A layer of the appropriate thickness is almost as important as having chosen the process for the job in hand.
Typical Thickness Ranges
For light-duty corrosion protection, a thickness of 5 to 15 μm is typical. Wear-critical parts need a thicker layer of wear protection, typically in the range of 25 to 75 μm or even more, depending on the specific requirements.
The Risk of Going Too Thin
Unless an unusually thick coating is intended (over 75 microns) deposits thinner than 5 microns typically contain pinholes and, consequently, have been corrosion tested to inferior levels. The wear surfaces, naturally, will soon wear through to bare metal.
Why Excessive Thickness Causes Problems
However, thick Electroless Nickel Plating above 75 microns can become brittle. Therefore, such plating can cause adhesion failure on steel surfaces. Moreover, thick plating can cause dimensional build-up as the coating grows equally on all surfaces. Therefore, even 10 to 15 microns per side can cause problems on press-fit bores or fine threads.
Close Tolerances Limit Your Options
For press-fit parts, such as bores (with or without locking features) and fine threads, the thickness of the plating on both surfaces (i.e. the plating on one surface plus the plating on the opposing surface) should be limited to around 10-15 microns per side to prevent dimensional problems (i.e. problems that result in the parts not fitting together as intended).
Post-Plate Heat Treatment
Baking the Electroless Nickel coating at approximately 190 to 200°C post plate will increase the hardness of the coating to approximately 70 HRC. The increase in thickness is minimal and is not a method of attempting to increase thickness.
Measuring and Specifying Correctly
For coating thickness measurement there are a number of surface coating measurement standards, maintained by the UK’s national measurement institute, which can be used for specifying coating thickness measurement. More on Electroless Nickel Plating can be found at www.swmf.co.uk/surface-coatings/electroless-nickel-plating/.
A simple callout on the drawing for the required.
