| abstract |
A substrate material to be coated with either a nitride, carbide, or oxide contains a small percent of a specific reactive element, like titanium, which forms very stable nitrides, carbides, or oxides. The material also contains larger percentages of elements, such as chromium, which form less-stable nitrides, carbides, or oxides. When the substrate material is immersed in a process medium which contains reactants, such as nitrogen, carbon, or oxygen, at a chosen elevated temperature and concentration, the less-stable nitrides, carbides, or oxides are reduced and cannot form a coating on the material surface. Thus, only a very stable nitride, carbide, or oxide can form a strong, adherent coating. As such, a stable compound forms on the surface, the surface concentration of the specific reactive element atoms (example: titanium) is depleted in relation to the atom concentration in the bulk material, and a concentration gradient results which causes more of the specific reactive element atoms to diffuse to the surface and react with the reactant in the process medium until a coating of the desired thickness is formed. |