REN Xuan-ru, WANG Wei-guang, SUN Ke, HU Yu-wen, XU Lei-hua, FENG Pei-zhong

（China University of Mining and Technology, Xuzhou 221116, China）

Abstract： Liquid-phase sintering combining an in-situ reaction method with a slurry method was used to prepare HfB2-MoSi2-SiC coatings of controllable composition and thickness. The effect of the MoSi2 content on the oxidation protection of HfB2-MoSi2-SiC composite coatings in a dynamic aerobic environment from room temperature to 1 500 °C and a static constant temperature at 1 500 °C in air was investigated. The relative oxygen permeability was used to characterize the oxidation resistance of the coatings.The results of dynamic oxidation test at room temperature～1 500 °C show that the initial oxidation weight loss temperature of the samples is increased from 775 to 821 °C, and the maximum weight loss rate is decreased from 0.9×10?3 to 0.2×10?3 mg·cm?2·s?1 with increasing MoSi2 content, the lowest relative oxygen permeability is reduced to 12.2% with the weight loss of the sample being decreased from 1.8% to 0.21%. The mechanism of MoSi2 improving the oxidation protection of the coatings is revealed. With an increase of the MoSi2 content, the amount of SiO2 glass phase in the coating is increased, and the dispersion of Hf-oxide on the surface is improved so that a Hf-Si-O glass layer with high stability is formed and the weight loss of the sample is reduced from 0.46% to 0.08% after 200 h oxidation at 1 500 °C in air.

Key words: MoSi2；Coating；Oxidation resistance；Multiphase glass layer

1 Introduction

Carbon structural materials, such as graphite and C/C composite materials, are widely used in aerospace and manufacturing industries due to their good physical and mechanical properties such as low density,high specific strength and specific modulus[1-4]. Carbon structural materials are considered as ideal ultrahigh-temperature thermal structural materials due to their high melting point (>3 500 °C), unique structural stability, excellent thermal stability and conductivity, low coefficient of thermal expansion, good corrosion resistance and wear resistance[5-11]. While the carbon structural materials are easily oxidized above 400 °C, which affects the thermal structure stability under high temperature and limits their applications[12,13]. Therefore, with the increasing applications of carbon structural materials, improving their oxidation resistance in a wide temperature range to meet the requirements is very important. Based on a large number of experimental and theoretical simulation results,using oxidation protection coating to prevent the oxidation corrosion of carbon materials is the most effective way[14-17].

In recent years, among many anti-oxidation ceramics, the ultra high temperature ceramic HfB2-SiC composite coating has attracted great attention.The HfB2component has super high melting point(3 250 °C), high hardness, high strength and good oxidation resistance[18-22]. Carbon structure materials have good compatibility and coefficient matching of thermal expansion with SiC, which can improve the oxidation resistance of materials by forming a selfhealing silicate glass protective layer in aerobic environment. Wang et al.[23]prepared a HfB2-SiC coating by in-situ reaction. The weight loss rate of the sample after static constant temperature oxidation at 1 500 °C for 753 h is only 0.48%. Jiang et al.[24]prepared a new double layer HfB2-SiC-Si/SiC-Si coating by combining a slurry method and a gas phase silicon permeation method. After oxidation at 1 600 °C for 230 h, the weight of the samples increased by 0.34%.After oxidation at 900 and 1 500 °C for 752 and 1 200 h, the weight gain rates are 0.099% and 0.26%,respectively. Wang et al.[25]prepared a HfB2-SiCSi/SiC coating using SiC whisker. The sample is protected by the HfB2-SiC-Si/SiC coating, and the weight loss rate is only 0.88% after oxidation at 1500 °C for 468 h. Therefore, the HfB2-SiC coating shows great potential for oxidation protection and has great advantages, However, the CO or CO2generated during the oxidation process of SiC will not only generate pores in the coating, but also easily cause the bubbling phenomenon of the self-produced glass film, and the pores or cracks will be generated in the glass layer after the bubbles break. These defects will increase the diffusion channel of oxygen to the matrix and weaken the oxidation protection ability of the composite glass layer. Therefore, in order to weaken the negative influence of gas by-products on the protection ability of the glass layer and improve the oxidation protection effect of the HfB2-SiC coating, it is necessary to find a substitute of SiC component.However, due to its good oxygen resistance, especially its good thermal expansion matching with carbon matrix, SiC is irreplaceable. In this case, it is very attractive to modify the HfB2-SiC coating by silicide with better oxygen resistance[26-29].

