Original Research Paper CoB-TiB 2 crystalline powders: Synthesis, microstructural analysis and their utilization as reinforcement agent Sina Khoshsima a , Zerrin Altıntas ß a , Ulrich Burkhardt b , Marcus Schmidt b , K.G. Prashanth c,d , Mehmet Somer a,e , Özge Balcı a,e,⇑ a Koç University Boron and Advanced Materials Application and Research Center, Rumelifeneri Yolu, 34450 Sarıyer, _ Istanbul, Turkey b Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany c Department of Mechanical and Industrial Engineering, Tallinn University of Technology, 19086 Tallinn, Estonia d Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, A-8700 Leoben, Austria e Koç University, Department of Chemistry, 34450 Sarıyer, _ Istanbul, Turkey article info Article history: Received 8 February 2020 Received in revised form 2 April 2020 Accepted 19 May 2020 Available online 12 June 2020 Keywords: Cobalt boride Titanium diboride Low temperature synthesis Microstructural characterization Bulk properties abstract Due to promising mechanical and chemical properties, transition metal borides have attracted attention, and numerous studies have investigated various combinations of transition elements in hopes of acquir- ing a final product with desired properties combined. In this study, novel low-temperature approach was adopted for the synthesis of cobalt-titanium-boron based crystalline powders. The method was based on the single-step direct reaction of CoCl 2(s) , TiCl 4(l) and NaBH 4(s) in a sealed reactor under autogenic pres- sure. After the reaction of the precursors at 850 °C by using the molar ratios of metal chlorides to NaBH 4 as 1:3, CoB and TiB 2 phases were formed in-situ. The subsequent annealing process at 1100 °C achieved a full conversion of metal chlorides to CoB-TiB 2 composite nanostructures. It was concluded that the binary forms of the borides tend to form as separate phases, which is illustrated in the SEM/EDS anal- yses with different morphologies. Amorphous boron layer surrounded TiB 2 particles with an average par- ticle size of 60 nm, whereas the CoB particles formed agglomerates with an average size of 450 nm. The use of synthesized composite powders as reinforcement in metal matrices resulted in enhanced hardness (506 HV) and compressive strength (1682 MPa) of the Ti6Al4V bulk samples. Ó 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 1. Introduction Having a combination of superior chemical, physical, and mechanical properties have made transition metal borides of dire importance in various scientific and industrial fields [1–3]. Transi- tion metal borides, thus, are utilized in automotive and aerospace industries and applied in numerous applications such as wear resistant coatings, magnets, catalyzers, cutting tools, and reinforce- ment agents in metal matrix composites or ceramics [3–8]. Numer- ous studies investigating properties of various binary and ternary boride ceramics have been conducted to obtain and/or improve the ceramics to meet the needs of industry and science. Co-based boride compounds have been used in various alloys as a coating material with high wear and corrosion resistance. There have been studies in the literature on significant improvements in the mechanical properties (elastic modulus, fracture toughness, etc.) of cobalt boride coated or impregnated alloys [9–11]. It is thought that Co-B based compounds developed by doping Ti will give superior properties compared to the binary system in terms of their mechanical properties [12]. On the other hand, being a well-known high temperature ceramic and possessing a set of highly desirable properties such as high melting point, strength, elastic modulus (>500 GPa), and hardness (>20 GPa), TiB 2 has attracted much attention in science and industry due in part to heavy duty wear applications and, thus, is considered amongst the most advanced and prominent ceramics [3,13–18]. Various studies have investigated binary boride compounds, and many synthesis methods have been applied to obtain them such as self-propagating high temperature synthesis (SHS), car- bothermal reduction, borothermal reduction, and mechanochemi- cal synthesis [19–24]. A comparison of various synthesis methods including the effect of the synthesis conditions and parameters on the feasibility of the method is illustrated in https://doi.org/10.1016/j.apt.2020.05.026 0921-8831/Ó 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. ⇑ Corresponding author at: Koç University Boron and Advanced Materials Application and Research Center, Rumelifeneri Yolu, 34450 Sarıyer, _ Istanbul, Turkey. E-mail address: obalci@ku.edu.tr (Ö. Balcı). Advanced Powder Technology 31 (2020) 2964–2972 Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt