11 August 2009

Self-healing metal surfaces

Human skin can suffer small scratches and cuts and heal quickly, leaving no trace of a scar after just a few days.

It is a different matter with materials like metals where if the electroplated layer protecting the metals from corrosion suffers some type of scratch, rust protection goes out the door.

The nano-capsules in the electroplated layer contain a fluid. If the layer is scratched, the layers burst, and the fluid escapes and repairs the scratch.

Engineers are now working on transferring the self-healing effect of skin to materials. The idea behind this is to introduce evenly distributed fluid-filled capsules into the electroplated layer, like raisins in a cake. If the layer suffers damage, the pellets at the point of damage burst, the fluid runs out and ‘repairs’ the scratch. Until now, these plans have failed due to the size of the capsules. At 10 to 15 micrometers, the capsules were too large for the electroplated layer, which is around 20 micrometers thick. The capsules altered the mechanical properties of the layer.

Researchers from the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart, Germany, together with colleagues from Duisburg-Essen University, developed a process for producing electroplated layers with nano-capsules. At only a few hundred nanometers in diameter, researchers measure the capsules on another scale entirely.

“The challenge lies in not damaging the capsules when producing the electroplated layer,” said Dr. Martin Metzner, head of Department at IPA. “The smaller the capsules, the thinner and more sensitive their casing. The electrolytes used for these electroplated-technical processes are extremely aggressive chemically and can easily destroy the capsules.” The researchers therefore had to find a compatible material for the capsule casing depending on the electrolytes used.

Mechanical bearings are one example of possible applications; the materials of the bearings usually have an electroplated coating, in which they can embed the capsules. If there is a temporary shortage of lubricant, part of the bearing’s coating is lost, the capsules at the top of the layer burst and release lubricant. The bearing therefore does not suffer damage if it temporarily runs dry. The researchers produced the first copper, nickel, and zinc coatings with the new capsules, although surface coverage does not extend beyond the centimeter scale. Experts estimate it will be another one and a half to two years before they can coat whole components. In a further step, the team worked on more complex systems, involving differently filled capsules whose fluids react with one another like a two component adhesive.

For related information, go to www.isa.org/manufacturing_automation.