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Dynamic oxidation protective behaviors and mechanisms of HfB2-20wt%SiC composite coating for carbon materials
Accepted Manuscript Title: Dynamic oxidation protective behaviors and mechanisms of HfB2 -20wt%SiC composite coating for carbon materials Authors: Xuanru Ren, Tianqi Shang, Wenhao Wang, Peizhong Feng, LiTong Guo, Ping Zhang, Ziyu Li PII: DOI: Reference:
S0955-2219(19)30048-2 https://doi.org/10.1016/j.jeurceramsoc.2019.01.033 JECS 12300
To appear in:
Journal of the European Ceramic Society
Received date: Revised date: Accepted date:
25 July 2018 19 January 2019 21 January 2019
Please cite this article as: Ren X, Shang T, Wang W, Feng P, Guo L, Zhang P, Li Z, Dynamic oxidation protective behaviors and mechanisms of HfB2 -20wt%SiC composite coating for carbon materials, Journal of the European Ceramic Society (2019), https://doi.org/10.1016/j.jeurceramsoc.2019.01.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dynamic oxidation protective behaviors and mechanisms of HfB2-20wt%SiC composite coating
for carbon materials
Xuanru Ren* [email protected], Tianqi Shang, Wenhao Wang, Peizhong Feng* [email protected],
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LiTong Guo* [email protected], Ping Zhang, Ziyu Li
School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou
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221116, China
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*Corresponding author. Tel.: +86 15505183876;
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Abstract
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HfB2-20%wtSiC composite coating was prepared by liquid phase sintering method. After modification of HfB2 phase, the initial oxidation consumptions of the SiC coated samples were delayed
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from 500 ℃ to 800 ℃. Due to the higher oxidation activity of HfB2, the sufficient generated B2O3 is
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capable of inhibiting oxidation consumption of carbon substrate in oxidation activation region (800 ℃1000 ℃) and fastest oxidation region (1000 ℃-1280 ℃). The enhanced oxidation activity of SiC above
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1000℃ leads to the increased generation of SiO2, inhibiting the evaporation of B2O3 through the formation of Hf-B-Si-O glass layer, improving its stability and oxidation resistance above 1000℃. The heterogeneous refractory Hf-Oxides embedded in Hf-B-Si-O glass layer play role of reinforcement phases, restricting generation and spread of cracks. The inerting effect of Hf-B-Si-O glass layer strengthened with the increase of thermogravimetric analysis (TG) recycle oxidation times, indicating 1
promising oxidation inhibition potential of the coating in dynamic aerobic environment.
Keywords: HfB2; Coating; Liquid phase sintering; Dynamic oxidation protection; Hf-B-Si-O glass layer;
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1. Introduction
HfB2, an excellent ultrahigh temperature ceramics (UHTCs) diboride, is well known for the
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outstanding properties such as high melting temperatures (3250°C), chemical stability, high hardness and thermal conductivity [1-4]. Hence, resulting from these wonderful ultrahigh temperature performances, it
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has attracted more attention for potential applications such as thermal protection systems, rocket engines,
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refractory crucibles, space vehicles, and so on [5-8]. To get better oxidation resistances, the SiC phase
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with self-sealing property is generally used as additive to prepare HfB2-SiC composites [9-13]. After the
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addition of SiC phase, the formation of oxidation protective borosilicate glassy layer is responsible for the
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sharply enhanced oxidation resistance of HfB2 composites below 1800°C. Carbon structural materials (graphite, carbon/carbon composites) possess many performances that
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are suitable in the aviation and aerospace field, including ultra-high melting temperature, low thermal
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expansion coefficient, high strength, high modulus. One of their most important performances is that mechanical properties increase along with the increase of temperature [14-16]. Nevertheless, though carbon possesses ultra-high melting temperature of about 3500℃, its oxidizability above 400℃ is the fatal
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flaw of the carbon structural materials, greatly restricting their broad applications. To solve this problem, oxidation resistant coating technique is considered as the most effective method [17-20]. Hence, recently, in view of the promising anti-oxidation resistance of HfB2-SiC materials, they were developed as oxidation resistant coatings to protect carbon structural materials from oxidation. 2
Wang et al. [21] have prepared HfB2-SiC coating by in-situ reaction method to protect carbon/carbon composites from oxyacetylene torch ablation at heat flux of 2400 kW/m2 for 60 s. The mass and linear ablation rate of the coated C/C substrate were only 0.147 mg/s and 0.267 μm/s. To protect carbon fiberbased composites, Richet et al. [22] synthesized HfB2-SiC coatings that exhibit good oxidation resistance
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up to 1700°C. Zhang et al. [23] prepared HfB2-SiC coating by in-situ synthesis method at 2373 K and
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found that the coating could protect the coated C/C substrate from oxidation at 1773K for 753 h with
0.487% mass loss. In our previous work, we also prepared HfB2-SiC/SiC coating with excellent antioxidation ability [24], and after oxidation at 1773 K in air for 265h, the mass loss of the coated C/C
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substrate is only 0.41×10-2 g/cm2. Hence, the application of HfB2-SiC coatings on carbon structural
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materials exhibits great potential.
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Furthermore, with the increase in demand for carbon materials, the applied coatings are facing more and more challenges [25-27]. The applications of carbon materials in multiple variable oxidation
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environments require that the coatings need to have better oxidation protection ability in a wider
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temperature range. Although the self-sealing property makes the Si-based ceramics effective antioxidation components in the temperature region from 1200℃ to 1600℃, the Si-based ceramics do not
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have good protection ability below 1000℃ [28-30]. While due to the existence of B atom in HfB2, the generated antioxidant B2O3 will make the HfB2 an ideal supplement for the anti-oxidation shortage of Si-
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based ceramics below 1000℃. Moreover, the borosilicate glass layer formed by the reaction between B2O3 and silicate glass layer will further enhance the oxidation protective ability of the HfB2-SiC coating. Hence, the suitable component design is crucial for the anti-oxidation protective ability of the HfB2-SiC coating for carbon materials in a wider temperature range. Generally speaking, the suitable content of SiC phase for the HfB2-SiC materials is about 20 3
wt./vol.% to obtain the best oxidation resistance [31-32]. However, until now, in the reported works, the HfB2 phase was generally used as additive for Si-based ceramic coating applied on the surface of carbon materials to enhance the anti-oxidation ability of Si-based ceramic coatings. Nevertheless, considering the excellent anti-oxidation potential of HfB2 phase in dynamic aerobic environment, the inadequate amount
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of HfB2 phase in the coating cannot ensure the sufficient oxidation protection ability to protect carbon
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materials from oxidation in dynamic environments. Thus, the preparation of HfB2-SiC composite coating with a high HfB2 phase content exhibits much potential to broaden the application of carbon structural materials in wider temperature range.
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Up to now, many coating preparation techniques can be utilized to synthesize HfB2-SiC coating,
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such as plasma spraying method, pack cementation method, in-situ reaction method, and so on. However,
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although the multiphase UHTCs borides-Si-based ceramic coatings prepared by the pack cementation method and in-situ reaction method exhibit excellent oxidation resistances, it is difficult to insert high
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amount of UHTCs borides into the coatings due to the limitation of coating technology. While for the
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plasma spraying method, although it is suitable to control the phase amount in coatings, the higher porosity of the synthesized coatings weakens the anti-oxidation effect of the coating. Hence, the obvious
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shortcomings of the above methods are not suitable to prepare HfB2-SiC coatings with high HfB2 phase content.
