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Mullite whisker-mullite/yttrium aluminosilicate oxidation protective coatings for SiC coated C/C composites ⁎
Lei Zhou, Qiangang Fu , Caixia Huo, Mingde Tong, Xuesong Liu, Dou Hu State Key Laboratory of Solidification Processing, Shaanxi Key Laboratory of Fiber Reinforced Light-Weight Composites, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: C/C composites Mullite whisker Thermal-shock resistance Oxidation resistance
A novel mullite whisker-mullite/yttrium aluminosilicate coating was designed and prepared through a three-step process including molten salt, supersonic atmospheric plasma spraying and sol-hot dipping. The mullite whiskers were used to enhance the interface bonding between mullite and SiC layer. A dense yttrium aluminosilicate outer coating was employed to seal the microholes in mullite coating. Results show that the bonding strength of the asprepared coating was enhanced and the coating present a dense and crack-free microstructure. After 30 thermalshock cycles from 1773 K to room temperature and oxidized for 185 h in atmospheric environment at 1773 K, the coated sample was only at a weight loss of 1.08% and 1.58%, respectively.
1. Introduction With the ever-increasing attention to high performance composites, carbon fiber reinforced composites, especially carbon/carbon (C/C) composites which possess excellent high temperature mechanical properties and stability, low density are regarded as an attractive thermal-structural materials [1–6]. Nonetheless, they have a fatal weakness that they are tend to be oxidized in air above 643 K, and oxidized rapidly above 773 K, leading to devastating damage to composites and then limiting their applications [7–10]. Thus, extensive research interest and efforts have been prompted to solve this problem, and an effective approach is to apply ceramic coating [11–15]. Among various promising ceramic coating candidates, SiC is the most successful widely-used coating materials [16]. However, the single SiC layer is difficult to meet the need of long time oxidation protection. To address this issue, multilayer coating was executed in recent years, for instance, MoSi2/SiC [17], ZrB2-SiO2/SiC [18], HfB2/ SiC [19], ZrSiO4/SiC [20], Y2SiO5/SiC [21], mullite/SiC [22]. Particularly, mullite has been selected as outer coating materials because it has similar thermal expansion coefficient with SiC and possesses low oxygen permeation and high melting point [23]. Up to now, lots of methods have been used to fabricate mullite coating, like hydrothermal electrophoretic deposition [24], supersonic atmospheric plasma spraying (SAPS) [25], hot dipping [26], sol-gel [27], etc. Although the as-prepared coating could provide protection for C/C composites to some extent, challenge still remains because the as-prepared coating
⁎
usually has high porosity and low interface bonding strength. Here, to enhance the interface bonding strength, mullite whiskers were in-situ synthesized to form a new layer, which not only can increase the wettability of the SiC interlayer and middle mullite layer, but also will toughen the middle mullite coating [28,29]. Then, to seal microholes in the middle mullite coating, the yttrium aluminosilicate coating was selected due to its good wettability with mullite coating [30]. In addition, the yttrium aluminosilicate has good oxidation resistance and possesses chemical durability at high temperatures [31–33]. In this work, a mullite whisker-mullite/yttrium aluminosilicate coating was designed and prepared through a three-step technique. A porous layer with mullite whiskers was firstly fabricated by in-situ molten salt reaction. Then, this composites were exposed to SAPS to prepare middle mullite coating. Finally, a dense yttrium aluminosilicate coating was applied to seal the as-prepared sample. Oxidation resistance of such multi-layer coating was tested. And the phase, microstructure of the coating, as well as the interface bonding between it and the C/C composites were also investigated. 2. Experimental 2.1. Specimen and coating preparation Cubic specimens as substrates (10 × 10 × 10 mm) were machined from 2D C/C composites whose density is 1.75 g/cm3. These specimens
Corresponding author. E-mail address: [email protected] (Q. Fu).
https://doi.org/10.1016/j.ceramint.2019.08.105 Received 25 July 2019; Received in revised form 8 August 2019; Accepted 10 August 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Lei Zhou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.105
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Fig. 1. Schematic of preparation of mullite whisker-mullite/yttrium aluminosilicate coating.
crucible was cooled naturally down to room temperature in furnace. The as-prepared sample was milled to obtain yttrium aluminosilicate powders. The silica sol was first mixed with distilled water according to a ratio of 1:5 in vol%. Afterwards, yttrium aluminosilicate powders were added into the silica sol solution, and then dispersed by ultrasonic bath and stirred by magnetic stirrer to acquire a homogeneous suspension. The coated specimens were heated up to 573 K for 5 min in the furnace, and then immediately dipped in the above yttrium aluminosilicate suspension. This process was repeated until the thickness of the coating is satisfied. Finally, a two-hour heat treatment at 1773 K in Ar was taken. The obtained coating sample is composed of SiC, mullite whisker, mullite layer and yttrium aluminosilicate layer, which was identified as SMMY. For comparison, the single mullite coating sample was also prepared by SAPS on the surface of SiC-C/C composites and named as SM.
were chamfered and abraded manually using the 300 and 600-grit SiC papers orderly, ultrasonically cleaned and dried in the oven. A SiC interlayer was synthesized on these samples by pack cementation and the precursor powders were as follows: Si (300 mesh) 60–80 wt%, graphite (300 mesh) 10–25 wt% and Al2O3 (325 mesh) 5–15 wt%. The C/C composites were embedded in the precursor powders, and then heated to 2073–2273 K for 2 h in an argon-protective atmosphere [34]. Fig. 1(a) and (b) present the process to synthesize mullite whiskers layer. The detailed fabrication process was as follow: SiC-C/C composites were embedded in an alumina crucible with the mixed powders of Al2(SO4)3·18H2O (40–50 wt%) and Na2SO4·9H2O (50–60 wt%). Subsequently, the alumina crucible was heated to 1173–1273 K for 30 min in air by an electric furnace. After cooling down to room temperature, the coated specimens were cleaned serveral times using boiling water. After that, the coated samples were dried in a 373 K-oven. Mullite coating was sprayed on the SiC-C/C substrate with mullite whisker layer by the SAPS method, as shown in Fig. 1(c). Mullite (5–10 μm) particles were selected as raw materials. Firstly, the raw particles were prilled by a comminutor. The slurry used in the prilling is composed of polymeric binder (4 wt%) and distilled water (96 wt%). Specific parameters of the spraying are listed in Table 1. The obtained coating sample contains SiC, mullite whisker and mullite layer. To simplify the discussion, it was identified as SMM. After fabricating the mullite coating, a sol hot dipping was selected to prepare yttrium aluminosilicate coating. The yttrium aluminosilicate powders were prepared as follows: 12.18% Y2O3-22% Al2O3-65.82% SiO2 mole% powders were mixed by tumbling in a ball mill and heated at 1673–1773 K for 2 h in a corundum crucible. Then, the corundum
2.2. Tests and characterization An alumina crucible with coated specimens was put into the furnace to test the isothermal oxidation property at 1773 K and held at that temperature for designated time. After the samples were cooled to room temperature, an electrical balance with the accuracy of ± 0.1 mg was used to measure their weight change. The formula for calculating the oxidation mass loss percentages (ΔW) is shown in Eq. (1). After that, the alumina crucible with the coated specimens was put into the furnace again for continue oxidation.
