Factors Affecting Frictional Properties of Plated Bolts

Q: What are the factors affecting the frictional properties of electroplated bolts?

A: Electroplated bolts typically have a triple-layer structure. The first layer, the bottommost, is the electrodeposit of zinc or zinc alloy, which provides good corrosion protection to underlying steel substrates. The second layer is the passivate, which prevents the early onset of “white rust.” The third layer is the topcoat or sealer, which stabilizes the passivate layer, enhances the corrosion resistance and modifies the surface.

The most widely used method to tighten a bolted joint is the torque controlling method. In this method, friction plays a critical role in controlling the torque-tension relationship of a bolted joint. It is critical to control friction in a certain range. Most automotive companies specify an acceptable range of coefficient of friction (CoF). General Motors and Tesla, for example, specify that the CoF of fasteners be within the range of 0.10 to 0.16.

To better control the friction in a bolted joint, it is essential to understand all the factors affecting it. One of the important factors is lubricants, which can significantly reduce the friction. The material of the washer plays a significant role, too. For example, when electroplated steel bolts are used, the aluminum washer will yield a dramatically higher CoF than the steel washer. The hardness difference between the bolt and washer causes increased plowing and/or microcutting of asperities of the hard bolt surface into the soft aluminum washer surface, increasing the real contact area and resistance to relative motion.

Another critical factor is the topcoat. The type, temperature and concentration of a topcoat will all greatly affect the friction. The CoFs of a bolted joint without a topcoat are high (0.27 to 0.41) and thus not acceptable for most automotive specifications. When a topcoat is applied, CoFs can be reduced to as low as 0.06. The topcoat can also reduce the scatter of the CoF. Generally, increasing the concentration or temperature of the topcoat will decrease the CoF. One can control the CoF range by selecting a proper topcoat and adjusting its concentration and temperature. 

Different topcoats have different strengths in reducing the friction depending on the type of wax used in the topcoat. After the topcoat is applied and dried, water in the topcoat evaporates and the topcoat layer becomes a dry self-lubricating film, which is composed of nanoparticles of organic polymer binder, wax and some additives. The mechanisms of wax behavior in topcoats are complex. One is that the wax migrates to the surface of the coating and forms a continuous film, providing a protective layer with properties of lubrication and abrasion resistance. Another is that the wax in the coating exists as discreet and independent nanoparticles on the surface. The wax particles protrude from the surface of the coating and become the first contact point with the contacting surface and act as spacers between two surfaces.

When the CoF needs to be more strictly controlled, it is advisable to consider some less pronounced factors. A black passivate usually gives a slightly lower CoF than a clear passivate. When zinc-plated steel washers and nuts are used, zinc-nickel alloy electrodeposits show slightly higher CoF than pure zinc electrodeposits. However, zinc-nickel alloy electrodeposits show slightly lower CoF than pure zinc electrodeposits.

It is worth noting that some topcoats are designed to increase the CoF for special applications. Inorganic topcoats usually give high CoFs. The plating layer thickness (with the range of 5 to 12 microns), the passivate dipping time, pH, concentration and temperature, and the topcoat dipping time and pH are expected to not affect the CoF. The minimum effect of the passivate layer may be due to the fact that it is the thinnest in the triple-layer structure, usually less than 500 nm.