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WSi2 compound coating on Nb-Ti-Si based alloy
Applied Surface Science 504 (2020) 144477
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Full Length Article
Isothermal oxidation and interdiffusion behavior of MoSi2/WSi2 compound coating on Nb-Ti-Si based alloy
T
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Gao Yue, Xiping Guo , Yanqiang Qiao, Fa Luo State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nb-Ti-Si based alloy MoSi2/WSi2 compound coating Diffusion barrier layer Oxidation behaviors
The oxidation and diffusion behaviors of MoSi2/WSi2 compound coating on the Nb-Ti-Si based alloy at 800, 1250 and 1350 °C have been investigated. The coating exhibited excellent oxidation resistance at 800 °C and 1250 °C, and the scale growth rates were about 0.335 μm2 h−1 and 0.939 μm2 h−1, respectively. At 800 oC, pesting oxidation was successfully suppressed and formation of Mo5Si3 was mainly resulted from the slow diffusion rate of Si atoms. Therefore, there was no Mo5Si3 at the scale/MoSi2 interface when oxidized at 1250 and 1350 °C. At 1250 °C, WSi2 layer showed strong restriction against outward diffusion of alloying element atoms. Thus, a pure SiO2-Al2O3 scale about 9.5 μm thick was obtained after oxidation for 100 h. At 1350 °C, the WSi2 layer degraded into porous W5Si3 in situ within a short duration and the outward diffusion was increased. As a result, oxides like Nb2O5, YNbO4 and AlNbO4 formed in the thick scale.
1. Introduction Moderate interfacial diffusion is considered to improve the boundary bonding between oxidation resistance coating and super alloy at high temperatures. Nevertheless, the accelerated interdiffusion is undesirable for it may cause loss of key elements (e.g. Al, Si) and then microstructure degradation of the coating, weakening its anti-oxidation property and decreasing service life consequently. Furthermore, it can also result in permanent jeopardizing of the mechanical property of the substrate [1,2]. Hence, it is necessary to develop a barrier layer at the interfacial region to suppress the intense elemental diffusion and prolong the service life of the integrate system [3–5]. Noble metals and their alloys have been selected as diffusion barrier in the coating system for their high melting point, good stability at high temperatures and favorable compatibility. Ru-based layer (RuNiAl) is fabricated between NiAl or NiAlDy coating and Ni-based super alloy and it could slow down the inward diffusion of Al from the coating and outward diffusion of alloying elements effectively, and then suppressed the formation of TCP phases and second reaction zone at high temperatures [6–8]. In addition, Re(W)-Cr-Ni barrier layer has also been synthesized on Ni and Nb based alloy against inward diffusion of Al derived from the outer NiAl coating. The σ-Re-Cr-Ni phase exhibits impressive diffusion suppression ability for both inward diffusion of Al and outward diffusion of alloying elements at the NiAl/super alloy interface [9–12]. However, the imitation of RuNiAl or Re(W)-Cr-Ni layer
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seems hard to succeed as the diffusion barrier at the MoSi2/Nb interface due to the higher application temperature and high reactivity of Si with the metallic materials [13,14]. Besides, the high expense and complicated preparation may also limit their general application in the aerospace field. The inert oxides barrier like Y2O3, ZrO2 and Al2O3 shows perfect suppression ability to the inward diffusion of Si at elevated temperatures [15,16]. However, spalling and rupture are inevitable while suffered intense thermal fluctuation due to the poor interfacial bonding and the great thermal expansion difference. Consequently, materials with admirable diffusion suppression ability against Si, good interfacial compatibility and close coefficient of thermal expansion (CTE) relative to both Nb-Si based alloys and MoSi2 coating are required [17]. It has been reported that oxidation resistance of MoSi2 coating could be improved by addition of WSi2 in terms of higher applied temperature and longer service life. Stability of the (Mo,W)Si2 composite layer on Nb system is more than twice of the MoSi2-Nb system, and 15–18 times of the MoSi2-Mo system [18]. The improvement is basically due to the high melting point of WSi2, enhanced mechanical property of (Mo,W) Si2 composite [19], slow degradation of WSi2 to W5Si3 and excellent oxidation resistance of WSi2 at ultra-high temperatures [20]. Transition of WSi2 to W5Si3 is on average four times slower than the transition rate for MoSi2 to Mo5Si3 according to the study of Glushko [20]. Moreover, Dayananda reported that the generated interdiffusion layer in W/MoSi2 couple was obvious thinner than that of Mo/MoSi2 and Nb/MoSi2
Corresponding author. E-mail address: [email protected] (X. Guo).
https://doi.org/10.1016/j.apsusc.2019.144477 Received 23 July 2019; Received in revised form 5 October 2019; Accepted 21 October 2019 Available online 31 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 504 (2020) 144477
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Table 1 Experimental parameters employed during the sputtering. Process steps
First deposition of W film Second deposition of Mo film
Working pressure
Sputtering power
Sputtering time
0.9 pa 0.7 pa
150 W 130 W
1h 2h
Table 2 Chemical composition of the MoSi2/WSi2 compound coating (by EPMA analysis). Layer MoSi2 WSi2
Composition (at. %) B Si Y 5.26 64.43 0.07 — 67.03 —
Mo 29.50 0.23
W — 32.35
Nb 0.01 0.09
Ti 0.41 0.14
Cr 0.02 0.12
Al 0.26 —
Hf 0.04 0.04
1350 °C were investigated. Moreover, the structure evolution of the compound coating, the oxidation mechanism and the failure at 1350 °C of this compound coating has been investigated too.
couple [21]. This observation suggests the slower reaction between W and MoSi2 and the hindering effect of W on the diffusion of Si. Bhattacharyya have calculated the crystal bonding energy of MoSi2 and WSi2 and it reveals that W-Si bound possesses higher bonding energy in comparison of Mo-Si bound (2058.1 kJ/mol and 1847.8 kJ/mol, respectively), indicating more difficult of Si atom diffusion in WSi2 layer than in MoSi2 layer [22,23]. Another advantage of WSi2 is its similar analogous crystal structure and lattice parameter with MoSi2 [23,24], which are helpful to improve the compatibility and combination between MoSi2 layer and WSi2 layer. Actually, the W atoms are able to substitute for Mo atoms in the lattice, made a complete solution and formation of (W, Mo)Si2 mixture [25]. In addition, the close CTEs of MoSi2, WSi2 and Nb-Ti-Si based alloy (8.4 × 10−6/oC, 8.4 × 10−6/oC and 8.5 × 10−6/oC, respectively) make it suitable to fabricate a duplex silicide coating assumed as MoSi2/WSi2 on the Nb-Ti-Si based alloy [26–28]. In this paper, this duplex structure has been successfully prepared on the Nb-Ti-Si based ultrahigh temperature alloy via magnetron sputtering (MS) technique and halide activated pack cementation (HAPC) treatment. Among the compound coating, MoSi2 layer is the major oxidation resistance coating and WSi2 layer is supposed to act as the diffusion barrier and buffering layer. Isothermal oxidation and interdiffusion behaviors of this compound coating at 800 °C, 1250 °C and
2. Experimental The substrates were cut into 8 × 8 × 3 mm cubes from a Nb-Ti-Si based ultrahigh temperature alloy block with a nominal composition of Nb-20Ti-16Si-6Cr-5Hf-4Al (at.%), and they were abraded with grit SiC papers. MS was applied to deposit W and Mo films on the surface of substrate successively through JPG450 sputtering system. Prior to sputtering, the abraded specimens were activated with mixture acid, composing of distilled water, HNO3, HF and H2SO4 with the proportion of 4:1:1:1, (vol. %, AR), and then were carefully washed with distilled water and in ultrasonic ethanol bath successively. The W and Mo target (with purity of 99.99%) plates of 3 mm in thickness and 60 mm in diameter were used and the target-substrate separation was kept 60 mm during the sputtering. The detailed sputtering parameters are listed in Table 1. HAPC was used to co-deposition of Si-B-Y onto the Mo/W complex films on the Nb-Ti-Si based alloy surface. The pack agent is consisting of 12Si-0.025B-5NaF-2.5Y2O3-Al2O3 (wt.%) and the pack cementation process was carried out at 1200 °C for 1 h. The short packing duration was applied in order to avoid the generation of NbSi2 layer for it may jeopardize the CTE match between the MoSi2/WSi2
(a)
(b) Mo layer
W layer
20 ȝP
Substrate
(c)
10 ȝP
(d) MoSi2 layer WSi2 layer Residual W
Substrate
20 ȝP
Fig. 1. Surface morphology (a), cross-sectional image (b) of the sputtered Mo/W film, (c) XRD pattern and (d) cross-sectional BSE image of the MoSi2/WSi2 compound coating on the substrate. 2
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Fig. 2. (a) Evolution of scale thickness and (b) linear plot of oxidation kinetics of MoSi2/WSi2 compound coating at 800, 1250 and 1350 °C, respectively.
film, which is resulted from the volume expansion in silication process [29]. The chemical composition of each layer was analyzed with EPMA and the results (Table 2) show distribution of B and Y in the MoSi2 layer. The WSi2 layer is quite compact while there are some micro pores in the MoSi2 layer. 3.2. Oxidation kinetics Oxidation kinetics of the MoSi2/WSi2 compound coating at 800, 1250 and 1350 °C are shown in Fig. 2 in terms of scale thickness evolution versus oxidation time. At 800 and 1250 °C, growth of the scale is restricted and the thickness is about 6 and 9.5 μm respectively after oxidation for 100 h. However, the scale grows faster at 1350 °C and it is about 19 μm thick after oxidation for 40 h. It can be viewed that growth of the scales follows parabolic laws. Remarkably, the approximate linear oxidation kinetics is detected at the initial oxidation stage (Fig. 2a). Fig. 2b lists the corresponding fitted linear equations and the growth rate constants are 0.335, 0.939 and 12.809 μm2 h−1 when oxidized at 800, 1250 and 1350 °C respectively.
Fig. 3. XRD patterns of the coating after oxidization at 800 °C for different time.
compound layer and Nb-Ti-Si based alloy [26]. After deposition, the specimen was cooled down in the furnace. The isothermal oxidation experiments were carried out at 800, 1250 and 1350 °C respectively in an open-ended tube electric furnace. Prior to oxidation, the coatings were grinded with used 1500 # grit SiC papers carefully to remove the residual pack agent as possible, then ultra-sonic cleaned in distilled water. The as-prepared and oxidized specimens were analyzed by X-ray diffraction (XRD, Panalytical X'Pert PRO, Cu Kα), scanning electron microscopy (SEM, TESCAN MIRA 3), energy dispersive X-ray spectroscopy (EDS, Inca X-sight) and electron probe micro analysis (EPMA, EPMA-1720, SHIMAZU) to investigate their constituent phases, microstructures and compositions.
