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Effect of mullite on the microstructure and oxidation behavior of thermal-sprayed MoSi2 coating at 1500 °C
Journal Pre-proof Effect of mullite on the microstructure and oxidation behavior of thermal-sprayed MoSi2 coating at 1500�°C Guangpeng Zhang, Jia Sun, Qiangang Fu PII:
S0272-8842(19)33775-7
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.273
Reference:
CERI 23926
To appear in:
Ceramics International
Received Date: 25 November 2019 Revised Date:
16 December 2019
Accepted Date: 31 December 2019
Please cite this article as: G. Zhang, J. Sun, Q. Fu, Effect of mullite on the microstructure and oxidation behavior of thermal-sprayed MoSi2 coating at 1500�°C, Ceramics International (2020), doi: https:// doi.org/10.1016/j.ceramint.2019.12.273. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Effect of mullite on the microstructure and oxidation
2
behavior of thermal-sprayed MoSi2 coating at 1500 oC
3
Guangpeng Zhang, Jia Sun*, Qiangang Fu*
4
a
State Key Laboratory of Solidification Processing, Carbon/Carbon Composites
5
Research Center, Northwestern Polytechnical University, Xi'an 710072, China
6 7
Abstract: MoSi2 coatings with different mullite additions (0, 10 wt.%, and 30 wt.%)
8
were prepared on siliconized Nb alloys and oxidation behaviors of the modified coatings
9
were comparatively investigated at 1500 oC in static air. After oxidation for 140 h, the 10
10
wt.% mullite modified coating presented an outstanding oxidation resistance with a mass
11
loss of 4.06 mg/cm2 in comparison to the unmodified coating and the 30 wt.% modified
12
case. A proper amount of mullite addition in MoSi2 coating can restrain the crystallzation
13
of SiO2 during the oxidation process, thus a crack-free oxide scale was obtained so as to
14
prevent the inward diffusion of oxygen. However, excessive addition of mullite (30 wt.%)
15
in MoSi2 coating can severeley hinder the formation of the crack-free oxide scale due to
16
the volume shrinkage of the amorphous mullite, which accelerated the failure of the
17
protective coatings.
18
Key words: MoSi2; Mullite; Coating; Plasma spray; Oxidation; Nb alloy.
19
1. Introduction
20
Niobium (Nb) alloys, one of the promising high temperature structural materials,
21
have received a great deal of attentions in recent years due to their many advantages
22
including high melting point (2468 oC), relative low density (8.57 g/cm3), and good
1
1
mechanical properties at elevated temperature [1-4]. However, sever oxidation for Nb
2
alloys at high temperature greatly restricts their engineering applications [5-7]. It has
3
been proved that employing oxidation resistant coating is an effective method to solve
4
this issue [8, 9]. Up to now, many coating systems, especially silicide coatings, were
5
developed to protect Nb alloys from oxidation [10-12].
6
MoSi2 has a superiority to be employed as a protective coating on Nb alloys for the
7
reason of its close CTE (thermal expansion coefficient, ~8.0×10-6/K) value to that of Nb
8
alloy (7.8-8.2×10-6/K) [5]. Furthermore, a bond layer, always made of the Nb-Si
9
intermetallics, is also needed before applying MoSi2 coating in order to improve the
10
adhesion between the coating and Nb substrate [6,13]. MoSi2 coatings can be prepared by
11
different methods, such as packing siliconizing [14,15], slurry method [16], molten salt
12
[17], and thermal spray [18-20]. Among them, the plasma spray technique possesses great
13
advantages to prepare MoSi2 coating with controllable thickness and composite
14
components which can contribute a good oxidation resistance [21]. However, the service
15
performance of thermal sprayed single MoSi2 coating is thermodynamically unstable
16
during a long-term oxidation period for the sake of cracking in the coating, elemental
17
inter-diffusion between coating and substrate, and the thermal degradation of the MoSi2
18
coating itself [22].
19
In order to improve the thermal stability of MoSi2 coating in terms of cracking and
20
structural integrity upon service, a variety of potential ceramics were introduced as
21
reinforcements, such as SiC, Al2O3, Si3N4, ZrO2 and mullite [23-26]. Among them,
22
mullite (3Al2O3·2SiO2) attracts a great deal of attentions due to its high melting point
23
(1830 oC), good chemical stability, excellent high-temperature strength, great oxidation
2
1
and thermal shock resistance [27,28]. For the coating application on Nb alloy, the mullite
2
can be expected to improve the oxidation resistance and creep strength of MoSi2 coating
3
at elevated temperatures. However, the systematical investigation on the thermal
4
degradation of mullite modified MoSi2 based composite coating is still not clear.
5
In the present work, MoSi2 coatings with different amount of mullite additions were
6
prepared by supersonic atmospheric plasma spraying (SAPS) on the siliconized Nb alloys.
7
MoSi2 coating without mullite reinforcement was prepared as well for comparison. The
8
microstructure of the coatings and their oxidation behaviors at 1500 oC were elaborately
9
studied. The role of mullite is carefully analyzed by XRD (X-ray diffraction) and TEM
10
(Transmission electron microscopy) to reveal the different oxidation mechanisms
11
compared to the pristine samples.
12
2. Experimental procedures
13
2.1. Specimen preparation
14
Nb521 alloys were used as substrate in this work for coating experiments, the
15
chemical composition (wt. %) of Nb521 is W 5-6, Zr 1.5-1.7, Mo 2.1-2.5 (Bal. Nb).
16
Commercial MoSi2 particles in size of below 5µm (provided by Songshan tungsten and
17
molybdenum materials Co., Ltd., Dengfeng, China) and mullite particles in size of 1µm
18
(provided by Fanrui new materials Co., Ltd., Zhengzhou, China) were used as the raw
19
materials for the coating. The spraying powders were obtained by the spray-drying
20
atomization treatment [29] and three different powders were employed in the present
21
work: pure MoSi2 powders (M0 powders), MoSi2 with 10 wt.% (M10 powders) and 30
22
wt.% mullite powders (M30 powders). Mixed powders of MoSi2 and mullite were
3
1
dispersed into 1~3 wt.% polyvinyl alcohol solutions and then milled by zircon balls for
2
3-7 h to get a homogeneous slurry. Subsequently, the slurry was transported to a
3
centrifugal spray-dry apparatus to get agglomerated powders with a proper fluidity. And
4
the obtained powders were sieved to be in a particle size distribution of 50-70 µm.
5
Before SAPS, a siliconized layer was prepared by HAPC (halide activated pack
6
cementation) as a bonding layer. The main details about the HAPC have been reported in
7
our previous work [6]. In this work, the HAPC process was conducted at 1250 oC for 1~3
8
h to get the siliconized layer with a thickness of 80 µm. The coated Nb alloys were
9
cleaned ultrasonically in ethanol for 30-45 min and dried at 70 oC. Then, the outer layer
10
was prepared by a high efficiency plasma spray system. And the working parameters of
11
the plasma spraying are shown in Table 1. The obtained coatings were denoted as M0,
12
M10, and M30 coatings, respectively.
13
2.2. Oxidation Tests
14
To evaluate the oxidation resistance of the coating, the isothermal oxidation tests of
15
the coated samples were carried out at 1500 oC under static air via an electrical furnace.
16
Prior to the oxidation test, the Al2O3 crucibles without loading samples were heated at
17
1500 oC to remove the impurity. Then, the specimens were loaded in the crucibles and put
18
into the electric furnace for oxidation tests. All three coated specimens, M0, M10, and
19
M30, were concurrently tested. And every kind of coating has three parallel samples for
20
statistical average. After a same certain period, the samples were taken out and cool down
21
to room temperature. Then the mass of the tested samples was weighed by an analytical
22
balance. Afterwards, the specimens were put back to the furnace for the next heating
4
1
cycle. The oxidation tests would be stopped as soon as the coating peeled off or obvious
2
oxides were observed on the sample surface.
3
The mass change rate (∆W, mg/cm2) of samples during oxidation was calculated by
4
the follow equation:
5
△W = (mt – m0)/S
6
in which the m0 represents the original mass (mg) of samples, mt represents the mass (mg)
7
after every cycle during isothermal oxidation process, and S (cm2) is the surface area of
8
the original specimens.
9
2.3. Characterization
(1)
10
The phases of the coatings were analyzed by an X-ray diffraction instrument with a
11
Cu target. The morphologies and the element composition of the coatings before and after
12
oxidation were investigated by the scanning electron microscopy (SEM, VEGAT-2,
13
Germany) equipped with an energy dispersive spectroscopy (EDS). And the transmission
14
electron microscopy analysis was performed using a FEI G2 instrument (FEI, TECNAL,
15
USA) at an acceleration voltage of 300 kV.