Among the silicide, MoSi2has high hardness,high oxidation resistance and corrosion resistance. In recent years, it has been widely used in the field of the anti-oxidation coating[30-36]. Zhang[37]et al. prepared a C/SiC/MoSi2-Si multilayer anti-oxidation protective coating by combing a slurry method and gas-phase silicon penetration method, which can effectively protect carbon matrix for 300 and 103 h in 1 500 and 1 600 °C, respectively. Jiang[38]et al. prepared a SiCSi/Mo-SiC-C coating by combing gas phase silicon infiltration technology and an impregnation method.The weight loss rate of the coating sample was only 0.96% when the sample was oxidized in air at 1 600 °C for 220 h. Li et al.[39]prepared a MoSi2-SiCSi anti-oxidation coating for protecting the C/C composite material by a two-step embedding method. The weight loss rate of the coating is only 1.04% after oxidation in air for 200 h at 1 500 °C. In addition, for MoSi2and SiC with the same amount, the amount of SiO2produced by MoSi2oxidation is twice that of SiC. In view of the sealing and oxygen blocking effect of SiO2, more SiO2can effectively improve the overall sealing and oxygen blocking ability of the coating, weaken the oxidation loss of the coating, and thus reduce the generation of gas by-products. Therefore, it is very hopeful to improve the oxidation protection effect of the composite glass layer by modifying the HfB2-SiC composite coating with MoSi2.

In our previous study[40], we used Si, C, B2O3,HfO2and Mo powder as raw materials to prepare a HfB2-MoSi2-SiC anti-oxidation coating on the surface of the C/C composite with in-situ formed SiC as an inner coating layer. The weight loss of the coating matrix is only 0.76% after 408 h of isothermal static oxidation protection at 1 500 °C. However, it is difficult to precisely control the content and thickness of each component of the coating, and it is difficult to further study the effect of MoSi2modification on the oxygen resistance and protection mechanism of the HfB2-SiC composite coating, which greatly limit its application.

In view of the potential of the HfB2-MoSi2-SiC coating prepared by high temperature reaction sintering technology in the construction of high oxygen resistance coating structure on a carbon substrate, we developed a liquid-phase sintering method by combining an in-situ reaction method with a slurry method,using HfB2powder, MoSi2powder, SiC powder, Si powder and C powder as raw materials and silica sol(SiO2·nH2O) as a binder to overcome the shortcomings on the control of coating component content and thickness. The HfB2-MoSi2-SiC coating with controllable composition, content and thickness was prepared by this method. The composition and content of the coating can be controlled by adjusting the slurry ratio, the coating thickness can be controlled by the brushing time, and the pressureless sintering can be realized by high temperature liquid phase reaction sintering at 2 100 °C. By adjusting the composition of the HfB2-MoSi2-SiC coatings, the effect of the MoSi2content on the oxygen resistance, oxidation protection behavior and oxidative protection mechanism of the HfB2-SiC composite coating in the dynamic aerobic environment from room temperature to 1 500 °C and in the static constant temperature air at 1 500 °C were studied, and the film-forming mechanism and oxygen resistance performance of the composite glass layer on the coating surface were analyzed.

