Magnesia tuned multi-walled carbon nanotubes–reinforced alumina nanocomposites Iftikhar Ahmad a, ⁎, Mohammad Islam a , Mushtaq Ahmad Dar a , Fang Xu b , Syed Ismat Shah c , Yanqiu Zhu d a Center of Excellence for Research in Engineering Materials, Advanced Manufacturing Institute, King Saud University, P.O. Box. 800, Riyadh 11421, Saudi Arabia b Division of Materials, Mechanics and Structure, Faculty of Engineering, University of Nottingham, University Park, NG7 2RD, United Kingdom c Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA d College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, United Kingdom abstract article info Article history: Received 18 June 2014 Received in revised form 29 November 2014 Accepted 2 December 2014 Available online 3 December 2014 Keywords: Nanocomposite Carbon nanotube Alumina Microstructure Mechanical properties Magnesia Magnesia tuned alumina ceramic nanocomposites, reinforced with multi-walled carbon nanotubes, were con- densed using pressureless and hot-press sintering processes. Densification, microstructure and mechanical proper- ties of the produced nanocomposites were meticulously investigated. Electron microscopy studies revealed the homogenous carbon nanotube dispersion within the alumina matrix and confirmed the retention of carbon nano- tubes' distinctive tubular morphology and nanoscale features during the extreme mixing/sintering processes. Pres- sureless sintered nanocomposites showed meagre mechanical responses due to the poorly-integrated microstructures with a slight improvement upon magnesia addition. Conversely, both the magnesia addition and application of hot-press sintering technique resulted in the nanocomposite formation with near-theoretical densi- ties (~99%), well-integrated microstructures and superior mechanical properties. Hot-press sintered nanocompos- ites incorporating 300 and 600 ppm magnesia exhibited an increase in hardness (10 and 11%), flexural strength (5 and 10%) and fracture toughness (15 and 20%) with respect to similar magnesia-free samples. Compared to monolithic alumina, a decent rise in fracture toughness (37%), flexural strength (22%) and hardness (20%) was ob- served in the hot-press sintered nanocomposites tuned with merely 600 ppm magnesia. Mechanically superior hot- press sintered magnesia tailored nanocomposites are attractive for several load-bearing structural applications. © 2014 Elsevier Inc. All rights reserved. 1. Introduction Carbon nanotubes (CNTs) have colossal interest due to their ex- ceptional mechanical properties and functional traits thus being the front-runner reinforcing nanomaterial for contemporary ceramic composite technology [1,2]. Alumina (Al 2 O 3 ) is an important struc- tural ceramic material that is widely used in conventional industries and has promises for advanced load bearing structural applications like military armour system, aircraft engine parts and space engi- neering after curtailing its brittleness [3]. Therefore, a rigorous re- search work was conducted, over the last few years, to transfer the exceptional elasticity and strength of the CNTs to the brittle Al 2 O 3 with numerous reports claiming success in this regard [4–10]. Although CNTs have refined the matrix grains and enhanced the strength, fracture toughness, wear resistance and electrical/thermal properties of the monolithic Al 2 O 3 [11–13], the fabulous properties of CNTs could not be fully transferred to the nanocomposites due to hitches in obtaining even CNT distribution and near theoretical densi- ties in the final nanocomposites, thus new strategies to address these challenges are in demand [4,5,14]. From literature, it seems that the mechanical properties of the CNT– reinforced Al 2 O 3 nanocomposites could be further improved by tuning their microstructures using rare earth metal oxides [15,16]. In this per- spective, microstructural modification of pure Al 2 O 3 and Al 2 O 3 -based composites containing rare-earth metal oxides such as magnesia (MgO), cerium oxide (Ce 2 O 3 ), lithia (Li 2 O) and yttria (Y 2 O 3 ) is an effec- tive practice [17–20]. For example, small amounts (b 1000 ppm) of Y 2 O 3 doping into monolithic Al 2 O 3 and ZrO 2 –reinforced Al 2 O 3 nanocompos- ites have shown notable improvements, resulting in high densities, de- fect-free microstructures and enhanced mechanical properties [19].A recent study on the Y 2 O 3 tuned Al 2 O 3 –CNT nanocomposites reported a higher (N 99%) density, fivefold grain refinement and toughness en- hancement of 40%, compared with the reference monolithic Al 2 O 3 [20]. Similarly, small amounts of MgO (300 ppm) have been reported to re- duce the effective sintering temperatures and restrict grain growth of Al 2 O 3 reinforced with 5 vol.% silicon carbide (SiC) [21,22]. Besides these successes, the microstructural tweaking of the CNT– reinforced Al 2 O 3 nanocomposites with MgO is rarely attempted. For this purpose, MgO-tuned Al 2 O 3 ceramic nanocomposites reinforced with 2 wt.% multi-walled carbon nanotubes (MWCNTs) were fabricated Materials Characterization 99 (2015) 210–219 ⁎ Corresponding author at: Center of Excellence for Research in Engineering Materials (CEREM), Advanced Manufacturing Institute (AMI), King Saud University, P.O. Box: 800, Riyadh 11421, Saudi Arabia. E-mail address: ifahmad@ksu.edu.sa (I. Ahmad). http://dx.doi.org/10.1016/j.matchar.2014.12.002 1044-5803/© 2014 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Materials Characterization journal homepage: www.elsevier.com/locate/matchar