Microstructure and oxidation behavior of plasma sprayed WSi2-mulliteMoSi2

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Surface & Coatings Technology
journal homepage: www.elsevier.com/locate/surfcoat
Microstructure and oxidation behavior of plasma sprayed WSi2-mullite-
MoSi2 coating on niobium alloy at 1500 °C
Guangpeng Zhang, Jia Sun⁎, Qiangang Fu⁎
State Key Laboratory of Solidification Processing, Shaanxi Province Key Laboratory of Fiber Reinforced Light Composite Materials, Northwestern Polytechnical University,
Xi'an 710072, China
A R T I C L E I N F O
Keywords:
Nb alloy
MoSi2 coating
Oxidation
Plasma spray
A B S T R A C T
WSi2 and mullite co-modified MoSi2 (WMM) coatings were prepared by atmospheric plasma spraying (APS) on
the siliconized Nb alloy, and the oxidation behavior was investigated at 1500 °C in air. Single MoSi2 (M) and
single mullite modified MoSi2 (MM) coatings were also studied for comparison. The effective protection time
(≥500 h) of WMM coating was at least 2.8 and 1.5 times longer than that of M (175 h) and MM (346 h) coatings,
respectively. The mass loss of the WMM sample was only 4.41 mg/cm2 after oxidation up to 500 h, which was
77% and 75% lower than that of M (18.88 mg/cm2) and MM (17.59 mg/cm2) samples, respectively. Such an
attractive long-term oxidation protection performance of WMM coating was attributed to the incorporation of
mullite and WSi2. The presence of mullite and, in particular, of glassy SiO2 contributed to seal cracks. On the
other hand, WSi2 gave rise to the growth of a new transition layer of (Nb, Mo, W)5Si3 in the coating, which
restrained the outward diffusion of Nb atoms, thus kept the oxide layer intact and stable.
1. Introduction
Niobium (Nb) alloys are considered as promising candidates for hot-
end components of advanced turbine engines, due to their high melting
points, low densities, and good mechanical properties at elevated
temperatures [1–4]. However, the poor oxidation resistance seriously
restricts their practical application [5,6]. Usually, multi-component
alloying and coating are the two main approaches against oxidation of
Nb alloys [7,8]. For multi-component alloying method, the elements
such as Al, Cr, and Ti were added in Nb substrate for purpose of im-
proving the spallation resistance of the surface scale, therefore, giving
rise to the oxidation resistance of Nb alloys upon exposure to elevated
temperatures in air [9,10]. Unfortunately, such improvements by al-
loying method are limited. Improper alloying in Nb alloys would lead to
decline of mechanical properties [11,12]. For instance, the room tem-
perature tenacity of Nb-Si alloy decreased from 14.5 MPa·m1/2 to
8.5 MPa·m1/2 when the content of Cr increased from 2 at.% to 17 at.%
[13]. Coating technology can achieve a balance of oxidation resistance
and mechanical properties. Regard on this, it is considered as the most
effective method to protect Nb alloy at elevated temperatures [14].
MoSi2 is one of the promising coatings on Nb alloy since it presents
good oxidation resistance by forming a continuous protective SiO2 film
at high-temperature oxidizing environment to protect against the in-
ward diffusion of oxygen [15,16]. Moreover, MoSi2 possesses a similar
coefficient of thermal expansion (CTE) value (8.0 × 10−6/°C) to Nb
alloys (7.2–8.2 × 10−6/°C) [17]. A two-step process consisting of the
initial depositing Mo layer followed by siliconizing was usually em-
ployed to prepare MoSi2 coating on Nb alloy. Yan et al. [18] prepared
MoSi2 coating by depositing Mo layer via air plasma spraying followed
by siliconizing, the coating exhibited good oxidation resistance at
1200 °C. Zhou et al. [19,20] prepared B modified MoSi2 coating by two-
step process and demonstrated that B2O3 can decrease the viscosity of
SiO2 and promote the rapid sealing of pores at high temperature. But
the two-step process is costly and time-consuming, thus showing the
limitation in practical application. Employing atmospheric plasma
spraying (APS) method can achieve the one-step preparation of MoSi2
coating [4]. Sun et al. prepared MoSi2 coating on the siliconized Nb
alloy by APS, and the service life of the coating reached to at least 128 h
at 1500 °C [21]. However, the self-healing capability of the SiO2 glassy
layer is insufficient for long-term service since the crystallization of
SiO2, and the elemental inter-diffusion between the coating and sub-
strate would damage the coating structure. So, it is still a challenge to
prolong the service life of APS-MoSi2 coating on Nb alloy.