16
3. Results and discussion
17
3.1. Microstructure and phase composition of the coatings
18
Fig. 1(a) shows the XRD patterns of the as-sprayed coatings. The main phases of the
19
sprayed coatings were t-MoSi2, h-MoSi2, Mo5Si3 and Mo. MoSi2 exists in the tetragonal
20
crystalline structure (t-MoSi2) below 1900 oC, and it will turn to the hexagonal crystalline
21
structure (h-MoSi2) when temperature is between 1900 oC and its melting temperature
22
(2030 oC) [5]. During the spraying process, the powders were melted fully in plasma 5
1
torch, and then ejected onto substrate rapidly. It is reasonable that some h-MoSi2
2
remained at room temperature because of the high cooling rate. For the same reason, it
3
has been reported that the mullite mainly existed in a glass state after the thermal
4
spraying [30,31]. So, the diffraction peak intensity of mullite is weak. Mo5Si3 was
5
generated because of the slight oxidation of MoSi2 during the spraying process (Eq. (2)).
6
The reflection peak of Mo was detected possibly due to the thermolysis of in-flight
7
droplet during the plasma spraying process and the oxidation of MoSi2 [32, 33] (Eq. (3)).
8
Figs. 1(b-d) show the surface morphologies of the coatings. A relative rough and undulate
9
surface was obtained after spraying. Unlike M0 coating (Fig. 1(b)), no obvious defects
10
such as holes were found on M10 and M30 (Figs. 1(c-d)) coating surfaces. The main
11
reason might be that the mullite became glassy during plasma spraying process, which
12
can seal the partial voids to a certain extent.
13
5 MoSi2 (s) + 7 O2 (g) → Mo5Si3 (s) + 7 SiO2 (s)
(2)
14
MoSi2 (s) + 2 O2 (g) → Mo (s) + 2 SiO2 (s)
(3)
15
Cross-sectional micro-structures of the as-sprayed coatings are shown in Fig. 2.
16
From Figs. 2(a-c), the inner siliconized layer bonds well with the outer sprayed layer in
17
all samples. It has been proved in our previous work, that the rough surface of siliconized
18
layer could greatly improve the adhesion between the sprayed coating and substrate [6].
19
According to the point EDS analysis results of the as-sprayed coating (Figs. 2(d-f)) listed
20
in Table 2, it can be analyzed that the mullite distributed relatively evenly in the coating.
21
In the M30 coating, a lot of unmolten particles stacked loosely (Fig. 2(f)) were clearly
22
observed. The possible reason is that the melting point of mullite is relatively lower than
23
MoSi2, so excessive mullite might restrain the melting degree of MoSi2 during spraying
6
1
process, resulting in the formation of more unmolten MoSi2 particles and larger holes in
2
the M30 coating compared with other two species.
3
3.2 Oxidation behaviors of the coated samples
4
3.2.1. Oxidation kinetic
5
The isothermal oxidation kinetic curves of three kinds of samples are illustrated in
6
Fig. 3, in which the insets are the corresponding macrographs of the coated samples after
7
oxidation. During the oxidation test at 1500 oC, M0 and M30 coatings failed to service
8
after untilizing for 130 h and 37 h, respectively, while M10 coating still provided an
9
effective and continuous protection for the Nb alloy substrate even for 140 h. The final
10
recorded mass change rates of M0, M10 and M30 samples were 21.38 mg/cm2, -4.06
11
mg/cm2 and -2.77 mg/cm2 (the minus represents the mass loss of the samples),
12
respectively.
13
Different samples exhibited different mass change tendency during oxidation period.
14
M0 samples displayed a mass loss at initial stage of oxidation (< 2 h), then their mass
15
kept relatively stable, while a rapid mass gain was observed after 118 h. As shown in the
16
inserted picture of Fig. 3, the dark yellow acicular substance is niobium oxide, which
17
indicates that M0 coating has been destroyed and a violent oxidation occoured to
18
substrate. M10 samples exhibited a slow and continuous mass loss during the whole
19
oxidation process. A visible smooth and dense glassy film was formed during oxidation
20
and provided a long-term protection for Nb alloy substrate. Compared with M0 and M10
21
samples, M30 samples gained mass at the initial stage and then lost mass until the coating
7
1
peeled off after 37 h. Further comparing with M10 coating, M30 coating displayed a dark
2
surface after oxidation, which might result from its much thicker glassy layer.
3
Apart from Eq. (2), the possible chemical reactions during the oxidation are as
4
follows:
5
2 MoSi2 (s) + 7 O2 (g) → 2MoO3 (g) + 4 SiO2 (s)
(4)
6
2 Mo5Si3 (s) + 21 O2 (g) → 10MoO3 (g) + 6 SiO2 (s)
(5)
7
2 Mo (s) + 3 O2 (g) → 2MoO3 (g)
(6)
8
In the initial stage of oxidation, M10 samples prestented a lower mass loss and M30
9
samples presented a slight mass gain compared with M0 case. The mullite in the M10 and
10
M30 coating prevented the effective contact between the MoSi2 and air, and then the
11
oxidation of MoSi2 may be restrained in some degree, which might cause the lower mass
12
loss of M10 samples at early period. Besides, more pores existed in M30 coating. The
13
main oxidation products of MoSi2 at the pores were Mo5Si3 and SiO2 due to the lower
14
oxygen pressure inside the coating (Eq. (2)). And small amount of MoO3 generated at the
15
pores may not pass through the coating and volitilize immediately in a short time. So, the
16
mass of M30 samples slightly increased at the initial oxidation stage.
17
3.2.2. Microstructure of the coating after oxidation
18
To clearly show the difference of oxidation resistance, M0, M10, and M30 samples
19
after oxidation were comapred at the service life of 37 h. Fig. 4(a) shows the XRD
20
patterns of the three kinds of coatings after oxidation for 37 h. The main phases of three
21
coatings after oxidation are Mo5Si3, MoSi2, and SiO2. A reflection of mullite was
22
obviously detected in M30 coating because of the recrystallization of amorphous mullite
23
after oxidation at 1500 oC [23]. It is obvious that crystalline SiO2 was generated in M0 8
1
coating. Both XRD patterns of M10 and M30 coatings present an amorphous
2
back-ground in the range of 10~35°. Moreover, the intensity of SiO2 diffraction peak of
3
M30 coating is much lower, which implies that the existence of mullite might restrain the
4
crystallization of SiO2 [34]. Furthermore, the diffraction peaks of Mo5Si3 and MoSi2 tend
5
to become lower in turn from M0 to M30, which suggests that M10 and M30 coatings
6
possess a denser glassy film than M0 coating.
7
Figs. 4(b-d) show the surface morphologies of M0, M10, and M30 coating after
8
oxidation for 37 h, respectively. All coating surfaces were covered mostly by a glassy
9
film. For the M0 coatings, some larger-size cracks were observed, and the rough bulges
10
caused by coating deposition still remained (Fig. 4(b)). By contrast, M10 coating (Fig.
11
4(c)) displayed a quite smooth, dense and integrated surface, and no defects were visible.
12
During oxidation, the mullite reacted with SiO2 according to Eqs. (7) and (8) [19,20]. A
13
new silicate glass with a good fluidity and self-healing effect was formed, thus M10
14
coating presented a crack-free surface. While for the M30 coatings, the interlaced
15
needle-shaped hollows which damaged the coating integrity were observed in local areas
16
(Fig. 4(d)). EDS analysis on such areas shows the chemical composition is 56.24 at.% O,
17
29.01 at.% Al, 14.75 at.% Si, which is close to 3Al2O3·2SiO2. So, it suggests that the
18
occourence of hollows might result from the precipitation of mullite during thermal
19
cycles.
20
The thicknesses of the outer layer in three coatings before oxidation and after
21
oxidation for 37 h are illustrated in Fig. 5(a). From Figs. 5(a-b), the thickness of outer
22
layer in M30 coating decreased by nearly a half after oxidation for 37 h, while the outer
23
layer thickness of M0 and M10 coating had no obvious change. The mullite mainly
9
1
existed in the form of amorphous phase after plasma spraying, and the recrystallization of
2
mullite at high temperature was accompanied with a volume contraction. It has been
3
reported that the associated stress resulted from the volume shrinkage in the sprayed
4
coating was the primary reason for the coating debonding [35-37] (Fig. 5(c)). So, M30
5
coating peeled off after oxidation for only 37 h.
6
3Al2O3·2SiO2 (s) + SiO2 (s) → Silicates glass (m)
(7)
7
Silicates (m) → Al2O3 (s) + SiO2 (s)
(8)
8
The oxidation difference of M0 and M10 samples was further investigated after a
9
lomg-term oxidation. XRD analysis of the coated samples after 130-140h exposure is
10
shown in Fig. 6. The phase composition of M0 coating (Fig. 6(a)) is similar to the one
11
oxidized for 37 h (Fig. 4(a)). Nb2O5 was detected, which suggests that the niobium oxides
12
were formed on the M0 coating surface. As for the M10 coating, the obvious amorphous
13
silica was detected since a hump peak occurred in Fig. 6(b). Al2O3 was also probed
14
because of the dissociation of mullite at high temperature (Eq. (8)).
15
Figs. 7(a-b) show the surface morphologies of M0 coating after oxidation for 130 h.
16
M0 coating has been destroyed extremely due to cracking and spallation of the layered
17
structure. From Fig. 7(b), the coating presented a scaly topography. EDS analysis of point
18
H and point G from the surface scale is given in Table 2. Combining with XRD analysis
19
(Fig. 6(a)), it can be identified that the dark phase is SiO2 and the light phase is niobium
20
oxide. As shown in Fig. 7(c), M10 coating still exhibited a quite smooth glassy surface
21
after oxidation for 140h, in which no defects were observed. In the high magnification
22
SEM view (Fig. 7(d)), there are none of fine cracks and small pores on the surface scale.
