Full Manuscript12
Full Manuscript12
Full Manuscript12
composites at 1773K
Imran Abbas1*, Muhammad Ali Siddiqui2, Fang-xu1 Niu, Wang Yanxiang1, Mudaser Ullah3,
Faisal Qayyum4
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Carbon Fiber Engineering Research Center, Faculty of Materials Science, Shandong
oxidation resistance at 1773K. The C/C composites were coated with a SiC inner layer and a
MoSi2-Si3N4 outer layer by the slurry and pack cementation method. The phase composition,
microstructure and spot elements of the coating were analyzed by SEM, XRD, EDS and
Raman spectra. Oxidation tests show that the multilayer coating could protect the C/C matrix
at 1773K for 150h and that coating could withstand 40 thermal cycles between 1773K and
room temperature. Si3N4 was found to play an important role in increasing the oxidation and
thermal cyclic oxidation resistance as a result of the formation of a dense SiO 2 layer at high
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temperature. The weight loss of the coated C/C composites has been ascribed to the formation
resistance
1. Introduction
Carbon/carbon (C/C) composites are candidate structural materials for aggressive service
environments in nuclear and aerospace industry owing to some of their attractive properties
such as better strength-to-weight ratio, thermal cyclic oxidation resistance, strength retention,
limitations such as susceptibility to oxidation beyond 773K and strength loss upon exposure
to high temperature, which make them highly unfavorable for certain high temperature
applications[6-8]. It was shown by previous researchers that the C/C composites can be
In the multilayered coatings, MoSi2 used as outer layer while SiC forms the inner buffer layer.
The peripheral multilayer coating containing MoSi2 exhibits an excellent oxidation protective
ability for C/C composites at 1500-1600 0C. However, generation of micro-cracks in the
MoSi2 coating cannot be competent for a long time at 800-1000 0C due to the mismatch of
thermal expansion coefficient between the SiC bonding layer and the MoSi 2 outer coating[9-
12]. Therefore, the protective temperature range of multilayer coating containing MoSi 2 is too
narrow, which limits its structural and high temperature applications. The coefficient of
thermal expansion of MoSi2 (~8.1×10-6/K) is much larger than that of C/C composites
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(~1.0×10-6/K). Therefore, the mismatch between CTEs of the outer MoSi 2 layer and the inner
SiC coated C/C matrix causes degradation of the coating under thermal cycling and ultimately
lowers the coating durability[13, 14]. Another major reason of coating failure is the ‘pest
phenomenon’, occurring due to its incompatibility between the matrix and the coating.
Actually, the pest phenomenon means the structural disintegration of MoSi 2 coating at a
temperature range of 400 to 6000C during oxidation, where MoSi2 reacts easily with oxygen
to form MoO3 and SiO2. These oxidation productions would create a considerable volume
expansion of MoSi2 matrix, which leads to the disintegration of bulk MoSi 2 into powders and
Earlier reports have suggested that the addition of Si 3N4 to MoSi2 can minimize CTE
mismatch between the outer MoSi 2 and inner SiC coating. Si3N4 exhibits the high flexural
strength, good compatibility with MoSi2 and SiC, and a reasonable resistance to creep,
oxidation and thermal cyclic oxidation[17]. The MoSi2-Si3N4 coating exhibited excellent
oxidation resistance with a minute weight loss. At high temperature, MoSi 2-Si3N4 forms a
dense glassy SiO2 film on the coating surface. Owing to its low oxygen-diffusion coefficient,
SiO2 film serves as an oxygen diffusion barrier and efficiently protects C/C composites from
oxidation at 1773K[18, 19]. Due to good fluidity at high temperatures, SiO 2 can seal all the
micro-cracks formed during the volatilization of MoO 3, CO2 and N2[20]. Furthermore, the
addition of Si3N4 provides a better refractory and some oxidation resistant properties,which
decreases the possibility of pest disintegration of MoSi 2. Yu Huang and Houan Zhang et al.