In order to improve the long-term oxidation protective ability of
APS-MoSi2 coating, various attempts by introducing second phase have
been made to modify MoSi2 coating [22,23]. A mullite modified APS-
MoSi2 coating was prepared on siliconized Nb alloy in previous work,
and a crack-free oxide layer with good self-healing ability was obtained
https://doi.org/10.1016/j.surfcoat.2020.126210
Received 3 April 2020; Received in revised form 12 July 2020; Accepted 15 July 2020
⁎ Corresponding authors.
E-mail addresses: j.sun@nwpu.edu.cn (J. Sun), fuqiangang@nwpu.edu.cn (Q. Fu).
Surface & Coatings Technology 400 (2020) 126210
Available online 18 July 2020
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to enhance the oxidation resistance of the modified coating [24].
Though obvious improvement of the self-healing ability of oxide layer
was obtained, the penetrating cracks in MoSi2 coating during thermal
cycles and the severe elemental diffusion have a negative effect on the
protection performance of the MoSi2 based coating. WSi2 possesses
good oxidation resistance as well as mechanical properties, and it has
been reported that the oxidation resistance of MoSi2 could be improved
by addition of WSi2 in terms of higher applied temperature and longer
service life. The (Mo,W)(Si,Ge)2 coating presented better thermal shock
resistance than Mo(Si,Ge)2 coating [25]. And the (Mo,W)Si2-Si3N4
coating can protect Mo substrate for 360 h at 1600 °C, which possessed
a longer antioxidant life than MoSi2-Si3N4 coating [26]. The inter-dif-
fusion layer in W-MoSi2 couple was obviously thinner than that of Mo-
MoSi2 and Nb-MoSi2 couple, which suggested the hindering effect of W
on diffusion of Si [27]. It was reported that the energy of W-Si bond
(~2000 kJ/mol) is higher than Mo-Si bond (~1800 kJ/mol), which
indicates that it is more difficult for Si atom to diffuse in WSi2 layer
than in MoSi2 layer [28]. P. Glushko investigated the kinetics of phase
redistribution in the (Mo, W)Si2-Nb system at 1500–1800 °C. The
growth of lower silicides (Mo, W, Nb)5Si3 + Nb5Si3 and the thickness
decrease of the higher silicide (Mo, W)Si2 layer as functions of the
oxidation temperature were determined. Their specific investigations
demonstrated that the thermal stability of (Mo, W)Si2 coating on Nb
system is higher than that of MoSi2-Nb system and MoSi2-Mo system
[29]. Inspired by above results, the oxidation performance of APS
MoSi2 coating is expected to be improved by introducing WSi2 to re-
strain the elemental diffusion, thus to prolong the service life of MoSi2
coating.
In the present work, WSi2 and mullite co-modified MoSi2 coating
was prepared on siliconized Nb alloy by APS. The microstructure and
oxidation behavior under long-term service in air at 1500 °C of the
composite coatings were investigated. In order to evaluate the oxida-
tion resistance of the co-modified coating, the single MoSi2 coating and
mullite modified MoSi2 coating were also investigated for comparison.
2. Experimental
Nb521 alloy with a nominal composition of (wt%) W 5–6, Zr
1.5–1.7, Mo 2.1–2.5 (Bal. Nb) was used as substrate. Specimens with a
size of 10 mm × 10 mm × 3
mm were cut from the ingots by wire-
electrode cutting. All six sides of the samples were grit-blasting with
Al2O3 under 0.8 MPa to get a relatively rough surface. After grit-
blasting treatment (JCK-1010FK, Jichuan Machinery Technology Co.,
Ltd. Shanghai, China), the average surface roughness of substrate was
17.77 ± 1.39 μm. Nb-Si bonding layer was prepared by halide acti-
vated pack cementation (HAPC) method to improve the adhesion of the
coating before APS. The preparation details of siliconized layer have
been described in previous work [21].
