Mechanical properties of MWNT/Ni bulk composites: Influence of the microstructural refinement on the hardness Sebastian Suarez, Federico Lasserre, Frank Mücklich n Department of Materials Science, Saarland University, Campus D3.3, D-66123 Saarbrücken, Germany article info Article history: Received 18 June 2013 Received in revised form 22 July 2013 Accepted 24 August 2013 Available online 5 September 2013 Keywords: Multiwalled carbon nanotubes Metal matrix composites Mechanical properties Microstructure Powder metallurgy abstract The grain growth stagnation produced by the addition of multiwalled carbon nanotubes to bulk nickel composites manufactured by hot uniaxial pressing is investigated. A hardness improvement up to 27% compared to pure nickel was achieved, mainly based on the microstructural refinement. However, this enhancement is only observable up to concentrations of 3 wt%. Beyond this point, no significant improvement is observed, thus setting an empirical reinforcing limit. The main strengthening mechan- ism identified was the Hall–Petch effect, which hinders the dislocation movement and is strongly dependent on the final grain size of the composites. Finally, a model for the prediction of the final grain size is proposed based upon the Zener pinning equation for the particular system studied in this work. This model is intended to provide a basic tool for a further tailoring of the mechanical properties, adjusted for a certain application. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Since composite materials were first used in engineering, researchers focused on tailoring the mechanical, electrical or thermal properties to the needs of a certain field of application. In the case of metal matrix composites (MMC), great advances have been made towards the improvement of the mechanical properties of materials with low intrinsic strength such as alumi- nium or copper. Carbon nanotubes (CNT) and their outstanding mechanical, thermal and electronic properties have been at the centre of scientific research since day one. These characteristics lead many scientists to foresee CNTs as the ultimate reinforcement for composite materials in a wide span of applications. In the particular case of multiwalled carbon nanotubes (MWNT), their high specific strength [1], thermal conductivity [2], electrical characteristics [3] and increasing availability as well as low cost make them a suitable candidate for bulk applications in MMCs. For instance, MWNTs reach a tensile strength of about 63 GPa [1], making them one of the highest specific tensile strength material known today. Nickel is a moderate-strength metal with a density comparable to that of copper but with a higher melting point, hardness and tensile strength. Many studies of the hardness and wear properties of nickel-based coating films focused on samples prepared by electroless [4] or electrochemical techniques [5–7]. They observed that the addition of MWNTs to a metallic matrix leads to a hardness increment of the samples. In this contribution, the work is focused on powder metallurgy manufactured bulk Ni/CNT samples. It is expected to observe also an increment of the hardness which could be tailored by varying the amount of reinforcement. The addition of MWCNTs is sup- posed to hinder the grain growth, and therefore increases the hardness verifying the Hall–Petch relationship. The composites are characterized using X-Ray diffraction (XRD), Raman spectroscopy, Electron Backscattered Diffraction (EBSD) and Vickers microhard- ness. The relationship between the hardness and the mean grain size of composites with four different MWNT concentrations (1 wt %, 2 wt%, 3 wt% and 5 wt%) is investigated. An approach to predict the final grain size of the composites comparing the experimental data to the theoretical models is proposed. 2. Material and methods MWNT commercially available as Baytubes C150P (purity: 495%, outer diameter distribution: 5–20 nm, agglomerate size: 0.1–1 mm) and dendritic Ni powder (Alfa Aesar, mesh À 325) were used as starting materials. A previously developed blending process called colloidal mixing process was utilized [8]. For this study, the original solvent was replaced by Ethylene Glycol (EG) fixing the concentration ratio MWNT/EG at 0.2 mg/ml due to better dispersion stability. The different MWNT/Ni weight ratios were 1, 2, 3 and 5 wt%. The dried powders are subsequently cold Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.08.058 n Corresponding author. Tel.: þ49 681 302 70500; fax: þ49 681 302 70502. E-mail address: muecke@matsci.uni-sb.de (F. Mücklich). Materials Science & Engineering A 587 (2013) 381–386