Small Deformable Ceramics

Unlike the general belief that ceramics are hard and brittle, they become very deformable when cut into nano-sized pieces. Dahye Shin and Dongchan Jang in the NQe department discovered the physical origin of this unusual flexibility in ceramics and demonstrated that it can be used to tune the mechanical responses of otherwise brittle materials.

Article | Spring 2021

Since their first use in the Stone Age, the nonmetallic inorganic materials, i.e., ceramics, have been considered to be hard, brittle, and rigid. This is because the threshold for fracture is much lower than that for deformation in most ceramics, so materials shatter into small pieces before they begin to change the extrinsic shapes. Such brittle fracture is frequently observed in our everyday lives, e.g., breakage of window glasses, windshields, potteries, or kitchen bowls. From these experiences, one can observe the following two points: i) if there is a deep scratch, e.g., inscribed by a glass cutter, ceramics break easily (consider how you split a bar of cold chocolate with a grooved pattern), and ii) after cracking, the total surface area increases by the amount of the fracture surfaces. In scientific language, one can say that the pre-existing crack accelerates the fracture, while the extra energy from the newly-formed crack surface imposes the resistance. The breakage of ceramics occurs when the former prevails over the latter under a given force. From the theory of classical fracture mechanics, Dr. Dahye Shin and Prof. Dongchan Jang in the NQe department recognize that the crack driving force scales with the volume of the material, but crack resistance is proportional to the surface area. Consequently, for tiny pieces having a substantial surface-to-volume ratio, the surface effects, such as the crack resistance, dominate, and the brittle ceramics become hardly breakable, as visualized by the electron micrograph in Figure 1.

Starting from this simple scientific deduction, Dr. Shin and Prof. Jang came up with an idea to control the mechanical response of an object by forming the nano-scaled structures capable of imposing exceptional deformability on the base material, which is otherwise brittle. For example, the 2D nano-patterns composed of the trapezoidal walls shown in Figure 2 behave differently under the frictional loading condition, i.e., dragging laterally across the walls while pushing them downward. Especially in geometries that generate relatively large shear forces, i.e., those with a lower aspect ratio and larger taper angle, more plastic deformation takes place (see Figure 3), resulting in the lowering of the frictional resistance. This discovery suggests that the shape and size of nano-scaled features can serve as an additional design parameter with which to tune the tribological performances of the surface, complementing the conventional ones such as the contact area or chemical adhesion. The non-traditional inter-correlation between the materials’ properties and geometric traits offers additional opportunities to expand the range of attainable properties over the currently-available ones, e.g., the deformable ceramics demonstrated by Dr. Shin and Prof. Jang in this work.

 

 

 

 
Reprinted with permission from ” Shin, D., Kang, D. G., Woo, K. Y., Cho, Y.-H., Han, S. M., & Jang, D. (2021). Friction Control by Deformation Mode in Nanopatterned Amorphous Carbon. Nano Letters , 21 (1), 107–113. Copyright 2021 American Chemical Society ”