2 Experimental

2.1 Sample preparation

In order to study the effect of the MoSi2content on the oxidative protection perfromance of the HfB2-SiC coating, three kinds of HfB2-MoSi2-SiC coatings with MoSi2contents of 0, 20 wt.% and 40 wt.% were prepared. Firstly, the SiC inner coating layer was prepared on the surface of graphite matrix (Shanxi Xi'an Carbon Factory, 3 × 3 × 3 mm3) by an embedding method with Si powder (Shanghai Jiuling smelting Co., Ltd, 3-5 μm, ≥99.9%) and C powder (Shanxi Xi'an Carbon Factory, 2-3 μm, ≥99.9%) as raw materials. The specific preparation process can be seen in our previous work[41]. Secondly, MoSi2powder (Hunan Zhuzhou Cemented Carbide Group Co., Ltd, 3-5 μm, ≥99.9%), SiC powder (Beijing Zhongxin metal material technology Co., Ltd, 200 mesh, ≥99.9%), Si powder, C powder and self-made HfB2powder[42]were used as raw materials, and silica sol (SiO2·nH2O)(Shandong Dezhou Jinghuo Technology Glass Co., 6-10 μm, 20 wt.%.) was used as a binder. The magnetic mixer was used to stir the raw material for 12 min at 80 °C at a rate of 600 r s?1to obtain the mixed slurry.Then, the mixed slurry (Vsilicasol:Motherrawmaterials=1 mL g?1) was evenly brushed on the surface of graphite matrix with the SiC inner coating layer, and dried at 100 °C for 30 min, and this step was repeated three times after the slurry coating was completely dried.Finally, the sample was placed in a sintering furnace at 2 100 °C, and the sample was sintered under Ar gas atmosphere protection for 120 min to obtain the sample with a HfB2-SiC-MoSi2/SiC coating. The chemical reactions (Fig. 1) that occur during liquid phase sintering are as follows:

2.2 Material characterization

The crystal structure of the prepared HfB2powder and HfB2-MoSi2-SiC coating before and after oxidation was detected and analyzed using X-ray diffractometer (Bruker D8 ADVANCE XRD, Bruker AXS, Germany). The microstructure of HfB2-MoSi2-SiC coating before and after oxidation was studied by field emission scanning electron microscopy (JSM-6700F FE-SEM, JEOL, Japan). The crystal structure and microstructure of HfB2powder were detected by transmission electron microscopy (JEM-3010, TEM,JEOL, Japan). The dynamic oxidation protection performance of the HfB2-MoSi2-SiC coating was tested by thermogravimetry (STA 449 F3, TGA, Netzsch,Germany) from room temperature to 1 500 °C. In addition, a static constant temperature oxidation test of the coating at 1 500 °C was conducted in a high-temperature resistance furnace (M4/18AE, SGM, China).

3 Results and discussion

3.1 Analysis of phase and microstructure of the coating

Fig. 2a shows the TEM image of HfB2powder.As can be seen from Fig. 2a, the size of HfB2crystal is in the range of about 40-320 nm. In order to further

study the microstructure of HfB2powder, the high resolution TEM image is shown in Fig. 2b. Clear lattice fringes can be observed in Fig. 2b, and the distances between crystal faces are about 0.273 and 0.214 nm,which are consistent with the (100) and (101) crystal faces of the HfB2PDF standard card (01-089-3651),indicating HfB2powder has a good crystallinity.

In order to study the phase composition of the HfB2-MoSi2-SiC composite coating, XRD pattern is shown in Fig. 3. As can be seen from Fig. 3, the coating is composed of three phases of HfB2, SiC and MoSi2, and no by-products are generated in the preparation process, indicating the successful preparation of the HfB2-MoSi2-SiC composite coating.

In order to have a clearer understanding of the microstructure of the HfB2-MoSi2-SiC composite coating, the micro morphology of the sample surface is analyzed by backscatter SEM technology, as shown in Fig. 4. It can be seen that the surface of the HfB2-MoSi2-SiC composite coating is relatively dense, and there are no visible cracks and other defects. In addition, the dense coating is mainly composed of the phases with light gray, black and white three different colors. As shown in Fig. 4(b-d), the light gray phase in the area 1 is MoSi2, the white phase in the area 2 is HfB2, and the black phase in the area 3 is SiC, which are consistent with the XRD test results shown in Fig. 3.

Fig. 5 shows the cross-section backscatter SEM morphology of the SiC inner coating layer and the HfB2-MoSi2-SiC/SiC composite coating. It can be seen from Fig. 5a that the inner coating thickness of pure SiC is about 60-80 μm, and it is well bonded with carbon matrix. After the outer coating layer is prepared on the surface, as shown in Fig. 5b, the area A is the HfB2-MoSi2-SiC coating, the area B is SiC inner coating and the area C is graphite matrix. The thickness of the coating is about 120-150 μm. There is no obvious boundary between the inner and outer layers. This is because during the preparation of the outer layer, the liquid silica sol slurry carries HfB2powder, MoSi2powder, SiC powder, Si powder, C powder and other raw materials into the inner coating through the pores of the inner coating layer, and then in the process of liquid-phase sintering, SiC is generated through the solid-phase reaction between Si powder and C powder.

