January 12, 2021
According to information provided by Virginia Polytechnic Institute and State University (Virginia Tech), researchers have now discovered the ‘how’ behind the hidden strengthening mechanism in biological ceramics.
Utilizing partial support provided by the Institute for Critical Technology and Applied Science at Virginia Tech and the Air Force Office of Scientific Research, Ling Li, an assistant professor in mechanical engineering at Virginia Tech, has developed new perspectives into building stronger and tougher ceramics by studying the shells of bivalve mollusks.
Looking beyond the shape and chemistry of the structure, Prof. Li focused on the capacity of the basic mineral building blocks in the shell to anticipate fractures.
Prof. Li’s team conducted an in-depth analysis of the microscopic structures of the shells of pen shell mollusks. The shells consist of two layers, an inner nacre layer and a brown-colored outer layer. The research team focused their attention to the outer layer, which is composed of prism-shaped calcite crystals arranged in a mosaic pattern. Between adjacent mineral crystals, very thin organic interfaces, approximately 0.5 micrometers, are present that glue the crystals together. The calcite crystals measure approximately half a millimeter in length and 50 micrometers in diameter, resembling elongated prisms.
In the study’s findings, published in the journal Nature Communications, the research group explained that unlike many geological or synthetic crystals, where the atoms within their crystalline grains are perfectly arranged in a periodic fashion, the calcite crystals in the pen shells contain many nanoscopic defects, primarily composed of organic substances.
“You can consider the biological ceramic, in this case the pen shells’ calcite crystals, as a composite structure, where many nanosized inclusions are distributed within its crystalline structure,” Prof. Li noted. “This is especially remarkable as the calcite crystal itself is still a single crystal.”
Normally, the presence of structural defects means a site of potential failure. Consequently, the normal approach is to minimize the structural discontinuities or stress concentrations in engineering structures. However, Prof. Li’s team has demonstrated that the size, spacing, geometry, orientation, and distribution of these nanoscale defects within the biomineral is highly controlled, improving not only the structural strength but also the damage tolerance through controlled cracking and fracture.
When these shells are subjected to an outside force, the crystal, aided by those internal nanoscopic defects, minimizes plastic yielding by impeding the dislocation motion, a common mode for plastic deformation in pure calcite. This strengthening mechanism has been applied in many structural metal alloys, such as aluminum alloy.
In addition to adding strength, this design allows the structure to use its crack patterns to minimize damage into the inner shell. The mosaic-like interlocking pattern of the calcite crystals in the prism layer further contains large-scale damage when the external force is spread across the individual crystals. The structure is able to crack to dissipate the external loading energy without failing.
“Clearly these nanoscopic defects are not a random structure, but instead, play a significant role in controlling the mechanical properties of this natural ceramic,” Prof. Li stated. “Through the mechanisms discovered in this study, the organism really turns the originally weak and brittle calcite to a strong and durable biological armor. We are now experimenting possible fabrication processing, such as 3D printing, to implement these strategies to develop ceramic composites with enhanced mechanical properties for structural applications.”