"Barrier Layer"

When plating is done onto a substrate, depending on what the two metals are, it is possible for them to diffuse into each other over time, especially at higher temperatures. The result of this interdiffusion is formation of an alloy which may not retain the desired characteristics of the intended plating.

When this is a serious possibility, a "barrier layer" of a 3rd metal (often nickel) which will not interdiffuse with the plating, is plated onto the substrate before the final layer of plating.