ΔW =
Table 1 Detailed spray parameters for SM, SMM and SMMY coating. Content
Parameters
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 diameter (mm)
40–55 75–80 8–12 5–10 25 100 5.5
m 0 − mt × 100% m0
(1)
where m0 and mt are the original mass of coated samples and oxidized for a designated time, respectively. For the thermal cycling test, the coated specimens in corundum crucible were put into and taken out from the furnace after they were kept in high temperature and room temperature for 5 min, respectively. Subsequently, this process was repeated. The mass of the samples was weighted every five times and the weight change percentages were also calculated. To reduce the error, three specimens were used for oxidation and thermal cycling test. X-ray diffraction (XRD, X-Pert PRO, PANalytical, The Netherlands) 2
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Fig. 2. (a) SEM surface image of the SiC inner coating on C/C composites; (b) SEM surface image of the prepared porous mullite whisker layer on the SiC inner coating (inset of figure refers XRD pattern of the mullite whisker layer sample); (c) magnified image of (b); (d) surface EDS analysis of spot A.
are uniform with a diameter about 100 nm, which are much shorter than their length. The large ratio of length to diameter may be conducive to enhance interface bonding strength between middle mullite and SiC inner coating. Clear lattice stripes can be observed in the HRTEM of the mullite whisker and the lattice spacing is 0.254 nm (Fig. 3(c)), which accords well with that of the (111) crystal plane of mullite. According to the FFT pattern, it can be concluded that the whisker is a single crystal and grows along the [111] direction. Fig. 4 shows the surface XRD patterns of SM, SMM and SMMY. The mullite and SiO2 phases were both identified for SM and SMM (Fig. 4(a) and (b)), which present the similar peaks at designated angles. The SiO2 phase is produced owing to the decomposition of mullite under high plasma arc temperature. Interestingly, the peak intensity of SMM is higher than that of SM, suggesting that incorporation the mullite whisker layer could improve compactness of the mullite coating. Three kinds of phases, including Y2Si2O7, mullite and SiO2 phases can be founded on the surface of the SMMY (Fig. 4(c)). Excessive SiO2 result in the appearance of the SiO2 diffraction peaks. This rich SiO2 is good for sealing the defects of the coating in high temperature. Surface morphology of SM is presented in Fig. 5(a). SM coating displays a rough surface and many homogeneously distributed microaggregates accompanied with micropores could be observed clearly. Such micropores are resulted from the stacking of particles. In addition, the coating is consisted of the molten zone and semi-molten zone, shown as inset of Fig. 5(a). The molten area is formed due to the ultrahigh temperatures under SAPS. Moreover, a microcrack can be observed, which may be formed due to the inherent brittleness of the coating and inferior wettability between the middle mullite and SiC inner layer. As for SMM (Fig. 5(b)), it also presents a typical characteristics of the sprayed coating, while the less protrusions on the surface than that of SM. In addition, there are no obvious microcracks, indicating that the mullite whiskers can increase both the toughness of the middle mullite coating and the wettability of the SiC and mullite layer. After covering SMM with yttrium aluminosilicate coating, the pinmicroholes on the surface were thoroughly sealed (Fig. 5(c)). Such SMMY-coating surface exhibits a dense and crack-free morphology with
analysis was performed to confirm the crystallographic phase of the coated specimens. Their microstructure was investigated by applying the scanning electron microscopy (SEM, JSM-6460) with energy dispersion spectroscopy (EDS). Microstructure of the mullite whisker was characterized by the transmission electron microscopy (TEM, Tecnai F30G2). The roughness of the as-prepared coating was examined by a 3D confocal laser microscopy (Optelics C130). The porosity of the specimens was evaluated using a mercury porosimeter (AutoPore IV 9500). Scratch tester (WS-2005 multi-functional tester. China) was used to evaluate the interface bonding strength between the as-prepared coating and the C/C substrate. 3. Results and discussion Surface SEM image of the SiC coating (Fig. 2(a)) presents a rough surface consisting primarily of big particles, while it is worth noting the smooth surface of the particle itself. This smooth surface is not conducive to the bonding of the middle mullite to SiC inner coating. Fig. 2(b) displays surface morphology of the in-situ synthesized porous whisker layer on the SiC inner coating at low magnification. It exhibits a porous structure with whiskers completely covering the SiC inner layer. By analysis of XRD displayed in inset of Fig. 2(b)), clearly, the diffraction peaks can be assigned to SiC and mullite, indicating that mullite whisker can be successfully incorporated on SiC inner coating. Both the survival big particles and the strong and sharp diffraction peaks of SiC indicate a thin mullite whisker layer is formed and the corrosion of inner SiC coating due to the molten salt is little. The highmagnification SEM image of mullite whisker layer (Fig. 2(c)) shows a continuous porous architecture, where the length of the self-assembled mullite-whisker bundles is 5–10 μm. More interestingly, the bundles are existing perpendicular to SiC inner coating. This porous hierarchical structure is expected to enhance the interface bonding strength between the middle mullite and SiC coating [35]. Based on the EDS results (Fig. 2(d)), the whisker denoted as spot A is composed of O, Al and Si elements, and their atom ratio are almost in line with the composition of 3Al2O3·2SiO2, demonstrating the whisker is mullite once again. The TEM images in Fig. 3(a) and (b) illustrate that mullite whiskers 3
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Fig. 3. (a) TEM image of bundle mullite whiskers; (b) TEM image of a single mullite whisker; (c) HRTEM image and the corresponding Fast Fourier transform (FFT) of the mullite whisker.