Commercial MoSi2 particles (Songshan Tungsten and Molybdenum
Materials Co., Ltd., Dengfeng, China), mullite and WSi2 particles
(Fanrui New Materials Co., Ltd., Zhengzhou, China) were used as the
raw materials for the APS coating. A spray-drying method (centrifugal
spray dryer, LGZ-5, Wuxi, China) with inlet temperature of 350 °C and
outlet temperature of 120 °C was utilized to achieve the required
feedstock. The mixed powders containing 2 wt% mullite, 18 wt% WSi2
and balanced MoSi2 were dispersed into the 1–3 wt% polyvinyl alcohol
solutions and subsequently milled for 3–7 h to get a homogeneous
slurry. Then, the slurry was transported into the centrifugal spray dryer
to get agglomerated powders. MoSi2 powders containing 10 wt%
mullite without WSi2 for mullite modified coating were also obtained
by above methods. Then the obtained powders were sieved, and the
particle size distribution of powders for plasma spraying was 15–65 μm
as shown in Fig. 1.
The coating deposition was conducted by a high efficiency APS
system (HEPJ-II, China) with a DC arc torch. During APS, the ag-
glomerated powders were transported into plasma torch by argon and
injected into plasma jet and sprayed on every side of siliconized Nb
alloy to form a coating. The main process parameters for APS were
shown in Table 1. The deposited single MoSi2 coating, mullite modified
coating and the co-modified coating were labeled as M, MM, and WMM
coating, respectively.
The isothermal oxidation tests were conducted at 1500 °C in an
electric furnace under atmosphere to evaluate the oxidation resistance
of the coatings. Every kind of coating has three samples for statistical
average. All the coated samples for oxidation tests were taken out from
the furnace to weight the mass change at intervals by an analytical
balance with an accuracy of 0.1 mg. The average mass change per unit
area ΔW was recorded and plotted as a function of oxidation time. It is
noted that the test is a kind of oxidation-resistant endurance running,
which means that the coating was continuously tracked unless the
failure came out. In particular, the maximum tested duration is mean-
while set in advance if the service time of one kind obviously exceeded
other kinds at least 100 h.
The phase composition of the coating before and after oxidation
were characterized by X-ray diffraction (XRD, PANalytical X'Pert PRO,
Netherlands) using a Cu Kα radiation. The surface and cross-sectional
morphologies were investigated via a scanning electron microscopy
(SEM, VEGAT-2, Germany), and the energy dispersive spectroscopy
(EDS, Oxford INCA) was used to examine the chemical composition of
the coatings. The crystalline structure characterizations of the oxides
were carried out using transmission electron microscopy (TEM, FEI
Talos, USA) at an accelerate voltage of 300 kV.
3. Results and discussion
3.1. Phase and microstructure of the as-sprayed coating
Figs. 2(a–c) show the cross-sectional SEM images of three kinds of
as-sprayed coatings. The as-sprayed coating bonds well with inner layer
(NbSi2 layer), and the thickness of the as-sprayed coating is
150 ± 5 μm. Fig. 2(d) shows the XRD pattern of WMM coating. The
Fig. 1. SEM image of WSi2-mullite-MoSi2 agglomerated powders.
Table 1
Parameters of the plasma spraying process for three coatings.
Parameters Values
Spraying power, kW 40–45
Primary gas Ar, slpm 75
Carrier gas Ar, slpm 10
Second gas H2, slpm 3.5
Powder feed rate, g/min ~20
Spraying distance, mm 100
Nozzle inner diameter, mm 5.5
G. Zhang, et al. Surface & Coatings Technology 400 (2020) 126210
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main phases of the coating are h-MoSi2, t-MoSi2, t-WSi2, Mo5Si3 and
W5Si3. Besides, there also exist a small amount of Mo, W and Mo3Si in
the coating. During the plasma spray process, t-MoSi2 (tetragonal
MoSi2) would turn to be h-MoSi2 (hexagonal MoSi2) since that the
temperature in the plasma torch was above 1900 °C [4,30]. Some h-
MoSi2 would be remained at room temperature due to the high cooling
rate of the plasma spray process. A small amount of MoSi2 and WSi2
would be oxidized under the atmosphere condition [31,32]. So, the
diffraction peaks of Mo5Si3 and W5Si3 were detected.
