Nanoindentation characterizes mechanical behavior in layered materials

In this podcast episode, MRS Bulletin’s Sophia Chen interviews Henry Quansah Afful, a postdoctoral research associate at the University of Illinois at Urbana-Champaign about a general framework for understanding how layered materials respond to stress. Afful is interested in the threshold conditions where the material goes from plastic deformation to fracture. Using nanoindentation, his group could learn, at the atomic scale, when plastic deformation starts and how various temperatures affect the mechanical behavior of layered materials. This work was published in a recent issue of Communications Materials.

Transcript

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Sophia Chen. 

Look around your home, take the wood in your sofa, or the glass in your windows. These conventional materials are useful for their bulk properties and 3-dimensional structure. But for emerging technologies such as flexible electronics, such as a tiny sensor implant for monitoring chronic illness, or a foldable smartphone, researchers are developing materials with properties on a smaller, flatter, scale. For these applications, they are developing materials made of thin layers, such as graphene. The layers stick together via Van Der Waals forces. To develop a layered material for a particular application, it’s important to measure its mechanical properties, such as how easily it bends and breaks. But layered materials are distinctively complicated to study. Henry Quansah Afful, a postdoctoral researcher at the University of Illinois at Urbana-Champaign explains.

HENRY QUANSAH AFFUL: Using layered materials are very challenging because you have so many mechanisms that can occur at the same time.

SOPHIA CHEN: As you apply force to the layered material, it might stretch or crack due to many different mechanisms, such as the layers separating, or atoms rotating or being pushed to form structures known as kink bands. In new work, Afful’s team demonstrate a way to characterize the mechanical properties of layered materials. The technique can elucidate some of the mechanisms behind the material’s deformation. They use a particular technique known as nanoindentation.

HENRY QUANSAH AFFUL: You are almost like driving a nail into the material, and then just getting like the mechanical fingerprints of the material. The proverbial nail here is like it's a diamond tip, and then the hammer is just like, you know, just some fixture that we are using to drive some currents to move the diamond tip into the material.

SOPHIA CHEN: This technique characterizes the material’s properties at localized spots on the sample. This is in contrast to conventional mechanical tests, where you compress or stretch a bulk sample. This localized characterization makes nanoindentation an efficient characterization technique.

HENRY QUANSAH AFFUL: Compared to the bulk test, is that you don't need to make so many samples to get the mechanical information, you just need one sample, and you can run like hundreds of tests on them. Because it's very localized, you can get more information about when, like, various, like, deformation events start on the atomic scale.

SOPHIA CHEN: When you drive a sharp tip into a layered material, it first starts to undergo an elastic deformation. This means that if you stop driving it, the material will revert back to its original shape. But drive that tip in past a certain point, and the material begins to deform permanently in what’s known as plastic deformation. Drive it in some more, and it fractures. Afful was interested in the threshold conditions where the material goes from elastic deformation to plastic deformation to fracture.

HENRY QUANSAH AFFUL: Under which conditions will it fracture, right? And does fracture get easier as we increase temperature? So, those are some of the questions that we were trying to answer, and apart from that, we were also looking at, like, how hard the material is.

SOPHIA CHEN: His team studied a naturally occurring layered material called muscovite as a model material. Muscovite is abundant in Earth’s crust.

HENRY QUANSAH AFFUL: The reason we chose it instead of a material like say graphene is that it's not as expensive, it's very abundant in the X crust, and so easily attainable. It has all of the characteristics representative of other layer materials, but you can acquire them very easily as well.

SOPHIA CHEN: When they drove the diamond tip into the muscovite, at certain points the tip would go into the material suddenly, which they referred to as a “pop-in.” When the tip would go in suddenly more than 5 nanometers, that was a sign of fracture rather than plastic deformation. They also studied how the muscovite’s response changed under different temperature conditions.

HENRY QUANSAH AFFUL: As we are increasing temperature, the material easily fractures. It's not as resistant to fracturing,

SOPHIA CHEN: The point of the study was not to study muscovite specifically, but to present a general framework for identifying how layered materials deform and fail in response to stress. Studying the fracturing of layered materials has applications beyond flexible electronics. It’s also useful for understanding Earth’s crust, which contains layered materials. This type of mechanical information is important in applications such as carbon sequestration or hydraulic fracturing, where fluids are forced into Earth’s crust. 

This work was published in a recent issue of Communications Materials. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.