3.3. Oxidation at 800 °C Fig. 3 shows the XRD patterns of the coating after oxidation at 800 °C for different hours. It can be viewed that the major oxidation products are SiO2, Mo5Si3 and B2O3·SiO2. Intensity of the diffraction peaks increase with the prolonging exposure time. Remarkably, MoSi2 shows the dominant diffraction peaks, indicating the outstanding oxidation resistance of the compound coating at 800 °C. Fig. 4 shows the surface morphology and cross-sectional BSE images of the specimen after oxidation at 800 °C for 5, 50 and 100 h respectively. No sign of pesting oxidation is observed at 800 °C. After oxidation for 5 h, a discontinuous SiO2 scale with dark gray contrast forms on the MoSi2 surface. As oxidation proceeds, the scale grows and MoSi2 layer is gradually covered. Thus, there is no bare MoSi2 after oxidation for 50 and 100 h. Nevertheless, bump and pores are detected and they may probably be caused by evaporation of MoO3 (Fig. 4b). The bump could be filled gradually and then a continuous scale with a few of pores form after oxidation for 100 h. The profile images indicate that there is no oxidation of the substrate due to protection of the coating. In addition, the thickness and compactness of the scale is improved with oxidation time and it shows perfect adhesion to the coating without cracks and spallation (Fig. 4a′-c′). As a result, oxidation of MoSi2 could be effectively restricted. A few particles with light gray contrast are observed at the scale/MoSi2 interface and the amount increases with exposure time. According to XRD and EDS analysis results (Table 3), these particles could be identified as Mo5Si3. Noticeably, pores and tubules are observed near the MoSi2/WSi2 interface and some tubules
3. Results 3.1. Characterization of the coating Surface morphology and cross-sectional BSE images of the deposited Mo/W duplex film are displayed in Fig. 1a–b. The layer is compact and free from cracks and holes, composing of densely packed micro particles. The Mo and W film shows good interfacial bonding and their thicknesses are estimated about 7.5 and 2 μm respectively. Nevertheless, cracks can be observed at the deposit/substrate interface, which are probably formed during the polishing process due to the weak adhesion. After HAPC, the MoSi2/WSi2 duplex coating has formed in situ. The XRD pattern shows dominant peaks of MoSi2 and a few weak peaks of Al2O3 (Fig. 1c). Remarkably, the residual W is observed at the bottom of the coating from Fig. 1b. The MoSi2/WSi2 compound layer is approximately 25 μm thick, about 2.5 times of the as-sputtered Mo/W 3
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(a)
Bare MoSi2
(c)
(b)
Pores
SiO2 Bump
50 ȝP
(aƍ)
(bƍ)
MoSi2
50 ȝP
50 ȝP
(cƍ)
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20 ȝP
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6-NbSi2
7-IDZ
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(cƍƍ)
1
2
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Mo5Si3 3
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4-W5Si3
10 ȝP
10 ȝP
10 ȝP
5
Fig. 4. Surface and cross-sectional BSE images of the compound coating after oxidation at 800 °C for 5 (a-a′′), 50 (b-b′′) and 100 h (c-c′′), respectively. Table 3 Chemical composition of the marked sites in Fig. 4 (by EDS analysis). Site Site Site Site Site Site Site Site
1 2 3 4 5 6 7
in in in in in in in
Fig. Fig. Fig. Fig. Fig. Fig. Fig.
Composition (at. %) O Mo 64.8 — 10.9 35.9 — 34.0 — 7.7 — 3.6 — — — 0.2
5c′′ 5c′′ 5c′′ 5c′′ 5c′′ 5c′ 5c′
Si 31.4 51.4 63.3 51.6 58.9 62.4 35.7
B — — — — — — —
W — — 0.5 36.9 33.8 — 0.5
Nb — — — — — 31.9 41.9
Ti 0.1 — 0.3 0.8 0.6 4.3 14.2
Cr — — — — — 0.5 4.9
Al 3.4 1.8 1.4 1.3 1.2 0.9 1.9
Al
O
B
Si
Y
W
Nb
Ti
Cr
Hf
15 ȝP Mo
Fig. 5. EPMA elemental maps of the coating after oxidation at 800 °C for 100 h. 4
Y 0.3 — — 0.6 0.7 — 0.6
Hf — — 0.5 1.1 1.2 — 0.1
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oxygen, which could result in their outward diffusion through the defects in the coating at this temperature, driven by the chemical potential. Low level distribution of O is observed at the coating/substrate interface, suggesting the diffusion of oxygen along the interfacial cracks. 3.4. Oxidation at 1250 °C Fig. 6 is the XRD patterns of the coating after oxidation at 1250 °C for different hours and it suggests that there is no Mo5Si3 formes at the interface. The oxidation products are SiO2, B2O3·SiO2 and Al2O3 mainly. Due to the efficient anti-oxidation property, MoSi2 still shows the dominant diffraction peaks after oxidation at 1250 °C for 100 h. Fig. 7 shows the surface morphology and cross-sectional BSE images of the specimen after oxidation at 1250 °C for 5, 50 and 100 h respectively. Likewise, the MoSi2 has not been covered completely after oxidation for 5 h (Fig. 7a). After that, the coating is covered gradually with the prolonging oxidation time. The scale is dense and there are some Al2O3 and Y2O3 particles observed (Fig. 7b). After oxidation for 100 h, the scale appears glass-like and lots of Y2O3 particles are distributed on its surface (Fig. 7c). The profile images show that the scales are about 2.1, 8.1 and 9.5 μm thick after oxidization for 5, 50 and 100 h, respectively (Fig. 7a′-c′). The limited growth of scale suggests the seriously restricted inward diffusion of oxygen and oxidation of coating. Mo5Si3 is absent at the scale/MoSi2 interface, which is accorded with the XRD results. Apart from some micro pores, no obvious degradation is observed for the WSi2 layer after oxidation for 5 and 50 h. Whereas, part of the WSi2 layer degrades after oxidation for 100 h. A porous W5Si3 layer is observed between the residual WSi2 layer and substrate,
Fig. 6. XRD patterns of the coating after oxidation at 1250 °C for different time.
stretch across the MoSi2 layer. A few pores and white contrast particles are observed in the WSi2 layer near the interface. The EDS results suggest these white particles are Si-lean and they should be W5Si3 (Table 3). However, there is no obvious degradation and the WSi2 layer is basically maintained. Fig. 5 is the EPMA elemental distribution of the compound coating after oxidation at 800 °C for 100 h. The results indicate the formation of a SiO2-Al2O3 based scale on the coating surface. Distribution of Y and B in the scale is also observed. Remarkably, there is nearly no distribution of Nb, Ti and Cr in the coating or scale but distribution of Hf and Al in the compound coating. This is probably due to their high affinity with
(a)
(b) SiO2
(c) Y 2O 3
SiO2
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4
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6
10 ȝP
Fig. 7. Surface and cross-sectional BSE images of the coating after oxidation at 1250 °C for 5 (a-a′′), 50 (b-b′′) and 100 h (c-c′′), respectively. 5
Applied Surface Science 504 (2020) 144477
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Table 4 Chemical composition of the marked sites in Fig. 7 (by EDS analysis). Site Site Site Site Site Site Site
1 2 3 4 5 6
in in in in in in
Fig. Fig. Fig. Fig. Fig. Fig.
Composition (at. %) O Mo 63.9 0.1 65.5 0.1 — 35.0 — 1.3 — 1.0 — 0.2
8c′′ 8c′′ 8c′ 8c′′ 8c′′ 8c″
Si 23.3 27.9 64.3 60.7 36.5 35.2
B — — — — — —
W — — — 33.2 44.1 —
Nb 1.2 0.1 0.3 — 2.0 26.7
Ti 1.0 — 0.3 — 7.2 27.0
Cr 0.1 0.4 0.1 — 1.4 7.2
Al 5.8 5.9 — 0.8 — 0.2
Al
O
B
Si
Y
W
Nb
Ti
Cr
Hf
Y 4.6 — — 3.3 7.0 1.7
Hf — 0.1 — 0.7 0.8 1.8
15 ȝP Mo
Fig. 8. EPMA elemental maps of the coating after oxidation at 1250 °C for 100 h.
elements and there is obvious distribution of Ti and Cr in it. Remarkably, low level distribution of O is observed at the coating/substrate interface, and it may cause oxidation of Hf in this area. According to Fig. 7, formation of HfO2 is beneficial to improve the coating/substrate adhesion during oxidation. 3.5. Oxidation at 1350 °C The XRD diffraction patterns of the coating after oxidation at 1350 °C depend on the oxidation time seriously and the oxidation products are more complicated (Fig. 9). After oxidation for 5 h, the major oxidation products are SiO2 and Al2O3. However, Nb2O5, AlNbO4, YNbO4 and Al2SiO5 are observed after oxidation for 10 h and intensity of their peaks increases with the prolonging exposure time, indicating that more and more oxides have formed. On the contrary, intensity of the MoSi2 diffraction peaks decrease gradually and they are not the dominant peaks any more after oxidation longer than 10 h. Fig. 10 shows the surface morphology and cross-sectional BSE images of the specimen after oxidation at 1350 °C for 5, 20 and 40 h respectively. After oxidation for 5 h, the coating is mostly covered except for a few MoSi2 protuberances. Noticeably, some white phases identified as YNbO4 and Nb2O5 are observed on the coating surface after oxidation for 20 h (Fig. 10b and Table 5). As oxidation proceeds, the scale becomes smooth and dense while a few pores could still be detected after oxidation for 40 h. The globular YNbO4 particles and rodlike AlNbO4 particles are observed on the scale matrix (Fig. 10c). It can be viewed that the scales is about 4.5, 14 and 19.5 μm thick after oxidization for 5, 20 and 40 h, respectively (Fig. 10a′-c′). Remarkably, the scales show two-layered structure, consisting of the upper layer with many oxide particles and the under layer with less particles. The upper layer is much thicker than the under layer. The EDS results show that
Fig. 9. XRD patterns of the coating after oxidation at 1350 °C for different time.
which is generated from the degradation of WSi2 layer in situ. Remarkably, the interfacial cracks are sealed gradually and the path for the oxygen is hindered consequently. After oxidation, the interdiffusion zone (IDZ) forms and it shows concentration of Ti and Cr (Table 4). Thickness of this layer is uneven but it increases with the extension of oxidation generally. Fig. 8 shows the EPMA elemental distribution of the compound coating after oxidation at 1250 °C for 100 h. The scale is mainly composed of SiO2 and Al2O3. Besides, concentration of Y is also observed in the scale. Outward diffusion of alloying elements has been suppressed basically. However, low level distribution of Cr and Hf is observed at the scale/MoSi2 interface and in the MoSi2 layer, respectively. The porous W5Si3 layer cannot suppress the outward diffusion of alloying 6
Applied Surface Science 504 (2020) 144477
G. Yue, et al.
(b)
(a)
(c)
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IDZ
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8
10 ȝP
Fig. 10. Surface and cross-sectional BSE images of the compound coating after oxidation at 1350 °C for 5 (a-a′′), 20 (b-b′′) and 40 h (c-c′′), respectively. Table 5 Chemical composition of the marked sites in Fig. 10 (by EDS analysis). Site Site Site Site Site Site Site Site Site Site
1 2 3 4 5 6 7 8 9
in in in in in in in in in
Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.