Fig. 2(e) shows the surface morphology of the WMM coating. The
sprayed coating presents a relatively rough surface due to the existence
of some bulges. Fig. 2(f) shows the enlarged view of the sprayed
coating. From Fig. 2(f), a typical lamellar structure of the sprayed
coating was observed. The EDS analysis results of spots 1–5 in Fig. 2(f)
are listed in Table 2. It can be identified that the black phases include
mullite (spot 1) and Al2O3 (spot 2) according to the element composi-
tion. The presence of Al2O3 might be resulted from the pyrolysis of
mullite, which decomposed into SiO2 and Al2O3 at high temperature.
SiO2 was easy to be volatilized due to its relative low melting point
(1710 °C), and Al2O3 with higher melting point (2050 °C) was deposited
into the coating. Similarly, Si was volatilized at high temperature after
the thermolysis of MoSi2 and WSi2, which resulted in the presence of
Mo, W, small amount Mo3Si and W-rich phase (spot 3) [33,34].
Therefore, the grey phases in Fig. 2(f) presents several different con-
trasts under BSE (backscattered electron) mode.
3.2. Oxidation behaviors of the coated samples
3.2.1. Oxidation kinetic
Fig. 3 shows the mass change curves of three kinds of coated sam-
ples during the oxidation period. It can be found that three kinds of
coatings presented an obvious mass loss at the initial oxidation stage. M
coating failed after oxidation for 175 h, and its final mass loss rate is
18.88 mg/cm2. The service life of MM coating is almost twice that of M
coating, which failed after oxidation for 346 h with a mass loss rate of
17.59 mg/cm2. Specially, WMM coating can protect the niobium alloy
from oxidation for more than 500 h. The mass loss rate of WMM coating
is only 4.41 mg/cm2. It can be seen in Fig. 3 that WMM coating can still
provide effective protection for substrate without rapid mass loss. In a
word, WMM coating possessed the best oxidation resistance among
three coatings at 1500 °C.
3.2.2. Microstructures of the coatings after oxidation
Fig. 4 shows the surface morphologies of M and MM coatings after
Fig. 2. Cross-sectional SEM images of M (a), MM (b) and WMM (c) coatings, XRD pattern (d), surface (e) and high magnification cross-sectional BSE (f) images of the
as-sprayed WMM coating.
Table 2
Element analysis of phases at spots 1–5 in Fig. 2(f).
Si (at.%) Mo (at.%) W (at.%) Al (at.%) O (at.%)
Spot 1 27.94 – – 15.58 50.71
Spot 2 – – – 43.55 51.25
Spot 3 8.80 – 60.64 – 10.35
Spot 4 51.99 27.63 5.97 – 4.29
Spot 5 22.25 44.38 12.03 – 8.06
Fig. 3. Mass change curves of three kinds of coated samples during oxidation.
G. Zhang, et al. Surface & Coatings Technology 400 (2020) 126210
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oxidation, which presents the surface structural change process of the
failing coatings. As shown in Fig. 4(a), the surface of M coating presents
lots of petal-like structures. The needle-like niobium oxides first grew at
the center of the petal-like structure, and the healing glassy SiO2 de-
generated gradually. The growth of niobium oxides and the release of
vapor oxides should be responsible for the formation of the petal-like
structure as mentioned. The loose needle-like niobium oxides growing
on the surface provided a diffusion channel for oxygen. Therefore, the
MoSi2 and Mo5Si3 beneath the SiO2 layer would be oxidized
rapidly
under sufficient oxygen condition. The gaseous oxides such as MoO3
volatilized intensely at 1500 °C, which accelerated the consumption of
the glassy SiO2 and the rupture of oxide scale. The similar structure has
also been reported in the case of MoSi2 coating on Mo [35]. According
to Fig. 4(b), niobium oxides continued to grow at the grain boundaries
of SiO2, and the glassy phase completely disappeared. Then the surface
of coating turned into a fragmented structure. Finally, the surface was
completely covered by the Nb2O5 (Fig. 4(c)) and M coating completely
lost its protective ability.
