Engineered man-made composite (inhomogeneous) materials are well known for their superior structural properties. Man-made composite materials and multilayered systems are widely used in civilian and military applications. The combined multilayered systems are attractive because they have the characteristics of being energy absorbent, lightweight, high-strength, high-stiffness, and can provide good fatigue and corrosion resistance. Although the engineered composites are promising and offer mutual exclusive material properties that are not found in other structural materials, they are prone to delamination at the glued layered interface.
In contrast to man-made composites, most superior performing materials found in nature possess a hierarchal biomineralized composite structure that tends to be delamination resistant. These delaminate resistant biocomposite structures [e.g. alligator gar’s (Atractosteus Spatula) exoskeleton fish scale] have mechanical properties that vastly exceed the properties of their relatively weak constituents. The fish scale is made up of 90 percent hard (inorganic minerals) and 10 percent soft (polymer-like organic collagen fibers) by volume. Nature integrates hard and soft materials at different length scales to form a two-layered composite that better resists delamination.
The objective of this research was to use scanning electron microscopy (SEM) and nanoindentation to investigate the delamination resistant behavior occurring at the layered interface for the alligator gar fish scale composite. The SEM imagery showed at the micron level the collagen bundles + B-Ap crystals (C/B-Ap) form a distinctive two-layered system that is connected by what is described as sawtooth geometrically structured interface. The outermost layer for the exoskeleton fish scale is called ganoine while the inner layer called bone. The layers interface seems to be mainly bonded by mechanical means using sawtooth notches, rather than the chemicals adhesives used in the man-made laminated planar interfaced composites. The notched regions for ganoine+bone materials overlap and are embedded at various depths within each layer to form periodic “repeating” bonded connections.
The indentation measurements taken at the nano-level showed that elastic moduli have property gradients occurring through the interfacial transition zone. Noticeably the ganoine layer has elastic moduli ranging from [98–67] GPa while the bone layer elastic moduli ranged from [20–13] GPa. The research findings indicate the sawtooth connections perhaps provide enhanced shear resistance at the interface and may help inhibit debonding. Additionally, the notched interlocking provides a less discrete (graded) interface, which seems to promote durability and delamination resistance.
