ZrB₂powders (~10 μm, >90wt% pure, Dandong Chemical Co., Ltd., Dandong, China) and phenolic resin (Shanghai Qinan Adhesive Material Factory, China) were mixed in alcohol through ball-milling to form a slurry for later use. C/SiC composites were chosen as the carbon fiber composites matrix. The C/SiC composites with a dimension of 5 mm×5 mm×20 mm were chamfered with no further machining, and the pre-coating was prepared by slurry-dipping method.

After dipped in the above slurry and dried, the slurry coated composite was cured at 100-150℃ for 1-3 h. Then, it was pyrolyzed in argon atmosphere at 1000℃ for 30 min at a heating rate of 5℃/min to convert the phenolic resin into carbon. In order to ensure the pre-coating thicknesses, several repetition of slurry dipping and pyrolysis process may be performed until the expected thickness was reached.

C/SiC composites covered with the pre-coating were placed in a graphite crucible containing some silicon powder at the bottom. For VSI coating, the specimens and the silicon powders should be separated appropriately. Then the crucible was heated in a vacuum furnace to 1600℃ at 5℃/min and dwelled for 30 min. Gaseous silicon would diffuse into the pre-coating and react with carbon to form SiC at 1600℃.

For LSI coating, the specimens bottom contacted with silicon bed were placed in another graphite crucible. Similarly, the crucible was heated in a vacuum furnace to 1500℃ at the same heating rate for the same dwell time as the VSI process. The liquid silicon would infiltrate into the pre-coating by the capillary force and react to form SiC at 1500℃.

The oxidation resistance of the ZrB₂-SiC coating was evaluated by static oxidation in air at 1500℃ using a muffle furnace. After each oxidation test, the specimens were weighed on an electrical balance with an accuracy of 0.1 mg. The composition variation was analyzed by X-ray diffraction (Model D/max 2550 V, Rigaku, Japan) with Cu-Kα source. The microstructure evolution was observed by electron probe micro-analyzer (EPMA, JXA-8100, JEOL, Tokyo, Japan) equipped with energy dispersive spectrum (EDS, INCA, Oxford, Tokyo, Japan).

Figure 1 shows the microstructure of pre-coating after pyrolyzed at 1000℃. The pre-coating shows a high integrity without obvious cracks despite of the high volume shrinkage associated with phenolic resin decomposition and the release of gas byproducts during the pyrolysis process. Generally, the microstructure of the pre-coating is relatively loose with many pores. Such a microstructure is beneficial for further liquid silicon infiltration and gaseous silicon diffusion. It only can be seen diffraction peaks of ZrB₂ phase from XRD pattern of the pre-coating, which indicates that the carbon derived from the pyrolysis of phenolic resin is in an amorphous state (Fig. 2(a)).

Fig.1

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	Fig.1 SEM images of the pre-coating on C/SiC composite surface after pyrolyzed at 1000℃

Fig.2

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	Fig.2 XRD patterns of the surfaces of various coatings(a) Pyrolyzed pre-coating, (b) VSI coating, (c) LSI coating, (d) VSI coating after oxidation test, (e) LSI coating after oxidation test

Figure 3 shows the microstructure of the polished cross sections of the as-fabricated VSI coating. As can be observed from Fig. 3(a), the coating has become dense and is mainly composed of two phases. The bright phase should be ZrB₂, while the continuous dark phase should be as-formed SiC, according to its XRD pattern (Fig. 2(b)). These two phases intimately connect with each other and no cracks can be found in the coating. The thickness of the coating is about 160 μm. The phase distribution of coating surface is very similar to that of coating inside, although the coating surface is rough (Fig. 3(b)).

Fig. 3

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	Fig. 3 Cross-section (a) and surface (b) SEM images of the VSI coating

Different from the VSI coating, a large amount of residual silicon is found in the LSI coating (Fig. 2(c)). A comparison between the cross section and surface SEM images of LSI coating finds that the population of ZrB₂ particle in the coating inside is much higher, while few ZrB₂ particles can be observed on the coating surface (Fig. 4). This means that the residual silicon may mostly exist in the surface region of the coating. In addition to residual silicon, cracks are found in the LSI coating, and the particle size of ZrB₂ in it is also much larger than that in the VSI coating, although the fabrication temperature of the former coating is much lower. The reasons for such differences in the microstructures between these two coatings can possibly be analyzed as follows.