Figs. 4(d–f) show the surface morphologies of MM coating after
oxidation for 346 h. It can be found that MM coating presents a similar
failing process to the M case. Many speckled niobium oxides were ob-
served at the marked areas in Fig. 4(d), and then these niobium oxides
grew up and formed a flaky structure. According to the element com-
position of spot A (58.53O–3.0Al–30.01Si–8.45Nb (at.%)) and spot B
(65.42O–1.91Al–3.56Si–29.11Nb (at.%)), the black and grey phases are
aluminosilicate and Nb2O5, respectively. From the illustration in
Fig. 4(e), many micropores were found in the flaky Nb2O5, which
should be resulted from the volatilization of some gaseous oxides such
as MoO3 and SiO [36]. Compared with M coating, MM coating pos-
sessed a thicker SiO2 layer, which contained more amorphous SiO2
[24]. Therefore, the oxide layer on the MM coating surface presented a
better flow ability at high temperature. As a result, the flaky niobium
oxides inlaid in the surface of coating (Fig. 4(e)), which presented a
different structure compared with the M coating. But the surface of MM
coating was also covered by Nb2O5 finally (Fig. 4(f)).
The nano-crystal structure of the niobium oxides was analyzed by
TEM, as shown in Fig. 5. It was identified that the niobium oxides
growing on the surface of M coating were H-Nb2O5, the most stable
structure of Nb2O5 (Fig. 5(a)). And the distribution of elements in-
dicates that SiO2 and Nb2O5 did not form compounds (Fig. 5(b)), which
means that the growth of Nb2O5 would accelerate the destruction of
SiO2 layer.
Microstructures and phases composition of WMM coating were in-
vestigated at different oxidation stages as shown in Figs. 6 and 7.
Figs. 6(a–b) show the surface morphologies of WMM coating after
oxidation for 50 h. The coating presents a relatively rough surface, and
some micro-cracks can be observed. The SiO2 protective layer just
formed in the initial stage, and some micro-cracks generated during the
heating cycles. As shown in Fig. 6(c), the coating surface turned to be
smooth after oxidation for 222 h, and no crack was observed. And
Fig. 7(a) shows the XRD pattern of the coating after oxidation for 222 h.
The hump peak appearing at the low angle (15–30°) proves that much
glassy phase existed on the surface. The glassy SiO2 possessed a good
self-healing effect, which contributed to the crack-free surface. So, the
mass change of the coated samples kept stable during long-term oxi-
dation.
Fig. 6(d) shows the surface morphology of the WMM coating after
oxidation for 500 h. There was still no crack found on the surface of
WMM coating, and only some micro-destruction structures were ob-
served at the local place. Such structure may result from the stress
concentration at the bulges during thermal cycles. From Fig. 7(b), the
hump diffraction peak at low angle tends to be sharper, which indicates
that the glassy SiO2 crystallized after long-term oxidation. Therefore,
the micro-destruction structures can't be sealed by glassy phase im-
mediately, so WMM samples presented a slight mass loss at this oxi-
dation stage. XRD also indicates that MoSi2 and WSi2 in the composite
coating formed a solid solution phase of (Mo, W)Si2, and no niobium
oxide was detected. Therefore, the oxide scale on WMM coating kept
intact after being exposed up to 500 h.
Cross-sectional microstructures of three kinds of coatings were also
characterized to further analyze the phase evolution of the coating
upon oxidation. Fig. 8 shows the cross-sectional morphologies of the
coatings after oxidation. From Fig. 8(a, c), severe interface debonding
was observed in M and MM coatings, and the protective SiO2 layer has
peeled from the substrate almost. Fig. 8(b) shows the SEM image of M
coating observed in the area where oxide scale has not peeled off. The
oxide layer in M coating has been replaced by Nb2O5, and the large
blowhole was found in the oxide scale, which suggests that large
amounts of gas products escaped from the oxide scale. Both the M and
MM coatings were penetrated by cracks after oxidation, which directly
Fig. 4. Surface SEM images of the coatings: (a–c) M coating after oxidation for 175 h, (d–f) MM coating after oxidation for 346 h.
G. Zhang, et al. Surface & Coatings Technology 400 (2020) 126210
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accelerated the oxidation at the interface of coating and substrate.
