Diffusion carbide coatings formed in molten borax systems

Abstract

Carbide coatings can be formed on carbon containing materials such as steels, nickel and cobalt alloys, cemented carbides, and carbon itself, by immersion in a molten borax based bath, often used as a basis for bonding. Coatings, thus formed, consist of VC, NbC, TaC, TiC, and Cr7C3. They are superior to coatings formed by conventional surface hardening techniques in resistance to wear, seizure, oxidation, and corrosion. Selected additives to a borax bath can produce either a carbide layer, a borided layer (the boride of the main element in the substrate), or no layer at all. An understanding of additive behavior as dictated by the free energies of carbide formation and oxide formation of elements added to molten borax will explain such behavior. Materials carbide coated by the process have excellent tribological, mechanical, and chemical properties, and are very similar to Titanium carbide coatings produced by chemical vapor deposition, in performance in industrial applications. Unlike some conventional surface hardening techniques, the improved surface properties are not achieved at the expense of the mechanical strength; there is no significant reduction in strength between conventionally hardened steel and that coated by this process.