Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Thermodynamical evaluation, microstructural characterization and mechanical properties of B 4 C–TiB 2 nanocomposite produced by in-situ reaction of Nano-TiO 2 Mina Khajehzadeh a,* , Naser Ehsani a , Hamid Reza Baharvandi a , Alireza Abdollahi a , Mostafa Bahaaddini a , Abbas Tamadon b a Faculty of Material & Manufacturing Technologies, Malek Ashtar University of Technology, P.O. Box, 15875-1774, Tehran, Iran b Department of Mechanical Engineering, University of Canterbury, Christchurch, 8041, New Zealand ARTICLE INFO Keywords: Nanocomposite B 4 C TiB 2 In-situ sintering Pressureless sintering TiO 2 ABSTRACT This work discusses the pressureless sintering of a boron carbide-titanium diboride (B 4 C– TiB 2 ) nanocomposite via in-situ reaction of the boron carbide/titanium dioxide/carbon system. Attempting to sinter pure boron carbide leads to poor mechanical properties. In this work, the effect of adding TiO 2 to B 4 C on mechanical properties of the boron carbide was investigated. Thermodynamic simulations were performed with HSC chemistry software to determine the phases which were most likely to form during the sintering process. The reaction thermodynamics suggested that during the sintering process, formation of TiB 2 occurs preferentially over formation of TiC. For examination of the microstructural evolution of the samples, Scanning Electron Microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized. The density, porosity, Young's modulus, microhardness and fracture toughness of the specimens were compared. Optimum properties were achieved by adding 10 wt% TiO 2 . In the sample possessing 10 wt% TiO 2 , the relative density, Young's modulus, hardness and fracture toughness were 94.26%, 428 GPa, 23.04 GPa and 5.19 MPa m 0.5 , respectively, and the porosity was decreased to 5.73%. Furthermore, phase analysis via XRD confirmed that the final product was free of unreacted TiO 2 or carbon. 1. Introduction Boron carbide is one of the most thermodynamically stable com- pounds, with a stoichiometry ranging from B 10.4 C to B 4 C[1]. Boron carbide is a desirable candidate for a variety of industrial applications [2–4], including welding electrodes, neutron-absorbing components, solid fuel, and body armour [4–6]. It has excellent mechanical and physical properties such as a high hardness (40–60 GPa), a low density (2.52 g/cm 3 ), and a high melting point (2427 °C), etc. Boron carbide requires densification to be useful. Industrially, this can be achieved through a number of methods such as hot pressing (HP), hot isostatic pressing (HIP), spark plasma sintering (SPS), mi- crowave and pressureless sintering (PS). Pressureless sintering has re- ceived a large amount of attention because of its relatively low cost and ability to produce complex shapes [4,7]. However, during this process, the relative density of sintered boron carbide is affected by abnormal grain growth and surface melting [2,3,8–11]. Utilizing certain additives can reduce the sintering temperature, and subsequently increase the relative density and oxidation stability, thus preventing unwanted grain growth [1,12]. To improve the sinterability and toughness of the ceramic, additives can be used which include oxides, such as Cr 2 O 3 ,Y 2 O 3 , TiO 2 , SiO 2 , ZrO 2 , and La 2 O 3 ; carbides and borides, TiB 2 , TiC, and SiC; and metals, Al, Ni, Si, and Ti [13]. By adding titanium diboride, TiB 2 , to boron carbide, B 4 C, the sintering temperature decreases, and the mechanical properties are improved through modification of the microstructure [14]. Adding carbon to the B 4 C–TiB 2 system can improve the toughness of the composite, as it prohibits the formation of relatively weak in- terfacial phases that are susceptible to microcrack generation [15–17]. When carbon is added to the B 4 C–TiB 2 system, it segregates at boundaries between the phases and creates a weak interface suitable for initiation and propagation of cracks at the interface of the B 4 C–TiB 2 [15]. Moreover, the existing B 2 O 3 oxide layer at the surface of the carbide particles disrupts the reactions during the sintering process and https://doi.org/10.1016/j.ceramint.2020.07.174 Received 2 November 2019; Received in revised form 9 July 2020; Accepted 16 July 2020 * Corresponding author. E-mail addresses: mina.khajehzadeh6462@gmail.com (M. Khajehzadeh), ehsani@mut.ac.ir (N. Ehsani), hrbahar@ut.ac.ir (H.R. Baharvandi), alirezaabdollahi1366@gmail.com (A. Abdollahi), mostafabahaaddini@gmail.com (M. Bahaaddini), abbas.tamadon@pg.canterbury.ac.nz (A. Tamadon). Ceramics International 46 (2020) 26970–26984 Available online 22 July 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved. T