During LSI coating fabrication, the molten silicon migrates upward along the pre-coating by means of strong capillary diffusion or reactive surface tension motion associated with surface diffusion^{[ 8]}, then reacts with carbon to form SiC. The strong exothermic effect associated with the reaction leads to local overheating, which drives the reaction to proceeds in such a violent way that stress induced cracking occurs in the coating, while the surplus silicon is left over on the coating surface. Furthermore, the infiltration of liquid silicon into the pre-coating may also enhance the coalescence of ZrB₂. This, together with the local overheating, leads to a rapid growth of ZrB₂ particles. Hereby, cracked LSI coating with relatively large ZrB₂ particles and residual silicon on surface is obtained. As to the VSI coating, the things are quite different. Since the silicon vapor pressure at 1600℃ is low (<100 Pa), SiC formation reaction proceeds slowly during the VSI process. The local overheating effect in the coating, if it exists, is weak. This not only prevents the formation of intensive stress and cracks in the coating, but also prevents the rapid growth of ZrB₂ particles. Under suitable conditions, crack free VSI coating with small ZrB₂ particles and almost no residual silicon can be fabricated.

Fig. 4

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	Fig. 4 Cross-section (a) and surface (b) SEM images of the LSI coating

When the LSI and VSI coatings are oxidized in air at 1500℃ for 64 h, the following reactions will occur:

(1)

(2)

(3)

(4)

The oxidation reactions of ZrB₂ and SiC will result in weight gain, while the evaporation of B₂O₃ will cause weight loss. As the mass gain is much larger than the weight loss, a net weight gain is expected.

The weight changes of C/SiC composites with LSI and VSI coating after exposure in air at 1500℃ are shown in Fig. 5. It is evident that both samples show weight gain during the whole oxidation process up to 64 h. This is in good agreement with the expectation. However, a great fluctuation in the weight gain of the composite with LSI coating is observed. This sample shows a maximum weight gain of 2.34wt% after oxidation for 16 h, then the weight gain declines with the prolongation of oxidation time. When the oxidation time is prolonged to 64 h, the weight gain decreases to 0.5wt%. In contrast, the composite with VSI coating exhibits much more superior oxidation resistance. This sample shows a weight gain of 0.6wt% after oxidation for 2 h because of surface oxidation, then the weight gain increase slowly with the increase in oxidation time. Nevertheless, its total weight gain is no more than 1wt% after 64 h oxidation.

Fig. 5

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	Fig. 5 Weight change of the surface coated with C/SiC composites during air oxidation at 1500℃

Figure 6 shows the SEM images of the VSI and LSI coating after the oxidation test. For both samples, a surface scale resulting from the coating oxidation is observed. XRD analysis indicates the existence of ZrSiO₄ and ZrO₂ crystalline phases in the surface scale, which are the oxidation products of SiC and ZrB₂ (Fig. 2(c) and (d)). For the VSI coating, residual ZrB₂ is also found in the surface scale. The oxidation scale on the VSI coating surface is rather compact without obvious cracking. This can effectively restrict the oxygen inward diffusion, resulting in the superior oxidation resistance of this sample. Elements line scanning result indicates that the oxide scale on the coating surface is rather thin, and oxidation only occurs in the outmost part of the VSI coating (Fig. 6(a)).

Fig. 6

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	Fig. 6 SEM images and elements line scanning of the VSI and LSI coatings after air oxidation at 1500℃ for 64 h(a, b) VSI coating; (c, d) LSI coating

In comparison, the microstructure of the oxidized LSI coating is quite different. A rather thick SiO₂ rich layer with cracks is formed on the LSI coating surface. This layer mainly arises from the oxidation of residual silicon and contributes to the large weight gain at the initial stage of the oxidation process. What’s more, the cracks in the original coating together with the cracks in the SiO₂ rich layer act as high speed channels for oxygen inward diffusion. Hereby, oxidation reactions can easily take place not only in the coating surface but in its interior as well, which causes a significant deterioration in the oxidation resistance of the coating. The interior oxidation of the LSI coating can be readily deduced from the result of elements line scanning shown in Fig. 6(c).