[21] prepared MoSi2/Si3N4 coating on Mo substrate and described the effect of Si 3N4 content
on microstructure and antioxidant properties of multilayer coatings. Niu et al. [22] prepared a
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MoSi2-SiOC-Si3N4/SiC anti-oxidation coating for C/C composites prepared at relatively low
temperature.
In this study, coatings composed of SiC bonding layer and MoSi 2-Si3N4 outer layer were
prepared by a simple and low cost slurry method. The as-prepared coatings (selected
compositions) showed excellent oxidation properties for a longer period of time (150h) with a
minute weight loss of (0.9%). The phase composition, microstructure and oxidation resistance
of the above multilayer coatings at 1773K in air were investigated in detail.
2. Experimental Procedure
Technology Co. Ltd., Jinan, China) used as substrates were cut from bulk piece of 2D C/C
composites with density of 1.79g/cm3. In this process, 30-40% of PAN based carbon fibers
The surface of cut samples was made more active by abrading them with commercially
available rough SiC papers. The specimens were later cleaned ultrasonically with distilled
water and dried in oven at 1000C for 2h to remove any debris and moisture.
A SiC inner coating was applied on the specimens by pack-cementation method and the
powder compositions were as follows: (Si 75 wt. %, graphite 20 wt. %, Al 2O3 5wt. %). C/C
composites and pack mixtures were put in a graphite crucible and heated at 1873K for 2h in
argon atmosphere to form SiC coating. After the completion of inner coating, SiC coated
specimens were cleaned ultrasonically and dried in oven at 1000C for 2h.
Liquid slurry mixture method was adopted for applying outer coating of MoSi 2-Si3N4 coating
on inner SiC coating. High purity MoSi 2 and Si3N4 powder supplied by Turnnano Ltd. were
used. 55 wt. % MoSi2 and 45 wt. % of Si3N4 were mixed together in 10ml aqueous solution of
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H-PSO and homogenous slurry solution was prepared. SiC coated C/C specimens were put in
slurry mixture of MoSi2 and Si3N4. The as-coated specimens were put in a graphite crucible
and heated at 1873-2073k for 2 hours in nitrogen atmosphere to apply the outer MoSi 2-Si3N4
The slurry method was preferred for the outer coating process due to its compositional
flexibility, ease of processing and low cost. Also, it is known that the coating deposited via
slurry method exhibits reasonable resistance to oxidation and thermal cyclic oxidation[9]. A
sample with only MoSi2-SiC multilayer coating without addition of Si 3N4 to outer coating
layer was prepared to using similar approach to elucidate the effect of Si 3N4 addition on the
Oxidation tests of the MoSi2-Si3N4/SiC multilayer coated C/C specimens were performed in a
corundum tube furnace at 1773K. After due time (every 10 hours to a maximum time of 150
hours) oxidation, the samples were taken out of the furnace directly and cooled to room
temperature in air. Then the samples were weighed by an electronic balance with sensitivity
of ±0.1 mg before they were put into the furnace again for the next oxidation period. The
percentage weight loss (wt %) was calculated as follows using Eq. 1. Where, m0 and m1 are
the weights of specimen before and after the oxidation test, respectively.
m0−¿m
wt %= 1
×100 % ¿ …………………………………………………………Eq. 1
m0
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Thermal shock resistance of the MoSi2-Si3N4/SiC multilayer coating was investigated by
conducting test between 1773K and room temperature. In this test, the coated C/C specimens
were heated at 1773 K for 10 min and then, cooled to the room temperature in the box furnace
at an average cooling rate of ⁓50K/min. The testing was carried out of 40 cycles.
Morphology of the MoSi2-Si3N4/SiC multilayer coating before and after the oxidation test
were analyzed using a scanning electron microscope (SEM), equipped with energy dispersive
spectroscope (EDS).Raman spectroscopy (LabRam-1B, 633nm line of the He-Ne laser) was
used to identify the chemical composition and phase transformation. X-Ray diffraction (XRD)
technique was used for crystallographic characterization of the coating before and after the
oxidation test.