11c′′ 11c′′ 11b′ 11c′′ 11c′′ 11c′′ 11c′ 11c′′ 11c′′
Composition (at. %) O Mo 54.7 — 65.4 — 74.6 — 62.7 — 66.5 — — 43.0 — 32.8 — 0.5 — —
Si 1.5 1.2 — 24.6 30.4 36.3 62.7 36.9 34.8
B — — — — — — — — —
W 0.1 0.1 — 0.3 0.4 9.4 0.4 55.6 —
the turbid SiO2 layer is with higher content of Nb and Al (Table 5). The micro particles in the upper layer are around the big particles, indicating the possible growth process of the big particles (Fig. 10c″). Likewise, Mo5Si3 is absent at the scale/MoSi2 interface but it is observed at the MoSi2/W5Si3 interface after oxidation for 20 h and 40 h, when the WSi2 layer has degraded completely. Meanwhile, the Kirkendall vacancies are observed beneath the IDZ layer, which are resulted from the intense outward diffusion of alloying elements. Elemental distribution of the coating after oxidation at 1350 °C for 40 h was characterized by EPMA (Fig. 11). The results show good accordance with the EDS analysis results (Table 5). Outward diffusion for alloying elements (Nb, Ti and Cr) becomes accessible since the degradation of WSi2 barrier layer. Hf is detected in the YNbO4 while Ti and Cr are concentrated in the AlNbO4. According to Fig. 11, B prefers to distribute in the YNbO4 and AlNbO4 particles rather than in the SiO2
Nb 21.5 17.2 18.5 6.3 2.4 5.4 2.3 0.7 26.2
Ti — 2.9 4.8 0.5 0.1 4.5 0.5 4.8 26.7
Cr — 0.4 0.1 — — 0.7 — 0.7 8.3
Al 1.2 12.0 1.3 4.4 0.2 0.3 0.5 0.6 1.2
Y 19.0 0.5 0.3 0.6 — — — — —
Hf 2.0 0.3 0.4 0.6 — 0.4 0.8 0.3 2.8
matrix scale. 4. Discussion In this compound coating, the outer MoSi2 layer is designed as antioxidation layer and the inner WSi2 layer is designed as diffusion barrier layer respectively. The compound coating shows excellent performance at 800 and 1250 °C while limited protection at 1350 °C. The different oxidation products indicate the distinguishing oxidation processes at each temperature. The following reactions display the probable oxidation process to take place on the coating surface during oxidation, and their standard free energy change values with temperature are listed in Fig. 12. Remarkably, the value of SiO2 in this plot is obtained according oxidation of pure Si, seen in reaction (9). 7
Applied Surface Science 504 (2020) 144477
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Al
Y B
Si
O
10 ȝm
W
Mo
Cr
Ti
Hf Nb
Fig. 11. EPMA elemental maps of the coating after oxidation at 1350 °C for 40 h.
Fig. 13. The specimen keeps integrated and a discontinuous SiO2 film doped with Al2O3 particles has formed on the coating surface (Fig. 13a,b). EDS results (Fig. 13c-d) show the higher distribution of Al in the covered areas, indicating formation of Al2O3 (reaction (3)) may promote the growth of SiO2 scale [34]. Subsequently, B2O3 could be generated (reaction (4)) due to its higher concentration and favorable diffusion aspects in the MoSi2 bulk [35]. The fluidity of SiO2 could be obviously improved by B2O3 even with a rather low content, then SiO2 nuclei could flow sideways to cover the coating continuously, insulating it from oxidizing ambient [26,31,33]. At this temperature, Y is readily to be oxidized as reaction (5) due to its high affinity with oxygen (Fig. 12) [36]. It is reported that reaction (1) is thermodynamically favored at high temperatures and reaction (2) is kinetically favored at low temperatures [30,31]. Nevertheless, Mo5Si3 is absent at the scale/MoSi2 interface when oxidized at 1250 and 1350 °C, indicating this phenomenon should be due to kinetics factors. According to the literatures [30,31], oxidation of MoSi2 is a diffusion controlled process. The diffusion coefficient of Si could be illustrated as the Arrhenius type relationship of reaction (6).
Fig. 12. Calculated standard free energy change values for the probable reactions (e.g. 4/3Y + O2 = 2/3Y2O3) at different oxidation temperatures.
5/7MoSi2 + O2 → 1/7Mo5Si3 + SiO2
(1)
2/7MoSi2 + O2 → 2/7MoO3 + 4/7SiO2
(2)
4/3Al + O2 → 2/3Al2O3
(3)
4/3B + O2 → 2/3B2O3
(4)
4/3Y + O2 → 2/3Y2O3
(5)
D = D0 exp(
At 800 °C, due to the low temperature and the barrier effect of WSi2 layer, outward diffusion of the alloying elements is mostly restricted except for Al. Oxidation of the alloying elements is avoided and the reaction on the coating surface should be reactions (1)–(5). Reactions (1) and (2) are the main oxidation process of MoSi2 and they are both identified at 800 °C for observation of pores in the scale and Mo5Si3 at the scale/MoSi2 interface simultaneously (Fig. 4) [30,31]. Pesting oxidation is successfully suppressed at this temperature. Due to the strict polishing process before oxidation, influence of the residual pack agent could be ignored and the achievement seems to be the effect of addition of B [32,33]. Whereas, effect of Al2O3 should be considered in view of concentration of Al in the scale (Fig. 5). For better understanding, the coating was oxidized at 800 °C for 2 h and the results are listed in
Q ) RT
(6)
5/7WSi2 → 1/7W5Si3 + Si
(7)
1/7Mo5Si3 + Si → 5/7MoSi2
(8)
Si + O2 → SiO2
(9)
where D0 is the diffusion constant, Q is the activation energy, R is the gas constant and T is temperature in Kelvin. At 800 °C, the diffusion rate of the released Si is too low to sustain the oxidation, and the generated Mo5Si3 according reaction (1) is retained after oxidation. On the contrary, the diffusion could be increased at 1250 and 1350 °C, and the released Si from the WSi2 layer could diffuse onto the scale/MoSi2 interface, suppress the formation of Mo5Si3 and to sustain the oxidation. This process could be illustrated as reactions (7) and (8). Combining reaction (1), oxidation essence of MoSi2/WSi2 compound coating at high temperatures is diffusion and preferential oxidation of Si (reaction (9)), like mono MoSi2 at high temperatures [31]. The underneath WSi2 layer could be a reservoir for Si and to buffer oxidation of MoSi2 at high temperatures. This similar phenomenon has also been reported by 8
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(b)
(a)
Al2O3 particle
Spot A
Spot B
Before oxidation
After oxidation
10 ȝm (c)
Element
(d)
Content (at. %)
Element
Content (at. %)
O
66.2
O
72.5
Spot A Si Al
23.0
Si
20.7
Al
3.6
Mo
8.0
Mo
2.6
Ti
0.1
Y
0.3
Hf
0.1
W
0.2
Ti
0.1
Spot B
0.6
Fig. 13. (a) Macroscopic morphologies of the coated sample before and after oxidation at 800 °C for 2 h, (b) surface morphology of the coating and (c) (d) the corresponding EDS analysis results of the marked sites in (b).
Fig. 14. Sketch map of oxidation and interfacial diffusion of the MoSi2/WSi2 compound coating at 800, 1250 and 1350 °C.
from formation of TiO2 and Nb2O5 particles in the scale (Figs. 7 and 8). At 1350 °C, the WSi2 layer degrades into porous W5Si3 layer completely within 20 h, which cannot suppress the outward diffusion of alloying elements any more (Fig. 10b′′). Accordingly, Nb and Ti, the major component elements of the substrate could diffuse onto the scale/ MoSi2 interface driven by the chemical potential and then react with oxygen. As a result, Kirkendall voids leave in the substrate and the scale become thicker by doping with the oxide particles like Nb2O5, YNbO4 and AlNbO4. Remarkably, the individual TiO2 and Cr2O3 particles are not detected and these elements distribute in the complicated oxides mainly (Fig. 11 and Table 5). Formation of these complicated oxides probably follows reactions (10)–(12)
Meheriz in CoWSi-WSi2 case in which CoWSi act as Si reservoir [37]. Nevertheless, this process will lead degradation of WSi2 and formation of W5Si3 particles accordingly (seen in Fig. 4b′′-c′′). At high temperatures, oxidation and diffusion of Si accelerate, so the growth of W5Si3 could be increased. Diffusion reaction between WSi2 layer and substrate is inevitable and it could be obviously accelerated at elevated temperatures. This process will increase loosing of Si and then lead degradation of WSi2 (formation of W5Si3) at the interface preferentially for the near location [17]. With the prolonging oxidation, the W5Si3 particles could grow up preferentially and incorporate with each other, then forming a continuous W5Si3 layer between residual WSi2 layer and substrate eventually. Decomposition could endanger the suppression ability of WSi2 layer at high temperatures. Nevertheless, degradation rate of WSi2 layer is quite slow, and the W5Si3 layer becomes obvious after oxidation longer than 50 h. Moreover, there is still a residual WSi2 layer left even after oxidation at 1250 °C for 100 h. This residual layer still shows admirable suppression to the alloying elements, preventing 9
4/5Nb + O2 → 2/5Nb2O5
(10)
Nb2O5 + Y2O3 → 2YNbO4
(11)
Nb2O5 + Al2O3 → 2AlNbO4
(12)
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(a)
(b)
AlNbO4
YNbO4
Scale
20 ȝm 20 ȝm
(d)
Crack
77.3O-14.2Nb-6.2Ti-0.9Cr0.6Al-0.6Hf-0.2Y
Fig. 15. (a) Macroscopic morphology of the specimen after oxidation at 1350 °C for 50 h, SEM morphology of the (b) residual coating and (c) exfoliated scale and (d) composition of the brown scale.
5. Conclusions
Formation of the layered scale is probably related to the diffusion and oxidation of Si. It could be viewed that the oxide particles and SiO2 could be formed at the scale/MoSi2 interface simultaneously (Fig. 10a′′c′′). However, due to the larger amount of Si, the amount of SiO2 generated at the interface is larger too. Thus, a clear SiO2 layer forms at the interface and it is retained perpetually. The oxides particles generate as the result of oxidation of alloying elements and they could be the nuclei to grow up with prolonging oxidation. In addition, the new formed small particles could move upwardly and then be incorporated by the existed bigger particles (Fig. 10c“). Extensive oxidation of Si not only leads the degradation of WSi2 layer, but also is responsible for the degradation of MoSi2 layer. After the WSi2 degrades completely, the near MoSi2 degrades as well due to the possible inducement effect of W5Si3 layer. In addition, considering the small thickness of IDZ layer, it could be inferred that oxidation is the main cause for the degradation. Sketch map of oxidation and interfacial diffusion of the MoSi2/WSi2 compound coating at 800, 1250 and 1350 °C is presented in Fig. 14. Fig. 15a shows the macroscopic morphology of the specimen after oxidation at 1350 °C for 50 h. It can be viewed that there is extensive peeling off of brown scales around the side faces. Moreover, the coating ruptures and the exposed areas show the same appearance of the exfoliated scales. XRD and EDS analysis results suggest the Nb2O5 + TiO2 + TiNb2O7 nature of the brown scale (Fig. 15c,d). These observations indicate that oxidation at the coating/substrate interface results in failure of the compound coating. In addition, exhaustion of the lateral NbSi2 layer is also related. Once the side protection fails, the oxygen can readily diffuse along the coating/substrate interface and react with the substrate. Formation of these oxides can cause volume expansion at the interface, which may result in exfoliation of the compound coating, as shown in Fig. 15. Another potential problem of the compound coating is that the bonding between MoSi2/WSi2 coating and alloy is not firm enough; scraping or crashing may lead rupture of the coating and then disastrous oxidation of the substrate. Thus, the future focus is trying to improve the interfacial adhesion and prolong the service life of WSi2 layer at ultra-high temperatures as possible.