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Recently, in our previous work [33], we have designed a kind of liquid phase sintering method to
modify the SiC coating by HfB2 phase. Although our previous work was aimed to develop a Si-based ceramic coating modification technique, the coating preparation technique presents great potential to deal with the challenges to prepare HfB2-SiC coating with high HfB2 phase content. Hence, in this paper, in view of the great potential of the liquid phase sintering method, to enhance the anti-oxidation protective 4
ability of the HfB2-SiC coating in wider temperature range, a HfB2-20%wtSiC composite coating was prepared by liquid phase sintering method using HfB2, graphite powders and silica sol as raw materials. The microstructure, dynamic anti-oxidation resistance and behaviors the HfB2-20%wtSiC coating were investigated. To disclose the oxidation protective mechanisms of the HfB2-20%wtSiC coating in wider
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temperature range, the oxidation resistance of HfB2 powders, SiC powders and HfB2-20%wtSiC composite powders were also analyzed.
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2. Experimental procedures
Fig. 1 illustrates the preparation of the outer HfB2-20%wtSiC coating. Applied on the surface of the
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graphite substrates with size of 3mm×3mm×3mm, the inner SiC transition layer was prepared by the pack
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cementation technique, the process of which was shown elsewhere [24]. The HfB2 powders were
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synthesized by carbothermal reduction reaction using C, HfO2 and B2O3 as raw materials, as shown
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elsewhere [32]. At first, the synthesized HfB2 powders (80 wt.%), graphite powders (20 wt.%) (Carbon
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Plant, Xi’an, China) and silica sol (SiO2· nH2O) (the value of Vsilica sol : M other raw materials is 0.5-1.5 ml/g) (City Fire Crystal Glass Co., Ltd, Dezhou, China) were used as raw coating materials, which were further
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ball milled for 2h. Then, the mixture was used as slurry to be brushed onto the surface of the SiC layer to
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produce the pre-fabricated HfB2-20%wtSiC coating. Next, the pre-fabricated HfB2-20%wtSiC coating was dried at 363 K for 1 h. Afterwards, the dried samples were repeatedly painted for another two runs. Finally, the coated specimens were placed in the graphite crucible with pure argon protection and heated
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to 2373K for 2h, with a heating rate of about 5-10 K/min. During the heat treatment, the silica sol and the C powders reacted on the basis of Equation (1) to form SiC phase. In addition, since the melting temperature (about 2000K) of the silica sol is far below the synthetic temperature (2373K) of the outer coating, the silica sol would be presented as liquid phase, creating liquid phase sintering conditions for
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the preparation of the coating. SiO2(s)+C(s)→SiC(s)+CO(g)
（1）
An X-ray diffractometer (XRD, BRUKER AXS, Germany) was used to analyze the produced HfB2 powders and the composition of the outer coatings. The morphology of the HfB2-20%wtSiC
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coating was measured by a scanning electron microscope (SEM, JSM6460, JEOL, Japan). The
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surface element mapping of the outer HfB2-20%wtSiC coating after oxidation was analyzed by
energy-dispersive spectroscopy (EDS, EDAX, octane plus, USA). Transmission electron microscopy (TEM, JEM-3010, JEOL, Japan) was utilized to analyze crystal structure of the synthetic HfB 2
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powders. The dynamic anti-oxidation ability of the coatings was investigated by a
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thermogravimetric analyzer (TGA, Netzsch, Germany). In addition, TG tests were carried out in an
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aerobic environment that consists of argon and oxygen. The volume of oxygen to argon is 22:78. To further analyze the anti-oxidation mechanisms of the coatings, the coated samples were oxidized at
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1773K for another 50h.
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3. Results and discussion
As we can see, Fig.2 exhibits the XRD pattern of the chemosynthetic HfB 2 powders. The XRD
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pattern matches the data of hexagonal HfB2 (JCPDS Card reference code #03-065-8678). The high
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intensity of the diffraction peaks indicates a good crystalline state of the hexagonal HfB2 powders.
The TEM photograph is shown in Fig.3 (a). The particle size of the synthesized HfB2 powders is about 200-650nm as shown in Fig.3 (a), only a small number of the particles being larger than 650nm. To further investigate the crystalline feature of the HfB 2 powders, a high-resolution TEM 6
photograph and a selected-area electron diffraction pattern of the HfB 2 powders are exhibited in Fig.3 (b) and (c). It can be seen that both the interplanar spacings of the lattice stripes and the diffraction rings are consistent with the XRD pattern of HfB 2 powders, reflecting the polycrystalline
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hexagonal nature of HfB2 powders.
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The XRD pattern of the outer HfB2-20%wtSiC coating is shown in Fig.4. From the pattern, the diffraction peak of HfB2 and SiC can be found at the same time, proving that the outer HfB220%wtSiC coating has been successfully prepared. In addition, no obvious SiO2 peak can be
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detected, indicating the successful transformation from silica to SiC.
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For the sake of analyzing the surface microstructure of the outer HfB 2-20%wtSiC coating, a
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surface backscatter SEM micrograph is exhibited in Fig. 5(a). A close-knit structure can be clearly observed. Moreover, many white particles covered on the surface of the coating can be observed. In
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Fig.5 (b), it can be seen that the white and black grains are bound to each other with no obvious
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cavity and microcrack. To analyze what the white and black grains really are, spot EDS analyses are done and the results are shown in Fig.5 (c). On the basis of these results, the white grains are
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confirmed as HfB2, while the other ones are SiC.
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As illustrated in Fig.6 (a), cross section SEM backscatter micrograph of the graphite substrate
coated with the HfB2-20%wtSiC/SiC coating shows that the thickness of the inner SiC coating and outer HfB2-20%wtSiC coating are approximately 120μm and 200μm, respectively. The HfB2 phases mainly concentrate on the thin top surface region of the outer coating, forming a kind of crust with roughness about 100 µm high; while the inner part of the outer coating is a kind of porous SiC layer 7
with few dispersed HfB2 particles. Furthermore, the white HfB2 grains can still be observed in the dense internal SiC coating. This is because during the preparation of the pre-fabricated outer coating, some raw outer coating materials slurry would infiltrate into the inner SiC coating, which not only brings the HfB2 powders into the inner SiC coating, but also effectively connects the internal and
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external coatings during the heat-treatment process. Thus, the double layer coatings exhibit good
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compatibility, no obvious large cavity or microcrack could be seen at its interface. In order to clearly
observe the quality of the coating, the cross section of the coating is magnified, as illustrated in Fig.6 (b). Due to the accumulation of large SiC grains in the outer coating, some small voids can be
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observed in the outer coating. In addition, no penetrating crack could be observed. As we know, the
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cracks act like diffusion channels for oxygen to diffuse into the carbon substrates, whose existence
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provide advantages for the entry of oxygen. A characteristic feature of oxygen corrosion of carbon substrate is the formation of oxidation holes in substrates. It has been reported that cracks, especially
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those penetrating in coatings, are always associated with oxidation holes generated in the carbon
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substrate during oxidation [34-36]. Fu et al. found that the pre-existing microcracks in the coating would provide diffusion channels for oxygen to corrode C substrate [37], which would form large
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voids, thus resulting in the failure of carbon substrate. The absence of cracks suggests that the
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coating described here may offer a better protection against oxidation than in these previous cases.
For the sake of measuring the oxidation resistance of the pure graphite substrate in dynamic condition from 25℃ to 1500 ℃, a TG test was carried out. As shown in Fig.7, there are mainly two 8
weight-loss zones in the figure, the gentle weight-loss zone (Fig.7 (a)) and the fastest weight-loss zone (Fig.7 (b)), respectively. The former one is a slower oxidation process from 500℃ to 800℃, and the latter a drastic one from 800℃ to 1080℃. However, when it comes to about 1100℃, the graphite
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Thus, the fastest weight-loss zone is actually an oxidation activation zone of carbon.
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will be completely oxidized, which indicates the poor oxidation resistance of the graphite substrate.