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3. Results and Discussions
Fig.1. Structure of the inner SiC coating (a) XRD pattern; (b) surface SEM image
XRD pattern of the as-coated inner SiClayer is shown in Fig.1a. As we can see, the three
major peaks at 2θ≈35.6°, 60.0° and 71.7°correspond to (111), (220) and (311) crystalline
planes of β-SiC, indicating the formation of β-SiC crystalline plane during the pack
faults (SF) in the β-SiC coating. The stacking faults might be induced due to the thermal
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stresses produced in the coating process[23]. Fig.1b shows the surface microstructure of the
as-coated inner SiC layer. The granular structure formed on the coating surface can eliminate
the thermal stresses and minimize the CTE between the coating and the matrix[24].
Fig.2 XRD spectra of MoSi2-Si3N4/SiC coated C/C specimen before the oxidation test
Fig. 2 shows the XRD spectra of the as-coated specimen, i.e., MoSi 2-Si3N4/SiC coated C/C
specimen before the oxidation test. In the spectra, MoSi 2, Si3N4, and SiC phases are obvious.
It is known that addition of Si 3N4 to MoSi2 hinders gas diffusion through MoSi2[25]. Hence,
MoSi2 and Si3N4 phases ensure that the as prepared coating has excellent adhesion with the
C/C matrix [26-28]. Lack of gas diffusion through MoSi 2 prevents the composites from
producing some undesirable residual phases which would otherwise appear due to a gas-C/C
composite interaction. The stronger peak intensity of the MoSi 2 phase in Fig. 2 indicates that
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Fig.3 Surface of the as-coated MoSi2-Si3N4 outer layer (a) SEM image, (b) EDS pattern corresponding to spot A
Of Fig. 3a
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Fig.5 Elemental distribution of the cross-section of C/C composite coated with MoSi 2-Si3N4/SiC
Fig.3a shows the surface microstructure of the as-coated MoSi 2-Si3N4 outer layer before the
oxidation test. The granular surface is without holes or micro cracks, indicating that the as-
prepared coating is dense and homogeneous. In Fig. 3b, the EDS pattern of ‘spot A’ clearly
shows that the smooth area of the as-coated MoSi 2-Si3N4 is composed of the elements, Si, C
and Mo.
Fig.4 shows an overlay of element line scanning over the SEM image of the cross-section of
MoSi2-Si3N4/SiC coated C/C composite before the oxidation test.The multilayer coating has
an average thickness of ~140 µm. The overlaidline scanning results indicate that the as-
prepared coating is composed of an MoSi 2-Si3N4 outer layer, an SiC inner layer and an SiC
transition layer of thicknesses ~50µm, ~50µm and ~40µm, respectively. The SiC transition
layer, formed possibly due to the infiltration of Si into the C/C substrate at high temperature,
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increases the oxidation and thermal cyclic oxidation resistance of the coating. In Fig. 5, the
elemental distribution of the cross-section of the MoSi 2-Si3N4/SiC coated C/C composite
before the oxidation testshows that its structure is dense, homogeneous and without
penetration crack or hole. Lack of voids between the outer and inner coating layers indicates
two things:
The elemental distributions images, shown in Fig. 5, indicate that the elements Mo, Si and C
distribution would play significant role in reducing the CTE mismatch between the coating
Fig.6 XRD pattern of MoSi2-Si3N4/SiC coated C/C composites after oxidation at 1773
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Fig.7 Raman spectra of the MoSi2-Si3N4/SiC coating after oxidation at 1773K for 150h
Fig.6 shows the XRD spectra of the MoSi2-Si3N4/SiC coating after the oxidation test at
1773K. It is clear that the oxidation test resulted in the formation of several new phases. The
producing some new phases, SiO2, Mo5Si3 and MoO3. Appearance of the large fractions of Si-
based new phases indicates that MoSi2 is completely oxidized. But, the partial oxidation of
Si3N4 has occurred at oxidation temperature resulted in the formation of Si 2N2O phase that is
in accordance with the XRD pattern [30]. The possible reactions are as follows.