Thus, a form of mechanical occlusion and chemical combination is formed between the inner and outer layers, which improves the binding force between the inner and outer layers. In addition, the HfB2-MoSi2-SiC/SiC composite coating does not show obvious holes and cracks, which reflects the good thermal expansion coefficient matching between the coating and the substrate, indicating that the liquidphase sintering process effectively promotes the integration of the inner layer and the outer layer.

3.2 Study on the oxidative protection performance of the coatings

Fig. 6 shows the TG curves of the HfB2-MoSi2-SiC composite coatings from room temperature to 1 500 °C in air. It can be seen from the figure that the quality of the three composite coatings does not change significantly from room temperature to 700 °C,which indicates that these coatings have good oxidation protection ability below 700 °C. When the temperature is over 700 °C, the weight of all the coating samples shows the trend of a first increase and then decrease. The difference is that the temperature at which the sample begins to lose weight is different.The HfB2-SiC coatings containing 0, 20 wt.% and 40 wt.% MoSi2have initial weight loss temperatures of 775, 808 and 821 °C, respectively.

Therefore, compared with the HfB2-60SiC coating, the initial oxidation temperatures of the HfB2-20MoSi2-40SiC coating and HfB2-40MoSi2-20SiC coating are delayed by 4.2% and 5.9%, respectively,indicating the improvement the protective ability of the coatings containing MoSi2in the middle temperature range. In addition, at the end of TG test, the weight loss rate of the HfB2-60SiC composite coating on the surface is 1.8%, while the final weight loss rates of the samples with 20 wt.% and 40 wt.%. MoSi2are only 0.45% and 0.21%, 75% and 88% lower than the sample without MoSi2, respectively, indicating that the addition of MoSi2significantly improves the oxidation protection ability of the samples in wide temperature range.

In addition, it can be seen from Fig. 6 that although the curves of all coating samples show the state of weight loss when the temperature is higher than 850 °C, the downward shift trend of the samples with MoSi2becomes more slow as the temperature increases. In order to further study the oxidation protection mechanism of the composite coating, the weight loss rate curves of the HfB2-MoSi2-SiC composite coating samples are calculated according to formula(2), as shown in Fig. 7. The weight loss rate of all samples increase rapidly, and almost reach the maximum weight loss rate near 1 000 °C. Under the protection of HfB2-60SiC coating, the maximum weight loss rate of the samples is 0.9×10?3mg·cm?2·s?1. After adding 20 wt.% and 40 wt.% of MoSi2, the maximum weight loss rates of the samples are 0.36×10?3and 0.2×10?3mg·cm?2·s?1, 60% and 78% lower than the sample without MoSi2, respectively. In addition, there is an obvious fastest weight-loss temperature region in the temperature range of 1 000-1 200 °C, but after adding 20 wt.% and 40 wt.% of MoSi2, there is almost no fastest weight-loss temperature region after the maximum weight-loss rate is reached. Therefore,the addition of MoSi2can effectively inhibit the maximum weight-loss rate and the fastest weight-loss area in the wide temperature range. The narrow fastest weight loss temperature region and low maximum weight loss rate will delay the oxidation process of the coating and reduce the risk of carbon matrix failure,thereby improving the oxidation protection ability of t he coating in a wide temperature range.

WhereVis the weight loss rate of the coating, Δm(g)is the weight change of the sample to be tested,s(cm2) is the surface area of the sample to be tested and Δt(s) is the unit time of sample oxidation to be tested.

Since the penetration of oxygen through the coating into the substrate is the main reason for the oxidation loss of the carbon substrate, we propose a calculation method of the relative oxygen permeability, as shown in formula (3), by which the influence of the addition of MoSi2on the relative oxygen permeability of the coating is further studied. The mass loss of the sample can be used as the measurement of oxygen permeability. Firstly, we choose the HfB2-60SiC coating as the standard sample, and set the mass loss per unit area as a benchmark of the oxygen permeability.Secondly, the ratio of mass loss per unit area between the sample to be tested and the reference sample is taken as the relative oxygen permeability. Finally, according to formula (3), the relative oxygen permeabilities of the coatings containing MoSi2are shown in Fig. 8.