shown Fig. 7(c). Obviously, the whole yttrium aluminosilicate coating presents a crack-free and dense morphology. A lot of precipitates embed in the coating to arrest the cracks and baffle the propagation of cracks [36]. It is noteworthy that the part of porous mullite coating was infiltrated by yttrium aluminosilicate. The interface between the middle mullite and yttrium aluminosilicate coating is highly corroded, confirming the good wettability of between the mullite and yttrium aluminosilicate coating. Moreover, three kinds of phases can be found in the yttrium aluminosilicate coating. The white phase contains O, Si and Y elements (denoted as spot C); the elemental compositions of black phase are O, Si, and Al (denoted as spot D); the grey phase includes O, Al, Si and Y elements (denoted as spot E). Based on the above analyses, together with their XRD results, the spot C is confirmed to Y2Si2O7, the spot D is mullite and the spot E is Y-Al-Si-O glass. To probe the novel structure of SMMY more deeply, EDS mapscanning analyses were conducted for the cross-sectional SMMY, as shown in Fig. 8. It shows that O and Si elements are uniformly scattered in the middle and outer coating, indicating the whole coating is homogeneous. To compare Fig. 8(d) and (f), penetration of Y element into the middle mullite coating is clearly observed, indicating significant dissolution reactions between mullite and molten glass during the heat treatment. It is important to note that the cross-sectional SMMY can be divided into three parts: the dense SiC inner coating, the porous mullite middle coating and the dense outer yttrium aluminosilicate coating. The SiC inner coating acted as buffer layer to alleviate the incompatibility between mullite and C/C composites. The porous mullite middle coating was used for relieving the concentration of stress in the coating. The dense yttrium aluminosilicate outer coating not only prevented oxygen from diffusion to the matrix, but also served as the intergranular phase in the design of in-situ toughened mullite [30]. The mercury porosimeter was used to further disclose the porosity of different coatings (Fig. 9). It can be found that the variation trend of the porosity is nearly identical with surface roughness. For SM, a lot of un-melted or semi-melted particles are prone to stack owing to the poor
lots of precipitates. A suitable content of precipitates was inseted into the glass layer forming an “inlaid structure”, which could effectively pin the glass phase to restrain the propagation of cracks and prevent the quick gasification of glass [36,37]. Based on the EDS analysis, the elemental compositions of spot B is consisted of O, Si and Y (Fig. 5(d)). This, combined with the XRD results, indicate that the bright phase is Y2Si2O7. To further probe the roughness evolution of different kinds of coating, 3D confocal laser scanning microscopy was employed. The corresponding images and the average roughness values are presented in Fig. 6. For SM, the wettability between mullite and SiC is poor, and the sprayed mullite particles are easy to form protrusions, inducing higher surface roughness (10.5 μm). With the mullite whisker layer prepared, the sprayed mullite particles tend to spread out on the inner surface. Therefore, the roughness of SMM decreased (8.7 μm). When applying the yttrium aluminosilicate coating, the defects on the surface of SMM were sealed completely and a dense and smooth outer coating was formed, leading to lower surface roughness (5.8 μm). This result agrees well with surface SEM analyses (Fig. 5). The cross-sectional morphology of the as-prepared coating samples and their corresponding EDS analyses are presented in Fig. 7. A homogeneous mullite coating without cracks is obtained and its thickness is about 200 μm, as shown in Fig. 7(a). Amounts of microholes exist in this coating and provide a channel to diffuse oxygen, resulting in the corrosion of SiC and C/C composites. Furthermore, an evident interface between the mullite and SiC coating can be observed, inferring the bonding of the coating is poor. As for SMM, the coating thickness is about 200 μm (Fig. 7(b)). Although some microholes can be found in the mullite coating, the size and number of the microholes are less than those of SM. Especially, no distinct interface was detected between the mullite and SiC coating, indicating the good bonding and wettability between middle mullite and SiC inner coating. To further seal the microholes in the SMM, the yttrium aluminosilicate coating was selected and its cross-sectional backscattered electron image is
Fig. 4. XRD patterns of the as-prepared coating samples: (a) SM; (b) SMM; (c) SMMY. 4
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Fig. 5. Surface SEM images of the as-prepared coating samples: (a) SM; (b) SMM; (c) SMMY; (d) surface EDS analysis of spot B.
wettability between mullite and SiC, leading to lager porosity (12.6%). For SMM, the porosity decreased to 9.8%, which demonstrates once again that incorporating the mullite whisker layer is beneficial for
spreading out of the mullite particles. After applying the yttrium aluminosilicate coating, the porosity of the coating significantly decreased (3.9%), indicating that the yttrium aluminosilicate could heal the
Fig. 6. Confocal laser scanning microscope images of the as-prepared coating samples: (a) SM; (b) SMM; (c) SMMY; (d) average roughness values for as-coated samples. 5
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Fig. 7. Cross-section SEM images of the as-prepared coating samples: (a) SM; (b) SMM; (c) SMMY. (d) cross-section EDS analysis of spot C, D and E.
results are displayed in Fig. 11. Obviously, all of the specimens exhibit an initial slightly mass gain process. After that, their weight loss curves can be regarded as straight lines. It is notable that the slopes for three coated specimens are different. For SM, it owns the maximum slope among the three samples. After going through 30 thermal cycles, the weight loss reached to 2.16%, indicating that the SM has inferior thermal shock resistance owing to the high porosity and more defects in the coating. After construction of the mullite whisker layer, such slope is decreased, suggesting the thermal shock resistance of the specimens was enhanced by preparing mullite whisker layer between the inner SiC and middle mullite coatings. The coating with mullite whisker layer and yttrium aluminosilicate outer layer presents the best thermal shock resistance, whose weight loss is only 1.08% after 30 thermal cycles. Fig. 12 exhibits the surface morphology of the different specimens after 30 thermal cycles. A large number of microcracks and microholes
defects of the SMM effectively. To more intuitively compare the bonding strength of different coating samples, a scratch test was carried out and the corresponding results are presented in Fig. 10. Generally, the first sudden increase of acoustic emission signal and the abrupt vanish of friction signal indicate the disbanding of coating [38,39]. According to Fig. 10(a), the first obvious signal for SM is detected at about 13.6 N. After the mullitewhisker layer was introduced, the interface bonding strength increased to 16.7 N as shown in Fig. 10(b). Such improvement is attributed to the introduction of mullite whiskers and their tougheness effects. After fabricating the yttrium aluminosilicate coating, the interface bonding strength further increased to 20.4 N (Fig. 10(b)), which can be ascribed to the sealing of microholes and the penetration of yttrium aluminosilicate into the middle mullite layer. Thermal cycles test was executed in air and the corresponding
Fig. 8. The elemental mapping images of cross-sectional SMMY. 6
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Fig. 11. Thermal cycling curves of the coated specimens between 1773 K and room temperature in air.
