Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond Spark plasma sintered Al-0.5 wt% MWCNT nanocomposite: Efect of sintering pressure on the densifcation behavior and multi-scale mechanical properties Lavish K. Singh, Alok Bhadauria, Akash Oraon, Tapas Laha ⁎ Department of Metallurgical & Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, India ARTICLEINFO Keywords: Al-MWCNT composite Sparks plasma sintering Sintering pressure Densifcation behaviour Mechanical properties ABSTRACT The current study aims to understand the efect of sintering pressure on densifcation behaviour and mechanical properties of multi walled carbon nanotube (MWCNT) reinforced aluminum (Al) based nanocomposite syn- thesized via spark plasma sintering (SPS). Physio-chemical functionalization, a novel dispersion technique, was followed which led to homogenous dispersion of MWCNTs in Al matrix. The crystallite size and density of the synthesized Al-0.5 wt% MWCNT nanocomposites increased with increase in sintering pressure. The relative density increased from 90.4% to 95.1% upon increasing the sintering pressure from 30 MPa to 80 MPa. To understand the densifcation behaviour, the movement of graphite punch within the die flled with Al-MWCNT powder mixture during sintering was monitored. Higher sintering pressure led to early onset of sintering and the total punch displacement during sintering increased from 0.38 mm to 1.48 mm upon increasing the applied pressure from 30 MPa to 50 MPa. The impact of high sintering pressure, leading to enhanced densifcation, was also translated in the improvement in mechanical properties. Compressive strength and microhardness increased by ~30% and ~13%, respectively as sintering pressure was increased from 30 MPa to 80 MPa. Higher sintering pressure leading to signifcantly enhanced mechanical properties at micro and macro scale was also replicated at nano-scale as confrmed by the results of nanoindentation and nanoscratch tests. 1. Introduction Aluminum (Al) possess excellent mechanical and physical proper- ties such as good formability, high corrosion resistance, lightweight and low melting temperature which makes it an ideal candidate for appli- cation in desalination machined components, turbines, aircraft, building materials, aerospace and automobile industries [1,2]. Carbon nanotubes (CNTs) have emerged as an ideal reinforcement to design Al- CNT nanocomposite owing to the excellent mechanical properties such as enormously high elastic modulus (~1 TPa) and tensile strength (~30 GPa) along with low density (1.8 g/cc) [3–5]. Components with nanometre-sized or ultrafne grains possess sig- nifcantly higher mechanical properties than the microcrystalline grained counterparts. Conventional sintering requires fairly high sin- tering temperature to obtain a highly densifed compact which also leads to grain growth causing the transformation of initial ultrafne grains to microcrystalline grains [6,7]. Spark plasma sintering (SPS) has emerged as one of the most advanced sintering process which leads to consolidation of high density compact at low sintering temperature. Three distinguished factors that contribute to this low temperature-high densifcation phenomenon are: (i) the use of pulsed direct current, (ii) application of very high heating rates, and (iii) the application of very high uniaxial pressure [8,9]. This technique resembles conventional hot-pressing, but in this case low-voltage, high intensity pulsed DC current fowing directly through the powder particles and the die heat up the sample which makes it possible to attain signifcantly higher heating rate. Apart from Joule heating, short pulses generated during SPS produce plasma discharges which assist in the early stages of sin- tering [10]. The application of mechanical pressure enhances mass transfer and helps in removing pores, thereby, improving density of the compacts. But, the correlation between the pressure applied during SPS and sin- tered materials' properties is not well-defned. Rumman et al. observed that the hardness of WC-7.5 wt% nano Co powder consolidated by SPS increases with increasing sintering pressure [11]. Libardi et al. also inferred that with the increase in pressure, grain size reduces, whereas, density and hardness increases for SPS consolidated milled FeMo powder [12]. However, Khali et al., while studying the efect of SPS https://doi.org/10.1016/j.diamond.2018.11.021 Received 14 September 2018; Received in revised form 8 November 2018; Accepted 21 November 2018 ⁎ Corresponding author. E-mail address: laha@metal.iitkgp.ernet.in (T. Laha). Diamond & Related Materials 91 (2019) 144–155 Available online 23 November 2018 0925-9635/ © 2018 Elsevier B.V. All rights reserved. T