Nanohardness of Sintered and Shock Deformed Alumina
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ALUMINA is one of the most important structural ceramics suitable for applications involving both highload and high–strain rate impact events.[1–27] All of such applications involve the measurement of hardness[1–9] at macro-, micro-, or nanoscale. Most of these measurements,[6–9] barring a few,[5] are on fine-grain and submicrometer grain-sized alumina ceramics. The other major problem with most of the hardness data of alumina reported as a function of load is that it exhibits an indentation size effect,[10,11] thereby making it difficult to provide unique hardness data of alumina for design purposes. The indentation size effect (ISE) is usually manifested as a decrease in macro-, micro-, and nanohardness with an increase in the indentation load and depth. It is well known that a wide variety of materials (e.g., metallic, ceramic, and quasi-crystalline)[10–13] exhibit ISE. Although numerous explanations have been proposed,[14–25] the basic cause of ISE in structural ceramics (e.g., polycrystalline alumina) is yet to be RIYA CHAKRABORTY, Project Fellow, ASHOK K. MANDAL, Technical Officer, and ANOOP K. MUKHOPADHYAY, SANDIP BYSAKH, and SAMPAD K. BISWAS, Scientists, are with the Central Glass and Ceramic Research Institute, Council of Scientific and Industrial Research, Kolkata 700032, India. Contact e-mail: [email protected] ARJUN DEY, formerly Senior Research Fellow, with the Central Glass and Ceramic Research Institute, Council of Scientific and Industrial Research, is now a Scientist with the Thermal System Group, ISRO Satellite Centre (ISAC), Indian Space Research Organisation, Department of Space, Government of India, Bangalore 560017, India. KESHAW D. JOSHI, AMIT RAV, and SATISH C. GUPTA, Scientists, are with the Applied Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India. Riya Chakraborty and Arjun Dey contributed equally to this research work. Manuscript submitted February 28, 2011. Article published online October 15, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
unequivocally established[12] because all of such explanations[14–22] or models[23–25] for ISE have their own limitations.[13,26] Thus, the basic objective of the present work was to use the nanoindentation technique to evaluate nanohardness of shock-recovered fragments of a coarse-grain (10 lm), high-density (3.978 gm cc–1) alumina obtained after a carefully conducted flyer-plate shock experiment,[27] and to examine if the nanohardness was similar to or degraded in comparison with that of the as-sintered alumina ceramics. As a result of this effort, we report for the first time a new explanation for the presence of strong ISE in shock-recovered alumina fragments obtained from an earlier study.[27]
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MATERIALS AND METHODS
The samples used in the present work were alumina discs prepared by pressureless sintering at 1583 K (1310 C) in air (dia ~48 mm, thickness ~2.5 mm, density ~3.978 gm cc–1, and grain size ~10 lm). The commercially available 99.99 pct pure a-alumina powder (Morimura Bros. Inc., Tokyo, Japan), with an averag
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