The MoSi2/WSi2 compound coating was prepared on the surface of Nb-Ti-Si based alloy through MS and HAPC techniques. The coating showed perfect oxidation resistance at 800 °C and 1250 °C and the oxidation kinetics followed parabolic laws. The scale growth rate constants were 0.335 μm2 h−1 and 0.939 μm2 h−1, respectively. It is believed that suppression of pesting oxidation at 800 oC should be the result from B addition. Besides, formation of Al2O3 was beneficial to improve the growth of scale and it may be helpful too. Generation of Mo5Si3 at the scale/MoSi2 interface was mainly resulted from the slow diffusion rate of Si at 800 °C. Thus, this phenomenon could be suppressed at 1250 and 1350 °C. WSi2 layer shows admirable diffusion suppression effect at 1250 °C, and a dense SiO2-Al2O3 based scale formed on the surface. In addition, the WSi2 layer also played the role of Si reservoir, through that service life of the MoSi2 coating was increased. At 1350 oC, the WSi2 layer completely degraded within 20 h and it cannot restrict the outward diffusion of alloying elements. As a result, Nb2O5, YNbO4 and AlNbO4 form in the scale, and the scale growth rate is increased. The invalidation test at 1350 °C indicated that exhaustion of the lateral NbSi2 coating and interfacial oxidation causing the failure of the coating system.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51431003, U1435201, 51971177 and 51971181) and the Fundamental Research Funds for the Central Universities (No. 3102019TS0401).
Declaration of Competing Interest The authors declare no competing financial interest.
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References [19]
[1] J.A. Haynes, Y. Zhang, K.M. Cooley, L. Walker, K.S. Reeves, B.A. Pint, High-temperature diffusion barriers for protective coatings, Surf. Coat. Technol. 188–189 (2004) 153–157. [2] Y. Matsumura, M. Fukumoto, S. Hayashi, A. Kasama, I. Iwanaga, R. Tanaka, T. Narita, Oxidation behavior of a Re-base diffusion-barrier/β-NiAl Coating on Nb5Mo-15W at high temperatures, Oxid. Met. 61 (2004) 105–124. [3] R. Ghasemi, Z. Valefi, Electrodeposition of rhenium-base layer as a diffusion barrier between the NiCoCrAlY coating and a Ni-based superalloy, J. Alloy Compd. 732 (2018) 470–485. [4] R. Ghasemi, Z. Valefi, The effect of the Re-Ni diffusion barrier on the adhesion strength and thermal shock resistance of the NiCoCrAlY coating, Surf. Coat. Technol. 344 (2018) 359–372. [5] K. Kurokawa, G. Ochiai, H. Takahashi, S. Ohta, H. Takahashi, Effects of sputterdeposited materials (W, Ti and SiC) on interfacial reaction between MoSi2 and Nb, Vacuum 59 (2000) 284–291. [6] B. Bai, H.B. Guo, H. Peng, L.Q. Peng, S.K. Gong, Cyclic oxidation and interdiffusion behavior of a NiAlDy/RuNiAl coating on a Ni-based single crystal superalloy, Corros. Sci. 53 (2011) 2721–2727. [7] Y. Wang, H.B. Guo, H. Peng, L.Q. Peng, S.K. Gong, Diffusion barrier behaviors of (Ru, Ni)Al/NiAl coatings on Ni-based superalloy substrate, Intermetallics 19 (2011) 191–195. [8] Z.M. Bai, D.Q. Li, H. Peng, J. Wang, H.B. Guo, S.K. Gong, Suppressing the formation of SRZ in a Ni-based single crystal superalloy by RuNiAl diffusion barrier, Prog. Nat. Sci-Mater. 22 (2012) 146–152. [9] T. Narita, K.Z. Thosin, L. Fengqun, S. Hayashi, H. Murakami, B. Gleeson, D. Young, Development of Re-based diffusion barrier coatings on nickel based super alloys, Mater. Corros. 56 (2005) 923–929. [10] T. Izumi, T. Yoshioka, T. Narita, Effect of Re-W-Cr-Ni diffusion barrier on high temperature oxidation of Pt-modified β-NiAl coated superalloy, J. Japan Inst. Metals. 72 (2008) 728–732. [11] C.P. Lungua, I. Mustataa, G. Musaa, A.M. Lungua, V. Zaroschia, K. Iwasakib, R. Tanakab, Y. Matsumurab, I. Iwanagab, H. Tanakab, T. Oib, K. Fujita, Formation of nanostructured Re-Cr-Ni diffusion barrier coatings on Nb superalloys by TVA method, Surf. Coat. Technol. 200 (2005) 399–402. [12] E. Sugiarti1, Y.M. Wang, N. Hashimoto, S. Ohnuki, T. Narita, Phase characterization of Re-based diffusion barrier layer on Nb substrate, Mater. Trans. 52 (2011) 319–323. [13] A. Littner, M. Francois, B. Malaman, J. Steinmetz, M. Vilasi, E. Elkaim, Isothermal section at 1673 K of the Mo-Ru-Si diagram and crystallographic structures of ternary phases, Intermetallics 11 (2003) 1223–1228. [14] H. Bei, Y. Yang, G.B. Viswanathan, C.J. Rawn, E.P. George, J. Tiley, Y.A. Chang, Formation, stability and crystal structure of the δ phase in Mo-Re-Si alloys, Acta Mater. 58 (2010) 6027–6034. [15] T.C. Lu, A.G. Evans, R.J. Hecht, R. Mehrabian, Toughing of MoSi2 with a ductile (niobium) reinforcement, Acta Metall. Mater. 39 (1991) 1853–1862. [16] L. Shaw, R. Abbaschian, Control of the interfacial reactions in Nb-toughened MoSi2, J. Amer. Chem. Soc. 76 (1993) 2305–2311. [17] J.H. Yan, Y. Wang, L.F. Liu, Y.M. Wang, Oxidation and interdiffusion behavior of Niobium substrate coated MoSi2 coating prepared by spark plasma sintering, Appl. Surf. Sci. 320 (2014) 791–797. [18] P.I. Glushko, V.I. Zmii, N.A. Semenov, B.M. Shirokov, Stability and heat resistance
[20]
[21]
[22]
[23]
[24] [25]
[26]
[27]
[28]
[29]
[30] [31] [32]
[33]
[34] [35] [36]
[37]
11
of silicide coatings on refractory metals. III. stability of silicide coatings on niobium heated in air at 1500–1800oC, Powder Metall. Met. Ceram. 42 (2003) 154–157. F. Chen, J.G. Xu, Y.B. Liu, L. Cai, In situ reactive spark plasma sintering of WSi2/ MoSi2 composites, Ceram. Int. 42 (2016) 11165–11169. V.I. Zmii, N.V. Kovtun, P.I. Glushko, S.G. Ruden’kii, Stability and heat resistance of silicide coatings on refractory metals. II. stability and Heat resistance of silicide coatings on tungsten and molybdenum at 1500–2000°C, Powder Metall. Met. Ceram. 42 (2003) 50–53. P.C. Tortorici, M.A. Dayananda, Interdiffusion and diffusion structure development in selected refractory metal silicides, Mater. Sci. Eng.: A 261 (1-2) (1999) 64–77, https://doi.org/10.1016/S0921-5093(98)01050-8. B.K. Bhattacharyya, D.M. Bylander, L. Kleinman, Comparison of fully relativistic energy bands and cohesive energies of MoSi2 and WSi2, Phys. Rev. 32B (1985) 7973–7976. H.A. Zhang, J.X. Lv, Y.H. Wu, S.Y. Gu, Y. Huang, Y. Chen, Oxidation behavior of (Mo, W)Si2-Si3N4 composite coating on molybdenum substrate at 1600oC, Ceram. Int. 41 (2015) 14890–14895. J.O. Olowolafe, E.G. Colgan, C.J. Palmstr, J.W. Mayer, Interfacial reaction of Mo-W alloys with silicon, Thin Solid Films 138 (1986) 245–254. Y. Harada, Y. Funato, M. Morinaga, A. Ito, Y. Sugita, Solid solubilities of ternary elements and their effects on microstructure of MoSi2, J. Japan Inst. Metals. 58 (1994) 1239–1247. L.F. Su, O.L. Steffes, H. Zhang, J.H. Perepezko, An ultra-high temperature Mo-Si-B based coating for oxidation protection of Nbss/Nb5Si3 composites, Appl. Surf. Sci. 337 (2015) 38–44. T.F. Retajczyk, A.K. Sinha, Elastic stiffness and thermal expansion coefficients of various refractory silicides and silicon nitride films, Thin Solid Films 70 (1980) 241–247. T.S.R.Ch. Murthy, J.K. Sonber, C. Subramanian, R.C. Hubli, A.K. Suri, Densification, characterization and oxidation studies of TiB2-WSi2 composite, Int. J. Refract. Met. Hard Mater. 33 (2012) 10–21. O.J. Lu-Steffes, R. Sakidja, J. Bero, J.H. Perepezko, Multicomponent coating for enhanced oxidation resistance of tungsten, Surf. Coat. Technol. 207 (2012) 614–619. Y.Q. Liu, G. Shao, P. Tsakiropoulos, On the oxidation behavior of MoSi2, Intermetallics 9 (2001) 125–136. A.A. Sharif, High-temperature oxidation of MoSi2, J. Mater. Sci. 45 (2010) 865–870. B.V. Cockeram, G. Wang, R.A. Rapp, Growth kinetics and pesting resistance of MoSi2 and germanium-doped MoSi2 diffusion coatings grown by the pack cementation method, Mater. Corros. 46 (1995) 207–217. B.V. Cockeram, R.A. Rapp, Oxidation-resistant boron- and germanium-doped silicide coatings for refractory metals at high temperature, Mater. Sci. Eng. A 192 (193) (1995) 980–986. T. Maruyama, K. Yanagihara, High temperature oxidation and pesting of Mo(Si, Al) 2, Mater. Sci. Eng. A 239–240 (1997) 828–841. S. Majumdar, B. Gorr, H.J. Christ, D. Schliephake, M. Heilmaier, Oxidation mechanisms of lanthanum-alloyed Mo-Si-B, Corros. Sci. 88 (2014) 360–371. S. Majumdar, A. Kumar, D. Schliephake, H.J. Christ, X. Jiang, M. Heilmaier, Microstructural and micro-mechanical properties of Mo-Si-B alloyed with Y and La, Mater. Sci. Eng. A 573 (2013) 257–263. M.Z. Mehrizi, M. Shamanian, A. Saidi, R. Shoja-Razazvi, R. Beigi, Evaluation of oxidation behavior of laser clad CoWSi-WSi2 coating on pure Ni substrate at different temperatures, Ceram. Int. 41 (2015) 9715–9721.