Compared with the vertical cracks in M coating, the crack deflection
was observed in MM coating. The reason might be that mullite dis-
persed uniformly in MM coating and provided many phase interfaces,
which restrained the cracks growth by absorbing the fracture energy
during the thermal cycles.
As seen in Fig. 8(d), WMM coating transformed into a four-layer
structure composing of Nb5Si3 layer, porous layer, outer layer and oxide
layer after oxidation for 500 h. As the continuous inter-diffusion of Si
and Nb elements, the inner NbSi2 layer transformed into Nb5Si3 layer
during oxidation. The thickness of the coating is about 300 μm, and the
thickness of the outer layer has decreased to about 40 μm. Fig. 8(e)
shows the morphology of the outer layer and oxide layer, the chemical
compositions at the spots 1–6 are listed in the Table 3. It can be found
that the outer layer was composed of (Mo, W)Si2 (spot 4) and (Mo,
W)5Si3 (spot 5). (Mo, W)5Si3 possesses a CTE value between (Mo, W)Si2
and SiO2, which can help to alleviate the mismatch of thermal expan-
sion between the oxide layer and outer layer. Therefore, the oxide scale
(spot 6) still kept dense and bonded well with the outer layer after
oxidation.
In the porous layer, the main phase at spot 2 is determined as (Nb,
Mo, W)5Si3, which indicates that Nb has diffused into the composite
coating and formed the solid solution phase of (Nb, Mo, W)5Si3 during
oxidation process [29]. While, no Nb element was detected near the
Fig. 5. TEM analysis of the niobium oxides on the surface of M coating: (a) HRTEM image; (b) HAADF image and element mappings.
Fig. 6. SEM images of the WMM coating after oxidation for different time: (a, b) 50 h; (c) 222 h; (d) 500 h.
G. Zhang, et al. Surface & Coatings Technology 400 (2020) 126210
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body layer (spot 3). The content of Nb element decreased in gradient
from substrate to the surface of coating (Fig. 8(f)), which suggests that
the porous layer effectively prevented the outward diffusion of Nb
element.
Fig. 9, the magnified image of area B in Fig. 8(d), presents the micro
morphologies of the porous framework of WMM coating after 500 h
exposure. It is reported that Si not only diffused towards the surface of
coating to form protective SiO2 layer but also into substrate to form sub-
silicide during oxidation [37]. This phenomenon led to the formation of
Kirkendall voids because of the large difference in the diffusion rate of
Mo and Si and the consequent unequal mass flow between the coating
and the substrate [38]. Therefore, the elemental interdiffusion was re-
sponsible for the formation of such a porous framework. Compared
with M and MM coatings, only few micro-cracks were found in porous
layer of WMM coating (Fig. 9(b)). The mismatch of coefficients of
thermal expansion (CTEs) of mullite (4.4–5.6 × 10−6/°C), WSi2
(8.4 × 10−6/°C) and MoSi2 (8.0 × 10−6/°C) resulted in thermal stress
during heating cycles, which caused few cracks in the porous layer.
According to literature [25], the porous framework would decrease the
effective elastic
modulus of the coating, which benefits to improve the
thermal shock and crack prolongation resistance of the coating. Besides,
it has reported that WSi2-MoSi2 composites presented a remarkable
improved fracture toughness and hardness than MoSi2 due to the solid-
solution strengthening [39]. In particular, the fracture toughness of
WSi2-MoSi2 composite is nearly twice that of MoSi2 [40]. Therefore, no
penetrating crack was observed in the WMM coating after multiple
thermal cycles.
Fig. 10 shows the cross-sectional elemental mappings of three kinds
of coatings. From Fig. 10(a–b), there is an obvious element-depleted
region in M and MM coatings, especially for the Si element. For the M
and MM coatings, Nb element has passed through the coating, reached
at the surface and formed niobium oxides. From Fig. 10c, Mo element
has diffused into the substrate and dispersed homogeneously
throughout the WMM coating. Si and W present a decreasing gradient
from the coating to substrate. It is noted that Nb element concentrated
inside the coating and oxygen concentrated at the surface. Therefore,
the formation rate of niobium oxides at surface greatly decreased,
which was a positive factor for the long-term oxidation protection of
WMM coating.
