Please cite this article in press as: Ang C, et al. SPS densification and microstructure of ZrB 2 composites derived from sol–gel ZrC coating. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.015 ARTICLE IN PRESS +Model JECS-9615; No. of Pages 9 Available online at www.sciencedirect.com ScienceDirect Journal of the European Ceramic Society xxx (2014) xxx–xxx SPS densification and microstructure of ZrB 2 composites derived from sol–gel ZrC coating Caen Ang a , Aaron Seeber b , Tim Williams c , Yi-Bing Cheng a,∗ a Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia b CSIRO Process Science and Engineering, Clayton, VIC 3168, Australia c Monash Centre for Electron Microscopy, Monash University, Clayton, VIC 3800, Australia Received 12 February 2014; received in revised form 2 April 2014; accepted 8 April 2014 Abstract A technique for densifying ultra high temperature ceramic composites while minimising grain growth is reported. As-purchased ZrB 2 powder was treated with a zirconia-carbon sol–gel coating. Carbothermal reduction at 1450 ◦ C produced 100–200 nm crystalline ZrC particles attached on the surface of ZrB 2 powders. The densification behaviour of the sol–gel coated powder was compared with both the as-purchased ZrB 2 and a compositionally similar ZrB 2 –ZrC mixture. All three samples were densified by spark plasma sintering (SPS). The ZrB 2 reference sample was slow to densify until 1800 ◦ C and was not fully dense even at 2000 ◦ C, while the sol–gel modified ZrB 2 powder completed densification by 1800 ◦ C. The process was studied by ram displacement data, gas evolution, SEM, and XRD. The sol–gel coated nanoparticles on the ZrB 2 powder played a number of important roles in sintering, facilitating superior densification by carbothermal reduction, nanoparticle coalescence and solid-state diffusion, and controlling grain growth and pore removal by Zener pinning. The sol–gel surface modification is a promising technique to develop ultra-high temperature ceramic composites with high density and minimum grain growth. © 2014 Elsevier Ltd. All rights reserved. Keywords: Sol–gel; Zirconium diboride; UHTC; Spark plasma sintering; Grain boundary pinning 1. Introduction Zirconium diboride (ZrB 2 ) belongs to a class of materials known as ultra high temperature ceramics (UHTCs) whose melt- ing points exceed 3000 ◦ C. They possess high thermomechanical strength, chemical resistance and high thermal conductivity, making them candidate materials for thermal protection sys- tems, refractories and high temperature energy generation. 1,2 Due to their highly covalent bonding, processing of monolithic UHTCs is difficult without additives because temperatures of ∼2000 ◦ C are required to achieve appreciable diffusion required for densification. 3 Applied pressure and rapid heating tech- nologies such as spark plasma sintering (SPS) were employed to overcome these hurdles. Processing was still mediated by ∗ Corresponding author. Tel.: +61 3 9905 4930; fax: +61 3 9905 4940. E-mail addresses: caen.ang@monash.edu (C. Ang), Yibing.Cheng@monash.edu (Y.-B. Cheng). additives. These included high fractions of refractory metals like Mo and Nb at 10–25 vol.%, or the silicides of Zr, Ti, Mo, Ta at up to 20 vol.%, which led to glassy grain boundary phases that may promote early intergranular liquid phases. 4–9 At ultra-high service temperatures, they may form liquid phases or are ther- mal insulators, compromising thermal stability and conductivity. Both can result in structural deformation to failure. Thus it is a challenge to use additives without significantly compromising thermal stability. Composites comprising of two UHTC phases are not a new idea. 10,11 Dual-UHTC composites (ZrB 2 –ZrC, TaC–TaB 2 , NbC–NbB 2 ) are a logical step for more refractory compositions. 10,11 In reducing environments, a composi- tion such as ZrB 2 –ZrC would be very stable. Both phases are UHTCs, with high liquidus transitions (ZrB 2 + L and ZrC 1-x + L transitions occur at ∼2800 ◦ C) and high thermal conductivity. 12 In oxidative environments approaching 3000 ◦ C, ZrB 2 would rely on zirconia retention in the oxide scale. At these temperatures, its vapor pressure is low. Densification http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.015 0955-2219/© 2014 Elsevier Ltd. All rights reserved.