Fig. 9. The porosity of the as-prepared coating samples.
were found on the SM coating (Fig. 12(a)). By further observation, the width of large crack is approximately 8 μm. These large-size cracks and holes cannot be sealed since the dwell time is short (5 min). Ultimately, these defects can provide the channel for the oxygen diffusing and penetrating into the substrate, leading to the oxidation of C/C composites. While for SMM, the number and size of the defects decreased and the crack with the width about 5 μm can be observed, inferring that the mullite whisker layer plays a positive role in relieving thermal stress and decreases the size of cracks. After construction the yttrium aluminosilicate coating, a few defects can be found (Fig. 12(e)) and the maximum crack width further decreased to 2 μm (Fig. 12(f)). In addition, numerous pinning phases occurred in the coating. The pinning phases are beneficial for enhancing the stability of the glass and have a significant effect on blocking the extension of microcracks. When cracks encounter the pinning phase, their original track will be changed, namely crack deflection, and the increased crack propagation path would consume more energy. Eventually, the cracks were arrested so as to restrict the spread of cracks resulting in a better thermal shock resistance [40]. In general, the improvement of the thermal shock resistance was thought to be the pinning effect of the precipitates and the toughening by the mullite whiskers. Fig. 13 presents the isothermal oxidation curves of different samples. It exhibits an increased trend at the primal oxidation stage for all the coated samples (Fig. 13(a)). Locally magnified view showed that the SM presents maximum weight gain (Fig. 13(b)), which is resulted from the more defects and higher porosity in it. These defects provide channels for the diffusion of oxygen. And then SiC layer was oxidized according to Eqs. (2) and (3), and it was a weight gain process. So the SM presents maximum weight gain eventually.
2SiC(s) + 3O2 (g) → 2SiO2 (s) + 2CO(g)
SiC(s) + 2O2 (g) → SiO2 (s) + CO2 (g)
(3)
With the increase of oxidation time, more and more defects were formed, and a plenty of oxygen not only react with SiC inner coating, but also react with C/C substrate, causing the mass loss of the specimens increased more quickly. The weight loss is 2.17% after oxidation 105 h. As for SMM, the weight loss is 1.87% after testing for 128 h at 1773 K, indicating that construction of the mullite whisker layer contributes to oxidation resistance of the mullite. With the yttrium aluminosilicate as an oxidation barrier, the oxidation protective ability is further enhanced, and the weight loss of C/C composites under oxidation for 185 h is only 1.58%. To understand on the oxidation behavior of the SMMY, the microstructure evolution of the coating was examined by SEM. The results are given in Fig. 14. At beginning, the precipitates of Y2Si2O7 and mullite are formed continuously from the Y-Si-Al-O glass, resulting the quantity of precipitate is much more than that of the raw coating. These precipitates are embedded in the Y-Si-Al-O glass, serving as immiscible phase in it, which could increase stability of the glass. After 13 h oxidation, the surface of coating is still dense and crack-free (Fig. 14(a)). More interesting, the interface between the mullite middle layer and yttrium aluminosilicate outer layer is vanished. In addition, the microholes in the mullite layer were sealed thoroughly. Lots of elongated, rodlike grains exist in the outer coating (Fig. 14(b)), which can in-situ toughen the mullite coating. In this stage, the dense glass could prevent the oxygen diffusing to C/C substrate. The precipitates act as a wild phase to toughen the coating. This coupling effect gives rise to better protection for C/C composites. In the later oxidation process (13–93 h), the average weight loss rate keeps a low level. After 93 h oxidation, it can be seen that cracks exist in the coating (Fig. 14(c)), while such cracks with small size concentrate on the surface and they will be sealed
(2)
Fig. 10. Acoustic emission and friction signals of as-prepared coating: (a) SM; (b) SMM; (c) SMMY. 7
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Fig. 12. Surface SEM micrographs of the coated specimens after 30 thermal cycles between 1773 K and room temperature: (a, b) SM; (c, d) SMM; (e, f) SMMY.
Fig. 13. (a) Isothermal oxidation curves of the coated samples at 1773 K in air; (b) magnified of (a).
Fig. 14. Surface SEM micrographs (a, c, e) and cross-section backscattered micrographs (b, d, f) of SMMY after oxidation at 1773 K in air for different hours: (a, b) 13 h; (c, d) 93 h; (e, f) 185 h.
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in time owing to the enough glass phase. Thus, the coating still possesses a good oxidation protective ability. From Fig. 14(e), more and more defects were detected after long time oxidation. More seriously, penetrating cracks were formed (Fig. 14(f)), which provide channel for oxygen to react with substrate according to Eqs. (4) and (5). The escape of gaseous byproducts (CO and CO2) results in holes and the volatilization of yttrium aluminosilicate layer causes the thickness of the coating decreased. In general, it can be draw a conclusion that the failure of coating during the oxidation test was thought to be due to the volatilization of the yttrium aluminosilicate layer and the formation of penetrating cracks.
2C(s) + O2 (g) → 2CO(g)
(4)
C(s) + O2 (g) → CO2 (g)
(5)
[12]
[13]
[14]
[15]
[16]
[17]
4. Conclusions
[18]
A novel mullite whisker-mullite/yttrium aluminosilicate coating with dense and crack-free structure was designed and prepared through a three-step process including molten salt, SAPS and hot dipping. The mullite whiskers in this coating acted as transition layer to enhance the bonding strength between the inner SiC and middle mullite layers. The porous mullite middle coating was used for relieving the concentration of stress. And the dense yttrium aluminosilicate coating not only served as a barrier to prevent the diffusion of oxygen, but also as the intergranular phase to toughen mullite by crack arrest and deflection. The synergistic effect of different layer contributed to the good interface bonding strength and oxidation protective ability. After 30 thermal shock cycles from 1773 K to room temperature and oxidation for 185 h in atmospheric environment at 1773 K, the coated sample was only at a weight loss of 1.08% and 1.58%, respectively. The failure of coating is mainly ascribed to the formation of penetrating cracks in the coating and the volatilization of glass layer.
[19]
[20]
[21]
[22] [23]
[24]
[25]
Acknowledgements
[26]
This work has been supported by the National Natural Science Foundation of China (No. 51872239, 51572223 and 51802244), and the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 142-TZ-2016).