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As shown in Fig.8 (a), TGA test was used to assess the anti-oxidation protective ability of the HfB2-20%wtSiC/SiC coating. Due to the oxidation consumption of carbon substrate, the sample
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coated with SiC coating begins to lose weight from about 580 ℃, and the final weight loss of the sample is 17.5% after the TGA test. On the other hand, for the HfB 2-20%wtSiC coating, although a
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slight weight loss region (900 ℃-1100 ℃) can be observed in the TG curve of the coated sample, the weight of the coated sample has been increased about 0.23% after the whole dynamic TG oxidation test. The dynamic anti-oxidation performance of the HfB2-20%wtSiC coating is far beyond the 10.29% weight loss reported by Wang et al. [38] and 7.5% weight loss in our previous work [33]. To disclose the role of HfB2 playing in the coating during the dynamic TG oxidation test, the TG curves 9
of the HfB2-SiC coatings with different weight fractions of HfB 2 were presented in Fig.8 (b). As shown in Fig.8 (a), the pure SiC coating exhibits the worst anti-oxidation protective ability in the medium temperature region from 600 ℃ to 1400 ℃, which results in a major weight loss of the carbon substrate in the dynamic oxidation environments. As shown in Fig.8 (b), with the increase of
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weight content of HfB2, the weight losses of the coated samples were significantly restricted in this region. Hence, the HfB2 phase exhibits an excellent anti-oxidation modification ability in the
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medium temperature region from 600 ℃ to 1400 ℃, showing obvious complementary advantages
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combined with Si-based ceramic coatings.
In order to further analyze the anti-oxidation protective processes of the coated samples, the
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mass loss rate curves of the coated samples during TG tests were shown in Fig.9. From Fig.7 we can see that the temperature region from 500 ℃ to 800 ℃ is the gentle weight loss region of carbon
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substrate, in which period the carbon substrate would lose about 10% weight percent due to the oxidation of carbon. As shown in Fig.9, although the sample coated with pure SiC coating starts to lose mass at about 580℃, that is, delayed with respect to the initial oxidation temperature of the carbon substrate, the samples lose weight quickly to the fastest mass-loss zone about 800℃, showing weak anti-oxidation ability in the gentle weight loss region of carbon substrate. However, after the 10
addition of HfB2, the initial mass loss temperature of the coated samples is delayed to about 800℃, testifying the inhibited oxidation consumption of the carbon substrates in the gentle weight loss region of carbon. Analyzing the different mass loss rates of the coated samples modified with HfB2 phase in the oxidation activation zone of carbon (>800 ℃), the curves can be roughly divided into
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three stages: the oxidation activation region (A, 800 ℃-1000 ℃), the fastest oxidation region (B,
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1000 ℃-1280 ℃) and the oxidation inerting region (C, 1280 ℃-1500 ℃). It is easy to see the main
oxidation consumption of the carbon substrates occurred in periods A and B. In period A, though the oxidation consumptions of nearly all the coated samples is activated, it can be seen that with the
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increase of weight content of HfB2, the trend of the weight loss is to go down gradually. When the
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weight content of HfB2 is 80 wt.%, the activation oxidation of carbon substrate has been almost
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inhibited. In period B, the samples underwent the fastest oxidation consumptions. When the weight content of HfB2 is less than 20%, the weight loss rates of the coated samples are still increasing
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slowly. Nevertheless, when the weight content of HfB 2 is above than 40%, the weight loss rates of
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the coated samples present a trend of stabilization, reduction and even no weight loss. Hence, the coated sample modified with high content of HfB2 phase almost suppresses the oxidation
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consumption of carbon substrate in the oxidation activation region.
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In order to evidence the anti-oxidation protective mechanisms of the HfB2-20%wtSiC coating in
dynamic aerobic environment with wide temperature region, TG curves of coating components were conducted from 25℃ to 1500℃ in air condition. As can be seen in Fig.10, the region of fastest weight increase of HfB2 begins at approximately 750℃, while the curve of SiC begins at about 1200℃. Thus, the HfB2 phase presents higher oxidation activity than SiC phase below 1200℃. When the HfB2 phase was 11
added into the SiC powders, the oxidation activity of the composite powders was significantly enhanced with the increase of weight content of HfB2 in the temperature region from 800℃ to 1280℃. Moreover, the temperature region is in accordance with the main oxidation consumption of the coated carbon substrates shown in Fig.9. As we know, the oxidation products of HfB2 are B2O3 and HfO2, while the
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product of SiC is only SiO2. B2O3 and SiO2 are all well-known sealants, however, their application
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temperature regions are completely different. B2O3 is mainly used below 1000℃, while SiO2 is above
1000℃. Hence, for the composite coating components with increased content of HfB 2, due to the higher oxidation activity of HfB2, the sufficient generation of B2O3 sealant is responsible for the significantly
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enhanced oxidation inhibition ability of the coating in oxidation activation region (A, 800 ℃-1000 ℃) and
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fastest oxidation region (B, 1000 ℃-1280 ℃), shown in Fig.9. Furthermore, when the temperature is
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above 1280℃, the curves of the pure HfB2 and SiC powders present quite different trends. Due to the elevated oxidation activity of SiC, the generation of SiO2 with high melting temperature results in the
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continuous weight gain trend of the pure SiC powders, whereas due to the evaporation of B2O3, the curve
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of pure HfB2 powders presents a weight loss trend. When the SiC powders were added into the pure HfB2 powders to make composite powders, the weight loss trend of the pure HfB2 powders above 1280℃ was
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clearly inhibited. This is because that owing to the inerting role of SiO2 above 1280℃, the generated SiO2 not only can seal and prevent B2O3 from volatilizing, but can also merge with B2O3 to generate
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borosilicate glass layer, thus enhancing the anti-oxidation ability of the coating, resulting in the slow weight loss trend in Fig.8 and in the appearance of an oxidation inerting region in Fig.9. Therefore, because of the synergistic anti-oxidation protection of the HfB2 and SiC phase, the coating containing a large fraction of HfB2 exhibits great potential in a dynamic aerobic environment. As we know, compound glass layers would form on the surface of the coatings during oxidation, 12
whose sealing effect are crucial for the anti-oxidation protection of the coatings for carbon substrates. In order to study the surface phase compositions of the HfB 2-20%wtSiC coating after oxidation tests, XRD was conducted and we can see the result in Fig.11. From the pattern, no diffraction peak of the HfB2 phase can be observed, and some newly generated diffraction peaks of the phases HfO2, SiO2 and HfSiO4 can be
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clearly seen, caused by the oxidation of HfB2 and SiC. But the diffraction peaks of any B2O3 phase cannot
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be observed. To further explore the composition of the generated compound glass layer, the surface EDS
element mapping of the outer coating after oxidation was conducted, which is shown in Fig.12. It appears that Hf, Si, B and O elements can be detected in the compound glass layer generated on the surface of the
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HfB2-20%wtSiC coating, with the following weight fractions: 43.73wt.%, 23.89wt.%, 12.06wt.% and
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20.32wt.%, respectively. B is present in lower amount, thus appearing darker on the EDS map, and the
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distribution of the B element is relatively uniform. Hence, the absence of a pure B2O3 phase in Fig.11 originates from the dissolution of B2O3 into the generated silicate glass to form the borosilicate glass
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layer.
In order to further investigate the anti-oxidation protective behavior and mechanisms of the
HfB2-20%wtSiC coating, the surface backscattered micrographs of the compound coatings with different weight amount of HfB2 after oxidation were done and a series of results are illustrated in Fig.13. Fig.13 (a)-(d) show the micrographs of the coatings after TG oxidation test. Numerous white 13
particles are dispersed on the surface of the coatings, whose amount increase with the weight content of HfB2. By spot EDS analysis, the white particles can be recognized as Hf-Oxides, while the black parts are SiO2. Since the samples tested in TG oxidation test undergo a short oxidation time, the generated glass layer is not enough to completely cover the surface of the coating, meaning the
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preliminary reactive of the coating. The white Hf-Oxides particles are mainly dispersed on the grain
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gaps and embedded in SiO2. As we know, along with the continuous oxidation of SiC, the SiO2 glass layer should gradually covers the surface of the coatings. To observe the microstructures of the
compound coatings under sufficient oxidation, surface backscattered micrographs of the compound
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coatings after isothermal oxidation for another 50h at 1773 K are shown in Fig.13 (f)-(i). It can be
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seen that with sufficient oxidation of the coatings, the SiO2 glass has completely incorporated the
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Hf-Oxides. Generally speaking, cracks, especially the long ones, are the main channels through which oxygen permeates into the carbon substrate, resulting in its oxidation loss. Due to the different
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coefficients of thermal expansion among the substrates, coatings and the generated glass layers,
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microcracks are inevitable for the outermost glass layer generated on the surface of the coatings. However, few long cracks can be observed in Fig.13 (f)-(i), indicating good stability of the
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compound Hf-B-Si-O glass layer.