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Full Length Article
Isothermal oxidation and interdiffusion behavior of MoSi2/WSi2 compound coating on Nb-Ti-Si based alloy
T
⁎
Gao Yue, Xiping Guo , Yanqiang Qiao, Fa Luo State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nb-Ti-Si based alloy MoSi2/WSi2 compound coating Diffusion barrier layer Oxidation behaviors
The oxidation and diffusion behaviors of MoSi2/WSi2 compound coating on the Nb-Ti-Si based alloy at 800, 1250 and 1350 °C have been investigated. The coating exhibited excellent oxidation resistance at 800 °C and 1250 °C, and the scale growth rates were about 0.335 μm2 h−1 and 0.939 μm2 h−1, respectively. At 800 oC, pesting oxidation was successfully suppressed and formation of Mo5Si3 was mainly resulted from the slow diffusion rate of Si atoms. Therefore, there was no Mo5Si3 at the scale/MoSi2 interface when oxidized at 1250 and 1350 °C. At 1250 °C, WSi2 layer showed strong restriction against outward diffusion of alloying element atoms. Thus, a pure SiO2-Al2O3 scale about 9.5 μm thick was obtained after oxidation for 100 h. At 1350 °C, the WSi2 layer degraded into porous W5Si3 in situ within a short duration and the outward diffusion was increased. As a result, oxides like Nb2O5, YNbO4 and AlNbO4 formed in the thick scale.
1. Introduction Moderate interfacial diffusion is considered to improve the boundary bonding between oxidation resistance coating and super alloy at high temperatures. Nevertheless, the accelerated interdiffusion is undesirable for it may cause loss of key elements (e.g. Al, Si) and then microstructure degradation of the coating, weakening its anti-oxidation property and decreasing service life consequently. Furthermore, it can also result in permanent jeopardizing of the mechanical property of the substrate [1,2]. Hence, it is necessary to develop a barrier layer at the interfacial region to suppress the intense elemental diffusion and prolong the service life of the integrate system [3–5]. Noble metals and their alloys have been selected as diffusion barrier in the coating system for their high melting point, good stability at high temperatures and favorable compatibility. Ru-based layer (RuNiAl) is fabricated between NiAl or NiAlDy coating and Ni-based super alloy and it could slow down the inward diffusion of Al from the coating and outward diffusion of alloying elements effectively, and then suppressed the formation of TCP phases and second reaction zone at high temperatures [6–8]. In addition, Re(W)-Cr-Ni barrier layer has also been synthesized on Ni and Nb based alloy against inward diffusion of Al derived from the outer NiAl coating. The σ-Re-Cr-Ni phase exhibits impressive diffusion suppression ability for both inward diffusion of Al and outward diffusion of alloying elements at the NiAl/super alloy interface [9–12]. However, the imitation of RuNiAl or Re(W)-Cr-Ni layer
⁎
seems hard to succeed as the diffusion barrier at the MoSi2/Nb interface due to the higher application temperature and high reactivity of Si with the metallic materials [13,14]. Besides, the high expense and complicated preparation may also limit their general application in the aerospace field. The inert oxides barrier like Y2O3, ZrO2 and Al2O3 shows perfect suppression ability to the inward diffusion of Si at elevated temperatures [15,16]. However, spalling and rupture are inevitable while suffered intense thermal fluctuation due to the poor interfacial bonding and the great thermal expansion difference. Consequently, materials with admirable diffusion suppression ability against Si, good interfacial compatibility and close coefficient of thermal expansion (CTE) relative to both Nb-Si based alloys and MoSi2 coating are required [17]. It has been reported that oxidation resistance of MoSi2 coating could be improved by addition of WSi2 in terms of higher applied temperature and longer service life. Stability of the (Mo,W)Si2 composite layer on Nb system is more than twice of the MoSi2-Nb system, and 15–18 times of the MoSi2-Mo system [18]. The improvement is basically due to the high melting point of WSi2, enhanced mechanical property of (Mo,W) Si2 composite [19], slow degradation of WSi2 to W5Si3 and excellent oxidation resistance of WSi2 at ultra-high temperatures [20]. Transition of WSi2 to W5Si3 is on average four times slower than the transition rate for MoSi2 to Mo5Si3 according to the study of Glushko [20]. Moreover, Dayananda reported that the generated interdiffusion layer in W/MoSi2 couple was obvious thinner than that of Mo/MoSi2 and Nb/MoSi2
Corresponding author. E-mail address: [email protected] (X. Guo).
https://doi.org/10.1016/j.apsusc.2019.144477 Received 23 July 2019; Received in revised form 5 October 2019; Accepted 21 October 2019 Available online 31 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Experimental parameters employed during the sputtering. Process steps
First deposition of W film Second deposition of Mo film
Working pressure
Sputtering power
Sputtering time
0.9 pa 0.7 pa
150 W 130 W
1h 2h
Table 2 Chemical composition of the MoSi2/WSi2 compound coating (by EPMA analysis). Layer MoSi2 WSi2
Composition (at. %) B Si Y 5.26 64.43 0.07 — 67.03 —
Mo 29.50 0.23
W — 32.35
Nb 0.01 0.09
Ti 0.41 0.14
Cr 0.02 0.12
Al 0.26 —
Hf 0.04 0.04
1350 °C were investigated. Moreover, the structure evolution of the compound coating, the oxidation mechanism and the failure at 1350 °C of this compound coating has been investigated too.
couple [21]. This observation suggests the slower reaction between W and MoSi2 and the hindering effect of W on the diffusion of Si. Bhattacharyya have calculated the crystal bonding energy of MoSi2 and WSi2 and it reveals that W-Si bound possesses higher bonding energy in comparison of Mo-Si bound (2058.1 kJ/mol and 1847.8 kJ/mol, respectively), indicating more difficult of Si atom diffusion in WSi2 layer than in MoSi2 layer [22,23]. Another advantage of WSi2 is its similar analogous crystal structure and lattice parameter with MoSi2 [23,24], which are helpful to improve the compatibility and combination between MoSi2 layer and WSi2 layer. Actually, the W atoms are able to substitute for Mo atoms in the lattice, made a complete solution and formation of (W, Mo)Si2 mixture [25]. In addition, the close CTEs of MoSi2, WSi2 and Nb-Ti-Si based alloy (8.4 × 10−6/oC, 8.4 × 10−6/oC and 8.5 × 10−6/oC, respectively) make it suitable to fabricate a duplex silicide coating assumed as MoSi2/WSi2 on the Nb-Ti-Si based alloy [26–28]. In this paper, this duplex structure has been successfully prepared on the Nb-Ti-Si based ultrahigh temperature alloy via magnetron sputtering (MS) technique and halide activated pack cementation (HAPC) treatment. Among the compound coating, MoSi2 layer is the major oxidation resistance coating and WSi2 layer is supposed to act as the diffusion barrier and buffering layer. Isothermal oxidation and interdiffusion behaviors of this compound coating at 800 °C, 1250 °C and
2. Experimental The substrates were cut into 8 × 8 × 3 mm cubes from a Nb-Ti-Si based ultrahigh temperature alloy block with a nominal composition of Nb-20Ti-16Si-6Cr-5Hf-4Al (at.%), and they were abraded with grit SiC papers. MS was applied to deposit W and Mo films on the surface of substrate successively through JPG450 sputtering system. Prior to sputtering, the abraded specimens were activated with mixture acid, composing of distilled water, HNO3, HF and H2SO4 with the proportion of 4:1:1:1, (vol. %, AR), and then were carefully washed with distilled water and in ultrasonic ethanol bath successively. The W and Mo target (with purity of 99.99%) plates of 3 mm in thickness and 60 mm in diameter were used and the target-substrate separation was kept 60 mm during the sputtering. The detailed sputtering parameters are listed in Table 1. HAPC was used to co-deposition of Si-B-Y onto the Mo/W complex films on the Nb-Ti-Si based alloy surface. The pack agent is consisting of 12Si-0.025B-5NaF-2.5Y2O3-Al2O3 (wt.%) and the pack cementation process was carried out at 1200 °C for 1 h. The short packing duration was applied in order to avoid the generation of NbSi2 layer for it may jeopardize the CTE match between the MoSi2/WSi2
(a)
(b) Mo layer
W layer
20 ȝP
Substrate
(c)
10 ȝP
(d) MoSi2 layer WSi2 layer Residual W
Substrate
20 ȝP
Fig. 1. Surface morphology (a), cross-sectional image (b) of the sputtered Mo/W film, (c) XRD pattern and (d) cross-sectional BSE image of the MoSi2/WSi2 compound coating on the substrate. 2
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Fig. 2. (a) Evolution of scale thickness and (b) linear plot of oxidation kinetics of MoSi2/WSi2 compound coating at 800, 1250 and 1350 °C, respectively.
film, which is resulted from the volume expansion in silication process [29]. The chemical composition of each layer was analyzed with EPMA and the results (Table 2) show distribution of B and Y in the MoSi2 layer. The WSi2 layer is quite compact while there are some micro pores in the MoSi2 layer. 3.2. Oxidation kinetics Oxidation kinetics of the MoSi2/WSi2 compound coating at 800, 1250 and 1350 °C are shown in Fig. 2 in terms of scale thickness evolution versus oxidation time. At 800 and 1250 °C, growth of the scale is restricted and the thickness is about 6 and 9.5 μm respectively after oxidation for 100 h. However, the scale grows faster at 1350 °C and it is about 19 μm thick after oxidation for 40 h. It can be viewed that growth of the scales follows parabolic laws. Remarkably, the approximate linear oxidation kinetics is detected at the initial oxidation stage (Fig. 2a). Fig. 2b lists the corresponding fitted linear equations and the growth rate constants are 0.335, 0.939 and 12.809 μm2 h−1 when oxidized at 800, 1250 and 1350 °C respectively.
Fig. 3. XRD patterns of the coating after oxidization at 800 °C for different time.
compound layer and Nb-Ti-Si based alloy [26]. After deposition, the specimen was cooled down in the furnace. The isothermal oxidation experiments were carried out at 800, 1250 and 1350 °C respectively in an open-ended tube electric furnace. Prior to oxidation, the coatings were grinded with used 1500 # grit SiC papers carefully to remove the residual pack agent as possible, then ultra-sonic cleaned in distilled water. The as-prepared and oxidized specimens were analyzed by X-ray diffraction (XRD, Panalytical X'Pert PRO, Cu Kα), scanning electron microscopy (SEM, TESCAN MIRA 3), energy dispersive X-ray spectroscopy (EDS, Inca X-sight) and electron probe micro analysis (EPMA, EPMA-1720, SHIMAZU) to investigate their constituent phases, microstructures and compositions.
