The researchers, led by materials science and engineering professor Julia Greer, found that by adding a secondary material that readily melts to the surface of thin films, they could stop cracks from forming and propagating.
In a paper published in the June 15 issue of Nature Communications, Greer and her colleagues describe how adding an ultrathin layer of gallium to gold makes gold surfaces much more fracture resistant.
“This is about controlling material failure at the smallest scales,” said Greer, who also serves as director of the UC San Diego Materials Research Laboratory (MRL). “We use the melting behavior of gallium to inhibit crack nucleation, and because it’s a conformal layer, it works in different geometries and over a range of crack sizes.”
In most engineering materials, cracks start at defects and grow under load until the material breaks. According to Greer, this conventional picture of fracture is incomplete. She suggests that cracks nucleate not just from larger-scale defects, but also from smaller-scale surface roughness.
“Fracture has traditionally been thought of as happening at the microscale or larger,” Greer said. “But cracks are created by atomic-scale processes. We’re accounting for these processes, which are normally ignored.”
The researchers tested their hypothesis using thin gold films deposited on a glass substrate. The films were then subjected to tensile loading, and the team observed the fracture behavior of the films using electron microscopy.
They found that the gold films with the gallium layer exhibited significantly higher fracture toughness than the pure gold films. The gallium layer prevented the formation of cracks, even when the gold films were subjected to high tensile loads.
The team’s findings suggest that a material’s fracture toughness can be significantly improved by simply adding a layer of material that melts at a lower temperature than the material itself. This approach could be used to improve the reliability and durability of a wide range of materials and structures, from aircraft components to electronic devices.
“We’re talking about thin coatings — less than a millionth of a meter — but they have a profound impact on fracture behavior,” Greer said. “This insight has implications for manufacturing and materials design.”
In addition to Greer, the research team included MRL graduate students Xiaoyue Ma and Qiang Yu. The research was supported by the National Science Foundation and the Air Force Office of Scientific Research.
The aerospace industry currently uses rivets to join together sheets of metal in aircraft structures. However, the use of rivets creates stress concentrations, which can lead to cracks and eventual failure. The addition of a thin layer of gallium to the surfaces of these sheets could help to inhibit crack formation and improve the overall safety and reliability of aircraft structures.
Electronic devices are also susceptible to cracking, particularly at the nanoscale. The use of a gallium layer could help to prevent cracks from forming in these devices, improving their reliability and performance.
The discovery by Greer and her team has important implications for the aerospace and electronics industries, as well as other industries that rely on thin films. By adding a secondary material that readily melts to the surface of thin films, engineers can significantly improve the fracture toughness and reliability of these materials.