High magnification surface backscattered micrographs of the compound coatings after isothermal oxidation are shown in Fig.14. From Fig.14 (a)-(d), it can be seen that the Hf-B-Si-O 14
glass layer has covered the grain gaps and presents a dense protective layer. As shown in Fig.14 (e), the Hf-oxides are perfectly embedded in the compound glass layers, and no gap can be observed between the glass layers and the heterogeneous Hf-oxides, indicating their good compatibility. Moreover, the melting temperature of Hf-Oxides (about 2850℃) are far beyond that of SiO2 (about
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1650℃), which makes them embed themselves in SiO2 glass layer as refractory heterogeneous
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phases. With the increase of weight content of HfB2, the amount of the dispersed and embedded Hfoxides gradually increases. As a matter of fact, although long cracks are absent in Fig.13 (f)-(i),
winding microcracks are observed in the compound glass layers after enlarging the micrographs. It
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can be seen that crack termination, bifurcation, and turning phenomena occur in the Hf-B-Si-O glass
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layer around the embedded Hf-oxides. In addition, it can be seen that the microcracks mainly
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concentrate in the areas of the compound glass layers without the embedded Hf-oxides. With the increase of weight content of HfB2, the amount of the existed microcracks decreases. Hence, the
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embedded Hf-oxides actually play a kind role of preventing the propagations of the cracks,
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demonstrating well the mechanism of crack limitation. This interesting phenomenon actually testifies that the heterogeneous refractory Hf-Oxides phases play a role of reinforcement phase in the
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Hf-B-Si-O compound glass layer, and are able to restrict the generation and spread of the cracks that
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exist as diffusion channel of oxygen.
As discussed above, the essence of the generated compound Hf-B-Si-O glass layer on the 15
surface of the HfB2-20%wtSiC coating during oxidation is a kind of oxidation inerting layer. The newly formed inerting layer is capable of providing enhanced anti-oxidation protective ability for the carbon substrate. To investigate the sustainable oxidation inhibition capability, cyclic TG analysis curves of HfB2-20%wtSiC coating from 25℃ to 1500℃ in air condition are shown in Fig.15.
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It can be seen that all the curves exhibit the same trend before 520℃, however, from 520℃ to 1500℃,
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the curves of the samples after cyclic TG oxidation tests do not present the similar trend as curve of 1st run, but show a linear trend in the whole TG oxidation tests from 25℃ to 1500℃. As discussed above, the weight gain of the coated sample is owing to the oxidation of the coating materials.
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Therefore, since the protective glass layer has not been generated on the surface of the HfB2-
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20%wtSiC coating for the first TG run, the TG curve of the coated sample presents a weight gain
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trend above 600℃ due to the oxidation of coating materials and its effective anti-oxidation protection. For the subsequent TG runs, the oxidation of coating materials during the first TG
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oxidation test generates an inerting glass layer, whose shielding effect actually prevents the
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oxidation erosion of oxygen to coating materials and the carbon substrate. Thus, the subsequent TG curves present a stable linear trend in the whole TG oxidation tests from 25℃ to 1500℃, which
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further testifies the inerting effect of the Hf-B-Si-O glass layer, originating from the diffusion inhibition of oxygen. As we know, as the oxidation time increases, the risk of coating failure also
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gradually increases, which could lead to a sudden drop in the TG curve. Nevertheless, with the increase of the TG cycle number, the slope of the curves does not increase but decreases, indicating that the inerting effect of the Hf-B-Si-O glass layer was actually strengthened. Hence, the enhanced anti-oxidation protective ability of the coated sample in cyclic TG oxidation tests further indicates the greatly promising oxidation inhibition potential of the HfB 2-20%wtSiC coating in dynamic 16
aerobic environment. 4. Conclusions HfB2-SiC composite coatings with a high HfB2 mass fraction and a close-knit structure were prepared by a liquid phase sintering method and their behavior under oxidation has been characterized.
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The main oxidation consumption of the SiC coated carbon substrates occurred in the oxidation activation
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region (800 ℃-1000 ℃) and in the fastest oxidation region (1000 ℃-1280 ℃). With the increase of weight content of HfB2, the oxidation activity of the composite powders were significantly enhanced, gradually inhibiting the activation oxidation consumption of carbon substrates. The sufficient generation of B2O3
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sealant is responsible for the significantly enhanced oxidation inhibition ability of the coating in oxidation
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activation region (800 ℃-1000 ℃) and fastest oxidation region (1000 ℃-1280 ℃). In the oxidation inerting
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region (1280 ℃-1500 ℃), the oxidation activity of SiC elevates. Owing to the inerting role of the generated SiO2, Hf-B-Si-O glass layer was generated and prevented B2O3 from volatilizing. The
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numerous generated heterogeneous refractory Hf-Oxides embedded in Hf-B-Si-O glass layer as
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reinforcement phase, demonstrating mechanism of crack limitation. With the increase of the TG recycle oxidation times, the inerting effect of the Hf-B-Si-O glass layer was actually strengthened, indicating
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promising oxidation inhibition potential of the HfB 2-20%wtSiC coating in dynamic aerobic environment. Acknowledgements
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This work has been supported by the National Natural Science Foundation of China (Grant No.
51602342, 51874305), Fundamental Research Funds for the Central Universities (No. 2017QNA03, 2018GF14), bilateral project of NSFC-STINT (51611130064), the QingLan Project of Jiangsu Province, Natural Science Foundation of Jiangsu Province, China (No. BK20160261). References 17
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722 (2017) 69-76.