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Mullite whisker-mullite/yttrium aluminosilicate oxidation protective coatings for SiC coated C/C composites ⁎
Lei Zhou, Qiangang Fu , Caixia Huo, Mingde Tong, Xuesong Liu, Dou Hu State Key Laboratory of Solidification Processing, Shaanxi Key Laboratory of Fiber Reinforced Light-Weight Composites, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: C/C composites Mullite whisker Thermal-shock resistance Oxidation resistance
A novel mullite whisker-mullite/yttrium aluminosilicate coating was designed and prepared through a three-step process including molten salt, supersonic atmospheric plasma spraying and sol-hot dipping. The mullite whiskers were used to enhance the interface bonding between mullite and SiC layer. A dense yttrium aluminosilicate outer coating was employed to seal the microholes in mullite coating. Results show that the bonding strength of the asprepared coating was enhanced and the coating present a dense and crack-free microstructure. After 30 thermalshock cycles from 1773 K to room temperature and oxidized for 185 h in atmospheric environment at 1773 K, the coated sample was only at a weight loss of 1.08% and 1.58%, respectively.
1. Introduction With the ever-increasing attention to high performance composites, carbon fiber reinforced composites, especially carbon/carbon (C/C) composites which possess excellent high temperature mechanical properties and stability, low density are regarded as an attractive thermal-structural materials [1–6]. Nonetheless, they have a fatal weakness that they are tend to be oxidized in air above 643 K, and oxidized rapidly above 773 K, leading to devastating damage to composites and then limiting their applications [7–10]. Thus, extensive research interest and efforts have been prompted to solve this problem, and an effective approach is to apply ceramic coating [11–15]. Among various promising ceramic coating candidates, SiC is the most successful widely-used coating materials [16]. However, the single SiC layer is difficult to meet the need of long time oxidation protection. To address this issue, multilayer coating was executed in recent years, for instance, MoSi2/SiC [17], ZrB2-SiO2/SiC [18], HfB2/ SiC [19], ZrSiO4/SiC [20], Y2SiO5/SiC [21], mullite/SiC [22]. Particularly, mullite has been selected as outer coating materials because it has similar thermal expansion coefficient with SiC and possesses low oxygen permeation and high melting point [23]. Up to now, lots of methods have been used to fabricate mullite coating, like hydrothermal electrophoretic deposition [24], supersonic atmospheric plasma spraying (SAPS) [25], hot dipping [26], sol-gel [27], etc. Although the as-prepared coating could provide protection for C/C composites to some extent, challenge still remains because the as-prepared coating
⁎
usually has high porosity and low interface bonding strength. Here, to enhance the interface bonding strength, mullite whiskers were in-situ synthesized to form a new layer, which not only can increase the wettability of the SiC interlayer and middle mullite layer, but also will toughen the middle mullite coating [28,29]. Then, to seal microholes in the middle mullite coating, the yttrium aluminosilicate coating was selected due to its good wettability with mullite coating [30]. In addition, the yttrium aluminosilicate has good oxidation resistance and possesses chemical durability at high temperatures [31–33]. In this work, a mullite whisker-mullite/yttrium aluminosilicate coating was designed and prepared through a three-step technique. A porous layer with mullite whiskers was firstly fabricated by in-situ molten salt reaction. Then, this composites were exposed to SAPS to prepare middle mullite coating. Finally, a dense yttrium aluminosilicate coating was applied to seal the as-prepared sample. Oxidation resistance of such multi-layer coating was tested. And the phase, microstructure of the coating, as well as the interface bonding between it and the C/C composites were also investigated. 2. Experimental 2.1. Specimen and coating preparation Cubic specimens as substrates (10 × 10 × 10 mm) were machined from 2D C/C composites whose density is 1.75 g/cm3. These specimens
Corresponding author. E-mail address: [email protected] (Q. Fu).
https://doi.org/10.1016/j.ceramint.2019.08.105 Received 25 July 2019; Received in revised form 8 August 2019; Accepted 10 August 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Lei Zhou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.105
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Fig. 1. Schematic of preparation of mullite whisker-mullite/yttrium aluminosilicate coating.
crucible was cooled naturally down to room temperature in furnace. The as-prepared sample was milled to obtain yttrium aluminosilicate powders. The silica sol was first mixed with distilled water according to a ratio of 1:5 in vol%. Afterwards, yttrium aluminosilicate powders were added into the silica sol solution, and then dispersed by ultrasonic bath and stirred by magnetic stirrer to acquire a homogeneous suspension. The coated specimens were heated up to 573 K for 5 min in the furnace, and then immediately dipped in the above yttrium aluminosilicate suspension. This process was repeated until the thickness of the coating is satisfied. Finally, a two-hour heat treatment at 1773 K in Ar was taken. The obtained coating sample is composed of SiC, mullite whisker, mullite layer and yttrium aluminosilicate layer, which was identified as SMMY. For comparison, the single mullite coating sample was also prepared by SAPS on the surface of SiC-C/C composites and named as SM.
were chamfered and abraded manually using the 300 and 600-grit SiC papers orderly, ultrasonically cleaned and dried in the oven. A SiC interlayer was synthesized on these samples by pack cementation and the precursor powders were as follows: Si (300 mesh) 60–80 wt%, graphite (300 mesh) 10–25 wt% and Al2O3 (325 mesh) 5–15 wt%. The C/C composites were embedded in the precursor powders, and then heated to 2073–2273 K for 2 h in an argon-protective atmosphere [34]. Fig. 1(a) and (b) present the process to synthesize mullite whiskers layer. The detailed fabrication process was as follow: SiC-C/C composites were embedded in an alumina crucible with the mixed powders of Al2(SO4)3·18H2O (40–50 wt%) and Na2SO4·9H2O (50–60 wt%). Subsequently, the alumina crucible was heated to 1173–1273 K for 30 min in air by an electric furnace. After cooling down to room temperature, the coated specimens were cleaned serveral times using boiling water. After that, the coated samples were dried in a 373 K-oven. Mullite coating was sprayed on the SiC-C/C substrate with mullite whisker layer by the SAPS method, as shown in Fig. 1(c). Mullite (5–10 μm) particles were selected as raw materials. Firstly, the raw particles were prilled by a comminutor. The slurry used in the prilling is composed of polymeric binder (4 wt%) and distilled water (96 wt%). Specific parameters of the spraying are listed in Table 1. The obtained coating sample contains SiC, mullite whisker and mullite layer. To simplify the discussion, it was identified as SMM. After fabricating the mullite coating, a sol hot dipping was selected to prepare yttrium aluminosilicate coating. The yttrium aluminosilicate powders were prepared as follows: 12.18% Y2O3-22% Al2O3-65.82% SiO2 mole% powders were mixed by tumbling in a ball mill and heated at 1673–1773 K for 2 h in a corundum crucible. Then, the corundum
2.2. Tests and characterization An alumina crucible with coated specimens was put into the furnace to test the isothermal oxidation property at 1773 K and held at that temperature for designated time. After the samples were cooled to room temperature, an electrical balance with the accuracy of ± 0.1 mg was used to measure their weight change. The formula for calculating the oxidation mass loss percentages (ΔW) is shown in Eq. (1). After that, the alumina crucible with the coated specimens was put into the furnace again for continue oxidation.