Fig. 11 displays the elemental line scan of Nb in M, MM and WMM
coatings. The niobium content in WMM coating is far lower than that of
M and MM coatings, which implies that the diffusion rate of Nb in
Fig. 7. XRD patterns of WMM coating after oxidation for 222 h (a) and 500 h
(b).
Fig. 8. Cross-sectional SEM images and EDS analysis of three coatings after oxidation: (a–b) M coating; (c) MM coating; (d–e) WMM coating; (f) elemental line scan of
WMM coating.
G. Zhang, et al. Surface & Coatings Technology 400 (2020) 126210
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WMM coating is lower than the other two species. Guo et al. [41]
prepared a WSi2 layer between the Nb-Si-Ti alloy and MoSi2 coating,
and it was found that WSi2 effectively restrained the outward diffusion
of alloying elements at 1250 °C, thus the oxide scale kept intact during
oxidation. However, the diffusion suppression effect of WSi2 layer de-
creased at 1350 °C because WSi2 degraded into porous W5Si3. In this
work, no niobium oxide was found in the oxide scale of WMM coating
after oxidation for 500 h, which suggests that WSi2 and mullite co-
modified coating shows stronger ability on diffusion suppression effect
than single WSi2 layer. Besides, P. Glushko et al. also demonstrated that
the thermal stability of (Mo, W)Si2-Nb system was more than twice that
of MoSi2-Nb system because the multicomponent and multiphase sili-
cide of (Mo, W, Nb)5Si3 created a better barrier for the elemental dif-
fusion [29]. So, the WMM coating achieved the best oxidation re-
sistance among three kinds of coatings.
3.2.3. Oxidation mechanism of the coatings
The schematic graphs of oxidation process for M, MM, WMM
coatings are illustrated in Fig. 12. For M and MM coatings, niobium
would continue to diffuse towards the surface of coating due to the
chemical potential during oxidation period. The niobium oxides would
form and grow in the oxide scale once the niobium and oxygen reached
a certain concentration (Eq. (1)). Niobium oxides damaged SiO2 pro-
tective layer and then MoSi2 and Mo5Si3 would be oxidized rapidly.
Large amounts of gaseous oxides such as MoO3 escaped from the
coating, which accelerated the volatilization of glassy SiO2 (Eqs. (2) and
(3)). Furthermore, the severe elemental interdiffusion between the
coating and substrate resulted in the formation of large quantities of
large-sized holes, which caused the debonding of the M and MM coat-
ings. The catastrophic cracks in M and MM coatings also accelerated the
collapse of the coating structure.
4Nb (s) + 5 O2(g) →2Nb2O5 (s) (1)
2MoSi2 (s) + 7O2 (g) → 2MoO3 (g) + 4SiO2 (s) (2)
2Mo5Si3 (s) + 21O2 (g) → 10MoO3 (g) + 6SiO2 (s) (3)
WMM coating presented more mass loss at initial oxidation stage
due to the volatilization of more gaseous oxides such as MoO3 and WO3
(Eqs. (2)–(7)). As the oxidation prolonging, mullite contributed the
formation of a crack-free surface, which blocked the penetration of
oxygen (Eqs. (8)–(10)). Moreover, the uniform porous layer of (Nb, Mo,
W)5Si3 was formed to play a key role of elemental diffusion barrier by
introducing WSi2 (Eqs. (11) and (12)). As a result, the protective layer
on WMM coating kept dense and intact for long periods. Besides, WSi2
and MoSi2 formed solid solution of (W, Mo)Si2, thus WMM coating
presented a better crack prolongation resistance, which contributed an
intact structure even after oxidation for 500 h. Therefore, WMM coating
achieved an excellent long-term oxidation protection performance re-
lied on its excellent thermal stability.