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Fig.2 XRD pattern of the chemosynthetic HfB2 powders
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Fig.1The process of the preparation of the outer HfB2-20%wtSiC coating
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Fig.3 (a) TEM photograph and (b) the high-resolution TEM photograph, (c) the selected-area
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electron diffraction pattern of the HfB2 powders
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Fig.4 XRD pattern of the outer HfB2-20%wtSiC coating
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Fig.5 (a) Surface backscatter SEM micrograph of the outer HfB 2-20%wtSiC coating; (b) the
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magnification of (a); (c) Spot EDS analyses
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Fig.6 (a) Cross section backscatter SEM micrographs of the graphite substrate coated with the HfB 2-
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20%wtSiC/SiC coating (b) magnification of (a)
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Fig.7 TG curves of pure graphite substrate in dynamic aerobic condition from 25 ℃ to 1500 ℃
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Fig.8 (a) TGA curves of the coated substrate in air from room temperature to 15
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00℃; (b) TGA curves of the coatings with different amounts of HfB2
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Fig.9 Mass loss rate curves of the coated samples during TG tests
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Fig.10 TG curves of coating components from 25℃ to 1500℃ in air condition
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Fig.11 XRD pattern of the outer HfB2-20%wtSiC coating after oxidation
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Fig.12 Surface EDS element mapping of the outer HfB 2-20%wtSiC coating after oxidation
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Fig.13 Low magnification surface backscattered micrographs of the compound coatings with different weight content of HfB2 after TG test ((a)- (d)) and oxidation at 1500℃ for 50h in air
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((f)- (i)); (e) spot EDS analyses
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Fig.14 High magnification surface backscattered micrographs of the compound coatings with different weight content of HfB2 after oxidation at 1500℃ for 50h in air: (a) 20%wt.; (b) 40%wt.;
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(c) 60%wt.; (d) 80%wt.; (e) magnification of A in (d); (f) magnification of B in (d);
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Fig.15 TG circulation curves of HfB2-20%wtSiC coating from 25℃ to 1500℃ in air condition
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S0955-2219(19)30048-2 https://doi.org/10.1016/j.jeurceramsoc.2019.01.033 JECS 12300
To appear in:
Journal of the European Ceramic Society
Received date: Revised date: Accepted date:
25 July 2018 19 January 2019 21 January 2019
Please cite this article as: Ren X, Shang T, Wang W, Feng P, Guo L, Zhang P, Li Z, Dynamic oxidation protective behaviors and mechanisms of HfB2 -20wt%SiC composite coating for carbon materials, Journal of the European Ceramic Society (2019), https://doi.org/10.1016/j.jeurceramsoc.2019.01.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dynamic oxidation protective behaviors and mechanisms of HfB2-20wt%SiC composite coating
for carbon materials
Xuanru Ren* [email protected], Tianqi Shang, Wenhao Wang, Peizhong Feng* [email protected],
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LiTong Guo* [email protected], Ping Zhang, Ziyu Li
School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou
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221116, China
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*Corresponding author. Tel.: +86 15505183876;
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Abstract
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HfB2-20%wtSiC composite coating was prepared by liquid phase sintering method. After modification of HfB2 phase, the initial oxidation consumptions of the SiC coated samples were delayed
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from 500 ℃ to 800 ℃. Due to the higher oxidation activity of HfB2, the sufficient generated B2O3 is
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capable of inhibiting oxidation consumption of carbon substrate in oxidation activation region (800 ℃1000 ℃) and fastest oxidation region (1000 ℃-1280 ℃). The enhanced oxidation activity of SiC above
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1000℃ leads to the increased generation of SiO2, inhibiting the evaporation of B2O3 through the formation of Hf-B-Si-O glass layer, improving its stability and oxidation resistance above 1000℃. The heterogeneous refractory Hf-Oxides embedded in Hf-B-Si-O glass layer play role of reinforcement phases, restricting generation and spread of cracks. The inerting effect of Hf-B-Si-O glass layer strengthened with the increase of thermogravimetric analysis (TG) recycle oxidation times, indicating 1
promising oxidation inhibition potential of the coating in dynamic aerobic environment.
Keywords: HfB2; Coating; Liquid phase sintering; Dynamic oxidation protection; Hf-B-Si-O glass layer;
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1. Introduction
HfB2, an excellent ultrahigh temperature ceramics (UHTCs) diboride, is well known for the
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outstanding properties such as high melting temperatures (3250°C), chemical stability, high hardness and thermal conductivity [1-4]. Hence, resulting from these wonderful ultrahigh temperature performances, it
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has attracted more attention for potential applications such as thermal protection systems, rocket engines,
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refractory crucibles, space vehicles, and so on [5-8]. To get better oxidation resistances, the SiC phase
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with self-sealing property is generally used as additive to prepare HfB2-SiC composites [9-13]. After the
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addition of SiC phase, the formation of oxidation protective borosilicate glassy layer is responsible for the
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sharply enhanced oxidation resistance of HfB2 composites below 1800°C. Carbon structural materials (graphite, carbon/carbon composites) possess many performances that
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are suitable in the aviation and aerospace field, including ultra-high melting temperature, low thermal
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expansion coefficient, high strength, high modulus. One of their most important performances is that mechanical properties increase along with the increase of temperature [14-16]. Nevertheless, though carbon possesses ultra-high melting temperature of about 3500℃, its oxidizability above 400℃ is the fatal
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flaw of the carbon structural materials, greatly restricting their broad applications. To solve this problem, oxidation resistant coating technique is considered as the most effective method [17-20]. Hence, recently, in view of the promising anti-oxidation resistance of HfB2-SiC materials, they were developed as oxidation resistant coatings to protect carbon structural materials from oxidation. 2
Wang et al. [21] have prepared HfB2-SiC coating by in-situ reaction method to protect carbon/carbon composites from oxyacetylene torch ablation at heat flux of 2400 kW/m2 for 60 s. The mass and linear ablation rate of the coated C/C substrate were only 0.147 mg/s and 0.267 μm/s. To protect carbon fiberbased composites, Richet et al. [22] synthesized HfB2-SiC coatings that exhibit good oxidation resistance
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up to 1700°C. Zhang et al. [23] prepared HfB2-SiC coating by in-situ synthesis method at 2373 K and
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found that the coating could protect the coated C/C substrate from oxidation at 1773K for 753 h with
0.487% mass loss. In our previous work, we also prepared HfB2-SiC/SiC coating with excellent antioxidation ability [24], and after oxidation at 1773 K in air for 265h, the mass loss of the coated C/C
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substrate is only 0.41×10-2 g/cm2. Hence, the application of HfB2-SiC coatings on carbon structural
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materials exhibits great potential.
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Furthermore, with the increase in demand for carbon materials, the applied coatings are facing more and more challenges [25-27]. The applications of carbon materials in multiple variable oxidation
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environments require that the coatings need to have better oxidation protection ability in a wider
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temperature range. Although the self-sealing property makes the Si-based ceramics effective antioxidation components in the temperature region from 1200℃ to 1600℃, the Si-based ceramics do not
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have good protection ability below 1000℃ [28-30]. While due to the existence of B atom in HfB2, the generated antioxidant B2O3 will make the HfB2 an ideal supplement for the anti-oxidation shortage of Si-
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based ceramics below 1000℃. Moreover, the borosilicate glass layer formed by the reaction between B2O3 and silicate glass layer will further enhance the oxidation protective ability of the HfB2-SiC coating. Hence, the suitable component design is crucial for the anti-oxidation protective ability of the HfB2-SiC coating for carbon materials in a wider temperature range. Generally speaking, the suitable content of SiC phase for the HfB2-SiC materials is about 20 3
wt./vol.% to obtain the best oxidation resistance [31-32]. However, until now, in the reported works, the HfB2 phase was generally used as additive for Si-based ceramic coating applied on the surface of carbon materials to enhance the anti-oxidation ability of Si-based ceramic coatings. Nevertheless, considering the excellent anti-oxidation potential of HfB2 phase in dynamic aerobic environment, the inadequate amount
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of HfB2 phase in the coating cannot ensure the sufficient oxidation protection ability to protect carbon
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materials from oxidation in dynamic environments. Thus, the preparation of HfB2-SiC composite coating with a high HfB2 phase content exhibits much potential to broaden the application of carbon structural materials in wider temperature range.
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Up to now, many coating preparation techniques can be utilized to synthesize HfB2-SiC coating,
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such as plasma spraying method, pack cementation method, in-situ reaction method, and so on. However,
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although the multiphase UHTCs borides-Si-based ceramic coatings prepared by the pack cementation method and in-situ reaction method exhibit excellent oxidation resistances, it is difficult to insert high
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amount of UHTCs borides into the coatings due to the limitation of coating technology. While for the
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plasma spraying method, although it is suitable to control the phase amount in coatings, the higher porosity of the synthesized coatings weakens the anti-oxidation effect of the coating. Hence, the obvious
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shortcomings of the above methods are not suitable to prepare HfB2-SiC coatings with high HfB2 phase content.
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Recently, in our previous work [33], we have designed a kind of liquid phase sintering method to
modify the SiC coating by HfB2 phase. Although our previous work was aimed to develop a Si-based ceramic coating modification technique, the coating preparation technique presents great potential to deal with the challenges to prepare HfB2-SiC coating with high HfB2 phase content. Hence, in this paper, in view of the great potential of the liquid phase sintering method, to enhance the anti-oxidation protective 4
ability of the HfB2-SiC coating in wider temperature range, a HfB2-20%wtSiC composite coating was prepared by liquid phase sintering method using HfB2, graphite powders and silica sol as raw materials. The microstructure, dynamic anti-oxidation resistance and behaviors the HfB2-20%wtSiC coating were investigated. To disclose the oxidation protective mechanisms of the HfB2-20%wtSiC coating in wider
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temperature range, the oxidation resistance of HfB2 powders, SiC powders and HfB2-20%wtSiC composite powders were also analyzed.
