Microstructure and physicomechanical properties of pressureless sintered multiwalled carbon nanotube/alumina nanocomposites Soumya Sarkar, Probal Kr. Das * Non-oxide Ceramics and Composites Division, Central Glass and Ceramic Research Institute (A CSIR Laboratory), Kolkata 700032, India Received 18 April 2011; received in revised form 12 July 2011; accepted 12 July 2011 Available online 23rd July 2011 Abstract Multiwalled carbon nanotube (MWCNT)/alumina (Al 2 O 3 ) nanocomposites containing CNT from 0.15 vol.% to 2.4 vol.% have been successfully fabricated by simple wet mixing of as-received commercial precursors followed by pressureless sintering. Extent of densification of nanocomposites sintered at low temperature (e.g. 1500 8C) was <90%, but increased up to 99% when sintered at 1700 8C and offered superior performance compared to pure Al 2 O 3 . Nanocomposites containing 0.3 vol.% MWCNT and sintered at 1700 8C for 2 h in Argon led to 23% and 34% improvement in hardness and fracture toughness, respectively, than monolithic Al 2 O 3 . In addition, the highest improvement (20%) in bending strength was obtained for 0.15 vol.% MWCNT/Al 2 O 3 nanocomposite compared to pure Al 2 O 3 . Weibull analysis indicated reliability of nanocomposites increased up to 0.3 vol.% MWCNT, whereas, beyond that loading consistency was the same as obtained for pure Al 2 O 3 . Detailed microstructure and fractographic analysis were performed to assess structure-property relationship of present nanocomposites. # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Sintering; B. Electron microscopy; B. Nanocomposites; C. Mechanical properties 1. Introduction Carbon nanotubes having outstanding physicomechanical properties are presently considered as next generation reinforcing phase in polymers, metals and ceramics [1–5]. However, fabrication of a useful CNT reinforced nanocompo- site also requires homogeneous filler dispersion, proper adhesion between filler and matrix and optimum interface properties [6,7]. Although, reports are available on CNT/ polymer and CNT/metal nanocomposites fulfilling above mentioned features [2,3], for making structural CNT/ceramic nanocomposites particularly using Al 2 O 3 which is the most ancient, widely available and cost-effective structural ceramic, major limitations are chemical incompatibility between these phases due to dissimilar bonding character and high clustering tendency of fibriform CNTs due to strong Van der Waals attractive forces and hydrophobicity [7,8]. Instead of con- tributing in property improvement, such agglomerated CNTs severely act as strength limiting defects in nanocomposites [9,10]. In literature, although, a variety of techniques have been proposed to improve dispersibility of CNTs in Al 2 O 3 to enhance interface properties [6–20], these routes involved additional stringent steps, thus, limit easy processing and wider use of such nanocomposites. In addition, spark plasma sintering (SPS) [7–13] and hot-pressing (HP) [14–19] are the two techniques that have been commonly employed to densify CNT/Al 2 O 3 nanocomposites. However, due to high cost involvement, incapability of continuous production and limited product shapes and sizes, SPS and HP are practically inappropriate for fabricating nanocomposites of various geometries having satisfactory performance at affordable price [20,21]. More surprisingly, even after employing above techniques, fabrication of CNT/Al 2 O 3 nanocomposites with optimum CNT dispersion and maximum interface performance is still not achieved and significant divergences in available mechanical property data of CNT/Al 2 O 3 nanocomposites have been noticed [6–19]. Zhang et al. [20] have recently prepared MWCNT/Al 2 O 3 nanocomposites by pressureless sintering considering its commercial viability towards manufacturing of complex geometries to near net-shape using typical ceramic powder processing techniques. However, the initial precursor www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 38 (2012) 423–432 * Corresponding author. Tel.: +91 33 2473 3469/76/77/96; fax: +91 33 2473 0957. E-mail address: probal@cgcri.res.in (P.Kr. Das). 0272-8842/$36.00 # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2011.07.023