3.3. Oxidation at 800 °C Fig. 3 shows the XRD patterns of the coating after oxidation at 800 °C for different hours. It can be viewed that the major oxidation products are SiO2, Mo5Si3 and B2O3·SiO2. Intensity of the diffraction peaks increase with the prolonging exposure time. Remarkably, MoSi2 shows the dominant diffraction peaks, indicating the outstanding oxidation resistance of the compound coating at 800 °C. Fig. 4 shows the surface morphology and cross-sectional BSE images of the specimen after oxidation at 800 °C for 5, 50 and 100 h respectively. No sign of pesting oxidation is observed at 800 °C. After oxidation for 5 h, a discontinuous SiO2 scale with dark gray contrast forms on the MoSi2 surface. As oxidation proceeds, the scale grows and MoSi2 layer is gradually covered. Thus, there is no bare MoSi2 after oxidation for 50 and 100 h. Nevertheless, bump and pores are detected and they may probably be caused by evaporation of MoO3 (Fig. 4b). The bump could be filled gradually and then a continuous scale with a few of pores form after oxidation for 100 h. The profile images indicate that there is no oxidation of the substrate due to protection of the coating. In addition, the thickness and compactness of the scale is improved with oxidation time and it shows perfect adhesion to the coating without cracks and spallation (Fig. 4a′-c′). As a result, oxidation of MoSi2 could be effectively restricted. A few particles with light gray contrast are observed at the scale/MoSi2 interface and the amount increases with exposure time. According to XRD and EDS analysis results (Table 3), these particles could be identified as Mo5Si3. Noticeably, pores and tubules are observed near the MoSi2/WSi2 interface and some tubules
3. Results 3.1. Characterization of the coating Surface morphology and cross-sectional BSE images of the deposited Mo/W duplex film are displayed in Fig. 1a–b. The layer is compact and free from cracks and holes, composing of densely packed micro particles. The Mo and W film shows good interfacial bonding and their thicknesses are estimated about 7.5 and 2 μm respectively. Nevertheless, cracks can be observed at the deposit/substrate interface, which are probably formed during the polishing process due to the weak adhesion. After HAPC, the MoSi2/WSi2 duplex coating has formed in situ. The XRD pattern shows dominant peaks of MoSi2 and a few weak peaks of Al2O3 (Fig. 1c). Remarkably, the residual W is observed at the bottom of the coating from Fig. 1b. The MoSi2/WSi2 compound layer is approximately 25 μm thick, about 2.5 times of the as-sputtered Mo/W 3
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Bare MoSi2
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Fig. 4. Surface and cross-sectional BSE images of the compound coating after oxidation at 800 °C for 5 (a-a′′), 50 (b-b′′) and 100 h (c-c′′), respectively. Table 3 Chemical composition of the marked sites in Fig. 4 (by EDS analysis). Site Site Site Site Site Site Site Site
1 2 3 4 5 6 7
in in in in in in in
Fig. Fig. Fig. Fig. Fig. Fig. Fig.
Composition (at. %) O Mo 64.8 — 10.9 35.9 — 34.0 — 7.7 — 3.6 — — — 0.2
5c′′ 5c′′ 5c′′ 5c′′ 5c′′ 5c′ 5c′
Si 31.4 51.4 63.3 51.6 58.9 62.4 35.7
B — — — — — — —
W — — 0.5 36.9 33.8 — 0.5
Nb — — — — — 31.9 41.9
Ti 0.1 — 0.3 0.8 0.6 4.3 14.2
Cr — — — — — 0.5 4.9
Al 3.4 1.8 1.4 1.3 1.2 0.9 1.9
Al
O
B
Si
Y
W
Nb
Ti
Cr
Hf
15 ȝP Mo
Fig. 5. EPMA elemental maps of the coating after oxidation at 800 °C for 100 h. 4
Y 0.3 — — 0.6 0.7 — 0.6
Hf — — 0.5 1.1 1.2 — 0.1
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oxygen, which could result in their outward diffusion through the defects in the coating at this temperature, driven by the chemical potential. Low level distribution of O is observed at the coating/substrate interface, suggesting the diffusion of oxygen along the interfacial cracks. 3.4. Oxidation at 1250 °C Fig. 6 is the XRD patterns of the coating after oxidation at 1250 °C for different hours and it suggests that there is no Mo5Si3 formes at the interface. The oxidation products are SiO2, B2O3·SiO2 and Al2O3 mainly. Due to the efficient anti-oxidation property, MoSi2 still shows the dominant diffraction peaks after oxidation at 1250 °C for 100 h. Fig. 7 shows the surface morphology and cross-sectional BSE images of the specimen after oxidation at 1250 °C for 5, 50 and 100 h respectively. Likewise, the MoSi2 has not been covered completely after oxidation for 5 h (Fig. 7a). After that, the coating is covered gradually with the prolonging oxidation time. The scale is dense and there are some Al2O3 and Y2O3 particles observed (Fig. 7b). After oxidation for 100 h, the scale appears glass-like and lots of Y2O3 particles are distributed on its surface (Fig. 7c). The profile images show that the scales are about 2.1, 8.1 and 9.5 μm thick after oxidization for 5, 50 and 100 h, respectively (Fig. 7a′-c′). The limited growth of scale suggests the seriously restricted inward diffusion of oxygen and oxidation of coating. Mo5Si3 is absent at the scale/MoSi2 interface, which is accorded with the XRD results. Apart from some micro pores, no obvious degradation is observed for the WSi2 layer after oxidation for 5 and 50 h. Whereas, part of the WSi2 layer degrades after oxidation for 100 h. A porous W5Si3 layer is observed between the residual WSi2 layer and substrate,
Fig. 6. XRD patterns of the coating after oxidation at 1250 °C for different time.
stretch across the MoSi2 layer. A few pores and white contrast particles are observed in the WSi2 layer near the interface. The EDS results suggest these white particles are Si-lean and they should be W5Si3 (Table 3). However, there is no obvious degradation and the WSi2 layer is basically maintained. Fig. 5 is the EPMA elemental distribution of the compound coating after oxidation at 800 °C for 100 h. The results indicate the formation of a SiO2-Al2O3 based scale on the coating surface. Distribution of Y and B in the scale is also observed. Remarkably, there is nearly no distribution of Nb, Ti and Cr in the coating or scale but distribution of Hf and Al in the compound coating. This is probably due to their high affinity with
(a)
(b) SiO2
(c) Y 2O 3
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Fig. 7. Surface and cross-sectional BSE images of the coating after oxidation at 1250 °C for 5 (a-a′′), 50 (b-b′′) and 100 h (c-c′′), respectively. 5
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Table 4 Chemical composition of the marked sites in Fig. 7 (by EDS analysis). Site Site Site Site Site Site Site
1 2 3 4 5 6
in in in in in in
Fig. Fig. Fig. Fig. Fig. Fig.
Composition (at. %) O Mo 63.9 0.1 65.5 0.1 — 35.0 — 1.3 — 1.0 — 0.2
8c′′ 8c′′ 8c′ 8c′′ 8c′′ 8c″
Si 23.3 27.9 64.3 60.7 36.5 35.2
B — — — — — —
W — — — 33.2 44.1 —
Nb 1.2 0.1 0.3 — 2.0 26.7
Ti 1.0 — 0.3 — 7.2 27.0
Cr 0.1 0.4 0.1 — 1.4 7.2
Al 5.8 5.9 — 0.8 — 0.2
Al
O
B
Si
Y
W
Nb
Ti
Cr
Hf
Y 4.6 — — 3.3 7.0 1.7
Hf — 0.1 — 0.7 0.8 1.8
15 ȝP Mo
Fig. 8. EPMA elemental maps of the coating after oxidation at 1250 °C for 100 h.
elements and there is obvious distribution of Ti and Cr in it. Remarkably, low level distribution of O is observed at the coating/substrate interface, and it may cause oxidation of Hf in this area. According to Fig. 7, formation of HfO2 is beneficial to improve the coating/substrate adhesion during oxidation. 3.5. Oxidation at 1350 °C The XRD diffraction patterns of the coating after oxidation at 1350 °C depend on the oxidation time seriously and the oxidation products are more complicated (Fig. 9). After oxidation for 5 h, the major oxidation products are SiO2 and Al2O3. However, Nb2O5, AlNbO4, YNbO4 and Al2SiO5 are observed after oxidation for 10 h and intensity of their peaks increases with the prolonging exposure time, indicating that more and more oxides have formed. On the contrary, intensity of the MoSi2 diffraction peaks decrease gradually and they are not the dominant peaks any more after oxidation longer than 10 h. Fig. 10 shows the surface morphology and cross-sectional BSE images of the specimen after oxidation at 1350 °C for 5, 20 and 40 h respectively. After oxidation for 5 h, the coating is mostly covered except for a few MoSi2 protuberances. Noticeably, some white phases identified as YNbO4 and Nb2O5 are observed on the coating surface after oxidation for 20 h (Fig. 10b and Table 5). As oxidation proceeds, the scale becomes smooth and dense while a few pores could still be detected after oxidation for 40 h. The globular YNbO4 particles and rodlike AlNbO4 particles are observed on the scale matrix (Fig. 10c). It can be viewed that the scales is about 4.5, 14 and 19.5 μm thick after oxidization for 5, 20 and 40 h, respectively (Fig. 10a′-c′). Remarkably, the scales show two-layered structure, consisting of the upper layer with many oxide particles and the under layer with less particles. The upper layer is much thicker than the under layer. The EDS results show that
Fig. 9. XRD patterns of the coating after oxidation at 1350 °C for different time.
which is generated from the degradation of WSi2 layer in situ. Remarkably, the interfacial cracks are sealed gradually and the path for the oxygen is hindered consequently. After oxidation, the interdiffusion zone (IDZ) forms and it shows concentration of Ti and Cr (Table 4). Thickness of this layer is uneven but it increases with the extension of oxidation generally. Fig. 8 shows the EPMA elemental distribution of the compound coating after oxidation at 1250 °C for 100 h. The scale is mainly composed of SiO2 and Al2O3. Besides, concentration of Y is also observed in the scale. Outward diffusion of alloying elements has been suppressed basically. However, low level distribution of Cr and Hf is observed at the scale/MoSi2 interface and in the MoSi2 layer, respectively. The porous W5Si3 layer cannot suppress the outward diffusion of alloying 6
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(b)
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Fig. 10. Surface and cross-sectional BSE images of the compound coating after oxidation at 1350 °C for 5 (a-a′′), 20 (b-b′′) and 40 h (c-c′′), respectively. Table 5 Chemical composition of the marked sites in Fig. 10 (by EDS analysis). Site Site Site Site Site Site Site Site Site Site
1 2 3 4 5 6 7 8 9
in in in in in in in in in
Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.