10
1
3.3. Oxidation mechanism of the mullite modified coating
2
Figs. 8(a-b) show the cross-sectional morphologies of the M0 and M10 coating
3
respectively, where a four-layer structure was formed after oxidation. Combined with
4
EDS analysis (Fig. 8(e)), both in M0 and M10 coatings that the diffusion layer and the
5
porous layer in the four-layer structure are detected as Nb5Si3 and (Nb,Mo)5Si3,
6
respectively. Mullite dispersed uniformly in the outer layer of M10 coating. The light
7
grey phase which concentrated at the upper region of outer layer (3` zone in Fig. 8(b)) is
8
Mo5Si3. Compared with the M10 coating, there is no obvious Mo5Si3 found beneath the
9
oxide scale of M0 coating. The absence of Mo5Si3 may be ascribed to the rapid oxidation
10
of MoSi2 for that the protective glass layer has been destroyed.
11
A porous layer was formed both in M0 and M10 coating. During oxidation period,
12
Si in the coating diffused towards the surface to react with oxygen and formed SiO2
13
protective layer, and it also diffused into substrate to generate sub-silicide phases. It has
14
been revealed that Si diffuses faster than Nb and Mo in MoSi2 and NbSi2 [38,39], so the
15
NbSi2 and MoSi2 transformed into Nb5Si3 and Mo5Si3 respectively as the diffusion of Si.
16
While, the diffusion of Si through the silicide requires a counterflow of vacancies, so a
17
great number of voids were formed through vacancy condensation. As more and more
18
voids gathering, many larger holes were formed. These voids may decrease the effective
19
Young’s modulus of the coating to improve the resistance to thermal cycling under the air
20
[40].
21
As shown in Figs. 8(a)(c), many obvious defects like large holes, vertical cracks,
22
were obviously observed in M0 coating. From Fig. 8(d), niobium oxides were found in
23
oxide layer. During the thermal cycles, the thermal stress due to the severe mismatch of
11
1
CTE between SiO2 (0.55 × 10-6 /K) and MoSi2 coating induced the crack extension in the
2
oxide scale [41], then MoSi2 layer would be oxidized. Such a CTE mismatch situation
3
may be reduced due to the closer CTE value by adding some mullite into the coating.
4
Simultaniously, the thermal stress concentration occured to the interface of coating and
5
substrate [5]. And shear stress along the interface would result in vertical cracks, which
6
accelerated the inward diffusion of oxygen and deteriorated the oxidation of the
7
NbSi2/MoSi2 interface. Furthermore, the niobium elements diffused towards the coating
8
surface during the oxidation process. The niobium oxides would generate once the
9
niobium and oxygen reached up to a certain concentration. Unfortunately, niobium oxides
10
grow disorderly, and tremendous stress will generate at the interface of coating and
11
substrate because of its large volume ratio to substrate [42], which would break up the
12
integrity of the coating and weaken the adhesion of coating. Niobium does not generally
13
react with SiO2 to form a compound [43], the flowable self-healing SiO2 glass film would
14
be consumed rapidly, leaving behind Nb2O5 and discontinuous SiO2 without protective
15
ability (Fig. 8(d)). By contrast, the crack-free surface of M10 coating effectively
16
prevented the inward diffusion of oxygen, greatly slowed down the oxidation rate of
17
niobium. Thus a long-term oxidation resistance of the coated Nb alloy was achieved.
18
More details about M0 and M10 coating after oxidation were further investigated by
19
TEM, and the results are shown in Fig. 9. Obvious difference between M0 and M10
20
coating was observed from Fig. 9. Well-crystallized SiO2 (Figs. 9(a-b)) was formed in
21
M0 coating after oxidation, while from HRTEM images (Fig. 9(c)) and SAED pattern
22
(Fig. 9(d)), the oxide scale of M10 coating is composed of fine crystal SiO2 and a great
23
number of amorphous SiO2. These results are consistent with the XRD analysis (Fig. 6),
12
1
which suggests that it is more difficult for SiO2 to crystallize due to the introduction of
2
mullite into MoSi2 based coating. The amorphous SiO2 possesses a lower softening
3
temperture, so the oxide layer may possess a better fluidity. It has been reported that Al
4
atom diffuses into SiO2 and substitutes a Si atom, causing the network structure of SiO2
5
disorder [44,45]. According to the model for estimating viscosities of aluminosilicate
6
melts [46], the introduction of Al2O3 coming from mullite works as a network modifier
7
and adjust the viscosity of pure SiO2 [20]. Compared to the pure SiO2 formed on M0
8
coating, an oxide scale with the improved sealf-healing capacity was obtained on M10
9
coating to provide a long-term protection at 1500 oC.
10
4. Conclusions
11
In order to improve the oxidation resistance of MoSi2 coating on the siliconized Nb
12
alloys at high temperature, MoSi2, MoSi2-10 wt.% mullite and MoSi2-30 wt.% mullite
13
coatings were prepared by SAPS. After exposure at 1500 oC, M0 samples were oxidized
14
for 130 h with a mass gain of 21.38 mg/cm2. While M10 samples presented a mass loss of
15
only 4.06 mg/cm2 even after oxidation for 140 h. M30 coating could merely service 37 h
16
due to the spallation of the coating. The outstanding oxidation resistance of M10 coating
17
was attributed to the improved surface oxide scale with no cracking by the mullite
18
addition. A proper amount of mullite can restrain the crystallization of the SiO2, then the
19
crack resistance and healing ability of the oxide scale would be obtained, which can
20
provide a good protection for substrate. Excessive mullite (30 wt.%), however, led to a
21
great volume shrinkage of the composite coating as a result of recrystallization of mullite,
22
which caused the coating debonding after a short-time oxidation.
23 13
1
Acknowledgements
2
We would like to thank the Analytical & Testing Center of Northwestern
3
Polytechnical University for the characterization of our samples. This work has been
4
supported by the Fund of State Key Laboratory of Solidification Processing
5
(SKLSP201819) in Northwestern Polytechnical University (NPU), and by the National
6
Natural Science Foundation of China under Grant No. 51572223 and 51872239.
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
14
1
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[10] S. Majumdar, J. Kishor, B. Paul, R.C. Hubli, J.K. Chakravartty, Isothermal oxidation behavior and growth kinetics of silicide coatings formed on Nb-1Zr-0.1C alloy, Corros. Sci. 95 (2015) 100-109. [11] Y.Y. Zhang, Y.G. Li, C.G. Bai, Microstructure and oxidation behavior of Si-MoSi2 functionally graded coating on Mo substrate, Ceram. Int. 43 (2017) 6250-6256. [12] L. Liu, H. Lei, J. Gong, C. Sun, Deposition and oxidation behaviour of molybdenum disilicide coating on Nb based alloys substrate by combined AIP/HAPC processes, Ceram. Int. 45 (2019) 10525-10529. [13] J. Sun, Q.G. Fu, L.P. Guo, Influence of siliconizing on the oxidation behavior of plasma sprayed MoSi2 coating for niobium based alloy, Intermetallics. 72 (2016) 9-16. [14] J. Pang, W. Wang, C.G. Zhou, Microstructure evolution and oxidation behavior of B modified MoSi2 coating on Nb-Si based alloys, Corros. Sci. 105 (2016) 1-7. [15] J.Y. Wu, W. Wang, C.G. Zhou, Microstructure and oxidation resistance of Mo-Si-B coating on Nb based in situ composites, Corros. Sci. 87 (2014) 421-426. [16] L.R. Xiao, D.Q. Yi, L. Yin, Z.G. Cai, Morphology and structure of high temperature MoSi2 coating on niobium, Trans. Nonferrous Met. Soc. China. 15 (2005) 18-22. [17] K. Tatemoto, Y. Ono, R.O. Suzuki, Silicide coating on refractory metals in molten salt, J. Phys. Chem. Solids. 66 (2005) 526-529. [18] G. Reisel, B. Wielage, S. Steinhäuser, I. Morgenthal, R. Scholl, High temperature oxidation behavior of HVOF-sprayed unreinforced and reinforced molybdenum disilicide powders, Surf. Coat. Technol. 146 (2001) 19-26. [19] Y. Wang, D.Z. Wang, J.H. Yan, A.K. Sun, Preparation and characterization of molybdenum disilicide coating on molybdenum substrate by air plasma spraying, Appl. Surf. Sci. 284 (2013) 881-888. [20] G.R. Li, L.S. Wang, G.J. Yang, A novel composite-layered coating enabling self-enhancing thermal barrier performance, Scripta Mater. 163 (2019) 142-147.
