Boron carbide based coatings on graphite for plasma-facing components

In the effort to evaluate boron-rich coatings as plasma-facing surfaces in fusion devices, a new process for applying boron carbide (B sub(4)C) coatings to graphite was developed. The process entails eutectic melting of the carbon (C) substrate surface with a precursor layer of B sub(4)C particles. Adherent coatings were achieved which consisted of two layers: a surface layer and a graded penetration zone in the outer portion of the substrate. The surface-layer microstructure was multiphase and ranged from reaction-sintered structures of sintered B sub(4)C particles in an eutectic formed matrix to that of hypereutectic carbon particles in a B sub(4)C-C eutectic matrix. Because of high surface energy, the coating generally developed a nonuniform thickness. Quantitative evaluations of the coating were performed with limiters in the TEXTOR fusion device and with coupons in electron beam tests. Test results revealed the following: good adherence of the coating even after remelting; and, during remelting, diagnostics detected a corresponding interaction of boron with the plasma.