11c′′ 11c′′ 11b′ 11c′′ 11c′′ 11c′′ 11c′ 11c′′ 11c′′
Composition (at. %) O Mo 54.7 — 65.4 — 74.6 — 62.7 — 66.5 — — 43.0 — 32.8 — 0.5 — —
Si 1.5 1.2 — 24.6 30.4 36.3 62.7 36.9 34.8
B — — — — — — — — —
W 0.1 0.1 — 0.3 0.4 9.4 0.4 55.6 —
the turbid SiO2 layer is with higher content of Nb and Al (Table 5). The micro particles in the upper layer are around the big particles, indicating the possible growth process of the big particles (Fig. 10c″). Likewise, Mo5Si3 is absent at the scale/MoSi2 interface but it is observed at the MoSi2/W5Si3 interface after oxidation for 20 h and 40 h, when the WSi2 layer has degraded completely. Meanwhile, the Kirkendall vacancies are observed beneath the IDZ layer, which are resulted from the intense outward diffusion of alloying elements. Elemental distribution of the coating after oxidation at 1350 °C for 40 h was characterized by EPMA (Fig. 11). The results show good accordance with the EDS analysis results (Table 5). Outward diffusion for alloying elements (Nb, Ti and Cr) becomes accessible since the degradation of WSi2 barrier layer. Hf is detected in the YNbO4 while Ti and Cr are concentrated in the AlNbO4. According to Fig. 11, B prefers to distribute in the YNbO4 and AlNbO4 particles rather than in the SiO2
Nb 21.5 17.2 18.5 6.3 2.4 5.4 2.3 0.7 26.2
Ti — 2.9 4.8 0.5 0.1 4.5 0.5 4.8 26.7
Cr — 0.4 0.1 — — 0.7 — 0.7 8.3
Al 1.2 12.0 1.3 4.4 0.2 0.3 0.5 0.6 1.2
Y 19.0 0.5 0.3 0.6 — — — — —
Hf 2.0 0.3 0.4 0.6 — 0.4 0.8 0.3 2.8
matrix scale. 4. Discussion In this compound coating, the outer MoSi2 layer is designed as antioxidation layer and the inner WSi2 layer is designed as diffusion barrier layer respectively. The compound coating shows excellent performance at 800 and 1250 °C while limited protection at 1350 °C. The different oxidation products indicate the distinguishing oxidation processes at each temperature. The following reactions display the probable oxidation process to take place on the coating surface during oxidation, and their standard free energy change values with temperature are listed in Fig. 12. Remarkably, the value of SiO2 in this plot is obtained according oxidation of pure Si, seen in reaction (9). 7
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Al
Y B
Si
O
10 ȝm
W
Mo
Cr
Ti
Hf Nb
Fig. 11. EPMA elemental maps of the coating after oxidation at 1350 °C for 40 h.
Fig. 13. The specimen keeps integrated and a discontinuous SiO2 film doped with Al2O3 particles has formed on the coating surface (Fig. 13a,b). EDS results (Fig. 13c-d) show the higher distribution of Al in the covered areas, indicating formation of Al2O3 (reaction (3)) may promote the growth of SiO2 scale [34]. Subsequently, B2O3 could be generated (reaction (4)) due to its higher concentration and favorable diffusion aspects in the MoSi2 bulk [35]. The fluidity of SiO2 could be obviously improved by B2O3 even with a rather low content, then SiO2 nuclei could flow sideways to cover the coating continuously, insulating it from oxidizing ambient [26,31,33]. At this temperature, Y is readily to be oxidized as reaction (5) due to its high affinity with oxygen (Fig. 12) [36]. It is reported that reaction (1) is thermodynamically favored at high temperatures and reaction (2) is kinetically favored at low temperatures [30,31]. Nevertheless, Mo5Si3 is absent at the scale/MoSi2 interface when oxidized at 1250 and 1350 °C, indicating this phenomenon should be due to kinetics factors. According to the literatures [30,31], oxidation of MoSi2 is a diffusion controlled process. The diffusion coefficient of Si could be illustrated as the Arrhenius type relationship of reaction (6).
Fig. 12. Calculated standard free energy change values for the probable reactions (e.g. 4/3Y + O2 = 2/3Y2O3) at different oxidation temperatures.
5/7MoSi2 + O2 → 1/7Mo5Si3 + SiO2
(1)
2/7MoSi2 + O2 → 2/7MoO3 + 4/7SiO2
(2)
4/3Al + O2 → 2/3Al2O3
(3)
4/3B + O2 → 2/3B2O3
(4)
4/3Y + O2 → 2/3Y2O3
(5)
D = D0 exp(
At 800 °C, due to the low temperature and the barrier effect of WSi2 layer, outward diffusion of the alloying elements is mostly restricted except for Al. Oxidation of the alloying elements is avoided and the reaction on the coating surface should be reactions (1)–(5). Reactions (1) and (2) are the main oxidation process of MoSi2 and they are both identified at 800 °C for observation of pores in the scale and Mo5Si3 at the scale/MoSi2 interface simultaneously (Fig. 4) [30,31]. Pesting oxidation is successfully suppressed at this temperature. Due to the strict polishing process before oxidation, influence of the residual pack agent could be ignored and the achievement seems to be the effect of addition of B [32,33]. Whereas, effect of Al2O3 should be considered in view of concentration of Al in the scale (Fig. 5). For better understanding, the coating was oxidized at 800 °C for 2 h and the results are listed in
Q ) RT
(6)
5/7WSi2 → 1/7W5Si3 + Si
(7)
1/7Mo5Si3 + Si → 5/7MoSi2
(8)
Si + O2 → SiO2
(9)
where D0 is the diffusion constant, Q is the activation energy, R is the gas constant and T is temperature in Kelvin. At 800 °C, the diffusion rate of the released Si is too low to sustain the oxidation, and the generated Mo5Si3 according reaction (1) is retained after oxidation. On the contrary, the diffusion could be increased at 1250 and 1350 °C, and the released Si from the WSi2 layer could diffuse onto the scale/MoSi2 interface, suppress the formation of Mo5Si3 and to sustain the oxidation. This process could be illustrated as reactions (7) and (8). Combining reaction (1), oxidation essence of MoSi2/WSi2 compound coating at high temperatures is diffusion and preferential oxidation of Si (reaction (9)), like mono MoSi2 at high temperatures [31]. The underneath WSi2 layer could be a reservoir for Si and to buffer oxidation of MoSi2 at high temperatures. This similar phenomenon has also been reported by 8
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(b)
(a)
Al2O3 particle
Spot A
Spot B
Before oxidation
After oxidation
10 ȝm (c)
Element
(d)
Content (at. %)
Element
Content (at. %)
O
66.2
O
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23.0
Si
20.7
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Mo
8.0
Mo
2.6
Ti
0.1
Y
0.3
Hf
0.1
W
0.2
Ti
0.1
Spot B
0.6
Fig. 13. (a) Macroscopic morphologies of the coated sample before and after oxidation at 800 °C for 2 h, (b) surface morphology of the coating and (c) (d) the corresponding EDS analysis results of the marked sites in (b).
Fig. 14. Sketch map of oxidation and interfacial diffusion of the MoSi2/WSi2 compound coating at 800, 1250 and 1350 °C.
from formation of TiO2 and Nb2O5 particles in the scale (Figs. 7 and 8). At 1350 °C, the WSi2 layer degrades into porous W5Si3 layer completely within 20 h, which cannot suppress the outward diffusion of alloying elements any more (Fig. 10b′′). Accordingly, Nb and Ti, the major component elements of the substrate could diffuse onto the scale/ MoSi2 interface driven by the chemical potential and then react with oxygen. As a result, Kirkendall voids leave in the substrate and the scale become thicker by doping with the oxide particles like Nb2O5, YNbO4 and AlNbO4. Remarkably, the individual TiO2 and Cr2O3 particles are not detected and these elements distribute in the complicated oxides mainly (Fig. 11 and Table 5). Formation of these complicated oxides probably follows reactions (10)–(12)
Meheriz in CoWSi-WSi2 case in which CoWSi act as Si reservoir [37]. Nevertheless, this process will lead degradation of WSi2 and formation of W5Si3 particles accordingly (seen in Fig. 4b′′-c′′). At high temperatures, oxidation and diffusion of Si accelerate, so the growth of W5Si3 could be increased. Diffusion reaction between WSi2 layer and substrate is inevitable and it could be obviously accelerated at elevated temperatures. This process will increase loosing of Si and then lead degradation of WSi2 (formation of W5Si3) at the interface preferentially for the near location [17]. With the prolonging oxidation, the W5Si3 particles could grow up preferentially and incorporate with each other, then forming a continuous W5Si3 layer between residual WSi2 layer and substrate eventually. Decomposition could endanger the suppression ability of WSi2 layer at high temperatures. Nevertheless, degradation rate of WSi2 layer is quite slow, and the W5Si3 layer becomes obvious after oxidation longer than 50 h. Moreover, there is still a residual WSi2 layer left even after oxidation at 1250 °C for 100 h. This residual layer still shows admirable suppression to the alloying elements, preventing 9
4/5Nb + O2 → 2/5Nb2O5
(10)
Nb2O5 + Y2O3 → 2YNbO4
(11)
Nb2O5 + Al2O3 → 2AlNbO4
(12)
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(a)
(b)
AlNbO4
YNbO4
Scale
20 ȝm 20 ȝm
(d)
Crack
77.3O-14.2Nb-6.2Ti-0.9Cr0.6Al-0.6Hf-0.2Y
Fig. 15. (a) Macroscopic morphology of the specimen after oxidation at 1350 °C for 50 h, SEM morphology of the (b) residual coating and (c) exfoliated scale and (d) composition of the brown scale.
5. Conclusions
Formation of the layered scale is probably related to the diffusion and oxidation of Si. It could be viewed that the oxide particles and SiO2 could be formed at the scale/MoSi2 interface simultaneously (Fig. 10a′′c′′). However, due to the larger amount of Si, the amount of SiO2 generated at the interface is larger too. Thus, a clear SiO2 layer forms at the interface and it is retained perpetually. The oxides particles generate as the result of oxidation of alloying elements and they could be the nuclei to grow up with prolonging oxidation. In addition, the new formed small particles could move upwardly and then be incorporated by the existed bigger particles (Fig. 10c“). Extensive oxidation of Si not only leads the degradation of WSi2 layer, but also is responsible for the degradation of MoSi2 layer. After the WSi2 degrades completely, the near MoSi2 degrades as well due to the possible inducement effect of W5Si3 layer. In addition, considering the small thickness of IDZ layer, it could be inferred that oxidation is the main cause for the degradation. Sketch map of oxidation and interfacial diffusion of the MoSi2/WSi2 compound coating at 800, 1250 and 1350 °C is presented in Fig. 14. Fig. 15a shows the macroscopic morphology of the specimen after oxidation at 1350 °C for 50 h. It can be viewed that there is extensive peeling off of brown scales around the side faces. Moreover, the coating ruptures and the exposed areas show the same appearance of the exfoliated scales. XRD and EDS analysis results suggest the Nb2O5 + TiO2 + TiNb2O7 nature of the brown scale (Fig. 15c,d). These observations indicate that oxidation at the coating/substrate interface results in failure of the compound coating. In addition, exhaustion of the lateral NbSi2 layer is also related. Once the side protection fails, the oxygen can readily diffuse along the coating/substrate interface and react with the substrate. Formation of these oxides can cause volume expansion at the interface, which may result in exfoliation of the compound coating, as shown in Fig. 15. Another potential problem of the compound coating is that the bonding between MoSi2/WSi2 coating and alloy is not firm enough; scraping or crashing may lead rupture of the coating and then disastrous oxidation of the substrate. Thus, the future focus is trying to improve the interfacial adhesion and prolong the service life of WSi2 layer at ultra-high temperatures as possible.
