Solid-state reactions of silicon carbide and chemical vapor deposited niobium.
Keywords Chemical vapor deposition, Thermodynamics, Solid-state reactions, Niobium, Silicon carbide
Because of their excellent thermomechanical properties, silicon carbide (SiC) ceramics are considered to be one of the most promising candidates for high temperature structural applications. (1) SiC is also a wide band gap semiconductor, which is suitable for high temperature electronic applications in hostile environments. (2) For many applications, solid-state reactions between SiC and metallic materials are frequently encountered. In the brazing process for joining C/SiC composites with Ti alloys and steel, and in the fabrications of SiC-reinforced metal matrix composites process, the reactions at the interfaces between SiC and metals have been extensively studied. (3-8) In SiC-based electronic-devices, on the other hand, the metallization process has also been investigated. (9) Nevertheless, most of the efforts have been focused primarily on the identification of the reaction products between SiC and metals.
SiC-metal connection is a general structure for high-temperature applications. Usually, Ti alloys and Ni alloys are used for such applications. With increasing the application temperatures, niobium with a higher melting point is preferred in such structures. Consequently, fundamental understanding of reactions between SiC and Nb becomes very important. Such understanding could lead to a guideline for improving connection properties by tailoring the interfacial chemistry.
In the present study, we investigated the reactions between the SiC and Nb that were deposited on the SiC substrate via chemical vapor deposition at different deposition temperatures. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the reaction products. The results are discussed on the basis of thermodynamics and kinetics.
The Nb films were deposited on SiC substrates by CVD process using niobium chloride (Nb[Cl.sub.5]), hydrogen ([H.sub.2]), and argon as starting materials. The deposition process was performed in a horizontal hot-wall deposition apparatus. Schematic representation of the deposition furnace is shown in Fig. 1. The horizontal tube is an aluminum tube with an inner diameter of 20 mm. The SiC substrates were obtained by sintering SiC powders at 1650[degrees]C From XRD results, it was learned that the substrates were polycrystalline without any preferred orientations. These SiC substrates were polished to 1 [micro]m finish and cleaned in ethanol by ultrasound before they were put into the deposition furnace. The furnace was then heated to the desired temperature by the heat elements with a rate of 5[degrees]C/min. After the setting temperature was reached, the deposition furnace was backfilled with [H.sub.2] to stabilize the temperature for a while. Simultaneously, the heat bells heated the Nb[Cl.sub.5] to 120[degrees] C in 5 min. and stabilized the temperature for 5 min. All the temperatures were controlled in a margin of error of [+ or -]5[degrees]C by programs. Subsequently, the flowing argon gas carried the sublimated Nb[Cl.sub.5] into the deposition chamber. The purities of precursors and deposition parameters are listed in Table 1. When the deposition process was completed, the heat belts were turned off at once. The [H.sub.2] gas was also shut down. Argon gas would be kept until the temperature in the chamber dropped to 100[degrees]C.
[FIGURE 1 OMITTED]
Table 1: The purities of precursors and deposition parameters Deposition precursors Nb[Cl.sub.5] [H.sub.2] Ar Purities (%) 99 99.999 99.999 Flow rates 4.7 g/h 200 ml/min 200 ml/min Deposition temperatures 900-1300[degrees]C Deposition pressure 3 atm Duration time 1 h
The obtained samples were analyzed by XRD. using a Rigaku D/max-2400 diffractometer (Tokyo, Japan) with copper K[alpha] radiation. Data were digitally recorded in a continuous scan mode in the angle (2[theta]) range of 15-75[degrees] with a scanning rate of 0.16[degrees]/s. The cross sections of the deposited samples were characterized by SEM (JEOL-6700F, Tokyo. Japan) equipped with energy dispersion spectroscopy (EDS). The samples for cross-section observation were prepared as follows: a glass sheet, about 0.2 mm thick, was first bonded to the coating surface using EpoxyBond 110[TM] (Allied, USA) to prevent the coatings from possible damage during polishing. The cross section of the specimen was then polished to 1 [micro]m finish. After polishing, the samples were cleaned by ethanol alcohol using ultrasound for cross-section observation.
Results and discussion
Figure 2 shows the XRD patterns of niobium films deposited at different temperatures. It shows that the film deposited at 900[degrees]C contains both [Nb.sub.3]Si and Nb. With increasing deposition temperatures, the intensity of [Nb.sub.3]Si decreases and almost disappears at 1000[degrees]C. There are two possible reasons to account for this: first, the [Nb.sub.3]Si at interface might become less detectable with the increase in the thickness of Nb films at elevated temperatures because XRD is limited by penetration depth. Second, the growth rate of Nb could be much faster than that of [Nb.sub.3]Si. According to the mixture rule, the signal of [Nb.sub.3]Si would become weak. Further increasing the deposition temperature to 1050[degrees]C, [Nb.sub.3]Si appears again. It indicates that the solid-state reactions proceed fast at high temperatures to get more products. A further increase of deposition temperatures to 1300[degrees] C leads to more products such as [Nb.sub.5][Si.sub.3], [Nb.sub.2]C, and NbC.