ΔW =
Table 1 Detailed spray parameters for SM, SMM and SMMY coating. Content
Parameters
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 diameter (mm)
40–55 75–80 8–12 5–10 25 100 5.5
m 0 − mt × 100% m0
(1)
where m0 and mt are the original mass of coated samples and oxidized for a designated time, respectively. For the thermal cycling test, the coated specimens in corundum crucible were put into and taken out from the furnace after they were kept in high temperature and room temperature for 5 min, respectively. Subsequently, this process was repeated. The mass of the samples was weighted every five times and the weight change percentages were also calculated. To reduce the error, three specimens were used for oxidation and thermal cycling test. X-ray diffraction (XRD, X-Pert PRO, PANalytical, The Netherlands) 2
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Fig. 2. (a) SEM surface image of the SiC inner coating on C/C composites; (b) SEM surface image of the prepared porous mullite whisker layer on the SiC inner coating (inset of figure refers XRD pattern of the mullite whisker layer sample); (c) magnified image of (b); (d) surface EDS analysis of spot A.
are uniform with a diameter about 100 nm, which are much shorter than their length. The large ratio of length to diameter may be conducive to enhance interface bonding strength between middle mullite and SiC inner coating. Clear lattice stripes can be observed in the HRTEM of the mullite whisker and the lattice spacing is 0.254 nm (Fig. 3(c)), which accords well with that of the (111) crystal plane of mullite. According to the FFT pattern, it can be concluded that the whisker is a single crystal and grows along the [111] direction. Fig. 4 shows the surface XRD patterns of SM, SMM and SMMY. The mullite and SiO2 phases were both identified for SM and SMM (Fig. 4(a) and (b)), which present the similar peaks at designated angles. The SiO2 phase is produced owing to the decomposition of mullite under high plasma arc temperature. Interestingly, the peak intensity of SMM is higher than that of SM, suggesting that incorporation the mullite whisker layer could improve compactness of the mullite coating. Three kinds of phases, including Y2Si2O7, mullite and SiO2 phases can be founded on the surface of the SMMY (Fig. 4(c)). Excessive SiO2 result in the appearance of the SiO2 diffraction peaks. This rich SiO2 is good for sealing the defects of the coating in high temperature. Surface morphology of SM is presented in Fig. 5(a). SM coating displays a rough surface and many homogeneously distributed microaggregates accompanied with micropores could be observed clearly. Such micropores are resulted from the stacking of particles. In addition, the coating is consisted of the molten zone and semi-molten zone, shown as inset of Fig. 5(a). The molten area is formed due to the ultrahigh temperatures under SAPS. Moreover, a microcrack can be observed, which may be formed due to the inherent brittleness of the coating and inferior wettability between the middle mullite and SiC inner layer. As for SMM (Fig. 5(b)), it also presents a typical characteristics of the sprayed coating, while the less protrusions on the surface than that of SM. In addition, there are no obvious microcracks, indicating that the mullite whiskers can increase both the toughness of the middle mullite coating and the wettability of the SiC and mullite layer. After covering SMM with yttrium aluminosilicate coating, the pinmicroholes on the surface were thoroughly sealed (Fig. 5(c)). Such SMMY-coating surface exhibits a dense and crack-free morphology with
analysis was performed to confirm the crystallographic phase of the coated specimens. Their microstructure was investigated by applying the scanning electron microscopy (SEM, JSM-6460) with energy dispersion spectroscopy (EDS). Microstructure of the mullite whisker was characterized by the transmission electron microscopy (TEM, Tecnai F30G2). The roughness of the as-prepared coating was examined by a 3D confocal laser microscopy (Optelics C130). The porosity of the specimens was evaluated using a mercury porosimeter (AutoPore IV 9500). Scratch tester (WS-2005 multi-functional tester. China) was used to evaluate the interface bonding strength between the as-prepared coating and the C/C substrate. 3. Results and discussion Surface SEM image of the SiC coating (Fig. 2(a)) presents a rough surface consisting primarily of big particles, while it is worth noting the smooth surface of the particle itself. This smooth surface is not conducive to the bonding of the middle mullite to SiC inner coating. Fig. 2(b) displays surface morphology of the in-situ synthesized porous whisker layer on the SiC inner coating at low magnification. It exhibits a porous structure with whiskers completely covering the SiC inner layer. By analysis of XRD displayed in inset of Fig. 2(b)), clearly, the diffraction peaks can be assigned to SiC and mullite, indicating that mullite whisker can be successfully incorporated on SiC inner coating. Both the survival big particles and the strong and sharp diffraction peaks of SiC indicate a thin mullite whisker layer is formed and the corrosion of inner SiC coating due to the molten salt is little. The highmagnification SEM image of mullite whisker layer (Fig. 2(c)) shows a continuous porous architecture, where the length of the self-assembled mullite-whisker bundles is 5–10 μm. More interestingly, the bundles are existing perpendicular to SiC inner coating. This porous hierarchical structure is expected to enhance the interface bonding strength between the middle mullite and SiC coating [35]. Based on the EDS results (Fig. 2(d)), the whisker denoted as spot A is composed of O, Al and Si elements, and their atom ratio are almost in line with the composition of 3Al2O3·2SiO2, demonstrating the whisker is mullite once again. The TEM images in Fig. 3(a) and (b) illustrate that mullite whiskers 3
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Fig. 3. (a) TEM image of bundle mullite whiskers; (b) TEM image of a single mullite whisker; (c) HRTEM image and the corresponding Fast Fourier transform (FFT) of the mullite whisker.