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[21] J. Sun, Q.G. Fu, G.P. Zhang, C. Wang, T. Li, Melting index of plasma sprayed molybdenum silicide characterized by constitutions and its effect on oxidation kinetics, J. Alloy. Compd. 766 (2018) 95-103. [22] P. Zhang, X.P. Guo, Y and Al modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy prepared by pack cementation process, Surf. Coat. Technol. 206 (2011) 446-454. [23] F. Chen, J.G. Xu, Z.F. Hou, In situ pressureless sintering of SiC/MoSi2 composites, Ceram. Int. 38 (2012) 2767-2772. [24] X.A. Fei, Y.R. Niu, H. Ji, 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. [ 25] 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 1600 oC, Ceram. Int. 41 (2015), 14890-14895. [26] Z.I. Zaki, N.Y. Mostafa, Y.M.Z. Ahmed, Synthesis of dense mullite/MoSi2 composite for high temperature applications, Int. J. Refract. Met. Hard Mat. 45 (2014) 23-30. [27] K.T. Wang, L.Y. Cao, J.F. Huang, J. Fei, A mullite/SiC oxidation protective coating for carbon/carbon composites, J. Eur. Ceram. Soc. 33 (2013) 191-198. [28] X.Z. Liu, C.M. Deng, C.G. Deng, M. Liu, K.S. Zhou, Mullite-modified ZrB2-MoSi2 coating for carbon/carbon composites to withstand long term ablation, Ceram. Int. 44 (2018) 4330-4337. [29] C.C. Wang, K.Z. Li, C.X. Huo, Q.C. He, X.H. Shi, Oxidation behavior and microstructural evolution of plasma sprayed La2O3-MoSi2-SiC coating on carbon/carbon composites, Surf. Coat. Technol. 348 (2018) 81-90. [30] E. Withey, C. Petorak, R. Trice, T. Taylor, Design of 7wt.% Y2O3-ZrO2/mullite plasma-sprayed composite coatings for increased creep resistance, J. Eur. Ceram. Soc. 27 (2007) 4675-4683. [31] C. Ma, H.J. Li, H. Wu, Q.G. Fu, C. Sun, X.H. Shi, Y.L. Zhang, Z.Z. Zhang, J. Tao, Z.H. Han, Mullite oxidation resistant coating for SiC-coated carbon/carbon composites by supersonic plasma spraying, J. Mater. Sci. Technol. 29 (2013) 29-33.
17
[32] J. Sun, T. Li, G.P. Zhang, Effect of thermodynamically metastable components on mechanical and oxidation properties of the thermal-sprayed MoSi2 based composite coating, Corros. Sci. 155 (2019) 146-154. [33] J.H. Yan, L.F. Liu, Z.Y. Mao, H.M. Xu, Y.M. Wang, Effect of spraying powders size on the microstructure, bonding strength, and microhardness of MoSi2 coating prepared by air plasma spraying, J. Therm. Spray Technol. 23 (2014) 934-939. [34] C.Y. Cheng, H.J. Li, Q.G. Fu, L. Li, L.P. Guo, Influence of Al2O3 on the oxidation resistance of SiC ceramic: First-principle study and experiment, Corros. Sci. 134 (2018) 57-63. [35] P. Ramaswamy, S. Seetharamu, K.J. Rao, K.B.R. Varma, Thermal shock characteristics of plasma sprayed mullite coatings, J. Therm. Spray Technol. 7 (1998) 497-504. [36] K.N. Lee, R.A. Miller, N.S. Jacobson, New generation of plasma-sprayed mullite coatings on silicon carbide, J. Am. Ceram. Soc. 78 (1995) 705-710. [37] Y.L. An, X.Q. Zhao, G.L. Hou, H.D. Zhou, J. Chen, J.M. Chen, Tribological and thermal properties of mullite coating prepared by atmospheric plasma spraying, J. Therm. Spray Technol. 23 (2014) 410-419. [38] S. Prasad, A. Paul, Growth mechanism of phases by interdiffusion and atomic mechanism of diffusion in the molybdenum–silicon system, Intermetallics. 19 (2011) 1191-1200. [39] S. Prasad, A. Paul, Growth mechanism of phases by interdiffusion and diffusion of species in the niobium–silicon system, Acta Mater. 59 (2011) 1577–1585. [40] A. Mueller, G. Wang, R.A. Rapp, E.L. Courtright, T.A. Kircher, Oxidation behavior of tungsten and germanium-alloyed molybdenum disilicide coatings, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 155 (1992) 199-207. [41] Z.Y. Cai, X.J. Zhao, D.X. Zhang, Y.H. Wu, J.X. Wen, G.Y. Tian, Q.X. Cao, X.Y. Tang, L.R. Xiao, S.N. Liu, Microstructure and oxidation resistance of a YSZ modified silicide coating for Ta-W alloy at 1800 oC, Corros. Sci. 143 (2018) 116-128.
18
[42] J. Sun, T. Li, G.P. Zhang, Q.G. Fu, Different oxidation protection mechanisms of HAPC silicide coating on niobium alloy over a large temperature range, J. Alloys Compd. 790 (2019) 1014-1022 [43] M. Ibrahim, N.F.H. Bright, The Binary System Nb2O5-SiO2, J. Am. Ceram. Soc. 45 (1962): 221-222. [44] Y.G. Wang, W.F. Fei, Y. Fan, L.G. Zhang, Silicoaluminum carbonitride ceramic resist to oxidation/corrosion in water vapor, J. Mater. Res. 21 (2006) 1625-1628. [45] J. Lægsgaard, K. Stokbro, Electronic structure and hyperfine parameters of substitutional Al and P impurities in silica, Phys. Rev. B. 65 (2002) 075208. [46] M. Nakamoto, Y. Miyabayashi, L. Holappa, T. Tanaka, A model for estimating viscosities of aluminosilicate melts containing alkali oxides, ISIJ Int. 47 (2007) 1409-1415.
19
Table 1
Parameters of the plasma spraying process for M0, M10, and M30 coatings.
Parameters
Values
Spraying power, kW Primary gas Ar, L/min Carrier gas Ar, L/min Second gas H2, L/min Powder feed rate, g/min Spraying distance, mm Nozzle inner diameter, mm
40~45 75 10 3.5 20 100 5.5
Table 2
Composition analysis of the spots shown in Fig. 3, Fig. 5, Fig. 7 and Fig. 8.
Spots
Mo (at.%)
Si (at.%)
O (at.%)
Al (at.%)
Nb (at.%)
Phases determined
A
6.44
39.25
54.31
0
0
SiO2 + MoSi2
B
0
19.24
61.56
19.2
0
SiO2 + Mullite
C
35.06
64.94
0
0
0
MoSi2
D
34.33
13.84
46.51
5.32
0
Mo5Si3 + Mullite
E
0
22.66
53.23
24.11
0
Mullite + SiO2
F
0
14.75
56.24
29.01
0
Mullite
G
0
4.32
66.24
2.81
26.64
Nb2O5 + SiO2
H
0
36.92
63.08
0
0
SiO2
I J K
0 0 0
45.51 35.28 6.91
50.05 64.72 71.85
4.44 0 1.75
0 0 19.48
Aluminosilicate SiO2 Nb2O5 + SiO2
Figure Captions Fig. 1. Phase and morphology analysis of as-sprayed coatings: (a) XRD patterns and surface SEM images of (b) M0 coating, (c) M10 coating and (d) M30 coating. Fig. 2. Cross-sectional backscattered electron (BSE) images of as-sprayed coatings: (a, d) M0 coating; (b, e) M10 coating; (c, f) M30 coating. Fig. 3. Oxidation kinetic curves of coated samples at 1500 oC in static air. The macrographs show the actual status of the representative sample in every case after oxidation. Fig. 4. Phase and morphology analysis of 37h-oxidized coatings: (a) XRD patterns and surface SEM images of (b) M0 coating, (c) M10 coating and (d) M30 coating. Fig. 5. Comparative analysis of three kinds of coatings: (a) the histogram of the outer-layer thickness of coatings before and after oxidation for 37 h at 1500 oC; (b, c) the cross-sectional BSE images of M30 coating after oxidation. Fig. 6. XRD patterns of the coatings after oxidation: (a) M0 coating exposed for 130 h; (b) M10 coating exposed for 140 h. Fig. 7. Surface SEM images of the coating after oxidation: (a, b) M0 coating; (c, d) M10 coating. Fig. 8. Cross-sectional microstructure analysis of the coating after oxidation: (a, c, d) M0 coating after oxidation for 130h; (b) M10 coating after oxidation for 140h; (e) element distribution through the cross section of M10 coating. Fig. 9. TEM analysis of M0 and M10 coatings after oxidation: (a) HRTEM images of M0 coating; (b) FFT pattern of domain #1 in (a); (c, d) HRTEM images of M10 coating and the corresponding FFT and SAED patterns.
Fig. 1. Phase and morphology analysis of as-sprayed coatings: (a) XRD patterns and surface SEM images of (b) M0 coating, (c) M10 coating and (d) M30 coating.
Fig. 2. Cross-sectional backscattered electron (BSE) images of as-sprayed coatings: (a, d) M0 coating; (b, e) M10 coating; (c, f) M30 coating.
Fig. 3. Oxidation kinetic curves of coated samples at 1500 oC in static air. The macrographs show the actual status of the representative sample in every case after oxidation.
Fig. 4. Phase and morphology analysis of 37h-oxidized coatings: (a) XRD patterns and surface SEM images of (b) M0 coating, (c) M10 coating and (d) M30 coating.
