Electroless Nickel Plating Surface Finish Options
This information explains that the finish from an electroless nickel process is not a “one size fits all” solution. This information explains how finish varies with respect to chemistry, surface preparation and phosphorus content.
What Controls the Finish
The finish depends primarily on the phosphorus content of the deposit. Thus, low-phosphorus coatings (1-4% P) are bright and crystalline. Medium-phosphorus coatings (5-9% P) give a semi-bright appearance. High-phosphorus coatings (10-13% P) are dull and amorphous. Other parameters such as bath temperature, additives, and pre-plating surface cleaning affect the appearance of the deposit.
Bright and Mirror Finishes
Low-phosphorus plating is typically used for optical components, decorative parts and for contact with food. It is a hard coating that can be used for wear resistance. A very smooth surface is expected and the coating is of the highest quality.
Semi-Bright for Engineering Parts
Engineering parts, wear parts, e.g. shafts, moulds, hydraulic parts etc. are typically made with a mid-phosphorus content. This provides a clean looking surface without having a mirror-like finish. The coating has a good balance of hardness and corrosion protection.
Matte Finishes and Adhesion
High-phosphorus coatings (e.g. up to 13% P) will come out of the bath as a dull grey, flat, matt finish. This surface is of coarse roughness, and is very suitable for coatings which are to be painted, bonded to rubber etc.. Adhesives and paints will have a strong hold on a dull surface such as this. For parts to be used as instrument cases, or indeed as components of a military nature, a low-reflectivity surface can be of great benefit, preventing glare in a hostile environment.
Post-Plate Treatments
Although subsequent heat treatment can increase the hardness of an ENP coating to above 700 HV (e.g. 740 HV for a 9% P coating after heating to 350°C/665°F) it tends to produce a dull surface, unless subsequently polished. As a result, measurement of the resulting coating to agreed surface coating measurement standards for surface roughness and coating thickness is necessary to establish that the resulting coating meets the specified requirements.
A proper finish right from the start will save a lot of time and prevent rework.
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