Metal Healing

Metal Healing

Background

With their unique mix of strength, toughness, and electrical and thermal conductivity, metals have found use in numerous human creations. Although metallic tools have been made since about 4,200 B.C., high temperatures have been required to cast and heal them from fractures, which makes the healing process difficult and energy-intensive. This process of casting and healing metals has remained largely unchanged, with no successful demonstrations of room temperature healing of solid structural metals.

Most strategies for healing materials, including metals, derive from pioneering work on polymers, where healing matter (monomer and catalyst, for example) is stored throughout the material and locally used where fracture occurs. This same technique has proven impractical in solid metals because metal atoms diffuse very slowly at room temperature. Metals, therefore, are healed at temperatures near or above their melting points, which requires high temperatures and large amounts of energy (107 J to 109 J per 1 mm crack length for solute precipitation, for example). Strategies using low melting temperature alloys (102 to 103 J/mm), highly-localized joule heating (102 to 104 J/mm), and combined solute precipitation and phase transformation (106 to 107 J/mm) have been developed to reduce the healing energy input, but none have demonstrated effective room-temperature healing.

Biological structural materials, such as bone, on the other hand, can effectively heal near room temperature by transporting matter and energy (oxygen, nutrients, and cells) to and from areas where healing is needed. Bone is characterized by a porous structure which houses functional materials within the open pores, such as cells and blood vessels. These functional materials sense where fracture occurs and allow mass and energy transport to and from fracture locations.

From left to right: A strip of polymer-coated metal foam; a strip that has been cut in two; and a healed sample.

Capabilities

Inspired by how bone heals to recover its geometric integrity and mechanical strength, we use electrochemical transport of ions in polymer-coated porous metals to enable rapid, effective, and low-energy healing at room temperature. The cellular structure facilitates fast ion diffusion through an electrolyte. The polymer coating, which is insulating and chemically inert, restricts healing to fractured locations.

This healing technique enables not only full recovery of strength in cellular metals after fracture, but it also allows strengthening after limited damage (plastic deformation, for example) to prevent future fracture and extend service life.

Besides mechanical properties, this transport-mediated healing approach can also be used to recover other material properties, such as electrical and thermal conductivity. Combining this healing approach with advances in solid-state electrolytes, 3-D printing, self-healing polymers, and topology optimization can lead to useful applications in fields as varied as robotics and microfabrication.

When a bone breaks, nutrients and cells are transported to the site of the fracture through the bone's porous structure.
In this sample of polymer-coated cellular nickel, nickel ions are carried to the break through an electrolyte solution.

Challenges

  • Any self-healing metallic material or system using a liquid electrolyte must be designed to prevent electrolyte leakage. This may pose hurdles for potential biomedical applications due to the toxicity of the electrolyte.
  • The electrolyte may impose a mass penalty that can be problematic in weight-constrained systems.
  • A metallic material that heals autonomously has yet to be demonstrated, and it will likely require major advances in the performance and integration of functional materials.

References

  1. H. van Dijk; S. van der Zwaag. Self-Healing Phenomena in Metals. Advanced Materials Interfaces 2018, 5 (17). DOI: 10.1002/admi.201800226.
  2. J. Blaiszik; S. L. B. Kramer; S. C. Olugebefola; J. S. Moore; N. R. Sottos; S. R. White. Self-Healing Polymers and Composites. Annual Reviews of Materials Research 2010, 40, 179 – 211. DOI: 10.1146/annurev-matsci-070909-104532.
  3. T-.P. Huynh; P. Sonar; H. Haick. Advanced Materials for Use in Soft Self-healing Devices. Advanced Materials 2017, 29. DOI: 10.1002/adma.201604973.
  4. Hsain; J. H. Pikul. Low-energy Room-temperature Healing of Cellular Metals. Advanced Functional Materials 2019, in press.
  5. Nosonovsky; Ryoichi Amano; Jose M. Lucci; Pradeep K. Rohatgi. Physical Chemistry of Self-organization and Self-healing in Metals. Physical Chemistry Chemical Physics 2009, 11. DOI: 10.1039/B912433K.
  6. J. Markvicka; M. D. Bartlett; X. Huang; C. Majidi. An Autonomously Electrically Self-healing Liquid Metal–elastomer Composite for Robust Soft-matter Robotics and Electronics. Nature Materials 2018, 17, 618 – 624. DOI: 10.1038/s41563-018-0084-7.
A cut strip of metal foam is immersed in an electrolyte solution in preparation for healing.

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