B4C coating

The surface temperature excursion during an plasma exposure is shown in Fig. 13.8. The plasma was heated auxiliary by two neutral beams of 1.5 and 1.3MW/m2. The resulting load on the test limiter amounted to 4MW/m2. The 170 pm thick B4C layer was rapidly heated up to 1300 K and cooled down, as the pyrometer signals show. The surface layer withstood such an exposure and did not detach from the copper. Increasing the deposited heating power above 8MW/m2 led to melting of the B4C coating (see Fig. 9 in [33]). 

The main goals of these experiments were to test the materials themselves under plasma exposure and to see the influence on the plasma performance. Especially, it could be shown under which conditions and under which maximum power load the used materials survive (B4C coating on copper, W, 0.5 mm W on graphite, Ta). Most of these materials are very promising. B4C-coated copper is now build in Wendelstein 7-X and tungsten is a very suitable material for target plates for medium power load. Such experiments will soon be continued with a tungsten brush test limiter. This structure should reduce the surface stress under plasma power load. 

Boron carbide (B4C) Coating for jet engine nozzles, coatings for shielding in nuclear reactors. 

The recent research focus of CVD B4C is to reveal the deposition mechanism under different deposition conditions and establish the relationship between deposition parameter and deposition mechanism, for which thermodynamic, mass transfer and kinetic modeling attempts have been studied by several research groups". The CVD B4C from BCI3/CH4/H2 precursor is a very complex chemical reaction process, and the B4C can be deposited by different mechanisms. Some reasonable deposition mechanisms have been established, such as Thomas S. Moss et al, and Mustafa Karaman et al, their experiments were performed under a small variation of deposition parameters and thus all of the B4C deposits had similar microstriicture and phase composition, which probably suggest the B4C coatings were deposited by a single mechanism. 

M.J. Santos, A.J. Silvestre, and O. Conde, Laser-assisted deposition of r-B4C coatings using ethylene as carbon precursor. Surface and Coatings Technology, 151 -152, 160-164(2002). 

A final step in manufacture is to coat the fibre with SiC or B4C. This provides protection against reactions with other elements at high operating temperatures and ensures that the fibre retains its tensile strength at elevated temperature. Typically, the W wire substrate has an 8 pm diameter, the diameter of the boron fibre sl50pm, and the SiC or B4C coating is 4pm thick. 

X. M. Li, L. T. Zhang, X. W. Yin, Effect of chemical vapor deposition of Si3N4, BN and B4C coatings on the mechanical and dielectric properties of porous Si3N4 ceramic, Scripta Mater, 66, 33-36(2012). 

Figure 6.11 Lubricated sliding friction coefficient of a steel pin sliding against a B4C coating and a nano-scale boric acid film formed on the B4C coating, as illustrated in the inset...

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

Additional surface modifications on vapor deposited SiC fibers, including WC. TaC, TiN, B4C, Al, Ni and Fe, have been applied with varying degree of success (Wawner and Nutt, 1980 DeBolt, 1982 Wawner, 1988). After exhaustive trial and error, TiB is selected as an additional coating material to further prevent the diffusion-induced reactions between the SCS-6 fibers and matrix materials, including Ti alloys and Ti Al intermetallic alloys (e.g. Ti Al, TiAl and TiAl ) (Donncllan and Frazier, 1991 James et al., 1991). When the coated fiber is subjected to tensile... 

Another application concerning the liquid-metal-cooled fast breeder reactors (LMFBR) was studied as development of advanced control rod materials for FBR (22). Fabrication tests and out-of-pile measurements were made of B4C/Cu cermet to obtain high-performance neutron absorber materials for LMFBR. A coating layer of Cu was formed on the surface of B4C/Cu powder, and then the coated B4C... 

The SiC-coated or uncoated MWCNTs are dispersed in isopropyl alcohol (IPA) using ultrasonic vibration for 5 min. They are then mixed with nanometersized SiC (mean diameter 30 nm) and B4C (mean diameter 240 nm) powders in IPA using ultrasonic vibration for 10 min. The B4C is added at 2 wt% as a sintering aid. After the mixing, the IPA solution is evaporated and the mixed powders are dried at 100°C. The MWCNTs are uniformly dispersed without aggregation. The mixed powders are sintered at 1800°C at 40 MPa for 5 min under a vacuum by means of PECS. The content of the SiC-coated MWCNTs is varied between 1 and 5 vol%. 

Fig. 13.9. The surface of a test limiter (made of copper coated with 170 pm thick B4C) after 80 exposures in TEXTOR near the last-close flux surface. The melting zone is clearly seen, especially in the magnification at the right-hand side, where also a number of craters are visible, possibly caused by electrical arcs...

X = Cl, Br, I), CH4 and Hj. Single crystals of BgC can be obtained by reacting BI3 and CI4 below 1000°C These reactions can be used to apply a protective coating of B or the carbides by proper adjustment of conditions Finely divided B4C is prepared by reacting BCI3, CH4 and Hj in an RF plasma. ... 

C/C-SiC tubes for high temperature heat exchangers in coal combustion chambers could withstand highly aggressive environments including water vapour and liquid coal slags, especially coated with multilayer EBC based on B4C, SiC and Cordierite. 

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