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Mo5Si3 and SiO2 play an important role to increase the oxidation resistance of the coating.
Mo5Si3 phase increases the coating flexural strength, the coating-substrate compatibility and
enhances the creep strength [31]. SiO2 phase, owing to its inherent low viscosity, good
fluidity and low oxygen permeability, serves as a barrier against oxygen attack. In Fig.7, the
Raman spectra result also confirms the formation of SiO 2 in the MoSi2-Si3N4 coating after
oxidation at 1773K. It is found that peak with wave number of 228 and 414 cm -1 is the
crystallite while the wave number of 1034 cm -1 is the amorphous silica, which is accordance
The effect of Si3N4 on the oxidation resistance of multilayer coating was investigated by
comparing results from the microstructure analysis and weight loss of coatings constituting
MoSi2 and MoSi2-Si3N4 outer layers,i.e.,MoSi2-Si3N4/SiC multilayer coating and MoSi 2/SiC
multilayer coating, respectively. Fig.8a shows the microstructure of MoSi 2 surface of the
MoSi2/SiC multilayer coating after the oxidation test. Surface MoSi 2 particles form non-
uniform agglomerates with the fused silica. These particles degrade the surface of the coating
and gradually reduce its oxidation resistance. As a result, some deep cracks occur and
Fig.8b shows SEM micrograph of the cross section of MoSi2/SiC multilayer coated C/C
specimen. It is evident that the cross section is rough, porous and oxidized. This is because of
the large CTE gap between SiC and MoSi2, leading to the incomplete formation of SiO 2 and
pore/cavity formation [29]. These pores provide the diffusion channel to oxygen that
penetrates into the substrate and cause the failure of coating. It is also noteworthy that the
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infiltration of oxygen via the cavities would reduce the thickness of the coating. Therefore,
the porous morphology of MoSi2/SiC coating is detrimental towards its oxidation resistance.
Fig.8c shows the microstructure of the outer surface of MoSi 2-Si3N4/SiC coating after the
oxidation test at 1773K. A dense glassy SiO 2 layer on the surface is evident, which obviously
prevents the C/C matrix from oxygen penetration and protect the coating from oxidation. This
makes the MoSi2-Si3N4/SiC coating oxidation resistant and an ideal choice for high
temperature applications. The random gas bubbles formed on the glassy surface might be due
to the rapid cooling from high temperature to room temperature. In Fig. 8d, the cross sectional
morphology of the post-oxidation tested MoSi 2-Si3N4/SiC coating exhibits good compatibility
between the coating and the substrate, which is responsible for the high bonding and the
flexural strengths of coating in high temperature environments [32,33]. Further, the absence
of voids/holes at the coating-substrate interface indicates that the coating is resistant to high
temperature rupture. Also, Si3N4 forms a suitable combination with MoSi 2 in the slurry which
helps minimizing the CTE gap between the outer MoSi 2-Si3N4 layer and the inner SiC coated
C/C substrate. Therefore, Si3N4 plays an important role in the protection of coating at high
temperature[34]. Formation of micro defects (i.e., micro cracks and micro pores) is due to the
volatilization of MoO3. From experimental observations, these pores are far from detrimental
to the coating and can be cured by the glassy SiO2 layer at high temperatures.
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Fig.8 SEM image of MoSi2/SiC coating after oxidation at 1773K (a) surface, (b) cross-section; SEM image
of MoSi2-Si3N4/SiC coating after oxidation at 1773K (c) surface (d) cross-section.
Fig.9 shows a comparison of weight losses for the two coatings, with and without Si 3N4, after
the oxidation test at 1773K. It is apparent that MoSi 2-Si3N4/SiC coating experiences a weight
loss of 0.9% after 150h of oxidation treatment while the MoSi 2/SiC coating suffers a
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Fig.9 Isothermal oxidation curves of MoSi2-Si3N4/SiC coated C/C composites in air at 1773K.