WSi2 (s) + O2 (g) → WO3 (g) + 2SiO2 (s) (4)
2W5Si3 (s) + 21O2 (g) → 10WO3 (g) + 6SiO2 (s) (5)
2Mo (s) + 3O2 (g) → 2MoO3 (g) (6)
2W (s) + 3O2 (g) → 2WO3 (g) (7)
5MoSi2 (s) + 7O2 (g) → Mo5Si3 (s) + 7SiO2 (s) (8)
3Al2O3·2SiO2 (s) + SiO2 (s) → Silicates glass (m) (9)
Silicates (m) → Al2O3 (s) + SiO2 (s) (10)
WSi2 (s) + MoSi2 (s) → (W, Mo)Si2 (s) (11)
(W, Mo)5Si3 (s) + Nb (s) → (Nb, W, Mo)5Si3 (s) (12)
4. Conclusions
M, MM and WMM coatings were prepared on the siliconized Nb
alloy by APS. The oxidation behaviors of three kinds of coatings at
1500 °C were comparatively investigated in static air. Large amounts of
Nb2O5 grew on the surface of coating and large holes gathered in the
coating as the elemental diffusion, which destroyed the protective SiO2
layer and the coating structure of M and MM coatings. MM coating
possessed better oxidation resistance than M coating due to its crack-
Table. 3
Chemical compositions of WMM coating at the spots 1–6 in Fig. 8(d–e).
Si (at.%) Nb (at.%) Mo (at.%) W (at.%) O (at.%) Al (at.%) Phase
Spot 1 37.15 62.85 – – – – Nb5Si3
Spot 2 36.56 36.97 23.08 3.39 – – (Nb, Mo, W)5Si3
Spot 3 31.39 – 38.32 5.50 24.79 – (Mo,W)xSiy⁎ + SiO2
Spot 4 67.59 – 30.36 2.05 – – (Mo, W)Si2
Spot 5 37.37 – 55.19 7.46 – – (Mo, W)5Si3
Spot 6 31.65 – – – 64.10 4.25 Aluminosilicate
⁎ 5/3 < x/y < 3.
Fig. 9. SEM images of the porous layer in WMM coating after oxidation for 500 h.
G. Zhang, et al. Surface & Coatings Technology 400 (2020) 126210
7
free surface, while it suffered the similar failing process with M coating
after oxidation for 346 h. WMM coating provided an effective protec-
tion for substrate up to 500 h with a mass loss of only 4.41 mg/cm2,
which presented more superior oxidation resistance than the other two
coatings. The outstanding oxidation protection performance of WMM
coating owed to the incorporation of both mullite and WSi2. Mullite
contributed to the formation of glassy SiO2 and then to seal cracks in
the oxide scale, and WSi2 can restrain the outward diffusion of Nb
element to keep the oxide layer intact.
CRediT authorship contribution statement
Guangpeng Zhang:Methodology, Validation, Formal analysis,
Investigation, Data curation, Writing - original draft, Writing - re-
view & editing, Visualization.Jia Sun:Methodology, Resources,
Writing - review & editing, Supervision, Project
administration.Qiangang Fu:Conceptualization, Methodology,
Formal analysis, Resources, Supervision, Project administration,
Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge financial supports by
Innovation Talent Promotion Plan of Shaanxi Province for Science and
Technology Innovation Team (2020TD-003), and General Project
(Youth) of Shaanxi Provincial Natural Science Basic Research Program
(2020JQ-170), the Fund of Key Laboratory of National Defense Science
and Technology (JZX7Y201911SY008901 6142911190207) in
Fig. 10. Cross-sectional elemental mappings of three kinds of coatings: (a) M coating after oxidation for 175 h; (b) MM coating after oxidation for 346 h; (c)
WMM
coating after oxidation for 500 h.
Fig. 11. Cross-sectional elemental line scan of Nb element in three coatings
after oxidation.
G. Zhang, et al. Surface & Coatings Technology 400 (2020) 126210
8
Northwestern Polytechnical University (NPU), the Fundamental
Research Funds for the Central Universities (G2019KY05116) and
National Natural Science Foundation of China (51821091, 51872239).
The authors also thank the Analytical & Testing Center of Northwestern
Polytechnical University for the characterization of our samples.
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	Microstructure and oxidation behavior of plasma sprayed WSi2-mullite-MoSi2 coating on niobium alloy at 1500 °C
	Introduction
	Experimental
	Results and discussion
	Phase and microstructure of the as-sprayed coating
	Oxidation behaviors of the coated samples
	Oxidation kinetic
	Microstructures of the coatings after oxidation
	Oxidation mechanism of the coatings
	Conclusions
	CRediT authorship contribution statement
	Declaration of competing interest
	Acknowledgements
	References