A thin, uniform coating on long segments of monofilament could drastically improve the functionality of many complex fibers. A length of fishing line, microtubing, or polylactic acid (PLA) coated with copper could be left to cure within an epoxy, and upon removal of the monofilament, a narrow channel with a thin outer wall of copper would remain. That channel would be open for fluid flow, and also have a conductive shell. The “vascularized” material could be used for thermal management or self-healing composites.

Coated (top) and uncoated (bottom) 1.4-mm diameter monofilament.
Two apparatuses were created to coat filaments using the magnetron sputter deposition (MSD) process that applies relatively uniform, nanometer-thick coatings. The first apparatus simultaneously rotated four monofilaments about their own axes. The monofilaments were attached to two sets of rotating axles. In the driven set, one of the axles was elongated and coupled to an external motor via a rotary vacuum feed-through. Each of the four driven axles was directly connected to its neighbor by nylon gearing. In the free set of axles, there were neither gears nor couplings, and each axle was free to rotate independently.

As the motor turned, the rotation was transferred to each of the four driven axles, which rotated each monofilament. The torsion in each monofilament was released as each opposing axle rotated freely. In the experiment, the distance between the two sets of axles was 5.25”, though that distance could be modified up to 8.25”.

The second apparatus was designed to rotate and translate a single monofilament. The added functionality of translating the monofilament allowed for longer sections of monofilament to be coated. With the second apparatus, each point on the surface of the monofilament followed a spiral path about the longitudinal axis of the monofilament. Two independent motors, each attached to a rotary vacuum feed-through, drove the apparatus. Each motor was coupled to a 0.625” wide roller of Buna-N polymer. The roller surfaces were separated by 0.04” and the axes of the wheels were offset by 90°.

Aluminum faceplates guided the monofilament between the rollers, constraining the monofilament axis to a 45° angle with respect to each roller axis. As the monofilament passed between the two rollers, it was both rotated and translated along its longitudinal axis. Both motors were driven at similar speeds, measured in revolutions per minute (RPM). After passing under the MSD source, the monofilament passed through a small hole in an aluminum plate placed 7” from the aluminum faceplate attached to the rollers.

Though many methods exist to measure the rate of coating thickness growth on a flat surface, no procedure was found to explicitly measure the coating thickness of a cylinder. The simplest estimation was found by comparing the surface area of a flat rectangle and a cylinder. The resistivity of copper is 18 orders of magnitude smaller than the resistivity of the monofilament material. Therefore, practically all of the current flows through the copper coating and the area through which electric current passes can be defined as the cross-sectional area of the coating.

To test uniformity and coating thicknesses, a set of simple experiments was performed on 6” long monofilament segments. Three types of monofilaments were used: a commercially available nylon fishing line with a diameter of 0.028”, a larger complex polycarbonate fiber with a diameter of 0.06”, and a PLA monofilament with a diameter of 0.02”. Apparatus 1 was used in this experiment set, coating two monofilament segments at a time. Copper coatings were applied using 120–150 W of direct current (DC) power for time periods of 2–4 minutes. In each test, the segments were rotated at 3–4.5 RPM. The segments were mounted 6” below the deposition source.

Experiment 1 yielded five measurable data points. Copper coatings were successfully applied to three types of filaments, each with a distinct material and diameter. Each coated monofilament was conductive and completely coated. The data collected in experiment set 2 demonstrate a definitive positive correlation between the rotation rate of the drive wheels and the resistance measured through the coating of each monofilament. After concluding experiment set 1, very few conclusions could be made with respect to the coating growth rate on a rotating monofilament. However, it was conclusive that coatings applied were conductive and the resistance of each coating could be easily measured with the use of Ag paint and an LCR meter.

The results of experiment set 2 support the hypothesis that the slower a monofilament is fed through the MSD process, the less resistive the coating will be. The lower resistance corresponds to the presence of a thicker coating of copper. The consistent conductivity of each coating suggests a uniform coating process with thorough coverage.

These two experiments provide consistent data to suggest proof-of-concept and possibilities for future research. Either device may be used to coat monofilaments with the MSD process.

This work was done by William G. Pritchett, Daniel M. Baechle, and Eric D. Wetzel of the Air Force Research Laboratory. ARL-0169

Aerospace & Defense Technology Magazine

This article first appeared in the December, 2014 issue of Aerospace & Defense Technology Magazine.

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