In the MoSi2/SiC coating, the absence of Si 3N4 cause the incomplete oxidation of MoSi 2
results in randomly oxidized surface. The increasing weight loss with oxidation time is due to
the insufficient amount of SiO2 and some large crack formed on the surface of coating. This
severely affects the coating surface homogeneity and allows oxygen an access to the C/C
composites, causing gaseous byproducts. At the high temperature, these gaseous byproducts
quickly evaporate to cause sudden the weight loss of coating. This also generates some micro
cracks and deep cavities in the coating surface which are the principal contributors to the
weight reduction.
In the MoSi2-Si3N4 coating treated at 1773K, a momentary weight gain is observed, which
might be due to formation of the dense glassy layer of SiO 2. A continued heat treatment
causes weight loss, possibly due to the volatilization of MoO 3, CO2 and N2. The lower weight
loss in the MoSi2-Si3N4/SiC coating indicates that addition of Si 3N4 is beneficial for the
1773K indicates that Si3N4 in the coating yields a better coating-substrate compatibility at
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high temperatures. This suggests that the MoSi 2-Si3N4 coating is more resistant to high
temperature oxidation.
Fig.10a shows the SEM image of MoSi2-Si3N4/SiC coating surface after the thermal cyclic
oxidation test. It can be observed that whiskers are formed throughout the surface. Exposure
of Si3N4 coating to high temperature can foster formation of Si 2N2O whiskers [35].These
whiskers play an important role in increasing the oxidation resistance of the coating and
prevent the coating from quick oxidation[36]. The cross sectional view of the multilayer
coating in Fig.10b shows small pores, mainly caused by the rapid cooling from1773k to room
temperature. These pores can be sealed by the formation of Si 3N4 whiskers in the subsequent
thermal cyclic oxidation process, which further protect the coating from high temperature
oxidation.
Fig. 10 (a) Surface SEM image, where red arrows represent whiskers (b) cross-section of multilayer coatings
after thermal cyclic oxidation test b/w 1773K and room temperature for 40 times .
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Fig.11 shows the weight loss curve of MoSi2-Si3N4/SiC coated C/C specimen during its
repeated thermal cycling between 1773K and room temperature. It can be found that the
weight loss of the coatedspecimen was only 0.04% even after 40 thermal cycles. During the
thermal cycling, the coating remained intact and no oxidation or spallation was found. This
indicates that the coating exhibits an excellent oxidation and thermal cyclic oxidation
resistance. The thermal cyclic oxidation resistance of the coating can be attributed to the
filling of micro cracks under oxidizing environments by the abundant glassy oxides. Besides,
the addition of Si3N4 reduces the thermal stresses caused by the rapid cooling from high
Fig.11Thermal cycling oxidation curves of the MoSi2-Si3N4/SiC coating between 1773K and room temperature.
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4. Conclusions
Effect of Si3N4 addition in MoSi2/SiC coating on the oxidation and thermal cyclic oxidation
resistance at 1773K was investigated. The 0.9% weight loss in MoSi 2-Si3N4/SiC coating after
its oxidation for 150h in comparison to the 4.0% weight loss in MoSi 2/SiC coating after 90h
proves that the coating with Si3N4 has a better high temperature oxidation resistance. The
negligible weight loss after 40 thermal cycles between 1773K and room temperature proves
that of MoSi2-Si3N4/SiC coating has an excellent thermal cyclic oxidation resistance. The
dense glassy SiO2 film. The SiO2 film improves the resistance to high temperature oxidation of
coating by preventing the gas diffusion into the coating, thus, shielding the C/C substrate from
byproducts-forming-gases and retarding the generation of micro cracks. Si 3N4 addition in the
multilayer coating is found beneficial for both coating integrity and coating-substrate
compatibility.
Acknowledgement
This work was supported by National Natural Science Foundation of China (51573087), and
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