J. Cent. South Univ. (2014) 21: 472−476 DOI: 10.1007/s117710141962z
A zirconium carbide coating on graphite prepared by reactive melt infiltration YANG Xin(杨鑫) 1, 2 , SU Zhean(苏哲安) 2 , HUANG Qizhong(黄启忠) 2 , CHAI Liyuan(柴立元) 1 , ZHONG PING(钟平) 2 , XUE Liang(薛亮) 2 1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China; 2. State Key Laboratory of Powder Metallurgy (Central South University), Changsha 410083, China © Central South University Press and SpringerVerlag Berlin Heidelberg 2014 Abstract: A dense ZrC coating with the thickness of 130 μm is prepared on graphite by reactive melt infiltration. XRD and SEM analyses show that the phase composition of the coating is ZrC and it adheres well with the substrate. The influence of ZrC coating on mechanical properties of the graphite was investigated by compression tests and the results show that after the coating process, the compression strength of the coated sample is improved by 13.64% as compared with graphite sample. The improvement of the compression strength for ZrC coated sample can be associated to the increased density and the ZrC particle reinforcement due to the infiltration and reaction of the melted Zr with carbon substrate in the coating process. Key words: graphite; compression strength; coating; ZrC
1 Introduction Carbon materials are ideal hightemperature structural materials for engineering and aerospace applications, due to their advantages of lightweight, high strength, excellent thermal shock and thermal erosion resistance, and they are widely used in electrical contact, heating unit, fusion reactors, space shuttle, rocket nozzles and propulsion systems [1−3]. However, with the development of new generation rocket and advanced space vehicles, their service condition becomes harsh. For carbon materials used in the high heat flux and high pressure gas flow environment, the oxidation and ablation of the material will lead to the degradation of physicalchemical properties and thus restrict their further applications. Therefore, to exert their full potential, additional protection that can extent their service life under severe condition is necessary. To protect carbon materials from oxygen attacking at high temperature, ceramic coatings have been explored and are considered as an efficient method because they can effectively separate the carbon substrate from oxidizing environment. However, the design of new generation flight vehicles with sharp nosetip, engine cowl inlets and leading edges requires that the materials used in this field can operate in air from 2000 to 2400 °C and be reusable [4]. At such high
temperatures, SiC and Sibased coatings cannot meet the requirement any more [5−8], because the transition from the passive to active oxidation will inevitably accelerate ablation rate and cause unacceptable shape change. To further improve the ablation resistance of carbon material at ultrahigh temperature, refractory metal carbides such as TaC, ZrC and HfC have been applied on their surface and achieved great success [9−14]. Among these refractory metal carbides, zirconium carbides have attracted much attention because of their relatively low density, low cost, excellent ablation resistant, good strength at high temperatures, and the ability to form refractory zirconium oxide scales with a melting point of 2770 °C [15−17], which are typical thermal barrier materials and can protect them withstanding temperatures up to 2500 °C. At present, ZrC coatings are usually deposited on carbon material by chemical vapor deposition (CVD) [18−21]. However, due to the low deposition rate, the coating process is time consuming, thus increasing the manufacturing cost and their applications are limited. Recently, WEN et al [22] developed a novel tungsten inert gas cladding technique to prepare ZrC coating on graphite through the reaction of graphitic carbon with ZrO2. However, to expand their applications in ultrahigh temperature environment, new methods that can prepare dense ZrC coatings for carbon materials are essential and urgent.
Foundation item: Project(51304249) supported by the National Natural Science Foundation of China; Project(14JJ3023) supported by Hunan Provincial Science Foundation of China; Project(2011CB605801) supported by the National Basic Research Program of China; Projects(2012M511752, 2013T607767) supported by the China Postdoctoral Science Foundation; Project(2012QNZT004) supported by the Fundamental Research Funds for the Central Universities of China; Project supported by the Freedom Explore Program of Central South University, China Received date: 2012−09−04; Accepted date: 2012−12−24 Corresponding author: HUANG Qizhong, Professor; Tel: +86−731−88836078; Email:
[email protected]
J. Cent. South Univ. (2014) 21: 472−476
In this work, the ZrC coating with dense structure was prepared by reactive melt infiltration at high temperature. Compared with CVD process, the reactive melt infiltration is more suitable to prepare ceramic coatings for its advantages of low cost, easy operation, and no special requirements to the complex gas feed stream system. More importantly, the reactions occurring between the substrate and coating materials also enhanced the bonding ability of the coating with substrate. The microstructure feature and phase composition of the ZrC coating prepared by reactive melt infiltration were present. Meanwhile, the influence of ZrC coating on the compression properties of the graphite was also investigated.
2 Experimental The graphite with the density of 1.76 g/cm 3 was used as substrate for coating and then cut into small specimens of 10 mm×10 mm×10 mm. Before the coating process, the specimens were handpolished using 600 grit SiC paper, then cleaned ultrasonically with ethanol and dried at 120 °C for 2 h. The zirconium powder of 48 μm was placed around the graphite samples in a graphite crucible. Then, the samples were heated up to 2000 °C under the pressure of 200 Pa. At the temperatures higher than the zirconium melting point, the zirconium powder melted and reacted with the graphitic carbon to form the ZrC coating. The whole coating process was conducted at the temperature within the range of 2000−2300 °C for 1 h, followed by a natural cooling course. To evaluate the mechanical properties of the coated samples, compression tests were carried out on a servohydraulic machine of 3369 (INSTRON Co., Ltd., USA). The size of the tested sample was 10 mm× 10 mm×10 mm and the crosshead speed was kept at 2 mm/min. The compression strength was calculated from the values of five specimens under each test condition. The surface morphologies of the coating and the fracture characteristic of the coated sample after compression test were observed with scanning electron microscope (SEM). Xray diffraction (XRD) and energy dispersive spectroscopy (EDS) were also used to identify crystalline structures and analyze element distribution in the coating, respectively.