Fig. 5. Comparative analysis of three kinds of coatings: (a) the histogram of the outer-layer thickness of coatings before and after oxidation for 37 h at 1500 oC; (b, c) the cross-sectional BSE images of M30 coating after oxidation.
Fig. 6. XRD patterns of the coatings after oxidation: (a) M0 coating exposed for 130 h; (b) M10 coating exposed for 140 h.
Fig. 7. Surface SEM images of the coating after oxidation: (a, b) M0 coating; (c, d) M10 coating.
Fig. 8. Cross-sectional microstructure analysis of the coating after oxidation: (a, c, d) M0 coating after oxidation for 130h; (b) M10 coating after oxidation for 140h; (e) element distribution through the cross section of M10 coating.
Fig. 9. TEM analysis of M0 and M10 coatings after oxidation: (a) HRTEM images of M0 coating; (b) FFT pattern of domain #1 in (a); (c, d) HRTEM images of M10 coating and the corresponding FFT and SAED patterns.
Declaration of Interest Statement We would like to submit the enclosed manuscript entitled “Effect of mullite on the microstructure and oxidation behavior of thermal-sprayed MoSi2 coating at 1500 o
C”, which we wish to be considered as a full article for publication in Ceramics
International. The manuscript is approved by all authors for publication and there is no conflict of interest in the submission of this manuscript. We certify that the submission has not been published previously and it is not under review at any other publication.
S0272-8842(19)33775-7
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.273
Reference:
CERI 23926
To appear in:
Ceramics International
Received Date: 25 November 2019 Revised Date:
16 December 2019
Accepted Date: 31 December 2019
Please cite this article as: G. Zhang, J. Sun, Q. Fu, Effect of mullite on the microstructure and oxidation behavior of thermal-sprayed MoSi2 coating at 1500�°C, Ceramics International (2020), doi: https:// doi.org/10.1016/j.ceramint.2019.12.273. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Effect of mullite on the microstructure and oxidation
2
behavior of thermal-sprayed MoSi2 coating at 1500 oC
3
Guangpeng Zhang, Jia Sun*, Qiangang Fu*
4
a
State Key Laboratory of Solidification Processing, Carbon/Carbon Composites
5
Research Center, Northwestern Polytechnical University, Xi'an 710072, China
6 7
Abstract: MoSi2 coatings with different mullite additions (0, 10 wt.%, and 30 wt.%)
8
were prepared on siliconized Nb alloys and oxidation behaviors of the modified coatings
9
were comparatively investigated at 1500 oC in static air. After oxidation for 140 h, the 10
10
wt.% mullite modified coating presented an outstanding oxidation resistance with a mass
11
loss of 4.06 mg/cm2 in comparison to the unmodified coating and the 30 wt.% modified
12
case. A proper amount of mullite addition in MoSi2 coating can restrain the crystallzation
13
of SiO2 during the oxidation process, thus a crack-free oxide scale was obtained so as to
14
prevent the inward diffusion of oxygen. However, excessive addition of mullite (30 wt.%)
15
in MoSi2 coating can severeley hinder the formation of the crack-free oxide scale due to
16
the volume shrinkage of the amorphous mullite, which accelerated the failure of the
17
protective coatings.
18
Key words: MoSi2; Mullite; Coating; Plasma spray; Oxidation; Nb alloy.
19
1. Introduction
20
Niobium (Nb) alloys, one of the promising high temperature structural materials,
21
have received a great deal of attentions in recent years due to their many advantages
22
including high melting point (2468 oC), relative low density (8.57 g/cm3), and good
1
1
mechanical properties at elevated temperature [1-4]. However, sever oxidation for Nb
2
alloys at high temperature greatly restricts their engineering applications [5-7]. It has
3
been proved that employing oxidation resistant coating is an effective method to solve
4
this issue [8, 9]. Up to now, many coating systems, especially silicide coatings, were
5
developed to protect Nb alloys from oxidation [10-12].
6
MoSi2 has a superiority to be employed as a protective coating on Nb alloys for the
7
reason of its close CTE (thermal expansion coefficient, ~8.0×10-6/K) value to that of Nb
8
alloy (7.8-8.2×10-6/K) [5]. Furthermore, a bond layer, always made of the Nb-Si
9
intermetallics, is also needed before applying MoSi2 coating in order to improve the
10
adhesion between the coating and Nb substrate [6,13]. MoSi2 coatings can be prepared by
11
different methods, such as packing siliconizing [14,15], slurry method [16], molten salt
12
[17], and thermal spray [18-20]. Among them, the plasma spray technique possesses great
13
advantages to prepare MoSi2 coating with controllable thickness and composite
14
components which can contribute a good oxidation resistance [21]. However, the service
15
performance of thermal sprayed single MoSi2 coating is thermodynamically unstable
16
during a long-term oxidation period for the sake of cracking in the coating, elemental
17
inter-diffusion between coating and substrate, and the thermal degradation of the MoSi2
18
coating itself [22].
19
In order to improve the thermal stability of MoSi2 coating in terms of cracking and
20
structural integrity upon service, a variety of potential ceramics were introduced as
21
reinforcements, such as SiC, Al2O3, Si3N4, ZrO2 and mullite [23-26]. Among them,
22
mullite (3Al2O3·2SiO2) attracts a great deal of attentions due to its high melting point
23
(1830 oC), good chemical stability, excellent high-temperature strength, great oxidation
2
1
and thermal shock resistance [27,28]. For the coating application on Nb alloy, the mullite
2
can be expected to improve the oxidation resistance and creep strength of MoSi2 coating
3
at elevated temperatures. However, the systematical investigation on the thermal
4
degradation of mullite modified MoSi2 based composite coating is still not clear.
5
In the present work, MoSi2 coatings with different amount of mullite additions were
6
prepared by supersonic atmospheric plasma spraying (SAPS) on the siliconized Nb alloys.
7
MoSi2 coating without mullite reinforcement was prepared as well for comparison. The
8
microstructure of the coatings and their oxidation behaviors at 1500 oC were elaborately
9
studied. The role of mullite is carefully analyzed by XRD (X-ray diffraction) and TEM
10
(Transmission electron microscopy) to reveal the different oxidation mechanisms
11
compared to the pristine samples.
12
2. Experimental procedures
13
2.1. Specimen preparation
14
Nb521 alloys were used as substrate in this work for coating experiments, the
15
chemical composition (wt. %) of Nb521 is W 5-6, Zr 1.5-1.7, Mo 2.1-2.5 (Bal. Nb).
16
Commercial MoSi2 particles in size of below 5µm (provided by Songshan tungsten and
17
molybdenum materials Co., Ltd., Dengfeng, China) and mullite particles in size of 1µm
18
(provided by Fanrui new materials Co., Ltd., Zhengzhou, China) were used as the raw
19
materials for the coating. The spraying powders were obtained by the spray-drying
20
atomization treatment [29] and three different powders were employed in the present
21
work: pure MoSi2 powders (M0 powders), MoSi2 with 10 wt.% (M10 powders) and 30
22
wt.% mullite powders (M30 powders). Mixed powders of MoSi2 and mullite were
3
1
dispersed into 1~3 wt.% polyvinyl alcohol solutions and then milled by zircon balls for
2
3-7 h to get a homogeneous slurry. Subsequently, the slurry was transported to a
3
centrifugal spray-dry apparatus to get agglomerated powders with a proper fluidity. And
4
the obtained powders were sieved to be in a particle size distribution of 50-70 µm.
5
Before SAPS, a siliconized layer was prepared by HAPC (halide activated pack
6
cementation) as a bonding layer. The main details about the HAPC have been reported in
7
our previous work [6]. In this work, the HAPC process was conducted at 1250 oC for 1~3
8
h to get the siliconized layer with a thickness of 80 µm. The coated Nb alloys were
9
cleaned ultrasonically in ethanol for 30-45 min and dried at 70 oC. Then, the outer layer
10
was prepared by a high efficiency plasma spray system. And the working parameters of
11
the plasma spraying are shown in Table 1. The obtained coatings were denoted as M0,
12
M10, and M30 coatings, respectively.
13
2.2. Oxidation Tests
14
To evaluate the oxidation resistance of the coating, the isothermal oxidation tests of
15
the coated samples were carried out at 1500 oC under static air via an electrical furnace.
16
Prior to the oxidation test, the Al2O3 crucibles without loading samples were heated at
17
1500 oC to remove the impurity. Then, the specimens were loaded in the crucibles and put
18
into the electric furnace for oxidation tests. All three coated specimens, M0, M10, and
19
M30, were concurrently tested. And every kind of coating has three parallel samples for
20
statistical average. After a same certain period, the samples were taken out and cool down
21
to room temperature. Then the mass of the tested samples was weighed by an analytical
22
balance. Afterwards, the specimens were put back to the furnace for the next heating
4
1
cycle. The oxidation tests would be stopped as soon as the coating peeled off or obvious
2
oxides were observed on the sample surface.
3
The mass change rate (∆W, mg/cm2) of samples during oxidation was calculated by
4
the follow equation:
5
△W = (mt – m0)/S
6
in which the m0 represents the original mass (mg) of samples, mt represents the mass (mg)
7
after every cycle during isothermal oxidation process, and S (cm2) is the surface area of
8
the original specimens.
