ABSTRACT
Natural biological materials have developed hierarchical and heterogeneous composite structures over millions of years of evolution in order to sustain the mechanical loads experienced in their specific environments. To perform diverse mechanical, biological, and chemical functions, these nanobiohybrid composite materials, e.g., tooth, bone, scale, shell, horn [1], etc., provide a varied arrangement of material structures at various length scales that work in concert. The science behind these structures and correlation with the property are yet to be well understood [1-10]. In this connection, it is now well known that the tooth is, in practicality, a ceramic-organic nanohybrid composite comprised of a hard enamel, the more ductile dentin, and a soft connective tissue, the dental pulp. Further, enamel is the hardest and the most highly mineralized structure in the human body, with approximately 96 wt% HAp being the primary mineral. The enamel and dentin regions of the tooth are interlocked by an irregular interface called the dentin-enamel junction (DEJ). Furthermore, the tooth has a hierarchical architecture spanning from macrostructure to microstructure to nanostructure [1]. The microstructure of the enamel reveals closely packed enamel rods or prisms encapsulated by an organic protein called enamel sheath. The prisms or rods consist of nanostructured inorganic HAp crystals, the nanocrystals being oriented at various angles inside the rod. The macrostructural properties are often determined by the respective micro-and/or nanostructures. Further, HAp is a brittle ceramic and, hence, is prone to suffer from contact damage. Therefore, in this chapter we aim to study the nanohardness of enamel at each region, starting from the DEJ to the end of the outer enamel region, by the nanoindentation technique [2]. We shall try to correlate the nanomechanical property with its microstructural and compositional properties.