3 Results and discussion Figure 1 shows the typical XRD pattern of the ZrC coating prepared by reactive melt infiltration. The result indicates that the phase composition of the coating is ZrC. No detectable zirconium peak is seen on the XRD pattern, which may be attributed to the fast diffusion of carbon in
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Fig. 1 XRD pattern of ZrC coating prepared by reactive melt infiltration
the reaction course. All the strong intensity peaks of the coating (2θ=33.0°, 38.3°, 55.3°, 65.9°, 69.3°, 82.1°) can be indexed as ZrC (111), (200), (220), (311), (222) and (400) planes according to the JCPDS card No. 350784. Figure 2 displays the surface images and EDS analysis of the ZrC coating. It is obvious that the coating surface is even and smooth, which is mainly composed of ZrC crystals. Meanwhile, it should be noted that these ZrC crystals reveal a hexagonal characteristic to some extent (Fig. 2(a)). Though no obvious holes can be found on the coating surface, some microcracks are inevitably formed due to the rapid cooling from high temperature to room temperature after the coating process (Fig. 2(b)). Figure 2(c) shows the EDS spectrum of the asprepared ZrC coating. It can be seen that the coating only contains Zr and C elements, which further confirms the formation of the ZrC coating and the result is in good agreement with the XRD analysis. Figure 3 presents crosssectional micrographs and element line scanning EDS analysis result of the ZrC coating. From Fig. 3(a), it is clear that the ZrC coating is 130 μm in thickness and it reveals a uniform thickness distribution. Meanwhile, no penetration cracks or holes are visible, indicating the formation of a perfectly dense structure. To have a more detailed insight of the structure of the ZrC coating, the crosssectional image with higher magnification is also conducted to observe the combination feature of the coating. As shown in Fig. 3(b), an evident interface is found between the coating and the substrate, unlike the SiC coatings prepared by other reactive methods [23−25]. To further understand the composition of the coating in the whole crosssectional direction, the element line scanning EDS analysis of the coating was conducted and the result is displayed in Fig. 3(c). From Fig. 3(c), it is apparent that the concentration of the Zr and C elements does not change obviously across the coating, while at the interface, the concentration of the Zr and C elements suddenly changes.
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Fig. 2 SEM images ((a) and (b)) and EDS analysis result (c) of ZrC coating
Moreover, it is noteworthy that Zr element is also detected in the graphite substrate, indicating that some Zr infiltrates into the graphite substrate in the coating process. Typical load−displacement curves derived from the compression tests for graphite and ZrC coated graphite are shown in Fig. 4. It can be observed that these two materials exhibit pseudoplastic fracture behavior in the compression process. Meanwhile, it is also indicated that the graphite sample shows more sudden drops after the maximum load is reached, suggesting a better mechanical property for the coated graphite. Furthermore, after the coating process, the calculated compression strength of the coated sample is increased from 30.8 to 35.0 MPa, and compared with graphite sample, the compression strength is improved by 13.64%. Figure 5 shows the fracture surface micrographs of
J. Cent. South Univ. (2014) 21: 472−476
Fig. 3 SEM micrographs and element line scanning EDS analysis result of ZrC coating: (a) Crosssection; (b) Interface between coating and graphite; (c) EDS element line scanning result of coating
Fig. 4 Typical load−displacement curves of graphite and graphite with ZrC coating
J. Cent. South Univ. (2014) 21: 472−476
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Fig. 5 Fracture surface micrographs of ZrC coated graphite after compression test: (a) Combination feature of ZrC coating with graphite; (b) Fracture surface; (c) Micrograph of Zr infiltrated region; (d) EDS analysis result of Zr infiltrated region
the ZrC coated graphite after compression test. The crosssection micrograph of Fig. 5(a) clearly shows that the coating is still integrate and adheres well with graphite even after compression test, indicating the good bonding ability between them. As shown in Fig. 5(b), the coated sample exhibits a rough fracture surface. Meanwhile, it should be noted that the defects such as holes and cracks that usually contain in graphite can rarely be found on the fracture surface (Fig. 5(c)). Moreover, the EDS result indicates that Zr element is also detected on the fracture surface (Fig. 5(d)), which means that the improved dense structure of the graphite substrate may be attributed to the infiltration and reaction of the melted Zr into the substrate. Therefore, based on the above analysis, it can be inferred that the improvement of the compression strength for ZrC coated sample can be associated to the increased density and the ZrC particle reinforcement due to the infiltration and reaction of the melted Zr with carbon substrate in the coating process.
the obtained ZrC coating is 130 μm and it adheres well with the substrate. 2) After the coating process, the average compression strength of the coated sample is increased from 30.8 to 35.0 MPa. Compared with the graphite sample, the compression strength of the coated sample is improved by 13.64%. 3) The improvement of the compression strength for ZrC coated sample can be attributed to the increased density and the ZrC particle reinforcement due to the infiltration and reaction of the melted Zr with carbon substrate in the coating process.
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4 Conclusions [3]
1) A dense ZrC coating is prepared on graphite by reactive melt infiltration. The coating is mainly composed of ZrC crystals which reveal a welldeveloped hexagonal characteristic to some extent. The thickness of
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