9
2.3. Characterization
(1)
10
The phases of the coatings were analyzed by an X-ray diffraction instrument with a
11
Cu target. The morphologies and the element composition of the coatings before and after
12
oxidation were investigated by the scanning electron microscopy (SEM, VEGAT-2,
13
Germany) equipped with an energy dispersive spectroscopy (EDS). And the transmission
14
electron microscopy analysis was performed using a FEI G2 instrument (FEI, TECNAL,
15
USA) at an acceleration voltage of 300 kV.
16
3. Results and discussion
17
3.1. Microstructure and phase composition of the coatings
18
Fig. 1(a) shows the XRD patterns of the as-sprayed coatings. The main phases of the
19
sprayed coatings were t-MoSi2, h-MoSi2, Mo5Si3 and Mo. MoSi2 exists in the tetragonal
20
crystalline structure (t-MoSi2) below 1900 oC, and it will turn to the hexagonal crystalline
21
structure (h-MoSi2) when temperature is between 1900 oC and its melting temperature
22
(2030 oC) [5]. During the spraying process, the powders were melted fully in plasma 5
1
torch, and then ejected onto substrate rapidly. It is reasonable that some h-MoSi2
2
remained at room temperature because of the high cooling rate. For the same reason, it
3
has been reported that the mullite mainly existed in a glass state after the thermal
4
spraying [30,31]. So, the diffraction peak intensity of mullite is weak. Mo5Si3 was
5
generated because of the slight oxidation of MoSi2 during the spraying process (Eq. (2)).
6
The reflection peak of Mo was detected possibly due to the thermolysis of in-flight
7
droplet during the plasma spraying process and the oxidation of MoSi2 [32, 33] (Eq. (3)).
8
Figs. 1(b-d) show the surface morphologies of the coatings. A relative rough and undulate
9
surface was obtained after spraying. Unlike M0 coating (Fig. 1(b)), no obvious defects
10
such as holes were found on M10 and M30 (Figs. 1(c-d)) coating surfaces. The main
11
reason might be that the mullite became glassy during plasma spraying process, which
12
can seal the partial voids to a certain extent.
13
5 MoSi2 (s) + 7 O2 (g) → Mo5Si3 (s) + 7 SiO2 (s)
(2)
14
MoSi2 (s) + 2 O2 (g) → Mo (s) + 2 SiO2 (s)
(3)
15
Cross-sectional micro-structures of the as-sprayed coatings are shown in Fig. 2.
16
From Figs. 2(a-c), the inner siliconized layer bonds well with the outer sprayed layer in
17
all samples. It has been proved in our previous work, that the rough surface of siliconized
18
layer could greatly improve the adhesion between the sprayed coating and substrate [6].
19
According to the point EDS analysis results of the as-sprayed coating (Figs. 2(d-f)) listed
20
in Table 2, it can be analyzed that the mullite distributed relatively evenly in the coating.
21
In the M30 coating, a lot of unmolten particles stacked loosely (Fig. 2(f)) were clearly
22
observed. The possible reason is that the melting point of mullite is relatively lower than
23
MoSi2, so excessive mullite might restrain the melting degree of MoSi2 during spraying
6
1
process, resulting in the formation of more unmolten MoSi2 particles and larger holes in
2
the M30 coating compared with other two species.
3
3.2 Oxidation behaviors of the coated samples
4
3.2.1. Oxidation kinetic
5
The isothermal oxidation kinetic curves of three kinds of samples are illustrated in
6
Fig. 3, in which the insets are the corresponding macrographs of the coated samples after
7
oxidation. During the oxidation test at 1500 oC, M0 and M30 coatings failed to service
8
after untilizing for 130 h and 37 h, respectively, while M10 coating still provided an
9
effective and continuous protection for the Nb alloy substrate even for 140 h. The final
10
recorded mass change rates of M0, M10 and M30 samples were 21.38 mg/cm2, -4.06
11
mg/cm2 and -2.77 mg/cm2 (the minus represents the mass loss of the samples),
12
respectively.
13
Different samples exhibited different mass change tendency during oxidation period.
14
M0 samples displayed a mass loss at initial stage of oxidation (< 2 h), then their mass
15
kept relatively stable, while a rapid mass gain was observed after 118 h. As shown in the
16
inserted picture of Fig. 3, the dark yellow acicular substance is niobium oxide, which
17
indicates that M0 coating has been destroyed and a violent oxidation occoured to
18
substrate. M10 samples exhibited a slow and continuous mass loss during the whole
19
oxidation process. A visible smooth and dense glassy film was formed during oxidation
20
and provided a long-term protection for Nb alloy substrate. Compared with M0 and M10
21
samples, M30 samples gained mass at the initial stage and then lost mass until the coating
7
1
peeled off after 37 h. Further comparing with M10 coating, M30 coating displayed a dark
2
surface after oxidation, which might result from its much thicker glassy layer.
3
Apart from Eq. (2), the possible chemical reactions during the oxidation are as
4
follows:
5
2 MoSi2 (s) + 7 O2 (g) → 2MoO3 (g) + 4 SiO2 (s)
(4)
6
2 Mo5Si3 (s) + 21 O2 (g) → 10MoO3 (g) + 6 SiO2 (s)
(5)
7
2 Mo (s) + 3 O2 (g) → 2MoO3 (g)
(6)
8
In the initial stage of oxidation, M10 samples prestented a lower mass loss and M30
9
samples presented a slight mass gain compared with M0 case. The mullite in the M10 and
10
M30 coating prevented the effective contact between the MoSi2 and air, and then the
11
oxidation of MoSi2 may be restrained in some degree, which might cause the lower mass
12
loss of M10 samples at early period. Besides, more pores existed in M30 coating. The
13
main oxidation products of MoSi2 at the pores were Mo5Si3 and SiO2 due to the lower
14
oxygen pressure inside the coating (Eq. (2)). And small amount of MoO3 generated at the
15
pores may not pass through the coating and volitilize immediately in a short time. So, the
16
mass of M30 samples slightly increased at the initial oxidation stage.
17
3.2.2. Microstructure of the coating after oxidation
18
To clearly show the difference of oxidation resistance, M0, M10, and M30 samples
19
after oxidation were comapred at the service life of 37 h. Fig. 4(a) shows the XRD
20
patterns of the three kinds of coatings after oxidation for 37 h. The main phases of three
21
coatings after oxidation are Mo5Si3, MoSi2, and SiO2. A reflection of mullite was
22
obviously detected in M30 coating because of the recrystallization of amorphous mullite
23
after oxidation at 1500 oC [23]. It is obvious that crystalline SiO2 was generated in M0 8
1
coating. Both XRD patterns of M10 and M30 coatings present an amorphous
2
back-ground in the range of 10~35°. Moreover, the intensity of SiO2 diffraction peak of
3
M30 coating is much lower, which implies that the existence of mullite might restrain the
4
crystallization of SiO2 [34]. Furthermore, the diffraction peaks of Mo5Si3 and MoSi2 tend
5
to become lower in turn from M0 to M30, which suggests that M10 and M30 coatings
6
possess a denser glassy film than M0 coating.
7
Figs. 4(b-d) show the surface morphologies of M0, M10, and M30 coating after
8
oxidation for 37 h, respectively. All coating surfaces were covered mostly by a glassy
9
film. For the M0 coatings, some larger-size cracks were observed, and the rough bulges
10
caused by coating deposition still remained (Fig. 4(b)). By contrast, M10 coating (Fig.
11
4(c)) displayed a quite smooth, dense and integrated surface, and no defects were visible.
12
During oxidation, the mullite reacted with SiO2 according to Eqs. (7) and (8) [19,20]. A
13
new silicate glass with a good fluidity and self-healing effect was formed, thus M10
14
coating presented a crack-free surface. While for the M30 coatings, the interlaced
15
needle-shaped hollows which damaged the coating integrity were observed in local areas
16
(Fig. 4(d)). EDS analysis on such areas shows the chemical composition is 56.24 at.% O,
17
29.01 at.% Al, 14.75 at.% Si, which is close to 3Al2O3·2SiO2. So, it suggests that the
18
occourence of hollows might result from the precipitation of mullite during thermal
19
cycles.
20
The thicknesses of the outer layer in three coatings before oxidation and after
21
oxidation for 37 h are illustrated in Fig. 5(a). From Figs. 5(a-b), the thickness of outer
22
layer in M30 coating decreased by nearly a half after oxidation for 37 h, while the outer
23
layer thickness of M0 and M10 coating had no obvious change. The mullite mainly
9
1
existed in the form of amorphous phase after plasma spraying, and the recrystallization of
2
mullite at high temperature was accompanied with a volume contraction. It has been
3
reported that the associated stress resulted from the volume shrinkage in the sprayed
4
coating was the primary reason for the coating debonding [35-37] (Fig. 5(c)). So, M30
5
coating peeled off after oxidation for only 37 h.
