SiC coatings

High temperature tensile tests of the C/SiC coated C-fiber showed very good oxidation resistance up to 650°C. The filaments of the fiber bundle were coated homogeneously. 

Sawa, K. et al., Investigation of irradiation behavior of SiC-coated fuel particle at extended burnup, /. Nucl. Tech., 142, 250, 2002. 

There are four types of SCS fibers depending on the thickness of the final SiC coating designed for different metal matrices. They are the standard SCS, SCS-2, SCS-6 and SCS-8. Fig. 5.30 illustrates schematically the cross sections of two commercially produced SiC fibers, the standard SCS and SCS-6 fibers, according to DiCarlo (1988). Both types of fibers consist of a carbon core of 37 pm in diameter, a SiC sheath of varying thickness and a carbon-rich surface coating of 0-4 pm in... 

A silane-based CVD reactor suitable for performing high-temperatnre anneals in an Si- rich ambient was used for these experiments [86]. The samples were placed on a SiC-coated graphite susceptor and an RF induction coil used to heat the susceptor to temperatures on the order of 1,600-1,800°C. Silane and argon were the two process gases used, where Ar not only serves as a dilutant gas but also as a carrier gas to transport silane molecules to the crystal surface. All the implant annealing experiments were performed at atmospheric pressure. 

However, carbon materials have a serious shortcoming. They are easily oxidized above 530°C in air. It is possible to protect graphite plates or carbon fibers with SiC coating by CVD or pyrolysis of polymer containing Si and C.16-18 SiC is known as an effective material to prevent oxidation and corrosion due to the strong covalent bond and the passive oxidation by forming a protective Si02 layer on SiC.19-24... 

In this chapter, a new and easy process for SiC coating on fine carbon materials is described28-30 and some applications of SiC-coated diamond particles and carbon nanotubes to create new composites are demonstrated.31 33... 

The SiC coating is processed based on the reaction of SiO vapor and carbon materials. Commercial SiO powders (99.9% pure) are provided as the silicon source. The carbon materials are placed on the SiO powder bed via a carbon felt as illustrated in Fig. 10.1. This assembly is covered with carbon sheets in an alumina crucible to keep the SiO gas pressure in the crucible, and heated in a vacuum furnace at various temperatures from 1150 to 1550°C in vacuum (about 0.03 Pa) for periods of time between 1 and 90 minutes. It is necessary to heat at a temperature greater than 1150°C for the vaporization of solid SiO. 

Diamond powders with a particle size of 1-30 pm are used for the SiC coating. Figure 10.2 shows a TEM image of a SiC-coated diamond with a particle size of 1 pm. Each diamond particle is completely covered with a polycrystalline SiC layer 60 nm thick. The grain size of SiC is several nanometers. Although a large thermal expansion mismatch exists between SiC (a = 4.6 x 10 6/K) and diamond (a = 3.1 x 10 6/K), no crack or debonding... 

The SiC-coated diamond particles can be characterized by X-ray powder diffractometry. The diffraction peak appears at 35.6° which is assigned as the /J-SiC (111) plane. 

Based on these analyses on the SiC coating, the growth mechanism of the SiC layer on diamond is considered as follows. In the early stage of the SiC formation on diamond, a very thin SiC layer is formed on the diamond surface according to reaction (10.2) between diamond and SiO(g). Once the SiC layer is formed, this reaction does not proceed due to the protective layer of SiC. The carbon sheet and felt in an alumina crucible act as the carbon source. The reaction of C02(g) with these carbon sources will produce further CO(g) and deposit SiC(s) by reaction (10.7). Thin j3-SiC whiskers are observed on the surface of the SiC-coated diamond, suggesting the vapor growth of SiC. 

Figure 10.4 shows SEM photographs of the surface of SiC-coated diamond particles coated at 1350°C. Tiny granules of SiC were deposited and aggregated with an increase in coating time. Even for samples treated for 1 min, the entire surface is considered to be covered with a thin SiC layer formed by the direct reaction of diamond and SiO(g) because the samples show good oxidation resistance, to be discussed later. EDX analysis shows a uniform distribution of Si atoms on the entire surface of the SiC-coated diamond particle. 

High-resolution TEM image of the SiC-coated MWCNTs treated at 1350°C for 15 min with carbon source. 

Two types of assembly were used for the SiC coating to investigate the growth mechanism of the SiC layer. In the first method, the SiO powders are set on the bottom of an alumina crucible and MWCNTs are placed upon SiO powders via a carbon felt, as shown in Fig. 10.6(a). In the second method, an alumina plate with a center hole is used instead of the carbon felt to separate the MWCNTs from SiO powders, as shown in Fig. 10.6(b). These assemblies... 

Assemblies for the SiC coating of MWCNTs (a) with carbon source, and (b) without carbon source. 

XRD patterns of the SiC-coated MWCNTs prepared (i) with carbon source, and (ii) without carbon source for 15 min, and (iii) with carbon source, and (iv) without carbon source for 30 min, at various coating temperatures. 

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