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2. Experimental procedures
Fig. 1 illustrates the preparation of the outer HfB2-20%wtSiC coating. Applied on the surface of the
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graphite substrates with size of 3mm×3mm×3mm, the inner SiC transition layer was prepared by the pack
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cementation technique, the process of which was shown elsewhere [24]. The HfB2 powders were
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synthesized by carbothermal reduction reaction using C, HfO2 and B2O3 as raw materials, as shown
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elsewhere [32]. At first, the synthesized HfB2 powders (80 wt.%), graphite powders (20 wt.%) (Carbon
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Plant, Xi’an, China) and silica sol (SiO2· nH2O) (the value of Vsilica sol : M other raw materials is 0.5-1.5 ml/g) (City Fire Crystal Glass Co., Ltd, Dezhou, China) were used as raw coating materials, which were further
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ball milled for 2h. Then, the mixture was used as slurry to be brushed onto the surface of the SiC layer to
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produce the pre-fabricated HfB2-20%wtSiC coating. Next, the pre-fabricated HfB2-20%wtSiC coating was dried at 363 K for 1 h. Afterwards, the dried samples were repeatedly painted for another two runs. Finally, the coated specimens were placed in the graphite crucible with pure argon protection and heated
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to 2373K for 2h, with a heating rate of about 5-10 K/min. During the heat treatment, the silica sol and the C powders reacted on the basis of Equation (1) to form SiC phase. In addition, since the melting temperature (about 2000K) of the silica sol is far below the synthetic temperature (2373K) of the outer coating, the silica sol would be presented as liquid phase, creating liquid phase sintering conditions for
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the preparation of the coating. SiO2(s)+C(s)→SiC(s)+CO(g)
（1）
An X-ray diffractometer (XRD, BRUKER AXS, Germany) was used to analyze the produced HfB2 powders and the composition of the outer coatings. The morphology of the HfB2-20%wtSiC
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coating was measured by a scanning electron microscope (SEM, JSM6460, JEOL, Japan). The
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surface element mapping of the outer HfB2-20%wtSiC coating after oxidation was analyzed by
energy-dispersive spectroscopy (EDS, EDAX, octane plus, USA). Transmission electron microscopy (TEM, JEM-3010, JEOL, Japan) was utilized to analyze crystal structure of the synthetic HfB 2
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powders. The dynamic anti-oxidation ability of the coatings was investigated by a
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thermogravimetric analyzer (TGA, Netzsch, Germany). In addition, TG tests were carried out in an
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aerobic environment that consists of argon and oxygen. The volume of oxygen to argon is 22:78. To further analyze the anti-oxidation mechanisms of the coatings, the coated samples were oxidized at
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1773K for another 50h.
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3. Results and discussion
As we can see, Fig.2 exhibits the XRD pattern of the chemosynthetic HfB 2 powders. The XRD
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pattern matches the data of hexagonal HfB2 (JCPDS Card reference code #03-065-8678). The high
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intensity of the diffraction peaks indicates a good crystalline state of the hexagonal HfB2 powders.
The TEM photograph is shown in Fig.3 (a). The particle size of the synthesized HfB2 powders is about 200-650nm as shown in Fig.3 (a), only a small number of the particles being larger than 650nm. To further investigate the crystalline feature of the HfB 2 powders, a high-resolution TEM 6
photograph and a selected-area electron diffraction pattern of the HfB 2 powders are exhibited in Fig.3 (b) and (c). It can be seen that both the interplanar spacings of the lattice stripes and the diffraction rings are consistent with the XRD pattern of HfB 2 powders, reflecting the polycrystalline
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hexagonal nature of HfB2 powders.
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The XRD pattern of the outer HfB2-20%wtSiC coating is shown in Fig.4. From the pattern, the diffraction peak of HfB2 and SiC can be found at the same time, proving that the outer HfB220%wtSiC coating has been successfully prepared. In addition, no obvious SiO2 peak can be
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detected, indicating the successful transformation from silica to SiC.
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For the sake of analyzing the surface microstructure of the outer HfB 2-20%wtSiC coating, a
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surface backscatter SEM micrograph is exhibited in Fig. 5(a). A close-knit structure can be clearly observed. Moreover, many white particles covered on the surface of the coating can be observed. In
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Fig.5 (b), it can be seen that the white and black grains are bound to each other with no obvious
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cavity and microcrack. To analyze what the white and black grains really are, spot EDS analyses are done and the results are shown in Fig.5 (c). On the basis of these results, the white grains are
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confirmed as HfB2, while the other ones are SiC.
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As illustrated in Fig.6 (a), cross section SEM backscatter micrograph of the graphite substrate
coated with the HfB2-20%wtSiC/SiC coating shows that the thickness of the inner SiC coating and outer HfB2-20%wtSiC coating are approximately 120μm and 200μm, respectively. The HfB2 phases mainly concentrate on the thin top surface region of the outer coating, forming a kind of crust with roughness about 100 µm high; while the inner part of the outer coating is a kind of porous SiC layer 7
with few dispersed HfB2 particles. Furthermore, the white HfB2 grains can still be observed in the dense internal SiC coating. This is because during the preparation of the pre-fabricated outer coating, some raw outer coating materials slurry would infiltrate into the inner SiC coating, which not only brings the HfB2 powders into the inner SiC coating, but also effectively connects the internal and
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external coatings during the heat-treatment process. Thus, the double layer coatings exhibit good
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compatibility, no obvious large cavity or microcrack could be seen at its interface. In order to clearly
observe the quality of the coating, the cross section of the coating is magnified, as illustrated in Fig.6 (b). Due to the accumulation of large SiC grains in the outer coating, some small voids can be
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observed in the outer coating. In addition, no penetrating crack could be observed. As we know, the
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cracks act like diffusion channels for oxygen to diffuse into the carbon substrates, whose existence
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provide advantages for the entry of oxygen. A characteristic feature of oxygen corrosion of carbon substrate is the formation of oxidation holes in substrates. It has been reported that cracks, especially
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those penetrating in coatings, are always associated with oxidation holes generated in the carbon
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substrate during oxidation [34-36]. Fu et al. found that the pre-existing microcracks in the coating would provide diffusion channels for oxygen to corrode C substrate [37], which would form large
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voids, thus resulting in the failure of carbon substrate. The absence of cracks suggests that the
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coating described here may offer a better protection against oxidation than in these previous cases.
For the sake of measuring the oxidation resistance of the pure graphite substrate in dynamic condition from 25℃ to 1500 ℃, a TG test was carried out. As shown in Fig.7, there are mainly two 8
weight-loss zones in the figure, the gentle weight-loss zone (Fig.7 (a)) and the fastest weight-loss zone (Fig.7 (b)), respectively. The former one is a slower oxidation process from 500℃ to 800℃, and the latter a drastic one from 800℃ to 1080℃. However, when it comes to about 1100℃, the graphite
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Thus, the fastest weight-loss zone is actually an oxidation activation zone of carbon.
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will be completely oxidized, which indicates the poor oxidation resistance of the graphite substrate.
