Achieving Coefficient of Friction Targets

Q: What topcoat should I use to obtain a CoF (coefficient of friction) of 0.12-0.16?

A. Some of the most common questions I get as a supplier of metal finishing chemicals from our customers involve achieving CoF targets. As a chemical supplier, there just isn’t a simple answer for that. While topcoats do play a major role in controlling the CoF, there are a lot of other testing details that we need to know before suggesting the right topcoat. Let’s dive in and look at all the different variables that will contribute to recommending one topcoat versus another.

The first variable is the plating deposit. Are the parts zinc or zinc-nickel plated? The difference between alkaline and chloride plating in either case is very marginal, but there is a large difference in CoF between zinc plated parts and zinc-nickel plated parts. This is because the deposits have very different surface hardness and roughness. With salt spray requirements being raised by several OEMs, Zn-Ni plating of fasteners is becoming more common.

The second important variable is the material of the test nuts and washers.  There are several different materials of the nuts and washers that are called for in different specifications. Zinc plated, unplated (steel), electroless nickel, e-coat, aluminum, and I am sure the list goes on. With all these different materials, you will see a significant difference in the CoF. An aluminum washer will often give a dramatically higher CoF than a steel washer due to its lower hardness. Below you can see the comparison studies done with zinc vs zinc-nickel, different topcoats, and different washer substrates.

Topcoat A 

Topcoat B

 

*µCoF is the total CoF

Another factor to consider would be if the test requires multiple drives of the same bolt. With multiple drives, the CoF tends to increase with each drive.  During the tightening process, the soft topcoat layer is quickly deformed by the forces of the tightening and all the three coating layers (zinc, passivate and topcoat) are damaged. Most topcoats can’t withstand this type of testing, so this is critical to know so we can suggest a topcoat and concentration that is able to hold the same CoF throughout the multiple drives.

So, before asking your supplier what topcoat will hit a specific CoF, make sure you have a copy of the specification you need to run to. This will contain all the pertinent details of the test that need to be known before making a proper recommendation. Some specifications even tell you a certain supplier to purchase the nuts, washers, and bolts from. Some also have a specific cleaning method listed for the nuts and washers. 

Having a specification on-hand will save significant time in your conversations about CoF targets and how to achieve them.