Understanding and Controlling Subcritical Crack Growth in Large Freestanding Metallic Nanosheets

Georgia Tech Research Corporation, Atlanta GA

Investigators

Abstract

Non-technical Abstract Thin metallic films have become a critical feature of many residential and industrial applications. They coat windows to keep our homes cool and energy efficient, enable the integrated circuits in electronic devices and appliances, allow for turbine engines to run more fuel efficiently, and are the basis of countless other products. However, their use is limited by the fact that the processing conditions for creating one film are often incompatible with those needed to make others, or even to deposit them on certain types of materials. This issue is problematic because thin films are extremely fragile, and thus far it has generally not been possible to remove them from one substrate and transfer them to another without tearing them apart. This research program will establish how to control crack growth in ultrathin films, so that they can be adhered to other materials after their production. This work will enable new materials to be fabricated and existing ones to be integrated to create a new generation of products and devices. These new scientific insights will be developed in concert with the education of graduate students, who will be prepared to use them to solve critical industrial and societal problems. Technical Abstract Ultrathin (< 100 nm) metal films have a variety of applications, but their use is limited by the fact that freestanding metallic films are prone to crack and tear. As a result, they generally cannot survive handling and assembly unless they are on substrates. However, if freestanding, ultrathin metal films could be fabricated and handled without damaging them, their nanoenabled optical, mechanical, and electrical properties could be integrated with materials that are usually considered incompatible. This new capability would lead to a richer use of the special properties of these films. This research program will enable the integration of these ultrathin metal films by using laser micromachining, small-scale mechanical testing systems, electron microscopy, and finite element modeling to establish the subcritical crack growth mechanisms of freestanding ultrathin Au films. A unique feature of this program is that large sheets (> 5 mm in-plane extent) are used to evaluate the anomalously large process zones that micrometer-scale specimens cannot capture. Experiments will be conducted on low force mechanical testing systems equipped with in situ optical systems to quantify deformation and damage accumulation. The relevant damage accumulation mechanisms will be identified using optical, scanning electron, and transmission electron microscopy. Guided by these insights, heat treatments and near-monolayer thickness metal layers will be used to engineer the tearing resistance of the ultrathin films. Furthermore, the development of thin sheet tearing crack tip parameters will enable fracture mechanics-based durability and life calculations. The resulting ability to engineer damage-tolerant freestanding metal films will allow engineers to create devices from virtually any metallic thin film material.