6
3Al2O3·2SiO2 (s) + SiO2 (s) → Silicates glass (m)
(7)
7
Silicates (m) → Al2O3 (s) + SiO2 (s)
(8)
8
The oxidation difference of M0 and M10 samples was further investigated after a
9
lomg-term oxidation. XRD analysis of the coated samples after 130-140h exposure is
10
shown in Fig. 6. The phase composition of M0 coating (Fig. 6(a)) is similar to the one
11
oxidized for 37 h (Fig. 4(a)). Nb2O5 was detected, which suggests that the niobium oxides
12
were formed on the M0 coating surface. As for the M10 coating, the obvious amorphous
13
silica was detected since a hump peak occurred in Fig. 6(b). Al2O3 was also probed
14
because of the dissociation of mullite at high temperature (Eq. (8)).
15
Figs. 7(a-b) show the surface morphologies of M0 coating after oxidation for 130 h.
16
M0 coating has been destroyed extremely due to cracking and spallation of the layered
17
structure. From Fig. 7(b), the coating presented a scaly topography. EDS analysis of point
18
H and point G from the surface scale is given in Table 2. Combining with XRD analysis
19
(Fig. 6(a)), it can be identified that the dark phase is SiO2 and the light phase is niobium
20
oxide. As shown in Fig. 7(c), M10 coating still exhibited a quite smooth glassy surface
21
after oxidation for 140h, in which no defects were observed. In the high magnification
22
SEM view (Fig. 7(d)), there are none of fine cracks and small pores on the surface scale.
10
1
3.3. Oxidation mechanism of the mullite modified coating
2
Figs. 8(a-b) show the cross-sectional morphologies of the M0 and M10 coating
3
respectively, where a four-layer structure was formed after oxidation. Combined with
4
EDS analysis (Fig. 8(e)), both in M0 and M10 coatings that the diffusion layer and the
5
porous layer in the four-layer structure are detected as Nb5Si3 and (Nb,Mo)5Si3,
6
respectively. Mullite dispersed uniformly in the outer layer of M10 coating. The light
7
grey phase which concentrated at the upper region of outer layer (3` zone in Fig. 8(b)) is
8
Mo5Si3. Compared with the M10 coating, there is no obvious Mo5Si3 found beneath the
9
oxide scale of M0 coating. The absence of Mo5Si3 may be ascribed to the rapid oxidation
10
of MoSi2 for that the protective glass layer has been destroyed.
11
A porous layer was formed both in M0 and M10 coating. During oxidation period,
12
Si in the coating diffused towards the surface to react with oxygen and formed SiO2
13
protective layer, and it also diffused into substrate to generate sub-silicide phases. It has
14
been revealed that Si diffuses faster than Nb and Mo in MoSi2 and NbSi2 [38,39], so the
15
NbSi2 and MoSi2 transformed into Nb5Si3 and Mo5Si3 respectively as the diffusion of Si.
16
While, the diffusion of Si through the silicide requires a counterflow of vacancies, so a
17
great number of voids were formed through vacancy condensation. As more and more
18
voids gathering, many larger holes were formed. These voids may decrease the effective
19
Young’s modulus of the coating to improve the resistance to thermal cycling under the air
20
[40].
21
As shown in Figs. 8(a)(c), many obvious defects like large holes, vertical cracks,
22
were obviously observed in M0 coating. From Fig. 8(d), niobium oxides were found in
23
oxide layer. During the thermal cycles, the thermal stress due to the severe mismatch of
11
1
CTE between SiO2 (0.55 × 10-6 /K) and MoSi2 coating induced the crack extension in the
2
oxide scale [41], then MoSi2 layer would be oxidized. Such a CTE mismatch situation
3
may be reduced due to the closer CTE value by adding some mullite into the coating.
4
Simultaniously, the thermal stress concentration occured to the interface of coating and
5
substrate [5]. And shear stress along the interface would result in vertical cracks, which
6
accelerated the inward diffusion of oxygen and deteriorated the oxidation of the
7
NbSi2/MoSi2 interface. Furthermore, the niobium elements diffused towards the coating
8
surface during the oxidation process. The niobium oxides would generate once the
9
niobium and oxygen reached up to a certain concentration. Unfortunately, niobium oxides
10
grow disorderly, and tremendous stress will generate at the interface of coating and
11
substrate because of its large volume ratio to substrate [42], which would break up the
12
integrity of the coating and weaken the adhesion of coating. Niobium does not generally
13
react with SiO2 to form a compound [43], the flowable self-healing SiO2 glass film would
14
be consumed rapidly, leaving behind Nb2O5 and discontinuous SiO2 without protective
15
ability (Fig. 8(d)). By contrast, the crack-free surface of M10 coating effectively
16
prevented the inward diffusion of oxygen, greatly slowed down the oxidation rate of
17
niobium. Thus a long-term oxidation resistance of the coated Nb alloy was achieved.
18
More details about M0 and M10 coating after oxidation were further investigated by
19
TEM, and the results are shown in Fig. 9. Obvious difference between M0 and M10
20
coating was observed from Fig. 9. Well-crystallized SiO2 (Figs. 9(a-b)) was formed in
21
M0 coating after oxidation, while from HRTEM images (Fig. 9(c)) and SAED pattern
22
(Fig. 9(d)), the oxide scale of M10 coating is composed of fine crystal SiO2 and a great
23
number of amorphous SiO2. These results are consistent with the XRD analysis (Fig. 6),
12
1
which suggests that it is more difficult for SiO2 to crystallize due to the introduction of
2
mullite into MoSi2 based coating. The amorphous SiO2 possesses a lower softening
3
temperture, so the oxide layer may possess a better fluidity. It has been reported that Al
4
atom diffuses into SiO2 and substitutes a Si atom, causing the network structure of SiO2
5
disorder [44,45]. According to the model for estimating viscosities of aluminosilicate
6
melts [46], the introduction of Al2O3 coming from mullite works as a network modifier
7
and adjust the viscosity of pure SiO2 [20]. Compared to the pure SiO2 formed on M0
8
coating, an oxide scale with the improved sealf-healing capacity was obtained on M10
9
coating to provide a long-term protection at 1500 oC.
10
4. Conclusions
11
In order to improve the oxidation resistance of MoSi2 coating on the siliconized Nb
12
alloys at high temperature, MoSi2, MoSi2-10 wt.% mullite and MoSi2-30 wt.% mullite
13
coatings were prepared by SAPS. After exposure at 1500 oC, M0 samples were oxidized
14
for 130 h with a mass gain of 21.38 mg/cm2. While M10 samples presented a mass loss of
15
only 4.06 mg/cm2 even after oxidation for 140 h. M30 coating could merely service 37 h
16
due to the spallation of the coating. The outstanding oxidation resistance of M10 coating
17
was attributed to the improved surface oxide scale with no cracking by the mullite
18
addition. A proper amount of mullite can restrain the crystallization of the SiO2, then the
19
crack resistance and healing ability of the oxide scale would be obtained, which can
20
provide a good protection for substrate. Excessive mullite (30 wt.%), however, led to a
21
great volume shrinkage of the composite coating as a result of recrystallization of mullite,
22
which caused the coating debonding after a short-time oxidation.
23 13
1
Acknowledgements
2
We would like to thank the Analytical & Testing Center of Northwestern
3
Polytechnical University for the characterization of our samples. This work has been
4
supported by the Fund of State Key Laboratory of Solidification Processing
5
(SKLSP201819) in Northwestern Polytechnical University (NPU), and by the National
6
Natural Science Foundation of China under Grant No. 51572223 and 51872239.
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
14
1
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[21] J. Sun, Q.G. Fu, G.P. Zhang, C. Wang, T. Li, Melting index of plasma sprayed molybdenum silicide characterized by constitutions and its effect on oxidation kinetics, J. Alloy. Compd. 766 (2018) 95-103. [22] P. Zhang, X.P. Guo, Y and Al modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy prepared by pack cementation process, Surf. Coat. Technol. 206 (2011) 446-454. [23] F. Chen, J.G. Xu, Z.F. Hou, In situ pressureless sintering of SiC/MoSi2 composites, Ceram. Int. 38 (2012) 2767-2772. [24] X.A. Fei, Y.R. Niu, H. Ji, 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. [ 25] 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 1600 oC, Ceram. Int. 41 (2015), 14890-14895. [26] Z.I. Zaki, N.Y. Mostafa, Y.M.Z. Ahmed, Synthesis of dense mullite/MoSi2 composite for high temperature applications, Int. J. Refract. Met. Hard Mat. 45 (2014) 23-30. [27] K.T. Wang, L.Y. Cao, J.F. Huang, J. Fei, A mullite/SiC oxidation protective coating for carbon/carbon composites, J. Eur. Ceram. Soc. 33 (2013) 191-198. [28] X.Z. Liu, C.M. Deng, C.G. Deng, M. Liu, K.S. Zhou, Mullite-modified ZrB2-MoSi2 coating for carbon/carbon composites to withstand long term ablation, Ceram. Int. 44 (2018) 4330-4337. [29] C.C. Wang, K.Z. Li, C.X. Huo, Q.C. He, X.H. Shi, Oxidation behavior and microstructural evolution of plasma sprayed La2O3-MoSi2-SiC coating on carbon/carbon composites, Surf. Coat. Technol. 348 (2018) 81-90. [30] E. Withey, C. Petorak, R. Trice, T. Taylor, Design of 7wt.% Y2O3-ZrO2/mullite plasma-sprayed composite coatings for increased creep resistance, J. Eur. Ceram. Soc. 27 (2007) 4675-4683. [31] C. Ma, H.J. Li, H. Wu, Q.G. Fu, C. Sun, X.H. Shi, Y.L. Zhang, Z.Z. Zhang, J. Tao, Z.H. Han, Mullite oxidation resistant coating for SiC-coated carbon/carbon composites by supersonic plasma spraying, J. Mater. Sci. Technol. 29 (2013) 29-33.
