CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International ] (]]]]) ]]]–]]] Role of synthesis method on microstructure and mechanical properties of graphene/carbon nanotube toughened Al 2 O 3 nanocomposites Bahareh Yazdani a , Harshit Porwal b , Yongde Xia a , Haixue Yan b , Mike J. Reece b , Yanqiu Zhu a,n a College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, UK b School of Engineering and Material Science, Queen Mary University of London, London E1 4NS, UK Received 6 April 2015; received in revised form 10 April 2015; accepted 10 April 2015 Abstract The effects of hot-pressing (HP) and spark plasma sintering (SPS) methods on the grain size, microstructural features, and mechanical behaviour of graphene nanoplatelet/carbon nanotubes (GNTs) reinforced Al 2 O 3 nanocomposites were comprehensively studied. Different graphene nanoplatelet to carbon nanotube ratios were selected as the overall reinforcement content of composites prepared using HP and SPS. Highly densified samples ( 498%) were obtained at 1650 1C under 40 MPa in Ar atmosphere, with dwell times of 1 h and 10 min for HP and SPS respectively. Both types of sample showed a mixture of inter- and transgranular fracture behaviour. A 50% grain size reduction was observed for samples prepared by HP compared to SPS samples. Both types of samples achieved a high flexural strength and fracture toughness of 4400 MPa and 5.5 MPa m 1/2 , whilst SPS samples peaked at relatively lower GNT contents than those for the HP samples. Based on analyses of the morphology, grain sizes and fracture mode, similar toughening mechanisms for both types of sample were observed, involving the complex characteristics of the combined GNT fillers. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: D. Al 2 O 3 ; Graphene nanoplatelets; Carbon nanotubes; Hot-press; Spark plasma sintering 1. Introduction During the last few years, 1-Dimensional carbon nanotubes (CNTs) have been widely used for producing advanced ceramic composites with improved thermal [1], electrical [2], and mechanical properties [3–7]. The wider application of CNTs as a filler in ceramics is still limited by the difficulty of uniformly dispersing them, due to the strong Van der Walls interaction between CNTs which make them tend to form agglomerates and bundles. Graphene, and in particularly the less expensive to produce graphene oxide (GO), has attracted increasing research interest as additives in ceramic composites. The 2-dimensional and flexible flakes with large surface area contribute to improved electrical conductivity, mechanical properties and thermal conductivity [8– 10]. For instance, a remarkable 235% improvement in fracture toughness with only 1.5 vol% of graphene addition in Si 3 N 4 was reported [11]; whilst 53% and 13 orders of magnitude improve- ments in fracture toughness and electrical conductivity respec- tively were documented in graphene nanosheet reinforced Al 2 O 3 composites [12]. However, re-stacking of the graphene sheets is a practical issue during the composite fabrication process. To use both graphene nanoplatelets (GNPs) and CNTs (together named as GNTs) as a hybrid reinforcement agent could combine the advantages of individual 2-D and 1-D reinforce- ments. This could potentially reduce the re-stacking issue of the GNPs, as insertion of CNTs amongst neighbouring graphene sheets would stop them stacking together; whilst the wrapping of CNTs by GNPs would also help to eliminate the agglomeration of CNTs to some extent. Therefore the uniform dispersion of GNTs may lead to further property improvements for the composites. This hybrid reinforcement has previously been investigated only for a few polymer-based systems and led to ultra-strong man- made fibres [13]. Very recently, we extended this concept and have successfully developed various GNT-reinforced Al 2 O 3 www.elsevier.com/locate/ceramint http://dx.doi.org/10.1016/j.ceramint.2015.04.054 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Corresponding author. E-mail address: Y.Zhu@exeter.ac.uk (Y. Zhu). Please cite this article as: B. Yazdani, et al., Role of synthesis method on microstructure and mechanical properties of graphene/carbon nanotube toughened Al 2 O 3 nanocomposites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.054