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As shown in Fig.8 (a), TGA test was used to assess the anti-oxidation protective ability of the HfB2-20%wtSiC/SiC coating. Due to the oxidation consumption of carbon substrate, the sample
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coated with SiC coating begins to lose weight from about 580 ℃, and the final weight loss of the sample is 17.5% after the TGA test. On the other hand, for the HfB 2-20%wtSiC coating, although a
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slight weight loss region (900 ℃-1100 ℃) can be observed in the TG curve of the coated sample, the weight of the coated sample has been increased about 0.23% after the whole dynamic TG oxidation test. The dynamic anti-oxidation performance of the HfB2-20%wtSiC coating is far beyond the 10.29% weight loss reported by Wang et al. [38] and 7.5% weight loss in our previous work [33]. To disclose the role of HfB2 playing in the coating during the dynamic TG oxidation test, the TG curves 9
of the HfB2-SiC coatings with different weight fractions of HfB 2 were presented in Fig.8 (b). As shown in Fig.8 (a), the pure SiC coating exhibits the worst anti-oxidation protective ability in the medium temperature region from 600 ℃ to 1400 ℃, which results in a major weight loss of the carbon substrate in the dynamic oxidation environments. As shown in Fig.8 (b), with the increase of
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weight content of HfB2, the weight losses of the coated samples were significantly restricted in this region. Hence, the HfB2 phase exhibits an excellent anti-oxidation modification ability in the
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medium temperature region from 600 ℃ to 1400 ℃, showing obvious complementary advantages
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combined with Si-based ceramic coatings.
In order to further analyze the anti-oxidation protective processes of the coated samples, the
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mass loss rate curves of the coated samples during TG tests were shown in Fig.9. From Fig.7 we can see that the temperature region from 500 ℃ to 800 ℃ is the gentle weight loss region of carbon
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substrate, in which period the carbon substrate would lose about 10% weight percent due to the oxidation of carbon. As shown in Fig.9, although the sample coated with pure SiC coating starts to lose mass at about 580℃, that is, delayed with respect to the initial oxidation temperature of the carbon substrate, the samples lose weight quickly to the fastest mass-loss zone about 800℃, showing weak anti-oxidation ability in the gentle weight loss region of carbon substrate. However, after the 10
addition of HfB2, the initial mass loss temperature of the coated samples is delayed to about 800℃, testifying the inhibited oxidation consumption of the carbon substrates in the gentle weight loss region of carbon. Analyzing the different mass loss rates of the coated samples modified with HfB2 phase in the oxidation activation zone of carbon (>800 ℃), the curves can be roughly divided into
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three stages: the oxidation activation region (A, 800 ℃-1000 ℃), the fastest oxidation region (B,
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1000 ℃-1280 ℃) and the oxidation inerting region (C, 1280 ℃-1500 ℃). It is easy to see the main
oxidation consumption of the carbon substrates occurred in periods A and B. In period A, though the oxidation consumptions of nearly all the coated samples is activated, it can be seen that with the
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increase of weight content of HfB2, the trend of the weight loss is to go down gradually. When the
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weight content of HfB2 is 80 wt.%, the activation oxidation of carbon substrate has been almost
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inhibited. In period B, the samples underwent the fastest oxidation consumptions. When the weight content of HfB2 is less than 20%, the weight loss rates of the coated samples are still increasing
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slowly. Nevertheless, when the weight content of HfB 2 is above than 40%, the weight loss rates of
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the coated samples present a trend of stabilization, reduction and even no weight loss. Hence, the coated sample modified with high content of HfB2 phase almost suppresses the oxidation
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consumption of carbon substrate in the oxidation activation region.
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In order to evidence the anti-oxidation protective mechanisms of the HfB2-20%wtSiC coating in
dynamic aerobic environment with wide temperature region, TG curves of coating components were conducted from 25℃ to 1500℃ in air condition. As can be seen in Fig.10, the region of fastest weight increase of HfB2 begins at approximately 750℃, while the curve of SiC begins at about 1200℃. Thus, the HfB2 phase presents higher oxidation activity than SiC phase below 1200℃. When the HfB2 phase was 11
added into the SiC powders, the oxidation activity of the composite powders was significantly enhanced with the increase of weight content of HfB2 in the temperature region from 800℃ to 1280℃. Moreover, the temperature region is in accordance with the main oxidation consumption of the coated carbon substrates shown in Fig.9. As we know, the oxidation products of HfB2 are B2O3 and HfO2, while the
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product of SiC is only SiO2. B2O3 and SiO2 are all well-known sealants, however, their application
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temperature regions are completely different. B2O3 is mainly used below 1000℃, while SiO2 is above
1000℃. Hence, for the composite coating components with increased content of HfB 2, due to the higher oxidation activity of HfB2, the sufficient generation of B2O3 sealant is responsible for the significantly
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enhanced oxidation inhibition ability of the coating in oxidation activation region (A, 800 ℃-1000 ℃) and
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fastest oxidation region (B, 1000 ℃-1280 ℃), shown in Fig.9. Furthermore, when the temperature is
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above 1280℃, the curves of the pure HfB2 and SiC powders present quite different trends. Due to the elevated oxidation activity of SiC, the generation of SiO2 with high melting temperature results in the
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continuous weight gain trend of the pure SiC powders, whereas due to the evaporation of B2O3, the curve
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of pure HfB2 powders presents a weight loss trend. When the SiC powders were added into the pure HfB2 powders to make composite powders, the weight loss trend of the pure HfB2 powders above 1280℃ was
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clearly inhibited. This is because that owing to the inerting role of SiO2 above 1280℃, the generated SiO2 not only can seal and prevent B2O3 from volatilizing, but can also merge with B2O3 to generate
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borosilicate glass layer, thus enhancing the anti-oxidation ability of the coating, resulting in the slow weight loss trend in Fig.8 and in the appearance of an oxidation inerting region in Fig.9. Therefore, because of the synergistic anti-oxidation protection of the HfB2 and SiC phase, the coating containing a large fraction of HfB2 exhibits great potential in a dynamic aerobic environment. As we know, compound glass layers would form on the surface of the coatings during oxidation, 12
whose sealing effect are crucial for the anti-oxidation protection of the coatings for carbon substrates. In order to study the surface phase compositions of the HfB 2-20%wtSiC coating after oxidation tests, XRD was conducted and we can see the result in Fig.11. From the pattern, no diffraction peak of the HfB2 phase can be observed, and some newly generated diffraction peaks of the phases HfO2, SiO2 and HfSiO4 can be
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clearly seen, caused by the oxidation of HfB2 and SiC. But the diffraction peaks of any B2O3 phase cannot
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be observed. To further explore the composition of the generated compound glass layer, the surface EDS
element mapping of the outer coating after oxidation was conducted, which is shown in Fig.12. It appears that Hf, Si, B and O elements can be detected in the compound glass layer generated on the surface of the
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HfB2-20%wtSiC coating, with the following weight fractions: 43.73wt.%, 23.89wt.%, 12.06wt.% and
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20.32wt.%, respectively. B is present in lower amount, thus appearing darker on the EDS map, and the
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distribution of the B element is relatively uniform. Hence, the absence of a pure B2O3 phase in Fig.11 originates from the dissolution of B2O3 into the generated silicate glass to form the borosilicate glass
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layer.
In order to further investigate the anti-oxidation protective behavior and mechanisms of the
HfB2-20%wtSiC coating, the surface backscattered micrographs of the compound coatings with different weight amount of HfB2 after oxidation were done and a series of results are illustrated in Fig.13. Fig.13 (a)-(d) show the micrographs of the coatings after TG oxidation test. Numerous white 13
particles are dispersed on the surface of the coatings, whose amount increase with the weight content of HfB2. By spot EDS analysis, the white particles can be recognized as Hf-Oxides, while the black parts are SiO2. Since the samples tested in TG oxidation test undergo a short oxidation time, the generated glass layer is not enough to completely cover the surface of the coating, meaning the
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preliminary reactive of the coating. The white Hf-Oxides particles are mainly dispersed on the grain
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gaps and embedded in SiO2. As we know, along with the continuous oxidation of SiC, the SiO2 glass layer should gradually covers the surface of the coatings. To observe the microstructures of the
compound coatings under sufficient oxidation, surface backscattered micrographs of the compound
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coatings after isothermal oxidation for another 50h at 1773 K are shown in Fig.13 (f)-(i). It can be
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seen that with sufficient oxidation of the coatings, the SiO2 glass has completely incorporated the
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Hf-Oxides. Generally speaking, cracks, especially the long ones, are the main channels through which oxygen permeates into the carbon substrate, resulting in its oxidation loss. Due to the different
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coefficients of thermal expansion among the substrates, coatings and the generated glass layers,
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microcracks are inevitable for the outermost glass layer generated on the surface of the coatings. However, few long cracks can be observed in Fig.13 (f)-(i), indicating good stability of the
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compound Hf-B-Si-O glass layer.