The MoSi2/WSi2 compound coating was prepared on the surface of Nb-Ti-Si based alloy through MS and HAPC techniques. The coating showed perfect oxidation resistance at 800 °C and 1250 °C and the oxidation kinetics followed parabolic laws. The scale growth rate constants were 0.335 μm2 h−1 and 0.939 μm2 h−1, respectively. It is believed that suppression of pesting oxidation at 800 oC should be the result from B addition. Besides, formation of Al2O3 was beneficial to improve the growth of scale and it may be helpful too. Generation of Mo5Si3 at the scale/MoSi2 interface was mainly resulted from the slow diffusion rate of Si at 800 °C. Thus, this phenomenon could be suppressed at 1250 and 1350 °C. WSi2 layer shows admirable diffusion suppression effect at 1250 °C, and a dense SiO2-Al2O3 based scale formed on the surface. In addition, the WSi2 layer also played the role of Si reservoir, through that service life of the MoSi2 coating was increased. At 1350 oC, the WSi2 layer completely degraded within 20 h and it cannot restrict the outward diffusion of alloying elements. As a result, Nb2O5, YNbO4 and AlNbO4 form in the scale, and the scale growth rate is increased. The invalidation test at 1350 °C indicated that exhaustion of the lateral NbSi2 coating and interfacial oxidation causing the failure of the coating system.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51431003, U1435201, 51971177 and 51971181) and the Fundamental Research Funds for the Central Universities (No. 3102019TS0401).
Declaration of Competing Interest The authors declare no competing financial interest.
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References [19]
[1] J.A. Haynes, Y. Zhang, K.M. Cooley, L. Walker, K.S. Reeves, B.A. Pint, High-temperature diffusion barriers for protective coatings, Surf. Coat. Technol. 188–189 (2004) 153–157. [2] Y. Matsumura, M. Fukumoto, S. Hayashi, A. Kasama, I. Iwanaga, R. Tanaka, T. Narita, Oxidation behavior of a Re-base diffusion-barrier/β-NiAl Coating on Nb5Mo-15W at high temperatures, Oxid. Met. 61 (2004) 105–124. [3] R. Ghasemi, Z. Valefi, Electrodeposition of rhenium-base layer as a diffusion barrier between the NiCoCrAlY coating and a Ni-based superalloy, J. Alloy Compd. 732 (2018) 470–485. [4] R. Ghasemi, Z. Valefi, The effect of the Re-Ni diffusion barrier on the adhesion strength and thermal shock resistance of the NiCoCrAlY coating, Surf. Coat. Technol. 344 (2018) 359–372. [5] K. Kurokawa, G. Ochiai, H. Takahashi, S. Ohta, H. Takahashi, Effects of sputterdeposited materials (W, Ti and SiC) on interfacial reaction between MoSi2 and Nb, Vacuum 59 (2000) 284–291. [6] B. Bai, H.B. Guo, H. Peng, L.Q. Peng, S.K. Gong, Cyclic oxidation and interdiffusion behavior of a NiAlDy/RuNiAl coating on a Ni-based single crystal superalloy, Corros. Sci. 53 (2011) 2721–2727. [7] Y. Wang, H.B. Guo, H. Peng, L.Q. Peng, S.K. Gong, Diffusion barrier behaviors of (Ru, Ni)Al/NiAl coatings on Ni-based superalloy substrate, Intermetallics 19 (2011) 191–195. [8] Z.M. Bai, D.Q. Li, H. Peng, J. Wang, H.B. Guo, S.K. Gong, Suppressing the formation of SRZ in a Ni-based single crystal superalloy by RuNiAl diffusion barrier, Prog. Nat. Sci-Mater. 22 (2012) 146–152. [9] T. Narita, K.Z. Thosin, L. Fengqun, S. Hayashi, H. Murakami, B. Gleeson, D. Young, Development of Re-based diffusion barrier coatings on nickel based super alloys, Mater. Corros. 56 (2005) 923–929. [10] T. Izumi, T. Yoshioka, T. Narita, Effect of Re-W-Cr-Ni diffusion barrier on high temperature oxidation of Pt-modified β-NiAl coated superalloy, J. Japan Inst. Metals. 72 (2008) 728–732. [11] C.P. Lungua, I. Mustataa, G. Musaa, A.M. Lungua, V. Zaroschia, K. Iwasakib, R. Tanakab, Y. Matsumurab, I. Iwanagab, H. Tanakab, T. Oib, K. Fujita, Formation of nanostructured Re-Cr-Ni diffusion barrier coatings on Nb superalloys by TVA method, Surf. Coat. Technol. 200 (2005) 399–402. [12] E. Sugiarti1, Y.M. Wang, N. Hashimoto, S. Ohnuki, T. Narita, Phase characterization of Re-based diffusion barrier layer on Nb substrate, Mater. Trans. 52 (2011) 319–323. [13] A. Littner, M. Francois, B. Malaman, J. Steinmetz, M. Vilasi, E. Elkaim, Isothermal section at 1673 K of the Mo-Ru-Si diagram and crystallographic structures of ternary phases, Intermetallics 11 (2003) 1223–1228. [14] H. Bei, Y. Yang, G.B. Viswanathan, C.J. Rawn, E.P. George, J. Tiley, Y.A. Chang, Formation, stability and crystal structure of the δ phase in Mo-Re-Si alloys, Acta Mater. 58 (2010) 6027–6034. [15] T.C. Lu, A.G. Evans, R.J. Hecht, R. Mehrabian, Toughing of MoSi2 with a ductile (niobium) reinforcement, Acta Metall. Mater. 39 (1991) 1853–1862. [16] L. Shaw, R. Abbaschian, Control of the interfacial reactions in Nb-toughened MoSi2, J. Amer. Chem. Soc. 76 (1993) 2305–2311. [17] J.H. Yan, Y. Wang, L.F. Liu, Y.M. Wang, Oxidation and interdiffusion behavior of Niobium substrate coated MoSi2 coating prepared by spark plasma sintering, Appl. Surf. Sci. 320 (2014) 791–797. [18] P.I. Glushko, V.I. Zmii, N.A. Semenov, B.M. Shirokov, Stability and heat resistance
[20]
[21]
[22]
[23]
[24] [25]
[26]
[27]
[28]
[29]
[30] [31] [32]
[33]
[34] [35] [36]
[37]
11
of silicide coatings on refractory metals. III. stability of silicide coatings on niobium heated in air at 1500–1800oC, Powder Metall. Met. Ceram. 42 (2003) 154–157. F. Chen, J.G. Xu, Y.B. Liu, L. Cai, In situ reactive spark plasma sintering of WSi2/ MoSi2 composites, Ceram. Int. 42 (2016) 11165–11169. V.I. Zmii, N.V. Kovtun, P.I. Glushko, S.G. Ruden’kii, Stability and heat resistance of silicide coatings on refractory metals. II. stability and Heat resistance of silicide coatings on tungsten and molybdenum at 1500–2000°C, Powder Metall. Met. Ceram. 42 (2003) 50–53. P.C. Tortorici, M.A. Dayananda, Interdiffusion and diffusion structure development in selected refractory metal silicides, Mater. Sci. Eng.: A 261 (1-2) (1999) 64–77, https://doi.org/10.1016/S0921-5093(98)01050-8. B.K. Bhattacharyya, D.M. Bylander, L. Kleinman, Comparison of fully relativistic energy bands and cohesive energies of MoSi2 and WSi2, Phys. Rev. 32B (1985) 7973–7976. H.A. Zhang, J.X. Lv, Y.H. Wu, S.Y. Gu, Y. Huang, Y. Chen, Oxidation behavior of (Mo, W)Si2-Si3N4 composite coating on molybdenum substrate at 1600oC, Ceram. Int. 41 (2015) 14890–14895. J.O. Olowolafe, E.G. Colgan, C.J. Palmstr, J.W. Mayer, Interfacial reaction of Mo-W alloys with silicon, Thin Solid Films 138 (1986) 245–254. Y. Harada, Y. Funato, M. Morinaga, A. Ito, Y. Sugita, Solid solubilities of ternary elements and their effects on microstructure of MoSi2, J. Japan Inst. Metals. 58 (1994) 1239–1247. L.F. Su, O.L. Steffes, H. Zhang, J.H. Perepezko, An ultra-high temperature Mo-Si-B based coating for oxidation protection of Nbss/Nb5Si3 composites, Appl. Surf. Sci. 337 (2015) 38–44. T.F. Retajczyk, A.K. Sinha, Elastic stiffness and thermal expansion coefficients of various refractory silicides and silicon nitride films, Thin Solid Films 70 (1980) 241–247. T.S.R.Ch. Murthy, J.K. Sonber, C. Subramanian, R.C. Hubli, A.K. Suri, Densification, characterization and oxidation studies of TiB2-WSi2 composite, Int. J. Refract. Met. Hard Mater. 33 (2012) 10–21. O.J. Lu-Steffes, R. Sakidja, J. Bero, J.H. Perepezko, Multicomponent coating for enhanced oxidation resistance of tungsten, Surf. Coat. Technol. 207 (2012) 614–619. Y.Q. Liu, G. Shao, P. Tsakiropoulos, On the oxidation behavior of MoSi2, Intermetallics 9 (2001) 125–136. A.A. Sharif, High-temperature oxidation of MoSi2, J. Mater. Sci. 45 (2010) 865–870. B.V. Cockeram, G. Wang, R.A. Rapp, Growth kinetics and pesting resistance of MoSi2 and germanium-doped MoSi2 diffusion coatings grown by the pack cementation method, Mater. Corros. 46 (1995) 207–217. B.V. Cockeram, R.A. Rapp, Oxidation-resistant boron- and germanium-doped silicide coatings for refractory metals at high temperature, Mater. Sci. Eng. A 192 (193) (1995) 980–986. T. Maruyama, K. Yanagihara, High temperature oxidation and pesting of Mo(Si, Al) 2, Mater. Sci. Eng. A 239–240 (1997) 828–841. S. Majumdar, B. Gorr, H.J. Christ, D. Schliephake, M. Heilmaier, Oxidation mechanisms of lanthanum-alloyed Mo-Si-B, Corros. Sci. 88 (2014) 360–371. S. Majumdar, A. Kumar, D. Schliephake, H.J. Christ, X. Jiang, M. Heilmaier, Microstructural and micro-mechanical properties of Mo-Si-B alloyed with Y and La, Mater. Sci. Eng. A 573 (2013) 257–263. M.Z. Mehrizi, M. Shamanian, A. Saidi, R. Shoja-Razazvi, R. Beigi, Evaluation of oxidation behavior of laser clad CoWSi-WSi2 coating on pure Ni substrate at different temperatures, Ceram. Int. 41 (2015) 9715–9721.