RPO2is the relative oxygen permeability of the coating, Δm1(g) is the weight change in unit time of the sample to be tested, Δm0(g) is the weight change in unit time of the sample to be tested,S1(cm2) is the surface area of the sample after oxidation in unit time of the sample to be tested, andS0(cm2) is the surface area of the sample after oxidation in unit time of the sample to be tested. The lower theRPO2value, the better the oxygen resistance of the coating.

It can be seen from Fig. 8 that after adding 20 wt.% and 40 wt.% of MoSi2into the coating, the maximum relative oxygen permeabilities of the HfB2-MoSi2-SiC coatings are 40.1% and 19.4%, respectively. That is to say, compared with the HfB2-SiC coating, the HfB2-20MoSi2-40SiC coating and HfB2-40MoSi2-20SiC coating block 59.9% and 80.6% oxygen diffusion to the substrate at most. In addition,with the increase of temperature, the relative oxygen permeabilities of the coating decrease, indicating that the oxygen resistances of the coatings increase with the increase of temperature. At about 1 300 °C, the relative oxygen permeabilities of the coatings tend to be stable because the silicate glass layers formed by oxidation begin to play a role gradually. The minimum relative oxygen permeabilities of the coating containing 20 wt.% and 40 wt.% of MoSi2are 24.7% and 12.2%, respectively. It can be seen that the addition of MoSi2can effectively reduce the oxygen permeability of the coating, thus reducing the oxidation corrosion of oxygen on the substrate and improving the oxidation protection ability of the coatings in a wide temperature range.

In order to further study the high temperature stability and oxidation protection ability of the coatings.Fig. 9 shows the isothermal oxidation curves of the HfB2-MoSi2-SiC composite coatings oxidized for 200 h at 1 500 °C in air. In the early stage of oxidation, the coatings are in full contact with oxygen, and chemical reactions (4-7) take place, resulting in the phenomenon of weight gain of the initial samples.Subsequently, the amount of SiO2glass formation increases gradually, and due to its good sealing effect,the oxidation of the coatings are obviously inhibited,so the samples do not show the phenomenon of weight gain continuously. In addition, the coatings began to lose weight after 40 h of oxidation, which indicates that the coatings could not completely block the oxygen permeation, and oxygen diffusion channels exist. When the coating iss oxidized at a constant temperature for 200 h, the weight loss ratio of the sample protected by the HfB2-SiC coating is 0.46%,while the weight loss rate of the samples protected by the HfB2-20MoSi2-40SiC coating and HfB2-40MoSi2-20SiC coating are 0.27% and 0.08%, 41.3% and 79.2% lower than the sample without MoSi2, respectively. The results show that the high temperature oxidation protection stability of the coatings is significantly improved due to the replacement of a part of SiC in the coating by MoSi2, thus providing more reliable oxidation protection for carbon matrix.

In order to further study the phase composition of the coatings after oxidation, the XRD patterns of HfB2-MoSi2-SiC composite coatings after static oxidation for 200 h at 1 500 °C in air are shown in Fig. 10.It can be seen that after oxidation, all the coatings contain SiC, SiO2, HfO2and HfSiO4phases, indicating that a composite glass layer is formed on the surface of the coating. However, the diffraction peaks of B2O3and MoO3are not detected in Fig. 10. On the one hand, the vaporization temperature of B2O3is around 1 300 °C[43], while the boiling point of MoO3is 1 155 °C. The vaporization of B2O3and MoO3at the oxidation test temperature results in the absence of two phases on the surface. On the other hand, B2O3can react with SiO2to form borosilicate glass, further weakening the diffraction peaks on the surface. As HfSiO4and HfO2are very stable at high temperature[44],the stability of SiO2glass layer can be improved with the formation of a Hf-Si-O glass layer, so their formation can improve the high temperature oxidation protection ability of the coatings. The chemical reaction equations during oxidation are as follows:

In order to further explore the surface microstructure of the coatings after oxidation, Fig. 11(a-c)shows the surface backscatter SEM morphology of the composite coatings containing HfB2-MoSi2-SiC after TG dynamic oxidation from room temperature to 1 500 °C. It can be seen that the coating surfaces are mainly composed of light gray and white areas. It can be seen from Fig. 11(d-e) that the white phase in Fig. 11a is Hf oxide, while the main elements of light gray phase in the area 2 are Si and O, so the gray phase is SiO2, which is consistent with the XRD test results shown in Fig. 10. In addition, it can be seen from Fig. 11a that the oxides of Hf are mainly dishancement of the coating stability.persed on the surface of the coating with large particles. It can be seen from Fig. 11(b-c) that with the increase of the MoSi2content, there is a ring of oxide dispersion around the large particles of Hf oxide. With the increase of the MoSi2conten in the composite coatings, the dispersion of the oxide becomes more and more obvious. This is because with the increase of the amount of SiO2glass phase on the surface of the coating, the flow of SiO2glass phase is intensified,and the strength of Hf oxide dispersed on the surface of the coatings is increased, so as to accelerate the homogeneous distribution of Hf oxide on the surface of the coatings. EDS analysis of Fig. 11f shows that the main elements in the light gray phase area 3 of Fig. 11c are Hf, Si and O. Due to the dispersion of Hf oxide, the glass layer on the coating surface becomes a Hf-Si-O composite glass layer.

In order to further study the anti-oxidation protection mechanism of the composite glass layers in long-term service at high temperature, Fig. 12(a-c)shows the surface backscatter SEM morphology of the composite coatings containing HfB2-MoSi2-SiC after static constant temperature oxidation for 200 h at 1 500 °C.

It can be seen from Fig. 12(a-c) that after full oxidation, the glass layers of the coatings have completely covered by the coating surface. Because of the poor matching of the thermal expansion coefficient,cracks can be observed in the glass film when the samples are cooled to room temperature. Although these cracks will be self-healed by the flow dynamic glass layer at high temperature in the alternative environment of high and low temperature. When the cracks are not healed, these cracks will be used as the diffusion channel of oxygen, resulting in the oxidation loss of carbon matrix. Therefore, the less the number of cracks in the glass layer, the better the oxidation protection effect. With the increase of the MoSi2content,the dispersion of Hf oxide on the surface of Hf-Si-O composite glass layer is intensified, and the cracks in the coating become less and less, which shows that the Hf-Si-O composite glass layer has excellent stability,showing the mechanism of Hf oxide dispersion en-

In addition, it can be seen from Fig. 12(a-c) that the cracks mainly exist in the region with less Hf oxide (black region). At the same time, the deflection and termination of the cracks can be clearly observed around the oxides of large white particles Hf in the glass layer, which indicates that when the oxide layer changes to glass state, the large Hf oxide particles are embedded in the glass layer in the form of "pinning phase", which limits the crack growth to a certain extent, thus reducing the number of the oxygen diffusion channels in the glass layer and reducing the glass layer oxygen permeation during the transition from glassy state to molten state, which further increases the oxidation protection of Hf-Si-O composite glass layer on carbon matrix.

4 Conclusions

HfB2-MoSi2-SiC composite coatings were successfully prepared by a liquid phase sintering method.The surface of the coatings is compact, and there are no visible cracks, holes and other defects. It is proved that the coatings have good thermal expansion coefficient matching with the matrix. The results of dynamic oxidation test from room temperature to 1 500 °C show that with the increase of the MoSi2content, the initial oxidation temperatures of the samples are delayed from 775 to 821 °C, and the maximum weight loss rate is reduced from 0.9×10?3to 0.2×10?3mg·cm?2·s?1, the fastest weight loss temperature area is narrowed, and the lowest relative oxygen permeability is reduced to 12.2%, which makes the weight loss rate of the samples reduce from 1.8% to 0.21%, improving the oxidation protection ability of the coatings in the wide temperature range. The results of static isothermal oxidation test at 1 500 °C show that the increase of the MoSi2content can increase the formation of SiO2glass phase in the coating, promote the dispersion of Hf oxide on the coating surface, and form a Hf-Si-O composite glass layer with higher stability and self-healing ability. The composite phase glass layers can effectively inhibit the crack formation, force the crack termination or deflection, thus reducing the weight loss rate of the samples after static constant temperature oxidation at 1 500 °C from 0.46% to 0.08%, significantly enhancing the high temperature stability of the coating, and providing reliable oxidation protection for the carbon matrix.

Acknowledgements

The Fundamental Research Funds for the Central Universities (2018GF14).