Ultra high-pressure spark plasma sintered ZrC-Mo and ZrC-TiC composites Der-Liang Yung a ,Sławomir Cygan b , Maksim Antonov a , Lucyna Jaworska b , Irina Hussainova a,c,d, ⁎ a Department of Mechanical Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia b Institute of Advanced Manufacturing Technology, 30-011 Krakow, ul. Wroclawska 37A, Poland c ITMO University, Kronverksky 49, St. Petersburg 197101, Russian Federation d Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801, USA abstract article info Article history: Received 17 July 2016 Received in revised form 18 September 2016 Accepted 27 September 2016 Available online 29 September 2016 Ultra-high-pressure spark plasma sintering was applied to ZrC-20 wt%Mo and ZrC-20 wt%TiC composites with a pressure up to 7.8 GPa and temperatures of 1550 °C and 1950 °C. Mechanical performance of the composites was benchmarked against a plain ZrC produced by the same method. Both composites outperformed the pure ZrC with superior hardness and indentation fracture toughness of 2239 HV1 and 5.4 MPa m 1/2 , and 1896 HV1 and 5.9 MPa m 1/2 , respectively, for ZrC-Mo and ZrC-TiC composites. It was shown that ultra-high compaction pressure affected the ZrC-20 wt%TiC miscibility gap by lowering the temperature threshold from the usually applied 1800 °C down to 1550 °C resulting in formation of the solid state solution of (Zr,Ti)C. In contrast, the high pressure does not inhibit the carburisation of Mo with ZrC to form MoC, even when experiments were performed in a graphite free environment. The equiaxed morphology of ZrC grains along with a right-shift in XRD peaks for ZrC indicates dissolution of Mo in ZrC resulting in formation of the solid solution of (Zr,Mo)C. High-temperature X-ray diffraction analysis under oxidation conditions was performed on the samples showing degradation of ZrC- 20 wt%Mo due to the oxidation of Mo at high-temperature leading to MoO 3 vaporisation. Conversely, the oxida- tion of ZrC-20 wt%TiC composites was characterised by formation of ZrO 2 and TiO 2 remaining stable up to 1500 °C. © 2016 Elsevier Ltd. All rights reserved. Keywords: Spark plasma sintering HPHT Mechanical properties ZrC-Mo ZrC-Ti 1. Introduction Zirconium carbide (ZrC) is regarded as an important material either as an additive or a base material for high-temperature composites. The ultra-high melting point (~3420 °C) of ZrC along with its high hardness (~25.5 GPa), low electrical resistivity (4.3 × 10 -7 Ω·cm), high modulus of elasticity (~ 400 GPa), and relatively low density (6.73 g cm -3 ) make this carbide attractive candidate for many applications that require ex- posure to extreme thermal and chemical environments [1]. Moreover, ZrC-based alloys display strong resistance to irradiation damage and are widely used as cladding nuclear materials [2]. However, the strong covalent Zr\\C bonding and a low self-diffusion coefficient makes pro- cessing of dense monolithic ZrC bulks a difficult task which, combined with a low fracture toughness of the pure ZrC (~4.0 MPa m 1/2 ), limits usage of the material for mechanically rigorous applications [3,4]. As for an ultra-refractory compound, very high-temperatures and pres- sure-assisted techniques are usually required to achieve dense bulks of zirconium carbide [5]. Various attempts have been made to improve toughness and me- chanical performance of zirconium carbides by introducing a second phase or appropriate sintering aids to the ZrC matrix. For example, ZrC–Mo composites of N 98% relative density were produced by hot iso- static pressing at 1800 °C and 200 MPa for 1 h [6,7]. However, the pro- cessing requires a non-carburising environment below 2100 °C since molybdenum readily reacts with carbon producing Mo 2 C or MoC [4]. Therefore, the goal to achieve a liquid phase sintering and to produce vi- able ZrC-Mo cermet is compromised by a lack of metallic Mo in the sys- tem. Another approaches are either fabrication the solid-state solution with titanium carbide (TiC) [8–10], which requires adjusting molar ra- tios and high-temperature sintering (N 2000 °C) [11], or incorporation of a tetragonal zirconia in an attempt to utilize a phase transformation toughening mechanism [3]. According to Markström et al. [12], the thermodynamic evaluation of the TiC-ZrC system has been performed to estimate a miscibility gap. The composite of ZrC-20 wt%TiC would have an approximate miscibility gap at ~1850 °C according to molar fraction ZrC X -TiC 1 - X , where x = 70 mol fraction). The mixed (Ti,Zr)C phase should be stable at high- temperatures, but decompose into TiC and ZrC at lower temperatures. Borgh et al. [13] suggested that mixed carbide could be used as a strengthening constituent in, for example, cemented carbides. It is be- lieved that the new superhard mixed carbide has a high potential in Int. Journal of Refractory Metals and Hard Materials 61 (2016) 201–206 ⁎ Corresponding author at: Department of Mechanical Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia. E-mail address: hussaino@illinois.edu (I. Hussainova). http://dx.doi.org/10.1016/j.ijrmhm.2016.09.014 0263-4368/© 2016 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM