Q. Some of our parts are plated with electroless nickel. The vendor bakes the parts to increase the hardness of the plated layer. The vendor also states that this will improve the adhesion of the electroless nickel to the base material. Does the baking improve the adhesion to the substrate? P.T.
A. It is well documented that heat treating of electroless nickel deposits both of the phosphorus and boron types shows an increase in hardness with baking. There is also evidence that shows that heat treatment of the parts at 180–200°C for at least one hour will increase adhesion. How does the baking increase hardness and adhesion?
When you raise the temperature you start a solid-state diffusion process in which the structure of the electroless nickel reverts to a thermodynamically more stable state. You end up with a mixture of various species that contain both nickel and phosphorus or nickel and boron. The change in species not only gives you a more stable nickel phosphorus or nickel boron layer but also seems to increase the bonding between the substrate and the nickel layer.
An excellent resource to learn more about electroless nickel is a book by Wolfgang Riedel, Electroless Nickel Plating, ASM International. The book is out of print, but you may be able to find a copy by searching the web.