Carbon nanotubes (CNTs) have been proposed for reducing fretting wear in connectors used in high-vibration environments like vehicles. They can also be used to make contacts with lower contact resistance and higher corrosion resistance compared with silver (Ag) plating, and they have been used to reduce the weight in some shielded cables by 25% or more for use in weight-sensitive applications like satellites, aircraft, and soldier-worn equipment.

Wear reduction
Fretting wear can be a challenge in any application subject to vibrations, including vehicles and industrial systems. It wears away contact material and can trap debris in the contacts, increasing contact resistance.

Lubrication of the contact surfaces can reduce fretting wear. It’s not usually an option in electrical connectors since available liquid lubricants have relatively high resistivity and increase the overall electrical contact resistance (ECR). Other options include greases and solid lubricants like graphite, both of which suffer from drawbacks of their own and may reduce wear but increase ECR.

Lubricants like carbon black and CNTs based on carbon nanoparticles (CNPs) have been investigated as a possible tool for reducing fretting wear without an accompanying increase in ECR. CNTs have the added benefit of providing superior oxidation protection. In one case, a series of CNT coatings with different thicknesses were tested for their effectiveness in reducing fretting wear.

As expected, thicker coatings demonstrated improved wear performance. As a surprise result, the thicker coatings also improved the ECR compared with bare copper. Under extended fretting cycles (up to 500,000 cycles), the CNT coatings were displaced, somewhat reducing the wear protection, but then stabilized and demonstrated reduced ECR compared with uncoated contacts. CNT-coated contacts can also withstand higher loads during fretting without experiencing excessive wear.

Corrosion resistance
In another series of tests, a silver (Ag) CNT composite film produced a lower ECR compared to a pure Ag plating after a hydrogen sulfide (H2S) gas corrosion test. Ag tarnishing or sulfidation is a common cause of corrosion and reduced performance in Ag-plated connector contacts. The Ag/CNT film was plated on the contacts using a non-cyanide plating process. It may provide a future alternative to current practices where Ag is plated onto electrical contacts using a cyanide plating system.

The pure Ag film had the lowest ECR before exposure to H2S. The better performance of the pure Ag plating was especially noticeable at lower contact loading forces. Before exposure to H2S, the Ag/CNT composite film had slightly higher ECR compared with pure Ag. However, after exposure to H2S, the Ag/CNT composite showed much lower ECR compared to pure Ag plating (Figure 1).

Light weight shielding
Lighter weight shielded cables using CNT shields can result in large weight savings in platforms like satellites where each pound of weight costs up to $50,000 to put into orbit. Unmanned aerial vehicles (UAVs) like drones can experience weight reductions of 300 pounds or more using CNT-based shielded cabling. In addition, weight reductions can extend the operational capabilities of manned aircraft, ground vehicles and even soldiers carrying lower weight equipment.

CNT-based shields have relatively high resistivity that results in lower shielding performance below about 100 MHz. They are also unable to provide protection against lightning. As a result, CNT-based shields are being developed for use in double-braided cables commonly found in aerospace systems. A conventional metal braid that provides low-frequency shielding and lighting protection is combined with a CNT tape shield that provides high-frequency shielding. The CNT foil weighs much less than the metal braid it replaces, and the combined system weighs about 25% less than a conventional shielding system using two metal braid shields (Figure 2).

Hybrid CNT/metal shielded cables are being developed for MIL-STD-1553B and IEEE 1394 applications. Improving the manufacturing processes for producing the CNT-shielded cables is still a hurdle to overcome. Current processes are not suited for high-volume manufacturing and suffer from limited yields.

Summary
CNTs are expected to improve the performance of interconnects in several ways. CNT plating on contacts has been demonstrated to increase contact resistance to fretting wear and reduce the effect of sulfidation corrosion on Ag contacts. Hybrid CNT/metal shielded cables are being developed to reduce the weight of shielded cables in aerospace and other applications.

References
Carbon nanotubes in interconnects, Wikipedia
Carbon Nanotube Technology Promise a Revolution in Cabling, TE Connectivity
Recent Advances in Metal/Alloy Nano Coatings for Carbon Nanotubes Based on Electroless Plating, Frontiers in Chemistry
Superior electrical contact characteristics of Ag/CNT composite films formed in a cyanide-free plating bath and tested against corrosion by H2S gas, Materials Letters
Wear Reduction via CNT Coatings in Electrical Contacts Subjected to Fretting, Tribology Letters