Three dimensional mechanical mapping of nano- and bio-composite materials using a combination of AFM and FIB

Abstract

Composites used in engineering applications are being made from smaller and smaller component phases; from macroscopic through micro- to nanometre length-scales. This trend has led man-made composites to mimic the length-scales and structures that biocomposites have developed in nature through millions of years of evolution. Biocomposites have evolved to perform specific engineering functions, such as piercing and grinding with wear resistance, impact resistance, and compressive load bearing. This work aims to map the 2D and 3D mechanical distribution in composite structures with a broad variation in localised stiffness in order to better understand the links between short and long scale mechanical performance. The approach undertaken in this work is to explore the use of Atomic Force Microscopy (AFM) to probe the mechanical performance of a specimen surface. The AFM can build a 2D map of the variation in contact stiffness at a resolution of the order of nanometres. Focussed Ion Beam (FIB) is used to precisely mill flat surfaces into specimens, which allows for the 2D mechanical mapping to be conducted on a flat plane. Quantitative AFM phase imaging is exploited to evaluate the elastic modulus variations on a simple FIB prepared nano-layered polymer composite surface, with variations in the elastic modulus exhibiting a bimodal distribution that is representative of the two polymers in the composite. Further demonstration of quantitative AFM phase imaging is made by evaluating the layered nanocomposite structure of snail shell. Extension of 2D nanomechanical imaging to 3D is presented by using FIB followed by subsequent AFM imaging to provide a stack of 2D AFM phase information that is reconstructed into a 3D map. This work therefore provides methods to systematically translate AFM phase information into quantitative elastic modulus measurements in 2D and 3D using FIB preparation, with examples of engineering and biological nanocomposites highlighting broad applicability.