High magnification surface backscattered micrographs of the compound coatings after isothermal oxidation are shown in Fig.14. From Fig.14 (a)-(d), it can be seen that the Hf-B-Si-O 14
glass layer has covered the grain gaps and presents a dense protective layer. As shown in Fig.14 (e), the Hf-oxides are perfectly embedded in the compound glass layers, and no gap can be observed between the glass layers and the heterogeneous Hf-oxides, indicating their good compatibility. Moreover, the melting temperature of Hf-Oxides (about 2850℃) are far beyond that of SiO2 (about
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1650℃), which makes them embed themselves in SiO2 glass layer as refractory heterogeneous
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phases. With the increase of weight content of HfB2, the amount of the dispersed and embedded Hfoxides gradually increases. As a matter of fact, although long cracks are absent in Fig.13 (f)-(i),
winding microcracks are observed in the compound glass layers after enlarging the micrographs. It
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can be seen that crack termination, bifurcation, and turning phenomena occur in the Hf-B-Si-O glass
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layer around the embedded Hf-oxides. In addition, it can be seen that the microcracks mainly
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concentrate in the areas of the compound glass layers without the embedded Hf-oxides. With the increase of weight content of HfB2, the amount of the existed microcracks decreases. Hence, the
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embedded Hf-oxides actually play a kind role of preventing the propagations of the cracks,
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demonstrating well the mechanism of crack limitation. This interesting phenomenon actually testifies that the heterogeneous refractory Hf-Oxides phases play a role of reinforcement phase in the
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Hf-B-Si-O compound glass layer, and are able to restrict the generation and spread of the cracks that
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exist as diffusion channel of oxygen.
As discussed above, the essence of the generated compound Hf-B-Si-O glass layer on the 15
surface of the HfB2-20%wtSiC coating during oxidation is a kind of oxidation inerting layer. The newly formed inerting layer is capable of providing enhanced anti-oxidation protective ability for the carbon substrate. To investigate the sustainable oxidation inhibition capability, cyclic TG analysis curves of HfB2-20%wtSiC coating from 25℃ to 1500℃ in air condition are shown in Fig.15.
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It can be seen that all the curves exhibit the same trend before 520℃, however, from 520℃ to 1500℃,
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the curves of the samples after cyclic TG oxidation tests do not present the similar trend as curve of 1st run, but show a linear trend in the whole TG oxidation tests from 25℃ to 1500℃. As discussed above, the weight gain of the coated sample is owing to the oxidation of the coating materials.
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Therefore, since the protective glass layer has not been generated on the surface of the HfB2-
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20%wtSiC coating for the first TG run, the TG curve of the coated sample presents a weight gain
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trend above 600℃ due to the oxidation of coating materials and its effective anti-oxidation protection. For the subsequent TG runs, the oxidation of coating materials during the first TG
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oxidation test generates an inerting glass layer, whose shielding effect actually prevents the
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oxidation erosion of oxygen to coating materials and the carbon substrate. Thus, the subsequent TG curves present a stable linear trend in the whole TG oxidation tests from 25℃ to 1500℃, which
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further testifies the inerting effect of the Hf-B-Si-O glass layer, originating from the diffusion inhibition of oxygen. As we know, as the oxidation time increases, the risk of coating failure also
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gradually increases, which could lead to a sudden drop in the TG curve. Nevertheless, with the increase of the TG cycle number, the slope of the curves does not increase but decreases, indicating that the inerting effect of the Hf-B-Si-O glass layer was actually strengthened. Hence, the enhanced anti-oxidation protective ability of the coated sample in cyclic TG oxidation tests further indicates the greatly promising oxidation inhibition potential of the HfB 2-20%wtSiC coating in dynamic 16
aerobic environment. 4. Conclusions HfB2-SiC composite coatings with a high HfB2 mass fraction and a close-knit structure were prepared by a liquid phase sintering method and their behavior under oxidation has been characterized.
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The main oxidation consumption of the SiC coated carbon substrates occurred in the oxidation activation
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region (800 ℃-1000 ℃) and in the fastest oxidation region (1000 ℃-1280 ℃). With the increase of weight content of HfB2, the oxidation activity of the composite powders were significantly enhanced, gradually inhibiting the activation oxidation consumption of carbon substrates. The sufficient generation of B2O3
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sealant is responsible for the significantly enhanced oxidation inhibition ability of the coating in oxidation
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activation region (800 ℃-1000 ℃) and fastest oxidation region (1000 ℃-1280 ℃). In the oxidation inerting
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region (1280 ℃-1500 ℃), the oxidation activity of SiC elevates. Owing to the inerting role of the generated SiO2, Hf-B-Si-O glass layer was generated and prevented B2O3 from volatilizing. The
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numerous generated heterogeneous refractory Hf-Oxides embedded in Hf-B-Si-O glass layer as
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reinforcement phase, demonstrating mechanism of crack limitation. With the increase of the TG recycle oxidation times, the inerting effect of the Hf-B-Si-O glass layer was actually strengthened, indicating
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promising oxidation inhibition potential of the HfB 2-20%wtSiC coating in dynamic aerobic environment. Acknowledgements
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This work has been supported by the National Natural Science Foundation of China (Grant No.
51602342, 51874305), Fundamental Research Funds for the Central Universities (No. 2017QNA03, 2018GF14), bilateral project of NSFC-STINT (51611130064), the QingLan Project of Jiangsu Province, Natural Science Foundation of Jiangsu Province, China (No. BK20160261). References 17
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722 (2017) 69-76.
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Fig.2 XRD pattern of the chemosynthetic HfB2 powders
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Fig.1The process of the preparation of the outer HfB2-20%wtSiC coating
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Fig.3 (a) TEM photograph and (b) the high-resolution TEM photograph, (c) the selected-area
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electron diffraction pattern of the HfB2 powders
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Fig.4 XRD pattern of the outer HfB2-20%wtSiC coating
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Fig.5 (a) Surface backscatter SEM micrograph of the outer HfB 2-20%wtSiC coating; (b) the
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magnification of (a); (c) Spot EDS analyses
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Fig.6 (a) Cross section backscatter SEM micrographs of the graphite substrate coated with the HfB 2-
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20%wtSiC/SiC coating (b) magnification of (a)
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Fig.7 TG curves of pure graphite substrate in dynamic aerobic condition from 25 ℃ to 1500 ℃
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Fig.8 (a) TGA curves of the coated substrate in air from room temperature to 15
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00℃; (b) TGA curves of the coatings with different amounts of HfB2
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Fig.9 Mass loss rate curves of the coated samples during TG tests
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Fig.10 TG curves of coating components from 25℃ to 1500℃ in air condition
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Fig.11 XRD pattern of the outer HfB2-20%wtSiC coating after oxidation
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Fig.12 Surface EDS element mapping of the outer HfB 2-20%wtSiC coating after oxidation
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Fig.13 Low magnification surface backscattered micrographs of the compound coatings with different weight content of HfB2 after TG test ((a)- (d)) and oxidation at 1500℃ for 50h in air
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((f)- (i)); (e) spot EDS analyses
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Fig.14 High magnification surface backscattered micrographs of the compound coatings with different weight content of HfB2 after oxidation at 1500℃ for 50h in air: (a) 20%wt.; (b) 40%wt.;
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(c) 60%wt.; (d) 80%wt.; (e) magnification of A in (d); (f) magnification of B in (d);
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Fig.15 TG circulation curves of HfB2-20%wtSiC coating from 25℃ to 1500℃ in air condition
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