shown Fig. 7(c). Obviously, the whole yttrium aluminosilicate coating presents a crack-free and dense morphology. A lot of precipitates embed in the coating to arrest the cracks and baffle the propagation of cracks [36]. It is noteworthy that the part of porous mullite coating was infiltrated by yttrium aluminosilicate. The interface between the middle mullite and yttrium aluminosilicate coating is highly corroded, confirming the good wettability of between the mullite and yttrium aluminosilicate coating. Moreover, three kinds of phases can be found in the yttrium aluminosilicate coating. The white phase contains O, Si and Y elements (denoted as spot C); the elemental compositions of black phase are O, Si, and Al (denoted as spot D); the grey phase includes O, Al, Si and Y elements (denoted as spot E). Based on the above analyses, together with their XRD results, the spot C is confirmed to Y2Si2O7, the spot D is mullite and the spot E is Y-Al-Si-O glass. To probe the novel structure of SMMY more deeply, EDS mapscanning analyses were conducted for the cross-sectional SMMY, as shown in Fig. 8. It shows that O and Si elements are uniformly scattered in the middle and outer coating, indicating the whole coating is homogeneous. To compare Fig. 8(d) and (f), penetration of Y element into the middle mullite coating is clearly observed, indicating significant dissolution reactions between mullite and molten glass during the heat treatment. It is important to note that the cross-sectional SMMY can be divided into three parts: the dense SiC inner coating, the porous mullite middle coating and the dense outer yttrium aluminosilicate coating. The SiC inner coating acted as buffer layer to alleviate the incompatibility between mullite and C/C composites. The porous mullite middle coating was used for relieving the concentration of stress in the coating. The dense yttrium aluminosilicate outer coating not only prevented oxygen from diffusion to the matrix, but also served as the intergranular phase in the design of in-situ toughened mullite [30]. The mercury porosimeter was used to further disclose the porosity of different coatings (Fig. 9). It can be found that the variation trend of the porosity is nearly identical with surface roughness. For SM, a lot of un-melted or semi-melted particles are prone to stack owing to the poor
lots of precipitates. A suitable content of precipitates was inseted into the glass layer forming an “inlaid structure”, which could effectively pin the glass phase to restrain the propagation of cracks and prevent the quick gasification of glass [36,37]. Based on the EDS analysis, the elemental compositions of spot B is consisted of O, Si and Y (Fig. 5(d)). This, combined with the XRD results, indicate that the bright phase is Y2Si2O7. To further probe the roughness evolution of different kinds of coating, 3D confocal laser scanning microscopy was employed. The corresponding images and the average roughness values are presented in Fig. 6. For SM, the wettability between mullite and SiC is poor, and the sprayed mullite particles are easy to form protrusions, inducing higher surface roughness (10.5 μm). With the mullite whisker layer prepared, the sprayed mullite particles tend to spread out on the inner surface. Therefore, the roughness of SMM decreased (8.7 μm). When applying the yttrium aluminosilicate coating, the defects on the surface of SMM were sealed completely and a dense and smooth outer coating was formed, leading to lower surface roughness (5.8 μm). This result agrees well with surface SEM analyses (Fig. 5). The cross-sectional morphology of the as-prepared coating samples and their corresponding EDS analyses are presented in Fig. 7. A homogeneous mullite coating without cracks is obtained and its thickness is about 200 μm, as shown in Fig. 7(a). Amounts of microholes exist in this coating and provide a channel to diffuse oxygen, resulting in the corrosion of SiC and C/C composites. Furthermore, an evident interface between the mullite and SiC coating can be observed, inferring the bonding of the coating is poor. As for SMM, the coating thickness is about 200 μm (Fig. 7(b)). Although some microholes can be found in the mullite coating, the size and number of the microholes are less than those of SM. Especially, no distinct interface was detected between the mullite and SiC coating, indicating the good bonding and wettability between middle mullite and SiC inner coating. To further seal the microholes in the SMM, the yttrium aluminosilicate coating was selected and its cross-sectional backscattered electron image is
Fig. 4. XRD patterns of the as-prepared coating samples: (a) SM; (b) SMM; (c) SMMY. 4
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Fig. 5. Surface SEM images of the as-prepared coating samples: (a) SM; (b) SMM; (c) SMMY; (d) surface EDS analysis of spot B.
wettability between mullite and SiC, leading to lager porosity (12.6%). For SMM, the porosity decreased to 9.8%, which demonstrates once again that incorporating the mullite whisker layer is beneficial for
spreading out of the mullite particles. After applying the yttrium aluminosilicate coating, the porosity of the coating significantly decreased (3.9%), indicating that the yttrium aluminosilicate could heal the
Fig. 6. Confocal laser scanning microscope images of the as-prepared coating samples: (a) SM; (b) SMM; (c) SMMY; (d) average roughness values for as-coated samples. 5
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Fig. 7. Cross-section SEM images of the as-prepared coating samples: (a) SM; (b) SMM; (c) SMMY. (d) cross-section EDS analysis of spot C, D and E.
results are displayed in Fig. 11. Obviously, all of the specimens exhibit an initial slightly mass gain process. After that, their weight loss curves can be regarded as straight lines. It is notable that the slopes for three coated specimens are different. For SM, it owns the maximum slope among the three samples. After going through 30 thermal cycles, the weight loss reached to 2.16%, indicating that the SM has inferior thermal shock resistance owing to the high porosity and more defects in the coating. After construction of the mullite whisker layer, such slope is decreased, suggesting the thermal shock resistance of the specimens was enhanced by preparing mullite whisker layer between the inner SiC and middle mullite coatings. The coating with mullite whisker layer and yttrium aluminosilicate outer layer presents the best thermal shock resistance, whose weight loss is only 1.08% after 30 thermal cycles. Fig. 12 exhibits the surface morphology of the different specimens after 30 thermal cycles. A large number of microcracks and microholes
defects of the SMM effectively. To more intuitively compare the bonding strength of different coating samples, a scratch test was carried out and the corresponding results are presented in Fig. 10. Generally, the first sudden increase of acoustic emission signal and the abrupt vanish of friction signal indicate the disbanding of coating [38,39]. According to Fig. 10(a), the first obvious signal for SM is detected at about 13.6 N. After the mullitewhisker layer was introduced, the interface bonding strength increased to 16.7 N as shown in Fig. 10(b). Such improvement is attributed to the introduction of mullite whiskers and their tougheness effects. After fabricating the yttrium aluminosilicate coating, the interface bonding strength further increased to 20.4 N (Fig. 10(b)), which can be ascribed to the sealing of microholes and the penetration of yttrium aluminosilicate into the middle mullite layer. Thermal cycles test was executed in air and the corresponding
Fig. 8. The elemental mapping images of cross-sectional SMMY. 6
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Fig. 11. Thermal cycling curves of the coated specimens between 1773 K and room temperature in air.
