Orientational Effect in Nanohardness of Functionally Graded Microstructure in Enamel
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Orientational Effect in Nanohardness of Functionally Graded Microstructure in Enamel Nilormi Biswas • Arjun Dey • Saugata Kundu • Himel Chakraborty • Anoop K. Mukhopadhyay
Received: 10 July 2012 / Accepted: 31 August 2012 / Published online: 23 October 2012 Ó The Institution of Engineers (India) 2012
Abstract Nanoindentation experiment was conducted on the enamel surface of extracted premolar tooth. The nanohardness constantly increased as the distance from a region close to the enamel dentine junction to the outer enamel zone was traversed. The plastic energy dissipated during the nanoindentation process also continuously increased as measurements were taken from the outer enamel zone to the inner enamel zone. The percentages of calcium and phosphorous measured by energy dispersive X-ray technique decreased from the outer enamel zone to the inner enamel zone. Further, local variation of nanohardness was explained in terms of different orientations of enamel rods as experimentally observed by the scanning probe microscopy. Keywords Enamel Nanoindentation Nanohardness Energy Microstructure
Present Address: N. Biswas A. K. Mukhopadhyay (&) CSIR-Central Glass and Ceramic Research Institute, 196, Raja S.C. Mullick Road, Kolkata 700032, India e-mail: [email protected]; [email protected] S. Kundu H. Chakraborty School of Materials Science and Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India Present Address: A. Dey Thermal Systems Group, ISRO Satellite Centre, Vimanapura, Post, Bangalore 560017, India
Introduction Natural nano-bio-hybrid composite materials e.g., teeth, bone, scale, shell, horn [1–9] etc. provide a varied arrangement of material structures at various length scales which work in concert to perform diverse mechanical, biological and chemical functions. These multi objective properties of the natural structures often lie in a functionally graded fashion either locally or in the entire body. The science behind those structures and correlation with the property are not yet fully understood. However, understanding the science would be certainly of tremendous technological and societal importance. For instance, it is well known now that teeth are practically ceramic-organic nano-hybrid composites comprising of hard enamel, the more ductile dentin and a soft connective tissue, the dental pulp. Further, enamel is the hardest structure in the human body with approximately 96 wt% hydroxyapatite (HAp). An irregular interface called enamel-dentin junction (EDJ) interlocks the two tissues namely, enamel and dentine. Furthermore, tooth has a hierarchical architecture (Fig. 1) e.g., from macrostructure to microstructure to nanostructure [1, 2]. The building block of enamel microstructure is closelypacked enamel prisms or rods encapsulated by an organic protein called enamel sheath. These prisms or rods consist of nanostructured inorganic HAp crystals with different orientations inside. The macro level properties are often determi
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