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[32] J. Sun, T. Li, G.P. Zhang, Effect of thermodynamically metastable components on mechanical and oxidation properties of the thermal-sprayed MoSi2 based composite coating, Corros. Sci. 155 (2019) 146-154. [33] J.H. Yan, L.F. Liu, Z.Y. Mao, H.M. Xu, Y.M. Wang, Effect of spraying powders size on the microstructure, bonding strength, and microhardness of MoSi2 coating prepared by air plasma spraying, J. Therm. Spray Technol. 23 (2014) 934-939. [34] C.Y. Cheng, H.J. Li, Q.G. Fu, L. Li, L.P. Guo, Influence of Al2O3 on the oxidation resistance of SiC ceramic: First-principle study and experiment, Corros. Sci. 134 (2018) 57-63. [35] P. Ramaswamy, S. Seetharamu, K.J. Rao, K.B.R. Varma, Thermal shock characteristics of plasma sprayed mullite coatings, J. Therm. Spray Technol. 7 (1998) 497-504. [36] K.N. Lee, R.A. Miller, N.S. Jacobson, New generation of plasma-sprayed mullite coatings on silicon carbide, J. Am. Ceram. Soc. 78 (1995) 705-710. [37] Y.L. An, X.Q. Zhao, G.L. Hou, H.D. Zhou, J. Chen, J.M. Chen, Tribological and thermal properties of mullite coating prepared by atmospheric plasma spraying, J. Therm. Spray Technol. 23 (2014) 410-419. [38] S. Prasad, A. Paul, Growth mechanism of phases by interdiffusion and atomic mechanism of diffusion in the molybdenum–silicon system, Intermetallics. 19 (2011) 1191-1200. [39] S. Prasad, A. Paul, Growth mechanism of phases by interdiffusion and diffusion of species in the niobium–silicon system, Acta Mater. 59 (2011) 1577–1585. [40] A. Mueller, G. Wang, R.A. Rapp, E.L. Courtright, T.A. Kircher, Oxidation behavior of tungsten and germanium-alloyed molybdenum disilicide coatings, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 155 (1992) 199-207. [41] Z.Y. Cai, X.J. Zhao, D.X. Zhang, Y.H. Wu, J.X. Wen, G.Y. Tian, Q.X. Cao, X.Y. Tang, L.R. Xiao, S.N. Liu, Microstructure and oxidation resistance of a YSZ modified silicide coating for Ta-W alloy at 1800 oC, Corros. Sci. 143 (2018) 116-128.
18
[42] J. Sun, T. Li, G.P. Zhang, Q.G. Fu, Different oxidation protection mechanisms of HAPC silicide coating on niobium alloy over a large temperature range, J. Alloys Compd. 790 (2019) 1014-1022 [43] M. Ibrahim, N.F.H. Bright, The Binary System Nb2O5-SiO2, J. Am. Ceram. Soc. 45 (1962): 221-222. [44] Y.G. Wang, W.F. Fei, Y. Fan, L.G. Zhang, Silicoaluminum carbonitride ceramic resist to oxidation/corrosion in water vapor, J. Mater. Res. 21 (2006) 1625-1628. [45] J. Lægsgaard, K. Stokbro, Electronic structure and hyperfine parameters of substitutional Al and P impurities in silica, Phys. Rev. B. 65 (2002) 075208. [46] M. Nakamoto, Y. Miyabayashi, L. Holappa, T. Tanaka, A model for estimating viscosities of aluminosilicate melts containing alkali oxides, ISIJ Int. 47 (2007) 1409-1415.
19
Table 1
Parameters of the plasma spraying process for M0, M10, and M30 coatings.
Parameters
Values
Spraying power, kW Primary gas Ar, L/min Carrier gas Ar, L/min Second gas H2, L/min Powder feed rate, g/min Spraying distance, mm Nozzle inner diameter, mm
40~45 75 10 3.5 20 100 5.5
Table 2
Composition analysis of the spots shown in Fig. 3, Fig. 5, Fig. 7 and Fig. 8.
Spots
Mo (at.%)
Si (at.%)
O (at.%)
Al (at.%)
Nb (at.%)
Phases determined
A
6.44
39.25
54.31
0
0
SiO2 + MoSi2
B
0
19.24
61.56
19.2
0
SiO2 + Mullite
C
35.06
64.94
0
0
0
MoSi2
D
34.33
13.84
46.51
5.32
0
Mo5Si3 + Mullite
E
0
22.66
53.23
24.11
0
Mullite + SiO2
F
0
14.75
56.24
29.01
0
Mullite
G
0
4.32
66.24
2.81
26.64
Nb2O5 + SiO2
H
0
36.92
63.08
0
0
SiO2
I J K
0 0 0
45.51 35.28 6.91
50.05 64.72 71.85
4.44 0 1.75
0 0 19.48
Aluminosilicate SiO2 Nb2O5 + SiO2
Figure Captions Fig. 1. Phase and morphology analysis of as-sprayed coatings: (a) XRD patterns and surface SEM images of (b) M0 coating, (c) M10 coating and (d) M30 coating. Fig. 2. Cross-sectional backscattered electron (BSE) images of as-sprayed coatings: (a, d) M0 coating; (b, e) M10 coating; (c, f) M30 coating. Fig. 3. Oxidation kinetic curves of coated samples at 1500 oC in static air. The macrographs show the actual status of the representative sample in every case after oxidation. Fig. 4. Phase and morphology analysis of 37h-oxidized coatings: (a) XRD patterns and surface SEM images of (b) M0 coating, (c) M10 coating and (d) M30 coating. Fig. 5. Comparative analysis of three kinds of coatings: (a) the histogram of the outer-layer thickness of coatings before and after oxidation for 37 h at 1500 oC; (b, c) the cross-sectional BSE images of M30 coating after oxidation. Fig. 6. XRD patterns of the coatings after oxidation: (a) M0 coating exposed for 130 h; (b) M10 coating exposed for 140 h. Fig. 7. Surface SEM images of the coating after oxidation: (a, b) M0 coating; (c, d) M10 coating. Fig. 8. Cross-sectional microstructure analysis of the coating after oxidation: (a, c, d) M0 coating after oxidation for 130h; (b) M10 coating after oxidation for 140h; (e) element distribution through the cross section of M10 coating. Fig. 9. TEM analysis of M0 and M10 coatings after oxidation: (a) HRTEM images of M0 coating; (b) FFT pattern of domain #1 in (a); (c, d) HRTEM images of M10 coating and the corresponding FFT and SAED patterns.
Fig. 1. Phase and morphology analysis of as-sprayed coatings: (a) XRD patterns and surface SEM images of (b) M0 coating, (c) M10 coating and (d) M30 coating.
Fig. 2. Cross-sectional backscattered electron (BSE) images of as-sprayed coatings: (a, d) M0 coating; (b, e) M10 coating; (c, f) M30 coating.
Fig. 3. Oxidation kinetic curves of coated samples at 1500 oC in static air. The macrographs show the actual status of the representative sample in every case after oxidation.
Fig. 4. Phase and morphology analysis of 37h-oxidized coatings: (a) XRD patterns and surface SEM images of (b) M0 coating, (c) M10 coating and (d) M30 coating.
Fig. 5. Comparative analysis of three kinds of coatings: (a) the histogram of the outer-layer thickness of coatings before and after oxidation for 37 h at 1500 oC; (b, c) the cross-sectional BSE images of M30 coating after oxidation.
Fig. 6. XRD patterns of the coatings after oxidation: (a) M0 coating exposed for 130 h; (b) M10 coating exposed for 140 h.
Fig. 7. Surface SEM images of the coating after oxidation: (a, b) M0 coating; (c, d) M10 coating.
Fig. 8. Cross-sectional microstructure analysis of the coating after oxidation: (a, c, d) M0 coating after oxidation for 130h; (b) M10 coating after oxidation for 140h; (e) element distribution through the cross section of M10 coating.
Fig. 9. TEM analysis of M0 and M10 coatings after oxidation: (a) HRTEM images of M0 coating; (b) FFT pattern of domain #1 in (a); (c, d) HRTEM images of M10 coating and the corresponding FFT and SAED patterns.
Declaration of Interest Statement We would like to submit the enclosed manuscript entitled “Effect of mullite on the microstructure and oxidation behavior of thermal-sprayed MoSi2 coating at 1500 o
C”, which we wish to be considered as a full article for publication in Ceramics
International. The manuscript is approved by all authors for publication and there is no conflict of interest in the submission of this manuscript. We certify that the submission has not been published previously and it is not under review at any other publication.