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Cu/Ni/Au diffusion barrier problem

Q. We are using a Cu bulk material with a Ni & Au layer (Au to protect against oxidation, and Ni as a diffusion barrier) at 350 °C. After a few hours (~100h) some of these parts turn brown/black. If we analyse the parts, we can see with EDX that Ni has diffused into the Au. What is the diffusion coefficient and activation energy of Ni/Au?

Can someone advise me, what metal would be a better diffusion barrier. Our target is that these parts can be used at 350 C-400 °C for more than 1 year.

A. Copper nickel and gold are mutually soluble, thus will diffuse together at high temperature. High phosphorus electroless nickel(1.2-1.5 mils) will perform much better than electroplated nickel because the deposit is amorphous. That is, it has few or no grain boundaries for diffusion to take place rapidly. However it is not likely that even electroless nickel would withstand 1 year at °350-400C without some diffusion. Increasing the EN thickness to about 2 mils would cause macrocracking, allowing diffusion to occur.

Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics"
           and "Plating ABS Plastics" eBay or AbeBooks affil links])
2004

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