Fig. 9. The porosity of the as-prepared coating samples.
were found on the SM coating (Fig. 12(a)). By further observation, the width of large crack is approximately 8 μm. These large-size cracks and holes cannot be sealed since the dwell time is short (5 min). Ultimately, these defects can provide the channel for the oxygen diffusing and penetrating into the substrate, leading to the oxidation of C/C composites. While for SMM, the number and size of the defects decreased and the crack with the width about 5 μm can be observed, inferring that the mullite whisker layer plays a positive role in relieving thermal stress and decreases the size of cracks. After construction the yttrium aluminosilicate coating, a few defects can be found (Fig. 12(e)) and the maximum crack width further decreased to 2 μm (Fig. 12(f)). In addition, numerous pinning phases occurred in the coating. The pinning phases are beneficial for enhancing the stability of the glass and have a significant effect on blocking the extension of microcracks. When cracks encounter the pinning phase, their original track will be changed, namely crack deflection, and the increased crack propagation path would consume more energy. Eventually, the cracks were arrested so as to restrict the spread of cracks resulting in a better thermal shock resistance [40]. In general, the improvement of the thermal shock resistance was thought to be the pinning effect of the precipitates and the toughening by the mullite whiskers. Fig. 13 presents the isothermal oxidation curves of different samples. It exhibits an increased trend at the primal oxidation stage for all the coated samples (Fig. 13(a)). Locally magnified view showed that the SM presents maximum weight gain (Fig. 13(b)), which is resulted from the more defects and higher porosity in it. These defects provide channels for the diffusion of oxygen. And then SiC layer was oxidized according to Eqs. (2) and (3), and it was a weight gain process. So the SM presents maximum weight gain eventually.
2SiC(s) + 3O2 (g) → 2SiO2 (s) + 2CO(g)
SiC(s) + 2O2 (g) → SiO2 (s) + CO2 (g)
(3)
With the increase of oxidation time, more and more defects were formed, and a plenty of oxygen not only react with SiC inner coating, but also react with C/C substrate, causing the mass loss of the specimens increased more quickly. The weight loss is 2.17% after oxidation 105 h. As for SMM, the weight loss is 1.87% after testing for 128 h at 1773 K, indicating that construction of the mullite whisker layer contributes to oxidation resistance of the mullite. With the yttrium aluminosilicate as an oxidation barrier, the oxidation protective ability is further enhanced, and the weight loss of C/C composites under oxidation for 185 h is only 1.58%. To understand on the oxidation behavior of the SMMY, the microstructure evolution of the coating was examined by SEM. The results are given in Fig. 14. At beginning, the precipitates of Y2Si2O7 and mullite are formed continuously from the Y-Si-Al-O glass, resulting the quantity of precipitate is much more than that of the raw coating. These precipitates are embedded in the Y-Si-Al-O glass, serving as immiscible phase in it, which could increase stability of the glass. After 13 h oxidation, the surface of coating is still dense and crack-free (Fig. 14(a)). More interesting, the interface between the mullite middle layer and yttrium aluminosilicate outer layer is vanished. In addition, the microholes in the mullite layer were sealed thoroughly. Lots of elongated, rodlike grains exist in the outer coating (Fig. 14(b)), which can in-situ toughen the mullite coating. In this stage, the dense glass could prevent the oxygen diffusing to C/C substrate. The precipitates act as a wild phase to toughen the coating. This coupling effect gives rise to better protection for C/C composites. In the later oxidation process (13–93 h), the average weight loss rate keeps a low level. After 93 h oxidation, it can be seen that cracks exist in the coating (Fig. 14(c)), while such cracks with small size concentrate on the surface and they will be sealed
(2)
Fig. 10. Acoustic emission and friction signals of as-prepared coating: (a) SM; (b) SMM; (c) SMMY. 7
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Fig. 12. Surface SEM micrographs of the coated specimens after 30 thermal cycles between 1773 K and room temperature: (a, b) SM; (c, d) SMM; (e, f) SMMY.
Fig. 13. (a) Isothermal oxidation curves of the coated samples at 1773 K in air; (b) magnified of (a).
Fig. 14. Surface SEM micrographs (a, c, e) and cross-section backscattered micrographs (b, d, f) of SMMY after oxidation at 1773 K in air for different hours: (a, b) 13 h; (c, d) 93 h; (e, f) 185 h.
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in time owing to the enough glass phase. Thus, the coating still possesses a good oxidation protective ability. From Fig. 14(e), more and more defects were detected after long time oxidation. More seriously, penetrating cracks were formed (Fig. 14(f)), which provide channel for oxygen to react with substrate according to Eqs. (4) and (5). The escape of gaseous byproducts (CO and CO2) results in holes and the volatilization of yttrium aluminosilicate layer causes the thickness of the coating decreased. In general, it can be draw a conclusion that the failure of coating during the oxidation test was thought to be due to the volatilization of the yttrium aluminosilicate layer and the formation of penetrating cracks.
2C(s) + O2 (g) → 2CO(g)
(4)
C(s) + O2 (g) → CO2 (g)
(5)
[12]
[13]
[14]
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4. Conclusions
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A novel mullite whisker-mullite/yttrium aluminosilicate coating with dense and crack-free structure was designed and prepared through a three-step process including molten salt, SAPS and hot dipping. The mullite whiskers in this coating acted as transition layer to enhance the bonding strength between the inner SiC and middle mullite layers. The porous mullite middle coating was used for relieving the concentration of stress. And the dense yttrium aluminosilicate coating not only served as a barrier to prevent the diffusion of oxygen, but also as the intergranular phase to toughen mullite by crack arrest and deflection. The synergistic effect of different layer contributed to the good interface bonding strength and oxidation protective ability. After 30 thermal shock cycles from 1773 K to room temperature and oxidation for 185 h in atmospheric environment at 1773 K, the coated sample was only at a weight loss of 1.08% and 1.58%, respectively. The failure of coating is mainly ascribed to the formation of penetrating cracks in the coating and the volatilization of glass layer.
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Acknowledgements
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This work has been supported by the National Natural Science Foundation of China (No. 51872239, 51572223 and 51802244), and the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 142-TZ-2016).
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