4a). The intermediate layer (18 µm) exhibits a typical columnar microstructure perpendicular to the Mo substrate with the Si concentration of 36.67 wt %, and the Si to Mo atomic ratio is about 2:1 (spot 3 & Fig. 4b).
Fig. 4. SEM images of surface (a) and cross-section (b) and the corresponding EDS patterns (c).
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Table 1 SEM-EDS analysis results. Microstructure
Surface
Intermediate layer transitional layer Substrate
Testing Spots
Fig. Fig. Fig. Fig. Fig. Fig.
2(a)-Spot 1 2(a)-Spot 2 2(a)-Area 3 2(b)-Spot 4 2(b)-Spot 5 2(b)-Spot 6
Weight (%)
Atom (%)
Si
Mo
Si
Mo
99.18 36.45 47.41 36.67 1.69 –
0.82 63.55 52.59 63.33 98.31 100
99.18 66.28 75.55 66.49 5.56 –
0.82 33.72 24.45 33.51 96.44 100
Phase composition
Grain size (μm)
Si MoSi2 Si+MoSi2 MoSi2 Mo5Si3+Mo3Si Mo
– 5–10 – 20 3–4 –
Fig. 5. Surface XRD pattern (a) and cross-section element distribution (b) of the coating obtained at 1460 °C for 20 min.
3.3. High temperature oxidation behavior
MoO3 as indicated by reactions (1) and (2). If the oxygen is insufficient during the oxidation, the oxidation behavior of MoSi2 coating becomes complicated, and the selective oxidation of silicon occurs as given by reactions (3) and (4). The reaction taking priority depends on the oxidation conditions and could be determined by the thermodynamic calculation. Reaction (3) is preferred in an oxygen-lean environment as shown in Fig. 9.
The variation of Gibbs free energies of some reactions related to the oxidation of Si–MoSi2 coating with oxidation temperature is shown in Fig. 6. When the Si–MoSi2 coating is exposed to sufficient oxygen at high temperatures, its oxidation behavior is very complex, molybdenum disilicide and silicon will be oxidized to stable SiO2 and volatile
Si + O2(g) = SiO2 ΔG θ = − 858.944 + 0.1754T
(1)
2/7MoSi2 + O2(g) = 2/7MoO3(g) + 4/7SiO2 ΔG θ = − 547.918 + 0.1146T (2)
5/7MoSi2 + O2(g) = 1/7Mo5Si3(g) + SiO2 ΔG θ = − 808.260 + 0.1672T (3)
Fig. 6. Variation of Gibbs free energies of some reactions related to the oxidation of MoSi2 coating with oxidation temperature.
Fig. 7. XRD patterns of coating surfaces exposed to the atmosphere for 2 h at different temperatures.
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Fig. 8. Surface microstructures of the oxidized coating at different temperatures in air atmosphere for 2 h. (a) 1200 ℃, (b) 1400 ℃, (c) 1600 ℃, and (d) EDS patterns.
3/5MoSi2 + O2(g) = 1/5Mo3Si(g) + SiO2 ΔG θ = − 800.038 + 0.1729T
at the reacting interface, the volatile MoO3(g) will be produced through reactions (2), (5), and (6). Fig. 7 shows the surface XRD patterns of the coated samples after oxidized at different temperatures for 2 h. It can be concluded that when the oxidation temperature is 1200 ℃, the reactions (1), (3), and (4) are the predominant reactions, whereas the reactions (2), (5), and (6) are negative. The oxidation products are mainly consist of SiO2 and small amounts of Mo5Si3. When the oxidation temperatures range from 1400 ℃ to 1600 ℃, the MoSi2 and Mo5Si3 phases are further oxidized to SiO2 and volatile MoO3, and the volatile MoO3 completely evaporates from the coating surface. Therefore, the oxidation products are mainly consist of SiO2. Since the oxide scale is very thin, the peaks of MoSi2 beneath the oxide scale appear in the XRD patterns. After the isothermal oxidation, the surface microstructures of the oxidized coating are shown in Fig. 8. It can be seen that the coating surface is very rough with small amount pores when the temperature is 1200 ℃ (Fig. 8(a)), the oxygen will be delivered through these pores and oxidize MoSi2 phase with a lower oxygen partial pressure. Therefore, the oxidation reaction preferable at this condition becomes Mo5Si3 and SiO2 formations in reaction (3). On the contrary, when the oxidation temperatures are 1400 ℃ and 1600 ℃, the samples are
(4) The further oxidation of Mo5Si3 in the oxidation atmosphere can be expressed by reactions (5) and (6). Reaction (5) is preferred in an excessive oxygen environment, while the preferential oxidation of silicon occurs in an oxygen-lean environment as indicated by reaction (6). 2/21Mo5Si3 + O2(g) = 10/21MoO3(g) + 2/7SiO2 ΔG θ = − 374.356 + 0.0795T
(5) 2/11Mo5Si3 + O2(g) = 6/11MoO3(g) + 2/11SiO2 ΔG θ = − 318.717 + 0.0616T
(6) It is apparent that the oxidation reaction of MoSi2 strongly depends on the oxygen partial pressure at the reacting interface from the thermodynamic consideration. It can be seen that the reactions (1), (3) and (4) were the predominant reactions, whereas the reactions (2), (5), and (6) were negative at the given experimental temperature range. Therefore, the initial oxidation products are mainly composed of Mo5Si3 and SiO2. When the partial pressure of oxygen was high enough
Fig. 9. The oxidation curves of Mo substrate (a), pure MoSi2, and siliconized sample (b).