[FIGURE 2 OMITTED]
As indicated by XRD, the formation of [Nb.sub.3]Si could happen at a temperature as low as 900[degrees]C. According to the reactions between SiC and Nb. another product should be carbon or carbides. However, no peaks from carbon/carbides are observed in XRD patterns at this temperature. One possible reason is that the Nb film is loo thick to observe the carbon or carbides (the thickness of films deposited at 900[degrees] C is about 30 [micro]m). When the thickness of deposited Nb films decreases to about 4 [micro]m at 1300[degrees]C, the carbon phase is clearly observed. Hence, it is believed that the solid-state reaction product between SiC and Nb is carbon besides [Nb.sub.3]Si. The carbon products were also observed in other metal/SiC systems, such as Ni/SiC, (7), (8) Fe/SiC, (10) and Co/SiC. (7) It is generally believed that these carbon products originate from the decomposition of SiC. As discussed in the previous study on the metal-SiC reactions, (8) the decomposition of SiC was required in order to proceed with the SiC and metal reactions. Since the decomposition of SiC exhibits a positive Gibbs free energy, the SiC-Nb reactions require surmounting a certain amount of activation barrier. As stated in Ref. 8, the metal atoms on the SiC surface were necessary for such a decomposition process. As for the chemical vapor deposition process, the Nb atoms prefer to deposit on the SiC surface at the beginning. SiC could decompose with the assistance of the deposited atomic Nb. After the decomposition, the silicon could diffuse out into Nb film to form niobium silicide. The driving force for silicon diffusion out is believed to originate from the negative Gibbs free energy of Nb silicide formation (Fig. 3). However, the kinetic process should also be considered for the solid state reactions. Since the silicon diffusion is slow at low temperatures (~ [10.sup.-20] [m.sup.2]/s at 900[degrees]C according to Ref. 11), the silicon diffusing into Nb is limited. This is the reason why the Nb-rich silicide ([Nb.sub.3]Si) is preferred to form at low temperatures, although the Si-rich silicide ([Nb.sub.5][Si.sub.3]) has more negative Gibbs free energy (Fig. 3). Since the Nb silicides are much more thermodynamically favorable than carbides (Fig. 3), the Nb prefers to connect with Si. Hence, the formed [Nb.sub.3]Si layer might stop the carbon diffusing out or Nb diffusing in to form niobium carbides.
[FIGURE 3 OMITTED]
The linear element distribution of samples deposited at 900[degrees]C is shown in Fig. 4. There is no obvious element concentration gradient across the films. It indicates that the silicon diffusion-out is limited and that the silicide layer is pretty thin. It gives experimental support to the aforementioned assumption. When increasing the temperature to 1100[degrees]C, the linear element distribution in the film cross section is almost the same except that there is a little more silicon diffusing into the Nb films.
[FIGURE 4 OMITTED]
At temperatures higher than 1200[degrees]C, the intensity of Nb decreases, but [Nb.sub.x]Si, [Nb.sub.2]C, and NbC are shown in XRD patterns. Based on our previous discussion, it is generally believed that silicon diffuses out faster at these temperatures than at low temperatures. As indicated by the linear element distribution in the film deposited at 1300[degrees]C (Fig. 5), the silicon shows a convex in the middle of the film, which demonstrates that it is diffusing out to the surface. This is also confirmed by the EDS analysis (Fig. 6) for the surfaces of the films deposited at various temperatures. As can be seen, there is scarce silicon in the films deposited at 1100[degrees]C, while the silicon begins to appear in the films deposited at 1200[degrees]C. The increase in temperatures results in an increasing intensity of silicon. It is known that the EDS analysis is a thickness-sensitive technology. The change of silicon intensity in the EDS results reflects the difference of silicon concentration in the film surface. The similar phenomenon of silicon diffusion-out to the surface is also observed in the other metal-ceramic reactions. (13) As shown in Fig. 5, the silicon diffusing-out results in a high Nb concentration at the rear of the silicon convex. The Nb would contact with carbon directly to form Nb carbide. According to XRD results, [Nb.sub.2]C is the only carbide phase appearing at 1250[degrees]C. Further increasing temperature to 1300[degrees]C results in the formation of NbC From this fact, it can be deduced that the formation of [Nb.sub.2]C is prior to NbC. A similar phenomenon was also observed in the previous study, (12) in which [Nb.sub.2]C formed at the temperature of 1100[degrees]C, and fully transformed to NbC at 1500[degrees]C. Based on the above discussion, it is believed that the Nb carbide could form only after silicon diffuses out to the surface. It also proves the previous assumption that the formation silicide blocks the carbide formation.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
In this paper, the niobium films were deposited on the surface of SiC substrates by chemical vapor deposition using niobium chloride and hydrogen. The solid-state reactions between SiC substrate and deposited Nb were studied by XRD and EDS. Based on these results, the following conclusions can be drawn:
(1.) At low temperatures, niobium silicides were preferred to form at the beginning of deposition, which blocked the further reactions between carbon and niobium to form carbides.
(2.) The increase in temperature resulted in the silicon diffusing out to the surface, which led to the formation of Nb carbides.
(3.) In order to obtain a metal Nb film on the SiC substrate, the deposition temperature should be controlled below 1150[degrees]C, to reduce the solid-state reactions between Nb and SiC substrate.
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Y. Wang ([??]), Q. Liu, L., Zhang. L. Cheng
National Key Laboratory of Thermostructure Composite
Materials, Northwestern Polytechnical University.
Xi' an 710072, China
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|Author:||Wang, Yiguang; Liu, Qiaomu; Zhang, Litong; Cheng, Laifei|
|Date:||Sep 1, 2009|
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