Metals Possess Self-Healing Capacity When Subjected to Fatigue Damage Could Be Due To 'Cold Welding'

Humans and plants can heal themselves, but surprisingly, metals possess the same skill. Scientists were baffled when they observed the same nature in platinum.

Self-Healing Metal From Fatigue Damage

In July, a study found that platinum can heal itself when subjected to a kind of strain called fatigue damage - repeated stress of motion resulting in microscopic breaks, eventually breaking the metal's structure.

The self-healing ability was observed at ultra-small scales in a 40-nanometer-thick piece of platinum suspended in a vacuum after 40 minutes of observation. The researchers were surprised when they noticed the platinum fused back to mend itself.

Materials scientist Brad Boyce from Sandia National Laboratories said they were "absolutely stunning to watch first-hand." They concluded that metals have an "intrinsic natural ability to heal themselves at least in the scale of fatigue damage."

The researchers at the time were baffled by what they observed and couldn't explain why it happened. However, an update revealed that one probable explanation is the phenomenon of cold welding, which happens at room temperature anytime two metal surfaces are near enough to one another for the atoms in each to entangle.

Thin air layers or other impurities usually obstruct the process; in vacuum spaces, for example, pure metals can be pushed so close to one another that they practically stick. Since the metal stuck together, the cracks disappeared, and the material seemed to heal.

What Is Cold Welding?

Cold welding or contact welding is a solid-state welding method that fuses two or more metals with little to no heat. Pressure is the energy that is utilized to create a weld instead. As can be observed in other techniques like arc welding, friction welding, or laser welding, no liquid or molten phase is present in the joint during the cold welding process, in contrast to fusion welding processes.

The oxide coatings on the surfaces of the materials must be removed before two or more metals can be joined by cold welding. Under typical circumstances, most metals have an oxide layer on their surface that is a barrier to stop the metal atoms from joining. Metallurgical bonds can be formed by pressing the metals together under intense pressure after removing this oxide layer. Other chemical or mechanical methods, such as wire brushing or degreasing, can remove the oxide coating.

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Nanoscale cold welding is also possible; experiments have demonstrated the ability to mechanically unite single-crystalline ultrathin gold nanowires (diameters less than 10 nm) in seconds. With the same crystal alignment, electrical conductivity, and strength as the remaining nanowire, the outcomes have been demonstrated to be almost flawless. The nanoscale sample size, mechanically aided surface diffusion, and orientated attachment processes are responsible for this excellent quality welding. It has been proven that gold and silver can be joined via nanoscale cold welding.

According to Richard Feynman, atoms "know" when they are not on the same section when other atoms are present in oxides, greases, and the more intricate thin surface layers of impurities in between.

Although cold welding has a far longer history, it was initially recognized as a method of joining metals without heat in the 1940s. This procedure is also referred to as cold-pressure welding. This method has several industrial applications and was frequently used to link wires and two metals together in space.