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Mo–MoSi2 functionally graded coating on Mo substrate shows an excellent oxidation resistance at high temperatures.
Table 2 Element composition of the coating after oxidized at different temperatures for 2 h. Oxidizing temperature (℃)
1200 1400 1600
Testing positions
Spot 1 Spot 2 Area 3 Area 4
Atom (%)
Phase composition
Si
Mo
O
36.29 24.21 20.86 24.91
9.26 1.48 0.87 –
66.54 72.20 78.27 75.09
4. Conclusions Si-MoSi2 functionally graded coating on Mo substrate consisting of a surface layer (Si–MoSi2 layer), an intermediate layer (MoSi2 layer) and a transition layer (Mo, Mo5Si3, and Mo3Si layer) was prepared by a liquid phase siliconizing technology. The coating has a dense layered structure and no micro-cracks and holes. The Si element mainly enriches on the surface at the highest content of about 50 wt%, and inhibits the formation of Mo5Si3 and volatile MoO3 and improves the high temperature oxidation resistance of MoSi2 coating. The mass gain of Si–MoSi2 coating is only 0.17 wt% after oxidized at 1600 ℃ for 70 h. Si–MoSi2 functionally graded coating exhibits better high temperature oxidation resistance than pure MoSi2 coating, which can effectively protect Mo substrate for long-term application above 1600 ℃.
SiO2+MoSi2+Mo5Si3 SiO2 SiO2 SiO2
completely coated by a continuous and dense film without micro-cracks and holes (Fig. 8(a) and (b)). In order to confirm the phase constitution of the oxidized sample, the EDS analysis is performed, and the EDS analysis results are shown in Table 2. It can be seen that the surface element compositions of the spot 1 and 2 are 9.26Mo–36.29Si– 66.54 O (at%), 1.48Mo–24.21Si–72.2 O (at%), respectively. The dark gray phase can be identified as SiO2, and the light gray phase can be identified as the mixtures of SiO2, MoSi2, and Mo5Si3. The surface element composition of the area 3 and 4 are 0.87Mo–20.86Si–78.27 O (at%), 24.91Si–75.09 O (at%), respectively. The smooth glassy film can be identified as SiO2, which prevents the inward diffusion of oxygen and the further oxidation of MoSi2. The oxidation process of Mo, MoSi2 and Mo–MoSi2 materials in air atmosphere was characterized by thermogravimetric analysis (TGA) system at the temperatures from room temperature to 1873 K. Fig. 9(a) shows the thermogravimetric curve of pure Mo substrate. It can be seen that the oxidation process of pure Mo was divided to three stages. During the initial stage (stage A), there is a slightly mass loss with the increasing temperature from 100 ℃ to 650 ℃, this is due to the contents of solid state molybdenum oxides are lower than the contents of the volatile intermediates (MoO2(g), MoO3(g), Mo3O6(g), Mo6O9(g), Mo4O12(g), Mo5O15(g)). For the stage B, there is a sharply mass gain with the increasing temperature from 650 ℃ to 840 ℃, this is due to the contents of solid state molybdenum oxides sharply increase and are much higher than the contents of the volatile intermediates. For the stage C, there is a severe mass loss with a lot of white smoke when the temperature is greater than 840 ℃, this is due to the contents of volatile MoO2(g) and MoO3(g) sharply increase and eventually lead to oxidation failure of the pure Mo [24,25]. In addition, the Si–MoSi2 coating was used to protect Mo substrate at high temperature in air for long-term application. For comparison, some pure MoSi2 samples were also oxidized under the same conditions. Fig. 9(b) shows the thermogravimetric curves of pure MoSi2 sample and siliconized sample at 1600 ℃ for 120 h in air atmosphere. For the pure MoSi2 sample, a rapid mass loss phenomenon is observed during the initial oxidation stage (Stage A), and then the mass loss becomes slow at the stage B. This is due to the higher oxygen partial pressure at the interface between oxide film and MoSi2 accelerates further formation of volatile MoO3. The volatilization of MoO3 prevents a dense film from forming, which provides a pathway for oxygen diffusion [26]. Therefore, it is thought that the loose oxidation film acts as a path for simultaneous oxidation of Mo and Si. The formation of the MoO3 whiskers causes volume expansion and leads to the oxidation film separating from the Mo substrate. However, for the Si-MoSi2 functionally graded coating, a relatively rapid mass gain phenomenon can be observed during the initial oxidation stage (Stage A), and then the weight of sample became stable (Stage B and C). The mass gain of siliconized sample is only 0.17 wt% after oxidized at 1600 ℃ for 70 h, which results from a continuous and dense of protective SiO2 scale forming on the coating surface. Because the silicon is oxidized to SiO2 easier than MoSi2, the oxidation of silicon effectively reduces the oxygen partial pressure of coating surface. Therefore, the reactions (2), (5), and (6) are prevented effectively. To some extent, the dense and continuous SiO2 film prevents Mo5Si3 from further oxidation. The
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